An investigation on longitudinal residual strains distribution of thin-walled press-braked cold formed steel sections using 3D FEM technique

Abstract

Steel sections are normally shaped via cold work manufacturing processes. The extent of cold work to shape the steel sections might induce residual stresses in the region of bending. Previously, researchers had performed studies on the influences of local buckling on the failure behavior of steel compression members which shown that failure will happen when most of the yielding has extended to the middle surface in the bend region of the sections. Therefore, these cold work methods may have major effect on the behavior of the steel section and also its load-bearing capability. In addition, another factor may play significant role in formed section's load-bearing capacity which is the longitudinal residual strain. The longitudinal residual strain raised during forming procedure can be used to define the section imperfection of the formed section and its relation to the existence of defects. Therefore, the main motivation of this research paper is to perform three-dimensional finite element (3D-FE) to investigate peak longitudinal residual strains of a thin-walled steel plate with large bending angle along member length. A 3D finite element simulation in ABAQUS has been employed to simulate this forming process. The study concluded that the longitudinal residual strain at the section corner edge was higher than those at the rest of the corner region. These strains at the edge were higher than the yield strain true(εytrue) of the formed section which occurred due to the lack of transverse restraint. This made the plate edge tended to bend toward the normal direction when it was under a high transverse bending. This causes a significant difference in longitudinal strain at the plate edge.

Numerical Prediction of Residual Stresses Distribution in Thin-Walled Press-Braked Stainless Steel Sections

“…The 3D-FE technique presented in [24,25] was employed in this study to examine the residual stress behavior along the corner region and also in through-thickness direction of press-braked stainless steel sections giving due consideration to the influence of the material anisotropy. The outcomes was compared with two 2D-FE results from the literature: [14] which considered specimen's material anisotropy and [17] that assumed material isotropy of the same specimen.…”

“…The simulation of press-braking covers two steps (forming and springback). Many studies preferred the dynamic explicit method over the implicit method in the sheet metal forming stage [17,24,25,[28][29][30]. One advantage of the explicit algorithm is resolving complicated contact problems and convergence with improved efficiency.…”

“…On the other hand, ε x at the edge zone (z = 0 mm) is lower than the mid-corner (z = 50 mm) in the inner surface. This differences are due to the Poisson's effect, where formed corners tend to bend at the edge due to the lack of transverse restraint [25]. As the outer surface in compressive mode and the inner surface in tensile mode, hence, the edge zone exhibited different strains than the mid-corner zone.…”

“…They stated in their conclusion that residual stresses near corners have different variations. In addition, Mutafi et al [25] investigated the surface longitudinal residual strains of thin-walled press-braked cold-formed steel and showed that the longitudinal residual strain (compressive and tensile strain) near the edge is greater than the rest of the corner region. Longitudinal residual stress distribution prediction and element types adopted in the prediction: (a) 2D-FE technique [12], (b) 3D-FE technique [24].…”

“…Therefore, this paper aims to extend the investigation of the distribution of residual stresses along the bending region of stainless steel press-braked sections by implementing the same technique in [24,25] giving due consideration to the influence of coiling-uncoiling process. The study proposed a full finite element procedure to predict residual stress starting from coiling-uncoiling to press-braking.…”