Experimental Observations on the Delamination Behavior in Composite Structures

Abstract

The objective of the current paper is to present experimental data of interface cracks (delaminations) in layered Glass/Epoxy and Graphite/Epoxy composites under cyclic compressive (fatigue) and static (Mixed Mode Bending and Double Cantilever Beam) loads. The growth behavior ranges from self-similar growth along the interface in the unidirectional configuration to branching out of the interface in 90 deg. and cross-ply configurations. In the case of cyclic compressive loading, the specimens undergo repeated buckling/unloading of the delaminated layer with a resulting reduction of the interlayer resistance. Also, for this case, equations describing the growth of the delaminations under cyclic loads are obtained on the basis of a combined delamination buckling/post-buckling and fracture mechanics model. The latter is based on a mode-dependent critical fracture energy concept and is expressed in terms of the spread in the energy release rate in the pre- and post- buckling state. The growth laws developed in this manner are integrated numerically, in order to produce the delamination growth vs. number of cycles curves. An important characteristic in all the configurations tested is that the state of stress near the delamination tip is of mixed mode, I and II. In the cases where branching (intra-layer cracking) occurs, there is some self-similar crack growth from the initial delamination (simulated as a Teflon sheet inserted in the plate) followed by branching of the crack through the thickness of the plate. Observed behavior ranges from sudden and catastrophic failure induced by the intra-layer crack in the 90 deg. specimen to the alternate successive initiation of intra-layer cracks and secondary interlayer cracks (delaminations) in the cross-ply specimens. Experiments and statistics show that there is a critical branching angle above which crack growth is greatly accelerated. The particular details of the experimental study, including the difficulties and challenges, are discussed.