軟鋼の溶着部,熱影響部における疲れ強さに関する研究

Abstract

It is well known that the fatigue strength of welded part reduces to almost half that of base metal of a mild steel. The conceivable factors to reduce the fatigue strength of welded part are the metallugical discontinuity and the stress concentration at the toe, the undercut of reinforcement etc.In this study, basic experiments were carried out with small notched plate specimens subjected to plane bending in order to determine whether the metallugical discontinuity or the stress concentration at welded joint plays more important parts on their fatigue strength. The notch was located respectively at the base metal, the weld metal and the heat-affected zone (fine-grained zone, spheroidized pearlite zone and embrittled zone) of butt welded joint, the reinforcement cut off by machine.From the test results, it is clear that the fatigue strengths of weld metal and heat-affected zone are larger than that of base metal unless they contain defects like a notch (Fig. 8). The weld metal and heat affected zone are more notch-sensitive than the base metal, especially the spheroidized pearlite zone (Fig.10). The rate of crack propagation on respective zones is less in the intermediate stage of stationary crack growth because of work hardening, while it is large at the initial stage of crack and the prior stage to fracture. The fatigue strength of welded parts depends on the rate of crack propagation in the stationary stage and the crack propagation rate is largest in the spheroidized pearlite zone of welded part (Figs.14, 15). This fact corresponds to the lowest fatigue strength of this structure. It is observed by microscope that the cracks in the heat-affected zone propagate along grain boundary more remarkably than those on base metal.