21 st International Conference on Composite Materials Xi’an, 20-25 th August 2017 DAMAGE TOLERANCE OF CARBON/EPOXY QUASI-INTERWOVEN COMPOSITES SUBJECTED TO LOW VELOCITY IMPACTS Bryony V. Waddington 1 *, Alvaro Silva-Caballero 1 , Sree S. Roy 1 , William R. Kennon 1 and Prasad Potluri 1 1 Robotics & Textile Composites Group, North West Composites Centre, School of Materials, University of Manchester, UK * bryony.waddington@manchester.ac.uk Keywords: Damage tolerance, Textile composites, Three-dimensional fibre reinforcement, Impact, Low velocity ABSTRACT Robotic dry fibre placement (RDFP) is a preforming technology with the potential of mass- producing laminar composite preforms at the speed and quality required for the automotive and aerospace industries. The RDFP technology enables the placement on unidirectional (UD) fibres in arbitrary in-plane directions, producing a laminate with tailored mechanical properties. Due to the layered nature of these lay-ups, the resulting composites exhibit poor out-of-plane properties. Quasi-interwoven lay-ups produced by the RDFP methodology present a potential method for creating preforms with layer connectivity and crimp. The RDFP carbon fibre preforms produced for this work were based on the Advanced Placed Ply (AP-Ply) structure, with pin spacing ranging from 2.5mm to 10mm. The narrowest pin spacing creates a cross-ply non-crimp preform whilst the widest spacing creates a quasi-interwoven composite upon consolidation, with high levels of layer crimp. Rectangular test coupons were cut according to the Queen Mary test protocol and subjected to low velocity impact and compression after impact testing. A distinguishable improvement in damage resistance was observed for the quasi-interwoven lay-up when compared with the non-crimp lay-up. Despite the high layer undulation levels seen with the quasi-interwoven lay-ups, these specimen have comparable compression after impact strengths to those with lower levels of undulation. 1 INTRODUCTION The automotive, marine and aerospace industries, to name a few, are working to replace traditionally metal parts with lightweight composite alternatives, as already seen with BMW's "project i" cars with carbon fibre chassis. This is expected to drive an increase in demand for composite materials over the coming years, with there being no doubt about the advantages that fibre reinforced composites have over traditional structural materials. Their high strength-to-weight and stiffness-to- weight ratios make composites preferable in many instances, with their versatility in the design of stiffness and strength to match the specifications required by the application. Most composite preforms are produced through the stacking of 2D fabric layers or the placement of pre-impregnated tape layers onto a mould. These layups, when impacted or loaded in the out-of-plane direction, exhibit poor mechanical properties and delamination of the layers often occurs. As a result the structure becomes less able to withstand loading. This poor performance is due to a significant proportion of the load being carried by the matrix, rather than the fibres [1]. When delamination damage occurs the composite's in-plane mechanical properties degrade and thus their load bearing performance. The level of property degradation is indicative of the composite structure's damage tolerance. Damage tolerance is defined as the material's ability to continue functioning after a damaging event [2], and is a design philosophy that was introduced during the 1970's for the design of aircraft structures. The classic methodology of assessing a composite structure's compressive strength after impact to determine damage tolerance may be largely influenced by the fact that the fuselage of an aircraft is subjected to significant compressive loading during flight. The typically poor damage