THICK CARBON FIBRE REINFORCED COMPOSITE MATERIALS: INVESTIGATION OF AN INDUSTRIAL CASE-STUDY

Carbon Fibre Reinforced Materials (CFRMs) are presently applied for a wide range of advanced applications such as in automotive, aerospace, construction, yachts, leisure and sport. CFRMs may have just an aesthetic role, mainly in luxury industry, but their main applications are still as structural materials where the particularly high strength to weight and stiffness to weight ratios provide the final objects with light weight and outstanding mechanical properties. CFRMs are generally produced as relatively thin objects, where layers of fibers and resin precursors, are stuck one on the other: playing on the composition of the layers, orientation of the fibers and on the stacking sequence, physical and mechanical properties can be controlled and tuned for specific applications. When the design of the final object is particularly challenging, or the dimension is quite large, the best choice for producing CFRM is hand lay-up of pre-impregnated fabrics. In principle an infinitestack of layer may be added to improve mechanical properties; however, when the number of layers is increased, though the composite can reach outstanding properties, in particular regarding the high strength to weight and stiffness to weight ratios, some problems arise in the manufacturing of the parts. There is no clear definition of what a thick composite material is, but Secord et al. defined1 “parts that have large unfavorable temperature overshoots are classified as “thick,” while parts that have little or no overshoot during cure are classified as “thin”. They claim that distinction of a CFRM into “thick” or “thin” classes depends just on how much this affects part quality. Indeed, owing to the exothermicity of the curing reaction in epoxy resins and to poor thermal conductivity across the section of single layer (due to the anisotropy of thermal conductivity in pre-pregs, which is high along the fiber axis, but definitely scarce perpendicularly to it), when thickness exceeds few millimetres overheating of the inner layers may happen, inducing thermal stresses and resin degradation in the final product and thus affecting the final object properties. When this happen, and care has to be taken in avoiding or, at least, in limiting the overheating effects, the object is considered a “thick composite”. Additional problems might arise in the work up of big CFR objects, where not only the thickness section is increased, but the overall dimension of the final part extends over a meter scale. In this case the hand lay-up of the whole part is particularly time-requiring and the ageing of raw materials during work up might affect the kinetics of the curing reaction. Additionally the curing process, when performed at industrial scale is far form ideal laboratory condition, thus making thick composite industrial production an actual challenge.2 In this context, the good knowledge of the thermal curing behaviour of the resin prepreg is of paramount importance to design a full production and curing cycle for a thick composite object avoiding defects that might compromise the final structural properties. Though modelling of the complex phenomena occurring in the thick composite production gives a rough estimation of what is occurring during the production of parts, the industrial reality is often far from the ideal conditions represented in the models. In the present work, a case-study is presented, in cooperation with RI-BA Composites srl, where the industrial production of a thick part intended for primary structural application is investigated, starting form the analysis of the raw material, up to the characterization of the obtained thick and large object.