Evaluation of Residual Stress Corrections to Fracture Toughness Values

This paper describes tests carried out to evaluate whether comparable values of fracture toughness may be determined from coupons that contain a range of residual stress. Coupons for fracture toughness testing are taken from a single rolled plate of 7075-T6 aluminum alloy and are fabricated using identical methods. Following fabrication, residual stresses are induced in coupon subsets using laser shock peening. Each coupon subset has a unique laser peening treatment design, such that following fatigue precracking, the stress intensity factor due to residual stress varies over the coupon subsets created from positive to neutral to negative. Subsequent fracture toughness tests show expected trends in the varying toughness values for each coupon subset. Measurements of residual stress on the crack plane, and measurements of the residual stress intensity factor, enable correction of the toughness data through superposition and linear elastic fracture mechanics. The resulting corrected fracture toughness values provide data useful for evaluating residual stress corrections with potential for application to standard fracture test methods. Such corrections would enable measurements of intrinsic fracture toughness in samples that contain significant levels of residual stress.