A FATIGUE CRACK-GROWTH THRESHOLD

Fatigue crack growth (FCG) threshold has been determined on Ti6A14V, A1 7475-T7351, quenched and temepered steel 24CrMo4 as well as on the Ni-base alloys Inconel 617 and Nicrofer 5219 Nb. The main objective of the studies was to prove the transferability of the experimentally determined FCG threshold to service type fatigue loading. For FCG threshold testing, the following requirements must be fulfilled: (A) a FCG threshold has to be entirely above K(op); (B) a FCG threshold has to exist and has to be measured over the fatigue loading range K(max), i.e. K(max)* < K(max) < K(Ic); (C) a FCG threshold has to be the upper bound amplitude of cycles at a given K(max), the action of which does not produce FCG no matter how these threshold amplitudes are applied: (D) a FCG threshold must be a material parameter useful for material characterization and FCG design methodology under all fatigue loading conditions. It was shown that all the experimental procedures, i.e. type I, III and IV tests, furnished DELTA-K(T) values for A1 7475-T7351, Ti6A14V and the quenched and tempered steel 42CrMo4. The results suggest that this might be true for all A1 alloys, Ti alloys and steels. Therefore, any one test method can be used to determine DELTA-K(T) for these materials. The ASTM E 647 recommended procedure for threshold testing furnishes DELTA-K(T) values for these materials as well, if the R-ratio is equal to or higher than R(c). For the Ni-base alloys (Inconel 617 and Nicrofer 5219 Nb) only type III and IV tests can be used to determine a FCG threshold DELTA-K(T). This is due to the different behaviour of Ni-base alloys with respect to K(op).