1. Impact test report
Objective
To determine ductile to brittle transition temperature using V notch Charpy impact test.
Apparatus
Charpy impact testing machines
Steel samples
Thermometer
Boiling water for high temperature
Ice + salt for sub-zero temperature
Theory and Background
Background
During World War II a great deal of attention was directed to
the brittle failureof welded Liberty ships and T-2 tankers
Some ofthese ships broke completely intwo, while, in other
instances, the fracture did not completely disable the ship.
Most of the failure occurred during the winter months,
Failures occurred bothwhen the ships were in heavy seas and
Figure 1: titanic ship
when they were anchored atdock Thesecalamities focused attention on the fact that normally
ductile mild steel canbecome brittle under certain conditions1.
The Titanic began its maiden voyage to New York just before noon on April 10, 1912, from
Southampton, England. Two days later at 11:40 p.m., Greenland time, it struck an iceberg that was
three to six times larger than its own mass, damaging the hull so that the six forward compartments
were ruptured. The flooding of these compartments was sufficient to cause the ship to sink within
two hours and 40 minutes, with a loss of more than 1,500 lives. The scope of the tragedy, coupled
with a detailed historical record, have fuelled endless fascination with the ship and debate over the
reasons as to why it did in fact sink. A frequently cited culprit is the quality of the steel used in the
ship's construction. A metallurgical analysis of hull steel recovered from the ship's wreckage
provides a clearer view of the issue.
Scientist observed several fractures in Second World War and predicted that
1. The fractures are of brittle type.
2. These occur below yield strength.
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2. Impact test report
Analysis of these fractures showed that the reason behind is the notches.
These may be due to
1. Design feature
2. Fabrication process (e.g. welding)
3. Flaws in the materials (e.g. porosity)
Theory
A broad research program was under-taken to find the causes of these failures and to prescribe the
remedies for their future prevention, In addition to research designed to find answers to a pressing
problem, other research was aimed at gaining a better understanding of the mechanism of brittle
fracture and fracture in general1.
Fracture
Fracture may be defined as the mechanical separation of a solid owing to the application of stress.
Fracture toughness
Fracture toughness is an indication of the amount of stress required to propagate a pre-existing
flaw. It is a very important material property since the occurrence of flaws is not completely
avoidable in the processing, fabrication, or service of a material/component. Flaws may appear as
cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or some combination
thereof
(http://www.ndt-
ed.org/EducationResources/CommunityCollege/Materials/Mechanical/FractureToughness.htm).
Toughness
Toughness is defined as the ability of a material to absorb energy. It is usually characterized by the
area under a stress-strain curve for a smooth (unnotched) tension specimen loaded slowly to
fracture (3).
Notch toughness
Notch toughness represents the ability of a material to absorb energy usually determined
underimpact loading in the presence of a notch. Notch toughness is measured by using a variety of
specimens such as the Charpy V-notch impact specimen, the dynamic-tear specimen, and planestrain fracture-toughness specimens under static loading (KId) and under impact loading (KIc)
(3asm).
Types of Fracture
There are various types of fracture which are encountered.
1. Ductile fracture
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3. Impact test report
2. Brittle fracture
3. Inter-crystalline
4. Fatigue fracture
Apart from these there fracture may be both
Brittle and ductile
Fracture dependence
Ductile-to-Brittle Fracture Transition. Traditionally, the notch-toughness characteristics of low- and
intermediate-strength steels have been described in terms of the transition from ductile to brittle
behavior as test temperature increases. Most structural steels can fail in either a ductile or a brittle
manner depending on several conditions such
1. Temperature
2. Strain rate (ϵ o)
3. State of stress
4. Section size
5. Notch acuteness
These variables may change a ductile fracture to a brittle fracture in service leading.
Notched-Bar Impact Tests
Various types of notched-bar impact tests are used to determine the tendency of a material to
behave in a brittle manner. This type of test will detect differencesbetween materials which are not
observable in a tension test. The results obtained from notched-bar tests are not readily expressed
in terms of design requirements, since it is not possible to measure the components of the triaxial
stress condition at the notch. Furthermore, there is no general agreement on the interpretation or
significance of results obtained with this type of test.
A large number of notched-bar test specimens of different design have beenused by investigators
of the brittle fracture of metals. Two classes of specimenshave been standardizedfor notchedimpact testing.
Charpay V-notch impact test (in USA)
Izod test (in UK)
Charpy V-notch impact test
The most widely used specimen for characterizing the ductile-to-brittle transition behavior of
steels. These specimens may be tested at different temperatures and the impact notch toughness
at each test temperature may be determined from the energy absorbed during fracture, the
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4. Impact test report
percent shear (fibrous) fracture on the fracture surface, or the change in the width of the specimen
(lateral expansion).
Figure 2: Charpy v-impact test
The notched-bar impact test is most meaningful when conducted over a range of temperature so
that the temperature at which the ductile-to-brittle transition takes place can be determined.
Note that the energy absorbed decreases with decreasing temperature but that for most cases the
decrease does not occur sharply at a certain temperature.
The principal advantage of the Charpy V-notch impact test is that it is a relatively simple test that
utilizes a relatively cheap, small test specimen. Tests can readily be carried out over a range of sub
ambient temperatures. Moreover, the design of the test specimen is well suited for measuring
differences· in notch toughness in low-strength materials such as structural steels. The test is used
for comparing the influence of alloy studies and heat treatment on notch toughness.
