Analytical, numerical and experimental investigation of wrinkling in flexible roll forming of variable cross-sectional channel profiles

Abstract

Flexible roll forming is an advanced sheet metal forming process for cost-effective innovative manufacturing of 3D profiles for automotive lightweight structures. In contrast to roll forming, the flexible roll forming process employs rolls that are not fixed in their position and can be moved along a path in order to describe the desired bend line of the profile. One of the most important defect in flexible roll forming is wrinkling that leads to an undesirable produc. This research work presents a new understanding on the deformation mechanics of the flexible roll forming and is focused on the occurrence of flange wrinkling. A new analytical-numerical model combining the energy method and the finite element simulation is developed to predict wrinkling. In this model, unlike previous models, the purpose is not the calculation of a critical stress assumed to be uniform over the compressive region, but that is the calculation of a factor to indicate the possibility of wrinkling considering the real distribution of stress in the compressive region. Thus, a wave function is assumed in accordance with the boundary conditions of the region under compression in the finite element simulation and the wrinkling factor is obtained as the ratio of the external work done by the in-plane membrane forces and the internal energy of the buckled sheet at each increment. The wrinkling factor depends on mechanical properties, geometry parameters, and current strain and stress field In this thesis, a new theoretical and experimental methodology based on the utilization of rectangular test specimens loaded in axial compression is developed to determine the wrinkling limit curve in the principal strain space. The wrinkling limit curve is then transformed into the space of effective strain versus stress triaxiality and checked against the finite element–predicted evolutions of effective strain with stress triaxiality of the deformed strip in the flexible roll forming process. Another wrinkling factor is proposed based on the wrinkling limit curve. This factor is defined in ABAQUS software by means of a subroutine and applied to determine the possibility of wrinkling during the simulation. A flexible roll forming setup is designed and built to perform the experiments. This setup has three major parts: Forming stand, Feeding mechanism and Control unit. Forming stand consists of two pairs of rolls that are integrated in a parallel kinematic system that allows following the varying contours of the profiles. The experiments were performed on the flexible roll forming setup and the history of longitudinal strain, the warping defect and the geometry of the variable cross-sectional profiles are measured by strain gauge, dial indicator and 3D scanner, respectively. The experimental observations and measurements give support to the numerical results and show that the proposed wrinkling criteria can correctly predict wrinkling in the flexible roll forming. Two proposed wrinkling criteria are applied to determine effects of the geometric parameters (Flange length, radius of bending line, width of transition zone and thickness) of the variable cross-sectional channel profiles and material properties (yield stress and hardening exponent) on the critical forming angle in the flexible roll forming. The results show that flange length, sheet thickness and yield stress have the greatest influence on the critical forming angle.