Material properties and residual stresses of Q690 high strength steel press-braked elliptical hollow sections: Experimental investigation and numerical modelling

Abstract

This paper presents an extensive experimental and numerical studies on the material properties and residual stresses distributions of Q690 HSS cold-formed EHS made from HSS plate with a nominal yield strength of 690 MPa to address the lack of information on the mechanical characteristics of these sections. A total of five Q690 HSS press-braked EHS were fabricated with 3 mm, 6 mm and 10 mm thick steel plates with transverse bending and longitudinal welding. The material properties were acquired by conducting tensile coupon tests with coupon specimens extracting from the critical regions within the cross-sections and from the parent plates as well. In addition, the membrane and bending residual stresses in the longitudinal direction were measured on half-section profile of representative EHS specimens. Sectioning method was utilised to manufacture the testing strips with a total of 71 strips cut by a wire-cutting machine and over 852 strain readings were recorded. In conjunction with the experimental tests, an integrated sequentially-coupled thermomechanical analysis was conducted by developing an advanced finite element (FE) model of manufacturing press-braked EHS specimen. The transverse bending was simulated with three-dimensional models underwent extensive plastic deformation to obtain the bending residual stress after press-braking and springback. The longitudinal welding was simulated using solid element and the technique of birth and death element was used to simulate the multi-passes welding and molten of welding electrode. Consequently, explicit residual stresses distributions in longitudinal direction against the perimeter due to the transverse bending and welding were ascertained. The experimental and numerical data were used to demonstrate and discuss the magnitudes and distribution patterns of the residual stresses. A predictive model for residual stress distribution within the Q690 HSS press-braked EHS was developed and proposed.