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

本研究探讨在金属未开始熔化或微结构未发生变化的条件下，利用激光照射来提高疲劳强度的可能性。使用的金属是SUS 304。结果表明，只要满足适当的条件，就可以提高疲劳强度。激光辐照金属表面时，影响疲劳强度的主要因素有两个：1。热应力引起的残余应力; 2.在金属表面形成的粗糙的几何形状。残余应力是指在引起金属组织变化的温度以下，由于热应力而产生的塑性变形。首先，通过非定常弹塑性有限元分析表明，残余应力的分布与试件的几何形状有很大关系。其次，通过电子显微镜观察，阐明了金属表面粗糙度是影响金属表面粗糙度的主要因素， ...更多信息 与激光照射形成的氧化膜相关。在这种情况下，表面出现许多裂纹，因此疲劳强度降低。如果激光照射在圆棒型试样上，则残余压应力保留在表面上，因此期望提高疲劳强度。事实上，如果压缩残余应力伴随着表面粗糙度，则疲劳强度会意外地降低。然而，如果获得不伴随表面粗糙度的压缩残余应力的状态，则预期疲劳强度得到提高。结果表明，这些条件是实现的，如果激光照射附近的缺口部分的圆棒试样从分析使用有限元分析。用缺口圆棒试件的实验结果证实了这一理论。激光辐照试样的疲劳强度比未辐照试样有一定的提高。这一原理有望在今后得到广泛应用。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