Study of Fatigue Strength Simulation under Corrosion Environments

本研究是在腐蚀环境下的模拟方法的发展和提出。构件在腐蚀环境下承受脉动载荷时的疲劳强度仅按计算要求计算，不进行腐蚀疲劳试验。这是基于对腐蚀环境下裂纹萌生和扩展行为的研究。提出了将静态腐蚀坑生长速率与断裂力学方法相结合并进行分析的方法。采用提出的仿真方法进行计算。并将模拟疲劳强度与腐蚀疲劳试验结果进行了比较。验证了所提出的仿真方法的有效性。发现了在腐蚀环境下脉动载荷条件下的疲劳强度模拟是可行的。主要结果如下。1)提出了一种基于断裂力学的基于早期坑尺寸随时间推移而变化的模拟方法，并通过静态腐蚀的方法要求在任意早期坑尺寸下的寿命。2)提出了一种要求裂纹发生后直接得到腐蚀环境下裂纹扩展速率图和假定裂纹发生条件的模拟方法。3)模拟得到的疲劳强度与腐蚀疲劳试验、模拟得到的定量到定性都能很好地吻合。4)在疲劳裂纹扩展寿命的大部分时间里，裂纹深度都是在厚度的一半范围内生长的。5)尽管腐蚀坑在材料表面大量发生，但如果不考虑这些动态干扰，腐蚀坑可能是由一个主坑扩展而来的表面裂纹的模拟。6)虽然提出的方法有可能在脉动载荷条件下进行仿真，但结果无法得到证实。

This research is the development and proposed of a simulation method under a corrosion environments. The fatigue strength of the member that receives a pulsating load under corrosion environment is requested by calculation, without carrying out a corrosion fatigue test. It did on the basis of a study of crack initiation and propagation behavior under a corrosion environment.The method that combines and carries out a static corrosion pit growth rate and a fracture mecanics method was proposed. It calculated by using a simulation method that proposed. And it went a comparison between simulated fatigue strength and corrosion fatigue test results. The efficacy of the simulation method that proposed was confirmed. That the simulation of the fatigue strength in a pulsating load condition under corrosion environment is possible was found out.The main result are as follows.1) A simulation method that it requests because an early period pit size change with the time progress and request the lifetime under an optional early period pit size by static corrosion by fracture mechanics was proposed.2) A simulation method that requests a crack propagation rate diagram under corrosion environment directly after crack occurrence and supposea crack occurrence condition was proposed.3) That the fatigue strength by the simulation and corrosion fatigue test, by simulation are agreeing well to quantitative to qualitative was able to confirm.4) As for most of a spread lifetime of a fatigue crack depth is spent in a growing period to a half extent of the thickness.5) A corrosion pit is possible a simulation with surface crack spread from one main pit, if those dynamic interference are disregarded, although it greatly occurs in a material surface.6) Although it is possible a simulation by the method that proposed even in pulsating load condition under the result was not able to confirm.