MPDB test - STCModifications of Car Crash Structure for Optimized Mobile Progressive Deformable Barrier Test Performance
Ladislav Dvorák∗1 1 CTU in Prague, Faculty of Mechanical Engineering, Department of Mechanics, Biomechanics and Mechatronics, Technická 4, 166 07
Prague 6, Czech Republic
Abstract The aim of this thesis is to explore possible improvements of the car crash structure to reach better results in the mobile progressive deformable barrier (MPDB) test. This work presents the new Euro NCAP’s frontal impact methodology, which encourages manufacturers to achieve better compatibility of vehicles during car-to-car crashes. Current design of car crash structure is studied and parameters having the strongest influence on the performance in the MPDB test are identified. Based on the findings, several design improvements are introduced and evaluated in an explicit FEM solver. Finally, the results are discussed and general suggestions are made.
Key-words: Crashworthiness, frontal impact, compatibility, MPDB
1. Introduction Safety of passenger cars has been increasing in the last decades [1]. This optimistic trend complies with the fact that despite the rising worldwide motorization, the rate of road traffic deaths per capita has remained constant between the years 2000 and 2016 and the number of deaths for 100,000 vehicles dropped more than twice over the period [2]. These facts imply that a good amount of progress has been made. Never- theless, with over 1.35 million deaths each year, road traffic remains a critical issue and more improvements in traffic safety should be made [2].
Safety of a car is ensured already by the homologa- tion process that requires every car to satisfy specific criteria before introduction to the market. Homolo- gation regulations are, however, changing slowly and do not necessarily keep up with the advancement of technology. They set a minimum standard, but are unable to evaluate how safe a particular car is.
From the point of more sensitive assessment, con- sumer organizations such as Euro NCAP, the Euro- pean New Car Assessment Programme, play an im- portant role. Their consumer tests encourage manu- facturers to invest more resources in the development of safer vehicles. Their assessment criteria are regu- larly updated and can thus keep up with the state of the art and encourage the manufacturers further.
One of such updated criteria is the Euro NCAP’s recently updated frontal impact assessment with the new test scenario with a mobile progressive de- formable barrier (MPDB). Previously, the tests were designed to assess vehicle self-protection with the as- sumption that the structure of the car is ideally hit. Real world crashes, however, indicate that the sup- porting structure is not always hit and the impact en- ergy causes cabin deformation in such cases [1]. The newly introduced MPDB test attempts to evaluate just the issue of compatibility of vehicles. [3]
2. Passive safety in front crashes 2.1. Active and passive safety
Safety of vehicles can be divided into two main areas. Active safety aims to avoid collisions, whereas pas-
sive safety describes the ability to mitigate collision consequences [4]. These two terms are identical to the terms primary and secondary safety, respectively, used in other literature.
Active safety is influenced by three main factors: the environment in the form of a suitable traffic in- frastructure, the human factor in the form of hu- man behaviour and knowledge, and finally the ve- hicle design. Current vehicles have numerous active safety systems including ABS (Anti-lock braking sys- tem), ESP (Electronic stability program), or more advanced systems such as EBA (Emergency braking assistant). Nevertheless, the most important active safety features are the fundamental systems enabling the driver to control the car, namely, good brakes, proper steering, or visibility from the driver’s seat [5].
Passive safety, aiming to minimize collision conse- quences, can be further divided into active measures and passive measures. The active measures repre- sent systems preventing injuries triggered in case of a crash, while passive measures do not need to be trig- gered and include most importantly the car structure with a crumple zone and safety cell [5].
Some of state-of-the-art technologies often act as features of both active and passive safety, that is, in some situations they avoid crashes, in the other they minimize the consequences. One of them is the radar which is incorporated in vehicle’s front end. It detects obstacles to initiate autonomous braking (to avoid collision) and alongside it actives electric seat belt pretensioners (to minimize injuries in case of a crash) [4].
2.2. Crumple zone and safety cell
As one of the most important features of passive safety, the vehicle body has two functions: absorb- ing energy in the crumple zone and securing integrity of the safety cell. The ultimate goal is not to make the vehicle structure as stiff as possible, the objective is rather to find a combination of a suitably deformable front end and a stiff passenger compartment.
