Experimental stress analysis module 5 Part-A (ME832) notes by Dr. Mohammed Imran

1. 1
GHOUSIA COLLEGE OF ENGINEERING
RAMANAGARAM-562159
EXPERIMENTAL STRESS ANALYSIS
[15ME832]
Dr. MOHAMMED IMRAN
ASST PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING

2. 2
Module-5 Part-A
Brittle Coating Technique
Principle of Brittle Coating
The principle of stress analysis involves the adherence of a thin coating (resin- or
ceramic-based) brittle in nature on the surface of a specimen. When the specimen is
subjected to external loads, the thin brittle coating cracks under tensile stresses.
Strains produced in specimen are transmitted to the coating resulting in coating
cracks. From the threshold strain of coating, i.e. minimum strain required to cause the
coating to crack, determined through calibration of coating, the stresses in specimen
are determined. The behaviour of the coating is quite complicated as it depends on the
number of parameters influencing the behaviour of the coating, such as coating
thickness, coating temperature, creep in coating during testing, moisture, velocity of
air flowing over coating, curing time of the coating, and load-time history; the
analysis through brittle coating is more of qualitative nature than of quantitative
nature. The use of the coating is limited to identifying the regions of high stresses and
regions of low stresses to economize the use of material in component, i.e. low
stressed regions of components are identified for weight reduction in component. This
technique provides a simple, direct approach for solving large class of industrial
problems such as pressure vessels. This technique provides whole field data, and at
the same time it is classified as non-destructive. There are various types of coatings as
resins based, ceramic based or glass lacquer based brittle coatings. The behaviour of
resin-based coatings is viscoelastic (i.e. creep during loading) because in addition to
resin it contains plasticizer which controls the sensitivity of the coating.
This technique has been used for (i) the determination of stress concentration in
components subjected to various types of loads, (ii) the measurement of thermal and
residual strains in components, and (iii) providing whole field data for the magnitude
and direction of principal stresses.
This method is based upon the perfect adhesion of a thin coating, brittle in nature on
the surface of a component to be analyzed for stresses as shown in Figure1. When the
specimen is stressed the surface strains of specimen (at the interface between the
specimen and coating) are transmitted to the coating and the coating cracks in a
direction perpendicular to maximum tensile principal stress.

3. Figure.1
This method is classified as non
and specimen is not over-stressed. Common examples of brittle coating are mill scale
on hot rolled steel bars, white wash on walls, but the strains required for such coa
to fail by cracking are large.
Historical background of Brittle Coating
Ellis in 1941 in United States developed a resin
coating. The formulation of this coating is (
disulphide as solvent, (iii) dibutyl pthalate as a plasticizer to control the plasticity of
the coating or to vary the degree of brittleness of the coating. This coating is known
by the TRADE name of `Stress coat‘. Ceramic coatings which can be employed in
high temperature applications have also been developed by the trade name of `All
temp’.
Advantages of brittle coating technique are as follows:
 It is nearly a whole field stress analysis technique.
 The technique can be directly applied to a prototype of actual machine
member and there is no necessity of any model.
 The technique is applied
there is no necessity for any simulation.
 Analysis for converting the data into stresses in component is not complicated
and only simple mathematical relations are needed.
Disadvantages of the coating are as
 Behaviour of the coating is strongly dependent on temperature and humidity
variations during testing.
 Number of variables affects the sensitivity of the coating, therefore the
behaviour of the coating has to be properly understood.
3
Figure.1 Cracks in brittle coating
his method is classified as non-destructive as the coating fails at very low stresses,
stressed. Common examples of brittle coating are mill scale
on hot rolled steel bars, white wash on walls, but the strains required for such coa
to fail by cracking are large.
Historical background of Brittle Coating
Ellis in 1941 in United States developed a resin-based commercially available
coating. The formulation of this coating is (i) zinc resinate as base, (
) dibutyl pthalate as a plasticizer to control the plasticity of
the coating or to vary the degree of brittleness of the coating. This coating is known
name of `Stress coat‘. Ceramic coatings which can be employed in
high temperature applications have also been developed by the trade name of `All
of brittle coating technique are as follows:
It is nearly a whole field stress analysis technique.
The technique can be directly applied to a prototype of actual machine
member and there is no necessity of any model.
The technique is applied to the actual machine component in operation and
there is no necessity for any simulation.
Analysis for converting the data into stresses in component is not complicated
and only simple mathematical relations are needed.
of the coating are as follows:
Behaviour of the coating is strongly dependent on temperature and humidity
variations during testing.
Number of variables affects the sensitivity of the coating, therefore the
behaviour of the coating has to be properly understood.
destructive as the coating fails at very low stresses,
stressed. Common examples of brittle coating are mill scale
on hot rolled steel bars, white wash on walls, but the strains required for such coatings
based commercially available
) zinc resinate as base, (ii) carbon
) dibutyl pthalate as a plasticizer to control the plasticity of
the coating or to vary the degree of brittleness of the coating. This coating is known
name of `Stress coat‘. Ceramic coatings which can be employed in
high temperature applications have also been developed by the trade name of `All-
The technique can be directly applied to a prototype of actual machine
to the actual machine component in operation and
Analysis for converting the data into stresses in component is not complicated
Behaviour of the coating is strongly dependent on temperature and humidity
Number of variables affects the sensitivity of the coating, therefore the

