Based upon extensive experimental testing and analytical modeling of compression test methods at the University of Wyoming and elsewhere during the past several years, a specific procedure is recommended for obtaining representative axial compressive strength design values for continuous fiber composite materials. A specimen in the form of an untabbed, straight-sided strip of a [90/0]ns cross-ply composite laminate is tested in a newly developed Wyoming combined loading compression (CLC) test fixture. The standard specimen is 140-mm (5.50-in.) long, 12.7-mm (0.50-in.) wide, and of uniform but arbitrary thickness. This length produces a 12.7-mm (0.50-in.) gage section in the fixture. By increasing the torque applied to the fixture clamping bolts, the ratio of shear-loading to end-loading is readily increased as required to avoid end crushing of the untabbed cross-ply specimen. Typically, the required torque is low, in the range of 2.26 to 2.83 N-m (20 to 25 in.-lb). Classical (linear) lamination theory is adequate to calculate a "back-out factor," which is used to determine the unidirectional ply axial strength. Using a Hercules AS4/3501-6 carbon/epoxy composite material, it is demonstrated that the axial compressive strength obtained is fully comparable to the stress at failure in the 0° plies of laminates of any general lay-up configuration, and also to that measured for unidirectional composites tested using special mini-sandwich and thickness-tapered test specimens.

The development of suitable specimen configurations and loading methods for the compression testing of high strength composite materials has received considerable attention during the past decade, and especially during the past five years. Both experimental and analytical investigations of very specific aspects of specimen and test fixture configurations have been performed. Many seemingly conflicting results have been presented, leading to considerable confusion within the composite materials testing community. However, a definite conclusion appears to now be emerging, viz., the use of tabs on compression test specimens has a detrimental influence on measured strength. This has been qualitatively suspected for some time since analytical studies and detailed finite element analyses consistently predict induced stress concentrations at the tab ends of the specimen gage section. Numerous approaches have been followed to minimize these stress concentrations, of course including the total elimination of tabs. Key analytical and experimental results, taken from the extensive published literature as well as from the author's own recent work, are presented and compared, to demonstrate the consistent trends that actually do exist in the seemingly scattered and confusing published literature. Finally, options currently available for the successful compression testing of high strength composite materials are presented.

An instrumented tensile impact test system with partial loading capabilities has been developed and tested. Hardware development included a low mass gripping system for tensile impact and a vibration-damped base and load cell. A standard pendulum type impact testing machine was modified for use. The rate sensitive material properties of unidirectional glass/epoxy and graphite/epoxy composite materials were investigated. The micromechanics of fracture within the composite caused by impact were studied, using partially loaded impact test specimens and scanning electron microscopy.

Graphite/epoxy composite laminates of T300/BP907 and AS6/HST-7 in layup orientations of [±35/0/90]s and [±302/90/ ̄9 ̄0]s were axial tension fatigue tested. Tests were conducted at 5 and 10 Hz, and at loading ratios of R = 0.1 and R = 0.5. Edge delamination as a function of number of fatigue cycles was monitored by monitoring stiffness reduction during fatigue testing. Delamination was confirmed and documented using dye-enhanced X-ray and optical photography. Critical strain energy release rates were then calculated. The composites delaminated readily, with loading ratio having a significant influence. Frequency effects were negligible.

Using a thickness-tapered cruciform specimen, uniaxial and biaxial tests were performed on 6061-T6 aluminum as well as biaxial and triaxial tests on an AS4/3501-6 carbon/epoxy cross-ply laminate. Designed in conjunction with a new triaxial test facility, the performance of the cruciform specimen was analyzed using an anisotropic, linear-elastic, finite element analysis. The results of this analysis, as well as the experimental data generated for both the aluminum and AS4/3501-6 carbon/epoxy material systems, are presented. An undesirable failure mode was observed in several biaxial tension tests, and the triaxial tension tests were not performed successfully. This study represents the initial effort using this specimen design and test facility, and several recommendations to improve the performance of the thickness-tapered cruciform specimen in future studies are presented. The potential of the triaxial testing facility and the thickness-tapered cruciform specimen has been demonstrated.

Three compression test fixtures, the IITRI (Illinois Institute of Technology Research Institute), Wyoming-Modified Celanese, and Wyoming-Modified IITRI fixtures, are directly compared by testing Hercules AS4/3501-6 carbon/epoxy unidirectional composites in axial compression. The principal characteristics of each fixture are described, and the relative advantages and disadvantages of each are discussed. It is demonstrated that, if proper fixtures and test procedures are used, all three test fixture configurations produce equivalent results for both axial compressive strength and modulus.