Abstract In order to study the residual stress induced by foreign object damage (FOD), the distribution of residual stress caused by the impact of a hard spherical body was measured via the sin 2 ψ technique, using synchrotron X‐ray. A steel sphere was impacted onto a flat surface of a Ti‐6Al‐4V alloy from an angle of either 90° or 45°, at a velocity of 180 m/s. The same sphere was also quasi‐statically pressed into the surface. In the cases of right‐angled impact and quasi‐static indentation, a compressive residual stress was extensively distributed inside the generated crater. No remarkable difference in residual stress distribution was noted between the dynamic case and the quasi‐static case. However, at an impact angle of 45°, a tensile residual stress that is more detrimental to fatigue strength was widely distributed inside the crater. Outside of the craters, tensile stress was generally observed in all cases.
To quantitatively estimate rolling contact fatigue strength as a crack problem, rolling contact fatigue tests were performed on JIS-SUJ2 bearing steel using specimen plates into which small holes of various diameters and depths had been drilled. In all the tests, fatigue cracks initiated at the edge near the bottom of the hole, and then propagated by the shear-mode. Even in the specimens that did not fail after being tested up to N = 2×108 cycles, short fatigue cracks were found at the edge. The fatigue life Nf plotted against the maximum contact pressure qmax varied greatly on the basis of the diameter and depth of the hole. The effect of the depth of crack initiation on Nf was uniquely characterized by using the nominal shear stress amplitude τa, instead of qmax Further, by considering the rolling contact fatigue strength as a small shear-mode crack problem, the fatigue life data was plotted using the novel parameter, τa/(√area)−1/6, where the area is a projected area of the hole. Consequently, all the fatigue life data was successfully fitted to a unified line irrespective of the diameter and depth of the hole, i.e., a defect size dependence on the rolling contact fatigue strength was manifested in a small crack regime.
A type 329Jl duplex stainless steel was gas tungsten arc welded without filler material in an Ar–N 2 gas mixture atmosphere with the aim of changing only the nitrogen content in the weld metal. The effect of nitrogen on the microstructure and corrosion properties of the weld metal was examined. An increase in nitrogen partial pressure increased the nitrogen content of the weld metal and brought reductions in the ferrite content and the quantity of Cr 2 N nitride precipitates. Three corrosion parameters, namely, critical pitting temperature (CPT), pitting potential, and corrosion rate, were measured for weld metals having different nitrogen contents. The CPT and pitting potential increased and corrosion rate decreased with increasing nitrogen content of the weld metal. The corrosion behaviour was explained in terms of changes in microstructure and pitting index depending on the nitrogen content of the weld metal.
To investigate the influence of hydrogen on the tensile and fatigue life properties of welded joints of 304/308 austenitic stainless steels, slow strain rate tensile (SSRT) tests and fatigue life tests were conducted in laboratory air using hydrogen exposed specimens. The specimens were fabricated from welded plates, and to elucidate the role of weld structure on hydrogen-induced degradation, the welded joint was solution-treated. In the SSRT tests of the as-welded (AW) joint, a non-exposed specimen failed at the base metal (BM), whereas a hydrogen-exposed specimen failed near the weld toe. In the case of the solution-treated-welded (STW) joint, the non-exposed specimen failed at the part of solution treated weld metal, whereas an H-exposed specimen failed near the weld toe. As a result, internal hydrogen significantly degraded the elongation of the AW joint. In the fatigue test, all the specimens failed near the weld toe. Internal hydrogen degraded the fatigue life considerably. However, the pre-charging led to little, if any, reduction in the fatigue limit. Similarly to the AW joint, hydrogen gas exposure notably degraded the fatigue life of the STW joint and led to little reduction in the fatigue limit. To investigate the relationship between the hydrogen-induced degradation and strain-induced martensitic transformation during fatigue testing, the volume fraction of ferrite in the broken specimens was measured by a ferrite scope. The volume fraction of martensitic transformation increased with an increase in the stress amplitude. These experimental results implied that the hydrogen-induced fatigue life degradation in the welded joint was closely related to the martensitic transformation during the fatigue process. The mechanisms of both the degradation in fatigue life and nondegradation in fatigue limit will be discussed further.
Effect of small defect on the flaking strength of rolling bearings (Part 2: Evaluation of the flaking strength of rolling bearing having a small drilled hole based on stress intensity factor)
