Relationships governing fatigue crack propagation in structural steels in dual-frequency loading

Abstract

1. In comparison with single-frequency loading, the magnitude of increase of the rate of FCG in dualfrequency loading of the 15Kh2MFA (I) and 15Kh2MFA (II) steels increases with an increase of the amplitude and frequency ratios. 2. The effect of dual-frequency loading on the rate of FCG starts to operate when the specific value of the summary SIF (Konset) is exceeded; this value decreases with an increase of the amplitude and frequency ratios. The start of operation of the effect of the high-frequency cycles on the rate of FCG in dual-frequency loading can be predicted using the dependence of the threshold ranges of SIF on the cycle asymmetry coefficient determined in single-frequency loading. 3. The effect of dual-frequency loading with the triangular form of the low-frequency cycle on FCG in 15Kh2MFA (I) steel is more damaging than that of dual-frequency loading with the trapezoidal form of the lowfrequency cycle in the amplitude ratio variation range from 0.05 to 0.2. This is expressed not only in a reduction of the level of Konset but also in an increase of the rate of FCG with other conditions being equal. 4. Application of the LDS hypothesis for predicting the rate of FCG in dual-frequency loading is justified in the conditions which do not lead to any changes in the failure mechanisms operating in single-frequency loadloading, and also in cases in which the range of SIF in the high-frequency cycles exceeds the threshold ranges of SIF which correspond to these cycles and were determined on the basis of the results of single-frequency tests. If these conditions are not fulfilled, the LDS hypothesis yields too low values of the rate of FCG in structural materials.