Testing of metal and alloy

1
Chapter I
HARDNESS TEST
Hardness is a property exhibited by all materials. If we hit a small block of copper
with a hammer the block elongates and a dent of the shape of the hammer tip can
be seen on the copper block. From this we can say that the hammer is harder than
the copper block.Similarly we can take any two materials and find out which of
them is harder.This can be done by hitting one with the other, by rubbing one
against another, or trying to pierce or dent one with the other.In all the cases,
harder material remains unaffected.Depending upon the way in which the hardness
is found out, the method is suitably termed.When we try to break one of the two
materials by sudden loading or impact, it is called impact hardness.The word
impact hardness is modified in technical usage and termed as rebound
hardness.Particularly for metals which are crystalline, sudden fracture is not
possible but the metal surface gives a rebound to the material which strikes it.The
amount of rebound is proportional to the hardness of the surface.In fact, the
rebound hardness is measured thus.
Scratch or wear hardness is that property of materials by virtue of which they resist
wear or abrasion.Similarly, the amount of resistance offered by a metal for
indentation or penetration is defined as the indentation hardness of the metal. Thus,
we define hardness as the property of a metal by virtue of which it resists scratch,
wear, abrasion or indentation.
In the industry, the indentation hardness is most commonly measured. In the
metallurgical sense, it has become a practice to understand hardness as the
indentation hardness only, unless otherwise specified.
The hardness of a material is a poorly defined term which has many meaning
depending upon the experience of the person involved. In general, hardness usually
implies a resistance to deformation, and for metals the property is a measure of
their resistance to permanent or plastic deformation. To a person concerned with
the mechanics of materials testing, hardness is most likely to mean the resistance to
indentation, and to the design engineer it often means an easily measured and
specified quantity which indicates something about the strength and heat treatment
of the metal.There arethree general types of hardness measurements depending on

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the manner in which the test is conducted.There are (i) scratch hardness, (ii)
indentation hardness and (iii) rebound or dynamic hardness. Only indentation
hardness is of major engineering interest for metals.
Scratch hardness is of primary interest to mineralogists. With this measure of
hardness, various minerals and other materials are rated on their ability to scratch
one on other. In dynamic-hardness measurements the indenter is usually dropped
onto the metal surface, and the hardness is expressed as the energy of impact. The
shore scleroscope, which is the commonest example of a dynamic hardness tester,
measures the hardness in terms of the height of rebound of the indenter.
SCRATCH HARDNESS
This is by far the oldest hardness testing method. Through seldom used except
mineralogical purposes,the testing is very simple according to hardness scale
known after Mohs. It consists of ten minerals arranged in increasing order of
hardness. The principle behind this method of testing is that when two materials
are scratched against each other, the harder of two scratches the other. The material
under test is scratched with the mineral number1. If the mineral fails to scratch the
material, then the next harder mineral, i.e. mineral number 2 is used and so on
until the material is scratched by a mineral. Suppose the mineral which scratches is
the one numbered 5, then the hardness of the material is expressed as lower than 5
and higher than 4. The Mohs scale of hardness is as follows:
1. Talc
2. Gypsum
3. Calcite
4.Fluorite
5.Apatite
6.Orthoclase(or feldspar)
7.Quartz

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8.Topaz
9.Corundum
10.Diamond.
Small pieces, of about two cm. size are arranged in numbered chambers in a
wooden box resembling the weight box of a conventional balance. This forms an
important appliance for testing in geological laboratories, besides serving as a
portable unit for geological expeditions.
BRINELL HARDNESS
The first widely accepted and standardized indentation-hardness test was proposed
by J.A. Brinell in 1900. The standard Brinell hardness test consists in indenting
the metal surface with a 10 mm-diameter steel ball at a load of 3000 kg. For soft
metals the load is reduced to 500 kg to avoid too deep an impression and for very
hard metals a tungsten carbide ball is used to minimize distortion of the indenter.
The load is applied for a ASTM (American Society for Testing and Materials)
specified standard time usually 15s for ferrous materials and 30s for non ferrous
materials. The diameter of the indentation is measured with a low-power
microscope after removal of the load. The average of two readings of the diameter
of the indentation at right angles should be made. The surface on which the
indentation is made should be relatively smooth and free from dirt or scale. The
Brinell hardness number(BHN) is expressed as the load P divided by the surface
area of the indentation.
Non standard Brinell hardness tester uses hardened steel balls of different diameter
(<10 mm) as the indenter with suitable loads. Nowadays, electrically operated
machines are available. After indenting, i.e.applying the load on the ball, for the
above mentioned ASTM specified time, the load is removed and the indentation
made is measured. In the Brinel machine, the surface area of the indentation is
calculated and used as an index of the hardnes of the metal. The surface area

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indentation is dependent upon the depth of indentation.The load applied (in kgf)
divided the spherical area of indentation (in square millimeters)is taken as the
Brinell Hardness Number.
B.H.N= (LOAD P) / (Spherical area of indentation)
Calculation of the Spherical or Surface Area of Indentation
The circle in Figure 1.1 is the periphery of the indenter whose diameter is D. The
indented portion of the material shown shaded is the replica of a part of the surface
of the indenter.
From mensuration principles, the surface area of segment of a sphere is =( *D*h)
Where D=diameter of the sphere,
and h=height of the portion under indentation,
From Figure 1-1, h={(D/2)-x}
Again, from the right angled triangle OCB,
(D/2)2
=(x)2
+ (d/2)2
x = { (D/2)2
– (d/2)2
}1/2
=1/2 * {D2
– d2
}1/2

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Substituting in (2) for 'x' we get,
h= D/2 - 1/2 * {D2
– d2
}1/2
Substituting in (1) for 'h', we get the spherical area of indentation
A = ∗ ∗ 1/2 ∗ √ D – d
The value of 'D' is known and the value of 'd' is measured with the help of a Brinell
microscope.The hardness of the metal can be calculated from the following
formula.
Where P = applied load,kg
D= diameter of ball indenter,mm
D1 = diameter of indentation,mm
h = depth of the indentation,mm
The hardness value can also be obtained by reading the standard charts which
indicate the hardness number directly based upon the load applied and the diameter
of the indetation.It will be noted that the units of the BHN are kilograms per square
millimeter.(1 kgf mm =9.8 MPa).

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From the Figure 1.2, it can be seen that d=D sino. Substitulion into Eqn (4) gives
an alternate expression for Brinell hardness number.
In order to obtained the same BHN with a nonstandard load or ball diameter it is
necessary to produced geometrically similar indentations. Geometric similitude is

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achieved so long as the included angle 2 remains constant. Eqn (5) shows that for
and BHN to remain constant the load and ball diameter must be varied in the ratio
P1/D1
2
= P2/D2
2
= P3/D3
2
Unless precautions are taken to maintain constant, which may be experimentally
inconvenient, the BHN generally will vary with load. Over a range of loads the
BHN reaches a maximum at some intermediate load. Therefore, it is not possible to
cover with a single load the entire range of hardness encountered in commercial
metals. The relatively large size of the Brine impression may be an advantage in
averaging out local heterogeneities. Moreover, the Brinell test is less influenced by
surface scratches and roughness than other hardness tests. On is the other hand, the
large size of the Brinell impression may preclude the use of this test with hand, the
small objects or in critically stressed parts where the indentation could be a
potential site of failure.
SCOPE AND APPLICATIONS:
The Brinell hardness test makes use of the steel ball as an identer.The hardness of
the steel ball should be sufficiently higher than the material under test, otherwise
the indenting ball itself will be deformed. Thus, the hardness of steels in the fully
or surface hardened condition can not be determined by this method. In other
words, the Brinell Hardness Numbers are not reliable above 350. Further,
depending upon the material under tesu, right selection of the bal and the load
should be made.
From the relation

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We can see that there is the constant P/D . If this ration is fixed, the indentations
produced or the values of 'd' will be in a particular range of 0.25D to 0.50D and in
the ideal situation d=0.375D. Hence, the ratio of P/D is maintained at different
values in order to obtain measurable indentations on the materials.
Usually P/D2
= 30 for iron and steels,
= 10 for copper and copper base alloys,
= 5 for aluminium and aluminium base alloys,
= 2 for soft melals like lead and lead base alloys.
The surface of the material should be even and smooth in order to obtain a clear
indentation. Irregularities induce difficulties not only in the proper distribution of
the load but also result in impression with no well-defined contours. Thus, an
accurate measuremenu of the diameter of the indentation becomes impossible.
Very thin specimens should not be tested by the Brinell method. When there is not
sufficient material to back up, the region of indentation may become unduly large
due to the least resistance offered to the ball. The thickness of the metal under test
should be at least equal to the diameter of the indenter or ten times depth of
indentation.
Similarly, the measurements at the edges and corners of the materials become
difficult. Maintain a distance of 2d to 3d from the edge/corner as well as between
the two adjacent indentations. Round objects should be filed out at a suitable place
so as to obtain a plain surface and tested by placing them on U or V shaped
backing anvil.

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The indenting ball should be frequently checked for its spherical shape and
accuracy by finding out the hardness of the standard test block supplied along with
the machine.
The Brinell hardness number is roughly related to the tensile strength. This
empirical relationship is as follows:
T.S = K x B.H.N. (tons / inch2
)
Where K = 0.217 for alloy steels and
K = 0.220 for plain carbon steels.
For wrought light alloys. the relationship is as follows:
T.S.= 0.25 x B.H.N. - 1 (tons/inch2
)
The Brinell hardness number (B.H.N.) also related to the rebound hardness
measured by Shore's Schleroscope (S) as per Beeching's formula (applicable for
steel) which is as follows:
S = 0.108 * BHN + 8
and S = 0.100* BHN + 15 when s is more than 55
MEYER'S HARDNESS
It is more rational to consider the projected area of indentation than the actual
surface area in the calculation of the hardness. This idea was put forth by Meyer in
1908. The mean pressure between the surface of the indenter and the indentation is
equal to the load divided by the projected area of indentation.
Pm = P /
Meyer proposed that this mean pressure should be taken as the measure of
hardness. It is referred to as the Meyer's hardness.
Pm = P /

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Like the Brinell hardness,Meyer hardness has units of kilograms per square
millimeter. Mayer proposed an empirical relation between the applied load and the
size of the indentation. This relationship is usually called Meyer's Law.
P = K* dn
Where P =load applied in kg,
K= a material constant based upon the resistance of the metal to penetration,
n= a material constant based upon the strain hardening of the metal.
d = diameter of the indentation in mm.
If log P is plotted against log d, a straight line graph is obtained. The slope of the
straight line is n' and K is the value of P when d = 1. The value of n' for fully
annealed metal is found to be 2.5 and for fully strain-hardened metals, about 2.0.
The exponent in Meyer's law is approximately equal to the strain-hardening
coefficient plus 2.
There is a lower limit of load below which Meyer's law is not valid. If the load is
too small the deformation around the indentation is not fully plastic and the Eqn
(2) is not obeyed. This load will depend on the hardness of the metal. For a 10 mm
diameter ball the load should exceed 50 kg for copper with a BHN of 100 and for
steel with a BHN of 400 the load should exceed 1500 kg. For balls of different
diameter the critical loads will be proportional to the square of the diameter
VICKERS HARDNESS
Vickers Hardness Testing Machine also functions on the same principle as the
Brinell hardness testing machine, but employs a square-base pyramid made of
diamond as the indenter. The size of the indenter is also very small. The included
angle between the opposite faces of the pyramid is 136°. This angle was chosen
because it approximates the most desirable ratio of indentation diameter to ball
diameter (i.e. d = 0.375D) in the Brinell hardness test, which results in cone angle
of 136°. Because of the shape of the indenter, this is frequently called the diamond-
pyramid hardness test.

