Joining of CFRT-steel hybrid parts via hole-forming and subsequent pin caulking

In times of increasing global warming, the awareness of the necessity for significant CO2 reduction is growing. Especially in the transport and aerospace sector, lightweight construction has potential to achieve emission reduction goals by reducing the overall vehicle weight. Thereby, adding lightweight fibre-reinforced composites to materials such as steel and aluminium is used to achieve weight savings. Furthermore, continuous-fibre-reinforced thermoplastics (CFRTs) begin to replace more traditional thermoset thermoplastics due to their easier bulk production and uncomplicated storage. Hybrid parts often consist of a CFRT and a higher strength metal component. Here, the joining process poses the main challenge, due to different chemical and physical properties of the components. In the current state of the art, riveted and bolted joints are commonly used, leading to increased weight due to auxiliary elements and requiring precise bolt holes often destroying load-bearing fibres. Joining with cold formed pin structures is an innovative and versatile joining process, which avoids the need for auxiliary elements. These pins are subsequently inserted in warm formed holes in the CFRT component and then caulked to create a form-fitting hybrid joint. To obtain a fundamental understanding of this joining process, hole-forming and pin-caulking, are investigated in this study. First, the hole-forming with IR-radiation is investigated with regard to suitable process parameters and resulting fibre morphology. The formed holes are consequently mechanically characterized. Second, the caulking-process is investigated by iteratively upsetting a pin and subsequently measuring the geometry. Based on these findings two different suitable caulking degrees are defined and samples for mechanical as well as microscopic investigations are manufactured. The created joints are first investigated via micro-sections and reflected light microscopy to identify possible damage in the CFRT component, which can result from the pin caulking process. Second, a mechanical characterisation under shear load as well as pin extraction loads normal to the sample surface is conducted and the normal load tests are compared with the bearing strength of CFRT samples.

Using taguchi approach for investigating mechanical properties of recycled carbon fibre reinforced thermoplastics for injection moulding applications

Demand for carbon fibre reinforced plastic (CFRP) increases due to its popular demand in sectors such as automotive and aerospace. This leads to high volume of manufacturing and end of life CFRP waste. The challenge is to recycle the heterogenous waste and utilise the recycled carbon fibre (rCF) in potential applications, including the injection moulding process. However, the effect of processing parameters such as type of new thermoplastics, filler weight loading and particle size on product mechanical properties is not well understood. This study carried out experimental trials based on L4 Taguchi orthogonal design. It is found that the mechanical and physical properties significantly depend on the selected parameters. Optimisation of the parameters should depend on final application of the product. This study highlights potential use of rCF in reinforcing pure thermoplastics, as well as an alternative material to virgin carbon fibre (CF).

Computed tomography investigation of the material structure in clinch joints in aluminium fibre-reinforced thermoplastic sheets

Recent developments in automotive and aircraft industry towards a multi-material design pose challenges for modern joining technologies due to different mechanical properties and material compositions of various materials such as composites and metals. Therefore, mechanical joining technologies like clinching are in the focus of current research activities. For multi-material joints of metals and thermoplastic composites thermally assisted clinching processes with advanced tool concepts are well developed. The material-specific properties of fibre-reinforced thermoplastics have a significant influence on the joining process and the resulting material structure in the joining zone. For this reason, it is important to investigate these influences in detail and to understand the phenomena occurring during the joining process. Additionally, this provides the basis for a validation of a numerical simulation of such joining processes. In this paper, the material structure in a joint resulting from a thermally assisted clinching process is investigated. The joining partners are an aluminium sheet and a thermoplastic composite (organo sheet). Using computed tomography enables a three-dimensional investigation that allows a detailed analysis of the phenomena in different joining stages and in the material structure of the finished joint. Consequently, this study provides a more detailed understanding of the material behavior of thermoplastic composites during thermally assisted clinching.

Inverse Fiber Reinforced Polymer/Metal-Hybrid Laminates for Structural Lightweight Applications

Composite multi-material structures for the automotive industry are another step forward. This is because they contribute to a significant reduction in the weight of structural elements, and thus to energy savings and, consequently, lower emissivity of toxic gases. The paper presents research on a new multi-material system made of fiber-reinforced thermoplastics (FRP) combined with metal elements. To improve the adhesion between the metal insert and the fiber-reinforced plastic, an innovative combination of mechanical fit and adhesive was used. As a result, a targeted use of the excellent mechanical properties of the proposed structure was achieved. Additionally, the proposed method shows advantages in mass production processes of mass-optimized products with high stiffness and load-bearing capacity. The paper presents the results of a new material bending test.