Development of a True Triaxial Testing Facility for Composite Materials

Abstract

An electromechanical testing facility capable of applying any combination of tensile and/or compressive forces to three mutually orthogonal axes of a thickness-tapered composite cruciform specimen was designed, fabricated, assembled, and evaluated. Any stress ratio in biaxial (σ1 - σ2) or triaxial (σ1 - σ2 - σ3) stress space can be explored using this computer-controlled test facility. A brief description of the testing machine and its capabilities as well as the present test specimen design is included. Once fully assembled, uniaxial and biaxial tests were performed on 6061-T6 aluminum using this facility. The excellent agreement between the uniaxial and biaxial results obtained in the present study for this material, with accepted handbook values and applicable failure theories, confirmed the performance of several aspects of the testing facility. These aspects included the intra-axis alignment, machine compliance, specimen fabrication and testing procedures, automated computer testing algorithms, data acquisition algorithms, and calibration values. In addition, biaxial and triaxial tests were performed on an AS4/3501-6 carbon/epoxy cross-ply laminate. While most of these tests are considered valid, they revealed aspects of the present thickness-tapered cruciform specimen design that could be improved. More specifically, an undesirable failure mode was encountered in some biaxial tension tests, and triaxial tension tests were not performed successfully. Nevertheless, the overall acceptable performance of the triaxial testing facility is believed to have been demonstrated.