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

Abstract

Abstract A fracture criterion for sheet metal subjected to draw bending is proposed based on 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 for commercial press forming machines was designed and built. The test materials were an ultralow-carbon steel sheet and a high-strength (590 MPa) steel sheet. Specimens underwent bending and unbending under tension when passing over a die. The magnitude of the true stress, σ D B , at which the stretch-drawn specimens fractured was precisely determined from the measured drawing force and the cross-sectional area of the specimen after fracture. Moreover, multiaxial tube expansion tests were performed to measure the forming limit stress, σ P T , for the test materials under plane-strain tension. It is found that σ D B is consistent with σ P T when the strain rate is properly taken into account in evaluating σ P T .