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This machine is more versatile than the Brinell hardness tester. Instead of changing
the as well as the load depending upon the nature of the material tested, only the
load is changed in the Vickers hardness tester. Varying loads from 1.0 kgf onwards
up to 120 kgf are s employed. Owing to the fineness and the small size of the
indentation obtaincd. the specimen needs a glossy surface finish for testing
As in the case of Brinell hardness tester, the diamond-pyramid bardness number
(DPH) or Vickers hardness number (VHN) or Vickers pyramid bardness (VPH) is
defined as the load divided by the surface area of the indentation. In practice, this
area is calculated from microscopic measurements of the lengths of the diagonals
of the impression. Thus the diamond pyramid hardness number
D.P.H. = (P/A)
Where P = Load applied, and
A = lateral area of pyramidal indentation in sq.mm
The area of the pyramidal impressions is calculated from the mensuration formula.
A = 4a * S/2
where 'S' is the slant height of the impression.

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From Figure 1.3., it can be seen that
D2
= 2*a2
A = (4*D/√2)*(S/2)
= √2 ∗ ∗
The included area of the indentation is the same as the included angle of the
pyramid. This angle is 136° (Figure 1.3). The slant height can be expressed in
terms of D as follows:
S =(a)/(2*sin(68°))

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Substituting the value of 'S' from (3) in (2) we get
A= D2
/ 2*sin(68)
If the value of 'A' as for(4) is substituted in the hardness number equation. Then,
DPH = P / A = P / ( D2
/ 2*sin(68))
Where P = applied load and
D = diagonal length of the impression.
Standard charts are supplied along with the Vickers machines, which give the
Vickers hardness number depending upon the diagonal length of the impression at
each load. Using these charts, the hardness rumher can be readily found.
The machine is the most accurate and versatile. Very high degree polishing of the
metal surface is required so that the impression can be perfectly focused and
viewed and the diagonal accurately measured. Very careful handling of the
indenter is recommended. Besides periodic checking of instrument against te
standard test blocks is required.
The Vickers hardness test has received fairly wide acceptance for research work
because it provides a continuous scale of hardness, for agiven load , from very soft
metals with DPHof 5 to extremely hard materials with a DPH of 1500. With the
Brinell hardness test, it is usually necessary to change either the load or the
indenter at some point in the hardness scale, so measurements at one extremeof the
scale cannot be strictly compared with those at the other end. Because de by the
pyramid indenter are geometrically similar no matter what thier size, the DPH
should be independent of load. This is generally found to be the case, except at
very light loads. In spite of these advantages, the Vickers hardness test has not
been widely accepted for routine testing because it is slow,requires careful surface
preparation of the specimen and allows greater chance for personal error in the
determination of the diagonal length. The Vickers hardness test is described in
ASTM standard E 92-72.

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A perfect identation made with a perfect diamond-pyramid indenter(Figure 1.4(b)
and 1.4(c)).The pincushion identation in figure 1.4(b) is the result of sinking in of
the metal around the flat faces of the pyramid. It results from ridging or pilling up
of the metal around the faces od the indeter.The diagonal measurement in this case
produces a low value of the contact that the hardness number is erroneously high.
Brinell cum Wickers Hardness Tester:
Machines capable of performing either Brinell or Vickers hardness tests are
arrailable. It is supplied with the hardened steel ball indenter as well as pramidal
diamond indenter.The indenter as well as image projection objective will come
into the position while loading and measuring respectively by means of electric
make and break mechanism inside the instrument. The indentation is magnified
and projected onto a ground glas fitted at thetop of the machine. A vernier
mechanism provided in the screen facilitates accurate measurement of the
indentation.

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The Brinell and Vickers hardness values are similar in their nature from the basic
principles. The two values will tally up us the hardness number400 and thereafter,
the Vickers values are found to be more (Figure 1.5). However, in this image, i.e.
above 400 vickers hardress values are more reliable.
ROCKWELL HARDNESS TEST
Probably the most widely used hardness test is Rockwell hardness Lest. This test
utilizes the depth of indentation, under constant load, as a measure of hardness. A
minor load of 10 kg is first applied to seat the specimen. This minimizes amount of
surface perparastaen needed and reduces the tendency for ridging in by the
indenter. The major load is then applied, and the depth of indentation is

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automatically recorded on a dial gage in terms of arbitrary hardness numbers. The
principle of operation concerning the Rockwell hardness tester is best described by
reference to Figure 1.6. The dial contains 100 divisions, each division
representinga penetration of 0.002mm. The dial is reversed so that a high hardness,
which correspond to a small penetration, reasults in a high hardness number. This
is in agreement with the other hardness numbers described previously, but unlike
the Brinell and Vickers hardness designation,which have units of kilogram per
square millimeter,the Rockwell hardness numbers are purely arbitrary.
One combination of load and indenter will not produce satisfactory results for
materials with a wide range of hardness. A 120° diamond cone with a slightly
rounded point, called a Brale indenter, and 1.6 and 3.2 mm diameter steel balls are
generally used as indenters. Major loads of 60, 100 and 150 kg are used. Since the
Rockwell dependent on the load and indenter,it is necesary to specify the
combination which is used. This is done by prefixing the hardness number with a
letter indicating the particular combination of load and indenter for the hardness
acale employed. A Rockwell hardness number without the letter prefix is
meaningless. Hardened sleel is tested the C scale with the diamond indenter and a
150 kg major load. The useful range for this scale is from about R20 to R70. Softer
materials are usually tested on the B scale with a 1.6mm-diameter steel hall and a
100-kg major load. The range of this scale is from R0 to R100. A scale (diamond
indenter,60-kg major load) provide the most extended Rockwell hardness scale,

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which is usable materialsfrom annealed brass to cemented carbides.Many other
scales are available for special purposes.
The Rockwell hardness test is a very useful and reproducible one provided that a
number of simple precautions are observed. Most of the point listed below apply
equally to other hardness tests:
1. The indenter and anvil should be clean and well seated.
2 The surface to be should be clean and dry,smooth, and free from oxides. A
rough-ground surface is usually adequate for the Rockwell test.
3. The surface should beflat and perpendicular to de indenter.
4. The thickness of specimen should be such that mark or bulge is not produced on
the reverse side of the piece. It is recommended that the thickness be at least 10
times the depth of indentation.Tests shauld be made on only a single thickness of
material.
5. The spacing between indentations should be three to five times the diameter of
the indentation.
6. The speed of application of the load should be standardized. This is done by
adjusting the dashpot on the Rockwell tester.Variations in hardness can be
applicable in vey soft materials unless the rate of load application is carefully
controlled. For such materials the operating handle of the Rockwell trster should be
brought back as soon as the major load has been fully applied.
Rockwell Hardness Scales and Prefix Letters
Scale symbol
and prefix
letter
Identer Major load,kg
Dial numerals Typical
applications of
scales
B 1/16 in. ball 100 Red Copper alloys soft steels
aluminum alloys, malleable
iron
C Diamond cone 150 Black Steel, hard cast iron,
pearlitic malleable iron,
deep case- hardened steel
A Diamond cone 60 Black Cemented carbides, thin
shallow case-hardened
steel
D Diamond cone 100 Black Thin steel, medium case-
hardened steel

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E 1/8 in. ball 100 Red Cast iron, aluminum and
magnesium alloys, bearing
metals.
F 1/16 in. ball 60 Red Annealed copper alloys,
thin soft sheet metals
Phosphor bronze,
beryllium copper,
malleable iron. Aluminum,
lead, zinc Bearing metal
and other very soft or thin
materials. Use smallest
ball and heaviest load that
does not give anvil effort.
G 1/16 in. ball 150 Red
H 1/8 in. ball 60 Red
K 1/8 in. ball 150 Red Bearing metal and other
very soft or thin materials.
Use smallest ball and
heaviest load that does not
give anvil effort
L 1/4 in. ball 60 Red
M 1/4 in. ball 100 Red
P 1/4 in. ball 150 Red
R 1/2 in. ball 60 Red
S 1/2 in. ball 100 Red
V 1/2 in. ball 150 Red
Minor load = 10 kg
Both the most commonly used Rockwell scales B and C consist of 0-100 divisons.
But they are staggered in such a fashion that the B30 is atC 0. It is made so, to
avoid the negaive hardness values on the B-scale if used to test very soft materials.
Thus also facilitates in establishing that the highest hardness the C-scale should be
employed.
Advantages of the process:
Listed below are the advantages of the process:
(l) The process is relatively fast.
(2) The hardness is directly read on the dial.
(3) There is greater latitude for soft to hard malerials.
(4) A small indentation is left on the object.
(5) Use of the initial minor load avoids the errors arising out of the uneven surface
of the metal, machine and table errors.
Disadvantages:
The process has its drawbacks which are as follows:

