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In this study, thermoset-based carbon fiber-reinforced polymer structures manufactured by the so-called modified co-curing process are analyzed and compared to well-established co-curing and co-bonding. The modified co-curing process allows manufacturing geometrically complex parts without traditional core technologies by producing laminates from a un-cured half and a pre-cured half in contrast to using two un-cured halves (co-curing) or a fully cured half plus an un-cured half (co-bonding). The interlaminar fracture toughness under Mode I loading, [Formula: see text], was determined in double cantilever beam (DCB) tests. [Formula: see text] displays a correlation of the degree of cure and the joint properties, with the co-curing laminates having 11% and 33% higher fracture toughness than the modified co-curing configurations. However, modified co-curing in all cases results is superior or equal to co-bonding. To assess the influence of surface properties for the bonding quality, different peel plies were compared with respect to the resulting joint properties. The results with up to 50% loss in [Formula: see text] values indicate the high importance of appropriate surface preparation. Subsequent tests also show that the negative influence of the peel ply on the joint properties can be reversed by abrasive surface treatment. It was found that at higher degrees of partial curing before co-curing, crack growth increasingly occurs in the interface of the bonded laminates. Therefore, the properties of the surface before joining were analyzed and modified to assess its relevance for the bonding properties and the potential for improvement.

The integration of continuous fiber-reinforced structures into short or long fiber-reinforced plastics allows a significant increase in stiffness and strength. In order to make the best possible use of the high stiffness and strength of continuous fiber-reinforcements, they must be placed in the direction of load in the most stressed areas. A frequently used tool for identifying the most heavily loaded areas is topology optimization. Commercial topology optimization programs usually do not take into account the material properties associated with continuous fiber-reinforced hybrid structures. The anisotropy of the reinforcing material and the stiffness of the base material surrounding the reinforcement are not considered during topology optimization, but only in subsequent steps. Therefore in this publication, existing optimization methods for hybrid and anisotropic materials are combined to a new approach, which takes into account both the anisotropy of the continuous fiber-reinforcement and the stiffness of the base material. The results of the example calculations not only show an increased stiffness at the same material input but also a simplification of the resulting reinforcement structures, which allows more economical manufacturing.

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Currently applied manufacturing technologies for hollow composite structures impose design restrictions, such as limited integration potential, whereas differential designs are limited regarding lightweight design and load transfer due to the need of joints. This paper introduces the patented concept of structural carbon fibre-reinforced polymer (CFRP) cores, intending to overcome limitations allowing for integral, yet complex designs. The hollow cores are initially manufactured and partially cured via resin transfer moulding (RTM) with inflatable rubber core. These cores are then integrated in a dry textile-based preform which is impregnated in another RTM process. Here, the cores stabilize the preform during injection and afterwards remain in the component as load-carrying elements. Thereby the partial degree of cure of the cores before the second RTM process is optmised to balance dimensional stability and bonding strength. The process route using structural CFRP cores was successfully applied to a generic structure and proven by structural testing.