Fracture toughness tests were carried out with blunt notched compact tension specimens of a high-strength low alloy steel, JIS SNCM 8 (equivalent to AISI 4340), tempered at several temperatures. The stress intensity factor Kρ at the initiation of crack extension detected by the DC potential method was related to the initial notch-tip radius ρ by
Kρ=K0(1+ρ/ρ0)1/2
where K0 and ρ0 are material constants. This was derived through the concept of the fictitious notch-tip radius proposed by Tanaka and Hirose. The values of material constants K0 and ρ0 took the lowest values for the material tempered at 300°C.
The residual stress near the fracture surface measured by the X-ray diffraction technique was tension. The depth of the plastic zone ωy was measured by the distribution of residual stress beneath the fracture surface. It was related to the stress intensity factor at crack initiation Ki by the following fracture mechanics equation:
ωy=α(Ki/σY)2
where σY is the yield stress and α is from 0.14 to 0.12. The values of residual stresses very close to the fracture surface tended to decrease with increasing Ki, and were found to be a single-valued function of the fracture strain at the notch-tip determined from K0 and ρ0 values based on the fictitious notch-tip radius concept.
