Investigation of high-strength steel component subjected to stretch-bending: effect of forming history

Abstract

The behaviour of a closed top-hat section made of Dogal 800 DP subjected to stretch-bending is studied both experimentally and numerically. The top-hat section was made by forming of a sheet using a FlexformTM fluid cell press and closed by a sheet using welding. Experiments were performed in a stretch-bending rig in two stages. During the first stage the profiles were bent under a constant horizontal stretch force. In the second stage the profiles were stretched back from the bent position. The force-displacement relations of the actuators involved in the experiments were recorded and the initiation and development of fracture during the stretching process after unloading of the die was detected by cameras. A finite element code was applied to model the forming operation and the stretch bending experiments including the unloading of the die and subsequent stretching of the profile. The elastic-plastic material model with calibrated parameters was adopted from previous studies on Dogal 800 DP, including the Cockcroft-Latham fracture criterion to detect initiation of fracture. The history variables were mapped from the forming model to the stretch-bending model. The forming process was simplified as a hydroforming operation and some deviations were observed regarding the thinning of the sheet between the model and the real process. The model of the stretch-bending experiment was able to capture the force-displacement relation during the first stage with reasonable accuracy. Some deviations between the experimental and simulated force-displacement relations were observed during the second stage, i.e. the stretch-back operation, but the initiation of fracture was well captured.