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Lightweight Design worldwide

, Volume 10, Issue 5, pp 38–41 | Cite as

Hybrid lightweight rear axle for electric vehicles

  • Paul Becker
  • Andreas Büter
Construction Multi-Material Construction
  • 345 Downloads

In the EU research project ”epsilon”, the Fraunhofer Institute for Structural Durability and System Reliability LBF has been tasked with the development, manufacturing, and testing of a hybrid lightweight rear axle. The main focus of the development was reducing mass while maintaining safety under operating conditions.

Introduction

Lightweight design plays an important role for the emerging electromobility. The significant increase in weight due to the necessary batteries is an enormous challenge for vehicle manufacturers. For this reason, a growing number of companies aim to optimise weight and use innovative solutions to do so. Thus, new materials, design concepts, and manufacturing processes have been introduced to the automotive sector over the last years. Increasingly, components of the structure (i.e. the car body) and of suspension systems traditionally made of metal are being replaced by those made of fibre reinforced polymers.

In doing so, it is important to determine, whether composite materials are suited for application in each individual part. Boundary conditions need to be analyzed carefully. Not any metal part can be substituted sensibly by a composite design. Loads and load-directions need to be considered in detail. The anisotropic properties of composite materials can then be used to locally reinforce load-paths in order to achieve further weight reduction of the overall component.

During the EU research project „epsilon“, which focuses on electromobility and lightweight design, Fraunhofer LBF has developed a composite rear axle, achieving a significant weight reduction, Figure 1. Compared to a conventional metal design, the axle’s weight has been reduced by 37 %, contributing significantly to the overall weight reduction and thus helping to compensate for the weight of the battery and to extend the range. Furthermore, the structure’s stiffness is positively influenced by the way the composite lay-up is designed. Road tests at the end of the project have demonstrated vehicle dynamics comparable to those of the conventional rear-axle.
Figure 1

Demonstrator of the lightweight rear axle (© Fraunhofer LBF)

Operational Loads Acting on the Rear Axle

Especially for the design of safety components such as the rear axle, it is important to know as much as possible about the operational loads and their directions. In use, the rear axle is mainly subjected to bending and torsion loading. Braking, acceleration, and steering manoeuvers can be distinguished. The analysis of the vehicle dynamics shows, that braking leads to a shifting of the load towards the front suspension. The rear axle’s load is decreased by this shift. The critical and design-driving load case is acceleration during a cornering manoeuver. In this case, bending as well as torsion loads act on the axle, leading to a multiaxial state of loading. A second relevant load case is the maximum of the vertical wheel-load resulting from running over a curb.

Design of the Rear Axle

In principle, the lightweight rear axle consists of metal parts on the sides and a central beam made of composite, Figure 2. This hybrid concept allows a straightforward design of the interfaces to the car body. Additionally, temperature effects and local stresses can be dealt with more easily. The composite beam is connected to the metal parts via so called T-Igel joints, invented by the Teufelberger company, Figure 3. Thanks to the form fitting connection between pins and laminate, very large forces and moments can be transferred between the metal parts and the composite. The T-Igel principle is also used at the central joint of the axle. In this region, the form fit reduces stress concentrations in the FRP around the bolts. Thus, machining of the laminate to realise the bolted joint is made possible without excessive weakening of the laminate around the holes.
Figure 2

Structure of the rear axle (© Fraunhofer LBF)

Figure 3

T-Igel element (© Fraunhofer LBF)

The load case “acceleration during cornering” is defined as the worst case scenario.

To achieve the required stiffnesses of the rear axle, the geometry of the CFRP beam has been optimised. The beam needs to withstand the critical torsion and bending loads. Based on numerical analysis, an optimised twisted shape of the beam’s profile is used to achieve sufficient stiffness. Stiffness is especially good under the multiaxial load case.

The design of the FRP component is an iterative process aiming for a compromise between manufacturability and requirements. To this end, the local laminate lay-up is optimised in several stages. The strength of the final design is analysed numerically.

Numerical Simulation

Numerical simulation is used for strength analysis and the determination of the resulting safety factors. Furthermore, it also supports the design of the laminated structure of the rear axle’s central beam. For the analysis, the critical load case is considered. As discussed above, the load case “acceleration during cornering” is defined as the worst case scenario, Figure 4. In this situation, the acceleration leads to a bending load in the lateral parts and torsion loading of the central beam.
Figure 4

Numerical simulation (acceleration during cornering) (© Fraunhofer LBF)

To validate the numerical model, a simplified truss model is defined. This is then considered analytically as well as numerically for a simple load case. A comparison of the results shows that the difference is acceptable.

