Oblique plies for steering through-thickness delamination migration in fibre reinforced polymers

The damage tolerance of safety critical aerospace structures is of paramount importance to ensure the airworthiness and safe operation of an aircraft. The ongoing transition from metallic to composite airframes requires a change in design philosophy to accommodate the fundamentally different behavior of composite structures in the event of damage, if future designs are not to be constrained or compromised, resulting in an inability to fully exploit the significant potential offered by composites. This work presents a feasibility study for an innovative method by which delaminations (a fundamental damage mechanism) can be steered through the thickness of a composite laminate. Plies with fibres oriented in the propagation direction are angled through the thickness in order to guide delamination migration. Static testing (Mode I) has shown the viability of this mechanism. In addition, a significant increase in fracture toughness, ranging from 88% to 269% depending on the configuration, is observed due to delamination branching and steering away from the preferential propagation plane. Tensile and flexural testing has been performed in order to quantify the effect of this innovative architecture on the global mechanical properties. A significant knockdown in longitudinal tensile strength by up to 57% has been observed, while the effect on stiffness and flexural properties seems to be less significant. In addition the potential of applying the steering mechanism in skin-stiffener debond specimens under fatigue loading has been successfully demonstrated. Similar stiffness decay as for the baseline configuration is observed over 40000 cycles. The work in this study illustrates the potential for tailored fibre architectures, with a through-thickness feature, to provide a viable mechanism for steering critical damage into designated regions which are non-structural or contain added functionality to mitigate that damage.