Validation of New Experimental and Analytical Tools to Address Residual Stress Effects Accurately in the Design Cycle: Part 1 - Material Characterization

摘要

Residual stress remains a major obstacle to capturing full benefits and broadening adoption of unitized structure solutions derived from a variety of thick and near net shape aluminum alloy product forms. For instance, the bulk residual stress created during forging thermo-mechanical processing can sometimes introduce bias and scatter into coupon-based fatigue crack growth rate (FCGR) and fracture toughness property determinations, which can lead to uncertainty in the data and its translation to design. The proper accounting of residual stress effects requires that their influence be understood, quantified, controlled, and accounted for throughout the entire product life cycle. Residual stress effects must be excluded from the material property database so their consequence can be managed in the appropriate design and sustainment phases of product life. Proper accounting for residual stress and its management throughout the product life-cycle should be determined based on a range of factors including the amount of conservatism necessary for the given application. The Air Force Research Laboratory sponsored Metals Affordability Initiative (MAI) consortium has conducted a detailed experimental and analytical study of bulk residual stress, fatigue crack initiation, and fatigue crack growth in aluminum alloy coupons extracted from material with intentionally induced residual stress. This Part 1 paper and a companion Part 2 paper summarize a portion of this work directed to validating a next-generation design capability to account for residual stress at the beginning of the design cycle. The main focus of this paper (Part 1) is an improved material FCGR characterization process that partitions residual stress effects from the material "true" FCGR behavior, leading to FCGR design curves that are free of residual stress bias. The Part 2 companion paper describes the fatigue crack growth (FCG) validation test program using design feature (DF) coupons, the analysis methods used to predict the FCG lifetime while addressing residual stress effects in the DF coupons, including variability in the residual stress, and the comparisons between experimental and model results. The advancements described in these papers not only enable quantification and management of residual stress during FCGR property determination, but when integrated and deployed in concert with new design methodologies described elsewhere, these technologies will enable new opportunities for optimization of final part design with respect to material properties, structural design, life management, etc. with the presence of residual stress.