Browsing by Subject "Fibrous composites testing"
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ItemDevelopment of an experimental facility for biaxial compressive testing of fiber composite materials(1991) Kotziapashis, Andreas Evangelou, 1962-; Kyriakides, S.The purpose of this study was the development of a testing facility and test specimen for the characterization of laminated composites under biaxial compressive stress states. A biaxial testing facility capable of applying almost arbitrary stress paths in the in-plane compressive stress regime, to circular cylindrical test specimens, was designed, assembled and calibrated. The required biaxial load is achieved by applying combined axial compression and external pressure to the cylinder. The performance of the biaxial facility was verified by performing several exploratory experiments on Aluminum and Graphite/Epoxy specimens. Test specimens and testing procedures were designed such that material failure rather than structural failure would prevail under a prescribed loading path. A circular cylindrical shell was selected over the other possible biaxial test specimens, for its relatively simple manufacture, its potential in achieving a relatively homogeneous stress state within the test section and a boundary region that is relatively free of stress concentrations. An experiment was performed on a circular cylindrical Graphite/Epoxy specimen designed to fail by material failure. The test was conducted under hydrostatic loading. Failure of the specimen occurred at a pressure of 9035 psi. Post failure evaluation of the specimen confirmed that failure was the result of local buckling or “kinking” of the hoop fibers at the outer layers of the specimen ItemThe compressive failure of aligned fiber composite materials(1993) Arseculeratne, Ruwan, 1968-; Kyriakides, S.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