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  • Influence of Crash Box on Automotive Crashworthiness

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

    CRISTIAN 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

    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

    Recent Advances in Civil Engineering and Mechanics

    ISBN: 978-960-474-403-9 49

    mailto:[email protected]

  • 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= 2W [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

    Recent Advances in Civil Engineering and Mechanics

    ISBN: 978-960-474-403-9 50

  • 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 rails 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 ma