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(1) The scale range is contracted.
(2) The use of more than one scale to express the hardness.
(3). Difficulty to readily convert the Rockwell hardness values either into Brinell
or Vickers.
ROCKWELL SUPERFICIAL HARDNESS TESTER
This machine is similar to the ordinary Rockwell machine. The indenters used are
also the same, viz. the 1/16" hardened steel ball and the conical based spherical
diamond indenter. For softer materials the ball is used and for harder material the
Brale isused.When used in the superficial tester the Brale is desiguated as 'N Brale'
andthe spherical end of the indenter is shaped to within closer tolerance limits.
The hardness reading dial is similar to that of the ordinary machine.It consists of
100 equal divisions. Each division represents 0.001 mm vertical motion of the
indenter. The dial is very sensitive to the motion of the indenter as the
immpressions obtained are very small. A minor load 3 kgf is used and major load
may be 15,30 or 45 kgf as against the 10 kgf and 60. al and the major inad may be
100 and 150 kgf, respectively, in the ordinary machine. Because of this, superficial
hardness scales are devised. These are defined by the indenter used. Thus when the
N Brale is used the hardness is read as an N-number and where the ball is used, as
the T number. Besides, the major load employed is also mentioned. Thus, if the N
Brale is used with a 30 kgf, major load the scale is prefixed as 30N and so on.
Rockwell Superficial Hardaess Scales
Scale Symbols
MAJOR
LOAD
N SCALE T
scale1/16-in
ball
W scale
1/8-in ball
X scale1/4-
in ball
Y scale1/2-
in ball
15 15 N 15 T 15 W 15 X 15 Y
30 30 N 30 T 30 W 30 X 30 Y
45 45 N 45 T 45 W 45 X 45 Y
Minor load 3 kg

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Standardization of the superficial tester is accomplished by means of special test
blocks similar to those used in the regular Rockwell, which are appropriate to the
N and T scales. The superficial tester is particularly adapted for testing thin sheets,
such as razor blades and for determining the hardness of lightly curburized or
decarburized steel surfaces, nitrided surfaces etc.
POLDI HARDNESS
A dynamic hardness tester which is very practically usable, even during the
processing of a material is Poldi Hardness tester (Figure 1.7). The instrument is
handy and can be taken to the job when it is unwieldy and can not be carried to the
conventional hardness tester and mounted on its table for testing. It is a simple
implement consisting of a hardened steel ball, located firmly in a tubular holder. A
movable plunger rigidly held by means of a spring works in the tubular holder. A
gap is provided between the ball and the plunger's bottom and into which a
'standard' rectangular bar is inserted. This bar is hardened to a predetermined value.
It has a tapered end to facilitate easy insertion into the space. The standard bar is
held in the proper position due to the action of the spring provided in the tubular
ball holder.
The surface of the material under test is polished enough to enable the tester to
accurately measure the indentation. The process is very simple. The indenter (i.e.,
the ball holder) is held vertically over the material surface (under test) with the

21
standard bar inserted into it, the ball touching the surface. A hammer blow is given
to the plunger from the top. The blow leads to the indentations on the test piece as
well as on the standard bar. Both the indentations are measured accurately using a
microscope, similar to the one used in Brinell hardness test.
Precautions:
1.The hammer used is an ordinary hand hammer of 1/2 kgf.
2. The blow given should be normal, not very severe nor very mild.
3. The ball holder should be held vertically on the specimen.
4.If the indentation obtained is not circular, the average diameter should be taken
to compute the hardness.
5. The standard test bar should be so inserted that the ball rest at a minimum
distance of 15 mm for any previous indentation.
6.Poldi hardness values obtained are within 10% of the accurately measured
hardness the material.
REBOUND HARDNESS
When a standard hammer weight with a diamond tip is made to fall to the surface
of a metal, there will be a great instantaneous load at the point of impact. The
kinetic energy of impact is expended in making an indentation at the spot and in
rebound of the hammer.If the metal is hard, the indentation made will be small for
it yields less, and the height to which the hammer rebounds will be more, and vice
versa. If the height of fall of the hammer and its weight are maintained an constant,
the height of the rebound of the hammer will give us an idea of the hardness of the
metal. Shore scleroscope operates on this principle.
The twoimportant models of Shore scleroscope are vertical scale type tester
(Model C-2) and dial recording type tester(Model D).The instrument(Model C-2)
esenlially consists of a graduated glass tube through which a 2.6 gm. standard
hammer falls through a standard height of 10". The whole length of the glass lube
is graduated into 100 or 140 equal parts. In all the instruments, the glass tube is
aligned to be perfectly vertical. The heiglit of the rebound may be noted thus:

22
(i) By watching the top of the hammer itsell.
(ii) By means of a magnifying glass and pointer attached to the tubes which are
moved lo the expected region of rebound.This is done by preliminary trials.
The principle of model D is the same as model C-2. In the model D, the rebound of
the hammer is automatically recorded on an instrument dial. The weight of the
hammer is 37 gmand the distance through the hammer falls is about a 0.75". The
circular scale indicates the scleroscope in addition to giving conversion into other
tyres of hardness scales.The instrument should be calibrated by using a standard
steel test block, supplied by the Shore Instrument and Manufacturing Company.
Sources of Error:
(l) The surace condition of the metal under test. If the surface is not perfectly plain,
a smaller amount of rebound will occur. Generally it is necessary that the surface
of the metal should be more perfect than is necessary in the case or eiher the
Brinell or Rockwell test.
(2) The specimen should be perfectly clamped to the base of the instrument;
otherwise the rebound will not be proper due to energy losses on account of the
movement of the specimen itself. This can be judged by the sound of the impact of
the hammer while testing. The sound should be the same as a characteristic
hammer blow an metal.
(3) Decarburized layers, dust, grease, etc., should not be present on the surface.
Advantages:
(i)The procedure is very short and simple
(ii) The impression left on the object is very small.
(iii) The instrument is portable.
Application
For routine inspection work, especially when 100% testing is necessary, the
instrument is invaluable. Very thin sections can also be tested.

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Limitations:
In this instrument the rebound property of hard surface is made use of. Since
rebound is a characteristic of elastic materials also, elastic materials like rubber,
etc, should not be tested by this instrument and the values obtained thereon should
not be compared to the hardness of metals .The very nature of the material under
test should be similar. Then only a comparison of the values obtained on testing
can be made.
MICROHARDNESS TESTS
Many metallurgical problems require the determination of hardness over very
small areas. The measurement at thin electroplated metals, carburized,
decarburized surface, nitrided or cyanided cases and the individual constituents of
a microstructure might be typical problems. By appropriate selection of loads with
respect to the hardness of the material under test, the depth of the indentation need
not exceed about 1 micron.
There are two standard indenters available for microhardness tester-the standard
Vickers tester-136° diamond pyramid described earlier and a diamond indenter
known as Knoop indenter. The Knoop indenter, a sketch of which is shown in
Figure 1.8(a), consists of pyramidal shaped diamond cut to an included transverse

24
angle of 130 deg o min, and an included longitudinal angle of 172 deg 30 min. The
impression formed by the knoop indenter, when viewed normal to the specimen
surface as illustrated in Figure 1.8(b) is rhombic in shape with one diagonal
perpendicular to and about seven times the length of the other. The depth of the
impression at the indenter apex is about one-thirtieth the length of the longer
diagonal.
There is some elastic recovery of the impression formed by the indenter when the
applied load is removed. The amount of such elastic recovery and the final
distorted shape of the impression depend upon the size and precise shape of the
indenter. Owing to the shape of the Knoop indenter, elastic recovery of the
projected impression occurs principally in a transverse direction, i e., along the
shorter diagonal rather than in a longitudinal direction. As a consequence, from the
measured length of the longer diagonal and the constants of the indenter,
dimensions of an impression closely related to the unrecovered length are secured.
The numerical value of the Knoop hardness number is equal to the applied load
divided by the unrecovered projected area of the impression. It should be noted
that the area referred to is the projected area and not the surface area of the
impression as in the vickers and Brinell equations.
Thus,
Knoop hardness number = P / Ap
Where P = Applied load, kg
Ap = area of unrecovered projected area of impression, sq mm.
The unrecovered projected area of impression, formed by an indenter of theoretical
dimensions and having perfect included transverse and longitudinal angles, is equal
to
A = w*l/2;
Where w = length of transverse (shorter) diagonal, mm
l = length of longitudinal (longer) diagonal, mm

25
Inasmuch as the unrecovered projected area Ap is more precisely related to the
unrecovered diagonal l, it is necessary to express this area in terms of the longer
diagonal only. From considerations of the transverse included angle of 130 deg 0
min and the longitudinal includedangle of 172 deg 30 min, the following
relationship may be established between the short ng diagonals:
tan 65/w = tan 86°15"/ l
Substituting appropriate numerical values in Eq.
l = 7.114*w;
or, w=0.14056*l;.....................................(5) The value of the w in terms of 1(Eqn
(5) may be substituted in Eqn (2) and the are ecovered projected area of the
impression may be expressed as
A= 0.14056*l / 2 = 0.07028*
Where 0.07028= Cp= indenter constant relating the longer diagonal 1 to the
unrecovered projected area.
If the value of Cp is now substituted into Eqn (l), the following working equation
is obtained that is applied for an impression made by a theoretically perfect
intenter, or for one within limits oferror defined by pecification for a certified
indenter:
where KHN = Knoop hardness number
P = applied load, kg
Cp = Knoop indenter constant
l = length of longitudinal, mm
The Knoop hardness number corresponding to a measured length of the long
diagonal may be determined from standard chard for a given load.
Precautions:
The instrument is very sensitive and great care should be taken to see that the
measurement of the diagonal is made accurately. Since only the projected area of
the indentation is taken into account, the measurement should be made exactly at
right angles to the surface under lest. To achieve this, the bottom portion of the
specimen should also be made very carefully so that it rests properly on the stage
of the instrument.

