Significance and Use
The parameters KEAC or KIEAC 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 KEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC or KIEAC 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 KEAC value, while in other material/environment combinations the measured KIEAC 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 KIEAC value.
The user may optionally determine and report a KEAC value or a KIEAC value. The specimen size validity requirements for a KEAC value meet the size requirements developed for Test Method E 647
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 KEAC 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 KISCC 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 KEAC 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 KEAC or KIEAC. This method can serve the following purposes:
In research and development, valid KEAC or KIEAC data can quantitatively establish the effects of metallurgical and environmental variables on the environment-assisted cracking resistance of materials.
In service evaluation, valid KEAC or KIEAC 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 KEAC or KIEAC 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.
1.1 This test method covers the determination of the environment-assisted cracking threshold stress intensity factor parameters, KIEAC and KEAC, for metallic materials from constant-force testing of fatigue precracked beam or compact fracture specimens and from constant-displacement testing of fatigue precracked bolt-load compact fracture specimens.
1.2 This test method is applicable to environment-assisted cracking in aqueous or other aggressive environments.
1.3 Materials that can be tested by this test method are not limited by thickness or by strength as long as specimens are of sufficient thickness and planar size to meet the size requirements of this test method.
1.4 A range of specimen sizes with proportional planar dimensions is provided, but size may be variable and adjusted for yield strength and applied force. Specimen thickness is a variable independent of planar size.
1.5 Specimen configurations other than those contained in this test method may be used, provided that well-established stress intensity calibrations are available and that specimen dimensions are of sufficient size to meet the size requirements of this test method during testing.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.