A study of damage accumulation in a knitted fabric reinforced composite.

The use of knitting technology with advanced fibres such as glass, carbon and aramid, to produce near-net-shape fabrics has in recent years received increasing interest from the composite materials community. Knitted fabrics have the potential of being used in engineering structures with complex shapes in conjunction with a suitable liquid moulding technique, such as Resin Transfer Moulding (RTM), due to their excellent drapeability and manufacturability. During previous studies in textile reinforced composites, an intimate relationship between the fabric architecture and the damage development has been demonstrated. In this work, the quasi-static tensile loading deformation behaviour and the relation between the fabric architecture and damage development have been studied for a weft knitted glass fabric. Tensile properties have been examined and the failure mechanisms have been identified experimentally by analysing the damage process in-situ with a camera and by studying fracture surfaces using scanning electron microscopy (SEM). The acoustic emission technique was used to support the microscopic analysis. The work has investigated the tensile properties and failure mechanisms of three knitted fabric reinforced composite laminates reinforced with a Milano weft knitted glass fabric. The three composites were (i) a single layer of fabric reinforcing epoxy resin, (ii) a single knitted fabric layer sandwiched between 0° glass fibre unidirectional plies (again with the glass reinforcing epoxy resin), and (iii) the same knitted glass fabric but this time used as the reinforcement in commercially produced high fibre volume fraction composites (using the RTM technique). The variation of mechanical properties with angle (from wale to course) has been measured for the single layer of the fabric reinforcing epoxy resin by orientating the wale direction of the fabric at different angles. Mechanical properties have been measured for each angular orientation and comparisons were made between them, especially with regard to the planes of final failure. The single layer composites failed as soon as the first damage was initiated. Hence, to investigate damage accumulation, a novel technique was employed to manufacture a sandwich laminate, which consisted in placing a single knitted fabric layer between 0° glass fibre unidirectional plies. The success of this method is that the accumulation of damage in the knitted architecture was allowed to be studied and some characteristics of crack initiation and crack propagation could be related to the fabric geometry and structure. Experiments on these model transparent materials have been complemented by tests on two types of commercial knitted fabric composite manufactured by the RTM process. Characterization of these materials under tensile loading has been carried out for monotonic and cyclic loading and the results have been compared with those found for the single layer and the sandwich model material. Various failure mechanisms such as cracking at loop cross-over points, resin matrix cracking, fibre bundle debonding and tensile fracture of fibre bundles in failed specimens were observed. The behaviour of the commercial RTM specimens has been discussed in the light of the results obtained from the model single layer and sandwich specimens