FIELD OF INVENTION
The present invention generally relates to a stress corrosion analysis of
engineering materials. More particularly, the invention relates to a method to
evaluate and apply a predetermined load on a tensile specimen including on the
base material in a process of stress corrosion analysis.
BACKGROUND OF THE INVENTION
Stainless steels are widely employed in boiler components wherever high
temperature and corrosion are expected. Stainless steels play an important role
as a corrosion resistant and an oxidation resistant material. Austenitic stainless
steels have high ductility, low yield stress and relatively high ultimate tensile
strength, when compared to typical carbon steel. The high chromium and nickel
content in austenitic steel suppress the phase transformation from austenite to a
mixture of ferrite and cementite, thus keeping the stainless material fully
austenite on cooling. The stresses required to cause SCC are small, usually below
macroscopic yield stress, and are tensile in nature.
The stresses can be externally applied, but residual stresses often cause stress
corrosion cracking (SCC) failures. However, compressive residual stresses can be
used to prevent this phenomenon. Static loading usually is considered to be
responsible for SCC, while environmentally induced crack propagation due to
cyclic loading is defined as corrosion fatigue.
The known testing method for predicting the stress corrosion performance of an
alloy in a particular service application is not conducted under a stress system
corresponding to that anticipated in service condition. Most of the known SCC

failure in service appear to cause from tensile stresses of unknown and high
magnitude that are usually generated. Accordingly, the testing methodology that
incorporate a high total strain are usually the most realistic in terms of
duplicating the service conditions.
Further, the test results are strongly influenced by the mechanical aspects of the
methodology, such as the method of loading including that specimen size. These
mechanical aspects produces variable effects on the initiation and propagation
life times of the boiler components and influence determined threshold stress
value. Therefore, the threshold stress value for SCC according to prior art does
not reflect the true material property and thus, any estimate of threshold value
must be qualified with regard to the test condition and the significance level.
United States Patent 4002510 discloses an austenitic stainless steel which is
virtually immune to stress-corrosion, cracking and intergranular stress -
corrosion. The disclosed steel material has higher resistance to pitting and
intergranular attack in a weld including the heat effect zone, and exhibits
improved hot and cold workability, weldability, and strength characteristics.
United States Patent 5571955 describes a method for determining the corrosive
effects of a metal in a chemical environment created in a vessel having an
environmental pressure. A probe is exposed to the chemical environment and is
made of a material substantially similar to that of the vessel. The probe has a
sealed internal chamber which contains a pressurized fluid which is monitored. A
pressure transducer is in communication with the sealed internal chamber of the
probe for translating any pressure change within the chamber into a (preferably
electrical signal which is indicated by an electrical meter or similar device. Stress

corrosion cracking of the probe causes fluid leakage and, therefore, a pressure
change.
United States Patent 4054447 describes an economical steel which is capable of
preventing intergranular stress corrosion cracking that occurs when the steel is
placed in contact with nitrogen oxides at temperatures below their dew point.
Accordingly, the prior art methods including Constant-strain (fixed displacement)
tests exhibit a poor reproducibility of the exposure stress.
OBJECTS OF THE INVENTION
Therefore sophisticated methodology of applying known stress is developed in
this invention.
Very accurate and reproducible on base as well as weld specimen could be used.
Therefore sophisticated methodology of applying known stress is developed in
this invention.
Very accurate and reproducible on base as well as weld specimen could be used.
Here are the major objectives :
It is therefore an object of the invention is to propose a method to apply a
known stress during estimation of stress corrosion performance of an alloy
corresponding to that of service conditions.

Another object of the invention is to propose a method to apply a known stress
during estimation of stress corrosion performance of an alloy corresponding to
that of service conditions, which adapts a strain gauge as sensor for measuring
the stress.
A still another object of the invention to propose a method to apply a known
stress during estimation of stress corrosion performance of an alloy
corresponding to that of service conditions, which is enabled to numerically
compute the stress using the measured strain.
Yet another object of the invention is to propose a method to apply a known
stress during estimation of stress corrosion performance of an alloy
corresponding to that of service conditions, which additionally provides the stress
corrosion property of the weld metal including that of the heat affected zone
(HAZ) of the weld material.
A further object of the invention is to propose a method to apply a known stress
during estimation of stress corrosion performance of an alloy corresponding to
that of service conditions, which provides the stress corrosion property of the
base metal of different compositions.

SUMMARY OF THE INVENTION
STRAIN GAUGE AND INSTALLATION PROCEDURE
Strain gauge is an electrical sensor, which converts mechanical strains into
electrical signals. It may be one of a unidirectional (single element), a bi-
directional (two element), and a rosette type (three element) resistance gauge
with individual element measuring either 120 ohms, 350 ohms or 1000 ohms
resistance and the elements are made of Constantan alloy.
For quantitative measurement of strains, a foil type of electrical resistance strain
gauges are used as the transducers. These strain gauges are highly sensitive
elements that accurately reproduce the strain experienced by the pressure part
and the resultant proportional resistance change is amplified by signal
conditioners and is directly recorded through multi channel strain meters.
The strain gauge with suitable gauge length and resistance is affixed using fast
curing adhesive at the location of interest and connected to a strain data
acquisition system (strain meter) to complete the Wheatstone bridge circuit
configurations. The strain gauge is capable of measuring strains of the order of
0.00005 mm/mm while the strain meter used is capable of recording strains with
an accuracy of one micro strain.
According to the invention, a single element strain gauge of gauge length of
6mm with 120 ohms resistance is used. Prior to affixing the strain on a reduced
tensile specimen, the area is marked and grinding is made on the marked area to
remove any scales, rust etc. The ground area is cleaned with acetone to remove

