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:
[email protected]
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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.
Student’s Conference 2021 | Czech Technical University in Prague |
Faculty of Mechanical Engineering
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.
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Active and passive safety
Restraint systems
Occupant Load Criterion
Vehicle structure and front crash management system
Vehicle structure
Requirements