Comparison

Table 5.13: U Model MPDB - intrusion measurements Direction or Intrusion

Position [mm]

Steering column

x 66

y -5.6

z 36

A pillar upper 29

lower 42

Firewall upper 135

lower 73

Door opening width - driver side upper 35

lower 14

Door opening width - passenger side upper 4

lower 1

Cross car beam - fixture point

x 63

y -6.2

z 39

Cross car beam - max deformation

x 68

y -5.4

z 25

5.4 – U Model

Figure 5.76: U Model - energy content comparison

Crash pulse

Looking at the crash pulse, the behaviour mentioned above is made even more evident:

in the first 20% of the MPDB crash, the vehicle has already gone through a high peak of acceleration, deforming the barrier front core and part of the middle core, damaging a great part of the upper front structures and also loading both main rails and crash boxes.

In ODB, during an equal percentage of the crash, the vehicle is only lightly loaded as it moves the barrier laterally and after 35ms (35%) finally hits the rigid wall and the lower structures start being loaded. The maximum acceleration peak achieved is similar for both tests, with a difference of only 1g, while the average is substantially higher for the MPDB, still mainly due to the peak occurring at the beginning of the crash. This is visible also in Figure5.80, where the slope of the line is much steeper for the new procedure in the initial stages of the impact, while the ODB shows a flat line. In addition, another substantial difference is represented by the behaviour of the engine mounts and consequent movement of the engine block. In the first ms of the crash the force exerted by the mobile barrier on the vehicle causes the engine mounts to deform and partially detach from their position, allowing the engine to move and hit the firewall. Part of the initial high peak can be attributed to this phenomenon, which affects the subsequent stages of the crash. This does not occur in ODB due to the softer initial contact with the barrier. It has to be further investigated if what is seen in the MPDB is in fact realistic. The two models are identical as they have not been modified in any way, apart for the dummy masses, between one procedure and the other, hence it seems that the comparison is meaningful; however, the engine mounts could have modeling issues which are made evident only in the new procedure.

With regards to the differences in deformation of the crash structures, it has to be

noted that the crash box appears to be working properly only in the ODB, as in the mobile barrier test it is pulled inwards by the cross beam and fails due to bending rather than axially. On the other hand, the load direction caused by the rigid block behind the barrier makes the component buckle perfectly in the current procedure. Furthermore, the main rail undergoes a much higher level of deformation, as it is the only element supporting the load against the rigid block. In MPDB the left hand side rail shares part of the load, failing axially only a few ms after the left hand one starts to buckle. These differences in behaviour of the main rails drive another point of discrepancy between the two tests, which consists in the loading of the sill and firewall by the wheel and tire assembly. In the ODB this component sees a much more reduced space and a high force being applied to it as it is squeezed between a rigid block and the cabin of the vehicle. In MPDB this is still present but to a lesser extent as the main rail has not been shortened as much, and the tire is blocked between the sill and the barrier’s deformable element.

The results of all the differences here reported cause major discrepancies between the loads that the cabin has to sustain at the end of the crash and the consequent levels of deformation and intrusion that will be shown at the end of this section.

Table 5.14: U model - ODB vs MPDB dynamic data

ODB MPDB Difference % Difference

Peak acc. [−g] 36.32 35.40 0.92 2.52

Mean acc. [−g] 15.96 20.76 -4.80 -30.11

Time to zero velocity [s] 103.10 69.50 33.60 32.59

Figure 5.77: U Model ODB - structural damage

5.4 – U Model

Figure 5.78: U Model MPDB - structural damage

Figure 5.79: U Model - crash pulse comparison

Figure 5.80: U Model - velocity comparison

Section forces

The section forces comparison shown in Figure 5.81highlights better the trends analysed in the previous paragraph, concerning the right hand side crash structures. The crash box in the MPDB procedure clearly has to support a much lower loading compared to that seen in ODB and its point of failure is not under the maximum allowable load that the component was designed for, but due to an incorrect failure mode. The front part of the main rail also sees a much lower force, which justifies the lower level of deformation in this section of the structure. Finally, the rear section forces appear to be similar in the peak force under which they collapse. A more interesting trend, however, is presented in Figure 5.81: it is possible to see how the left hand side rail is loaded in a relevant way during the MPDB impact. During the initial phases of the crash, this part of the structure is experiencing about 60% of the force of the directly loaded rail; in the final part of the crash, the structure is loaded again to about half the values seen by the rear segment of the rhs structures. In the ODB, on the other hand, this does not occur at all: the loads are extremely low for the whole duration of the impact, up until the point where the rail is pulled towards the centreline of the vehicle and fails due to bending. The overall contribution of this structure towards energy absorption and limitation of the damage to the cabin seems to be vastly different between the two tests. This trend was visible in the previous vehicles as well, but its significance was absolutely marginal, while in this case it is definitely worth noting. Regarding the section force trends for the locations further down the rail, the same behaviour can be found, but the difference between the two tests goes diminishing due to decreased values seen in the MPDB, while the ones for ODB remain very low throughout.

5.4 – U Model

(a) Crash box (b) Main Rail front

(c) Main Rail mir (d) Main Rail rear 2

Figure 5.81: U Model - RHS section force comparison

Figure 5.82: U Model - LHS rail section forces comparison

Intrusion and cabin deformation

As already mentioned, the deformation and intrusion levels of the U model are all around very high. The results of the ODB test however are considerably worse than those obtained from the mobile barrier procedure, under every aspect. Table5.15 shows the comparison between the values recorded for the two tests and there is no area showing a small discrep-ancy, while no value is low in the first place. The firewall deformation reaches values above 240mm for the current test procedure, with the pedals and the passenger area all being greatly affected, while the same deformation in the MPDB shows maxima of 170mm and a lower pedal backwards movement. Furthermore, it is evident that the deformation of the A-pillar, sill and consequent failure of the roof are much more pronounced in the ODB, where the deformation shows its effects until the rearmost parts of the cabin’s side and top panel. The B-pillar is also affected in the offset deformable barrier test, while it shows no damage at all in the new procedure. These phenomena result in much larger intrusion of the dashboard, steering wheel and instrument cluster, both on the upper section and in correspondence with the knee area, as it is visible in Figure5.84.

In conclusion, the vehicle’s performance is not adequate in both procedures. The results, however, show that the vehicle would need a more limited number of actions in order to achieve a satisfactory level of occupant protection, due to the lower intrusions and reduced cabin collapse obtained through MPDB procedure. Nonetheless, the whole frontal structure would have to be highly revisited to achieve this objective. A completely different problem is represented by the issue of partner protection, as the vehicle clearly deforms heavily the barrier with the main rail puncturing it deeply.

Table 5.15: U Model - intrusion measurement comparison

Direction or ODB MPDB Difference

Position [mm] [mm] [mm]

Steering column

x 108 66 -42

y -10.5 -5.6 -4.9

z 72 36 -36

A pillar upper 71 29 -42

lower 109 42 -67

Firewall upper 190 135 -55

lower 181 73 -108

Door opening width - driver side upper 99 35 -64

lower 91 14 -77

Door opening width - passenger side upper 9 4 -5

lower 9 1 -8

Cross car beam - fixture point

x 103 63 -40

y -7.9 -6.2 -1.7

z 91 39 -52

5.4 – U Model

(a) ODB 1

(b) MPDB

Figure 5.83: U Model - firewall intrusion comparison

(a) ODB

(b) MPDB

Figure 5.84: U Model - interior intrusion comparison