4.  The technique is more qualitative in nature than quantitative.
1. COATING STRESSES
Coating is sprayed over the surface of the specimen until a thickness of 0.1 to 0.25
mm is built up. Then, coating is dried either at room temperature or at an elevated
temperature in a hot air oven. After the coating is completely dried or cured, loads are
applied on the sample. Since the coating is very thin, it can be safely assumed that
surface strains of the specimen are faithfully transmitted from specimen to coating
without any magnification or attenuation. From the stresses in specimen, the stresses
in the coating can be obtained. Let us take
, ; stresses in the specimen in
, stresses in the coating in
vc, vs; Poisson’s ratio of coating and speci
Ec, Es; Young’s modulus for coating and specimen respectively.
Strains in the specimen at the interface
Figure.2
Strains in the coating
Assuming perfect adhesion between
4
is more qualitative in nature than quantitative.
1. COATING STRESSES
Coating is sprayed over the surface of the specimen until a thickness of 0.1 to 0.25
mm is built up. Then, coating is dried either at room temperature or at an elevated
ot air oven. After the coating is completely dried or cured, loads are
applied on the sample. Since the coating is very thin, it can be safely assumed that
surface strains of the specimen are faithfully transmitted from specimen to coating
ification or attenuation. From the stresses in specimen, the stresses
in the coating can be obtained. Let us take
; stresses in the specimen in x, y directions
stresses in the coating in x, y directions
; Poisson’s ratio of coating and specimen
; Young’s modulus for coating and specimen respectively.
Strains in the specimen at the interface abcd as shown in Figure .2
Figure.2 Stresses in specimen and coating
Assuming perfect adhesion between specimen and coating
Coating is sprayed over the surface of the specimen until a thickness of 0.1 to 0.25
mm is built up. Then, coating is dried either at room temperature or at an elevated
ot air oven. After the coating is completely dried or cured, loads are
applied on the sample. Since the coating is very thin, it can be safely assumed that
surface strains of the specimen are faithfully transmitted from specimen to coating
ification or attenuation. From the stresses in specimen, the stresses

5. Strain’s in x direction,
Strain’s in y direction,
From Eqs (1) and (2)
Equations (3) and (4) represent the plane state of stress in the coating (i.e.
produced in the coating by the plane state of stress in the specimen (i.e.
the sample is loaded only in
A brittle coating is generally calibrated by applying uniaxial stress to the specimen
which is in the form of cantilever
If = , threshold strain, that is the minimum strain required to crack the
coating, E* = Young’s modulus of the calibrating
calibrating beam
where = minimum uniaxial stress in the calibrating beam required to crack the
coating.
5
(1)
(2)
(3)
(4)
represent the plane state of stress in the coating (i.e.
produced in the coating by the plane state of stress in the specimen (i.e.
the sample is loaded only in x-direction, i.e. = 0, then
A brittle coating is generally calibrated by applying uniaxial stress to the specimen
which is in the form of cantilever
, threshold strain, that is the minimum strain required to crack the
= Young’s modulus of the calibrating beam. Then v* Poisson’s ratio of
= minimum uniaxial stress in the calibrating beam required to crack the
(3)
(4)
represent the plane state of stress in the coating (i.e. = 0)
produced in the coating by the plane state of stress in the specimen (i.e. = 0). If
A brittle coating is generally calibrated by applying uniaxial stress to the specimen
, threshold strain, that is the minimum strain required to crack the
Poisson’s ratio of
= minimum uniaxial stress in the calibrating beam required to crack the

6. The biaxial stress system in the coating due to uniaxial stress system in the specimen
results from the mismatch in
coating.
Problem 2 : Calculate coating stresses if the specimen stresses are 70 and 40 MPa,
assuming
Es = 200 kN/mm2
, Ec = 2 kN/mm
Solution: Specimen stresses are
Coating stresses are
Substituting the value of E
Coating stresses in x and y
Problem 2: Calculate the stresses in steels specimen if coating stresses are
= 0.7 / , = 0
Ans. [ = 79.2 /
6
The biaxial stress system in the coating due to uniaxial stress system in the specimen
results from the mismatch in the values of Poisson’s ratio between specimen and
Calculate coating stresses if the specimen stresses are 70 and 40 MPa,
= 2 kN/mm2
, vc = 0.44, vs = 0.30
Specimen stresses are
Es, Ec, vs, and vc, we get
y directions are 0.823 and 0.552 N/mm2
, respectively.
Calculate the stresses in steels specimen if coating stresses are
0.88 / ; If
. = 109 / ]
The biaxial stress system in the coating due to uniaxial stress system in the specimen
the values of Poisson’s ratio between specimen and
Calculate coating stresses if the specimen stresses are 70 and 40 MPa,
, respectively.
Calculate the stresses in steels specimen if coating stresses are
; If

7. 2. FAILURE THEORIES
Coating is brittle in nature and for brittle materials, Mohr has developed a theory of
failure. In all such brittle materials such as rock, concrete, cast
compressive strength is much more than the ultimate tensile strength and Mohr theory
is the ideal choice for brittle coating. Following cases can be considered for predicting
the failure of brittle coating.
, both stresses are
, one tensile and other compressive stress
both the stresses are compressive.
Case (i) Mohr theory of failure coincides with the maximum principal stress theory of
failure
where is the ultimate tensile strength of the coating.
Now,
where = minimum uniaxial stress in the specimen when the coating cracks
perpendicular to
From expressions (5) and
Equation (7) governs the stresses in the specimen and if
and can be worked out
Case (ii) Mohr theory of failure gives the following relationship
where = ultimate strength of the coating in compression.
7
2. FAILURE THEORIES
Coating is brittle in nature and for brittle materials, Mohr has developed a theory of
failure. In all such brittle materials such as rock, concrete, cast iron, ceramics ultimate
compressive strength is much more than the ultimate tensile strength and Mohr theory
is the ideal choice for brittle coating. Following cases can be considered for predicting
the failure of brittle coating.
, both stresses are tensile
, one tensile and other compressive stress
both the stresses are compressive.
Mohr theory of failure coincides with the maximum principal stress theory of
is the ultimate tensile strength of the coating.
(5)
(6)
= minimum uniaxial stress in the specimen when the coating cracks
(6)
(7)
governs the stresses in the specimen and if is known then stresses
can be worked out
Mohr theory of failure gives the following relationship
(8)
ngth of the coating in compression.
Coating is brittle in nature and for brittle materials, Mohr has developed a theory of
iron, ceramics ultimate
compressive strength is much more than the ultimate tensile strength and Mohr theory
is the ideal choice for brittle coating. Following cases can be considered for predicting
Mohr theory of failure coincides with the maximum principal stress theory of
(5)
= minimum uniaxial stress in the specimen when the coating cracks
is known then stresses

