MATRIX CRACKING AND DELAMINATION IN CROSS-PLY LAMINATES IN TENSILE FATIGUE Zahid R. Khokhar, Ian A. Ashcroft, and Vadim V. Silberschmidt Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK KEYWORDS: Cross-ply laminates, CFRP, matrix cracking, delamination, microstructural randomness, cohesive zone modelling INTRODUCTION Due to their high specific strength and stiffness fibre-reinforced composite materials are being increasingly used in structural applications where a high level of performance is important (e.g. aerospace, automotive, offshore structures, etc.). Performance in service of these composites is affected by multi-mechanism damage evolution under loading and environmental conditions. For instance, carbon fibre-reinforced cross-ply laminates demonstrate a wide spectrum of failure phenomena under tensile fatigue, with matrix cracking and delamination being initiated at relatively early stages of loading/service. The mutual effect of transversal (matrix) cracking and delamination can result in significant deterioration of residual stiffness and load-bearing capacity of composite components and should be thoroughly investigated. This paper describes the cohesive-zone modelling of the interaction between intra-laminar (matrix cracking) and interlaminar (delamination) damage mechanisms in CFRP cross-ply laminates. MATRIX CRACKING AND DELAMINATION: EFFECT OF MICROSTRUCTURE The first stage of failure development in CFRP [ ] s n m 90 0 -composites under these loading conditions is characterised by a multiple generation and growth of transverse cracks in inner 90° layers. The place of the matrix crack’s arrest at the interface between 0° and 90° layers is characterised by considerable stress concentration and often serves as a nucleus for initiation of delamination along the interface. These processes of matrix cracking and delamination depend both on the composite’s structure and loading history, but even identically loaded specimens of the same material demonstrate sufficiently different distributions of matrix cracks/delamination zones. It was shown that spatial non-uniformity of local (microstructural) properties at micro level results in a considerable extent of randomness in the axial distribution of matrix cracks in 90° layers, with a scatter in spacing reaching hundreds of per cents [1-4]. Standard approaches and schemes, exploiting equal spacing between neighbouring cracks or using representative volume elements, can not be used to analyse the effect of this randomness. So, the first stage of the study is analysis of ensembles, comprising similar numbers of matrix cracks with adjacent delamination zones along interfaces between 0° and 90° layers of laminates, for various types of spacing – minimum, maximum, average and random – between neighbouring transversal cracks. The respective magnitudes for spacings are taken from experimental data for CFRP cross-ply laminates. The obtained results for tensile and flexural stiffnesses of laminates with different types of spacing vividly demonstrate the necessity to account for material’s randomness.