On the determination of delamination toughness by using multidirectional DCB specimens

Abstract

This work covers the problems encountered in correctly determining mode I interlaminar fracture toughness of composite materials. Pure mode I tests were performed on double cantilever beam (DCB) specimens composed by quasi-homogeneous and uncoupled multidirectional (MD) laminates using 16 or 26-ply: [α/−α 2/α/−α/α 2/−α] sym or anti-sym and [0/α/−α/0 2/−α/0/α/0 2/α/−α/0] sym, with α = 0°,15°,30°,45°,90°. A finite element analysis shows that the non-uniformity ratio β=(G Imax−G Iav.)/G Iav% depends not only on the parameter D c = D 12 2/(D 11D 22), but also on the specimen geometrical ratios a/b and a/h. The condition of Dc<0.25 is not sufficient to assure a uniform G I width-wise distribution. If we want to study the crack growth between any ply angles, it is difficult to find lay-ups having β<10%. In fact, the crack initiation in MD DCB specimens usually occurred at the middle of the specimens, where G I attained a maximum. Hence, the critical energy release rate G IC has to be measured by the maximum instead of the mean of G I.