Development of novel form-locked joints for textile reinforced thermoplastices and metallic components

Abstract

Lightweight constructions in multi-material-design, especially the application of textile reinforced composites in combination with metallic components, gain increasing relevance, e.g., in automotive engineering. To ensure an optimal load bearing capability of the hybrid structure, it is essential to provide adapted joining technologies.

In this paper a new joining technology to produce hybrid structures with continuous fibre reinforced thermoplastics and metallic components is introduced, adapting the concept of classical clinching for thermoplastic composites. The technological concept of the thermoclinching process was successfully tested by manufacturing, testing and both non-destructive and destructive analyses of firstthermoclinched joints.

Introduction

In recent years, aspects like resource consumption and energy efficiency gain increasing relevance within the development of lightweight structures, e.g., for automotive applications. Currently, especially the multi-material design is focussed in many research and development activities, aiming to take advantage of the specific structural and functional properties of different materials. In this context, hybrid structures made of continuous fibre reinforced thermoplastics in combination with metallic materials have proven to be very efficient, due to their adjustable high-level mechanical properties and the ability of an effective and reproducible manufacture in short cycle times using the pressing technology. One of the main challenges during the development of such hybrid structures is to provide specific joining technologies, considering the load bearing capability, the specific properties of the combined materials and the associated manufacturing processes.

Classical joining technologies like bonding, riveting and joining via loop connections are established for manufacturing of continuous fibre reinforced composites with thermoset matrices. Hereby, bonding is known for its flexibility regarding the material combinations, but often requires an extensive surface pretreatment of the joining partners as Bishopp (2005) described for structural bonding applications used in the aerospace industry. Higashi and Lima (2012) and Wilson et al. (2012) have displayed that bolted and riveted connections are common and proven joining methods for aviation, according to their high reproducibility and fast joining processes. Since the holes which are needed for these connections are usually manufactured by drilling, the fibre reinforcement is locally interrupted and the component is structurally weakened in those areas (Thoppul et al., 2009). Beside this, Liu et al. (2012) have shown that drilling of composites can cause specific problems such as local delamination and high tool wear. Furthermore, the need of additional joining elements increases the weight of the joining area. To fasten continuous fibre reinforced composites without any pretreatments such as surface treatments or hole drilling, Ueda et al. (2012) tested a modified self-piercing riveting process. Although a significantly reduction of the process time was achieved, the process-related unpredictable interruption of the fibre reinforcement is inadequate to generate joints in structural components. The integration of special joining zones during processing of thermoset composites, like Hufenbach et al. (2006) have shown for loop connections or Rispler et al. (2000) for metallic inserts, is state of the art but very labour intensive.

In contrast, the manufacturing of such joining zones in fully automated process chains for thermoplastic composites is highly complex and often requires the adaption of the manufacturing technology and the process setup, as shown by Hufenbach et al. (2011) for producing bolted joints in textile thermoplastic composites or by Blaga et al. (2013) for joining thermoplastic composites and metallic parts with an adapted riveting process. Additionally, adhesive bonding of most thermoplastic composites is problematic due to low surface polarity and a lacking surface wettability as shown by Gotoh and Kikuchi (2005), resulting in an extensive surface pretreatment and therefore being not very efficient in the scope of high-volume applications.

A promising approach for the manufacturing of joining zones in thermoplastic composites is the use of its repeatable meltability to build form-locking joints by plastic deformation. Thereby, state of the art technologies to build short and long fibre reinforced hybrid joints are for example injection clinching joining, presented by Abibe et al. (2011), or ultrasonic staking, described by Troughton (2008). However, considering that continuous reinforced thermoplastic composites only provide a limited plastic deformation range, established joining technologies based on forming short and long fibre reinforced thermoplastics are only partly transferable.

In the presented investigations a novel joining technology for manufacturing hybrid components with continuous fibre reinforced thermoplastic composites and metallic joining partners was developed in accordance to the metallic clinching process and first laboratory scaled joining zones were produced. Furthermore, the joining zones were analysed by means of experimental and analytical methods.