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Experimental evaluation of effective tensile properties of laminated composites

Laminated structures are normally designed using a set of strength values that are determined experimentally. Such values are determined by carrying out experiments on unidirectional test coupons. In this paper it is argued that the predicting the failure of multidirectional laminates based on the lamina to laminate approach cannot be accurately done using lamina properties obtained from unidirectional coupon test. Instead, one needs to obtain the effective properties of laminae as deduced from multidirectional laminate test. A method is therefore proposed to determine the effective tensile properties of a composite laminate. In particular, two types of composite materials were examined, namely carbon fiber-reinforced epoxy and glass fiber-reinforced epoxy. The failure behavior of laminates made from these materials is explained and the effect of multiple cracking in strength reduction is outlined. The experimental results confirm that the failure of multidirectional laminates cannot be accurately predicted using unidirectional laminae properties. Instead, the paper describes how to obtain the effective properties and shows that these are quite different from those obtained using unidirectional configurations.