Reverse bend experiments on the base plate and discussion of the appropriate amount of reverse bending for precrack straightness (Effect of the reverse bend process on the CTOD toughness evaluation and understanding of its mechanisms)

Abstract

Measurements of the critical value of the crack-tip opening displacement (CTOD) of welded joints often suffer from weld residual stresses, which prevent the introduction of a straight precrack front. In this study, the effects of reverse bending, a proven method for straightening the crack front shape, on evaluations of the critical CTOD are investigated. To compare the differences in the evaluation results, the authors used a base plate without residual stress inside the material for their experiments. Five different reverse bending treatments were applied to a 50 mm thick base metal, and the effects of reverse bending were evaluated via CTOD tests at low temperatures. Additionally, numerical simulations were carried out via the finite element method to exclude the effects of a₀/W and a_f, which cannot be unified in the experiments, and to understand the mechanism. After reverse bending and unloading, the tensile residual stress is distributed at the notch edge, and a high stress intensity at the fatigue crack tip during CTOD testing can be expected. In addition, when the amount of reverse bending is increased to L_r = 1.35, the compressive residual stress distribution in front of the specimen expands, and the P–V_g curve clearly decreases, which may impair the precrack shape flatness. Therefore, the controlled range of L_r should be at least less than 1.35.