NON‐PROPAGATION CONDITIONS (Δ<i>K</i>_th) AND FATIGUE CRACK PROPAGATION THRESHOLD (δ<i>K</i>_T)

Abstract

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 Δ K th on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the d a /d N –Δ K eff methodology are applied to the non‐propagation condition Δ K th . Parameters such as K Op , the threshold Δ K T , and a parameter “K LL ” are investigated by numerical modelling of their individual influence on the Δ K th versus R curves. This modelling work shows that the individual Δ K th versus R curves are primarily dependent on the K op behavior of the respective material. Further, it is shown that the threshold Δ K T is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the Δ K 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 Δ K T values, the Δ K th versus R curves of the three materials investigated were accurately reconstructed. It follows that the Δ K th versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely Δ K eff