Advanced joining technology for the production of highly stressable lightweight structures, with fiber-reinforced plastics and metal

Abstract

Organic sheets made of fiber-reinforced thermoplastics can make a crucial contribution to increase the lightweight potential of a technical design. They show high specific strength- and stiffness properties as well as good damping characteristics, while being able to show a higher energy absorption capacity than comparable metal constructions. In addition, organic sheets provide good recycling capabilities. Nowadays, multi-material designs are an established way in the automotive industry to combine the benefits of metal and fiber-reinforced plastics (FRP). Currently used technologies for the joining of organic sheets and metals in large-scale production are mechanical joining and adhesive technologies. Both require large overlapping areas to achieve the desired joint strength and stiffness of the technical design. Additionally, mechanical joining is usually combined with “fiber-destroying” pre-drilling and punching processes. This will disturb the force flux at the joint zone by causing unwanted fiber- and inter-fiber failure and inducing critical notch stresses. Therefore, the multi-material design with fiber-reinforced thermoplastics and metals needs optimized joining techniques that don’t interrupt the force flux, so that higher loads can be induced and the full benefit of the FRP material can be used. This article focuses on the characterization of a new joining technology, based on the Cold Metal Transfer (CMT) welding process, that allows to join organic sheets and metals in a load path optimized design. This is achieved by realigning the fibers around the joint zone by the integration of a thin metal pin. The alignment of the fibers will be similar to load paths of fibers inside structures found in nature. A tree with a knothole is always going to align its fibers in principle stress direction. As a result of the bionic fiber design, high joining strengths can be achieved. The increase of the joint strength compared to blind riveting was performed and proven with stainless steel and orthotropic reinforced composites in tensile shear-tests, based on the DIN EN ISO 14273.

Defects and uncertainties of adhesively bonded composite joints

“…For instance, the aerospace industry exploits these benefits by replacing metals such as aluminium and titanium alloys with composites in primary structures such as the main wing and fuselage [1,2]. Similarly, the automotive industry is replacing more conventional materials with lighter and stiffer alternatives that meet the higher performance standards set by regulating bodies and tighter regulations and restrictions to produce environmentally friendly cars [3][4][5][6][7].…”

Fatigue behaviour of material-adapted fibre-reinforced polymer/metal joints

“…Due to the lack of alternatives, the production of such FRP/metal-structures is currently predominantly based on established joining methods, such as bonding or screws, rivets and bolts, known from metallic construction [2][3]. However, this procedure is associated with a poor exhaustion of the material's lightweight potential and high demands on the surface condition.…”

Characterization and Numerical Modelling of Through-Thickness Metallic-Pin-Reinforced Fibre/Thermoplastic Composites under Bending Loading

“…In particular, the application of material specific joining technologies plays an essential role for the exploitation of the load-bearing capacity of thermoplastic composites. Main technologies for joining thermoplastic composites are based on adhesive joining [4], fusion bonding [5] and mechanical fastening [6][7][8][9]. However, their application is challenging due to their sensitivity to delamination failure caused by typically very low through-thickness strength dominated by the thermoplastic matrix material [10].…”