| Voids are one of the most common flaws encountered during the fabrication of polymer composites. Mechanical properties of polymer composites are adversely influenced by the presence of manufacturing induced voids. Consequently, if polymer composites are to be successfully employed as structural materials, insight into the effect of voids on the performance of composites is highly desirable.; In this dissertation, systematic approaches are provided to investigate the effects of fabrication induced voids on the elastic properties of polymer composites. Firstly, micro-mechanics models which can predict the elastic properties of voided polymer composites have been developed. The void size, shape, content, and location are considered in the micro-mechanics models presented. Different scenarios for voids in the fiber reinforced composites are considered and their effects on deterioration of the elastic properties of the composites are analyzed. Secondly, finite element models based on the super-element and RVE have been developed. These models are extremely efficient and flexible and can be used to simulate composites with a wide variety of voids, even for some special cases, such as cracks at the interface between fibers and matrix, which current micro-mechanics models have difficulties dealing with. Several finite element simulations are conducted to validate the micro-mechanics models for fiber tow reinforced composites with voids. Very good correlations between micro-mechanics model prediction and finite element modeling results are demonstrated. For cracks at the fiber interface, the extremely deleterious effects of the cracks on the elastic properties of composites are observed. Thirdly, verification of the micro-mechanics models by experiment is provided. For composites reinforced by fiber filaments with large voids, the analytical predictions show excellent agreement with available experimental results. For the fiber tow reinforced composites containing micro voids, a unique experimental procedure, which can control both the porosity and the size of in-tow voids, is developed. The analytical predictions agree well with the experimental results. Last, a procedure for the analysis of voids in the composite structure is presented. This procedure will help the composite design engineer to successfully optimize the combination of reinforcement fiber, matrix, and manufacturing process. |
