A Microscale Approach to Identify Damage Mechanisms in Thermally Cycled CFRP

Kurzfassung

For the performance and the reliability of composite hydrogen vessels, the understanding and prediction of the fatigue behavior of carbon fiber reinforced plastics (CFRP) under both mechanical cyclic loading and thermal cyclic loading is essential. Cyclical thermal load induces internal stresses on two scales: (1) Internal stresses on the micro-scale occur due to the different thermal expansions of the carbon fibers and the matrix material; (2) on the macro-scale, the direction-dependent thermal expansion of unidirectional (UD) plies induces internal stresses in a multidirectional laminate. Under cyclically varying temperature, these thermally induced stresses may cause fatigue damage in the material. Studies comparingly investigating the damage mechanisms in mechanically and thermally cycled composites reveal different crack patterns and damage growth kinetics for each load type. These apparent differences in the observed damage mechanisms are not yet fully understood. The present research contributes to the understanding of the damage mechanisms by investigating the following hypothesis: The damage modes and the damage mechanisms in a UD composite are equal for mechanical and for thermal cyclic loading. The different internal stress states induced by the individual load types cause the observed differences in damage initiation and progression. For examining this hypothesis, a modelling approach is derived to simulate the micro-scale damage initiation and progression in UD composites under mechanical and under thermal fatigue loads. A representative volume element models the UD composites’ microstructure composed of carbon fibers and the surrounding epoxy matrix. The constitutive model of the matrix considers its non-linear stressstrain-response, its pressure-dependent failure, the mean stress effect of the fatigue behavior and the temperature dependency of the material parameters. By means of a progressive damage analysis, the initiation, progression and the final crack pattern of matrix cracks within the UD composite under cyclic mechanical loads (tension, compression, shear) and under cryogenic thermal cyclic loading are simulated. For these load cases the predicted damage mechanisms (localization of damage initiation, final crack pattern, kind of driving stress state) are evaluated and compared. This comparison verifies the formulated research hypothesis: The damage mechanisms occurring in UD composites are connected to the pressure-dependent failure behavior of the matrix. The local distribution of dilatory and deviatoric stress components in the composite determines the crack initiation and the final crack pattern. Thus, the damage mechanisms are the same for mechanical and for thermal loading. The different induced stress states in the matrix lead to the observed different damage initiation, progression and final crack pattern.