Influence of Crash Box on Automotive Crashworthiness

MIHAIL DANIEL IOZSA, DAN ALEXANDRU MICU, GHEORGHE FRĂȚILĂ, FLORIN-

CRISTIAN ANTONACHE

University POLITEHNICA of Bucharest

313 Splaiul Independentei st., 6th Sector,

ROMANIA

daniel_iozsa@yahoo.com; dan.micu@upb.ro; ghe_fratila@yahoo.com; antonache_florin@yahoo.com

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

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 crash-

box 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.

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.

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

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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. b.

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:

Ve= V [km/h] (1)

We= W [J] (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:

Ve=2

V[km/h] (3)

We= 2·W [J] (4)

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] (5)

We =21

21

WW

WW

[J] (6)

A frontal impact between a car and an obstacle

provided with a damping system (equivalent to a

deformable barrier), both moving, is presented in

Fig 2.d.

Ve=2

21 VV [km/h] (7)

We = 21

212

WW

WW

[J] (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).

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.

a. obstacle and bumper beam

b. crash boxes and flanges

c. front frame rail and a block representing the

car Fig. 3 Elements used to simulate the frontal impact

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Figure 4 shows the first model of the crash box

integrated in the frame rail during the impact with

the obstacle.

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. 5 Isometric view of the three modelled

geometric solutions for impact energy management

system

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. 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.

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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).

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).

Fig. 9 Frictional type of dynamic contact between

the frontal cross member and the contact surface of

the obstacle

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).

Fig. 10 Initial velocity conditions of the simulation

components

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

2

modmodmod

elelelc

VmE

[J] (9)

2

2

autoautocauto

VmE

(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.

Because: Ecmodel= Ecautol ⇒

]/[mod

2

mod hkmm

VmV

el

autoautoel

(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).

2.4 Meshing geometric model using finite

elements

The finite element mesh size of each component

of the model geometry can be chosen in the "Model"

part, "Mesh" menu.

Fig. 11 Meshing the assembly to simulate frontal

impact

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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. 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 Auto-

mobile

Frame

rails Flanges

Cross

member

Crash

Box Obstacle

1

Thickness

profile of the

cross section

[mm]

- 2.0 2.0 1.1 1.0 -

2 Material Structural

Steel

Structural

Steel NL 2

Structural

Steel NL 2

Structural

Steel NL 2

Structural

Steel NL 1

Structural

Steel

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 0

3 Results

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. 13 Plastic deformation variation of geometric

model M1 during the impact simulation

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. 16 Stress variation of geometric model M2

during the impact simulation

Fig. 17 Plastic deformation variation of geometric

model M3 during the impact simulation

Fig. 18 Stress variation of geometric model M3

during the impact simulation

Fig. 19 Stress and plastic deformation variations of

geometric model M1 during the impact simulation

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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

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

4 Conclusion Total plastic deformation growth during the

impact, reaches a maximum value and remain quasi-

constant 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.

References:

[1] Micu, D.A., Straface, D., Farkas, L., Erdelyi,

H., Iozsa, M.D., Mundo, D., Donders, S., A co-

simulation 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.

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ISBN: 978-960-474-403-9 54