To obtain a better understanding of the behavior of shallow cracks in structures the influence of crack depth on fracture toughness has been studied. Fracture toughness tests were carried out on 50-mm thick three-point bend specimens sampling HY130 high strength steel. The mode of crack growth in all cases was tearing, and fracture toughness values at initiation of crack growth were evaluated as well as resistance (R) curves. Fracture toughness values were calculated in terms of both the crack tip opening displacement (CTOD) and the J integral. Crack extension was deduced from unloading or reloading compliance and by use of an alternating-current potential drop technique.
It was found that the fracture toughness values at initiation of crack growth, and the R-curve slopes, were consistently higher for crack depth to width (a/W) ratios of 0.3 or less than for the a/W ratio of 0.5 commonly used for routine fracture toughness tests. For very deep cracks, a/W ≃ 0.8, the fracture toughness at initiation of tearing was little different from that for a/W ≃ 0.5. The R-curve slope, however, was higher for a/W ≃ 0.8 than it was for a/W ≃ 0.5. The latter observation (and the observed increase in fracture toughness with decrease in crack length for a/W less than 0.3) is considered to be consistent with loss of constraint for very short and very deep cracks.