Evaluation of a Biaxial Test Fixture.

This report evaluates a test system for examining laminated composite materials subjected to a biaxial state of stress. The system utilizes short, thin-walled cylindrical specimens loaded with combinations of axial compression, internal pressure and external pressure. Fourteen instrumented specimens were tested with gages of five different pattern designs and eleven different gage layouts depending on the type of load applied to the specimens. The test data is reported and the performance, validity and repeatability of the test system is evaluated in the report. It was found that the system was best suited for testing specimens with internal pressure only. The next best test results were obtained with axial load only. Combinations of internal pressure with axial load produced good results, however there was little correlation between the biaxial stress ratio and the shape of the failure envelopes. Very poor and unpredictable results were obtained from the tests using external pressure loads. These specimens failed in buckling at very low loads due to compressive hoop stresses. Author