8. where K is a constant.
Substituting values of
follows
where = minimum uniaxial stress in the specimen when the coating cracks
perpendicular to .
Case (iii) According to Mohr theory, coating will fail by cracking if
or
Substituting the values of
Equation (11) can be utilized to determine specimen stresses
3. CRACK PATTERNS IN
The behaviour of the brittle coating depends upon the nature and magnitude of
principal stresses σ1and σ2
, i.e. both the stresses
are tensile. Two families of cracks can
form depending upon the magnitude
of σ1 and σ2. First of all cracks
perpendicular to σ1 are developed and as
the load on the component increases,
cracks perpendicular to σ
shown in Figure.3. The example of this
case is of a thin cylindrical shell subjected
to internal pressure. On the surface of the
8
, the relationship (8) can be simplified as
(9)
= minimum uniaxial stress in the specimen when the coating cracks
According to Mohr theory, coating will fail by cracking if
(10)
and, we get
(11)
can be utilized to determine specimen stresses and
3. CRACK PATTERNS IN BRITTLE COATING
ehaviour of the brittle coating depends upon the nature and magnitude of
2. Following three cases will be discussed here:
, i.e. both the stresses
are tensile. Two families of cracks can
form depending upon the magnitude
. First of all cracks
are developed and as
the load on the component increases,
σ2 also appear as
. The example of this
case is of a thin cylindrical shell subjected
to internal pressure. On the surface of the Figure.3.
be simplified as
= minimum uniaxial stress in the specimen when the coating cracks
.
ehaviour of the brittle coating depends upon the nature and magnitude of
. Following three cases will be discussed here:
Figure.3.

9. cylinder, hoop stress is more than the axial
stress.
, i.e. one stress is
tensile and the other stress is compressive.
Only one set of cracks forms which are
perpendicular to σ1. Brittle coating is
strong in compression, therefore second
family of cracks perpendicular to
not appear; as shown in Figure 4
, when both the
stresses are the same, stress system is said
to be isotropic. Every direction is a
principal stress direction. Coating fails by
cracking but crack pattern will be random
in character, i.e. a craze pattern. This type
of crack patterns occur on a spherical shell
subjected to internal pressure, where hoop
stress occurs equally in all the di
as shown in Figure 5.
Problem 3: Brittle coating is applied
the pressure inside the vessel is
along longitudinal axis of the vessel. By calibration the threshold strain of coating is
560 μ strains. Now pressure in
cracks appears in circumferential direction and threshold strain of coating is
550 μ strains at this stage. Determine stress in coating and stresses in pressure vessel
when pressure inside is 2p.
9
cylinder, hoop stress is more than the axial
, i.e. one stress is
nsile and the other stress is compressive.
Only one set of cracks forms which are
. Brittle coating is
strong in compression, therefore second
family of cracks perpendicular to σ2 does
Figure 4.
Figure. 4
, when both the
stresses are the same, stress system is said
to be isotropic. Every direction is a
principal stress direction. Coating fails by
cracking but crack pattern will be random
in character, i.e. a craze pattern. This type
of crack patterns occur on a spherical shell
subjected to internal pressure, where hoop
stress occurs equally in all the directions
Figure.5 Random cracking in coating
Brittle coating is applied on the outer surface of a pressure vessel where
the pressure inside the vessel is p, first family of cracks appears in coating which are
along longitudinal axis of the vessel. By calibration the threshold strain of coating is
strains. Now pressure inside the vessel is increased to 2p, second family of
cracks appears in circumferential direction and threshold strain of coating is
strains at this stage. Determine stress in coating and stresses in pressure vessel
2p. Given
Figure. 4 Tension
Random cracking in coating
on the outer surface of a pressure vessel where
, first family of cracks appears in coating which are
along longitudinal axis of the vessel. By calibration the threshold strain of coating is
, second family of
cracks appears in circumferential direction and threshold strain of coating is
strains at this stage. Determine stress in coating and stresses in pressure vessel

10. Figure.3.
Ec = 1.45 × 10+3
N/mm2
, v
Es = 200 kN/mm2
, vs = 0.30
Solution: Say when pressure is
perpendicular directions, so when pressure is
or
or
Then
or
From Eqs (i) and (ii), we get
Stresses in coating
Specimen stress (at 2p)
10
Figure. 4 Tension Figure.5
cracking in coating
, vc = 0.43
= 0.30
Say when pressure is 2p, stresses in coating are
perpendicular directions, so when pressure is p, stresses would be 0.5
, we get
Figure.5 Random
cracking in coating
, , in two
, 0.5

11. Putting in values
or
or
From these equations = 282.454 N/mm
Stress in pressure vessel
Problem.4: A particular specimen of aluminium alloy is coated with Stress coat. At a
particular external load first family of cracks appears in coatings, and
570 μ strain. Now the load is
perpendicular to the first family of cracks appears in coating, and
Determine stresses in coating and stresses in specimen at the second stage. If
1.4GPa, Es = 70GPa, vc =0.42,
Ans. [ = 1.85 / ,
65.6 / ].
11
= 282.454 N/mm2
= 194.70 N/mm2
A particular specimen of aluminium alloy is coated with Stress coat. At a
particular external load first family of cracks appears in coatings, and
strain. Now the load is increased by 50%, a second family of cracks
perpendicular to the first family of cracks appears in coating, and
Determine stresses in coating and stresses in specimen at the second stage. If
=0.42, vs = 0.33
, = 1.55 / ; = 81.6
A particular specimen of aluminium alloy is coated with Stress coat. At a
particular external load first family of cracks appears in coatings, and =
increased by 50%, a second family of cracks
= 550 μ strain.
Determine stresses in coating and stresses in specimen at the second stage. If Ec =
/ . =