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5. Impact test report
It frequently is used for quality control and material acceptance purposes. The chief difficulty is that
the results of the Charpy test are difficult to use in design. Since there is no measurement in terms
of stress level, it is difficult to correlate Cv data with service performance. Moreover, there is no
correlation of Charpy data with flaw size. In addition, the large scatter inherent in the test may
make it difficult to determine well-defined transition-temperature curves.
Specimen in Charpay v-notch impact test
The Charpy specimen has a square cross section (10 X
10 mm) and contains a 45° V notch, 2 mm deep with a
0.25-mm root radius. The specimen is supported as a
beam in a horizontal position and loaded behind the
notch by the impact of a heavy swinging pendulum
(the impact velocity is approximately 5 ms
- 1
). The
specimen is forced to bend and fracture at a high
3
Figure 3: sample for charpy v-notch impact test
-1
strain rate on the order of 10 s .
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6. Impact test report
Figure 4: micrograph of charpy tested samples
INSTRUMENTED CHARPY TEST
The ordinary Charpy test measures the total energy absorbed in fracturing thespecimen. Additional
information can be obtained if the impact tester is
instrumented to provide a load-time 'history of the
specimen during the test.In figure an idealized loadtime curve foran instrumented Charpy test. Withthis
kindof record it is possible to determine the energy
required for initiatingfractureand the energy required
for propagating fracture. It also yields information on
the load for general yielding, the maximum load, and
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7. Impact test report
the fracture load.
Transition temperature
As mentioned previously, the absorbed energy of BCC metals changes drastically within the
transition region, we therefore have to identify a transition temperature, which can be used
todetermine the suitable service temperature of particular materials in order to avoid metal failure in
acatastrophic manner. There are several criteria for the identification of the transition temperature.
Transition temperature is the temperature at which the test sample absorbs the
mostfracture energy and possesses 100% fibrous fracture surfaces. This means brittle
fracture is neglected in this case and is considered to be the safest among other criteria.
The transition temperature is also called the fracture transition plastic or FTP.
Transition temperature is the temperature at which the percentage of cleavage and
ductilefractures are equal. This transition is also called fracture appearance transition
temperature orFATT because the fracture surface area is used as an indicator to determine
the transition temperature.
Transition temperature is the temperature correlating to an average absorbed energy
valueof upper and lower shelf energy absorption. At or above this temperature, there is
acorrelation that less than 70% of the brittle cleavage fracture that indicates a high
probability at which failure will not occur if the stress does not exceed about one-half of the
yield stress.
Transition temperature is the temperature at which the absorbed energy (C) equals 20J.
This criterion was introduced to determine toughness value of steels used during the World
War II. It is based on the idea that brittle fracture will not occur if the sample has the
absorbed energy above 20J. However this criterion might show no significant meanings
forother materials.
Transition temperature is the temperature at which there is none of the ductile
dimplesappearing on the fracture surfaces. This temperature is also called nil ductility
temperature orNDT since there is no plastic deformation during fracture.
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8. Impact test report
Metallurgical Factors Affecting Transition Temperature
The factors which affect the transition temperature are
Composition of steel
Grain size
Ageing phenomena
Notched size and direction
Tempering time
Presence of martensite
Lowest possible finish temperature of rolling
Procedure
Room temperature test is first carried out by placing the Charpy
impact specimen on the anvil and positioning it in the middle location
using a positioning pin where the opposite site of the notch is
destined for the pendulum impact.
Raise the pendulum to a height corresponding to the maximum
stored energy of 300J.
Release the pendulum to allow specimen impact. Safely stop the
movement of the pendulum after swinging back from the opposite
side of the machine.
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9. Impact test report
When the pendulum is still, safely retrieve the broken specimen
without damaging fracture surfaces. Repeat the test at the same test
condition using another specimen to average out the obtained
values.
Charpy impact testing at temperatures other than room temperature
is carried out following
Prior to specimen impact, specimen is submerged in the medium for
at least 5 minutes to ensure uniform temperature across the
specimens. Specimen impact must be within 5 seconds after
removing from the medium.
Repeat the test at the same test condition using another specimen
to average out the obtained values.
Data
Sample
Mild Steel
Angle during free falling
Condition
Temperature oC
135o
Room temperature
10
Ice + salt
0
Boil water
98
Calculation
Hammer lift angle
α
=
139.5o
Distance from axis to centre of gravity
D
=
0.694m
Weight of hammer
P
=
40.98kg
E = PD (cosβ-cosα)
Case I
Room temperature
β
E=
=
96o
energy calculated = 18.77 Kgfm
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10. Impact test report
Case II
Boiling water
=
95o
E
β
=
19.135 kgfm
Case III
Ice + salt
β
=
112
E
=
10.96 kgfm
Result
As we decrease temperature material fails with brittle manner. And absorbed energy also decrease. Because
material is much prone to brittle behaviour.
Reference
Mechanical metallurgy 'g.e. dieter' 3rd edition
Mechanical Testing and Evaluation was published in 2000 as Volume 8 of the ASM Handbook. The
Volume was prepared under the direction of the ASM Handbook Committee.
Hand out lecture
http://www.saecanet.com/calculation_page/000380_000509_Charpy_impact_test.php
ASTM standards e 2
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