Non-linear necking behaviour of S275 to S960 structural steels under monotonic tension

For cylindrical coupons of structural steels under tension, gradual reductions in their cross-sections, commonly known as necking, occurred at an elongation that the maximum force is attained. Non-uniform stress and strain distributions in the critical cross-sections of the necking regions have been known for more than 70 years; however, the values of these stresses cannot be directly measured in experiments so far. This paper presents a systematic investigation into the post-necking structural responses of the S275 to S960 steels under monotonic tension forces through the Instantaneous Area (iArea) Method proposed by the authors. In order to examine post-necking deformation characteristics of structural steels up to fracture, a total of 15 standard tensile tests on proportional cylindrical coupons of these steels were carried out carefully to obtain their force–elongation (F – e) curves while precision measurements were made to obtain their instantaneous dimensions. Complementary advanced finite element models are then established to predict non-linear stress and strain distributions at the critical cross-sections of the necking regions. Through a process of successive approximation, a series of correction factors are obtained to allow for the effects of these non-uniform stress and strain distributions respectively onto the post-necking structural responses of the coupons. Hence, the predicted force–elongation (F - e) curves of the coupons are demonstrated to follow the measured curves closely throughout the entire deformation ranges. More importantly, the predicted instantaneous diameters d_gi are shown to have a good agreement with the measured values at various deformation stages up to fracture.