Development of an experimental facility for biaxial compressive testing of fiber composite materials

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