NON-PROPAGATION CONDITIONS (DELTA-K(TH)) AND FATIGUE-CRACK PROPAGATION THRESHOLD (DELTA-K(T))

Fatigue crack propagation threshold values have been determined with two experimental methods, i.e., the constant R method and the constant K(max) method. Three materials, namely Al7075-T7351 and Ti6Al4V 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 DELTAK(th) on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the da/dN - DELTAK(eff) methodology are applied to the non-propagation condition DELTAK(th). Parameters such as K(op), the threshold DELTAK(T), and a parameter ''K(LL)'' are investigated by numerical modelling of their individual influence on the DELTAK(th) versus R curves. This modelling work shows that the individual DELTAK(th) versus R curves are primarily dependent on the K(op) behavior of the respective material. Further, it is shown that the threshold DELTAK(T) is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the DELTAK(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 DELTAK(T) values, the DELTAK(th) versus R curves of the three materials investigated were accurately reconstructed. It follows that the DELTAK(th) versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely DELTAK(eff).