A TWO-PARAMETER FATIGUE CRACK GROWTH CORRELATION USING AK AND K_(MAX) PARAMETERS

摘要

The paper presents a correlation of fatigue crack growth rate (FCGR) behavior for different load ratios R (= min load/ max load). The proposed method of correlation is explained in detail using comprehensive experimental FCGR data from 2524 aluminum alloy. The consistency of the proposed method has been verified using test data taken from literature for more than 10 different materials including aluminum, steel, titanium and other alloys. For all the materials studied, a transition load ratio R_t was found, which marks the transition between a dominant influence of ΔK or K_(max) on FCGR behavior. For R＞R_t the FCGR is dominated by ΔK. On the other hand, for R＜R_t it is found that K_(max) is to be the dominating parameter. Two equations, in terms of ΔK or K_(max), have been developed to represent FCGR curves for various load ratios. ΔK_(driving) is used to represent FCGR behavior for R＞R_t and K_(max driving for) R＜R_t. The study reveals that the FCGR curves for different R-ratios can be collapsed into two narrow scatter bands, where each band is influence by either ΔK or K_(max). The final correlation was further simplified to a single equation, which represents "the master curve" corresponding to the transition load ratio, R_t. Thus by knowing FCGR data for one load ratio and using the proposed method, FCGR curves for any other load ratio may be predicted within a narrow scatter band.