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Why bother testing a car's resilience at low speeds? Because slight collision can

damage the bumper. Depending on the its design, that bumper could actually

cost thousands of dollars to fix. Therefore, low-speed crash tests are conducted

to produce effective bumper

systems and reduce the cost of insurance.

Automotive

Low-speed crash test (RCAR bumper test)

Why do we need low-speed crash test?

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Right chart shows how auto-

insurance premiums are

calculated . First, low-speed

crash tests are conducted.

Second, announce estimated

repair cost . Then, insurance

contract premiums are decided

based on the estimated repairs.

The table shows repairs of two different models: for K5, the front and rear collision

repair costs 2 million won (1800 $). On the other hand, for New SM5 in the same

condition, 3.6 million won (3200 $) repairs will occur.

Therefore, it makes more

sense for insurance companies

to charge an extra premium for

people who purchase New

SM5.

Class Model Front Rear Total Index *

Medium-

sized

K5 1,408 549 1,957 100

YF Sonata 1,484 569 2,053 105

New SM5 2,709 949 3,658 187

Formati

on

Al peon 1,435 591 2,026 100

K7 1,739 891 2,630 130

Grandeur

HG 1,776 1,013 2,789 138

SUV Sportage R 1,914 532 2,446 100

TUCSON IX 2,500 1,118 3,618 148

* Index the lowest repair cost of vehicle repairs compared to the

corresponding vehicle in its class is

※ Repairs in January 2011 calculated on the basis

RCAR of low-speed crash

test standards

Announced repair estimation

result

Calculate car insurance

contract premiums

By optimizing the bumper

system to reduce damages and

car reparation, vehicle

manufacturers can sell vehicles

more economically and

competitively.

Automotive

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Typical low-speed crash tests are described as the following pictures. 2 main

factors are measured in the test: damageability representing the level of

external chock that a vehicle can endure, and repairability which measures the

possible restoration of damaged vehicle.

Automotive

Process of RCAR test

Front low-speed collision test

Front collision with an angle of 10˚

Collision speed: 15 ~ 16km / h

Front low-speed collision test

Rear-end collision on a wall inclined

with an angle of 10˚

Collision speed: 15 ~ 16km / h, M

Moving wall weight: 1,400 kg

40% offset

wall

40% offset

Design Guide of RCAR

In an effort to reduce damage, RCAR offers a design guide.

Front shock absorption structure described below defines some of the terms.

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With regard to the front crash test there are two design guides.

The first is in the rear impact beams and the proper distance between

components must be maintained. Take the bottom center of the picture as an

example. If the cooling system is too close to impact bumper. A bumping damage is

likely to occur.

The second is proper collision energy absorption. For example crush cans are

used to absorb collision energy and minimize damage to important side members.

CAE tools are used to analyze and define the appropriate amount of distance and

energy absorption in any situation.

Automotive

Picture: Front Shock-absorbing system

Impact Bumper

Used to protect the

components located in the

rear.

Crush cans

Crush cans are located at the

two ends of the impact

bumper in order to absorb the

energy of the impact and

minimize the damage of the

side members

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For the rapid development of bumper system, a brief model has been configured

as above. The finite element model is composed of fixed wall for front-end collision,

impact bumper, crush cans and side member. 14mm wide and 3mm deep crush

cans which charge absorbing collision energy are applied to the bead.

Displacement, energy absorption and peak force are adjusted.

Automotive

Modeling of the front bumper

Test mass : 1,119kg

Collision velocity : 16km/h

Kinetic energy : 11.1kJ

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Although it’s a simplified model, overall vehicle weight has to be considered.

1119kg was applied to the center point of the mass. During the test, vehicle speed

is 15~16 km/hour. Considering the worst condition, we set collision velocity as 16

km/hour. And

Impact speed 15 ~ 16km per hour in the test procedure defined in, but considering

the worst conditions have been set to 16km per hour. And total kinetic energy is set

to 11.1kJ

The yield stress of the beam impact is 800MPa. And crush cans are made of

material with yield stress of 240MPa. Therefore, non-linearity needs to be

considered.

