Prediction of Fracture Toughness of Steels by Local Fracture Criterion

Abstract

Fracture toughness of a material is determined by its fracture characteristics as well as its plastic flow properties which control the stress-strain state at the crack tip.
The maximum tensile stress criterion is usually supposed as the fracture criterion for cleavage fracture of steels. So it was assumed in this study that cleavage fracture at the crack tip is initiated when the stress at a certain characteristic distance from the crack exeeds a certain limit. Based on this fracture criterion, the fracture toughness for cleavage fracture initiation can be given from the HRR solution by
J_c=C(n)X₀σ_ys(σ_c/σ_ys)ⁿ⁺¹
where σ_c is the cleavage fracture stress, C(n) is a function of strain hardening of materials, σ_ys is the yield stress and X_o is the characteristic distance.
By the use of this relation, the correlations have been obtained between the fracture toughness and flow/fracture properties of the materials, σ_ys and σ_c, obtained by the smooth and notched round bar tensile tests. A good agreement has been shown between these predictions and experiments on 8 kinds of low carbon structural steels which have various microstructures with the yield strength ranging from 280MPa to 1110MPa.