Stress corrosion or environmentally-assisted cracking data which support standard damage integration schemes, as well as materials evaluation and selection studies, are based on either constant load or constant displacement type tests of fatigue cracked specimens placed in simulated service environments.  There are two types of stress corrosion cracking data properties measured by such tests: 

1)      the threshold property (K_IEAC), which is the level of the stress-intensity factor associated with no cracking in the given environment, and

2)      the crack growth rate resistance property (da/dt as a function of the static stress-intensity factor K).

ASTM E1681 covers determination of stress corrosion threshold.  Figure 7.2.1 describes the three types of test specimen configurations utilized in the ASTM standard:

·        a bolt-loaded, compact [MC(W)] specimen,

·        a constant load single-edge specimen [SE(B)], and

·        a compact tension specimen [C(T)]. 

As can be noted from the figure, the bolt-loaded MC(W) specimen is a self-loading specimen.  The force loaded SE(B) and C(T) specimens must be placed in a test figure that supports the specimen while under load, which is typically applied using weights attached to one end of the specimen.  (Note that ASTM E1681 does not describe da/dt testing, but does mention da/dt information may be obtained on such tests.)

As with other sub-critical crack growth resistance tests, the materials test engineer must pay particular attention to the pre-cracking, loading, and crack size measurement details.  In addition, because the environment has a more important influence on the crack growth resistance of many materials, specific controls must be instituted here also.

Crack growth tests conducted in aqueous or similar deleterious environments lead to difficult crack length measurement problems since typically the direct use of visual techniques is restricted to conditions whereby the specimen is removed from the environment.  Use of visual techniques under these conditions is acceptable if it can be shown that removing the specimen from the environment introduces no major crack growth transient effects.  Collecting crack length data using electric potential difference (EPD) methodology and the relationships between crack size and potential voltage difference has gained credibility in recent years as a means of automating the measurement of crack size in both SE(B) and C(T) specimens.  Since stress-corrosion cracking tests are conducted over longer periods of time (~ 10,000 hours) than other mechanical tests, stability of the crack size measurement system must be given a great deal of attention.

Differences of opinion exist between the experts relative to the use of either the increasing (constant load) or decreasing (constant displacement) stress-intensity factor (K) type specimens for collecting threshold stress corrosion cracking data.  These differences result from the influences of test conditions and of crack growth transients.  Since the objective of the K_IEAC test is to obtain a threshold level of K associated with a preset growth rate limit, a series of tests should be conducted which would minimize these effects.

The K_IEAC results obtained using constant load specimens are influenced somewhat by the fact that the test time includes both the time associated with initiating the crack movement from the sharp precrack and that associated with subsequent propagation.  For K_IEAC data collection programs using increasing K specimens, a number of tests should be conducted such that the precracked specimens are loaded above and below the level of the expected stress-intensity factor condition associated with zero crack movement.  Subsequently, each unbroken specimen should be broken open and examined for evidence of crack movement during the test period.  In all cases, the K_IEAC value is lower than the lowest value of the stress-intensity factor associated with the broken specimens.  If no stress-corrosion cracking movement is observed when the unbroken specimens are examined, the K_IEAC is taken as the highest stress-intensity factor level associated with the unbroken specimen group.  When stress-corrosion cracking movement is observed in the unbroken specimen group, the amount of crack movement should be divided by the test time in order to ascertain if the average growth rate associated with any test is below that required to obtain the K_IEAC value.  The highest level of stress-intensity factor that yields an average growth rate below that required is taken as the K_IEAC value.

The K_IEAC results obtained using the bolt-loaded (K-decreasing type) specimen can be influenced by crack growth transients that occur after loading.  (For additional information see the discussion in ASTM E1681 on stress relaxation influences in Section 5.1.7)  For K_IEAC data collection programs using decreasing K specimens, a number of tests should be conducted such that the precracked specimens are loaded to levels that are slightly above (10 to 25 percent) the level of expected K_IEAC.  High initial stress-intensity factor levels (relative to K_IEAC) result in a number of problems in determining K_IEAC accurately.  These problems sometimes result from the fact that once the precrack starts to move it has a longer distance to travel before arresting as a result of the high initial K condition and the slowly decaying K gradient associated with the bolt-loaded conditions.  Another problem associated with high initial K conditions is that cracks will sometimes initiate and arrest prematurely due to crack blunting (under first loading) and crack front tunneling.  In the decreasing K specimen, as soon as crack movement occurs from the precrack, the crack front loses the sharpness of a fatigue crack; this sometimes results in a value of K_IEAC that is somewhat above that measured in the increasing K specimen.

Some of the problems in estimating K_IEAC using either constant-load (increasing K) and bolt-loaded (decreasing K) specimens are alleviated when crack growth measurements are continuously made throughout the test.  Specifically, measurement of the first crack movement that occurs in constant-load specimens provide a better time basis for estimating the crack growth rate from unbroken specimens.  Even periodic measurement of the crack length in the bolt-loaded C(T) specimens will increase the test engineer’s confidence that transient or abnormal crack growth behavior has not occurred during the test. Crack growth rate data used for sensing a material’s resistance to environmental attack is collected and reduced in a manner similar to fatigue crack growth rate data.  The principal difference in an environmental attack testing program is that the loads or displacements are held constant during the test.  K_IEAC is used primarily for ranking materials for sub-critical crack growth resistance in environments.  Because fatigue testing is conducted extensively in similar environments during the design of airframe structures, a high level of interest continues in combining the time dependent rate information with the cyclic dependent data into a common predictive model.  It is therefore suggested that when such tests are necessary to support damage integration packages, that stress-corrosion cracking rate tests follow the basic guidelines of the fatigue crack growth rate tests in ASTM E647.