Recent Advances in Civil Engineering and Mechanics
Influence of Crash Box on Automotive Crashworthiness
MIHAIL DANIEL IOZSA, DAN ALEXANDRU MICU, GHEORGHE FRĂȚILĂ, FLORINCRISTIAN ANTONACHE
University POLITEHNICA of Bucharest
313 Splaiul Independentei st., 6th Sector,
ROMANIA
[email protected]; [email protected]; [email protected]; [email protected]
Abstract: In this paper, frontal impact behaviours of three car frontal parts with a rigid obstacle at rest is
presented. The purpose is to analyze the best crashworthiness. The models have different crash boxes and are
analyzed using Explicit Dynamics module of Ansys software. Shape and dimensions of the model were
obtained from repeated simulations and constant improvements. Finite element mesh size for each part of the
model varies, depending on its role. Velocity of the car model was computed by equalizing the kinetic energy
of the modelled geometry with the kinetic energy of a considered automobile. The results present a comparison
of deformations and stress, resulting an analyze of absorbed energies values during the impact.
Key-Words: crash box, frontal impact, crashworthiness, Ansys, deformation, car structure
Initial conditions of frontal impact simulations
and meshing settings are presented in the last two
subsections of section 2.
Variations and comparisons of stress and plastic
deformations of the all three models are analyzed in
section 3.
1 Introduction
Crashworthiness is the ability of a structure to
protect its occupants in the event of a crash. Frontal
impact cars is one of the most often crash types.
Automotive manufactures increasingly employ
computer simulation, because physical vehicle
crash-testing is highly expensive [1]. Currently,
dynamic explicit integration is commonly used for
the simulations like impact and collision.[2]
A 2D concept model of a detailed automotive
bumper model was introduced and it was discretized
by using lumped mass spring elements in [3]. The
time efficiency and the good approximation of
results proved its utility in crash analysis,
confirming that early stages of product design can
make use of the simplifications and rapid decisions
can be taken for early improvements.
It is useful to utilize mathematical optimization
by altering the geometry and the material and
structural properties of the bumper- beam and crashbox to improve the low speed performance[4].
When a vehicle impacts in less than 15 km/h
velocity, the insurance companies require that the
damage of the vehicle should be as small as
possible.
Section 2 presents the steps necessary to simulate
frontal impact. The first step consists in establishing
a mathematical model to use in crash analyze of a
car frontal part. Three models of crash boxes that
belong to geometry of the impact energy
management system are described in the second part
presented in subsection 2.2.
ISBN: 978-960-474-403-9
2 Simulating frontal impact
2.1 Study of mathematical models used on
impact analyze of a car frontal part
Simple or complex mathematic models can be
used to study structure dynamics, depending on
complexity of simulated phenomena, precision
and/or computation rate.
Figure 1 shows four of most usual mathematic
models used to test bumper beams in impact
computations.
a.
b.
c.
d.
Fig. 1 Usual mathematical models used to test
bumper beam in impact computations [5]
The mathematic model with one damping
element (c1) and one elastic element (k1) in serial
communication is the most used (Fig 1.a). One
damping element (c2) and one elastic element (k2) in
49
Recent Advances in Civil Engineering and Mechanics
deformable barrier), both moving, is presented in
Fig 2.d.
parallel communication is another mathematical
model (Fig 1.b).
Complex structures or particular situations can
be modelled using elastic elements (k31) in parallel
communication with a damping element (c3) and an
elastic element (k32) in series communication (Fig
1.c), or with a damping element (4) in parallel
communication with a spring element (k41), both in
series communication with a spring element (k42)
(Fig 1.d).
An impact of an vehicle can be defined by four
cases which are presented in Fig 2.
a.
V1  V2
[km/h]
2
2  W1  W2
We =
[J]
W1  W2
V e=
2.2 Modelling geometry of the impact energy
management system
Geometry modelling was performed using
ANSYS, a structural analysis software, and the
elements were defined by the surface type. Elements
whose geometry is necessary to simulate a frontal
impact are: an obstacle, a front bumper beam, crash
boxes, flanges, front frame rail and a block
representing the car.
Figure 3 shows the components used to simulate
the frontal impact.
c.
d.
Fig. 2 Typical cases to study the impact of
vehicles [5]
The first case (Fig 2.a) is a frontal impact
between a moving car and a rigid obstacle at rest. In
this case the impact velocity (Ve) and impact energy
(We) are those of the car:
(1)
(2)
The second case (Fig 2.b) is a frontal impact
between a moving car and a barrier equipped with a
dampening impact energy (equivalent to a
deformable barrier) at rest. To study this case the
impact velocity (Ve) and the impact energy (WE) is
calculated using formulas:
V e=
V
[km/h]
2
We= 2·W [J]
a.
W W
We = 1 2 [J]
W1  W2
(4)
b. crash boxes and flanges
(5)
(6)
c.
