Abstract
The mechanical performance of composite materials is intricately related to the arrangement of reinforcing constituents and defects within a matrix material. In particle-reinforced syntactic foam composites, as well as fiber-reinforced polymer- and ceramic-matrix composites, damage tends to propagate rapidly from pores, cracks or other intrinsic weak points in the material, such that composites express strong flaw sensitivity. As conventional, surface-based experimental measurement techniques cannot access these internal features, it has been challenging to precisely quantify the influence of defects on the overall mechanical response of the composite. To address this limitation, a new 3D, quantitative experimental technique based on in situ X-ray Computed Tomography (XCT) mechanical testing coupled with Digital Volume Correlation (DVC) has been developed. The in situ XCT volumetric images capture the composite’s original microstructure as well as damage features that develop under mechanical loading, while DVC provides local deformation measurements in the vicinity of these key microstructural features. In this way, the coupled in situ XCT/DVC approach can uniquely establish the influence of microstructural features on the damage behavior.
This dissertation develops the in situ XCT/DVC technique to study the damage behavior in syntactic foams, polymer-matrix composites and ceramic matrix composites. First, the accuracy of DVC in these materials is validated experimentally, numerically and theoretically, since the microstructures of these materials violate commonly held requirements for accurate DVC measurement. Next, this approach is applied to study the damage mechanisms in elastomer-matrix syntactic foams with glass microballoon (GMB) reinforcement. Through quantitative assessment of the original GMB arrangement, measurements of the deformed GMB shapes, and the DVC strain fields, we show that the damage is strongly influenced by the original arrangement of GMBs. Collapse is favored in tightly-packed, transverse “chains” of GMBs, such that a striated strain pattern develops in the DVC measurement. Further experiments showed that this trend held for syntactic foams over a range of GMB volume fractions, suggesting that damage is explained not only by the global volume fraction of reinforcement but also by the particle clustering. Next, in situ experiments on stir-processed polymer matrix composites identified that local metallic particles controlled the compression response and onset of buckling. Finally, expanding plug tests of braided ceramic matrix composite tubes revealed that damage initiated at tow overlaps, and was influenced by irregular spacing between adjacent tows. These experiments demonstrate the ability of in situ XCT/DVC techniques to capture the effects of local microstructure on damage in composites, and offer new routes to validate numerical and analytical models of composite performance.