Safety cell, sometimes denoted as survival space, is a part of the car which should remain undeformed and where there is space for the passengers to survive
∗Corresponding author: Ladislav.Dvorak@fs.cvut.cz
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in crashes. In any case, it should not be penetrated by any other part of the vehicle (e.g., steering col- umn, pedals). The safety cell must provide enough space for the passengers’ bodies to decelerate and the exposed surfaces should be soft and without sharp edges [6].
Crumple zone is the part of the car which is capa- ble of absorbing the energy of the impact. It serves as packaging for the safety cell. The structure is de- signed to deform in a way that the safety cell should experience the lowest achievable deceleration. It aims to absorb as much impact energy as possible to stop the deformation before reaching the safety cell. Be- sides, integrity of the principal parts of the construc- tion, e.g., longitudinal and bumper beams, should be ensured during the impact so that the vehicle can withstand a possible secondary impact. Lastly, there are other requirements for low-speed impacts. Most parts of the construction should remain without any plastic deformation and the related repair should be affordable [4].
2.3. Restraint systems
During collision, the safety cage is exposed to high de- celerations. Restraint systems, part of passive safety, ensure that occupants move only in a designed way during this phase. The main goal is to slow down the occupants’ bodies so that they do not get injured, and prevent them from hitting any part of the structure, e.g., dashboard.
Seat belts, together with airbags and possibly child seats, are the primary restraint devices. In frontal impact, they tie the occupants to the decel- erating safety cage and reduce forward displacement. In rollover, side, or more complex crashes, they sim- ply keep the occupants in place inside the safety cage [7].
In the majority of today’s cars, there are three- point belts with pretensioners and force limiters. These three devices cooperate in a specific manner with a precise timing. After the ECU, Engine Control Unit, detects a crash, it activates the seat belt pre- tensioners which tighten the belts and decrease the slack. As the occupant’s torso moves in the forward direction (relative to the decelerating safety cell), the force limiter gradually loosens the belt to ensure a limited pressure is applied on the thorax. Thus, the passenger does not get seriously injured by the belt and the path for deceleration is longer [7].
In Europe, airbags are perceived as a supplemen- tary restraint system and are designed to assist the seat belts. In frontal crashes, they provide a better protection of head, neck, and upper torso and catch the occupant in the last phase of the slowdown. The frontal airbags must be synchronized with the seat belts so that they can provide the best protection. Their vents are designed to deflate the airbag at a specific speed so that it acts in a similar way as the force limiter in the seat belts. The passenger’s torso can thus continue decelerating through the airbag in a controlled manner [7].
The timing of the restraint systems is crucial to provide a good level of safety. An approximate time sequence of the events can be shown in the case of driver’s seat and frontal collision as follows:
• At 0 ms - The collision occurs.
• At 10 ms - Based on data from sensors, the ECU detects the crash and activates the pretension- ers [8][9].
• At 15 ms - The ECU fires the frontal airbag [8].
• At 25 ms - The belts are fully tightened and the driver begins to move forward [9].
• At 50 ms - The frontal airbag is fully inflated. The driver’s torso is moving forward as the belt is loosened by the force limiter [8].
• At 80 ms - The driver’s torso touches the airbag, which slows it further down [8].
• At 150 ms - The driver is back in the initial position, after a rebound [8].
Fig. 1. Time sequence of activation of the restraint sys- tems [8].
The presented time sequence could be extended by mentioning the collapse of the steering column that extends the space for the driver’s torso to decelerate. This may occur in high speed crashes and happens when the torso reaches the steering wheel. Addition- ally, some modern cars are equipped with reversible seat belt pretensioners that can be autonomously ac- tivated based on radar readings before the collision occurs [10].
2.4. Impact speed, kinetic energy, and energy equivalent speed
There are various terms describing crashes. The most frequent ones are impact speed, delta-v, kinetic en- ergy, and energy equivalent speed. It is important to distinguish these terms, especially if there are two cars involved or the considered car has a nonzero ve- locity at the end of the collision.
For further text, it is necessary to define a coordi- nate system. In the automotive industry, a frequently used coordinate system is the vehicle coordinate sys- tem defined in the ISO 8855-2011 norm [11]. It is attached to the vehicle and its x axis points forward as shown in Fig. 2.