26
Chapter II
TENSILE TEST
The strength of a material when it is called upon to withstand loads which produce
a tensile stress in it, is defined as the tensile strength of the material. In routine
usage, the term "tensile" is omitted and tensile strength is referred to simply as
strength A load which tends to pull apart the two ends of an object said to be a
tensile laad.
Before proceeding to testing it is appropriate to recapitulate some elementary
terminology such as load, stress, deformation, strain, etc.
Load
Load is referred to as the weight applied to the body in testing. If a wire is
suspended from a nail on wall and a kilogram-weight is placed in a pan fixad to its

27
bottom end, it is said that a the wire is loaded. In this example, the magnitude ofthe
load applied is one kilogram.
In any test, load and its application are important. Load is applied to the exitent
desired. viz, to the exten the material under test can withstand it before it breaks. In
tensile testing the under load applied is tensile in nature, i.e. it tends to pull or
elongate the specimen. The units for the load are force units and the loads are
expressed in weight-kgf.
Stress
When a body is loaded, forces will be set up inside it. These forces will be in
direction opposite to that of the application of the external load. The magnitude of
these internal forces will be such that the effect of the external load is balanced and
the body remains in mechanical equilibrium. These internal forces are called
stresses.Stress is defined as the indemal reaction set up in the solid per unit cross-
sectinnal area. It should be further noted that the internal reaction is proportional to
the load applied. At any load,the value of the stress in the solid can be arrived at by
dividing the load by the crosssectional area over which it acts.
Stress is expressed in kg/mm.
Deformation
When a load is applied on solid, it tends to change the shape of the solid in its
direction. The change affected is called deformalion. In the case of tensile loading,
the formation suffered by the object is termed as extension or elongation.
Deformation is expressed in the same units as the particular dimension is
cxpressed, i.e. in mm or cm.
Strain
Strain in the deformation expressed the basis of the unit dimension. It is customary
to express the deformation as strain because the term strain gives a precise
measure. In the example illustrated (Figure 2.2), the strain is 1/L, the units for
strain are mm per mm thus, it is a number.

28
Gauge length = Lo
Distance between the gauge length marks after the failure =L
Elongation = (L - Lo)
Percentage elongation = (L – Lo)/Lo * 100
Tensile Deformatian of Ductile Metal
The data obtained from the tension test generally plotted as a stress strain diagram.
Figure 2.4 shows a typical stress-strain curve for a metal such as aluminium or
copper. The initial linear portion of the curve OA is the elastic region within which
Hooke's law is obeyed. Point A is the elastic limit, delined as the greatest stress
that the metal can withstand without experiencing a permanent strain when the

29
load is removed. The determination of the elastic limit is quite tedious, not all
routine, and dependent on the sensitivity of the strain-measuring instrument. For
these reasons it is often replaced by the prorortional limit, point A'. The
proponional limit is the stress at which the suess strain curve deviates fron
linearity. The slope of the stress-strain curve in this region is the modulus of
elasticity.
For engineering purposesthe limit of usable elastic behaviour is described by the
yield strength, point B. The yield strength is defined asthe stress which will
produce a small amount of permanent deformation generally equal to strain of
0.002. In Figure 2.4 this permanent strain, or offset is OC. Plastic deformation
begins when the elastic limit is exceeded. As the plastic deformation of the
specimen increases, the metal becomes stronger (strain hardening) so that the load
required to extend the specimen increases with further staining. Eventually the load
reaches a maximum value.The maximum load divided by the original area of the
specimen is the ultimate tensile strength. For a ductile metal the dianmeter of the
specimen begins to decrease rapidly beyond maximum load, so that the load
required to continue deformation drops of until the specimen fractures. Since the
average stress is based on the original area ofthe specimen, it also decreases from
maximum load to fracture.
Ductile vs. Brittle Behaviour
The general behaviour of materials under load can be classified as ductile or brittle
depending upon whether or not the material exhibits the ability to undergo plastic

30
deformation. Figure 2.5 illustrates the tension stress-strain curve of a ductile
material. A completely brittle material would fracture almost at the elastic limit
(Figure 2.5a) while a brittle metal, such as white cast iron, some slight measure of
plasticity before fracture.(Figure 2.5b) Adequate ductility is an important
engineering consideration, because it allows uhe material toredistribute localised
stresses. When localized stresses at notches and other accidental stress
concentrations do not have to be considered, it is possible to design for static
situations on the basis of average stresses. However, with brittle materials,
localized stresses continue to build up when there is no local yielding. Finally, a
crack forms at one or more points of stress concentration, and it spreads rapidly
over the section. Evem if no stress concentrations are present in a brittle material,
fracture will still occur suddenly because the yield stress and tensile strength are
practically identical.
It is important to note that brittleness is not an absolute property of a metal. Ametal
such as tungsten,which is brittle at room temperature, is ductile at an elevated
temperature. A metal which is brittle in tension may be ductile under hydrostatie
compression. Furthermore, a metal which is ductile in tension at room temperature
can become brittle in the presence of notches, low temperature, high rates of
loading, or embrittling agents such as hydrogen.
Stress-strain diagram

31
The general shape of the stress-strain curve is shown in Figure 2.6. The stress used
here is the average longitudinal stress in the tensile specimen. lt is obtained by
dividing the load by the original area of the cross section ofthe specimen.
The strain used for the engineering stress-strain curve is the average linear strain,
which ia obtained by dividing the elongation of the gauge length of the specimen,
by its original length.
Strain = (L-Lo)/Lo
Since both the stress and strain are obtained by dividing the load and elongalian by
constant factors, the load-elongation curve will have the same shape as the
engineering stress-straincurve.
Tensile Strength
The tensile suength or ultimate tensile strength (UTs) is the maximum load divided
by the cross sectional area of the specimen.
Measures of Yielding
The stress at which plastic deformation of yielding is observed to begin depends on
the sensitivity of the strain measurements. With most materials there is a gradual
transition from elastic to plastic behaviour and the point at which plastic
deformation begins is hard to define with precision. Various criteria fot initiation
of yielding are used depending on the sensitivity of the strain meauurements and
the intended use of the data.

32
1. True elastic limit based on micro-xtrain measurements at strains on order of
2x10. This elastic limit is very low value and is related to the motion of a few
hundred dislocations.
2. Proportional limit is highest stress at which stress is directly propotionalto strain.
It is obtained by observing the deviatian fron the straight-line portion of the stress
strain curve.
3.Elastic limit is the greatest stresa the material can withstand without any
measurable permanent strain remaining on the complete release of load.With
increasing strain measurement, the value of the elastic limit is decreased until at
the limit itequals the true elastic limit determined frorn micro-strain measurements.
With the sensitiviry of the strain usually employed in engineering studies, the
elastic limit is greater than the proportional limit. Determination of the elasticlimit
requires a tedious incremental loading unloading test procedure.
4. The yield strength is the stress required to produce a small specified amount of
deformation. The usual definition of this property is the offset yield strength
determined by the stress corresponding to the interaction of the stress-strain curve
and a line parallel to the elastic part of the curve offset by a strain of 0.2 percent.
Reduction of area
The reduction of area at fracture is expressed as q.
Here q = ( Ao – Af)/(Ao)
Where, Ao and Af are initial and final cross sectional are respectively.
Modulus of Elasticity
The slope of the initial linear portion of the stress strain curve is the modulus of
elasticity or Young's modulus. Modulus of elasticity is a measure of the stiffness of
the material. The modulus of elasticity is determined by the binding forces
between atoms. Since these forces cannot be changed without changing basic
nature of the material, it follows that the the modulus of elasticity is one of the
most structure insensitive of the mechanical properties. It is only slightly affected
by alloying additions, heat treatment, or cold-work. However, increasing the
temperature decreases the modulus of elasticity.

33
Resilience
The ability of a material to absorb energy when deformed elastically and to return
it when unloaded is called resilience. This is usually measured by the modulus of
resilience which is the strain energy per unit volume to stress the material from
zero stress to the yield stress.
Toughness
Toughness is aits ability to absorb energy in the plastic range. Toughness is
commonly used concept which is difficult to pin down and define. One way of
looking at toughness is to consider that it is the area under the stress-strain curve.
This area is an total indication of the amount of work per unit volume which can be
done on the material without causing it to rupture.
Figure 2.7 shows the stress-strain curves for high and low toughness materials.The
high carbon spring steel has a higher yield stength and tensile strength than the
medium carbon structural steel.However,the structural steel is more ductile and has
a greater total elongation. The total area under the stress-strain curve is greater for
a structural steel,and therefore it is tougher material.This illustrates that toughness
is a parameter which comprises both strength and ductility. The crosshatched
regions in Figure 2.7 indicate modulus of resilience for each steel. Because of its
higher yield strength the spring steel has the greater resillience.
Several mathematical approximations for the area under the stress strain curve
have been suggested. For ductile metals which have a stress straincurve like that of
structural steel the area under the curve can be approximated by either of the
following equations:
UT = su * ef
Or UT = (so + su)/2 * ef

34
For brittle materials the stress-strain curve is sometimes assumed to be a parabola,
and the area under the curve is given by
UT = 2/3* su * ef
All these relations are only approximations to the area under the stres-strain curve.
Further the curves do not represent the true behavior in the plastic range, since they
are all based on the original area of the specimen.
TRUE-STRESS. TRUE-STRAIN CURVE
The engineering stress-strain curve does not give a true indication of the
deformation characteristics of a metal because it is based entirely on the orginal
dimensions of the specimen, and these dimensions change continously during the
test.Also, ductile metal which is pulled in tension becomes unstable and necks
down during the course of the test. Because the cross section area of the speeimen
is decreasing rapidly at the stage in the test, the load required to continue
deformation fails off. The average stress based on original area likewise decreases,
and this produces the falloff in the stress-strain curve beyond the point of
maximum load. Actually,the metal continues to strain harden all the way up to
fracture.so that the stress requiredto produce further deformatian should also
increases.
If the true stress,based on the actual cross-sectional area of the specimen,is used,it
is found that the stress-strain curve increases continuously up to fracture. If the
strain measurements is also based on instantaneous measurements, the curve which
is otrained is known as a true-stress-strain curve.This is also known as flow curve
since it represents the basic plastic flow characteristics of material. Any point on

35
the flow curve can be considered the yield stress for metal strained in tension by
the amount shows on the curve. Thus, if the load is removed at this point and then
reapplied,the material will behave elastically throughout the entire range of
reloading.
True stess is the load at any instant divided by the cross-sectional area over which
it acts.True stress and engineering stress are expresses by symbol and s
respectively.
Stress = (P/A) and strain = (P/Ao)
The true stress may be determined from the engincering stres as follows:
True stress = (P/ A)
By constancy of volame relationship
Ao/A = L/Lo = e +1
The true stress can be shown in terms of engineering stress s by
True stress = s* (e+1)
The derivation of Eq.(2.1) assumes both constancy of volume and a homogeneous
distribution of strain along the gage length of the tension specimen. Thus Eq (2.l)
should only be used until the onset of necking.Beyondmaximum load the true
stress should be determined from actual measurements of load and cross sectional
area.
The true strain may be determined from the enrineering of conventional strain e by
True strain = ln(e+1)
This equation is applicable only the onset of necking for the reasons discussed
above. Beyond the maximum load the true stain should be based on actual area or
diameter measurements.

