NON-PROPAGATION CONDITIONS (ΔKth) AND FATIGUE CRACK PROPAGATION THRESHOLD (δKT)

Abstract— Fatigue crack propagation threshold values have been determined with two experimental methods, it., the constant R method and the constant K_max method. Three materials, namely A17075-T7351 and Ti6A14V STA in the LT- and TL-orientations, and a Ti-turbine disk material (IMI 685) in the CR-orientation, were investigated. The paper is divided into 3 parts. In the first part the test conditions, the experimental results and the conclusions drawn from the experimental results are presented, namely that the three different functional dependencies of ΔK_th on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the da/dN–ΔK_eff methodology are applied to the non-propagation condition ΔK_th. Parameters such as K_Op, the threshold ΔK_T, and a parameter “K_LL” are investigated by numerical modelling of their individual influence on the ΔK_th versus R curves. This modelling work shows that the individual ΔK_th versus R curves are primarily dependent on the K_op behavior of the respective material. Further, it is shown that the threshold ΔK_T is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the ΔK_eff concept is applied to the experimental results obtained in the first part. Using either experimentally or semi-empirically determined K_op functions and the measured ΔK_T values, the ΔK_th versus R curves of the three materials investigated were accurately reconstructed. It follows that the ΔK_th versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely ΔK_eff