Keywords

1 Introduction

In the last decades, the percentage of people living in urban environments has increased from 65 to 71%, and this trend is expected to continue in the following years [1]. In terms of mobility, this means that traffic congestion and air pollution will intensify. As a result, the EU has emphasized the need to develop lighter and cleaner passenger vehicles, while still providing safety for its occupants, as well as for other road users [2].

The Multi-Moby project, a European initiative within the Horizon 2020 Programme, aims at develo** a cost-effective and safe vehicle structure for urban electric vehicles. The structure proposed is based exclusively on high-strength steel tubes, in such a way that it is modular and easily scalable. In practice, this means that the vehicle length, and even the vehicle purpose can be modified just by adapting a few tubular elements, at a much lower cost and time compared to the development of conventional vehicles (Fig. 1).

From a safety point of view, the objective is to design a vehicle structure providing comparable safety performance to that of conventional vehicles of higher dimensions, hence M1 homologation tests are taken as reference for the safety assessment. Besides, the protection of Vulnerable Road Users (VRUs) is considered, therefore the structure designed shall minimize the damage caused to pedestrians in the event of a run over.

Fig. 1.
figure 1

Modular approach: minimum structural changes lead to different vehicle concepts.

Full size image

2 Methodology

The structural design was optimized via Finite Element analysis. The development process followed several iterative steps until meeting the target key performance indicators concerning the structural stiffness (2.000 N/mm bending stiffness, 3.000 Nm/° torsional stiffness), durability (250.000 km) and crashworthiness (UNECE Regulations). Firstly, the structure was modified until ensuring sufficient stiffness and durability, to enable a proper operation of the vehicle’s safety systems, and then the complete vehicle was simulated and physically tested to evaluate its crashworthiness.

2.1 Torsional and Bending Stiffness

The torsional stiffness test serves as reference to assess the robustness of a vehicle structure. From a functional point of view, a minimum torsional stiffness is needed to ensure a correct vehicle behavior, especially in transverse maneuvers such as turns. In this work, the torsional stiffness measurement was carried out without installing the vehicle suspension, to avoid external influences on the results. Tailored tools were manufactured to introduce the torsional loads, concretely by locking the rear axle and allowing only the rotation of the front axle around the longitudinal vehicle direction.

Fig. 2.
figure 2

Torsional (left) and bending (right) test setups.

Full size image

On the other hand, the bending stiffness test provides information about the robustness of the vehicle structure when subjected to vertical loads due to the weight that is carried, to acceleration and braking maneuvers, or to the driving over road irregularities. The testing setup also required the manufacturing of specific tools, in this case locking both vehicle axles, with an additional tool for introducing the vertical load in the central part of the structure. Both test setups are depicted in Fig. 2.

2.2 Durability Tests

The durability (fatigue) tests of the structure were carried out taking daily driving maneuvers as starting point for the loads definition. A representative multi-body vehicle model was developed for this purpose and introduced in CarSim software for the estimation of the loads induced to the structure, in the following maneuvers: acceleration, corner braking, driving over a speed bump, and moose test. Additionally, all maneuvers were simulated at different driving speeds, representing different driving profiles. Three aggressiveness levels were defined for the maneuvers: level 1, with longitudinal and transverse accelerations around 0.25 g; level 2, around 0.5 g; and level 3, around 0.75 g. From the simulation results, the cyclic loads were defined for the real tests, in such a way that they induce similar damage to the vehicle structure.

2.3 Crashworthiness Performance

Occupant protection: after analyzing the applicable testing protocols, mainly the UNECE Regulations for the M1 vehicles and the EuroNCAP test procedures, the safety tests according to UNECE Regulations R137 [3], R94 [4], and R95 [5] were selected. Their characteristics are summarized in Table 1. In addition, regarding the biomechanical parameters, a target value for the Occupant Load Criterion (OLC) was set at 40 g.

Table 1. Summary of selected UNECE Regulations for the crashworthiness evaluation
Full size table

Vulnerable Road Users (VRUs) protection: in what concerns the VRU protection, the study was only performed through simulations. To constraint the great variety of run over events that can happen in real life, the following variables were considered as part of a parametric study: frontal geometry of the vehicle (sharp - aggressive or rounded - smooth edges), bumper stiffness, vehicle speed (20, 30 and 40 km/h), vehicle-dummy relative position (central, left side or right side position) and alert state (preparedness) of the dummy right before the crash (different stiffness of the body muscles).

3 Results

3.1 Torsional and Bending Stiffness

The torsional stiffness was simulated up to 2° of torsion measured in the frontal part of the vehicle, registering a torsional stiffness of 2847 Nm/°. In these conditions, the maximum stresses registered reached 350 MPa, well below the elastic limit of the material, hence ensuring the integrity of the structure. In the physical tests, 1.5° of torsion in the frontal part of the vehicle were reached, registering a torsional stiffness of 3.071 Nm/°, fully aligned with the simulation.

