Asymmetric Hole Expansion Test (aHET) for Direct Measurement of Orientation-Dependent Tensile Fracture Strain

The flangeability of sheared edges in sheet metal forming is commonly evaluated using the ISO 16630 Hole Expansion Test (HET), in which fracture initiates at the edge under predominantly uniaxial tensile loading. For high-quality edges, this test can be interpreted as providing the strain to fracture under proportional uniaxial tension; however, the measured fracture strain is restricted to a single material-dependent sheet orientation. In this work, a novel experimental approach is proposed to directly measure the uniaxial tensile fracture strain in a predefined sheet orientation using digital image correlation (DIC). The method, termed the Asymmetric Hole Expansion Test (aHET), is derived from the standard HET through the introduction of a novel asymmetric punch geometry. This modification promotes accelerated edge stretching along a controlled direction, enabling orientation-specific characterization of fracture strain. The capability of the aHET to characterize direction-dependent strains to fracture under uniaxial tension is demonstrated on a DP450 dual-phase steel. The consistent fracture initiation at the edge along the predefined fracture direction, combined with the low scatter of the measured fracture strains across repeated tests for all three investigated sheet orientations, demonstrates that the aHET is well suited for identifying the strain to fracture under proportional uniaxial tensile loading for the calibration of fracture initiation models.