36
Figure 2.8 compares the true stress true-strain curve with its corresponding
engineering stress-suain curve. Note that because of the relatively large plastic
strains, the elastic region has been compressed into the y axis.In agreement with
Eqs (2.1) and (2.2), true stress-true- strain curve is always to the left of the
engineering curve until the maximurn load is reached. However, beyond maximum
load the high localizad strains in the necked region thal are used in Eq (2.3.) far
exceed the engineering strain. Frequently the flow curve is linear from maximum
load to fracture, while in other cases its slope continuously decreases up to
fracture. The formation of a necked region introduces triaxial stresses which make
it difficult to determine accurately the longitudinal tensile stress on out to fracture.
The following parameters usually are determined from the true stress-true-strain
curve.
True Stress at Maximum Load
The true stress at maximum load correspond s to the true tensile strength. For most
malerials necking begins at maximum load at a value of strain where the true stress
equals the slop of the flow curve. Let o. and e denote the true stress and true strain
at maximum load when the cross-sectional area of the specimen is M.
The tensile strength is given by
True Facture Stress
The true fracture stress is the load at fracture divided by the cross-sectional area at
fracture.

37
True Fracture strain
The true fracture strain is the true strain based on the original cross-soctional area
Aand the area after fracture A.
Its value is given as ln(A0/Af).
True Uniform strain
The true fracture strain -is the true strain based up to the maximam load.
Its value given as ln(Ao/Af).
True Local Necking strain
The true local neck strain e, is the strain required to deform the specimen from
maximum load to fracture.
Its value given by ln(Au/Af).
The flow curve of many metals in the region of uniform plastic deformation can be
expressed by the simple power curve relation
Stress = k* en
Where n the strain hardening exponent and K is one strength coemcient. A log log
plot of true stress and true strain up to maximum load will result in a straight line.
The linear slope of this line is n and K is the ine stress at e-1.0 (corresponds q =
0.63) The strain- hardening exponent may have values form n = 0 (perfectly plastic
solid)to n= 1 (elastic solid). For most metals n has values betwern 0.10 and 0.50
(see Table 2.l)
Metal Condition n K,MPa
0.05 % C steel Annealed 0.26 530
SAE 4340 steel Annealed 0.15 640
0.6% C steel Quenched and
tempered 540 C
0.10 1570
0.6% C steel Quenched and
tempered 540 C
0.19 1230
Copper Annealed 0.54 320
70/30 brass Annealed 0.49 900

38
It is important to note the rate of strain hardening is not identical with the strain
hardening exponent(n).
INSTABILITY IN TENSION
Necking generally begins at maximum load during the tensile deformation of a
ductile material. An ideal plastic material in which no strain hardening occurs
would become unstable in tension and begin to neck just as soon yielding took
place. However a real metal undergoes strain hardening, which tends to increase
the load carrying capacity of the specimen as deformation increases. This effect is
opposed by the gradual decrease in the cross-sectional area of specimen as it
elongates.Necking or localized deformation begins at maximum load, where the
increase in stress due to decrease in cross-sectional area of the specimen becomes
greater that the increase in the load-carrying ability of the metal due strain
hardening.This condition of instability leading to localized deformation is defined
the condition dP = 0.

39
Therefore, the point of necking at maximum load can be obtained from the true
stress-true strain curve by finding the point on the curve having a sub tangent of
unity(Figure 2.9a) or the point where the rate of strain hardening equals thr stress.
(Figure 2.9b)
The necking criterion can be expressed more explicitly if engineering strain is
used. Starting with Eq.(2.4)
By substituting the necking criterion given in Eq (2.5) into Eq. (2.4), we obtain a
simple relationship for the strain at which necking occurs. This strain is the true
uniform strain.
Its value equal to be n.

40
Necking in a cylindrical tensile specimen is symmetrical around the tensile axis if
the material is isotropic. However, a different type of necking behaviour is found
for a tensile specimen with regular cross section that is cut from a shee. For a sheet
tensile specimen where width is much greater than thickness there are two types of
tensile flow instability. The first is diffuse necking,so called because its extent is
much greater than the sheet thickness.(Figure 2.11) This form of unstable flow in a
sheet tensile specimen is analogous to the neck formed in a cylindrical tensile
specimen. Diffuse necking may terminate in fracture but it often is followed by a
second instability process called localised necking. In this mode the neck is a
narrow band with a width about equal to the sheet thickness inclined at an angle to
thespecimen axis, across the width nr the specimen (Figure 2.11) In localised
necking there is no change in width measured along the trough of thr localised
neck, so that localised necking corresponds to a state of plane-strain deformation.
EFFECT OF STRAIN RATE ON FLOW PROPERTIES
The rate at which strain is applied to a specimen can have an important influence
on the flow stress strain rate is defined as , and is conventionally expressed in units
of s. i.e. "per second."

41
Figure 2.12 show that increasing strain rate increases flow stress. Moreover, the
strain-rate dependence of strength increases with increasing tempereture. The yield
stress and flow stress at lower plastic strains are more dependent on strain rate than
the tensile strength. High rates of strain cause the yield point to appear in test on
low-carbon steel that do not show a yield point under ordinary rates of loading.
The extension of cylindrical specimen requires one end fixed and the other end is
attached to the movable crosshead of the testing machine. The crosshead velocity
is v. The strain rate expressed in terms of conventional linear strain is
Thus, the conventional strain rate is proportional to the crosshead velocity. In a
modem testing machine the crosshead velocity can be set accurately and
controlled. It is a simple matter to carry out tensiontests at constant conventinal
strain rate.
The true strain rate is e is given by:
The true strain rate is relaled to the conventional strain rate by ihe followina
equation:

42
As Figure 2.12 indicates, a general relationship between flow stress and strain rate,
at coastunt strain and temperature is
Here m is known as the strain rate sensitivity. The exponent mcan be obtained
from he slope of a plot of log o log .However, a more sensitive way is rate change
test in which is m is determined by measuring the change in flow stress about by a
change in e at a constant e and T (see ligure 2.13).
Figure 2.13: Strain rate change to determine strain rate respectively.
Strain-rate sensitivity of metals is quile low(<0.1) roomtemperature but m
increases with temperature, especially at lemperatures above half of the absolute
melting point. In hot- working conditions m values of 0.1 to 0.2 are common.

43
High strain rate sensitivity is a characteristic of superplastic metals and
alloys.Superplasticity refers to extreme extensibility with elongations usually
between 100 and 1000 percent.Superplastic metals have a grain size or interphase
spacing of the order of 1 m. Testing at high temperalurca rates accentuates
superplastic behaviour. While the mechanism of superplastic deformation isnot yet
well established, it is clear that the large elongations result from the suppression of
necking in these malerials with high values of m. An extreme case in hot glass (m
= 1) which can be drawn from the melt into glass without the fibers necking down.
In a normal the geometric softening that constitutes the formation of a neck is
opposed by strain hardening, and so long as da/de o the tensile specimen will not
neck down. With a superplastic material the rate of strain hardening is low
(because of the high temperature or structure condition) but necking is prevented
by the presence of strain-rate hardening and Consider a superplastic rod with cross-
sectional area A that is loaded with an axial force P.
Eqn (2.10) states that so long as m is <1 the smaller the cross sectional area, the
more rapidly the area is reduced. Figure 2.14 shows how the area decrease varies
with m.When m = 1 the dA/dt is independent of A and any incipient neck is simply
preserved duringelongation and does not propagare inward. As m approaches
unity, the rate of growth of incipientnecks is drastically reduced. Figure 2.15 shows

44
how the tensile elongation of superplastic alloys increases with stain rate
sensitivity is agreement with the above analysis.
EFFECT OF TEMPERATURE ON FLOW PROPERTIES
The stress-strain curve and the flow and fracture properies derived from the tension
test are strongly dependent on the temperature at which the test conducted. In
general, strength decreases and ductility increases as the test temperature is
increased. However,structural changes such as precipitation, strain aging, or
recrystallisation may occur in certain temperature ranges to after this general
behavior. Thermally activated processes assist deformation and reduce strength at
elevated temperatures. At high temperatures and/or long exposure, structural
changes occur resulting in the time-dependent deformation or creep.

45
Figure 2.16 Changes in engineering stress-strain curves mild steel with
tempereature
The change with temperature of the engineering stress-strain curve in mild steel is
shown schematically in Figure 2.16, Figure 2.17 shows the variation of yield
strength with temperature for body-centered cubic tantalum, tungsten,
molybdenum and iron and face- centered cubic nickel. Note that for the bcc metals

46
the yield stress increases rapidly with decreasing temperature, while for nickel (and
other fcc metals) the yield stress is only slightly temperature-dependent. Figure
2.18 shows the variation of reduction of area with ternperature for these same
metals. Note that tungsten is brittle at 100°(373K), iron at -225° C (48 K), while
nickel decreases nickel decreases little in ductility over the entire temperature
interval.
In fcc metals flow stress is not strongly dependent on temperature but the strain-
hardening exponent decreases with increases temperature. This result in the stress-
strain curve flattening out with increases temperature and the tensile strength being
more temperature-dependent that the yield strength. Tensile deformation at
elevated temperature may be complicated by the formation of more than one neck
in the specimen.
The best way to compare the mechanical properties of different materials at
various temperatures is in terms of the ration of the test temperature to the melting
point, express in degree Kelvin. This ratio is often referred to as the homologous
temperature. When comparing the flow stress of two materials at an equivalent
homologous temperature, it is advisable to correct for the effect of temperature on
elastic modulus by comparing ratio of /E rather than simple ratio of flow stress.
NOTCH TENSILE TEST
Ductility measurements on standard smooth tensile specimens do not always reveal
metallurgical or environmental changes that lead to reduced local ductility. The
tendency for reduced ductility in the presence of a triaxial stress field and steep
stress gradients (such as arise at a notch) is called notch sensitivity. A common
way of evaluating notch sensitivity is a tension test using a notched specimen. The
notch tension test has been used extensively for investigating the properties of
high-strength steels, for studying hydrogen embrittlement in steels and titanium
and for investigating the notch sensitivity of high-temperature alloys.
The most common notch tensile specimen uses a 60° notch with a root radius 0.025
mm or less introduced into a round (circumferential notch) or flat (double-edge
notch) tensile specimen. Usually the depth of the notch is such that the cross-
sectional area at the root of the notch is one-half of the area in the un-notched
section. The specimen is carefully aligned and loaded in tension until failure. The
notch strength is defined as the maximum load divided by the original cross-
section area at the notch. Because of the plastic constrain at the notch, this value
will be higher than the tensile strength of an unnotched specimen if the material

