Effect of Residual Stress on Brittle Fracture Testing

Residual stress present in coupons used for fracture testing will influence the experimental determination of fracture toughness. Triaxial residual stress (e.g., due to welding) can alter both the driving force for fracture and the crack-tip constraint at a specified load, so that the apparent toughness of a coupon depends on the residual stress present. In this paper, we employ a micromechanical failure theory (the RKR model) to predict brittle fracture loads for three-point bend samples with and without triaxial residual stress. Finite element modeling is used in the study to introduce an idealized welding residual stress field, to simulate the crushing of the coupon by circular platens (referred to as homogenization or local compression), and to simulate three-point bend testing. Results show that the residual stress markedly changes load at fracture, which would be used to compute apparent fracture toughness. Furthermore, the method of superposition commonly used is not able to accurately predict this effect.