The compressive failure of aligned fiber composite materials




Arseculeratne, Ruwan, 1968-

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The present understanding of the compressive behavior of fibrous composites is somewhat limited. Reliable compressive test methods are one of the keys to understanding the complex compressive failure process in these materials. Compressive test devices that are currently in use often encounter difficulties with stress concentrations and instabilities. In addition, current micromechanical approaches have shown only moderate success in predicting the compressive strengths of these materials. This thesis examines some new experimental and micromechanical modeling aspects of compressive failure in fibrous composites. To achieve a better understanding of the compressive failure, two new specimen geometries were used to determine the compressive strength of an AS4 carbon fiber/PEEK composite. The first involved a thin-walled, hoop-wound ring loaded laterally in a confined ring loading device. These specimens reached maximum compressive strains as high as 1.08 % but exhibited a significant amount of scatter. The second involved circular cylindrical rod specimens with tapered and untapered test sections that were loaded axially in a special loading device. The tapered test specimens reached maximum compressive strains that were comparable to the ring specimens (1.04 %) and experienced less scatter. The constant cross section specimens did not achieve the same level of performance as the tapered specimens. However, the confined end condition was able to preserve the failed microstructure in these specimens by limiting the amount of post failure deformation. With the insight gained through these experiments and previous microbuckling modeling efforts, an alternate method of modeling this failure mode was undertaken. The model consists of individual fibers separated by matrix material. The model considers the fibers to be geometrically imperfect and also includes the nonlinearity of the matrix material. This analysis was able to show that a critical mechanism in microbuckling failure is the interaction between the geometric imperfections and material nonlinearity which produces a limit load type response