Residual Stresses Produced During Sharp Bending of Wide Sheets

Abstract

<p> Manufacturing processes such as cold-bending flat sheet to a small radius produces a rather complex residual stress state through the thickness of light gauge structural steel sections. When the load-application device is released the sheet exhibits a phenomenon called "spring-back".</p> <p> The purpose of this research was to develop an exact method of computing the residual stresses in a wide sheet of ideal plastic metal after spring back and subsequently to study their effect on the behaviour of cold-formed sections. An approximate analysis based on the assumption of elastic spring back is presented for comparison purposes. It is demonstrated that plastic flow occurs within a thin core of sheet below the neutral axis and that the spring back is not completely elastic beyond a certain curvature. Such behaviour is particularly significant in cases where the radius of bend is of the same order as the thickness. An exact analysis based on a more realistic approach is then performed considering the adjustments in the stress components due to the plastic core. Computer programs have been developed to calculate the residual stresses associated with any given radius of bend. It is found that the choice of one yield criterion rather than the other does not have any effect on the present analysis and the stress components for Von Mises' material are (2/√3) times the components for Tresca's material. Furthermore, the effect of lateral stretching on the behaviour of cold-formed sections is briefly viewed.</p> <p> The purpose of the experimental work in this thesis was to verify the validity of some of the assumptions made in the analysis.</p> <p> Conclusions are drawn and suggestions made for further research.</p>