On the prediction of compressive strength and propagation stress of aligned fiber-matrix composites
Abstract
The compressive strength of aligned fiber (carbon and glass) composites with polymeric matrices has been found to be only 50--60% of their tensile strength. Such composites have fiber imperfections in the form of small misalignments which under compression cause local shearing of the material. These composites exhibit a nonlinear shear response with low yield stress caused by the relatively low shear strength of the matrix. Tim combined effect of the geometric nonlinearity of misalignment coupled with this material nonlinearity am responsible for the reduction in compressive strength. The failure of the material is usually in the form of kink bands inclined to the fiber direction. A three dimensional model of the composite has been developed using an idealized hexagonal array of circular, elastic fibers surrounded by an elasto-plastic matrix. Because post failure examination of actual kink bands revealed that the fibers rotate only in the plane of loading, it is possible to limit attention to a characteristic section of the array. The model is used to predict the strength of the composite in compression and for combined compression and shear. The sensitivity of the critical load to imperfections and to the applied shear are investigated. The post-failure behavior of the composite is then explored. In recent experiments it was discovered that once a kink band is formed, it can grow in a steady-state manner at a constant stress level termed the propagation stress. The propagation stress is approximately 40% of the compressive strength of the material. Thus, the composite can exhibit significant post-failure strength and apparent ductility. A modified version of the 3-D FE model is used to simulate the steady-state axial propagation of an existing kink band. Here, the scope of the study is extended by modeling the matrix as an elastic-power law viscoplastic solid in order to account for the effect of the rate sensitivity of the PEEK matrix on the propagation stress. The model is used to establish the propagation stress and fiber rotation angles inside the band for different loading rates and band inclinations. A representative unit cell model is developed to study the nonlinearities exhibited by AS4/PEEK composites in transverse compression and shear, their interaction, and their rate dependence at room temperature. Two viscoplastic constitutive models, J2-type viscoplasticity and the non-associative Drucker-Prager model, are used to simulate biaxial experiments from three loading histories. The models provide insight into the interaction between shear and transverse compression which helps explain some unusual behavior observed in the experiments.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- November 1999
- Bibcode:
- 1999PhDT.......217H