For a preliminary dimensioning, forces and moments are scaled according to the following combination of load cases:
  • ▸ 50 % of the maximum acceleration

  • ▸ 87 % of the maximum steering angle.

The numerical analysis reveals very high shear loading of the lateral part between the wheel carrier and the spring damper interface. The reason for this are large transverse forces acting on the rather short section of the part. The maximum of the bending moment also occurs in the vicinity of these lateral parts.

Based on these considerations and the results of the truss simulation, it is obvious that designing the whole of the axle as an FRP part would not be reasonable. On the one hand, large shear and compression forces occur in the lateral parts, on the other hand heat in the region of the spring damper interface and the brake system could have negative effects on FRP material. Especially the shear stresses are regarded critical with respect to a risk of inter fibre failure.

The results of the numerical analysis are the determined safety factors which are summarised in Table 1.
Table 1

Determined safety factors for the loading situation „acceleration during cornering”

Load case

Computed safety factors (CFRP central beam)

25 % of maximum acceleration 97 % of maximum steering angle

4.02

50 % of maximum acceleration 87 % of maximum steering angle

3.6

75 % of maximum acceleration 66 % of maximum steering angle

2.9

© Fraunhofer LBF

Apart from the loading situation „acceleration during cornering”, the maximum vertical load (running over a curb) is simulated. The results are summarised in Table 2.
Table 2

Determined safety factors for the loading situation „running over a curb”

Load case

Computed safety factors (CFRP central beam)

Maximum vertical load (running over a curb)

1.75

© Fraunhofer LBF

As can be seen from the tables, the computed safety factors are relatively large. Since the lightweight rear axle is later mounted on the prototype for road testing (on a test course) without extensive testing, no further optimisation of the weight, which would lead to a reduction of the safety factors, is conducted.

Compared to a conventional metal design, the axle’s weight has been reduced by 37 %.

Manufacturing of the Lightweight Rear Axle

The choice of manufacturing technology for automotive components is strongly dependent on cost. These are mainly associated with material, tooling and cycle times. Especially cycle times are crucial. For this reason, automation of the manufacturing (e. g. by robots) is a goal. Hence, radial braiding is selected as the manufacturing process for producing the CFRP-parts, here. Metal parts are manufactured conventionally. The challenge in manufacturing the rear axle lies mainly in respecting the tolerances for assembly. By carefully choosing the joining procedures accordingly and by using specially made jigs, this is achieved.

To stabilise the structure and to increase strength, several inserts and the T-Igel elements are used to join the metal parts to the FRP beam. Due to their form fit, T-Igel connections are able to tolerate large forces and moments. Per pin, approximately 0.5 kN of shear force can be sustained.

For the lateral parts as well as the central joint connection, metal is chosen. Thus, these parts are better suited to the multiaxial loads and the temperatures expected under the given boundary conditions. In addition, this choice of material simplifies the design of the interfaces for the central joint as well as the wheels and spring dampers. During the concept phase it is determined, that the potential for weight saving of these parts is rather low so that substituting them with composite parts would not be worthwhile.

Testing of the Lightweight Rear Axle’s CFRP-structure

After development and manufacturing, tests in the laboratory as well as road tests on a test course are performed. Laboratory experiments aim to check the composite part’s behaviour under static loads of realistic magnitude. This also serves to validate the results of the numerical analysis. For validation, discrete locations on the rear axle’s central beam are defined. At these points, strains due to the static loads are determined in three directions via numerical analysis. In the experiment, the beam is subjected to realistic displacements. Loading is increased in several steps. Strains are measured at the previously defined locations using strain-gauges, Figure 5.
Figure 5

Experimental test of lightweight rear axle (© Fraunhofer LBF)

The measurement results reveal a slightly larger stiffness of the manufactured part compared to the numerical analysis.

The road tests with the lightweight rear axle have been successful. The driving dynamics is comparable to the use of a conventional axle.

Summary

Fraunhofer LBF has demonstrated the possibility to substitute highly loaded metal components of vehicle suspensions by CFRP parts. The lightweight rear axle weighing in at 12 kg and thus 37 % less compared to a conventional metal design, is an example. Testing of the axle under real operating conditions approve the applicability of composite design to highly-stressed components in the automobile industry with advantages like reduced weight and selective local reinforcement of the components.

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Paul Becker
    • 1
  • Andreas Büter
    • 1
  1. 1.Fraunhofer-Institute for Structural Durability and System Reliability LBFDarmstadtGermany

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