Fatigue damage characterization in carbon fibre composite materials using an electrical potential technique

Changes in electrical resistance during static and fatigue loading of unidirectional and cross ply carbon fibre reinforced polymer composites have been studied. The carbon fibres in the study were T300 and the matrix resins were Hexcel 914 and 920. It was found that changes in resistance during static tensile testing were about three per cent of the original resistance of the samples, while fatigue testing caused resistance changes of up to 10% of the original resistance, immediately prior to final failure. The initial linear portion of the resistance increase on static testing was reversible and could be attributed to reversible elastic strains in the fibres; later non-linear changes were a consequence of fibre fracture and were irreversible. Changes in resistance during fatigue also contained both reversible and non-reversible components. It was found that during fatigue testing the initial changes in resistance caused by the first few thousand cycles could be correlated with the eventual life. Samples with large initial resistance change had reduced lives compared with those with small changes in resistance. Fatigue lives of composite laminates may thus be predicted from monitoring of initial resistance changes. Many of the results could be explained via the parallel resistance model of conduction in composite laminates.