MESOSTRUCTURES AND THEIR MECHANICS IN FIBER COMPOSITES

Two structural levels have predominated in fibre composites: (1) The m icrostructure, i.e. the detailed structure at the nanometer-micrometer level, involving the fibre, the interface or interphase, and the imme diately surrounding polymer; (2) The macrostructure, which involves th e final design including laminate lay-up, core-skin structure, etc. Be tween these structural levels lies the mesostructure, a hitherto negle cted area, but one with great potential for improving composites and t hus making them more efficient and more economical. Nearly all models for fibre composites have straight fibres, regularly packed. This is f ar from the case in real composites, and this can have serious consequ ences. The concept of mesostructures include structural defects such a s fibre misalignments and uneven fibre packing. The concept can also b e applied to lower performance composites with randomly distributed sh ort fibres. In this case we have adventitious order rather than disord er, including preferred orientations and end synchronizations, etc. On e mesostructure has already been quite intensively researched, i.e. fi bre waviness. Most laminates are made with nominally straight fibres. However, when the structures are looked at carefully, they are always found to contain misaligned fibres. These misalignments are often quit e small (only a degree or so) in well made reinforced thermosets, but can become quite large in some thermoplastics. (Very significant wavin ess has been detected in carbon fibre reinforced polyarysulphone.) The waviness reduces tensile properties to some extent, but has a much mo re serious effect on compressive properties. On the other hand, it man y actually be beneficial for shear properties. Waviness can be control led by choice of initial material (the exact form of prepreg) and proc essing conditions such as the type of mold used (i.e. graphite as comp ared with nickel). Thus it provides a direct link between processing a nd properties. The identification and classification of mesostructures will be discussed and their effects on final properties will be revie wed.