Investigation of a carbon fiber/epoxy prepreg curing behavior for thick composite materials production: An industrial case-study

Carbon Fibre Reinforced Materials (CFRMs) are widely renown for their structural application in many different fields, such as automotive and aerospace. CFRMs are usually designed and produced as relatively thin objects, called “laminates”. In a laminate a number of layers, made up 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 number of layers is increased, the composite can reach outstanding properties, in particular regarding the high strength to weight and stiffness to weight ratios. Thick composites engineering, however, introduces some additional problem to the manufacturing of CFRMs based parts. Owing to the exothermic character of the curing reaction and to the anisotropic thermal conductivity (which is high along carbon fibers but low across their section) that characterize every single ply, when thickness exceed few millimetres overheating of the inner layers may happen, thus inducing thermal stresses and resin degradation in the final product. Further problems might rise from the ageing of raw materials during work up which is, in fact, particularly time-requiring for thick and complex objects. Additionally the curing process, when performed at industrial scale is far form ideal laboratory condition, thus making thick composite industrial production an actual challenge. 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 isothermal curing behaviour of the prepreg.