A frontal impact between a car and an obstacle
provided with a damping system (equivalent to a
ISBN: 978-960-474-403-9
obstacle and bumper beam
(3)
A frontal impact between a car and a rigid
obstacle, both moving, is presented in the third case
(Fig 2.c). Impact velocity (Ve) and impact energy
(We) can be determined using the following
formulas:
Ve= V1+V2 [km/h]
(8)
The mathematical model used is the one with
elastic and damping elements in series
communication (Fig 1.a) and the case to study is the
impact of the rigid obstacle at rest by a moving car
(case I)(Fig.2 a).
b.
Ve= V [km/h]
We= W [J]
(7)
front frame rail and a block representing the
car
Fig. 3 Elements used to simulate the frontal impact
50
Recent Advances in Civil Engineering and Mechanics
Figure 4 shows the first model of the crash box
integrated in the frame rail during the impact with
the obstacle.
The geometry was been modified by using
different crash boxes. The cross-section profile and
dimensions of the front cross beam were not been
modified during the initial geometric model
improvement.
A top view of the three modelled geometric
solutions for impact energy management system is
presented in Figure 6.
Fig. 4 Isometric view of first model of the crash box
integrated in the frame rail during the impact with
the obstacle
Shape and dimensions of the model were
obtained from repeated simulations and constant
improvements. The objective is to obtain a better
behavior if the structure is subjected to similar
stresses to those that occur in a frontal impact.
The model improvement in this phase was
obtained by choosing the measure to increase the
cross-section of the front frame rail and of crash
boxes, by the relative disposition of the vehicle
body block so that its center of gravity to be at an
usual distance above the assembly and by choosing
the front frame rail’s curvature radius from the
frontal part to the cockpit.
The model was chronology developed from
model M1, to model M2 and to model M3, as it can
be noticed in Figure 5.
Fig. 6 Top view of the three modelled geometric
solutions for impact energy management system
Figure 7 presents an isometric view of the
geometrical model solutions of crash boxes.
Fig. 7 Isometric view of the geometric model
solutions of crash boxes (removable ends of the
front frame rail)
Steels values of the physical parameters of
materials were introduced in the analysis software
library to model the impact energy management
system materials (HSLAS S300MC and S250MC).
The material models were saved separately with
specific names to be assigned to each component
separately.
The steel model H.S.L.A.S. S250MC, named
"Structural Steel NL 1" in the material library of the
software is assigned to crash boxes and model
HSLAS S300MC named "Structural Steel NL 2" is
assigned to bumper beam, flanges and to frame rails.
The "NL" suffix in the name of the steel refers to
the fact that the materials have nonlinear material
characteristics to simulate both material behaviours:
plastic and elastic. This is necessary because during
the simulation, the stress of the components exceed
their yield strength.
Fig. 5 Isometric view of the three modelled
geometric solutions for impact energy management
system
ISBN: 978-960-474-403-9
51
Recent Advances in Civil Engineering and Mechanics
Also, a fixed support was imposed on the outer
surface of the obstacle plane farthest from
automobile to represent the state of relative rest of
the obstacle (Figure 8).
The imposed velocity to car assembly was
inferred from equalizing the kinetic energies of the
modelled geometry and designed automobile as
follows:
2.3 Defining initial conditions to simulate
frontal impact
Particular conditions, such as rigid contacts with
or without friction, fixed supports, pretensions,
relative speeds etc., have to be imposed to the model
components. These conditions are necessary
because the results obtained from the dynamic
simulation should behave as close to reality.
Two static „Bonded” type contacts between the
left front frame rail and the car and between the
right front frame rail and car were established
surfing in the "Model" part of the "Explicit
Dynamics" module of Ansys software (Figure 8).
2
mmod el Vmod
el
[J]
Ec mod el 
2
m V 2
Ecauto  auto auto
2
(9)
(10)
where:
 Ecmodel [J] - kinetic energy of the modelled
geometry;
 Ecauto [J] - kinetic energy of the automobile;
 mmodel [kg] - mass of the modelled geometry;
 mauto [kg] - mass of the automobile;
 Vmodel [km/h] – impact velocity of the modelled
geometry corresponding to its kinetic energy;
 Vauto [km/h] – impact velocity of the
automobile corresponding to its kinetic energy.
Fig. 8 Rigid and static contacts established between
the front frame rails and the car body box
In ”Connections” menu, ”Body Interactions”
field, a Frictional type of dynamic contact was
established between the frontal cross member and
the contact surface of the obstacle (Figure 9).
Because: Ecmodel= Ecautol ⇒
Vmod el 
2
mauto  Vauto
[km / h]
mmod el
(11)
According to European regulations regarding
frontal impact test, the initial speed of the
automotive before impact must be kept constant
around 15 km / h (≈4,166 m / s).
