A first step that can be taken toward understanding structural interaction is to com-pare frontal fixed barrier crash tests and car-to-car head-on collisions.
In a collision with a fixed barrier, the interaction interface remains planar throughout the crash as the wall is also planar and, with respect to the magnitude of the front-end forces of the car, infinitely stiff. Each load path in the vehicle front-front-end is acti-vated and deformed to the same degree, even if the distribution of forces within the front-end, measured at the wall, is not homogenous. Figure 17 is an illustration of a EURO NCAP crash test involving a Rover 75 and reflects the planar nature of the loading of the front-end of the vehicle.
Figure 17 Rover 75 – EURO NCAP Crash Test – 64km/h. Overlap ratio 40%.
Post-crash photographs confirm the planar nature of the interaction, Figure 18. The dissipation of energy through deformation of the front-end of the vehicle can be as-sumed to be maximal for a fixed barrier collision. In Figure 17, a deformable element can be seen, located in front of the wall. The element is reasonably soft, so that bot-toming out occurs early in the collision. The majority of the interaction in this collision occurs between the vehicle front-end and the wall.
Figure 18 Post-crash deformation (Rover 75) – EURO NCAP Crash Test – 64km/h. Overlap ratio 40%
The fixed barrier acts as the ideal impact object, providing infinite support forces to deform each of the load paths within the vehicle front-end. In fixed barrier collisions, structural interaction can be considered to be perfect, as only one of the bodies de-forms.
When a passenger car collides frontally with an impact object other than a planar, rigid fixed barrier, the deformation of the front-end is fundamentally different. If the object is deformable, the structures interact together to dissipate the crash energy. In car-to-car collisions, for example, structural interaction is a relevant consideration. In such collisions, uneven deformation of the front-end of both vehicles occurs, resulting in a non-planar interface between the two bodies. The non-planar nature of the colli-sion interface is shown for a car-to-car head-on collicolli-sion involving a Rover 75, Figure 19.
Figure 19 Overhead view - Car-to-car head-on collision – Rover 75 (right) versus small passenger vehicle (left). Closing velocity 112km/h. Overlap ratio 50% of the width of the smaller vehicle
The non-planar nature of the collision interface is more evident in the post-crash pho-tograph of the Rover 75, Figure 20.
Figure 20 Post crash deformation (Rover 75) – Rover 75 versus small passenger vehicle.
Closing velocity 112km/h. Overlap ratio 50% of the width of the smaller vehicle
For a car-to-car, head-on collision, less than maximal structural interaction can be expected to occur. A reduction in structural interaction results in lower deforma-tion forces (on average) and a lower degree of energy being dissipated within the front-end than in a collision with a fixed barrier.
For a given vehicle, a front-end force-displacement characteristic can be calculated for a collision with a fixed barrier. This can be considered to represent the maximal force-displacement characteristic for this vehicle. Figure 21 shows two sketched force-displacement characteristics for a collision involving a light vehicle (vehicle A) and a heavy vehicle (vehicle B).
Due to the influence of the engine and transmission, the force at the rigid wall is higher than that calculated in the compartment. For this research, structural interac-tion will be calculated based on compartment forces and the influence of the engine will be neglected. See chapter 8.
Vehicle B Fixed-wall collision Vehicle A Fixed-wall collision
Force Force
Crush Crush
C D
A B
Figure 21 Sketched force-displacement characteristics for a light vehicle (vehicle A) and a heavy vehicle (vehicle B) involved in a collision with a fixed barrier
One method to predict the maximum possible degree of energy dissipation between two vehicles involved in a head-on collision is to statically combine the fixed barrier force displacement characteristics of these vehicles. The result is a force versus total displacement characteristic, where total displacement refers to the combined crush of both vehicle front-ends. This process is illustrated in Figure 22, based on the force-displacement characteristics for vehicle A and vehicle B, shown in Figure 21 above.
The first step is to mirror the fixed barrier force-displacement characteristics for vehi-cle A and vehivehi-cle B about a theoretical collision interface, Figure 22.
Crush Vehicle B A
Force
Vehicle A Vehicle B
C B
D
FA/B
FC/D
Crush
Crush Vehicle A
Force
Crush
C
Figure 22 Mirroring force-displacement characteristics of two vehicles about a common axis to act as the basis for developing predicted maximal force versus combined front-end displacement for a car-to-car, head-on collision
The force at the crash interface is theoretically equal for both vehicles (action and reaction) and the magnitude of displacement of the front-end of each vehicle in a fixed barrier collision, corresponding to a given force level, can be added as shown in Figure 23. The shaded areas in Figure 23 correspond to the shaded areas in Figure 22.
Vehicle A Vehicle B
Combined crush Vehicle A and Vehicle B F , FA B
F , FC D
(Vehicle A) + (Vehicle B)
Combined Crush
Force
Figure 23 Predicted maximal force versus combined front-end displacement for a car-to-car, head-on collision - Statically combined fixed barrier, front-end force-displacement charac-teristics
The statically combined characteristics shown in Figure 23 represent the highest possible level of energy dissipation that could occur between two vehicles involved in a head-on collision. As the level of structural interaction occurring in the collision de-creases, the deformation forces theoretically decrease as well. In order to dissipate the same amount of kinetic energy, a higher degree of deformation is required. This is illustrated in Figure 24, based on the statically combined force-displacement char-acteristic developed in Figure 23.
Total
Displacement Force
F FA B F FC D
Combined front-ends
Passenger Compartment
Cases representing reduced structural interaction
Figure 24 The theoretical influence of reduced structural interaction on the force versus total front-end crush characteristics exhibited by two vehicles in a head-on collision
Figure 24 represents a theoretical car-to-car, head-on collision of high severity in which the entire available front-end deformation travel is required for the case of maximal structural interaction. As the level of structural interaction decreases, in or-der to dissipate the same degree of kinetic energy through deformation, a higher de-gree of compartment deformation is required. The observations made in this chap-ter lead to the following definition for structural inchap-teraction which forms the basis of this research:
The degree of structural interaction that occurs in a head-on collision can be de-scribed as the proportion of the actual energy dissipation through structural defor-mation compared to the maximum possible energy dissipation, for a given colli-sion configuration. The maximum possible energy dissipation can be predicted by statically combining the force-displacement characteristics exhibited by each vehicle in a fixed barrier crash test at the given degree of barrier overlap.
All curves previously generated in this chapter are based on the assumption that the compartment strength of the lighter vehicle exceeds the front-end force of the heavier vehicle. This was already discussed in section 2.3.2 as the Bulkhead Concept was introduced. The implications of this assumption will be discussed in more detail in the conclusion to this chapter.