EVALUATION OF THE CRASH ENERGY ABSORPTION OF
HYDROFORMED BUMPER STAYS
B. J. Kim1, S. M. Sohn2 and Y. H. Moon1*
1
Department of Mechanical and Precision Engineering, Pusan National University,
Busan, 609-735, Korea
2
*
Technical Institute of Sungwoo Hitech Co. Ltd., Kijang,
Busan, 619-961, Korea
Corresponding author. Y. H. Moon (Young Hoon Moon)
Tel.:+82-51-510-2472; Fax:+82-51-512-1722. E-mail address: yhmoon@pusan.ac.kr
Poster address: Dept. of Mechanical Engineering, Pusan National University, Busan, 609-735, Korea
EVALUATION OF THE CRASH ENERGY ABSORPTION OF
HYDROFORMED BUMPER STAYS
B. J. Kim1, K. S. Park1 S. M. Sohn2 and Y. H. Moon1*
1
Department of Mechanical and Precision Engineering, Pusan National University,
Busan, 609-735, Korea
2
Technical Institute of Sungwoo Hitech Co. Ltd., Kijang,
Busan, 619-961, Korea
Abstract
Because flanges provide the sole area of contact between a conventional bumper stay and a bumper
beam, a substantial load concentrates on these flanges when an impact is applied to the bumper beam. It is
difficult to distribute the load from such impacts, since the traditional bumper stay is not capable of
absorbing a large amount of energy. In the present study, the crash energy absorption of bumper stays that
were produced by a hydroforming process has been evaluated with static compression tests and impact
tests. To hydroform a bumper stay, the tube specimen in the guide zone is fed into the expansion zone
with a linear increase in pressure, while a compression load is applied to the folded region. Static
compression tests were conducted to evaluate deformation behavior and the absorption energy of the
bumper stays under various strain rates. The impact tests were conducted under conditions stated in the
CMVSS215 specification. Results show that a hydroformed bumper stay exhibits good impact resistance,
as compared to a traditionally formed bumper stay.
keyword : Hydroforming, Bumper Stay, Bumper beam, Crash energy
1. Introduction
The conventional front structure of an automobile is primarily composed of left and right front frames,
connected with the inner side of a front wheel apron and a front bumper. The front bumper consists of a
bumper face, bumper beam for distributing the load from impacts, an absorber member located between
the bumper face and the bumper beam, and a pair of bumper stays which secure the bumper beam to the
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vehicle body (see Fig. 1(a)). When the automobile is hit from the front or behind, the bumper beam
collapses and the impact force is transmitted to the left and right front frames, respectively, through the
bumper beam and bumper stays. The impact energy is absorbed by plastic deformation of the bumper
beam and bumper stays. The conventional bumper structure is assembled from several parts, so several
manufacturing processing steps are needed, and the structure is somewhat complex. Because the flanges
provide the only area of contact between the bumper stay and the bumper beam, a substantial load
concentrates on these flanges when an impact is applied to the bumper beam. Most research work on
bumper stays has focused on using reinforcing members that have complicated shapes [1,2]. In the
present study, the tubular hydroforming process is used to produce a bumper stay (see Fig. 1(b)).
Fig. 1 Modeling of a conventionally produced stay and a hydroformed stay.
Hydroformed parts are often found in the motor vehicles. Such parts can include exhaust system
modules, axle and transmission components, body frame parts, and engine subframes. Hydroformed parts
have several advantages over conventionally formed parts, such as the reduction in the number of
processing steps and in the number of subassembly parts needed.
In the present work, static compression tests were conducted to evaluate deformation behavior and the
energy absorption of bumper stays deformed under various strain rates. Also, impact tests such as the
pendulum and the barrier tests were conducted under the conditions stated in the CMVSS215 (Canadian
Motor Vehicle Safety Standards)[3] regulation to compare the impact resistance of a hydroformed bumper
stay with that of traditionally formed bumper stay.
2. Experimental setup
2.1 Experimental procedure
The material used for the experiments is STKM11A, which was rolled formed into seamed tubes. The
tubes have a 50.8 mm outside diameter with a wall thickness of 2.1 mm (see Fig. 2(a)). The tube
hydroforming system, which is capable of applying pre-programmed paths of axial feed and internal
pressure, consisted of two 80-ton actuators mounted horizontally to supply the axial feed at both ends of
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the tube. A vertical hydraulic cylinder, capable of delivering a clamping force of up to 80 tons, provided
the upper die movement. A pressure intensifier unit with the maximum capacity of 180 MPa was used to
pressurize hydraulic fluid during the experiments (see Fig. 2(b)).
Fig. 2 Experimental setup for the hydroforming of a bumper stay.
