Detection of early plasticity in steel using large scale 4-point bending

Characterizing the early metallic plasticity of materials remains a challenge, as the stress–strain curve derived from a tensile test cannot accurately identify the first plastic deformations. Although tensile thermal camera coupling reveals an elastic limit lower than the conventional limit, it does not reveal incipient plasticity. The aim of this study is to develop an experimental setup and the method for detecting minimum plastic deformations before the classical yield strength determined by tensile testing is reached. An original large 4-point bending machine was designed to perform loading-unloading cycles and to analyze specimen profiles by detecting irreversible deformations, signs of early plasticity. Tests on the bending machine using dual-phase steels (DP600 and DP800) and cold-rolled low-carbon steel (DC01) specimens (1200 × 35 × (2 mm or 3 mm)) revealed the onset of irreversible strains occurring between 127–135, 154–169, and 64–68 MPa, respectively. These values are well below their respective yield strengths of 305, 360, and 158 MPa. These results confirm the existence of early plasticity and pave the way for the design of a behavioral model that takes this phenomenon into account, in response to the challenges posed by the prediction of springback during cold forming of metallic materials.