On the determination of delamination toughness by using multidirectional DCB specimens

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: [alpha/-alpha 2/alpha/-alpha/alpha 2/-alpha](sym) (or) (anti-sym) and [0/alpha/-alpha/0(2)/-alpha/0/alpha/0(2)/alpha/-alpha/0](sym), with alpha = 0 degrees,15 degrees,30 degrees,45 degrees,90 degrees. A finite element analysis shows that the non-uniformity ratio beta=(G(Imax)-G(Iav.))/G(Iav)% depends not only on the parameter D(c) = D(12)(2)/(D(11)D(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 beta<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). (C) 2010 Elsevier Ltd. All rights reserved.