Apparent Flexural Modulus and Strength of Multimodulus Materials

Abstract

The ASTM flexure test is commonly used to measure the flexural modulus and flexural strength of materials other than the plastics for which it was originally developed. The officially designated equations for reporting the test results are applicable only to materials that are linear elastic to failure. However, the test is often used for carbon-carbon and other composite materials which have different (and sometimes nonlinear) stress-strain curves under tension loading than under compression loading. Such materials are called multimodulus materials. The influence of the multimodulus characteristic on the behavior of a test beam is evaluated for two representative carbon-carbon composite materials. The ASTM flexure test is found to be inapplicable to this class of materials because of the multimodulus characteristic and the nonlinear stress-strain behavior.