Direct measurement of longitudinal compressive strength in carbon fibres

Abstract

A novel method for the direct measurement of the compressive strength of carbon fibres has been developed. PAN-based carbon fibres were examined at specimen gauge lengths from 45 to 200 μm; at long gauge lengths, the fibres exhibited a typical buckling deformation which invalidates longitudinal compressive strength measurement; at short gauge lengths, the measured compressive strength increased with decrease of the gauge length, probably due to irregular interior stress distributions resulting from end-clamp effects. However, a buckling-free and clamp-free compressive strength could be obtained over a limited range of gauge lengths. These directly measured longitudinal compressive strengths have been compared with results from the tensile recoil method. The inherent drawbacks of the recoil method were clearly indicated, particularly with respect to the buckling behaviour of long specimens. It was also confirmed that the compressive strengths of the carbon fibres studied were very much lower than the tensile strengths, potentially a serious limitation to their end-use performance.