【国外标准】 Standard Practice for Making and Using Precracked Double Beam Stress Corrosion Specimens

5.1 Precracked specimens offer the opportunity to use the principles of linear elastic fracture mechanics (1)4 to evaluate resistance to stress corrosion cracking in the presence of a pre-existing crack. This type of evaluation is not included in conventional bent beam, C-ring, U-bend, and tension specimens. The precracked double beam specimen is particularly useful for evaluation of materials that display a strong dependence on grain orientation. Since the specimen dimension in the direction of applied stress is small for the precracked double beam specimen, it can be successfully used to evaluate short transverse stress corrosion cracking of wrought products, such as rolled plate or extrusions. The research applications and analysis of precracked specimens in general, and the precracked double beam specimen in particular, are discussed in Appendix X1.5.2 The precracked double beam specimen may be stressed in either constant displacement or constant load. Constant displacement specimens stressed by loading bolts or wedges are compact and self-contained. By comparison, constant load specimens stressed with springs (for example, proof rings, discussed in Test Method G49, 7.2.1.2) or by deadweight loading require additional fixtures that remain with the specimen during exposure.5.3 The recommendations of this practice are based on the results of interlaboratory programs to evaluate precracked specimen test procedures (2, 3) as well as considerable industrial experience with the precracked double beam specimen and other precracked specimen geometries (4-8).1.1 This practice covers procedures for fabricating, preparing, and using precracked double beam stress corrosion test specimens. This specimen configuration was formerly designated the double cantilever beam (DCB) specimen. Guidelines are given for methods of exposure and inspection.1.2 The precracked double beam specimen, as described in this practice, is applicable for evaluation of a wide variety of metals exposed to corrosive environments. It is particularly suited to evaluation of products having a highly directional grain structure, such as rolled plate, forgings, and extrusions, when stressed in the short transverse direction.1.3 The precracked double beam specimen may be stressed in constant displacement by bolt or wedge loading or in constant load by use of proof rings or dead weight loading. The precracked double beam specimen is amenable to exposure to aqueous or other liquid solutions by specimen immersion or by periodic dropwise addition of solution to the crack tip, or exposure to the atmosphere.1.4 This practice is concerned only with precracked double beam specimen and not with the detailed environmental aspects of stress corrosion testing, which are covered in Practices G35, G36, G37, G41, G44, and G50.1.5 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.