Material properties and residual stresses of cold-formed octagonal hollow sections

Abstract

This paper presents an experimental investigation on the material properties and residual stress distributions in cold-formed steel octagonal hollow sections. Octagonal hollow sections (OctHSs) manufactured through two fabrication routes were considered. Two batches of steel plates with measured yield strengths of 546.5 MPa and 580.7 MPa were employed. A total of 46 tensile coupons taken from the parent plates and cold-formed hollow tubes were tested to obtain the material properties of parent metals and to assess strength variations within the sections caused by press-braking and welding process. A new bilinear plus nonlinear strain-hardening material model was proposed for the materials at flat regions and an existing material model for cold-formed steel was adopted for the materials at corners. These models yielded satisfactory agreements with the test results in terms of stress-strain relationships. Up to date, investigations on the residual stress distributions in cold-formed OctHSs remain limited. Subsequently, residual stress measurements on 4 cold-formed octagonal hollow sections with different fabrication routes and different flat side width-to-thickness ratios were performed. Sectioning method was adopted in this study with 74 strips extracted and more than 800 strain readings taken. Results of residual stress magnitudes and distributions are presented and discussed. Membrane residual stresses were typically found to be as high as 62% of the material yield strength, whilst bending residual stresses were found to be below 39% of the material yield strength. Simplified models for the residual stress magnitudes and distributions in the cold-formed octagonal hollow sections were proposed on the basis of test results.