Static and fatigue behaviour of stitched graphite/epoxy composite laminates

The paper studies the effect of edge stitching on tensile static and fatigue properties of graphite fiber reinforced laminates. The influence of Kevlar stitchings on edge delaminations, ultimate tensile strength and tension–tension fatigue life were examined on laminates of two stacking sequences ([±45/0/90]s and [±30/90]s), as representative of two classes of fiber-dominated and matrix-dominated laminates. Through-thickness stitching offered a significant improvement in the static delamination resistance of laminated composites, but had varying effects on the ultimate tensile stress of the laminates under study, with an increase in strength of [±30/90]s laminates and a decrease in strength of [±45/0/90]s laminates. Similarly the fatigue life of matrix dominated [±30/90]s specimens was considerably extended by stitching, while the fatigue resistance of fiber-dominated [± 45/0/90]s specimens was reduced, particularly at high fatigue stresses. Stitching appeared very efficient in arresting delamination in [±30/90]s specimens for both static and fatigue loads, but did not prove to be as efficient in stopping fatigue delaminations in [±45/0/90]s specimens, since Kevlar threads were found to be prone to breakage under cyclic loading. As a conclusion, while the static delamination resistance of laminated composites is improved by stitching, this does not automatically result in better fatigue performance for all lamination sequences.