47
process some ductility. Therefore, the common way of detecting notch brittleness
(or high notch sensitivity) is by determining the notch-strength ratio NSR.
NSR = Snot/Su = (for notched speciment at maximum load/ tensile strength for
notched specimen)
If the NSR is less than unity the material is notch brittle. The other property that is
measured in the notch tension test is the reduction ofarea at the notch.
As strength hardness, or some metallurgical variables restricting plastic flow
increases, the metal at the root of the notch is less able to flow and fracture
becomes more likely. A typical behavior is shown in Figure 2.19. Notch brittleness
may be considered to begin at the strength level where the notch strength begins to
fall off or more conventionally at the strength level where the NSR becomes less
than unity.
TENSILE PROPERTIES OF STEELS
The tensile properties of annealed and normalised steels are controlled by the flow
and fracture characteristics of the ferrite and by the amount, shape, and distribution
of the cementite. The strength of the ferrite depends on the amount of alloying
elements in solid solution and the ferrite grain size. The carbon content has a very
strong effect because it controls the amount of cementite in the microstructure. The
strength increases and ductiliry decreases with increasing carbon content because
of the increased amount of cementite in the microstructure. A normalised steel will
have higher strength than an annealed steel because the more rapid rate of cooling

48
used in the normalising treatment causes the transformation to pearlite to occur at a
lower temperature and a finer pearlite spacing results. One of the best ways to
increase the strength of annealed steel is by cold-working. Table 2.1 summarises
the tensile properties which result from the cold working of SAE 1016 steel bars
by drawing through a die.
Reduction of
area
Yield strength Tensile
strength
Elongation in
50 mm
Reduction of
area
0 276 455 34 70
10 496 517 20 65
20 565 579 17 63
40 593 655 16 60
60 607 703 14 54
80 662 793 7 26
Figure 2.20 shows the variation of tensile properties for a Ni-Cr-Mo eutectoid steel
with isothermal reaction temperature. In the region of 1000 to 800 K the
transformation product is lamellar pearlite. The spacing between cementite
platelets decreases with transformation temperature and correspondingly the
strength increases. In the region 700 to 559 K the structure obtained on
transformation is acicular bainite. The bainitic structure becomes finer with
decreasing temperature, and the strength increases almost linearly to quite high
values.
Good ductility accompanies this high strength over part of the bainite temperature
range. This is the temperature region used in the commercial heat-treating process
known as austempering. The temperature region 800 to 700 K is one which mixed
lamellar and acicular structures are obtained. There is a definite ductility minimum
and a leveling off of strength for these structures. The sensitivity of the reduction
of area to changes in microstructure is well illustrated by these results.

49
The best combination of strength and ductility is obtained in steel which has been
quenched to a fully martensitic structure then temperedn The best criterion for
comparing the tensileproperties of quenched and tempered steels is on the basis of
an as-quenched structure of 100 percent martensite. However, the attainment of a
completely martensite structure may, in many causes, be commercially impractical.
Figure 2.21 shows that the hardness of martnsite as a function of carbon content for
different total amounts of martensite in the microstructure.
The mechanical properties of a quenched and tempered steel may be altered by
changing the tempering temperature. Figure 2.22 shows how tensile properties
vary with tempering temperature for an SAE 4340 steel. This is the typical
behavior for heat-treated steel.

50
ANISOTROPY OF TENSILE PROPERTIES
It is frequently found that the tensile properties of wrought-metal products are not
the same in the directions. The dependence of properties on orientation is called
anisotropy. Two general types of anisotropy are found in metals. Crystallographic
anisotropy results from the preferred orientation of the grains which is produced by
severe deformation.Since the strength of a single crystal is highly anisotropic, a
severe plastic deformation which produces a strong preferred orientation will cause
a polycrystalline specimen to approach the anisotropy of a single crystal. The yield
strength, and to a lesser extent the tensile strength, are the most affected. The yield
strength in the direction perpendicular to the main (longitudinal) direction of
working may be greater or less than the yield strength in the longinudinal
direction,depending on the type of preferred orientation which exists. This type of
anisotropy is most frequently found in nonferrous metals, especially when they
have been severely worked into steel. Crystallographic anisotropy can be
eliminated by recrystallisation, although the formation of a recrystallization texture
can cause the reappearance of a different type of anisotropy.
Mechanical fibering is due to the preferred alignnment of structural
discontinuities such as inclusions, voids, segregation, and second phases in the
direction of working. This type ofanisotropy is important in forgings and plates.
The principal direction of working is defined as the longitudinal direction. This is
the long axis of a bar or the rolling direction in a sheet or plate. Two transverse
directions must be considered. The short-transverse direction is the minimum
dimension of the product, for example, the thickness of a plate. The long trans-
verse direction is perpendicular to both the longitudinal and short-transverse

51
directions. In a round or square, both or square, both these transverse directions are
equivalent, while in a sheet the properties in short-transverse direction cannot be
measured. ln wrought-steel products mechanical fibering is the principal cause of
directional properties. Measures of ductility like reduction of area most affected. In
general, reduction of area is lowest in the short-transverse direction, intermediate
in the long-transverse direction, anl highest in the longitudinal direction.
Transverse properties are particularly important in thick walled
tubes, like guns and pressure vessels, which are subjected to high internal
pressures. In these applications the greatest principal stress acts in the
circumferential direction, which correspond to the transverse direction of a
cylindrical forging. Where there is no direct method for incorparating the reduction
of area into the design of such a member, it is known that the transverse reduction
of area(RAT) is good index of steel quality for these types of applications.
An interesting graphic method for correlating the amount and
direction of deformation with the resulting directionally of tensile properties has
been presented by Huniscker. The procedure has been well documented directional
properties aluminium alloys, but it has yet been widely applied to other systems.
YIELD POINT PHENOMENON
May metals, particularty low carbon steel, show a localised, heterogeneous types
of transition from elastic to plastic deformation which produces a yield point in the
stress-strain curve. Rather than having a flow curve with a gradual transition from
elastic to plastic behaviour, metals with a yield point have a flow curve or, what is
equivalent a load-elongation diagram similar to Figure 2.23. The load increases
steadily with elasticstrain drops suddenly,fluctuates about some approximately
constant value of load and then rises with further strain. The load at which the

52
sudden drop occurs called the upper yield point. The constant load is called the
lower yield point, and the elongation which occurs at constant load is called the
yield-point elongation. The deformation occurring throughout the yield-point
elongation is heterogeneous. At the upper yield point a discrete band of deformed
metal, often readily the yield point visible with the eye, appears at a stress
concentration such as a fillet, and coincident with the formationof the band the
load drops to the lower yield point. The band then propagates along the length of
the specimen, causing the yield point elongation In the usual case several bands
will form at several point of stres concentration. These bands are generally at
approximately 45° to tessile axis.They are usually called Laders bands.When
several Luders bands. are formed,the flow curve during the yield point elongation
will be irregular, each corresponding to the formation of a new Luders band. After
the Laders bands have propogatedto cover the entire length of the specimen test
section, the flow will increase with strain tha usual manner. This marks the end of
the yield-point elongation.
The yield-point phenoemenen was found originally in low-carbon steel. A
pronounced upper and lower yield point and a yield point elongation of over 10
percent can be obtained with this material under proper conditions. More recently
the yield point has come to be accepted as a general phenomenon, since it has been
observed in a number of other metals and alloys. In addition to iron and steel, yield
points have been observed in polycrystalline molybdenum, titanium, and alumiman
alloys and in single crystals of iron, cadmium, zinc, alpha and bets brass, and
aluminium. Usually the yield point can be associated with smallamounts of
interstitial or substitutional impurities. For example, it has been shown that almost
complete removal of carbon and nitrogen from low carbon steel will remove the
yield point.However, only about 0.001 percent of either or these elements is
required ior a reappearance ofthe yield point.
BAUSCHINGER EFFECT
Generally a lower strees is required to reverse the directian of slip on a certain slip
plane than to continue slip in the original direction.The directionally of strain
hardening is called the Bauschinger effect. Figure 2.24 is an example of the type of
stress strain curve that is obtained when the Bauschinger effect is considered. The
Bauschinger effect is a general phenomenon in polycrystalline metals.
The initial yield stress of the matrial in tension is A. If the same ductile
material were tested in compression, the yield strength would be approximately the
same, point B on the dashed curve. Now, consider that a new specimen is loaded in

53
tension past the tensile yield to C along the path O-A-C. If the specimen is then
unloaded, it will follow the path C-D,small elastic-hysteresis effects being
neglected. If now a compressive stress is applied, plastic flow will being at the
stress corresponding to point E, which is appreciably lower than the original
compressive yield stress of material. While the yield stress in tension was
increased by strain hardening from A to C, the yield stress in compression was
decreased. This is the Bauschinger effect. The phenomenon is reversible, for had
the specimen originally been stressed plastically in compression, the yield stress in
tension would have been decreased.
One way of describing the amount of Bauschinger effect is by the Baschinger
strain (figure 2.24). This is the difference in strain between the tension and
compression curves at given stress.
If the loading cycle in Figure 2.24 is completed by loading further in compression
to point F. then unloading and reloading in tension, a mechanical hysteresis loop is
obtained. The area under the loop will depend on the initial overstrain beyond the
yield stress and the number of times the cycle is repeated. If the cycle is repeated
many times, failure by fatigue is likely to occur.
The Bauschinger effect can have important consequences in initial forming
applications. For example, it can be important in the bending of steel plates and
results in work-softening when severely cold worked metals are subjected to
stresses of reversed sign. The best example of the straightening of drawn bars or
rolled sheet by passing through rolls which subject the material to alternating
bending stresses. Such roller-levelling operations can reduce the yield strength and
increase the elongation from its cold-worked value.

