Micromechanisms in tension-compression fatigue of composite laminates containing transverse plies

For both monotonic and fatigue loading conditions, debonding is the subcrit ical micromechanism which leads to transverse cracking and ultimately influ ences final failure of the composite structure. Previous studies show that tension-compression fatigue is more detrimental than tension-tension fatigu e to transverse and multidirectional laminates. By analysing the debonding mechanisms and modelling thereof, the macroscopic fatigue behaviour can be better understood. Also, the dominant crack-propagation mode can be identif ied which may be of use in selection of constituent material properties to optimise fatigue resistance. In this study, glass-fibre-reinforced vinyl-es ter was used. The adverse effect of compressive load excursions was verifie d by counting the transverse cracks in cross-ply laminates. The mechanisms were studied in low-cycle fatigue of a specimen containing a single transve rse fibre. Compressive load cycles led to significantly increased debond gr owth. In tension, contact zones developed at the crack tips for sufficientl y large debonds. Because of the mismatch in elastic properties, an opening zone appeared at the tips of the interfacial crack when the same debond was subjected to a compressive load. Since debond propagation is more suscepti ble to mode I loading, the sensitivity to tension-compression fatigue is ex plained by the effective crack-tip opening in compressive loading for suffi ciently large debond cracks. This has also been verified by finite-element analysis. (C) 1999 Elsevier Science Ltd. All rights reserved.