Requirements and lightweight designs for EVs: Selecting and defining a lab-scale and industrial demonstrator
Within the Fatigue4Light project, different virtual and physical demonstrators were selected to be analysed.:
- Low Control Arm and Shock Tower for CRF research centre.
- Wheels for MW.
- Chassis cross beam for Gestamp.
- Lab demonstrator for Eurecat.
Lower Control Arm
The low control arm (LCA) is a component that aims to connect the wheel to the rest of the chassis frame. The LCA can be made in iron steel, sheet steel or cast aluminium depending on the vehicle’s mission and costs.
For the light commercial vehicle taken into consideration by CRF in the Fatigue4Light project, the current solution is a mono shell made using the MPH660Y760T steel sheet in accordance with the MS.50002 Stellantis specification.
The component shape, which could appear easy, due to high strength material, the flanges and holes is complicated to make, in fact usually there are needed 6-8 transfer dies to achieve the final shape without cracks. The edge quality is fundamental to guarantee the component performance, the fatigue cracks usually start from the holes or the edges and then propagate to the rest of the component.
During the Fatigue4Light project we are making this component using different materials:
- Steel Sheet: HR980CP
- Sandwich material: aluminium skin + carbon fibre core
The shock tower is a component that aims to link the chassis to the Body in White (BIW) structures, generally made by different grades of steel sheets spot-welded together or, for the premium car, by heat-treated high-pressure die-casting aluminium alloy. For the vehicle taken into consideration, the same as for the low control arm, the shock tower is done by low alloyed steel sheets.
The shock tower is a very important part in a vehicle because almost all the vertical loads are transferred from the front suspension to the body through it: bumps and rebounds of the suspension have to be sustained by the shock tower without any failure under fatigue and misuse loads. During the project a new design of the shock tower will be implemented, taking benefit of the higher structural performances of MP1000 grade in order to reduce the weight of the vehicle.
Figure 3: Tipical shock tower made in steel sheets.
The steel wheel is made of two components: the rim, which is the cylindrical peripherical element that is in contact with the tyre, and the disc, the central element that connects the rim with the hub. The disc and rim are connected through a joint that combines both an interference fit obtained by press-fit and a series of welding.
The purpose of the wheel is to sustain the weight of the vehicle and to connect it to the road via the tyre. The fatigue resistance is the key parameter to take into account in designing the wheel, but also the rigidity and deformation resistance are important. Being the wheel an unsprung mass, its weight reduction will also result in an improvement of overall vehicle handling and shock absorption, ultimately increasing the performance of the vehicle.
Two different sets of demonstrators are planned in the project for the wheel:
- Physical demonstrators are made with current grades and new grades in order to evaluate the advantages in fatigue performance and serve as a reference to correlate the fatigue calculation & algorithms.
- A virtual demonstrator will be designed based on the previous finding obtained by the projectand compared with a solution with current materials. The final weight reduction potential will be assessed in this demonstration.
Cross members are positioned between the two long-frame members in trucks. There are usually different designs of the cross members, depending on how much space is available due to parts like the drive shaft, gearbox and exhaust system. If no parts interfere with the design of the cross member, then the design can be quite simple, as showed in the green part of the image below. The red part of figure below shows an example of max interference, where the design needs to be much more complicated to be able to transfer the same forces and torque. The forces the cross member needs to transfer come from when the two frame members move independently of each other.
The current state-of-the-art solutions for cross-member beams are typically produced by cold forming in hot rolled steels up to 9 mm sheet thickness. During the project two different load cases on component level are being used to compare the performance of different designs or materials (22MnB5 steel of 6 mm sheet thickness will be evaluated) of the cross member beam. The hot-forming process of 22MnB5 steel will allow producing a lighter part with complex geometries and superior mechanical properties.
Lab scale demonstrator
Considering the complexity of the previous demonstrators the lab-scale demonstrator aims to get a simple shape easy to manufacture and test. The lab demonstrator contains the most relevant features of chassis parts that permit a fast validation of the developed materials. As described in the lower control arm demonstrator the most critical features in terms of fatigue strength are the punched holes and trimmed edges. The damage introduced in this manufacturing process reduces the fatigue resistance acting as crack initiation points. Once nucleated, the cracks will easily propagate throughout the part leading to complete failure. To reproduce this behaviour in a lab test, the L-shape geometry shown in the following figure was proposed.
Using this lab demonstrator, the following materials were tested:
- Stainless steel (AISI 410S) for press hardening
- Sandwich material: aluminium sheet 6082 or 6181A with high scrap content for the skin and the core made by GFRP