Abstract
The residual stress left on fracture surface is a useful information to analyse local plastic deformation near the crack tip. In the present study, X-ray residual stress measurements were conducted on the fatigue fracture surface of a low-alloy high strength steel (JIS SNCM 8) and a structural-low-carbon steel (JIS SM 41 B), and the results were analysed based on fracture mechanics. The followings are the main results obtained.
(1) The residual stress on the fatigue fracture surface was tension. The residual tensile stress increased with the maximum stress intensity factor Kmax in the case of the material tempered at 200°C, While it has a maximum value at about Kmax=30MPa√m in the case of the material tempered at 600°C. On the fracture surface of mild steel, the residual stress tended to decrease with increasing Kmax.
(2) The distribution of the residual stress beneath the fatigue fracture surface of the high strength steel was able to be decomposed into three parts: the distribution caused by monotonic plastic deformation near the crack tip, that relieved by surface roughness, and that resulting from the compressive deformation due to crack closure.
(3) The maximum depth of the plastic zone ω determined from the residual stress distribution was correlated to Kmax and the yield stress σY as follows.
ω=0.21(Kmax/σY)2
This equation holded for the cases of high strength steels tempered at 200 and 600°C. On the other hand, in the mild steel, the width of X-ray diffraction profiles was suggested to be a more appropriate parameter for plastic zone determination than the residual stress.