12. 4. REFRIGERATION TEC
Many components may be highly stressed in a particular region and remaining part of
the component may not be sufficiently stressed so
These low stressed areas are equally important while designing a component and the
thickness of the component can be reduced in such low stressed regions so as to affect
weight reduction.
It is possible to obtain cracks
the coating and reducing the value of its threshold strain
temperature). In this technique, first of all, the component is subjected to loads, then
cracks develop in some regio
coating is subjected to a rapid temperature drop while under load. This rapid
temperature drop introduces uniform thermal strains (or hydrostatic tension) in the
coating. Thermal stresses introduced in the
stresses in the coating due to applied load on component. Now these isotropic thermal
stresses do not have any preferential direction, therefore the direction of resulting
cracks is coincident with one of the princ
In refrigeration technique, two methods are generally used: (i) ice cold water is
sponged over the surface of the coating which has not cracked, but this method does
not produce sufficiently high thermal stresses, (ii) by passing a stream of
air through a box of dry ice before it is directed onto the surface of the coating.
5. LOAD RELAXATION
When both the principal stresses are compressive it is not possible to obtain crack
patterns in the coating because coating is
difficulty, a relaxation technique is used to obtain crack pattern. In this technique,
load is applied on the specimen. Then coating is sprayed on the specimen and it is
dried while the load is maintained on specim
is released gradually and cracks appear in directions perpendicular to
depending upon the magnitude of these stresses, as shown in
But if one principal stress is tensile and other is compressive, i.e.
families of cracks are obtained which are superimposed over each other.
12
4. REFRIGERATION TECHNIQUE
Many components may be highly stressed in a particular region and remaining part of
the component may not be sufficiently stressed so as to produce cracks in the coating.
These low stressed areas are equally important while designing a component and the
thickness of the component can be reduced in such low stressed regions so as to affect
It is possible to obtain cracks in the coating in such low stressed regions by sensitizing
the coating and reducing the value of its threshold strain (at ordinary room
temperature). In this technique, first of all, the component is subjected to loads, then
cracks develop in some region and the remaining portion is_ uncracked. Now the
coating is subjected to a rapid temperature drop while under load. This rapid
temperature drop introduces uniform thermal strains (or hydrostatic tension) in the
coating. Thermal stresses introduced in the coating are superimposed on the existing
stresses in the coating due to applied load on component. Now these isotropic thermal
stresses do not have any preferential direction, therefore the direction of resulting
cracks is coincident with one of the principal stresses.
In refrigeration technique, two methods are generally used: (i) ice cold water is
sponged over the surface of the coating which has not cracked, but this method does
not produce sufficiently high thermal stresses, (ii) by passing a stream of
air through a box of dry ice before it is directed onto the surface of the coating.
5. LOAD RELAXATION TECHNIQUE
When both the principal stresses are compressive it is not possible to obtain crack
patterns in the coating because coating is strong in compression. To circumvent this
difficulty, a relaxation technique is used to obtain crack pattern. In this technique,
load is applied on the specimen. Then coating is sprayed on the specimen and it is
dried while the load is maintained on specimen. When the coating is fully cured, load
is released gradually and cracks appear in directions perpendicular to
depending upon the magnitude of these stresses, as shown in Figure 6.
But if one principal stress is tensile and other is compressive, i.e. σ1>
families of cracks are obtained which are superimposed over each other.
Many components may be highly stressed in a particular region and remaining part of
as to produce cracks in the coating.
These low stressed areas are equally important while designing a component and the
thickness of the component can be reduced in such low stressed regions so as to affect
in the coating in such low stressed regions by sensitizing
(at ordinary room
temperature). In this technique, first of all, the component is subjected to loads, then
uncracked. Now the
coating is subjected to a rapid temperature drop while under load. This rapid
temperature drop introduces uniform thermal strains (or hydrostatic tension) in the
coating are superimposed on the existing
stresses in the coating due to applied load on component. Now these isotropic thermal
stresses do not have any preferential direction, therefore the direction of resulting
In refrigeration technique, two methods are generally used: (i) ice cold water is
sponged over the surface of the coating which has not cracked, but this method does
not produce sufficiently high thermal stresses, (ii) by passing a stream of compressed
air through a box of dry ice before it is directed onto the surface of the coating.
When both the principal stresses are compressive it is not possible to obtain crack
strong in compression. To circumvent this
difficulty, a relaxation technique is used to obtain crack pattern. In this technique,
load is applied on the specimen. Then coating is sprayed on the specimen and it is
en. When the coating is fully cured, load
is released gradually and cracks appear in directions perpendicular to σ1, and/or σ2,
> 0, σ2< 0, two
families of cracks are obtained which are superimposed over each other.

13. Figure .6
Figure.7 Showing cracks
perpendicular to σ1 during direct
loading
Figure.7 shows the family of cracks for
and Figure.8 shows the family of cracks for
During direct loading, cracks ap
relaxation, cracks appear perpendicular to
13
6 Cracks in coating during load relaxation
Showing cracks
during direct
Figure.8 Showing cracks perpendicular
to σ2 during load relaxation
shows the family of cracks for σ1 > 0 and σ2 < 0 during direct loading
shows the family of cracks for σ1 > 0 and σ1 < 0 during load relaxation.
During direct loading, cracks appear perpendicular to σ1 > 0 and during load
relaxation, cracks appear perpendicular to σ2 < 0.
Showing cracks perpendicular
during load relaxation
< 0 during direct loading
< 0 during load relaxation.
> 0 and during load

