Crack growth resistance characteristics determination algorithm for creep-fatigue interaction

A creep-fatigue crack growth rate interpretation method and algorithm and test results are described. The algorithm allows to obtain a crack growth rate diagram in terms of creep stress intensity factor. The proposed algorithm is realized on steel compact tension specimens (12.1MF). Crack growth rate was determined on standard compact specimens under temperature 550 degrees C. The waveforms for the loading and unloading portions were trapezoidal, and the loading/unloading times were held constant - 5 s. A hold time of predetermined duration, 60 s, was superimposed on the trapezoidal waveforms at maximum load. The potential drop and the unloading compliance methods were used to monitor crack length during the creep-fatigue tests. For the experimental crack paths in tested specimens the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral was calculated by finite element analysis along crack front. The governing parameter of the stress-strain fields in form of I-n-integral was used as the foundation of the creep stress intensity factor. The creep stress intensity factor approach was applied to the fatigue crack growth. The numerical and experimental results demonstrated that K-cr is the most effective crack tip parameter in correlating the creep-fatigue crack growth rates in power plant materials and can be used reliably for practical purposes. (C) 2016 The Authors. Published by Elsevier Ltd.