X-ray fractography is a useful technique for analyzing the cause and mechanism of fracture from the information obtained by X-ray irradiation on the fractured surface. In this report, the relations between the residual stress on the fatigue fractured surface and some parameters of fracture mechanics were investigated. Special attention was paid to the contact of fractured surfaces due to crack closure, as well as to the effect of surface roughness on the X-ray stress measurement.
The materials used were a Ni-Cr-Mo steel (SNCM 439) and a mild steel (SS 41). The test pieces were CT specimens with W=51mm and B=12mm. On the fractured surface the residual stress along the crack propagation direction was measured by a parallel beam X-ray stress measure Cent apparatus.
The main results obtained were as follows.
(1) In the ductile material the residual stress on the fatigue fractured surface increased at first and then gradually decreased with increasing crack length. In the brittle material the residual stress increased monotonically and kept almost a constant value do the final fracture surface. In both cases the residual stress could be correlated to the Kmax values.
(2) The residual stress on the fatigue fractured surface was not affected by the contact of fractured surfaces due to crack closure.
(3) The roughness of the fatigue fractured surface increased with increasing K value, and it was found that the stress measured by X-ray diffraction tended to give a lower value than that of the appplied stress.