14. 6. CRACK DETECTION
For the purpose of stress analysis or to determine stresses in the specimen through
coating cracks it is necessary that all coating cracks a
they occur are noted down. These coating cracks are so fine that these are hardly
visible through naked eye. These cracks are V
coating thickness and width ranging from 0.05 to 0.08 mm. I
fine cracks visually, a pencil of light is focused on the surface of the crack through
oblique incidence as shown in
of the crack as shown in the figure.
For stress analysis it is necessary to keep the permanent record of these cracks by
taking their photographs
techniques of crack detection, i.e. (i) statiflux method and (ii) red dye etchant
technique.
In the statiflux method, water containing wetting agents (to reduce its surface tension)
is spread over the cracked portion of the coating. This wet water flows inside the
cracks and fills these cracks, thus making electrical contact with the metallic
specimen. Then the surface of the cracked coating is rubbed dry with the help of a
facial tissue, so that water is
cracks. Now a talcum powder negatively charged is sprayed (through a special spray
gun) on the coating surface. These negatively charged particles of talcum powder are
attracted electrically towards the gr
The powder forms small white mounds over the cracks showing fine white lines of
powder over yellow background of resin coating or brown glassy background of
14
6. CRACK DETECTION
For the purpose of stress analysis or to determine stresses in the specimen through
coating cracks it is necessary that all coating cracks are located and the loads at which
they occur are noted down. These coating cracks are so fine that these are hardly
visible through naked eye. These cracks are V-shaped with thickness equal to the
coating thickness and width ranging from 0.05 to 0.08 mm. In order to observe these
fine cracks visually, a pencil of light is focused on the surface of the crack through
oblique incidence as shown in Figure.9. Light beam is focused normal to the surface
of the crack as shown in the figure.
Figure.9 Crack detection
For stress analysis it is necessary to keep the permanent record of these cracks by
taking their photographs and before that making them visible. There are two
techniques of crack detection, i.e. (i) statiflux method and (ii) red dye etchant
method, water containing wetting agents (to reduce its surface tension)
cked portion of the coating. This wet water flows inside the
cracks and fills these cracks, thus making electrical contact with the metallic
specimen. Then the surface of the cracked coating is rubbed dry with the help of a
facial tissue, so that water is removed from the surface but it remains
cracks. Now a talcum powder negatively charged is sprayed (through a special spray
gun) on the coating surface. These negatively charged particles of talcum powder are
attracted electrically towards the grounded water contained in the fine coating cracks.
The powder forms small white mounds over the cracks showing fine white lines of
powder over yellow background of resin coating or brown glassy background of
For the purpose of stress analysis or to determine stresses in the specimen through
re located and the loads at which
they occur are noted down. These coating cracks are so fine that these are hardly
shaped with thickness equal to the
n order to observe these
fine cracks visually, a pencil of light is focused on the surface of the crack through
. Light beam is focused normal to the surface
For stress analysis it is necessary to keep the permanent record of these cracks by
and before that making them visible. There are two
techniques of crack detection, i.e. (i) statiflux method and (ii) red dye etchant
method, water containing wetting agents (to reduce its surface tension)
cked portion of the coating. This wet water flows inside the
cracks and fills these cracks, thus making electrical contact with the metallic
specimen. Then the surface of the cracked coating is rubbed dry with the help of a
removed from the surface but it remains inside the
cracks. Now a talcum powder negatively charged is sprayed (through a special spray
gun) on the coating surface. These negatively charged particles of talcum powder are
ounded water contained in the fine coating cracks.
The powder forms small white mounds over the cracks showing fine white lines of
powder over yellow background of resin coating or brown glassy background of

15. ceramic coating. Figure.10
surface.
Figure.10
A red dye etchant can also be used for crack detection and increasing the visibility of
crack patterns so that cracks can be photographed. The red dye etchant is a mixture of
turpentine oil, machine oil, and a red dye (soluble in turpentine oil). The dye mixture
is applied on the surface of the cracked coating for approximately one minute. During
this time the etchant begins to attack the coating at the surfaces of the cracks, thus
making them wider. Now the etchant is wiped out from the surface of the coating and
the coating surface is cleaned with the help of an etchant emulsifier which is a soap
and water solution. The dye which has penetrated inside the cracks is not removed
during this cleaning process. The cracks appear as fine red lines over the surface of
brittle coating (yellow in colour) as shown in
15
Figure.10 shows the white lines of talcum powder over coating
Figure.10 White talcum powder in cracks
can also be used for crack detection and increasing the visibility of
crack patterns so that cracks can be photographed. The red dye etchant is a mixture of
turpentine oil, machine oil, and a red dye (soluble in turpentine oil). The dye mixture
on the surface of the cracked coating for approximately one minute. During
this time the etchant begins to attack the coating at the surfaces of the cracks, thus
making them wider. Now the etchant is wiped out from the surface of the coating and
ng surface is cleaned with the help of an etchant emulsifier which is a soap
and water solution. The dye which has penetrated inside the cracks is not removed
during this cleaning process. The cracks appear as fine red lines over the surface of
ting (yellow in colour) as shown in Figure.11.
Figure.11 Red dye in crack
shows the white lines of talcum powder over coating
can also be used for crack detection and increasing the visibility of
crack patterns so that cracks can be photographed. The red dye etchant is a mixture of
turpentine oil, machine oil, and a red dye (soluble in turpentine oil). The dye mixture
on the surface of the cracked coating for approximately one minute. During
this time the etchant begins to attack the coating at the surfaces of the cracks, thus
making them wider. Now the etchant is wiped out from the surface of the coating and
ng surface is cleaned with the help of an etchant emulsifier which is a soap
and water solution. The dye which has penetrated inside the cracks is not removed
during this cleaning process. The cracks appear as fine red lines over the surface of

