Design of a cylindrical specimen for biaxial testing of composite materials

The design and analysis results of a specimen and fixture to study the failure of composite materials under biaxial loading environments are presented. The objective of the current study was to achieve a capability of transmitting higher axial loads than had been achieved previously. The magnitude of the axial load is limited by failure and debonding of the end-tab which is bonded to the composite. Higher axial loads were achieved primarily by designing an end-tab geometry to put the bondline normal stress in compression, which was found to increase the bond shear strength, and by careful material selection for the end grip region. The end-tab was analyzed using finite elements to verify a smooth introduction of load in both the axial and hoop directions. Experiments showed that the grip and apparatus design was successful in achieving gage section failures over a wide range of biaxial test conditions. Biaxial test results are presented for tension-tension and compression-compression tests of carbon/epoxy braid specimens to illustrate the present test methods.