Fig. 2. Vehicle coordinate system according to ISO 8855- 2011 [11]. Adapted from [12].
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Impact speed vk is the translational velocity of the studied object at the beginning of collision [13].
Delta-v, denoted as v, describes the velocity dif- ference between the beginning and the end of colli- sion. Contrary to the impact speed, delta-v takes into account the situation after the collision.
v = vk − v′, (1)
where v′ is the velocity at the end of collision [13]. These variables yield completely different values,
for example, in case of rear-end collision of two cars travelling in the same direction.
Energy equivalent speed better estimates the severity of a crash. It is a velocity corresponding to the kinetic energy dissipated by the vehicle during the contact phase of the collision. First, recall the formulation for kinetic energy
Ek = 1
2 mv2k, (2)
wherem is the mass of an object and Ek is the kinetic energy of the object [14].
During collision, kinetic energy dissipates in the form of deformation energy. Any residual kinetic en- ergy that is not absorbed in the collision results in a nonzero velocity at the end of collision. Assuming a collision of two cars, this effect can be described by the following energy balance equation:
1
(3)
where the indexes 1 and 2 correspond to vehicle 1 and 2 respectively, ψ′ is the yaw velocity at the end of collision, J is the moment of inertia about the z axis, and WDef is the deformation energy absorbed by the respective vehicle [13].
Energy equivalent speed is then the speed that would induce the same deformation energy, if the ve- hicle experienced a fully plastic collision with a rigid wall bringing the car to a complete stop.
WDef = 1
2.5. Occupant Load Criterion
In crashes, there are two fundamental factors that put passengers in danger. Firstly, high intrusion into the car structure can deform the safety cell and thus harm the occupants. This can be assessed by mea- suring the deformation of specific points on the car or inside the cabin. Secondly, high deceleration of the safety cell can cause injuries either by the collision of the passengers with the indoor panels or by the direct effect of deceleration on the occupants’ bodies. The latter can be, in a simplified way, quantitatively esti- mated by the Occupant Load Criterion, often denoted as OLC.
The following definition of the Occupant Load Criterion is based on the Euro NCAP’s protocol de- scribed in [3]. The OLC is a measure of a vehicle’s, or possibly other object’s, deceleration in the x direc- tion, defined in Ch. 2.4., in units of standard gravity g. The input is the x-acceleration pulse of the cen- ter of gravity of the studied object, usually a car or mobile barrier. First of all, the pulse is filtered and, using (6), velocity data are obtained
Vx(t) =
∫ Ax(t)dt + V0, (6)
where V0 is the velocity before collision and t is time [3].
Fig. 3. Calculation of the Occupant Load Criterion [15].
Due to the slack in the seat belt, mentioned in Ch. 2.3., the driver’s torso moves freely for the first part of the distance. By the OLC calculation, this free-flight distance is set to 65 mm and marks the beginning of the restraining phase (t = t1). [3]. The restraining phase, shown in Fig. 3 as the middle segment of the red line, continues until the driver reaches the steer- ing wheel. The average distance between the driver’s chest and the steering wheel is 300 mm [16]. The end of the restraining phase (t = t2) is thus marked by reaching the displacement of 300 mm. The Occu- pant Load Criterion is then the average acceleration of the restraining phase [3]. The magnitude of the OLC corresponds to the slope of the red line in Fig. 3.
The start and the end of the restraining phase, t1 and t2 respectively, and the OLC can be obtained by solving the following set of equations, namely (7), (8), and (9).∫ t=t1
t=0
t=t1
− ∫ t=t2
t=t1
Vx(t)dt = 0.235,
V0 −OLC SI × (t2 − t1) = Vx(t2), (9)
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where OLC SI is the OLC in SI units [3]. The OLC is eventually converted to units of grav-
ity using (10).
OLC = OLC SI
2.6. Car to Car Compatibility
To achieve a higher level of road traffic safety, it is necessary to consider all traffic participants. This in- cludes occupants of passenger cars, pedestrians, two- wheelers, and occupants of larger vehicles such as vans, buses, and trucks [4]. Some of these challenges are addressed by Euro NCAP’s assessments or even by some homologation regulations. For instance, pro- tection of vulnerable road users, including pedestri- ans and cyclists, is covered directly by the main safety rating of passenger cars. In addition, for commercial vans, there is a new rating category introduced by Euro NCAP in 2021 [17]. There are, however, incom- patibilities even among cars in one category, in par- ticular among passenger cars. Geometrical incompat- ibilities of the vehicles’ structures together with high mass differences play an important role [4].
