Delamination migration in multidirectional composite laminates under mode I quasi-static and fatigue loading

Delamination migration is particularly critical in multi-directional composite laminates and is often observed in different loading scenarios and components. Further understanding on the migration mechanism, especially the similarities and differences in the quasi-static and fatigue delamination migrations, is important for the design of composite structures. In this study, the process of delamination migration under mode I quasi-static and fatigue loadings was experimentally investigated for specimens with a +θ/−θ centreline interface. Specimens, with a specially designed stacking sequence, which allows migration events using a simple Double Cantilever Beam set-up, were tested for θ = 75° and 60°. Delamination migration via intralaminar ply splitting has been observed and this was confirmed by the X-ray computed tomography scan results. All the specimens from both quasi-static and fatigue loadings had a fairly similar sequence of damage events; delamination grows through the −θ and +θ ply block successively until it reaches the 0° ply that prevents further migration. The delamination paths and shape of fracture surfaces were observed to be the same, while the Scanning Electron Microscope fractography results showed that the quasi-static fracture surface was rougher in comparison with that of fatigued specimens. In addition, the distances of migration points from the pre-crack tip were slightly smaller in the fatigue specimens, which may indicate a greater propensity for migration under fatigue loading. This study provides important guidelines to the damage tolerance design of multidirectional composite structures and the verification of advanced numerical modelling technologies.