Fracture Prediction of Mild Steel Sheet and High-Strength Steel Sheet Subjected to Draw-Bending Using Forming Limit Stress Criterion

Abstract

A fracture criterion for sheet metals subjected to draw-bending is investigated using the concept of the forming limit stress criterion. An experimental apparatus that is capable of draw-bending sheet specimens with forming speeds （approximately 100 mm・s^-1 ） comparable to those of real press forming machines is designed and built. The test materials are an ultralow-carbon steel sheet and a 590MPa high-strength steel sheet. Specimens undergo bending-unbending under tension when passing over a die profile. The magnitudes of true stress σ _DB at which the stretch-drawn specimens fractured are precisely determined from the measured data of the drawing force and the cross-sectional area of the specimen after fracture. Moreover, multiaxial tube expansion tests are performed to measure the forming limit stresses σ_PT of the test materials under plane-strain tension. It is found that σ_DB is larger than σ_PT by 6-18%. Therefore, it is concluded that the forming limit stress criterion is effective for the fracture prediction in draw-bending.