Compatibility of car to car crashes is not satis- fied most frequently due to weight difference, differ- ent front end rigidity, or different front end geome- tries [18]. All that should be addressed by the new Mobile Progressive Deformable Barrier (MPDB) test introduced by Euro NCAP in 2020.
2.6.1. Weight difference
In frontal collisions of two vehicles of a different mass at identical speed, the law of momentum conserva- tion causes a nonzero velocity at the end of collision in the direction of travel of the heavier vehicle. Based on (1), the lighter vehicle experiences higher delta v and the occupants are thus at a higher risk of injury.
Fig. 4. Self Protection and Partner Protection according to the mass of the vehicle (vehicles designed since 2000 or registered since 2004). The legend refers to vehicles’ mass. Lower severity rate indicates lower probability of severe injuries and fatalities either for the studied car (y axis) or for the partner car (x axis) [19].
This fact matches the observation in [19] arising from Fig. 4. In the figure, a total 1875 occupants in- volved in frontal car to car collisions was analyzed [19]. It shows that heavier vehicles tend to cause more serious consequences for the partner car then vice versa.
2.6.2. Different front end stiffness
Until 2020, Euro NCAP tested vehicles using a sta- tionary barrier or a stationary rigid wall simulating a collision with a vehicle of identical weight. Dur- ing this testing procedure, the crumple zone had to absorb all impact energy and should not damage the safety cell. This led to front ends that were stiff pro- portionally to the vehicles’ weight. Heavier vehicles’ front ends need consequently a higher force to col- lapse than front ends of lighter vehicles [18].
In a real crash with a vehicle of different weight, unequal front end stiffness leads to the effect that the front end of the lighter vehicle losses stability first. The front end of the heavier vehicle starts to col- lapse at earliest when the lighter vehicle is severely deformed [18].
2.6.3. Geometrical incompatibility of front ends
Another phenomenon that was not represented in the previous Euro NCAP’s tests is the geometrical incom- patibility of front ends. Crash tests using a rigid wall do not set any requirements on the location or the extent of the front bumper. On the contrary, by the offset deformable barrier test, also previously used by Euro NCAP, there should be an advantage of a bet- ter design of the front end element. However, [18] suggests that the deformable element used by Euro NCAP was too soft, modern vehicles penetrated it, and their front end structure usually leaned against the rigid metal plane behind the deformable element. For instance, there is a quite significant difference even in the vertical location of supporting structures, especially between SUVs and passenger cars, see Fig. 5. Hence, the geometry of the front end varies among different vehicles [4][18].
Fig. 5. Longitudinal member height [4].
Geometrical incompatibilities can lead to situa- tions that the supporting structure of the partner ve- hicle is not hit and the impact energy causes cabin de- formation, instead of dissipation in the crumple zone. Alternatively, the longitudinal beam can penetrate the partner vehicle [18].
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3. Vehicle structure and front crash man- agement system
Today’s mass-produced passenger cars are usually de- signed as integral structures composed of stamped steel or aluminium sheets joined mechanically or more frequently by welding [20]. Front crash management system is a part of the structure in the front. This important part of the front crumple zone consists of the bumper beam and crash boxes. It distributes the contact forces to the longitudinals and consequently to the whole vehicle.
3.1. Vehicle structure
Vehicle structures are most commonly integral struc- tures that provide both structural and other func- tions [20]. They are based on steel or aluminium panels connected to a skeleton providing stiffness and strength of the structure. Most parts are pressed and then spot-welded to form the skeleton and the whole integral structure [21]. The structure can be divided into platform and subframes [20].
The platform is a part of the structure, includ- ing the underbody in most cases, that is unified for more vehicle models of one or more manufacturers. It is centrally designed and produced, which reduces costs and allows the introduction of a higher number of distinct models with a similar engineering effort [22][23].
