On plastic notch effects in quenched and tempered steels

ABSTRACT: In 1971, Firrao and Spretnak performed a large experimental campaign to assess the elastic-plastic stress concentration factor at fracture as a function of the elastic stress concentration factor and of the tempering temperature, by using 25.4 mm wide, 1.14 mm thick AISI 4340 quenched steel sheet tensile specimens with variable tip radius central notches. The availability of finite element methods allows now to re-examine those results and overcome the simplifying assumptions that were originally used to evaluate notch stresses. The elastic stress concentration factors are obtained by three-dimensional solutions, which also evidence the gradual evolution from plane-stress to plane-strain that occurs by decreasing the notch radius while keeping the thickness constant. Moreover, both the actual stress state and the stress concentration factor in the notch immediately before the failure are evaluated by elastic-plastic solutions. Finally, the original conclusions on the notch sensitivity of the examined steel are re-assessed and reinterpreted.