ASTM E1681-03(2008)

The parameters K_EAC or K_IEAC determined by this test method characterize the resistance to crack growth of a material with a sharp crack in specific environments under loading conditions in which the crack-tip plastic region is small compared with the crack depth and the uncracked ligament. The less restrictive thickness requirements of K_EAC are intended for those conditions in which the results are a strong function of the thickness of the specimen and the application requires the testing of specimens with thickness representative of the application. Since the chemical and mechanical influences cannot be separated, in some material/environment combinations, the thickness must be treated as a variable. A K_EAC or K_IEAC value is believed to represent a characteristic measurement of environment-assisted cracking resistance in a precracked specimen exposed to an environment under sustained tensile loading. A K_EAC or K_IEAC value may be used to estimate the relationship between failure stress and defect size for a material under any service condition, where the combination of crack-like defects, sustained tensile loading and the same specific environment would be expected to occur. (Background information concerning the development of this test method can be found in Refs (3-18).

The apparent K_EAC or K_IEAC of a material under a given set of chemical and electrochemical environmental conditions is a function of the test duration. It is difficult to furnish a rigorous and scientific proof for the existence of a threshold (4, 5). Therefore, application of K_EAC or K_IEAC data in the design of service components should be made with awareness of the uncertainty inherent in the concept of a true threshold for environment-assisted cracking in metallic materials (6, 18). A measured K_EAC or K_IEAC value for a particular combination of material and environment may, in fact, represent an acceptably low rate of crack growth rather than an absolute upper limit for crack stability. Care should be exercised when service times are substantially longer than test times.

The degree to which force deviations from static tensile stress will influence the apparent K_EAC or K_IEAC of a material is largely unknown. Small-amplitude cyclic loading, well below that needed to produce fatigue crack growth, superimposed on sustained tensile loading was observed to significantly lower the apparent threshold for stress corrosion cracking in certain instances (7, 8). Therefore, caution should be used in applying K_EAC or K_IEAC data to service situations involving cyclic loading. In addition, since this standard is for static loading, small-amplitude cyclic loading should be avoided during testing.

In some material/environment combinations, the smaller the specimen, the lower the measured K_EAC value, while in other material/environment combinations the measured K_IEAC value will be the lowest value (5, 9, 10, 11, 12). If, for the material/environment combination of interest, it is not known which specimen size will result in the lower measured value, then it is suggested that the use of both specimen sizes should be considered; that is, specimens with thicknesses representative of the application and specimens in which the thickness meets the requirements (see 7.2.1) of a K_IEAC value.

The user may optionally determine and report a K_EAC value or a K_IEAC value. The specimen size validity requirements for a K_EAC value meet the size requirements developed for Test Method E 647 to achieve predominately elastic behavior in the specimen. Test Method E 647 size requirements for compact specimens should be applied to both the compact specimen and the beam specimen. The specimen size validity requirements for a K_IEAC value meet the size requirements developed for plane strain conditions for Test Method E 399.

Evidence of environment-assisted crack growth under conditions that do not meet the validity requirements of 7.2 may provide an important indication of susceptibility to environmental cracking but cannot be used to determine a valid K_EAC value (14).

Environment-assisted cracking is influenced by both mechanical and electrochemical driving forces. The latter can vary with crack depth, opening, or shape and may not be uniquely described by the fracture mechanics stress intensity factor. As an illustrative example, note the strong decrease reported in K_ISCC with decreasing crack size below 5 mm for steels in 3 % NaCl in water solution (15). Geometry effects on K similitude should be experimentally assessed for specific material/environment systems. Application modeling based on K_EAC similitude should be conducted with caution when substantial differences in crack and specimen geometry exist between the specimen and the component.

Not all combinations of material and environment will result in environment-assisted cracking. In general, susceptibility to aqueous stress-corrosion cracking decreases with decreasing material strength level. When a material in a certain environment is not susceptible to environment-assisted cracking, it will not be possible to measure K_EAC or K_IEAC. This method can serve the following purposes:

In research and development, valid K_EAC or K_IEAC data can quantitatively establish the effects of metallurgical and environmental variables on the environment-assisted cracking resistance of materials.

In service evaluation, valid K_EAC or K_IEAC data can be utilized to establish the suitability of a material for an application with specific stress, flaw size, and environmental conditions.

In acceptance and quality control specifications, valid K_EAC or K_IEAC data can be used to establish criteria for material processing and component inspection.

Test results will be affected by force relaxation in constant displacement bolt-loaded compact specimens for some material/environment conditions. For relatively low strength material, non-agressive environments, or high test temperatures, force relaxation can occur independently from environment-assisted cracking. Significant force relaxation would make cracking results difficult to interpret. If force relaxation is suspected of influencing the data, the following trial specimen test is recommended. Test a trial specimen with all the test conditions of interest, except with no environment applied. Monitor the force on the sample using a bolt with an electronic load cell attached. Instrumented bolts of this type are commercially available. A force relaxation of more than 5 % after 24 h indicates that the constant displacement test method may not be suitable for these test conditions, and a constant force test should be considered.

Residual stresses can have an influence on environment-assisted cracking. The effect can be significant when test specimens are removed from material in which complete stress relief is impractical, such as weldments, as-heat-treated materials, complex wrought parts, and parts with intentionally produced residual stresses. Residual stresses superimposed on the applied stress can cause the local crack-tip stress-intensity factor to be different from that calculated from externally applied forces or displacements. Irregular crack growth during precracking, such as excessive crack front curvature or out-of-plane crack growth, often indicates that residual stresses will affect the subsequent environment-assisted crack growth behavior. Changes in the zero-force value of crack-mouth-opening displacement as a result of precrack growth is another indication that residual stresses will affect the subsequent environment-assisted crack growth.

For bolt loaded specimens, the user should realize that material being tested at an non-ambient temperature may have a different displacement-to-force ratio from that at ambient temperature, and also the bolt material may have a different coefficient of thermal expansion from that of the material being tested. Care should be taken to minimize these effects.