Post on 04-May-2022
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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