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.

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The paper describes tests on a 0–28 per cent carbon steel in tension and compression, and in flexure of beams of rectangular cross-section, to a maximum strain about three times that at the initial yield. The object of these tests-was to investigate the shape of the stress-strain curve immediately after the initial yielding, and to determine whether in a case such as flexure the upper yield stress could be relied upon as a criterion of design. The results from this material indicate that the stress-strain curve falls rapidly but not immediately from the upper to the lower yield value, and that a beam is capable of withstanding a slightly greater bending moment than would be predicted by calculations based on the direct stress tests including the upper yield stress.