Local formability and bendability of UHSS: Correlations between bending and tensile fracture strains

Abstract

This study investigates the correlation between bendability and local formability properties of six ultra-high strength steel (UHSS) grades using small radius air-bending tests, uniaxial tensile tests, and plane-strain tensile tests with grooved specimens. Digital image correlation (DIC) was employed to measure surface strains, while post-mortem thickness measurements provided through-thickness strain data from the tensile tests. A novel strain-based method was used for bending fracture detection, offering improved accuracy over conventional methods such as the load drop thresholds defined in the VDA 238-100 specification. The bending fracture strain showed a moderate correlation (R² = 0.73) with the plane-strain tensile fracture thickness strain. In contrast, correlations between bending fracture strain and uniaxial tensile fracture measures were weak (R² = 0.38–0.53). No clear relationships were found between bending fracture strain and conventional tensile parameters such as total elongation or strain hardening exponent. The findings of this paper highlight the complexities in predicting bendability from tensile measures due to stress state differences and underscore the importance of bending tests for accurate characterization of sheet metal formability.