Offset Test Procedures

The ODB, PDB, and MPDB are evaluated regarding the requirements presented in Tab. 4.5:

Compartment strength: The strength of the deformable element in the ODB is much lower than the force levels of modern vehicles, which were developed after the implementation of ECE R94. Thus, the ODB will be fully crushed in almost every test with normal passenger cars [138, p. 26], and the test vehicle will hit the rigid wall behind the barrier and produce high severity loads for the occupant compartment of the vehicle.

A Japanese crash analysis [134, p. 7] showed that the crushing strength of the ODB is comparable with the force levels of minicars. Due to the light weight of minicars and their low crash energy, a part of the deformable element remains uncrushed and the minicar does not hit the rigid wall. Therefore, assessment of the compartment strength for microcars could be inadequate in the ODB test procedure. As discussed in Section 4.1.3, the crash severity of the ODB test raises with the vehicle mass. Therefore, it is not representative of compartment loads in car-to-car collisions, which decrease with vehicle mass.

The stiffness of the PDB is higher than the ODB and comparable with the force levels of the current vehicle fleet in Europe [142, p. 2]. The 50 % overlap and the progressive stiffness of the barrier in the PDB and MPDB test procedures normally result in high intrusions [138, p. 35]; [140, p. 26]; [141, p. 12] and an adequate assessment of the compartment integrity. However, simulation analyses [140, p. 26] and crash tests [138, p. 35] of the FIMCAR project have shown that the intrusions in the PDB tests with heavy vehicles is lower than in the ECE R94. This issue was also observed in one MPDB test with a SUV [141, p. 12]. Thus, it is doubtful that the PDB and MPDB test procedures can assess the compartment integrity of heavy vehicles adequately.

As described in Section 4.1.4, the objective of the PDB was to harmonize the collision severity in term of EES for the entire fleet of passenger cars in Europe. Thus, severity of the PDB test procedure is higher for light cars compared to the ODB test. However, the PDB test procedure cannot represent the kinematics of car-to-car collisions with unbalanced crash severities. On the contrary, the MPDB test can represent the crash kinematic of car-to-car collisions and therefore simulate the unbalanced compartment loadings in car-to-car collisions better.

Restraint systems: The ODB test procedure produces acceleration pulses that are similar to car-to-car offset collisions with mass ratios close to one (Fig. 4.16). However, the ODB cannot represent the unbalanced velocity differences and consequently the crash pulses of vehicles in car-to-car collisions with mass ratios higher than one.

Simulation [140, p. 36] and crash test results [142, p. 10] from the FIMCAR project showed that the PDB test procedure produces slightly higher acceleration pulses than the ODB test procedure. The acceleration pulses have the same form, particularly at the beginning of the collision, which is decisive for triggering the restraint systems (Fig. 4.17).

Therefore, the PDB is also assumed to have a representative acceleration pulse for car-to-car tests with a mass ratio close to one, as observed in the ODB test procedure. The acceleration pulses in the MPDB test depend on the mass ratio and are generally higher due to more kinematics in the test procedure. Thus, the MPDB can represent car-to-car collisions with different mass ratios.

Structural interaction: The deformable block of the ODB is unstable in tests with modern vehicles. Previous studies by EEVC WG15 [97, p. 35] showed that the ODB can be deformed differently in tests with the same vehicle model. Therefore, the ODB cannot assess the structural integrity of vehicles. The offset constellation of the ODB test provides the potential to reflect low horizontal structural interactions in higher deformation and intrusion values. However, the unstable deformable element cannot distinguish between low and high vertical structural interactions. The PDB and the MPDB are designed to assess the structural interaction of vehicles, and they showed repeatability and robustness in crash tests performed in the FIMCAR project [140, p. 39].

Our previous work [130] conducted simulation analyses to investigate the capability of different barriers to reflect structural properties in the test results. The structural

Figure 4.16: Minicar crash tests shown by airbag’s deployment time and the time when the unbelted occupant reaches 127 mm

According to [134, p. 7]

Figure 4.17: Acceleration pulse of the Fiat 500 normalized to the absolute value of maximum acceleration peak in the FWRB test at 56 km/h

According to [140, p. 39]

Time of the airbag's deployment in ms

Time when the unbelted occupant reaches 127 mm in ms ODB

properties of a validated simulation model, i.e., Toyota Yaris from National Crash Analysis Center (NCAC) [143], are varied to create four different vehicle models with varying structural properties (Tab. 4.7). Mini E-Car represents electrified microcars with low horizontal and vertical structural interaction. E-Car represents vehicles with low horizontal structural interaction, and Strong Car represents vehicles with high horizontal and vertical structural interaction.