The bending stiffness was simulated introducing a vertical load of 25 kN. In this scenario, a bending stiffness of 2.730 N/mm was measured, and the maximum stresses registered did not surpass 400 MPa, again comfortably below the elastic limit of the material. In what concerns the physical tests, a vertical load of 15 kN was introduced, registering a bending stiffness of 2.330 N/mm, hence validating the simulation results.

3.2 Durability Tests

As in the previous case, the fatigue performance of the structure was studied through simulations and real tests. For the simulation, a vehicle weighing 850 kg (includes batteries and passengers) was considered. The results obtained showed that maximum stresses are kept below 150 MPa in the most loaded areas, what guarantees the durability of the structure. After the simulation, physical tests were conducted. The structure was subjected to a million load cycles, representing the target 250.000 km, distributed as follows: 800.000 cycles under level 1 loads, 190.000 cycles under level 2 loads, and 10.000 cycles under level 3 loads. The structure withstood the test without any visible damage. The structure critical nodes were inspected by liquid penetrants test, to identify small crack that might have appeared during the test, no evidence of damage was found.

3.3 Crashworthiness Performance

Occupant protection: the crashworthiness of the structure was assessed according to the specifications of the aforementioned UNECE Regulations, including for each crash scenario the evaluation of both the structural behavior and the occupant protection.

Frontal impact according to Regulation 137: the structural behavior is highly satisfactory and the simulated performance correlates well with the actual performance of the prototype. The vehicle proves to be safe for its occupants in this load case, and the structural integrity was checked. The OLC value is 38.9g, below the limit values targeted. Likewise, other biomechanical parameters monitored in the dummy are below the threshold values set in the Regulation. Considering that this test configuration is the most severe among the frontal tests covered in the UNECE Regulations, it can be concluded that the structure is suitable to protect the occupants in this kind of crash events. The simulation and physical test results are presented in Fig. 3.

Fig. 3.
figure 3

Vehicle structure after the test according to R137, simulation (left) and real test (right).

Full size image

Frontal impact according to Regulation 94: since this load case is slightly less severe than the previous one (deformable barrier and partial overlap), acceptable results were expected. Indeed, the structure maintained its integrity and did not present any potentially dangerous deformation or break in the cabin. Additionally, no relevant intrusions were observed in the occupant surroundings. Figure 4 shows the physical test results.

Fig. 4.
figure 4

Vehicle structure after the test according to R94 (left), and detail of the front-end (right).

Full size image

Both frontal tests were also employed to assess the battery protection capabilities of the structure, trying to anticipate the potential triggering of thermal runaway phenomenon in the battery that may threaten the safety of the vehicle occupants after a frontal crash. In this direction, acceleration and intrusion levels in the surroundings of the battery pack were measured. In both cases, a relatively smooth behavior (progressive decelerations, no intrusions) was observed, slightly better for the R94 test scenario, mainly due to the lower energy involved in the crash.

Lateral impact according to Regulation 95: the results were also successful, although in this scenario the low weight of the vehicle causes it to be dragged during the crash, what limits deformations, and consequently the energy absorbed by the structure. This test also served to check the outstanding performance of the steel doors. Its stiffness and robustness help control the intrusion of the barrier into the cabin, therefore protecting the occupant. Similarly, the deployment of the side curtain airbag was satisfactory, also contributing to reduce the damage on the occupants. In what concerns the battery integrity, the structure design lets the barrier to slide over the sill, kee** the battery region almost intact. The simulation and physical test results are presented in Fig. 5.

Fig. 5.
figure 5

Vehicle structure after the test according to R95, simulation (left) and real test (right).

Full size image

Vulnerable Road Users (VRUs) Protection:

The simulation results of the run over scenarios show that the damage induced in the VRU is strongly related with the frontal dimensions of the vehicle. From a certain speed, the short vehicle bonnet forces the dummy head to crash against hard areas of the vehicle, such as the windscreen or the A-pillar, which translates into a severe injury potential. This event is by far the most critical; however, it is observed that at low speeds, if the vehicle has a smooth geometry, the damage induced in the legs and pelvis can be reduced to some extent.

4 Conclusions

Considering the results as a whole, it can be stated that it is feasible to design a safe urban electric vehicle based on a tubular structure made of high-strength steel. The appropriate design of the structure allows obtaining enough stiffness to increase occupant safety, as well as the deformation capability to absorb the energy involved in a crash event. However, it is worth noting that the influence of the vehicle design on the VRUs protection is almost negligible. This result highlights the need for integrating in the vehicle active safety systems specifically targeting VRUs protection.