In-situ SEM study of transverse cracking and delamination in laminated composite materials

Transverse microcrack growth and delamination are two key damage mechanisms in laminated composite materials, and while often treated separately in damage prediction studies, they are, in fact, highly coupled. Essentially, transverse cracks initiate around fibres, coalesce and grow until they extend to ply boundaries, at which point they initiate micro-delaminations. Under increasing load these micro-delaminations eventually coalesce to form macroscopic delaminations, which severely reduce material stiffness and lead to catastrophic failure of the composite structure. This paper presents an investigation into how altering transverse crack densities can influence the growth of delaminations. Novel in-situ SEM micromechanical testing and acoustic damage detection techniques were coupled and used to determine transverse crack initiation loads, transverse crack density, and local micro-delamination lengths for a number of cross-ply laminates. The laminates were loaded in a four-point bending mode to induce crack opening direct stresses on the tension side. To examine the effect of combined direct and shear stresses, the laminates were also loaded in a three-point bending mode, and suitable comparisons between both bending modes allowed for the influence of the shear stress to be isolated. The main variable under investigation is the thickness of the transverse ply block, and it is shown that increasing the number of transverse plies (i.e. thickness) can significantly increase the load carrying capacity of the laminate by reducing the transverse crack density. It was found that the lower transverse crack densities meant that the micro-delaminations which initiated at the ply boundary required significantly greater stress to fully coalesce as the distance between transverse cracks was greater. Once micro-delamination had initiated, its length was found to be linearly related to the load applied. For all layups investigated, the average micro-delamination length seen immediately prior to catastrophic failure was approximately 1.2 times the thickness of the tensile 90° ply portion of the laminate.