16. 16
7. TYPES OF BRITTLE COATING
There are various types of resin- and ceramic-based brittle coatings. Magnaflux
Corporation of USA markets two different coatings under the trade name of stress
coat. Photolastic Corporation markets a coating known as Tens lac. The exact
composition of Tens lack is proprietary; however the constituents in the non-
flammable coating are similar to those found in strain tec. This coating consists of
zinc resinate and calcium resinate dissolved in solvent methylene chloride with oleic
acid and the plasticizer is dibutyl pthalate as in stress coat. Stress coat is highly toxic
and flammable while strain tec is a new non-flammable, low toxicity coating
developed by General Motors. A glass lacquer was developed by Hickson at the
Royal Aircraft Establishment in England. This coating consists of a mixture of lithium
hydroxide, boric acid, and water. The coating dries at room temperature and is similar
to a resin-based coating in many respects.
A ceramic-based coating by the trade name of All-Temp is also marketed by
Magnaflux Corporation of USA. All-temp consists of finely ground ceramic particles
suspended in a volatile matter. This coating is sprayed onto the specimen.
When the ceramic coating dries in air after spraying, it has a chalk-like appearance
and is not suitable for use. The coating is fired at about 1100°F until the ceramic
particles melt and coalesce. After firing, the coating has a glass-like appearance and is
brown in colour.
Ceramic coating has several advantages over the resin-based coatings such as
 It is relatively insensitive to minor changes in temperature,
 It can be employed at higher temperatures up to 700°F,
 It is not influenced by the presence of oil or water which may be present in the
test environment,
 Coating can be used at cryogenic temperatures provided the coating is slowly
cooled from room temperature to test temperature.
Ceramic coatings suffer from a number of disadvantages such as
 High firing temperatures of 1100°F is difficult to obtain,
 Aluminium, magnesium, plastics, and highly heat-treated steels cannot be used
as components for testing,

17. 17
 Firing temperature has to be maintained properly as overheating by 25°F will
produce bubbles in coating and under heating by 25°F may produce partially
cured coatings,
 Cracks are so fine that visual observation of crack patterns is not possible.
There are various grades of All-temp, marketed by Magnaflux Corporation. The
coatings are designed to match the thermal coefficient of expansion of the metal used
in fabricating the components.
(a) Resin-based brittle coating
There are three resin-based coatings, i.e. stress coat, strain tec, and tens-lac. After the
resin-based coating is sprayed on the specimen, the coating is cured at room
temperature or at elevated temperature so that solvent is diffused out of the coating.
These resin-based coatings are highly sensitive to change in atmospheric conditions,
because the coefficient of thermal expansion of a resin-based coating is an order of
magnitude greater than the coefficients of thermal expansion of metals in common use
for engineering components. Relative humidity equally affects the behaviour of these
coatings. To account for predictable conditions of temperature and humidity, resin-
based coatings are available in different grades and a particular grade can be selected
for a particular application for specified value of threshold strain under given
conditions of temperature and relative humidity. Threshold strain is the minimum
value of uniaxial strain required to crack a coating. Selection charts are available for
different grades of these coatings.
Variation in temperature during the testing period causes changes in the value of
threshold strain for a resin-based coating. Figure. 12 shows that variation of threshold
strain with the increase in the testing temperature. With the increase in the testing
temperature, sensitivity of the coating decreases and the threshold strain required to
crack the coating is increased. As the temperature increases, residual compressive
stress is developed in coating because the coefficient of thermal expansion of coating
is much more than the coefficient of thermal expansion of specimen and so more
tensile stress is required to crack the coating. For consistent good results, it is
necessary that brittle coating laboratory must have at least a temperature-controlled
atmosphere. When the tests are performed outdoor, i.e. in the field, it is advisable that
weather bureau should be consulted to find when the temperature will be most stable

18. during the day. The test should be conducted in the minimum possible time over
which the temperature change is minimum. Moreover, there should b
calibration of the coating for the threshold strain so as to develop a relationship
between threshold strain, time, and temperature.
Coatings are sprayed over the specimen with the help of spray guns. For resin
coatings where carbon disulphide is the solvent, the recommended coating thickness
ranges from 0.15 to 0.20 mm and coating where methylene chloride is the solvent, the
recommended coating thickness ranges from 0.05 to 0.10 mm. These coatings require
24 hours for curing at room te
Figure .12
Variation of the thickness of the coating from point to point over the surface of the
specimen being analyzed can be a serious source of error. Solvent in the coating is
removed by a diffusion process; therefore thicker coatings take more time to dry than
thinner coatings. Thicker coatings may not be completely dry. So, the coating exhibits
variable sensitivity from point to point. To eliminate such errors, due to partial curing
of the coating, coating along with sample is heated in air circulating over at
28°C for 12 to 16 hours before testing. These heat
and the error caused due to the variation in strain sensitivity on account of variation in
concentration of residual solvent in the coating is eliminated. The resin
should never be overheated in the oven, as the overheated coatings start absorbing
moisture from the atmosphere when these coatings are taken out from the oven. The
18
during the day. The test should be conducted in the minimum possible time over
which the temperature change is minimum. Moreover, there should b
calibration of the coating for the threshold strain so as to develop a relationship
between threshold strain, time, and temperature.
Coatings are sprayed over the specimen with the help of spray guns. For resin
sulphide is the solvent, the recommended coating thickness
ranges from 0.15 to 0.20 mm and coating where methylene chloride is the solvent, the
recommended coating thickness ranges from 0.05 to 0.10 mm. These coatings require
24 hours for curing at room temperature.
Figure .12 Strain vs testing temperature curve
Variation of the thickness of the coating from point to point over the surface of the
specimen being analyzed can be a serious source of error. Solvent in the coating is
removed by a diffusion process; therefore thicker coatings take more time to dry than
thinner coatings. Thicker coatings may not be completely dry. So, the coating exhibits
variable sensitivity from point to point. To eliminate such errors, due to partial curing
of the coating, coating along with sample is heated in air circulating over at
28°C for 12 to 16 hours before testing. These heat-cured coatings are completely dry
and the error caused due to the variation in strain sensitivity on account of variation in
concentration of residual solvent in the coating is eliminated. The resin
should never be overheated in the oven, as the overheated coatings start absorbing
moisture from the atmosphere when these coatings are taken out from the oven. The
during the day. The test should be conducted in the minimum possible time over
which the temperature change is minimum. Moreover, there should be a continuous
calibration of the coating for the threshold strain so as to develop a relationship
Coatings are sprayed over the specimen with the help of spray guns. For resin-based
sulphide is the solvent, the recommended coating thickness
ranges from 0.15 to 0.20 mm and coating where methylene chloride is the solvent, the
recommended coating thickness ranges from 0.05 to 0.10 mm. These coatings require
Variation of the thickness of the coating from point to point over the surface of the
specimen being analyzed can be a serious source of error. Solvent in the coating is
removed by a diffusion process; therefore thicker coatings take more time to dry than
thinner coatings. Thicker coatings may not be completely dry. So, the coating exhibits
variable sensitivity from point to point. To eliminate such errors, due to partial curing
of the coating, coating along with sample is heated in air circulating over at about
cured coatings are completely dry
and the error caused due to the variation in strain sensitivity on account of variation in
concentration of residual solvent in the coating is eliminated. The resin-based coatings
should never be overheated in the oven, as the overheated coatings start absorbing
moisture from the atmosphere when these coatings are taken out from the oven. The

