INTERLAMINAR FRACTURE-BEHAVIOR OF SANDWICH STRUCTURES

The experimental determination of mode I interlaminar fracture toughne ss (G(IC)) values within the composite material core of sandwich struc tures, by using double cantilever beam (DCB) specimens, is strongly af fected by-amongst other factors-the strain rate applied to the specime n. On the other hand, phenomena such as fibre bridging and crack-tip s plitting are closely related to the applied strain rate, so that exper imentally determined G(IC) values are expected to be rate-dependent. A nother difficulty for G(IC) evaluation is the unknown difference betwe en the real and measured crack length. Thus, although a number of diff erent data reduction methods have been developed in the literature for the determination of G(IC) values, a great discrepancy between predic ted G(IC) values has emerged. In the present paper, sandwich structure s consisting of two aluminium skins bonded together with an epoxy-base d adhesive reinforced with woven E-glass fibres were tested in the for m of DCB specimens for the determination of their G(IC) values. Due to the non-linear and inelastic behaviour of the system, both the classi cal simple beam theory as well as the usual 5% compliance approach fai led to predict G(IC). Thus, a new approach based mainly on experimenta l observation is suggested here for the determination of G(IC) The res ults are in good agreement with those derived by applying a model deve loped by Bishop and Drucker, further developed by Devitt ed al. and re cently completed and presented in its final form by Williams and Freem an. Moreover, a strong effect of the applied strain rate on the G(IC) values was observed.