Ductile fracture toughness evaluation at high strain rates using stretch zones and shear lips

Fracture studies of high strength low alloy steels, at very high loading (strain rates) of up to K equal to 10(6) MPa root m/s were conducted using a specially designed Split Hopkinson Bar (SHB) equipped with a swing arm mechanism capable of fracturing compact tension (CT) specimens with a thickness of 12.7 mm. While it was possible to reproduce the load-displacement curves for the specimens, evaluation of the fracture toughness was mostly obtained from post-test examination of the stretch zone ahead of the crack using scanning electron microscopy. Several factors affecting the high strain rate stretch zone were identified including the effect of fatigue precracking level as well as the method of measurement of the stretch zone (nine point vs. three point approach). Significant differences in the stretch zone were observed which are attributed to the extent of the constraint factor ahead of the crack due to fatigue precracking level which demonstrates the occurrence of a zero shift in the initial part of the J-Stretch Zone Width relationship. Shear lips were measured on fractured specimens to create an indirect mearure of toughness. The effect of notch acuity on toughness (and toughness transition) was studied from the size of the shear lip.