Automotive

Item

Modulus o

f elasticity

(N/mm2)

Poisson'

s ratio

Density

(kg/m3)

Yield stress

(N/mm2)

Impact bumper 210000 0.3 7890 800

Crush Cans 210000 0.3 7890 240

RCAR Low Speed Impact analysis - front

Deformation modes of the front bumper and crush cans @ 100msec

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Analysis was performed by midas NFX - Nonlinear Explicit Dynamic Analysis. In

the analysis we tested 2 conditions respectively: crash can thickness 1.8mm and

2.0mm. From the result we can clearly observe that deformation of 1.8mm crash

can is larger than that of 2.0mm crash can.

Automotive

Picture on the right side shows

deformation mode of crush cans from

horizontal section view. The interior

deformation which can not be viewed

in the previous picture can also be

observed here.

Crushed cans of deformation

(horizontal section) @ 100msec

Deformation of the front bumper Plastic strain in the crush cans

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Above pictures describe deformation of whole bumper system and detailed

deformation of crash cans.

Crush cans

thickness

(mm)

Maximum

load

(kN)

Maximum

displacement

(mm)

Energy

absorbed

in the

collision

(kJ)

1.8t 172.2 80.7 5.9

2.0t 162.0 74.1 6.0

From the load-displacement curve we can see when thickness of crush can

increases, total displacement decreases, but load level increases ( total

displacement of red line is 8mm larger than that of blue line, therefore load level of

1.8 t crash can is lower than 2t crash can)

For optimization design of bumper system during low speed collision, when

distance between cooling system and impact beam is too small, we need to

decrease total displacement by increasing the thickness of crash can. On the other

hand, when side members are at risk of being damaged, we need to decrease the

impact load delivered to side members by decreasing thickness of crash can.

For reference, If the energy lost during the impact is 6.0kJ, the impact bumper

absorbed 54% of the total crash energy (11.1kJ)

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Now we look at rear structure of the vehicle. This simplified model contains

187,512 nodes and 185,351 elements.

For the omitted part rigid elements were used. And whole vehicle identical mass

was applied to the center of gravity.

Modeling of vehicle rear structure

Nodes: 187,512

Elements: 185,351 RBE2

Element

Barrier : 1,400kg

Concentrated mass

(COG)

Vehicle : 1,119kg

Concentrated mass

(COG)

RBE2

element

Modulus of elas

ticity

(N/mm2)

Poisson's

ratio

Density

(kg/mm3)

Elastic materials

210000 0.3 7.89e-6

210Y 210000 0.3 7.89e-6

240Y 210000 0.3 7.89e-6

300Y 210000 0.3 7.89e-6

800Y 210000 0.3 7.89e-6

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Different nonlinear material models have been applied to the components of the

system as the above table.

210Y Stress-Strain Curve 240Y Stress-Strain Curve

300Y Stress-Strain Curve 800Y Stress-Strain Curve

Four different nonlinear material models have been used, with yielding stress

going from 210 Mpa to 800 Mpa for the stiffer parts.

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Above analysis have been performed using midas NFX nonlinear explicit dynamic

analysis. We can observe displacement distribution from bottom view and side

view. Rear crush can (shown in minified picture) shows progressive collapse

deformation with bead.

RCAR low speed impact analysis - Rear

Displacement -Bottom View Displacement -Side View

Rear Structure Damage (Rear View)

Trunk Lid

Rear End Panel

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Pictures above show distribution of plastic strain. Red parts are where plastic

strain surpasses criteria, that is to say where damages occur.

This vehicle was produced in 2000 and didn’t follow RCAR low speed collision

standard. Therefore the rear structure are mostly damaged. This can be observed

from the pictures.

To improve this vehicle, we can first improve the rear side member and rocker pars.

And then decide if we need to improve the trunk lid which cost relatively more.

Rear Structure Damage (Bottom View)

Side member

Rear floor

Rocker RH

Rocker LH