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Handbook for Damage Tolerant Design

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Section 5.1.5. Stress-Corrosion Cracking and Stress Intensity

Many engineering materials exhibit some cracking behavior under sustained loading in the presence of an environment (thermal and/or chemical).  The type of cracking behavior for many chemical environments is referred to as stress-corrosion cracking behavior.  The mechanism for this attack process has been attributed to the chemical reactions that take place at the crack tip and to diffusion of reactive species (particularly hydrogen) into the high stressed region ahead of the crack.  The cracking process has been noted to be a function of time and it is highly dependent on the environment, the material, and the applied stress (or stress-intensity factor) level.

For a given material-environment interaction, the stress-corrosion-cracking rate has been noted to be governed by the stress-intensity factor.  Similar specimens with the same size of initial crack but loaded at different levels (different initial K values) show different times to failure [Brown, 1968; Sullivan, 1972; Chu, 1972], as shown in Figure 5.1.13.  A specimen initially loaded to KIc fails immediately.  The level below which cracks are not observed to grow is the threshold level that is denoted as KIscc.

Figure 5.1.13.  Stress Corrosion Cracking Data [Brown, 1968]

If the load is kept constant during the stress-corrosion-cracking process, the stress-intensity factor will gradually increase due to the growing crack.  As a result, the crack-growth rate per unit of time (da/dt) increases according to


When the crack has grown to a size so that K becomes equal to KIc, the specimen fails.  This is shown schematically in Figure 5.1.14.  In typical tests, specimens may be loaded to various initial K’s such as K1, K2, and K3.  The time to failure is recorded giving rise to the typical data point (t1, K1).  During the test, K will increase, as a result of crack extension, from its initial value to KIc, when final failure occurs.  The times t2 and t3 represent the time to failure for higher K’s such as K2 and K3.


Figure 5.1.14.  Stress Corrosion Cracking

The stress-corrosion threshold and the rate of growth depend on the material and the environmental conditions.  Data on KIscc and da/dt can be found in the Damage Tolerant Design (Data) Handbook [1994] .  Typical examples of KIscc and da/dt data presentation formats are shown in Figures 5.1.15 and 5.1.16.












Figure 5.1.15.  KIscc Data as Presented by the Damage Tolerant Design (Data) Handbook [1994]











































Figure 5.1.16.  Stress Corrosion Cracking Rate Data for 2024-T351 Aluminum as Presented by the Damage Tolerant Design (Data) Handbook [1994]

As illustrated in Figure 5.1.17, a component with a given crack fails at a stress given by


It will exhibit stress-corrosion-crack growth when loaded to stresses in excess of



Figure 5.1.17.  Stress Required for Stress Corrosion Cracking

In service, stress-corrosion cracks have been found to be predominantly a result of residual stresses and secondary stresses.  Stress-corrosion failure due to primary loading seldom occur because most stress-corrosion cracks favor the short transverse direction (S-L), which is usually not the primary load direction.  In many materials, the long transverse (T-L) and longitudinal (L-T) directions are not very susceptible to stress corrosion.

Prevention of stress corrosion cracking is preferred as a design policy over controlling it as is done for fatigue cracking.  This means that stress-corrosion critical components must be designed to operate at a stress level lower than


in which ai is the initial flaw size as specified in the Damage Tolerance Requirements of JSSG-2006.  However, if stress corrosion can occur, it must be accounted for in damage tolerance analyses by using an integral form of Equation 5.1.7.

Stress-corrosion cracking may occur in fatigue-critical components.  This means that in addition to growth by fatigue, cracks might show some growth due to stress corrosion.  In dealing with this problem, the following should be considered:

·        Stress-corrosion cracking is a phenomenon that basically occurs under a steady stress.  Hence, the in-flight stationary stress level (l g) is the governing factor.  Most fatigue cycles are of relatively short duration and do not contribute to stress-corrosion cracking.  Moreover, the cyclic crack growth would be properly treated already on the basis of data for environment-assisted fatigue-crack growth.  When stress corrosion cracking is expected, the stress corrosion cracking rate should be superimposed on the fatigue crack growth rate [Wei & Candes, 1969; Gallagher & Wei, 1972; Dill & Saff, 1978; Saff, 1980].

·        Stress-corrosion cracking is generally confined to forgings, heavy extrusions, and other heavy sections, made of susceptible materials.  Thus, the problem is generally limited to cases where plane strain prevails.

·        The maximum crack size to be expected in service is , where s  equals sLT or sDM, depending upon the inspectability level (see Section 1.3).

If stress-corrosion cracking is not expected at any crack size, the l-g stress, s1g, should be lower than .  With ac given as above, it follows that complete prevention of stress corrosion extension of a fatigue crack requires selection of a material for which: