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Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601
www.springerlink.com/content/1738-494x
DOI 10.1007/s12206-012-0640-z
Crumple zone design for pedestrian protection using impact analysis
†
Hyung-il Moon
1
, Young-eun Jeon
1
, Dae-Young Kim
1
, Heon Young Kim
1,*
and Yong-soo Kim
2
1
Department of Mechanical and Biomedical Engineering, Kangwon National University, Chuncheon-si, 200-701, Korea
2
Product Development Team, SL Corporation, Gyeongsan-si, 1208-6, Korea
(Manuscript Received October 26, 2011; Revised February 2, 2012; Accepted April 3, 2012)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Abstract
This paper describes the design process for an automobile crumple zone for pedestrian protection. The impact load and bending mo-
ments predicted by impact analysis were used to design a plastic structure that may help reduce pedestrian injuries to the thigh area. The
fracture effect was incorporated into the model by calculating the damage to the plastic material during impact, and the analysis was con-
ducted under the European New Car Assessment Program (Euro NCAP) test conditions, using the upper legform developed by ESI Cor-
poration. In addition, the values predicted by the analysis were validated by comparison with results of actual impact tests.
Keywords: Pedestrian protection; Upper legform; Bonnet leading edge test; Crumple zone; Impact analysis; Front end module; EURO NCAP
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Introduction
Recently, there has been increased interest in pedestrian
protection. Numerous studies have been conducted on reduc-
ing pedestrian injuries during collisions with automobiles, and
vehicle safety organizations are modifying the process by
which regulations or standards for pedestrian protection are
released. Such regulations or standards are now disclosed only
after they have been tested on an assortment of vehicles. Thus,
in the future, it will be important to satisfy appropriate vehicle
design criteria [1-3].
This paper describes the design and evaluation of a crumple
zone for the front-end module (FEM) of an automobile that
can effectively absorb the impact of a collision with a pedes-
trian. A 1D spring element model was used for the basic de-
sign of a plastic crumple zone. The upper-legform to bonnet
(hood)-leading-edge test (one of the pedestrian protection
evaluation tests specified by the European New Car Assess-
ment Program (Euro NCAP)) was implemented to design the
crumple zone in detail and predict its performance [4-6].
2. Upper legform to bonnet leading edge test
The Global Technical Regulation (GTR) proposed by the
automobile production sub-committee WP29 is based on stud-
ies carried out by the International Harmonized Research Ac-
tivities (IHRA) and the European Economic Community
(EEC) Directive 2003/103/EC. In Korea, if the GTR pedes-
trian crash safety act is enacted in 2012–2013, it will be
adopted and implemented. At present, the GTR is incomplete.
However, it is known that the overall content of the pedestrian
protection test will be similar to that of the Euro NCAP, and
the limitations will be mitigated.
There are five Euro NCAP pedestrian safety classifications
in a collision between a pedestrian and an automobile. Fig. 1
illustrates the pedestrian safety impact tests and their positions.
The test procedures include upper and lower legform tests,
upper-legform to bonnet leading edge tests, and adult and
child headform tests. The upper-legform to bonnet leading
edge test evaluates the damage to the thigh area of a pedes-
trian, using the upper legform shown in Fig. 2. The level of
damage is inferred from the impact load and the bending mo-
ment observed during a collision between the upper legform
impactor and the upper hood of the vehicle [7-11].
*
Corresponding author. Tel.: +82 33 250 6317, Fax.: +82 33 242 6013
E-mail address: khy@kangwon.ac.kr
†
Recommended by Associate Editor Kyeongsik Wo
© KSME & Springer 2012
Fig. 1. Pedestrian safety impact tests and their positions (Euro NCAP).
2596 H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601
3. Initial design of the crumple zone using a 1D
spring element
The crumple zone design process was divided into two
phases: the initial design using a 1D spring element model,
followed by the detailed design based on the results of impact
analysis under the Euro NCAP test conditions.
The 1D spring element model was used to select a basic de-
sign concept and material for the crumple zone that would
satisfy the Euro NCAP test conditions. The spring model con-
sisted of 1D springs connecting two parallel planes, as shown
in Fig. 3. Since the spring model yielded stable results regard-
less of spring placement, the springs were placed at regular
intervals along the length and width of the planes, as deter-
mined during the early stages of the FEM analysis of the
crumple zone height. The number of springs placed length-
wise ranged from 1–19, while 1–4 rows of springs were
placed widthwise, and the differences in the impact loads were
recorded (Fig. 4). The lower plane was fixed and the upper
legform was made to collide perpendicularly with the upper
plane using the Pam-Crash software package. The legform
collision speed was reduced by 30% from the Euro NCAP test
speed, in light of the deformation effect of the hood. It was
verified that consistent results could be obtained when 17
springs were placed lengthwise, with 3 rows of springs placed
widthwise, as indicated in Fig. 3.
Using the verified model, the deflection of the upper plane
and the legform load were investigated according to the stiff-
ness of the springs. In order to satisfy the impact load re-
quirements, the crumple zone structure must be designed with
a rigidity of less than 0.5 kN/mm (Fig. 5). Based on these
(a) Lengthwise
(b) Widthwise
Fig. 4. Changes in the impact force according to the spring model
configuration.
Fig. 5. Changes in the impact forces and deflections for various values
of the spring stiffness.
Fig. 2. Commercial upper legform and measurement points.
Fig. 3. 1D spring element model for selecting the design concept of the
crumple zone.
H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601 2597
simulation results, and considering the productivity of injection
forming, a ribbed structure made of plastic material was chosen
for the basic design concept for the crumple zone [12-14].
4. Upper legform impact test and analysis
4.1 Test conditions
The Euro NCAP conditions for the upper-legform to bonnet
leading edge test are as follows. The test is based on five con-
tact points, located at the center and at parallel positions on the
hood of the vehicle, each of which makes a vertical angle of
40° with the ground (this angle can be changed by a number
of factors). Because the impact locations change according to
the vehicle type and the shape of the FEM, the Euro NCAP
regulations specify that the test should be conducted at more
than three points having the potential to cause injuries, and
that the distance between these points must be greater than
150 mm. To pass the test, the impact load on the impactor at
each measurement point must be less than 7.5 kN, and the
bending moment measured at three sensing points in the leg-
form (upper UPR, center CTR, and lower LWR) must be less
than 510 Nm (Fig. 2). These values must be minimized to be
awarded additional stars (competitive level).
In this research, the tests were carried out at five impact
points, illustrated in Fig. 6. The test conditions (upper legform
position, impactor speed, angle, and energy) at these points
were determined according to the Euro NCAP formulas. The
test results were filtered using the CFC180 method to calcu-
late typical maximum values of the impact load and bending
moment [7, 8].
4.2 Impact analysis and verification
In order to verify the reliability of the impact simulations,
the results were compared with test results for vehicles with-
out a crumple zone. The analysis model was configured as
shown in Fig. 7. The garnish, grill, and front end of the hood
(the parts expected to have the largest impact deformations)
were modeled in detail (element size of 5–7 mm), while the
remaining parts were modeled roughly (10–15 mm). The as-
sembled parts were represented by a rigid-body condition
(multi-point constraint). As for the garnish, grill, bumper, and
air-duct parts (made of plastic) and the other parts (made of
steel), appropriate uniaxial tension test data for each material
(elastic–plastic data) were applied. In particular, the modeling
of the hood and carrier parts was based on test data that took
into account the strain rate effect.
The model was verified by comparing the analysis results
with averages obtained from three tests. All five positions
yielded similar average impact loads for the average values, as
indicated by Fig. 8. However, as shown in Fig. 9, there were
large differences in the bending moment at some positions (a
Fig. 6. Target points on the leading-edge line of the hood.
Fig. 7. Finite element model for impact analysis.
Fig. 8. Comparison of impact forces for existing models without a
crumple zone.
Fig. 9. Comparison of bending moments for existing models without a
crumple zone.
2598 H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601
maximum error of about 25% against the average experimen-
tal value). Nevertheless, since the test results at these positions
exhibited a wide range of values, and the analysis results were
located between the experimental maximum and minimum
values, the predicted bending moments were deemed reliable.
5. Detailed design of the crumple zone
The crumple zone designed in this study was to be mounted
between the frame and the hood, on the upper part of the FEM.
Therefore, it was to be made of plastic that had excellent heat
and oil resistance characteristics. After pricing various materi-
als, a particular kind of polypropylene plastic (PP) was chosen
for the design.
5.1 Design parameter study
In order to develop the basic structural design of the crum-
ple zone, nine cases were selected with varying intervals and
angles between the ribs, as listed in Table 1. A parametric
simulation study was conducted at point A. For the sake of
productivity, the rib thickness was fixed at 2 mm, and only the
effectiveness of the rib pattern was verified. Six points were
selected to assemble the crumple zone on the upper frame of
the FEM, considering the shape of the frame and the manufac-
turing capacity, and these were expressed in terms of the rigid-
body condition in the analysis model. Moreover, damaged
elements were used in the crumple zone model to incorporate
the fracture effect of the plastic material. The parameters of
the damage model were obtained from uniaxial tension test
results at a crosshead speed of 450 mm/min.
The crumple zone design specified by Case 2 was most ef-
fective for reducing the impact load and bending moment. In
particular, a bending moment reduction of up to 67% was
predicted. However, the impact load was quite significant
compared to that of the model without a crumple zone. It is
possible that the crumple zone spacing and joint locations
were set inappropriately.
5.2 Detailed design
Based on the results of the case study, the height of the crum-
ple zone was increased, as shown in Fig. 10, and its assembly
location was also changed. The results of the impact test and the
simulation for the modified crumple zone model are shown in
Figs. 11 and 12. There was a 90% level of agreement between
the analytical and average impact loads from the impact test
using the prototype model. The simulated bending moment re-
sults were within the margin of error obtained from the test re-
sults, which was equivalent to 35.0−194.3 Nm.
Figs. 13 and 14 show the analysis results with and without a
crumple zone at the five impact points. When a crumple zone
was used, the impact loads and bending moments at points B,
C, and D decreased. At each of the end points, however, the
crumple zone created a negative load path, and the predicted
values actually increased. Based on these results, the final
crumple zone design was completed with the structures at
points A and E removed, as shown in Fig. 15. As indicated in
Fig. 10. Modification of the initial crumple zone design.
Fig. 11. Comparison of test and analysis results for impact force with
the initial crumple zone.
Table 1. Analysis results for the case study.
Case Interval of rib Angle of rib
1 10 mm 30°
2 20 mm 30°
3 30 mm 30°
4 10 mm 60°
5 20 mm 60°
6 30 mm 60°
7 10 mm 90°
8 20 mm 90°
9 30 mm 90°
H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601 2599
Figs. 16 and 17, when this modified crumple zone was used,
the impact load and bending moment decreased by more than
10% on average at all locations, except for position E. Based
on an analysis of the load propagation path and the shape of
the deformation, we assumed that the impact characteristics at
point E were closely related to the shape of the air conductor
and its mounting position on the crumple zone. Accordingly,
we will try to modify the design of the air conductor to im-
prove the upper legform test results.
Fig. 12. Comparison of test and analysis results for the bending
moment at the CTR position (CTR had the worst results among the
three positions).
Fig. 13. Comparison of impact forces with and without the crumple zone.
Fig. 14. Comparison of bending moments at the CTR with and without
the crumple zone.
Fig. 15. Shape of the final modified crumple zone.
Fig. 16. Impact force predicted by the FEM of the final modified
crumple zone.
Fig. 17. Bending moment (CTR) predicted by the FEM of the final
modified crumple zone.
2600 H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601
6. Conclusion
In this study, a front-end module impact analysis was
conducted using the Euro NCAP upper-legform to bonnet
leading edge test. Based on the simulation results, a new
crumple zone design was proposed, which is expected to
decrease the impact load and bending moment by more than
27% and 36%, respectively, compared to existing models.
The following is a summary of our conclusions.
(1) Using a 1D spring model, a design concept for the
crumple zone was carried out. The stiffness, material, and
basic shape of the crumple zone were chosen based on the
analytical results from the spring model.
(2) Comparison to actual impact test results confirmed
that the reliability of the impact analysis was satisfactory.
The analytical predictions were between the experimental
maximum and minimum values.
(3) An effective ribbed model for the crumple zone was
developed from the results of a case study. A crumple zone
was then designed, incorporating the effects of various fac-
tors, including joint location and interference between sur-
rounding components, and its performance was predicted
analytically.
Acknowledgment
It is notified that this study has been for the achievement
for the “Development of Integrated Light Active Front End
Modules for Protecting Pedestrians” (supported by the Ko-
rean government - Ministry of Knowledge Economy -
10033831).
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Hyung-il Moon received his B.Sc.,
M.Sc., and Ph.D degrees from the De-
partment of Mechanical and Mechatron-
ics Engineering at Kangwon National
University in 2005, 2007, and 2011. His
research interests include structural
analysis, crash analysis, rubber and plas-
tic components, and optimum structural
design.
Young-eun Jeon received her B.Sc.
degree from Kangwon National Univer-
sity in 2010. She will receive her M.Sc.
degree from the Department of Me-
chanical Engineering at Kangwon Na-
tional University in 2012. Her research
interests include crash and safety analy-
sis.
Dae-Young Kim received his B.Sc. and
M.Sc. degrees from the Department of
Mechanical and Mechatronics Engineer-
ing at Kangwon National University in
2009. His research interests include
structural analysis, crash and safety,
welding analysis, and failure criteria.
H. Moon et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2595~2601 2601
Heon Young Kim received his B.Sc.,
M.Sc., and Ph.D. degrees from the De-
partment of Mechanical Design and
Production Engineering at Seoul Na-
tional University in 1985, 1987, and
1991. His research interests include
stamping, nonlinear analysis, crash and
safety, rubber and plastic components,
and biomedical equipment.
Yong-soo Kim received his B.Sc. de-
gree from Youngnam University in
1994. His major was mechanical engi-
neering, and his field of interest is
automotive structural design compo-
nents.
