Advancements in fracture toughness testing of ultra-high-strength steel sheets: Unraveling the crack-closure effect and unanticipated thickness independence

A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 similar to 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness.