Unified criterion for fatigue endurance modelling under combined loading
Résumé
Titanium compressors in an aero-engine suffer from very complex loading regimes in service. Rupture of any part in this thermo-mechanical system is life threatening and requires stringent fatigue endurance prediction methodologies. A conservative local approach is currently used in industries to justify the integrity of engine parts under combined static and dynamic loadings and it is deemed to be too limited to describe the progress of microcracks in a continuum. The reasons that any local stress invariant based fatigue criteria are limited are diverse, whether probabilistic or not. Among, is the utter obliteration of stress gradient effects which arise independently from geometrical effects substantiated by quasi uniform gradient and vibratory localised stresses with high stress gradients. The current study appeals to provide a methodology to consider these effects around a “hot-spot”, devised as the crack initiation site using a novel methodology based on the superposition of independent analytical and/or basic numerical stress domain functions and a multiplicative factor fitting the complex loading. Time/Space variables will be split using the Karhunen-Loève decomposition. This method will ease post-processing of complex stress fields so as to extract rupture mechanics parameters using criteria such as in [2], from coarsely meshed domains directly from the a priori known sets of analytical fields, otherwise computationally very expensive. The first phase of the project consists of acquiring a tool to examine a spatio-temporal stress field in a structure. A test-case consisting of a dissymmetric notched geometry is currently being examined under the effect of combined tension and torsion. Subsequent steps will be to post-process the local “hot-spot” zone and trial the current methodology.