19. chances of moisture absorption from air in thinner coatings are more than in
coatings. Therefore, thinner coatings become less sensitive than thicker coatings.
Figure.13 shows the variation of strain sensitivity of the coating with changes in
curing temperatures for different coating thicknesses. It is obvious that
 Room temperature curing produces a coating with threshold strain which
increases with increasing thickness,
 High temperature curing produces a coating with a threshold strain that
decreases with increasing thickness,
 Curing at moderate temperature, i.e. 28°C, produces a coating which shows
independence of threshold strain on coating thickness.
 No doubt these resin
viscoelectric material, therefore mechanical properties of a coating vary as a
function of time.
Figure.13 Threshold strain vs coating thickness curve
During calibration of a coating, a cantilever beam (with
deflected in about 1 second, load is maintained for 15 seconds then deflection
removed (or beam unloaded) in 1 second. The position of the crack nearest to the free
end of the beam is noted for threshold strain. The load time his
is shown in Figure.14. The threshold strain established in this manner is dependent on
the load – time relation. When the load is applied slowly to a specimen, the coating
exhibits viscoelastic effects and the stresses developed in the coating relax to some
degree depending upon the time of the load applied. The overall effect of this stress
relaxation in the coating is to increase the value of the specimen strain required to
19
chances of moisture absorption from air in thinner coatings are more than in
coatings. Therefore, thinner coatings become less sensitive than thicker coatings.
variation of strain sensitivity of the coating with changes in
curing temperatures for different coating thicknesses. It is obvious that
Room temperature curing produces a coating with threshold strain which
increases with increasing thickness,
erature curing produces a coating with a threshold strain that
decreases with increasing thickness,
Curing at moderate temperature, i.e. 28°C, produces a coating which shows
independence of threshold strain on coating thickness.
No doubt these resin-based coatings are brittle in nature, but resin is a
viscoelectric material, therefore mechanical properties of a coating vary as a
Threshold strain vs coating thickness curve
During calibration of a coating, a cantilever beam (with coating sprayed on it) is
deflected in about 1 second, load is maintained for 15 seconds then deflection
removed (or beam unloaded) in 1 second. The position of the crack nearest to the free
end of the beam is noted for threshold strain. The load time history during calibration
. The threshold strain established in this manner is dependent on
time relation. When the load is applied slowly to a specimen, the coating
exhibits viscoelastic effects and the stresses developed in the coating relax to some
degree depending upon the time of the load applied. The overall effect of this stress
relaxation in the coating is to increase the value of the specimen strain required to
chances of moisture absorption from air in thinner coatings are more than in thicker
coatings. Therefore, thinner coatings become less sensitive than thicker coatings.
variation of strain sensitivity of the coating with changes in
curing temperatures for different coating thicknesses. It is obvious that
Room temperature curing produces a coating with threshold strain which
erature curing produces a coating with a threshold strain that
Curing at moderate temperature, i.e. 28°C, produces a coating which shows
coatings are brittle in nature, but resin is a
viscoelectric material, therefore mechanical properties of a coating vary as a
Threshold strain vs coating thickness curve
coating sprayed on it) is
deflected in about 1 second, load is maintained for 15 seconds then deflection
removed (or beam unloaded) in 1 second. The position of the crack nearest to the free
tory during calibration
. The threshold strain established in this manner is dependent on
time relation. When the load is applied slowly to a specimen, the coating
exhibits viscoelastic effects and the stresses developed in the coating relax to some
degree depending upon the time of the load applied. The overall effect of this stress
relaxation in the coating is to increase the value of the specimen strain required to

20. crack the coating. The manufacturers of the coatings provide correction charts for
time of loading during test.
Figure.14
8. EQUIPMENT FOR BRITTL
The equipment necessary for stress analysis through brittle coating is listed below:
 A wide range of brittle coatings.
 An aluminium under coat paint.
 Red dye etchant.
 Etchant emulsifier.
 Two spray guns—one for aluminium
 A small portable air compressor.
 Respirator for operator.
 Focussed light for visual inspection.
 About a dozen calibrating beams.
 A beam bending device.
 A strain scale with beam bending device.
 Temperature and humidity m
 A storage cabinet.
9. PREPARATION OF SP
The surface of the specimen is lightly rubbed with sand paper, degreased with
gasoline, acetone, and finally with carbon disulphide (if coating contains carbon
disulphide as solvent) or me
chloride as solvent. Then aluminium under coat is sprayed over the surface to provide
uniform reflecting background, which increases the visibility of cracks. The surface is
then ready to be sprayed with t
20
crack the coating. The manufacturers of the coatings provide correction charts for
time of loading during test.
Figure.14 Load time history during calibration
EQUIPMENT FOR BRITTLE COATING METHOD
The equipment necessary for stress analysis through brittle coating is listed below:
A wide range of brittle coatings.
An aluminium under coat paint.
Etchant emulsifier.
one for aluminium under coat and another for coating.
A small portable air compressor.
Respirator for operator.
Focussed light for visual inspection.
About a dozen calibrating beams.
A beam bending device.
A strain scale with beam bending device.
Temperature and humidity measuring instruments.
9. PREPARATION OF SPECIMEN
The surface of the specimen is lightly rubbed with sand paper, degreased with
gasoline, acetone, and finally with carbon disulphide (if coating contains carbon
disulphide as solvent) or methylene chloride if the coating contains methylene
chloride as solvent. Then aluminium under coat is sprayed over the surface to provide
uniform reflecting background, which increases the visibility of cracks. The surface is
then ready to be sprayed with the proper grade of brittle coating.
crack the coating. The manufacturers of the coatings provide correction charts for
The equipment necessary for stress analysis through brittle coating is listed below:
under coat and another for coating.
The surface of the specimen is lightly rubbed with sand paper, degreased with
gasoline, acetone, and finally with carbon disulphide (if coating contains carbon
thylene chloride if the coating contains methylene
chloride as solvent. Then aluminium under coat is sprayed over the surface to provide
uniform reflecting background, which increases the visibility of cracks. The surface is

