Analysis of trimming die durability and sheared edge formability of ultra-high strength steels

Abstract

The increasing demand for lightweight, safe, and fuel-efficient vehicles has led to greater use of advanced high-strength steels (AHSS), especially dual-phase steels like DP980, in automotive structures. However, these materials are often sensitive to trimmed edge cracking if stretching along the sheared edge occurs during processes such as stretch flanging. Excessive wear and early chipping of trimming tools are also significant issues in mass production. This dissertation investigates these challenges by analyzing the sheared edge formability of DP980 and the durability of trimming dies using experimental, numerical, and fatigue modeling methods.Edge quality was assessed based on burr height, burnish depth, fracture morphology, and microhardness mapping. Stretchability was evaluated through tensile, side-bending, and hole-expansion tests, demonstrating that edge quality significantly affects the sheets ability to withstand secondary deformation. Two DP980 materials were studied, showing different stretchability results and highlighting the variability within the same grade. A distinctive fracture mechanism was observed during trimming of DP980 steel, resulting in a burr at the final stage. Finite element models were created using Abaqus/Explicit. Optimized meshes, kinematic contact algorithms, and appropriate fracture criteria enabled reasonable predictions of strain localization, fracture initiation, and punch load history, while also reducing the penetration issue between the die and sheet meshes. The simulation also provided stress data for fatigue analysis of the trimming tool. This study also analyzed die wear and fatigue life over 230,000 cuts for D2 trimming inserts. Gradual tool wear decreased edge quality and stretchability, while localized chipping on inserts aligned with local burrs on the part edge. Fatigue modeling using the Goodman, Morrow, and Findley criteria showed that shear-based approaches best matched actual failure modes, supported by microstructural evidence of carbide-band effects. Overall, the findings provide valuable metrics and predictive tools to optimize trimming processes, extend die life, and ensure reliable manufacturing of AHSS.