Fatigue crack growth rates in corner notched specimens of forged Ti-6Al-4V, used in the manufacture of rotating aero-engine components, have been measured for fatigue loadings that combine major and minor stress cycles. The loadings are simple representations of the flight cycle and the potential in-flight vibrations, each loading block consisting of high-frequency minor cycles superimposed on the major cycle dwell at maximum load. The crack growth rates are dependent on the number and stress ratio of the minor cycles, but only when they individually contribute to the growth of the crack. Estimates of the fatigue threshold values and near-threshold growth rates associated with the minor cycles have been made, all potential load history effects having been ascribed to the minor cycle component of the loading. Using this data, satisfactory crack propagation life predictions have been demonstrated for a wide range of test conditions involving the conjoint action of major and minor stress cycles.
NOMENCLATUREda/dBlock = crack advance produced by a loading block consisting of a major [da/dN],,l = crack advance produced by the application of the total or overall [da/dN Jminor = average crack advance produced by the application of a minor stress cycle and several superimposed minor stress cycles stress cycle in a loading block cycle within a loading block n = number of minor cycles per major cycle; the cycle ratio R = stress ratio R,,, = stress ratio of the major cycle &, , , or = stress ratio of the minor cycle K, , = maximum stress intensity factor AKGfl = that part of the applied range of stress intensity factor associated with minor cycle crack growth AK,,,, = range of stress intensity factor associated with the total or overall stress cycle AKminor = range of stress intensity factor associated with the minor stress cycle AKfh,O.l) = derived fatigue threshold value for a minor cycle stress ratio of 0.9 A&, = fatigue threshold value derived from tests involving a combination of major and minor stress cycles