Table 4.7: Variations of the Toyota Yaris with different structural properties [143, p. 4]

Model Name Structural Property Changes relative to the original model Mini E-Car Low horizontal and

vertical structural interaction

1- No motor block or radiator in the front to represent electric vehicles without the load path of the motor block in the middle

2- Reduced height by 50 mm to represent microcars with the risk of override

3- Added a battery pack to the rear section E-Car Low horizontal

structural interaction

1- No motor block or radiator in the front 2- Added a battery pack to the rear section

Basic Model Normal No changes

Strong Car High horizontal and vertical structural interaction

1- High strength material for front structural components (e.g., bumper and radiator frame) 2- Higher thickness for front structural components

(e.g., bumper and radiator frame)

3- The density of the materials is scaled to retain a similar mass as the basic model

The structural properties of the vehicle models are validated in two tests. Vertical structural interaction is tested against the bumper of the Research Council for Automobile Repairs [144] at 56 km/h, and the horizontal structural interaction is tested against the original Yaris model with 50 % overlap and a collision speed of 100 km/h (i.e., 50 km/h for each party). The test results confirmed the structural properties presented in Tab. 4.7, and the vehicles with higher structural interaction had more homogenous deformation patterns (Fig. 4.18).

These vehicle models are simulated in the ODB and PDB test procedures. The ODB test results [130, pp. 6-7] showed that the structural differences influence the

Mini E-Car E-Car Basic Model Strong Car

Vertical Horizontal

Figure 4.18: Test results corresponding to the structural properties [130, p. 4]

acceleration pulses, but do not affect the intrusion values. However, the PDB can reflect the structural properties in the acceleration pulses and intrusion values of the vehicles.

Force levels: As described in Section 4.1.3, the ODB cannot represent the force levels of the new generation of cars and is too soft to assess the force levels. The PDB and MPDB are intended to represent the opponent vehicle in a car-to-car collision [142, p. 3]

and should therefore represent the force levels of the vehicle fleet of passenger cars in Europe. However, some crash tests from ADAC [14, p. 9] showed that the load spreading in the deformable element of the MPDB test procedure is different from the deformation patterns that occur in comparable car-to-car collisions (Fig. 4.19). ADAC experts stated that the upper part of the PDB is stiffer than an average car.

Furthermore, the PDB and MPDB provide a high potential of deformation, which makes the deformable element unlikely to bottom out. The automotive industry criticized the large deformation potential, which provides a possibility of abuse to increase the vehicles’ force levels without influencing the test results [97, p. 26].

Conclusion: The baseline test of the ODB, PDB, and MPDB test procedures is a car-to-car collision. Therefore, constellation and test set-up of these barriers are appropriate for use in assessing the compatibility rate.

Tab. 4.8 summarizes the evaluation results for the offset test procedures.

Table 4.8: Evaluation of offset test procedures

Requirements ODB PDB MPDB

Assessment of the compartment integrity in an offset test procedure + + + High loads for light cars as observed in car-to-car accidents - + + Comparative severity with current crash tests for heavier vehicles + - - Higher acceleration pulse for light vehicles in the offset test

procedure - - +

Representative acceleration-time pulse + + +

Reflection of low and high structural interaction in intrusions and

acceleration pulses - + +

Barrier should represent the average force level of passenger cars - - - Representative crash constellation for car-to-car collisions + + +

As can be seen, none of the proposed offset test procedures fulfill the important requirements for a comprehensive assessment approach and evaluation of the

Figure 4.19: Deformations in car-to-car test (left) and car-to-MPDB test (right) [14, p. 9]

compatibility rate. The MPDB is the best candidate and might be able to fulfill all requirements with some improvements. ADAC made some changes to the MPDB test procedure [14, pp. 9−11] to improve its representability of the force levels of normal passenger cars. However, the issue of less severity for heavier vehicles and the potential for misuse remain open. Thus, an alternative offset test procedure is needed for assessing the compatibility rate.