Strain Redistribution in Composite Laminates Resulting from Off-Axis Ply Cracking
Katerelos, Dionisiοs G.
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The initial mode of damage in multidirectional composites is the accumulation of matrix cracks in the off—axis plies. This kind of damage develops until reaching a so-called saturation point. Several other damage modes, such as delamination, may appear concurrently or just upon termination of the matrix cracking process due to joining and growth of these cracks. Remote Laser Raman Spectroscopy (ReRaM) has been employed to monitor the local strains in cracked cross—ply composites. The experiment involves the incorporation of an aramid (Kevlar 49®) fibre Raman strain sensor into the 0° ply and near the 0°/θ° interface of glass—fibre reinforced epoxy resin laminates. This type of laminate is transparent due to the matching of the refractive indices of glass fibres and epoxy resin. Thus, the changes in the longitudinal strain in the 0° ply, caused by transverse cracking in the θ° ply, are quantified. In this work, the results obtained from [0/90/0], and [0/45/0]t laminates that have been produced under identical conditions, are reported. The strain magnification in the 0° ply caused by 90° or 45° cracking are measured at different levels of loading. A relationship between the values of strain magnification and the distance between the fibre and the 0°/θ° interface has been derived. Finally, important conclusions concerning the mechanisms of strain redistribution as well as the conditions of crack growth and crack—crack interaction are drawn from this work.