54
The mechanism of the Bauschinger effect lies in the structure of the cold-
worked stale During plastic deformation dislocation will accumulate at barriers in
tangles, and eventually form cells. Now, when the load is removed, the dislocation
lines will not move appreciably because the structure is mechanically stable
However, when the direction of loading isreversed, some dislocation lines can
move an appreciable distance at a low shear stress because the barriers to the rear
of the dislocations are not likely to be so strong and closely spaced as those
immediately in front. This gives rise to initial yielding at a lower stress level when
the loading direction is reversed.

55
Chapter III
COMPRESSION AND TORSION TEST
Theoretically, compression test is merely the opposite of the tensile test with
respect to the direction of the applied stress. The compression test can be done on
the same machinc on which the tension test is done like universal testing machine
or some other machine which is designed specially for the purpose. In general,
brittle materials are good in compression than in tension and therefore, they are
used for compression loads. Due to this, compression test is mainly used to test
brittle materials such as cast irons, concrete, stones, bricks and ceramic products.
During testing, fracture occurs in brittle materials and therefore, the ultimate
strength is determined corresponding to the fracture point; but no fracture occurs
for ductile materials and hence ultimate strength is found out for some arbitrary
amount of deformation. It has been observed that some errors are always bound to
come in the compression test due to the following practical difficulties.
I. Since the top and bottom faces of the specimens are seldom perfectly
parallel to each other and there is always a tendency for bending of the
specimen during testing, it is very difficult to a5 pply truly axial loads.
II. The friction between the ends of the specimen and heads of the testing
machine prevent the deformation of the specimen uniformly throughout the
length. This results in more lateral expansion in the central region that the
other regions giving a barrel like shape to specimen.
III. Since the length of specimen is kept short enough (not more than twice its
diameter) to avoid its bucking, it is difficult to obtain strain measurements
accurately.
IV. For a constant length to diameter (1/D) ratio, length can be increased with
proportionate increase in diameter. However, for testing large diameter
specimens, high capacity testing machines are required.
The test specimens can be square, rectangular or circular in cross section; but
circular section is preferred for uniform application of load. The length to diameter
(1/D) ratio is between 1.5 and 10 for different materials but a ratio of 2 is
commonly employed. For longer specimens(i.e. l/d > 10) bending is more which
reduces the compressive strength and for shorter specimens(<1.5) frictional effects
at the ends become more important which increase the compressive
strength(ASTM E9).

56
The compression of a short cylinder between anvils is a much better test for
measuring the flow stress in metalworking applications. There is no problem with
necking and the test can be carried out to strain in excess of 2.0 if the material is
ductile. However, the friction between the specimen and anvils can lead to
difficulties unless it is controlled. In the homogeneous upset test a cylinder of
diameter Do and initial height ho would be compressed in height to h and spread
out in diameter to D according to the law of constancy of volume:
`
During deformation, as the metal spreads over the compression anvils to increase
its diameter, frictional forces will oppose the outward flow of metal. This frictional
resistance occurs in that part of the specimen in contact with the anvils, while the
metal at specimen midheight can flow outward undisturbed. This leads to a
barrelled specimen profile and internally a region of undeformed metal is created
near the anvil surfaces (Figure 3.1). As these cone-shaped zones approach and
overlap,they cause an increase in force for a given increment of deformation and
the load deformation curve bends upward (Figure 3 2) For a fixed diameter, a

57
shorter specimen will require a greater axial force to produce the same percentage
reduction in height because of the relatively larger undeformed region (Figure 3.1)
Thus, one way to minimise barrelling and the nonuniform deformation is to use a
low value of DMo. However, there is practical limit of Doho 0.5. for below this
value the specimen buckles instead of barrelling. The true flow stress in
compression without friction can be obtained by plotting load versus Doha for
several values ofreduction and extrapolating each curve to Doha-0
The friction at the specimen-platen interface can be minimised by using smooth,
hardened platens, grooving the ends of the specimen to retain lubricant, and
carrying out the test in increments so that the lubricant can be replaced al intervals.
Teflon sheet for cold deformation and glass for hot deformation are especially
effective lubricants. With these techniques it is possible to reach a strain of about c
= 1.0 with only slight barrelling. When friction is not present the uniaxial
compressive force required to produce yielding is
The True compressive stress p produced by this force P is
p = 4*P/π*d2
and using the constancy of volume relationship
p = 4*P*h/π*D2
*ho
Where are the initial diameter and height and h is the height of the cylindrical
sample at any instant during compression. The true compressive strain is given by
ln(ho/h).
TYPES OF FRACTURE
Brittle materials commonly fracture by shear either along a diagonal plane or with
a cone (for cylindrical specimens) or a pyramidal (for square specimens) shaped
fracture, sometimes called as hourglass fracture(figure 3.3).Cast iron usually fails
along an inclined plane and concrete shows the cone type of fracture.

58
The resolved shear stress is maximum at 45° to the load axis and therefore, the
fracture should occur on a plane at 45° to the load axis. However, due to internal
friction, non-homogeneous composition and structure and friction at the ends of
the specimen, the fracture plane is between 50° and 60 to the load axis.
If the specimen is so short that a normal failure plane can not develop within its
length, then the strength is appreciable increased and other types of failures such as
crushing may occur.
Ductile and plastic materials bulge laterally and take a barrel shape as they are
compressed, provided the specimen does not bend or buckle. Due to this, the
compressive stress continues to increase almost without any limit as there is no
failure of the material.
EFFECT OF SIZE AND SHAPE OF SPECIMEN ON THE COMPRESSIVE
STRENGTH
In the majority of cases, compressive test specimens are either square or circular
in cross section and hence the important variable is length-to-width (or length -to-
diameter) ratio. Asthe length to diameter (0/D) ratio increases, compressive
strength decreases (Figure 3.4). This is due to increased amount of bending
stresses.

59
TORSION TEST
The torsion test is not so widely used as the tension test. However, it is useful in
many engineering applications and also in theoretical studies of plastic flow.
Torsion test is carried out on materials to determine such properties as the modulus
of elasticity in shear, the torsional yield strength and the modulus of rupture. It is
carried out on parts, such as shafts, axles, twist drills, rivets and keys used in
shafts, wires, and rods. Stress-strain diagrams are plotted from the actual test data
from which the above mentioned properties are determined. This test has not been
standardised in general and therefore, there is no accurate relationship between the
properties determined by the torsion test and the tensile test.However, for plain
carbon steels, the torsional strength is usually 75% of the lensile strength.

60
The torsion testing machine consists of a twisting head, with chuck for gripping
the a specimen and for applying the twisting moment to the specimen and a
weighting head, which grips the other end of the specimen and measures the
twisting moment or torque applied by the twisting head (Figure 3.5), The gripping
arrangements must be such that the axis of the specimen coincides with the axis of
rotation.
The deformation of the specimen is usually measured by a twist measuring device
(or troptometer) which indicates the angular displacement.
Specimens that have a circular cross section are generally used for torsion tests.
For solid specimens, stress varies from zero at the centre of the specimen to a
maximum at the surface. For tubular specimens, the stress distribution is nearly
uniform throughout the cross section. Therefore, it is desirable to test a thin walled
tubular specimen or wires which will have nearly uniform stress distribution over
the entire cross section area.
Fractures (or failures) are different in torsion test than in tensile test. In torsion
test, very small (or almost nil) localised reduction in cross section area occurs. A
ductile metal fails by shear along one of the planes of maximum shear stress and
generally the plane of fracture is normal to the longitudinal axis. (Figure 3.6(a)). A
brittle material fails along a plane perpendicular to the direction of the maximum
tensile stress and shown a helical fracture (Figure 3.6(b))
The test is specified for certain materials like wires, tubes, etc. by Indian standards
Institution (ISI) and it is therefore necessary to refer to the IS code of that material
and test according to the procedure given in the IS code. Usually, the result is
reported as the number of turns prior to failure over a specified length of wire or
tube (IS-1717).

61
Chapter IV
IMPACT TEST
A simple tensile test does not reveal the brittle nature of the metals and if only the
tensile test data are relied upon and the object put into use, failure is certain. In
actual use and practice usually loads are borne by the engineering components
suddenly. It is therefore, necessary to test the material under shock or sudden
loading conditions. There are two common types of impact tests Izod and Charpy.
These machines are standardised in all respects, including the specimens to be used
in them.
The principle employed in all impact testing procedures is that a material absorbs
certain amount of energy before it breaks. The
quantity of energy thus absorbed is characteristic of the physical nature of the
material. If it is brittle, it breaks more readily i e., absorbs a lesser quantity of
energy and if tough it needs more energy in order to fracture.
The methods of testing are also very similar. A swinging hammer is made to strike
the specimen held firmly in a vice. The hammer breaks the specimen on account of
its potential energy. The height of rise o uhe hammer on the other side indicates the
residual energy of the hammer The energy actually absorbed by the material
specimen in order in fracture is given by the difference between the initial and final
energies of the hammer.
THE IZOD TEST
The Izod testing machine consists of a heavy triangular frame. At its apex is a
smooth bearing in which heavy pendulum weight. The pendulum weight can be
lifted up to a height swings a and clamped. When released, it carries energy of120
lbs (16.36 kg Mts). At the base. there a vice to fix the standard specimen. There is
a pointer which moves on a scale at the top to read the energy in kgf metres. The
pointer is in turn moved by the swinging pendulum weight. The scale is marked
from centre to both the ends, so that the energy absorbed by the specimen is read
directly. (Figure 4. l)
The standard specimen for the izod test is a square rod of 10 mm side.(figure 4.1.)
There is 2 mm deep, 45° notch made at a distance of 28 mm from the end of the
specimen that root of the notch is finished with a 0.25 mm radius. The specimen is
fixed in such away thatthe hammer strikes at a point 6 mm from the top. The notch
of the specimen is fixed to be on level with the anvil and faces the pendulum.

62
The actual testing procedure is as follows:
(i) The pendulum weight is brought up and clamped.
(ii) The specimen is fixed in the vice.
(iii) The scale is adjusted to read zero.
(iv) The pendulum weight is released from the clamp.
(v) The energy absorbed by the specimen is read on the scale to give the impact
toughness of the material.
In the illustration (Figure 4.2)
W = weight of pendulum
R = length of the pendulum (distance from its centre of gravity to the point of
support).
A = angle of fall
B = angle of rise.
H=height of fall and
h = height of rise
Initial energy (the energy in the pendulum before it breaks the specimen) = W*H =
W*R*(1 - cos(A)).
Final energy(the energy remaining in the pendulum after it has broken the
specimen) = W*h = W*R*(1 - cos(B)).
Energy of rupture of specimen = W*(H-h) = W*R*(cosB - cos A).

