ＦＥＭによる疲労き裂の開口レベルの評価法に関する研究

Abstract

Recent researches have clarified that the fatigue crack propagation rate has a good relationship with the effective stress intensity factor range (ΔKeff). In order to estimate ΔKeff, however, the fatigue crack opening stress level must be known. Surely, by using the FEM analysis, it is possible to calculate the opening stress, but the calculated value is affected by the minimum mesh size used in the analysis or stress range etc..In this report, the new method for estimating the crack opening stress unconditionally from the calculated residual stress distribution was proposed, and the relation between crack opening stress and stress ratio (R≥0) was investigated based on the cyclic elasto-plastic analysis by FEM. The process of the proposed method was as follows: By using the compressive stress transmitted region on the crack plane and the total compressive stress obtained by FEM analysis, a certain ideal stress distribution on the crack plane was determined, and K value when the same stress distribution was applied on the crack plane was calculated (K*res1). On the other hand, from the stress distribution normalized so as that the loading direction stress of the element involving crack tip became the yield stress, another K value was calculated on the same process mentioned above (K*res2). K*res2/K*res1 was adopted as a correction factor, and K*res·K*res2/K*res1 was determined as K value at crack opening. (K*res was obtained by a superposition model.)As the results, it was recognized that the proposed method gave about the same result under different mesh devisions or stress ranges. And also, the following relationship between the opening stress and stress ratio was obtained.σ*op/Δσ=U0-0.396R+R/(1-R)where σ*op is the crack opening stress, Δσ is the stress range, R is the stress ratio and U0 is the experimental value of σ*op/Δσ under R=0. It was also clarified that the above equation has a good agreemeat with the experimental results.