Fig. 9 Frictional type of dynamic contact between
the frontal cross member and the contact surface of
the obstacle
2.4 Meshing geometric model using finite
elements
In ”Explicit Dynamics” module, ”Initial
Conditions” part, the initial linear and constant
velocity, its direction and its orientation were
established for components of both the car and
impact energy system group (Figure 10).
The finite element mesh size of each component
of the model geometry can be chosen in the "Model"
part, "Mesh" menu.
Fig. 10 Initial velocity conditions of the simulation
components
Fig. 11 Meshing the assembly to simulate frontal
impact
ISBN: 978-960-474-403-9
52
Recent Advances in Civil Engineering and Mechanics
Finite element mesh size for each part of the
model varies depending on its role: for crash boxes
a mesh as fine (10 mm), for cross member and
flanges a larger mesh (15 mm), for front frame rail a
large mesh (50 mm) and for car body block and
obstacle a coarse mesh (100 mm) (Figure 12).
"Generate mesh" button is used. A number of
4747 elements and 4290 nodes resulted following
the completion of the entire assembly meshing.
Fig. 14 Stress variation of geometric model M1
during the impact simulation
Fig. 15 Plastic deformation variation of geometric
model M2 during the impact simulation
Fig. 12 Finite element meshing of different sizes for
each component of the model
Table 1 The main parameters of each component
used to simulate the frontal impact
No
Criterion
Automobile
Frame
rails
Flanges
Cross
member
Crash
Box
Obstacle
1
Thickness
profile of the
cross section
[mm]
-
2.0
2.0
1.1
1.0
-
Structural Structural Structural Structural Structural
Steel Steel NL 2 Steel NL 2 Steel NL 2 Steel NL 1
2
Material
3
Mass [kg]
847.80
4.895
0.448
3.148
0.326
526.75
4
Mesh size
100
50
15
15
10
100
5
Velocity
[m/s]
4.190 ≈ 15 km/h
Fig. 16 Stress variation of geometric model M2
during the impact simulation
Structural
Steel
0
3 Results
Fig. 17 Plastic deformation variation of geometric
model M3 during the impact simulation
The demountable crash boxes deflection should
not do flaming but controlled by folding
deformation using initiators such as ribs, holes,
folds, cuts, different shapes of sections, elements
with variable thickness and constant increase of
sections and of inertia moments. After modelling the
geometry and imposing the initial conditions the
"Solve" button is used to run the simulation. The
results can be read and save in the "Explicit
Dynamics" module, "Solution" part.
Fig. 18 Stress variation of geometric model M3
during the impact simulation
Fig. 13 Plastic deformation variation of geometric
model M1 during the impact simulation
ISBN: 978-960-474-403-9
Fig. 19 Stress and plastic deformation variations of
geometric model M1 during the impact simulation
53
Recent Advances in Civil Engineering and Mechanics
4 Conclusion
Total plastic deformation growth during the
impact, reaches a maximum value and remain quasiconstant around this value (saturate) for all three
models. From this moment, it is considered that the
impact energy is not consumed any more by the
crash boxes, but the energy is sent to the front frame
rail.
The aim is to consume higher quantities of
energy away from the passenger compartment in a
short time interval. The amount of transmitted
energy to other body parts and/or to passenger
compartment should be minimized. It is observed
that the model M2 has the highest strain in the
shortest deformation time. A larger deformation
implies a higher consumption of impact energy and
a less time for this strain is an increased safety for
car occupants.
Stress is represented from blue to light blue on
the surface of crash boxes, and maximum stress
appear only in some points. That means the stress
values are small.
Fig. 20 Stress and plastic deformation variations of
geometric model M2 during the impact simulation
Fig. 21 Stress and plastic deformation variations of
geometric model M3 during the impact simulation
References:
[1] Micu, D.A., Straface, D., Farkas, L., Erdelyi,
H., Iozsa, M.D., Mundo, D., Donders, S., A cosimulation approach for crash analysis, UPB
Scientific Bulletin, Series D: Mechanical
Engineering, 76 (2), 2014, pp. 189-198;
[2] Micu, D.A., Iozsa, M.D., Stan, C., Quasi-static
simulation approaches on rollover impact of a
bus structure, WSEAS, ACMOS, Brașov, June
26-28, 2014;
[3] Sîrbu, A.D.M.,
Research on improving
crashworthiness of the frontal part of the
automotive
structure,
PhD
Thesis,
POLITEHNICA University of Bucharest,
Romania, 2012;
[4] Redhe, M., Nilsson, L., Bergman, F., Stander,
N., Shape Optimization of Vehicle Crash-box
using LS-OPT, 5th European LS-DYNA Users
Conference, Birmingham, 2005;
[5] Donald Malen, Fundamentals of Automobile
Body Structure Ddesign, SAE International,
2011.
Fig. 22 Comparison of plastic deformation
variations of geometric models during the impact
simulation
Fig. 23 Comparison of stress variations of geometric
models during the impact simulation
ISBN: 978-960-474-403-9
54