The importance of the mesostructure in toughening cellulosic short fibre composites

In tonnage terms the commercial production of engineering composites is dominated by glass reinforced systems, this is particularly the case in the automotive industry. Natural fibres have long been regarded as a viable lightweight replacement for glass, however the various shortcomings of natural/cellulosic fibres have so far, inhibited exploitation, where resistance to fast fracture during impact is a major failing. Composite mesostructure describes mid-scale structures in composites, such as fibre alignment patterns, bundling effects, and fibre end synchronisation. The mesostructure can dramatically affect final properties in some random short fibre systems where flow is involved, such as sheet moulding compounds (SMC), and can be the determining factor in, for example, the success of one fibre system over another. This study seeks to manipulate the fibre mesostructure in moulding compounds reinforced with natural/cellulosic fibres, where it is shown that by arranging mechanically inferior fibres in bundles, composite impact energy absorption can be substantially improved, where the reasons behind the toughening mechanism at work, is discussed and optimum bundle dimensions for several fibre systems are identified. Fibre bundling seems to be a highly interesting method for toughening composites made from mechanically inferior natural/cellulosic fibres, however no work in the area has been reported until now.