INFLUENCE OF SPECIMEN GAUGE LENGTH AND LOADING METHOD ON THE AXIAL COMPRESSIVE STRENGTH OF A UNIDIRECTIONAL COMPOSITE-MATERIAL

Various compression test methods for composite materials have been developed during the past few years incorporating specimens of different gage lengths and modes of load introduction. Most of these test methods are not recognized as standards at the present time. Thus the question arises as to whether these various test methods produce similar results. Two commonly used test fixtures are compared in detail here, viz., the ASTM Standard D 3410 IITRI compression test fixture, and another often referred to as the Boeing-Modified D 695 compression test fixture. The IITRI fixture loads a specimen of 12.7-mm gage length via shear through end tabs and wedge grips. The Boeing-Modified D 695 fixture also uses a tabbed specimen, but with only a 4.8-mm gage length and loaded directly on its ends. Hercules S2/3501-6 glass/epoxy and AS4/3501-6 carbon/epoxy unidirectional composites were used as the primary material configurations in the present comparison. High-strength, highly orthotropic materials such as these are among the most difficult composites to test successfully in compression. However, a carbon/epoxy quasi-isotropic laminate was tested as well. The results obtained are compared with similar data available in the literature. It is concluded that there is relatively little influence of specimen gage length as long as it is sufficiently small so that gross buckling does not occur. Also, the differences between shear loading and end loading of the specimen are small, as long as the ends of the end-loaded specimen do not crush, and that in either case the tabs do not debond.