Effect of Loading Method on Compression Testing of Composite Materials

Abstract

Compression testing methods for composite materials usually employ either shear loading or end loading to apply the compressive load to the specimen. The effect of both loading methods on compression testing of unidirectional composite materials was studied. A combined loading scheme, i.e., the compression specimen was subjected to a combined end and shear loading, was also investigated. The stress distributions within a compression specimen subjected to each of these three different loadings were studied using a three-dimensional finite element analysis. It was found that the shear loading method yielded the most severe stress concentration at the tab tip of the specimen, while the end loading yielded a high shear stress concentration at the loading end of the specimen. The combined loading method was found to be a good compromise to reduce the stress concentration at both the tab tip and the loading end of the specimen, when compared to the shear loading and the end loading methods, respectively. The effect of loading method on a thick-section compression specimen was also analyzed. The end loading method was found to be superior to the shear loading method. The analytical results were also correlated with available experimental data. End crushing of the end-loaded specimen is often experimentally observed, due to the high stress concentration at the end of the specimen as predicted in the present study. However, if the end crushing problem can be overcome, available experimental results show that the end loading method generally yields slightly higher compressive strengths for unidirectional composites than the shear loading method. This agrees with the present analytical results, the end loading method yielding less stress concentration at the tab tip.