oil, grease etc. The strain gauge is then installed using a fast curing two
compound adhesive, in the quarter bridge mode and connected to the strain
meter to complete the Wheatsone bridge circuitry. After connecting, the residual
strains is brought to zero. The specimen is then loaded on to a honsfield
tensometer. A shakedown operation is performed by initially applying an
exemplary load on the specimen and brought to zero. The shakedown operation
is repeated at least twice to ensure proper functioning of the strain gauge with
no drift in the data acquisition system. After completion of the shake down
operation, a tensile load is applied to the specimen by adjusting the screw in the
tensometer and the strain value is recorded in the strain meter. Using the
mechanical properties of the material under test, the specimen is loaded upto
90% of its yield strain. The stress is computed according to the standard
relationship between stress and applied strain. This process allows an application
of a known stress of about 90% of its yield strength on the specimen and the
specimen is then immersed in the corrosive media for further evaluation.
Thus, the present invention provides a method to evaluate and apply a
predetermined load on a tensile specimen in a process of stress-corrosion
analysis of a component formed of the test specimen material. According to the
invention, a known stress is applied through strain gauge that is installed on the
tensile specimen. The tensile specimen is loaded with a tensile load. The stress
experienced by the tensile specimen is computed using the standard relationship
between stress and strain, which is Young's modulus. Now the specimen is under
known load.
In this condition the specimen is immersed in the corrosive media. Now the
specimen under known stress as well as under corrosive environment.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1 - Schematically shows a methodology for applying known stress on a
tensile specimen in the weld region for SCC evaluation according to
the invention.
FIGURE 2 - Schematically illustrates a methodology for applying known stress
on the base metal region of a second specimen formed of base
material according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figures 1 & 2, the inventive method comprises the steps of :-
(a) A tensile specimen (1) is taken and a strain gauge (2) is installed on the
tensile specimen in the weld region (3). The strain gauge (2) is connected
to a strain meter (4). By applying at least two exemplary loads (5, 6), the
strain is measured in the strain meter (Figure 1). Using the standard
relationship between the stress and strain, the stress is computed. The
specimen (1) is now under known stress. Now the specimen (1) is
immersed in a corrosive media so that the specimen (1) is under load in a
corrosion environment. The specimen (1) is kept in the corrosion
environment for a specific duration depending upon the material
composition and physical properties;

As shown in figure 2, the identical steps are followed for the base metal
region (7) of another test specimen (1) except that the strain gauge (2) is
fixed on the base metal region (7).
The specimen (1) prepared from base metal as well as the welded specimen (1)
are taken for stress corrosion cracking evaluation. Tensile specimens are
prepared from both weld metal and base metal as per the ASTM standard: Then
the specimens (1) are taken for application of the known stress. Tensile
specimens (1) from base metal and/or weld metal are taken and a strain gauge
(2) is installed on the tensile specimen (1) in the weld region (3) in the weld
metal specimen (1). In case of the specimen formed of base metal the strain
gauge is installed on the base metal region. The strain gauge (2) is connected to
a strain meter (4). By applying known loads (5,6) and the strain is measured in
the strain meter (4). Using the standard relation ship between stress and strain,
the stress is computed. The specimens (1) are now under known stress. Now the
specimen (1) is immersed in a corrosive media so that specimens were under
known load with corrosion environment. The specimens are periodically and
visually checked for known cracks. The specimens are kept in the corrosive
medium for a specific duration depending upon the material.

WE CLAIM :
1. A method for determination and application of a known stress on a
base material including a tensile specimen formed of the base material
in a process of stress corrosion analysis, the method comprising the
steps of:
- preparing at least two specimens from a single base metal the
second specimen being a welded specimen welded as per ASTM
standard, while the first specimen being a tensile sample piece;
- installing a first strain gauge on the tensile specimen in a first
phase and connecting the first strain gauge to a first strain meter;
- installing a second strain gauge on the welded region of the welded
specimen in a second phase and connecting the second strain
gauge to a second strain meter;
- applying at least two known loads separately on the tensile
specimen in the first phase, the corresponding strain values being
measured in the first strain gauge;
- applying said at least two known loads separately on the welded
specimen in the second stage, the corresponding strain values
being measured in the second strain gauge;

- providing a corrosive media in a vessel to produce a corrosive
environment and individually and separately immersing the two
preloaded specimens in the corrosive media;
- allowing the specimens to remain under said corrosive environment
for a specified period depending upon the chemical composition
including physical properties of the base material, and
- periodically and visually checking the specimens to identify
development or otherwise of a crack which enables optimization of
the applicable load in the stress corrosion analysis.
2. A method for determination and application of a known stress on a
base material including a tensile specimen formed of the base material
in a process of stress corrosion analysis as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to a method for determination and application of a known
stress on a base material including a tensile specimen formed of the base
material in a process of stress corrosion analysis, the method comprising the
steps of preparing at least two specimens from a single base metal the second
specimen being a welded specimen welded as per ASTM standard, while the first
specimen being a tensile sample piece; installing a first strain gauge on the
tensile specimen in a first phase and connecting the first strain gauge to a first
strain meter; installing a second strain gauge on the welded region of the welded
specimen in a second phase and connecting the second strain gauge to a second
strain meter; applying at least two known loads separately on the tensile
specimen in the first phase, the corresponding strain values being measured in
the first strain gauge; applying said at least two known loads separately on the
welded specimen in the second stage, the corresponding strain values being
measured in the second strain gauge; providing a corrosive media in a vessel to
produce a corrosive environment and individually and separately immersing the
two preloaded specimens in the corrosive media; allowing the specimens to
remain under said corrosive environment for a specified period depending upon
the chemical composition including physical properties of the base material, and
periodically and visually checking the specimens to identify development or
otherwise of a crack which enables optimization of the applicable load in the
stress corrosion analysis.