Thermal effects on microstructural matrix variations in thick-walled composites

Thick composite laminates are increasingly applied in high-performance structures, such as wind turbine blades, pressure vessels, parts in aircrafts, bridges, etc. These structures can be manufactured with filament winding, vacuum infusion, and hand lay-up. During processing of thick-walled semi-crystalline thermoplastic composites, through-the-thickness gradients may arise in, for example, cooling rate and degree of crystallinity. This may lead to a residual stress distribution within the part as well as a gradient in material properties. Therefore, this study aims to investigate the cooling rate variation through the thickness and the resulting crystallinity gradient in 10 and 20 mm thick carbon fibre fabric reinforced polyphenylenesulfide (CF/PPS) laminates subjected to two different surface cooling rates. A parabolic cooling profile was found in both laminates, which resulted in a parabolic crystallinity gradient. This was established by means of Differential Scanning Calorimetry (DSC) as well as microVickers hardness testing. It was shown possible to determine crystallinity variations in a thick semi-crystalline laminate by means of this latter technique.