63
The presence of the notch in the specimen is very important, especially to detect
the brittleness. A highly localised stress value is built up at the root of the notch.
This causes the commencement of a crack at a much lower value of deformation
than for an unnotched specimen.
CHARPY IMPACT TESTING MACHINE
The charpy impact testing machine differs from the Izod machine in the method of
breaking the specimen. The specimen employed may be or any of the three types
illustrated in Figure 4.3. Specimen designs differ only in the shape of the notches.
A V-notch as in the case of the load specimen, a U notch or a keyhole are the three
commonly recommended for the charpy specimens.
The specimen is fixed in the machine as a simple beam (Figure 4.4). The notch in
the specimen does not face the hammer as in the Izod method. The opposite face of
the notch is fixed to receive the hammer blow. The hammer head, a pointed one of
8 mm radius, strikes the specimen just in the vertical axis of the notch.

64
The charpy machine is simillar to the Izod machine in the other aspects like the
design and method of operation. A hammer blow capacity of 220 ft.lbs.(30 kgf.m)
is very common.

65
EFFECT OF VARIABLES ON THE IMPACT TEST VALUES
Various impact test data do not agree. The variations are due to the follows:
A. A part of the energy of the pendulum being absorbed by the machine and its
foundation, instead of the specimen alone
B. Variations in the striking velocity of the pendulum.
C. Size and shape of the specimen.
D. Temperature
E. The gain size.
F. Composition of the metal.
G. Cold work.
A. Energy losses
Theoretically, all the energy of the swing hammer should be transferred to the
specimen. In practice, this does not happen. When the pendulum hammer strikes
the specimen,the vice and the foundation of the machine take up some of the
energy of the pendulum. However, this is minimised by the manufacturer of the
machines by providing a heavy structure for the machine Even while erecting the
machine, a very thorough foundation work is laid to minimise the damping and
vibrations.
B. Striking Velocity
The commonly used izod and Charpy impact testing machines develop with 16.36
and 30 kgf meters energy respectively; their respective striking velocities being 3.5
and 5.3 m/s.
Changes in the velocity of impact in these ranges are found to have no effect on the
impact values obtained. However, at very high volocities of impact, the resistances
are found to decrease remarkably. It is pointed out that, for a particular metal, there
is a critical velacity of impact above which it gives way very easily, Further, the
micro-structural condiuon is also found to have its influence on the critical striking
velocity; an annealed steel having a much lower critical striking velocity than a
handened steel.
C. Size and shape of the specimen
Photographical decrease in the size of the specimen may be and is some times
adopted when sufficient material to be tested in not available to prepare a specimen
of the standard size is experimentally established that the impact strength value is
not affected by the size of the specimen except when the steel is brittle. Brittle
steels, however, record a higher value of impact when tested on miniature sizes.
The same is found to be true when the testing is done at lower temperatures.

66
The presence of the notch and its sharpness has a marked effect on the impact
values. ISO recommended tolerance on the standard test specimen dimensions are
0.11 mm. Hence, it is advisable that the depth of 2 mm of the notch be carefully,
rather stringently, adhered to. It is observed that the behaviour of a metal is more
sensitive to the presence of the notch than its size. That is why the shallow V-notch
is preferred to the keyhole notch in the Charpy impact test.
D.Temperature
Impact strength of notched bars is greatly afrected by the temperature. Up to a
certain temperature, the metal exhibits a brittle fracture. After that temperature are
is exceeded and until a higher temperature is reached,the nature of fracrure will be
a mixed one, via, ductile and brittle. Again after passing this range , the metal
shows a ductile fracrure (Figure 4.5)
It can be noted that the changes of fracture from brittle to ductile is a gradual
process and occurs over a range of temperature. This is called the transition
temperature range. Further, it should be noted that the metal breaks very easily
below the transition temperature range (brittle failure).
E. Grain size
The effect of the grain size on transition temperature is quite marked. As the grain
size decreases (the ASTM grain size number incrcases) the transition lemperature
is found to be lowered considerably. Thus, it is beneficial to either adopt the lowest
possible finishing temperature in hot working or normalised the steel after hot
working.

67
F.Composition
Of all the alloying elements in steel, chromium has little effect on the transition
temperature while the maximum effect produced by carbon aud manganese. While
carbon pronouncedly raises the transition temperature, manganese, through to a
slightly lesser degree lowers it. Their effects are such that for satisfactory notch
toughness, they should be present in a proportion of M C- 3
Other elements which rise the transition temperature are phosphorus and to a lesser
degree, silicon, when present in quantities more than 0.25%.Of course the effect of
molybdenum on the transition temperature is as drastic as that of carbon, but
fortunately all the steels do not contain molybdenum unless it is intentionally
added.
Oxygen is another element which raises the transition temperature considerably.
Every thousandth percentage of it (0.001%) raises the transition temperature by
about 5° C (on a 0° C basis). Thus, the process of killing the steel has a bearing on
its impact properties. Again amongost, the killed steels exhibit better impact
properties than the semi-killed and rimming steels. Again amongst the killed ones
aluminium-killed steels are found to be tougher than the silicon-killed steels.
Aluminium forms the nitride also, besides oxide and thus seals off the nitrogen.
Nitrogen embrituement is thus avoided.
Besides manganese, which lowers the transition temperature, nickel appears to be
the only alloying element which lowers the transition temperature. Nickel in
quantities of about 2% is very useful in this regard.
Microstructure and heat treatment
Completely pearlitic structure imparts very poor impact properties to the steel,
while structure consisting of tempered martensite yields good notch impact
strength values. Bainitic structure is of a moderate quality in this respect.
Embrittlement phenomena
Temper embrittlement and hydrogen embrittlement are discussed below.
Temper embrittlement:
Steels are found to lose toughness remarkably when held between 350° C and the
transition temperature. This is referred to as to the temper embrittlement.
Tempering or slow cooling in the aforesaid temperature range should be avoided to

68
get rid of this phenomenon. While heat- treating heavy sections, becomes
practically impossible to avoid the embrittlement temperature range. Addition of
molybdenum is found to reduce the susceptibility to temper embrittlement.
Whether a steel is prone to temper embrittlement or not can be found out by
determining its transition temperature. The transition temperatures of temper brittle
steels are observed to be about 100° C or so above those for ordinary steels.
Further, fracture shows out to be intercrystalline, unlike the transcrystalline nature
of an ordinary brittle failure.
It is established that temper embrittlement results on account of grain boundary
weakness, Though no experimental evidence is available, it is assumed that a
precipitate or film of unobservable fineness is fomed at the gain boundaries.
Hydrogen Embrittlement
When hydrogen is present in a metal or alloy, it produces severe embrittlement.
Quantities of hydrogen as small as 0.0001% cause the steel to crack. Usually bcc
and hcp metals are highly susceptible to this effect.
Hydrogen embrinlement is affected by strain rate and temperature. The chief
evidence for this phenomenon is a delayed fracture (Figure 4.6). Slow tension and
bending test detect the hydrogen embrittlement during which the ductility will be
found to be low. Steels are prone to hydrogen embrittlement both at subzero
temperarure and high temperature. It is very marked at the room temperature.
Hydrogen dissolved in the steel is present in the atomic state. Under favourable
conditions, however these atoms diffuse into any voids or regions of high
dislocation concentrations and become molecular. High pressure develops and
starts to exert, which ultimately lead to the crack formation and propagation. The

69
dissolved hydrogen in the metal can be driven off to a considerable extent by
prolonged heating al 200-250 C.
G. Cold Work
The same metal in a cold worked condition and in an annealed condition exhibits
two types of impact toughness. Work hardened metals show brittle failures and
annealed ones ductile failures. The reason for the brittle failure in the case of cold
worked metal may be the orientation of the grains in the direction of work
(preferred orientation) and the consequent behaviour of the metal as if it were a
fibrous one (isotropy).
FRACTURE MECHANISM
When a material is loaded and the stress reaches a value that the material can not
withstand, it breaks. In material testing terminology, this is known as fracture.
Fracture can be defined as the separation of a material into two or more pieces
under stress.
A fundamental study of the process or mechanism of fracture can be made by
observing the two fracture surfaces. A brittle material shows no visible plastic
deformation at the time of its fracture (Figure 4.7(a)). The two fractured pieces
when joined together will exactly resemble the test specimen before loading. This
is not common in metals and alloys but occurs in non-metals and viscous ttalerials
like glass.
Metals and alloys show a different type of behaviour. A degree of plastic
deformation can be formed during or prior to the failure. A drastic case is complete
necking or drawing-down to two nearly conical surfaces. This is exhibited by very
ductile materials like annealed copper. A mixed type of fracture commonly known
as the 'cup and cone fracture' is exhibited by a majority of metals and alloys. The
cup and cone fracture consists of a core of plastic deformation and an annular ring
of brittle failure(Figure 4.7b). Whenever we refer to a ductile failure, it means
without saying the cup and cone type fracture only, as it is the most common in
metals.

70
We know that in the transition range, the fracture changes from ductile to
brittle.That is thefibrous fracture with deformation changes into the crystalline
state. The energy absorbed also decreases. In between the two extremes, the
fracture will be a mixed one. (Figure 4.8) It consists of a well defined crystalline or
brittle portion, generally away from the region of notch.
SIGNAFICANCE OF TRANSITION TEMPERATURE CURVE

71
The transition temperature behaviour of a wide spectrum of materials falls into the
three categories shown in Figure 4.9. Medium and low-strength fcc metals and
most hcp metals have such high notch toughness thal brittle fracture is not a
problem unless there is some special reactive chemical environment. High-strength
materials have such low notch toughness that brittle fracture can occur at nominal
stresses in the elastic range at all temperatures and strain rates when flaws are
present.High strength steel, aluminium and titanium alloys fall into this category.
At low tomperature fracture occurs by brittle cleavage,while at higher temperatures
fracture occurs by low-energy rupture. The notch toughness of low and medium-
strength on temperature bcc metals, as well as Be, Zn and ceramic materials is
strongly dependent on temperature.At low temperature the fracture occurs by
ductile rupture Thus, there is a transitionfrom notch brittle to notch tough
behaviour with increasing temperature.In metals this transition occurs at 0.1 to 0.2
of the absolute melting temperature T while in ceramics the transition occurs at
about 0.5 to 0.7 T.

Testing of metal and alloy

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