The measurement of the interlaminar shear strength of glass-reinforced plastic (GRP) laminates is proposed using an offset four-point bend test on a beam where two (P) of the four forces differ from the other two (Q) and the ratio λ = P/Q depends upon the relative positions along the beam. The paper describes the development of this test which appears simpler, more economic and more realistic in many aspects than other test methods. It is demonstrated theoretically and experimentally that the span length (L), thickness (t) and the ratio λ are important test parameters in determining whether the failure is by shear or by flexure. A test rig was designed and constructed to investigate the influence of these parameters and to determine the interlaminar shear strength of a GRP laminate. Specimens were made from polyester resin containing E-glass fibres in the form of chopped strand mat (CSM). Clear and repeatable interlaminar shear failures have been obtained where L/t was relatively small and Λ large. The crack formation mechanism was studied using photography.
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