Residual thermal stresses are an important factor considered in the production of directly joined hybrid structures arising from the difference between the thermal expansion coefficients of fibre and metal, and the difference between the curing temperature of the matrix and the operating temperature of the cured laminate. Although intrinsically joined layered composites have a decisive disadvantage of residual stresses occurrence after cooling from the curing temperature, theoretical and experimental study confirms that pre-stressing the fibers during the production process can be used as an active method to reduce residual stresses. Accordingly, if the fibres are loaded with a defined prestress during the curing process, the deviating thermal expansion coefficients in the fibre direction can be compensated. After cooling from the curing temperature, the pretension of the fibres is released, and the matrix is put under pressure. Subsequently, the amounts of the production-related residual stresses in the heterogeneous microstructure can be minimized. Based on this method, it is possible to apply mechanical pre-stress to the fibers of the CFRP component during the production process and transfer this method to metal-CFRP hybrid composites. This means that the fibres are subjected to a defined pretension before the manufacturing process. After the pretensioning, the heating of the tool, which is necessary for the curing of the matrix and the bonding with the metal component, takes place simultaneously. Thereby, the prestressing can be designed in such a way that residual stresses are minimized after processing.

In this study, for the hybrid joint, unidirectionally pre-impregnated semi-finished products (prepregs) with duromer matrix resin and micro-alloyed HC340LA steel were used. The prepregs were prestressed by applying a preloading force using a specially designed frame, Fig. 1a, and hybrid structures were manufactured by pressing tool, Fig. 1b. The resulting residual stresses and their distribution in the thickness direction are analysed experimentally by the incremental hole drilling method and numerically, and compared between the without fibre-prestressing and with fibre-prestressing conditions.