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

Abstract

Abstract— Fatigue crack propagation threshold values have been determined with two experimental methods, it., the constant R method and the constant Kmax 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 ΔKth on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the da/dN–ΔKeff methodology are applied to the non‐propagation condition ΔKth. Parameters such as KOp, the threshold ΔKT, and a parameter “KLL” are investigated by numerical modelling of their individual influence on the ΔKth versus R curves. This modelling work shows that the individual ΔKth versus R curves are primarily dependent on the Kop behavior of the respective material. Further, it is shown that the threshold ΔKT is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the ΔKeff concept is applied to the experimental results obtained in the first part. Using either experimentally or semi‐empirically determined Kop functions and the measured ΔKT values, the ΔKth versus R curves of the three materials investigated were accurately reconstructed. It follows that the ΔKth versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely ΔKeff