The platform is connected to subframes and other structures that are designed specifically for the par- ticular vehicle model. Subframes, or auxiliary frames, may offer suspension and power train mounts or con- tribute to crashworthiness [23]. Structures aimed at frontal crash protection are the front end, crash man- agement system, bonnet, and partially also the green house including all pillars and roof.
This work focuses on design improvements of the front crash management system. The key parts of the vehicle front crumple zone are thus introduced. A transverse front-engine layout is assumed in the following chapters.
3.1.1. Front crumple zone
The front part of the vehicle structure has several functions. It firstly carries the engine, transmission, front axle, and accessories. Secondly, the front crash management system with the longitudinals transmits the forces to the back and, lastly, it serves as a crumple zone and absorbs energy during collisions [5]. Key parts for crashworthiness are the bumper beam, crash boxes, longitudinals, firewall, upper longitudi- nals, and possibly subframes. All above are shown in Fig. 6.
Fig. 6. Key parts of the front crumple zone [24].
3.1.2. Load paths
During collision, the contact forces are distributed to the rest of the vehicle via load paths. Structural components on load paths are designed to sustain the load or, alternatively, to collapse at a specific moment to absorb impact energy, i.e., longitudinals or crash- boxes. As the load paths split, the force level in each branch proportionately decreases [5].
In case of a frontal collision, the first part to dis- tribute the contact forces is the bumper beam. The bumper beam must be designed to sustain diverse loads, to distribute the force, and most importantly not to collapse. It is the first point where the load path splits, in particular into the left and right crash box. As shown in Fig. 7, the load paths then lead via crashboxes and longitudinals to the firewall. At the firewall, part of the force continues to the sills and the rest goes on to the tunnel.
Fig. 7. Load paths activated in case of a frontal crash [24].
If there is a deeper intrusion into the crumple zone, the upper longitudinals get involved. They act as another load path that subsequently splits into the A pillars and the door struts [24].
3.2. Design of crash management system
The front crash management system consists of the bumper beam and crash boxes. The bumper beam distributes the contact force, while the crash boxes deform and absorb the first amount of impact energy.
3.2.1. Requirements
The front crash management has several functions in high speed crashes as well as in low speed crashes. Requirements are given by the homologation legis- lation, consumer organizations, and possibly by the manufacturer’s own criteria.
Low speed impacts are tested mostly because of insurance companies to evaluate the vehicle repair costs. In these tests, the goal is to ensure vehicle integrity and to minimize the number of parts dam- aged by the impact [4]. The list of required tests varies with each market or possibly each country.
One of the widely used low speed tests is the RCAR bumper test shown in Fig. 8. It encour- ages manufacturers to produce compatible bumper
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systems that protect the vehicle and are easy to re- place. It assesses three components of bumper perfor- mance, in particular geometry, stability, and energy absorption. In terms of geometry, the bumper should be positioned at common heights and should extend to the corners. Stability represents the certainty that the bumper works in different conditions, i.e., braking or unusual loads. Energy absorption stands for the ability to absorb the impact energy without damage to other parts of the vehicle, i.e., the cooler [25].
Fig. 8. RCAR bumber test [25].
The test comprises of a full width test, shown in Fig. 8, with an impact speed of 10 km/h and a corner test with an impact speed of 5 km/h. The corner test is performed with a bumper barrier that is impacted by the vehicle with a 15% overlap. Both tests are car- ried out for both front and rear bumper with slightly different ground clearances of the barrier [25].
Another RCAR test procedure, which also sets re- quirements on the crash management system, is the low-speed structural crash test. Similarly, it aims to assess vehicle damageability and repairability by an estimation of the vehicle damage in two scenarios. Firstly, it is a 15 km/h frontal impact with a rigid barrier and, secondly, a mobile 1400 kg barrier hit- ting the stationary vehicle from rear. Both scenarios are with a 40% overlap and at a 10° impact angle [26].
High-speed frontal crash tests required for the ho- mologation process in Europe are similar to those per- formed by Euro NCAP. They are covered in regula- tions of the United Nations Economic Commission for Europe, namely, it is:
• UN R137 - Full width frontal impact with a rigid barrier at 50 km/h impact speed
• UN R94 - 40% offset frontal impact with a de- formable barrier at 56 km/h impact speed
• UN R127, R (EC) 78/2009, and R (EC) 631/2009 - Regulations related to pedestrian protection [27].
