Evaluation and modelling of the fatigue damage behaviour of polymer composites at reversed cyclic loading
The project in application focusses the analysis of multi-scale damage of endless fibre reinforced composites under reversed cyclic loading. The aim of the project is the physically based generalisation of existing damage models for fibre reinforced composites as an essential part of the fatigue life prediction of composites structures. Due to the inhomogeneous structure of fibre composites on different scales a combined numerical/experimental and multiscale mathematical approach for the investigation of damage has been chosen. The type and quantity of damage due to cyclic loading is defined by the orientation and sign of the load vector in relation to the fibre direction. Specific challenges for realistic modelling of damage and degradation under reversed loading have been identified in the not well understood phenomena of tension-compression-asymmetry in stiffness and strength, the interaction of tension and compression induced damage (passive damage) and the specific influence of tension induced delaminations to subsequent compression loads.For that reason the damage phenomena, the degradation behaviour on filament level (micro level) and on layer level (meso level) are analysed with the help of microscopic and macroscopic cyclic tests accompanied by in-situ computer tomography and photoelastic stress analysis. From additional numerical analyses on micro level, physically based mathematical formulations for the varying stress-strain behaviour under cyclic loading with different stress ratios are formulated. The mathematical approaches developed are subsequently implemented in the FE-based Fatigue-Damage-Model of the Institut für Statik und Dynamik (ISD), for which first validations for pulsating cyclic loads have already been carried out.Thus the present limitation of the known fatigue analysis models to constant amplitude pulsating loads may be overcome and a significant step towards a realistic life time assessment of fibre reinforced materials under spectrum loading is taken.