The Sheet Thickness Dependency of Fracture Toughness in High Strength Steels

Abstract

The effect of specimen thickness B on the fracture toughness K_C as well as on the shear lip thickness B_SL of high strength steels was examined to obtain new models for explaining the thickness dependency of K_C, and the influence of tensile properties on the dependency was studied.
The results obtained are as follows.
(1) For each steel, K_C increases with the increase of B in a smaller thickness range where slant fractures occur. In the middle thickness range, however, where fractures of a slant-flat mix mode occur, K_C decreases with the increase of B, and in a larger thickness range where flat fractures predominate, K_C gradually decreases to a constant value (plane strain fracture toughness K_IC).
(2) The shear lip thickness B_SL decreases with the increase of specimen thickness B in the range of the mix mode fracture.
(3) The new models proposed in this study can express the thickness dependency of K_C relatively well.
(4) An approximate relation, B_SL/B=αβ^m_c holds between B_SL/B and the relative plastic zone size β_C (=K²_C/Bσ²_ys, where σ_ys, is yield strength). The exponent m does not much change with materials (m=0.6-1.0), but the coefficient α is dependent on the work hardening exponent n (α≈0.0016/n^1.6).