Tufting of carbon fibre/epoxy composites for improved damage tolerance
Laminated composite structures are susceptible to delamination and debonding during their service life due to their poor out-of-plane interlamellar properties. Such damage can occur either during manufacturing or during service due to foreign body impact. One way to improve the interlamellar properties is to develop these structures by introducing fibre/reinforcements in the Z direction, which can arrest the delamination growth to a large extent. Tufting is one such technique, which is carried out on dry preforms followed by liquid composite moulding for manufacturing of aircraft structural part. Research shows that tufting increases the fracture toughness of about 10 times and also reduces the damage propagation to a large extent. However, tufting reduces the in-plane properties due to damage of fabric yarn during the tufting process. Both in-plane and out-of-plane properties depend on tuft density. The out-of-plane properties increase with the increase in tuft density but in-plane properties decrease with increase in tuft density. In order to have a minimum impact on in-plane properties, tufting parameters were optimized. With optimized parameters, composite laminates were characterized for in-plane and out-of-plane mechanical properties. In-plane mechanical characterization was carried out in room temperature dry (RTD) condition and in elevated temperature wet (ETW) test condition. Specimens for ETW test were conditioned at 71⁰C & 85%RH and after saturation, testing of specimens was carried out in the same environmental condition. Majority of tufted specimens showed reduction less than 10% in RTD test condition except for flexural strength and quasi-isotropic tensile strength, where reduction was observed to be 12% and 18% respectively. A slightly higher reduction of 21% and 27% was observed for compression strength and flexural strength respectively in ETW test condition. The quasi-isotropic tensile strength showed similar reduction of 18% in RTD and ETW test condition. No appreciable change was observed in the notched specimens in both RTD and ETW test conditions. Mode-I fracture toughness and CAI strength test were carried out to ascertain the improvement by tufting on the out-of-plane properties. Fracture toughness of tufted specimens exhibited 4.5-10 times the fracture toughness of untufted specimen depending on tufting thread and number of tufting rows. In CAI specimens, on increasing the tuft density up to 0.56%, damage area decreased by 51% and CAI strength increased by 43% due to increase in apparent interlaminar strength attributed to the enhancement in bridging effect. Subsequently the tufting was utilized on T-joint, a basic element of cocured composite structure. Tufting enhanced the failure initiation load by 24% thereby improving the structural properties of T-joint. Tufting increased the toughness of T-joint by 120% and reduced the disbond growth rate by 24%. Thus, tufting significantly improved damage tolerance of T-joint. Further the failure modes and mechanisms were studied systematically to understand the failure of tufted composite which would led to understand the failure in composites.