Evaluation of Tearing Instability by Means of Recrystallization-Etch Technique (Comparison of Fracture Toughness Between Through Crack Specimens and Part-Through Crack Specimens)

Tearing instability crack growth in low alloy steels was evaluated in a regime of elastic plastic fracture mechanics by use of recrystallization-etch technique. Specimen geometries used here were standard compact and 3pt bend specimen, and 4pt bend type specimen having a part-through surface crack. In order to produce high compliance for a testing machine, disc springs were used where the compliance changes from lkN/mm to 5kN/mm. After the fracture experiments the crack tip strain energy dissipation rate d W_P/da (W_P : crack tip strain energy absorbed within the intense strain region, a : tearing crack extension) was determined by the recrystallization-etch technique. It is shown that the value of T_w (tearing modulus based on W_p) is constant before and after an onset of tearing instability. In the surface crack specimen, three dimensional profile of the intense strain region along the crack front was observed. This experimental results indicate that the T_w criterion is applicable to the crack extension from the surface flaw, although a singular extension behavior of the intense strain region was observed only near the specimen surface.