Experimental and theoretical investigation of thermal stress in CFRP

This paper focuses on the determination of thermal stress in advanced CFRP foam-core sandwich structures. The sandwich was manufactured by open mould vacuum infusion process with polymethacrylimide-(PMI-)foam core and NCF carbon fibres. The thermal residual stress was determined by Timoshenkos theory based on the measured curvature of bisected sandwich specimen. The theory firstly was proved for applicability by means of numerical calculations, because it was original derived from bi-metals. The sandwich specimen were cut into two half specimens at room temperature. The separated halved specimens were heated up to 180°C and cooled down to 30°C for several times with defined temperature ramps and the curvature was imaged with contactless optical measuring system. Based on the measured curvature the residual stress was calculated and compared to theoretical results. It was found out that the specific relaxation behaviour of the foam has an important influence of the residual stress in the investigated sandwich. To verify these results separated foam specimens were heated and cooled with the same proceedings and the thermal expansion behaviour of the foam was measured.