The growth of cracks in Ti–6Al–4V plate under combined high and low cycle fatigue

Abstract

The fatigue crack growth rates in cross-rolled Ti–6Al–4V plate subjected to combined major and minor stress cycles have been measured at room temperature. The concept of crack closure was used to model the data for a test sequence using 1000 minor cycles per major cycle, and the model validated by either the accurate or safe prediction of the crack growth rates for a second series of tests involving 10,000 minor cycles per major cycle. Fatigue threshold values for the minor cycles derived from the growth rate data for combined major and minor cycle loadings were lower than those determined by the conventional load shedding method. In comparison with the behaviour of Ti–6Al–4V disc material which had been forged, the cross-rolled plate material exhibited: first, a clearly defined bilinear growth rate curve under a separate major cycle loading; second, similar or lower derived threshold values with separate minor cycle loadings; and third, reduced crack propagation lives for loadings combining major and minor cycles.