Study of a Steel’s Energy Absorption System for Heavy Quadricycles and Nonlinear Explicit Dynamic Analysis of its Behavior under Impact by FEM

Longitudinal Energy Absorption in Conventional Vehicles
 Passive safety comes into play once an accident is inevitable and its function is to reduce as far as possible the fatal consequences of an impact. On impact, the structure of a vehicle has two main functions: ‚
- The first function is to absorb the kinetic energy of the vehicle while keeping those allowable decelerations for the survival of occupants. ‚
- The second function is to preserve the integrity of the passenger compartment and so avoid the intrusion of rigid components into it.

Energy Absortion Systems

 Studying the structural behavior of a vehicle in the event of longitudinal impact reveals that the structure of a vehicle is composed of components whose function is to deform in a programmed way under impact (deformable parts), i.e., to absorb energy. These components in first instance are the vehicle bumper, and the initial section of the front rails of the vehicle (see Figure 1) [5]. In most conventional vehicles the front rail is made up of two parts: a replaceable piece called the crash box (A), which folds in a controlled pattern under impact, and a second part, the frame rail, with an initial section (B) which also folds in a controlled pattern and a second section (C) which transmits the force to the vehicle structure.

The ideal behavior of the front structure of a vehicle would provide a constant resistance to deformation (while not reaching the passenger safety cell). This would cause a constant force producing deceleration that is not only great but which must also be bearable by the occupants. Various studies focus on the behavior of the axial crushing of tubes [6,7], where folding patterns of different geometries are studied, and the evolution of compressive force versus displacement is tested. The study of geometry buckling initiators [8] is also very common. These are small predeformations in the tube that will achieve a reduction in the initial peak force for the collapse of the tube. The use of buckling initiators is very common in crash boxes and in the initial section of frame rails in order to reduce the initial buckling force.

Energy Absorption Distribution between the Different Components of the Front Structure of an Automobile.

The distribution of energy absorption between different longitudinal components can be studied. Several works have studied this, particularly doctoral theses [9–11], which make a proposal for the division of energy for an impact at 56 km/h against a rigid wall as shown in Figure.

Figure 2 shows a top view of the front of a car, where the distribution of energy absorption can be seen. The front panel of the vehicle absorbs only 10% of the total energy, and the components 6895 Materials 2015, 8, 6893–6908 which absorb more energy are the front rail of the car with 50% of the energy, followed by the motor, which absorbs 20%. Within each of the two front rail components, 7.5% of the total energy is absorbed by the crash box (Figure 1A), 7.5% by the front section of the frame rail (Figure 1B) and another 10% by the frame rail (Figure 1C). These energy absorption percentages would be different depending on the impact speed. If the velocity is not high, the components which are in the first half of the structure would be able to absorb all the kinetic energy, without deformation of the second half. These percentages correspond to an impact that does not involve deformation of the passenger compartment and the impacted object is a rigid wall which does not absorb energy.

Impact Performance of Minicars

Now that the impact performance for conventional vehicles has been shown, we will focus on the performance for quadricycles and minicars. The literature on the impact crashworthiness of minicars has been mostly written relatively recently and in all events is very scarce. One of the most complete references is the study by Hardy [12], a European study of L category vehicles (unladen mass under 350 kg or 450 kg) and whether they could meet the same regulatory requirements as M1 cars (conventional cars). The study shows wide disparity between the results of frontal-impact tests for the quadricycles reviewed, and indicates that no current quadricycle would comply with M1 category safety requirements, which suggests that the evolution of safety in quadricycles still has a long way to go. Moreover, EuroNCAP (European New Car Assessment Programme) undertook a safety campaign for heavy quadricycles (L7e) [13] in 2014, which tested the following models: Renault Twizy 80, Ligier IXO JS Line 4 Places, Tazzari Zero and Club Car Villager 2+2 LSV. The study results indicate that “all of the quadricycles tested showed critical safety problems”, and according to the executive management of ETSC (European Transport Safety Council) “these vehicles already satisfy a minimum set of requirements which is clearly not enough as the tests show” [13]. Mizuno [2] produced a work on passive safety for minicars, which clearly states that their security is less than that of a conventional vehicle due to the technological challenge of their small size and mass. The work concludes that the lack of safety is mainly due to the lack of space, and that a lower mass implies larger decelerations, and it recommends that this type of vehicle have a force limiter and pre-tensioner system for the seat belt and shrinkable columns for the steering because intrusions into the passenger compartment are one of the main problems with this type of vehicle. In a way, the chassis structure of a minicar is similar to that of a conventional vehicle, with the handicap of having less space available. Figure 3 shows the chassis structure of two minicars whose impact performance was tested [5]. Minicar type A (see Figure 3a) has two front rails connected directly to the crossbeam that supports the bumper, while minicar type B (see Figure 3b) does not have a crossbeam for the bumper and front rails are joined together by a horizontal beam over the suspension. Materials 2015, 8, page–page 4 motor, which absorbs 20%. Within each of the two front rail components, 7.5% of the total energy is absorbed by the crash box (Figure 1A), 7.5% by the front section of the frame rail (Figure 1B) and another 10% by the frame rail (Figure 1C). These energy absorption percentages would be different.