Evaluation of the IITRI compression test method for stiffness and strength determination

Abstract

The objective of this paper is to evaluate the IITRI Compression Test Method for measuring the axial compressive modulus and strength of composite materials possessing a high degree of anisotropy in the axial direction. The influence of material anisotropy and test specimen geometry on the data reduction scheme used to characterize the axial compressive modulus and ultimate strength is examined. The findings demonstrate that ASTM D3410 specimen geometries, recommended for the IITRI and Celanese compression test methods, may not always be appropriate for materials that are highly anisotropic in nature. The significance of the findings on appropriate specimen design for thick section laminates is also discussed. An error investigation is included which demonstrates to what extent the experimentally determined compressive modulus may be affected if careful consideration of the specimen geometry is not made. This investigation utilizes the finite element method and an elasticity solution based on Saint-Venant's principle for an upper bound estimate on stress decay length in a parametric study involving combinations of specimen geometries with varying material anisotropy. A methodology is presented for sizing specimen geometries as a function of material anisotropy that ensures accurate determination of the linear elastic axial compressive modulus and provides an estimate of the maximum amount of axial strain that can be tolerated prior to Euler buckling induced failure. Based on the degree of material anisotropy and anticipated failure strain, the appropriateness of the test method for a given material system can be evaluated.