CONTROL OF RESIDUAL STRESS USING LASER AND DEVELEPMENT OF PREVENTION TEQUNIQUE FOR FATIGUE STRENGTH

本研究探讨了在金属不熔化或不发生微结构变化的情况下，利用激光辐照提高疲劳强度的可能性。使用的金属为SUS304。结果表明，只要满足适当的条件，疲劳强度的提高是可以实现的。激光辐照金属表面时，影响其疲劳强度的因素主要有两个：1.激光辐照强度；热应力引起的残余应力；金属表面上形成的粗糙几何形状。残余应力仅由低于温度的热应力引起的塑性变形给出，从而引起金属微观结构的变化。首先，通过非平稳弹塑性有限元分析表明，残余应力的分布在很大程度上取决于试件的几何形状。其次，通过电镜观察，明确了金属表面的粗糙度与激光照射形成的氧化膜有关。在这种情况下，表面出现许多裂纹，从而降低了疲劳强度。当激光照射在圆棒型试样上时，压缩残余应力仍保留在试样表面，从而有望提高试样的疲劳强度。事实上，如果压缩残余应力伴随着表面粗糙度，则疲劳强度会出乎意料地降低。然而，如果达到压残余应力不伴随表面粗糙度的状态，则有望提高疲劳强度。有限元分析表明，当激光照射在圆棒试件的缺口部分附近时，可以达到上述条件。用缺口圆棒试样验证了这一理论。激光辐照试样的疲劳强度比未辐照试样明显提高。这一原则有望在未来得到广泛应用。少

This research investigates the possibility of improvement of fatigue strength using the laser irradiation under the condition the melting or the variation of micro structure of metal does not start. Used metal is SUS304. As a result, it is clarified that the improvement of fatigue strength is achievable if appropriate conditions are satisfied. On irradiating laser on the surface of metals, two major factor that influence the fatigue strength exist : 1. Residual stress due to thermal stress, 2. Rough geometry formed on the surface of metals. Residual stress is given by only plastic deformation due to thermal stress below the temperature that provokes the variation of micro structure of metals. Firstly, by the analysis using non-stationary elastic-plastic finite element analysis, it is shown that the distribution of residual stress greatly depends on the geometry of test specimen. Secondly, by observation using electron microscope, it is clarified that the roughness of metal surface is r … More elated to the oxidation film formed by the irradiation of laser. In this case, many cracks appear of the surface and so the fatigue strength is reduced. If the laser is irradiated on circular bar type specimen, the compressive residual stress remains on the surface and so the improvement of fatigue strength is expected. In fact, the fatigue strength is unexpectedly reduced if the compressive residual stress is accompanied by the surface roughness. However, if the state of compressive residual stress not accompanied with surface roughness is attained, the fatigue strength is expected to be improved. It is shown that these conditions are achieved if the laser is irradiated near the notched portion of circular bar specimen from the analysis using finite element analysis. This derived theory is confirmed experimentally using notched circular bar specimen. The fatigue strength for laser irradiated specimen is surely improved compared with that for non-irradiated one. This prinsiple is expected to be applied widely in the future. Less