Comparison of dynamic fracture toughness and critical J-integral criteria under high loading rates

Dynamic fracture toughness (DFT) and the critical J-integral are two key criteria traditionally used to assess the onset of crack initiation in materials. These criteria, however, are not interchangeable within the elastic–plastic range. Consequently, understanding their validity becomes crucial, particularly when the material's failure mode changes. In this study, we employed the electromagnetic Hopkinson bar technique and Digital Image Correlation (DIC) to conduct compact tensile tests on two ultra-high-strength steels (UHSS) under high loading rates. DFT was determined through an experimental–numerical method, while the critical J-integral was obtained indirectly using the DIC technique. The crack initiation mode was identified by analyzing the fracture morphology at the crack tip. The findings reveal that as the crack initiation mode shifts, the DFT criterion becomes less reliable in characterizing a material's resistance to crack initiation. In contrast, the critical J-integral remains a robust and applicable measure.