Characterisation of fabric deformation mechanisms during preform manufacture
The use of composites for structural applications in the automotive industry has become more attractive due to the possible weight savings and part integration. Liquid moulding processes, where the reinforcement is prepared separately from the moulding operation, have been suggested as a suitable production method. However there are several obstacles to overcome before they can meet the high production volumes required. Whilst forming the preform, defects such as wrinkling and tearing can occur which can prevent successful moulding. This thesis addresses problems in the design and production of preforms. Current preform manufacturing processes and modelling techniques are reviewed. A model based on kinematic principles to predict fibre architectures for biaxial fabrics draped over arbitrary surfaces is described. A technique based on grid strain analysis was used to measure the deformation of various stitch bonded fabrics, and compared to the kinematic drape model results. The pure shear assumptions of the kinematic drape model assume the fabric has zero resistance to shear. Experimental measurements of fabric in-plane shear resistance were undertaken and compared for a range of fabrics. This highlighted some important criteria in fabric selection and possible problems in the kinematic modelling approach. The results from the in-plane shear tests were compared with those from the grid strain analysis to determine which fabric variables were important to fabric formability. Problems in the application of constraints within the kinematic model were discovered, and methods for overcoming them were suggested. Criteria which must be considered when selecting suitable fabrics for high drape preforms are discussed.