Damage-Based Assessment of Stress Corrosion Performances Among Aluminum Alloys

At Alcoa Laboratories the breaking load test was developed to provide a more discriminating accelerated laboratpry practice for rating stress corrosion cracking (SCC) performances of relatively resistant aluminum materials. The method involves residual strength measurements from tensile bar specimens previously exposed to sustained tensile stress in a corrosive medium. The degree of SCC degradation is measured by comparing the specimen postexposure strength with the original tensile strength (no exposure); the greater the strength loss, the more harmful the attack. In contrast to the more traditional pass/fail life testing approaches, the breaking load method enables numerical differentiation among materials and precludes long waiting periods for specimens to fail in the environment. In addition, the breaking load method provides descriptors amenable to flaw size and probabilistic fracture mechanics interpretations, thereby enabling expression of SCC ratings in meaningful terms of crack initiation and growth kinetics. Conventionally used descriptors of SCC behavior (e.g., time to failure, percent survival, threshold stress, KIscc) are influenced to varying degrees by both the strength and toughness of the test material and the conditions of the testing (e.g., specimen geometry and loading method). The damage-based evaluation of breaking load data is advantageous in that potential biases in SCC performance ratings caused by material property differences and specimen size effects can be normalized. Extensive work to refine and demonstrate the usefulness of the breaking load practice for aluminum SCC evaluation was completed in 1984 under a NASA-Langley contract to Alcoa Laboratories. Much of the development work to this point was largely based on single-lot evaluations of AA7075 plate/temper variations. Since this time, Alcoa has adopted the breaking load method in alloy development, and this paper provides new additional data to illustrate the general applicability of the approach. Included in these results are posttest fractography and fracture mechanics analysis, illustrating advantages of damage-based interpretation over conventional SCC descriptors developed from smooth and precracked (DCB or WOL-type) specimen tests. The significance of the findings is discussed in terms of general material selection guidance related to durability and damage-tolerant structural design objectives.