Besides, during the high-speed crash tests, the in- tegrity of the crash management system should never be lost so that it can protect the vehicle from possible secondary collisions.
All above-mentioned constrains must be taken into consideration in design of the bumper beam, crash boxes, as well as the rest of the vehicle.
3.2.2. Bumper beam
Bumper beam, often denoted as cross member or transverse beam, is incorporated in the front end and acts as the first part of the stiff structure that comes into contact with the barrier.
Supported by the crash boxes near the ends, the dominant type of loading is bending. It may absorb a certain amount of impact energy; however, it should always remain stable to keep transferring the contact force to both crash boxes. Additionally, in today’s vehicles, the front side is covered with a padding that ensures lower stiffness for pedestrian protection [5]. Location of the front bumper is shown in Fig. 9.
Fig. 9. Front bumper beam (red) with a reinforcement plate (yellow), forming an enclosed profile. Padding for pedestrian protection on the right side (black) [28].
To resist the contact forces, bumper beams are of- ten hot formed parts made of high strength or dual phase steels [16]. As in Fig. 9, they can be addition- ally reinforced by a plate to form an enclosed profile and thus feature a higher bending stiffness due to the increased cross-sectional moment of inertia.
3.2.3. Crash boxes
Crash boxes, also denoted as crash cans or defo el- ements, are responsible for the absorption of im- pact energy especially in low speed crashes. At im- pact speeds up to 15 km/h, the crash boxes and the bumper beam should be the only parts that plasti- cally deform in the structure [16]. They are thus de- signed to collapse at a significantly lower force level then the neighboring longitudinals.
The dominant type of loading is axial compres- sion, thus the crash boxes are exposed to buckling. Since energy is absorbed only at the buckling knees, to increase the energy absorption, it is desired to increase the number of knees. In the case of crash boxes, it is achieved by placing failure initiators, such as those shown in Fig. 10, that initiate a suitable deformation mode [29][30].
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Fig. 10. Crash boxes with three typical failure initiators: corner notches, corner holes, and surface beads [21].
Fig. 11. Crash box (purple) with a bumper beam (red) in the front and a longitudinal (yellow) behind. The surface beads act as failure initiators to initiate desired folding of the box [28].
In general, crash boxes are variously shaped thin- walled tubes placed between the bumper beam and the longitudinals. An example of a crash box of a SUV is shown in Fig. 11. For high-volume vehicles, they are usually made of high strength steels [21]. For low-volume luxury cars, the crash boxes, as well as the whole crash management system, may be made of extruded aluminium profiles to reduce weight. Alter- natively, crash boxes can be filled with various foams for better energy absorption capabilities [16].
3.3. Disadvantageous design for MPDB
Compatibility, which is assessed by the MPDB crash test, requires a novel approach to design of the crash management system. The goal is to ensure the same level of self-protection while protecting the partner vehicle. Literature suggests that some previous de- signs of the front structure are a priori unsuitable for MPDB.
Already the 24th International Technical Confer- ence on the Enhanced Safety of Vehicles in 2015 paid attention to compatibility and MPDB. The Euro NCAP’s Frontal Impact Working Group pointed out that some crash management systems fail in the MPDB test [18].
One of the disadvantageous designs is the front crash management system without any overlapping
ends. In such cases, the bumper beam tends to col- lapse and the longitudinal perforates the deformable barrier. This was presented in [18] and is shown in Fig. 12.
Fig. 12. Crash management system with a limited span tends to collapse under the load from the deformable bar- rier. In the barrier, it left deep intrusions. Front left [18].
The same behaviour was observed in [31] and [32], shown in Fig. 13 and 14 and Fig. 15 and 16 respec- tively.
Fig. 13. Crash management system of Volkswagen Golf lost stability and the left longitudinal perforated the bar- rier. Deformed front structure [31].
Fig. 14. Crash management system of Volkswagen Golf lost stability and the left longitudinal perforated the bar- rier. Barrier scan after collision [31].
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Fig. 15. An MPDB test conducted by Hyundai Motor Company with a compact car for the European market. The bumper beam collapses the same way as in the previ- ous examples. Deformed front structure [32].
