Hybrid lightweight rear axle for electric vehicles
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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.
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.
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
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.
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.
▸ 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.
Determined safety factors for the loading situation „acceleration during cornering”
Computed safety factors (CFRP central beam)
25 % of maximum acceleration 97 % of maximum steering angle
50 % of maximum acceleration 87 % of maximum steering angle
75 % of maximum acceleration 66 % of maximum steering angle
Determined safety factors for the loading situation „running over a curb”
Computed safety factors (CFRP central beam)
Maximum vertical load (running over a curb)
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
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.
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.