| An experimental study is conducted to examine the effects of the state of stress on the fundamental nature of distributed fatigue damage in a random short fiber composite. The project consists of two different tasks: first, to develop a new experimental method, for conducting combined shear and axial fatigue damage in composites which are difficult to fabricate in axisymmetric configurations; and second, to study the evolution of the multiaxial fatigue damage in the material. A sandwich specimen, consisting of SMC composite skins bonded to an aluminum honeycomb core, is proposed. Anisotropic finite element analyses are used to obtain detailed stress and deformation fields in the composite facing and in the core of specimens with optimum geometry. The macroscopic fatigue damage is defined and measured as the relative change in the elastic stiffness tensor of the composite. Multiaxial extensometry is developed for strain measurements, and an approximate analytical technique is evolved for online prediction of the multiaxial stress state in the damaged material. Measurements of residual modulus are conducted, after multiaxial fatiguing, to provide additional information on the residual stiffness of the composite. The magnitude and rate of stiffness degradation are found to depend not only on the level of stress but also on the state of stress. Combined stress states are found to be particularly detrimental to the degradation rate of shear modulus. Microscopic damage parameters such as crack length distribution and crack orientation distribution are monitored through non-destructive replication and subsequent quantitative microscopy techniques. |
