Experimental Study of Laminated Composites Containing Manufacturing Defects Under Combined Stress States

Manufacturing defects are known to cause a knock-down in the mechanical properties of composite materials. These knock-downs are however usually only tested in simple stress states and not the more complex loading as found in real structural applications. Here a modified Arcan test fixture has been designed and manufactured to apply interlaminar shear in combination with through thickness compression to carbon fibre/epoxy composite laminates. The fixture is designed to accommodate a 30 A— 30 mm size specimen and allows the ratio of interlaminar shear to through-thickness compression applied to the specimen to be varied by altering the angle of the fixture. The material used in this study is the Hexply® IM7/8552 and the ply stacking sequence of the laminates is [+452/902/-452/02]3S, giving a nominal specimen thickness of 6 mm. The study included pristine specimens as well as specimens with an artificially embedded out-of-plane wrinkle defect which was generated by inserting a precise pattern of narrow pre-preg strips with 90° orientation across the width of the laminate during lay-up, which during cure merged with adjacent 90° plies in the stack. This experimental study has investigated failure stresses and failure modes for both pristine and wrinkle defect specimens, tested over a range of interlaminar shear to through thickness compression stress ratios.