Fig. 16. An MPDB test conducted by Hyundai Motor Company with a compact car for the European market. The bumper beam collapses the same way as in the previ- ous examples. Barrier scan after collision [32].
Apart from the limited span of the bumper beam, a prevailing feature of these crash management sys- tems is the absence of multiple load paths at the front face. The only example with multiple load paths is the vehicle in Fig. 12, which has a secondary lower load path. It is an additional steel cross member whose outer edges rest on the chassis subframe [18]. Nevertheless, the main common problem of these de- signs is the limited strength of the bumper beam and the related poor energy absorption potential.
3.4. Advantageous design for MPDB
For an improved compatibility and better MPDB re- sults, a different approach to crash management de- sign must be taken. Several novel designs have been introduced in the literature and at recent conferences.
Fig. 17. Disadvantageous and advantageous crash man- agement system according to [18]. Current disadvanta- geous design of numerous crash management systems.
Fig. 18. Disadvantageous and advantageous crash man- agement system according to [18]. Proposed front shield with multiple bumper beams for an improved compatibility.
The Euro NCAP’s Frontal Impact Working Group suggests that the narrow bumper beam should be re- placed by a front shield with a set of wide bumper beams at multiple levels acting in multiple load paths. The shield should cover the area between 250 and 650 mm above the ground. Besides, the extended span, ideally over the entire width of the vehicle, should help with the small overlap scenarios. The suggested front shield is shown in Fig. 17 and 18, together with the original disadvantageous design.
The usage of multiple load paths is present also in the Advanced Compatibility Engineering Structure developed by Honda within its compatibility research. The structure, which is shown in Fig. 19, should re- duce aggressiveness and increase safety in car to car collisions [33].
Fig. 19. Advanced Compatibility Engineering Structure developed by Honda [33].
The Advanced Compatibility Engineering Struc- ture was incorporated also in Honda Civic whose crash results were presented in [31]. In terms of in- trusions into the barrier, the structure, shown in Fig. 20 and 21, performed very well.
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Fig. 20. Honda Civic with the Advanced Compatibility Engineering Structure. Deformed front structure [31].
Fig. 21. Honda Civic with the Advanced Compatibility Engineering Structure. Barrier scan after collision [31].
At the Enhanced Safety of Vehicles conference, there were more contributors that emphasized the im- portance of multiple load paths. For instance, [34] claims that a lower load path ensures a good struc- tural engagement and is the first step towards com- patibility. Lower load paths in Peugeot 3008 and Re- nault Clio are then shown as examples.
In addition, [35] introduces a compatibility struc- ture that should decrease the standard deviation of the barrier deformation. The bumper beam and the lower load path are both reinforced and extended out- ward. The presented FEM results show that this im- provement can significantly improve the MPDB score. However, in a comparative car to car simulation, the improvement assessed using an equivalent parameter, which substitutes the MPDB score, was marginal, raising questions about the validity of the MPDB pro- cedure.
Lastly, [18] suggests that larger vehicles, which tend to have stiffer crumple zones, feature usually a longer front end. This space can be thus divided into a soft partner protection and a stiffer self-protection area. Such compatibility of front end stiffness would guarantee that a similar amount of impact energy is absorbed in both cars.
3.5. Conclusion
The compatibility assessment in the mobile progres- sive deformable barrier test challenges manufacturers to completely revise the design of the front crash man- agement system. Previous approach, which secures only self-protection, often fails due to the collapsing bumper beam. Sole bumper beam acting as a single load path causes uneven intrusion into the barrier. If collapsed, it can even penetrate the barrier.
To improve the performance, multiple load paths must be introduced to cover a larger area of the front end. A higher compatibility can be achieved espe- cially by connecting the lower load path to the main one to form an extensive front shield. If only smaller modifications are possible, some improvements can be done too. A pure extension of the crash manage- ment system outwards should have a positive influ- ence, since it utilizes the unprotected area in front of the wheels. A significant progress can be made by re- inforcement of the bumper beam to avoid its collapse and deeper penetration of the barrier.
If properly implemented, these suggestion can im- prove the mobile progressive deformable barrier test performance and a higher level of compatibility can be achieved.
References
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Active and passive safety
Restraint systems
Occupant Load Criterion
Vehicle structure and front crash management system
Vehicle structure
Requirements