21. 10. TESTING PROCEDUR
The specimen with brittle coating is loaded, then load is maintained for 15 seconds on
the specimen and then load is released. After unloading the entire surface of the
coating is examined for coati
and the entire process is repeated. The crack patterns located after each loading cycle
are encircled with line (isoentatic line) and marked with a number corresponding to
load on the specimen which prod
of points of approximately constant principal stress.
Figure.15
If the test is performed on a cylindrical pressure vessel then first family of cracks is
perpendicular to σc(hoop stress) and second family of cracks are perpendicular
to σa (axial stress).
11. CALIBRATION OF B
In order to know the stress or strain associated with each isoentatic line (
is necessary to calibrate the coating every time when the isoentatic line is plotted. A
cantilever beam of rectangular section is fitted in a fixture or beam bending device as
shown in Figure.16. The cantilever beam has been sprayed with the same grade of
coating as on the specimen and has been subjected to the same curing cycle as the
coating on the specimen. The beam is defle
arrangement as shown and it is kept deflected for 15 seconds and then cam is
reversed. The cracks on the coating are inspected so as to locate the last crack which
separates the cracked and uncracked regions of the coating. T
21
10. TESTING PROCEDURE
The specimen with brittle coating is loaded, then load is maintained for 15 seconds on
the specimen and then load is released. After unloading the entire surface of the
coating is examined for coating cracks. Now the load on the specimen is increased
and the entire process is repeated. The crack patterns located after each loading cycle
are encircled with line (isoentatic line) and marked with a number corresponding to
load on the specimen which produced the strain (see Figure.15). Isoentatic line is loci
of points of approximately constant principal stress.
Figure.15 Isoentatic lines on coating
If the test is performed on a cylindrical pressure vessel then first family of cracks is
(hoop stress) and second family of cracks are perpendicular
11. CALIBRATION OF BRITTLE COATING
In order to know the stress or strain associated with each isoentatic line (
to calibrate the coating every time when the isoentatic line is plotted. A
cantilever beam of rectangular section is fitted in a fixture or beam bending device as
. The cantilever beam has been sprayed with the same grade of
coating as on the specimen and has been subjected to the same curing cycle as the
coating on the specimen. The beam is deflected through a cam and handles
arrangement as shown and it is kept deflected for 15 seconds and then cam is
reversed. The cracks on the coating are inspected so as to locate the last crack which
separates the cracked and uncracked regions of the coating. The strain corresponding
The specimen with brittle coating is loaded, then load is maintained for 15 seconds on
the specimen and then load is released. After unloading the entire surface of the
ng cracks. Now the load on the specimen is increased
and the entire process is repeated. The crack patterns located after each loading cycle
are encircled with line (isoentatic line) and marked with a number corresponding to
). Isoentatic line is loci
If the test is performed on a cylindrical pressure vessel then first family of cracks is
(hoop stress) and second family of cracks are perpendicular
In order to know the stress or strain associated with each isoentatic line (Figure.15) it
to calibrate the coating every time when the isoentatic line is plotted. A
cantilever beam of rectangular section is fitted in a fixture or beam bending device as
. The cantilever beam has been sprayed with the same grade of
coating as on the specimen and has been subjected to the same curing cycle as the
cted through a cam and handles
arrangement as shown and it is kept deflected for 15 seconds and then cam is
reversed. The cracks on the coating are inspected so as to locate the last crack which
he strain corresponding

22. to the last crack on a strain scale of 0 to 2000 microstrain gives the threshold
strain of the coating.
COMMON QUESTIONS
1 Explain how uniaxial
coating.
2 What are the merits and demerits of brittle coating technique over strain gage
technique?
3 Derive the expression for failure theory of the case
4 Describe how crack patterns are developed in co
specimen gradually increases.
5 Describe briefly the load relaxation and refrigeration techniques for getting
cracks in coating
6 Explain the following
a. Statiflux method
b. Red eye etchant method of crack detection
7 Compare the properties of
8 Explain the procedure for calibration of brittle coating.
22
to the last crack on a strain scale of 0 to 2000 microstrain gives the threshold
Figure.16 Strain scale
COMMON QUESTIONS
Explain how uniaxial stress system in specimen develops biaxial stresses in
What are the merits and demerits of brittle coating technique over strain gage
Derive the expression for failure theory of the case
Describe how crack patterns are developed in coating where the load on
specimen gradually increases.
Describe briefly the load relaxation and refrigeration techniques for getting
Explain the following
Statiflux method
Red eye etchant method of crack detection
Compare the properties of All Temp with the properties of Stress coat.
Explain the procedure for calibration of brittle coating.
to the last crack on a strain scale of 0 to 2000 microstrain gives the threshold
stress system in specimen develops biaxial stresses in
What are the merits and demerits of brittle coating technique over strain gage
ating where the load on
Describe briefly the load relaxation and refrigeration techniques for getting
All Temp with the properties of Stress coat.