Experimental demonstration of the in-situ effect under transverse shear

Failure of composite laminates, in load cases where transverse shear prevail, involves shear cracks and delaminations, and yet it is unclear which damage mechanism triggers the other or how it depends on ply-thickness. Combining interrupted interlaminar shear strength tests with X-ray tomography inspections, we compared the damage sequence of [45°/0°/-45°/90°]ns short-beam specimens manufactured with standard- (n=2) or thin-ply (n=4) non-crimp fabrics (fibre areal weights of 134 and 67 gsm per ply). Failure manifested as a load drop in the force-displacement curve. In both materials, we associated the onset of instability to a shear crack tunnelling across the central 902° ply-cluster. The intersection of this crack with the ones developing in the adjacent layers induced a delamination which, in turn, activated the load drop. A 3D elasto-plastic model predicted that, for the same applied load, the ply-cluster of both laminates would display a similar maximum principal stress distribution. However, the thin-ply samples improved the interlaminar shear strength by 34%, evidencing the so called in-situ effect: the resistance to matrix cracking under transverse shear (τ23 in the local coordinates of the 90° plies) increases when ply-thickness is reduced.