Mode i quasi-static delamination growth in multidirectional composite laminates with different thicknesses

Multidirectional DCB specimens with different thicknesses were manufactured and tested to have in-depth understanding of delamination behavior in composite laminates. The initiation crack growth is demonstrated to be ply orientation and thickness independent. However, interlaminar resistance and damage mechanisms in delamination growth are significantly related to the interface configuration as well as thickness. Fractography analysis demonstrated that resistance difference between unidirectional and multidirectional DCB specimens is related to damage mechanisms. Optical microscope observation revealed that crack path in the multidirectional specimens is zigzag. This phenomenon becomes vague with thickness increase. The appearance of zigzag crack indicates both interlaminar and intralaminar damage can occur in the delamination growth. However, interlaminar damage becomes the dominant failure mode in the thick specimens. SEM (Scanning Electron Microscope) observation demonstrated fibre prints and cusps are two typical morphologies located on the fracture surface. These features are even more significant in the multidirectional interface with thickness decrease. This paper will conclude that interface configuration and specimen thickness have significant effects on the damage mechanisms and interlaminar resistance during delamination growth in composite laminates. It is, therefore, insufficient to apply the unidirectional DCB specimen with a given thickness to determine delamination growth and damage mechanisms of a composite material.