Vorspannkraftrelaxationsverhalten von Schraubenverbindungen mit endlosfaserverstärkten Kohlenstofffaser-Kunststoff-Verbunden sowie dessen Verbesserung

Conventional joining technologies for fiber-reinforced plastics like rivet connections are not suitable for large-scale production in the automotive industry because of the high level of effort required to generate a reliable connection. It is common in the car assembly to use conventional force-locking bolted joints.
This work examines the preload relaxation behavior of conventional force-locking bolted joints with carbon fiber reinforced plastic (CFRP) in the clamping system. The relaxation was measured on a simple specimen of CFRP suitable for serial production with the variation of important parameters, like ambient temperature, preload, fiber volume content, etc. The measurement of the preload was done by an evaluation of the elastic length change of the screw with a tactile contacting and compensation of thermal elongations. The investigations show that bolted joints with CFRP and exposition at room temperature have a comparable preload relaxation behavior like conventional bolted joints with metal parts. With rising temperatures in the range of common car components until about 212 °F, a significant increase of the preload relaxation was detected. Most of the preload loss happens in the first hours of the temperature conditioning and the first temperature cycles. Reasons for this behavior are load plastifications because of a reduction of the yield strength with rising temperature, load plastifications because of thermal-induced additional forces due to the high thermal expansion coefficient of the CFRP, and increased creep deformation with increased temperature.
The findings of the preload relaxations test are the basis to define methods to reduce the preload relaxation. The methods are divided into mechanical, constructive, and assembly. Fiber reinforcement in the laminate thickness direction with the sewing method “tufting” leads to a slight reduction of the preload relaxation. The increased fiber content in thickness direction should lead to a clear increase of the positive effect. Belleville washers and sleeves incorporated into the bolted assembly are an effective way to raise the bolted joint resilience and therefore to reduce the preload relaxation. These solutions are well known from conventional bolted joints with metals in the clamping but also show good results with CFRP and the relatively high amounts of plastification. Two mounting methods have proved to be especially effective which are referred to as warmassembly and warmprepressing in this paper. Both procedures lead to a significant and sustainable reduction of the preload relaxation after the first temperature loading and also the complete product life.