The hydroforming process, which is composed of the two steps—tube bulging and folding—is applied
to the tube specimen, as depicted in Fig. 3. During the bulging step, the tube is expanded into the die
cavity as the internal oil pressure P increases to a preset value. The pre-form with contact parts
constrained by die wall is obtained. As shown in Fig. 3, during the folding process, while the internal oil
pressure P is maintained constant, an axial force is applied along the longitudinal direction of the bulged
tube in order to complete the final shape of the bumper stay[4]. Finally, the oil pressure and the axial
force are reduced to zero and the completed bumper stay has been formed.
Fig. 3
Illustration of the overall bumper stay hydroforming process.
2.2 Die setup for bumper stay hydroforming
Fig. 4 shows the hydroforming die set for the bumper stay consists of two parts: a tube-bulging die and
a folding die.
Fig. 4 Hydroformed bumper stay die for the folding step in the process.
A tube of 50.8 mm diameter (d0) and 2.1 mm thickness was used to form the part with a maximum
diameter (D) of 66 mm having an expansion ratio (D/d0) of 1.3. During the tube-bulging process, the oil
pressure increases linearly to smoothly form the pre-form while a folding die remains in its initial position.
During the folding process, the folding die moves in the longitudinal direction of the bulged tube applying
an axial load with the punch, while the interior pressure is maintained in order to form the final folded
shape. At the end of the process, the folding die returns to its initial position by the action of the
springs[5].
3. Experimental results
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3.1 Compression test
Static compression tests were conducted to evaluate the deformation behavior and the energy
absorption behavior of the bumper stay under stroke speeds of 10 to 60 mm/min. The total compression
displacement was approximately 63 mm. The testing machine was a hydraulic press with a 20-ton
capacity. Three tests were conducted for each condition to ensure the repeatability of the results. Static
load-displacement curves were measured, and the area under the curve (i.e. the impact absorption energy)
and the maximum load (i.e. the maximum compression-resistance force) were determined.
Fig. 5 Compression test of hydroformed bumper stay.
As shown in Fig. 5, under compressive loading, the hydroformed bumper stay deforms by folding in on
itself. The tube material in the guide zone region moves into the expansion zone of the tube under a
constant load. During the initiation of plastic folding, variations in the load-displacement curves are
observed, depending on the speed. Once the stoke reaches about 60 mm, a peak load of 5,500 kgf is
maintained and the differences due to displacement speed are not observed (see Fig. 6).
Fig. 6 Load-stroke and energy-stroke curves obtained from a static compression test of a
hydroformed bumper stay.
Hence, the stroke speed has little influence on energy absorption and maximum load, implying that the
impact energy is absorbed under a constant resistive force after first plastic fold has occurred.
3.2 Impact Test
From an assembly consideration, a weight reduction from 1.86 to 1.39 kg (33.8% decrease) can be
achieved with the hydroformed bumper stay. Fig. 7(a) and (b) show the pendulum and barrier impact tests,
which were performed according to CMVSS215 regulations.
Fig. 7 Impact test results of hydroformed bumper stay system.
The bumper stay and a commercial front bumper of LC F/L were placed on a car with weight of 1000
kg. There were three types of deformation modes: 1) center, 2) offset (300 mm from RH), and 3) right and
left corner (RH & LH). For the pendulum test, bumpers were impacted at the automotive standard speed
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of 3.2 and 5.2 mph (miles per hour)[6]. Fig. 7(c) shows the results from a pendulum impact test where the
resistance force is plotted as a function of time. For the center and the offset (300 mm from RH) impact
locations, the resistance force was 1600 to 1700kgf. For the corner impact test, the value was
approximately 1100kgf. In the barrier tests a car with a bumper is impacted perpendicular to the fixed wall
with 5.2 mph and 10.4 mph.
The conventional requirements on the intrusion (INT) and deflection (DEF) amount are 120 mm and 54
mm, respectively. Table 1 shows that both of conventional stay and hydroformed stay satisfy the
regulations at impact velocities of 3.2 and 5.2 mph.
Table 1. Comparison of impact test results between conventional stay and hydroformed stay.
It is observed that the hydroformed bumper stay is not easily folded and absorbs impact energy
similarly to the conventional stay. At low impact velocities, the hydroformed stay is quite stiff, but at
higher velocities the deflection amount of the hydroformed stay system is a little larger than that of the
conventional stay system. As shown in Fig. 7 (d), at the velocity of 10.4 mph in the barrier test,
symmetric folds are formed and the tube specimen in the guide zone is easily fed into the expansion zone
of the tube, causing the impact energy to be easily absorbed while the deflection requirement is not
satisfied (i.e. it is in excess of 42.6%). Nevertheless, it is expected that a hydroformed bumper stay can
meet the requirement, if the beam and stay assembly is optimally designed.
4. Conclusions
Compression tests and impact tests have been performed on hydroformed and conventional bumper
stays and from the results, some useful conclusions can be stated, as follows:
1. The hydroforming process can be used to manufacture a bumper stay with a two-step
operation—deformation followed by folding.
2. In compression tests, the hydroformed bumper stay can be easily folded and the impact energy is
linearly absorbed under constant resistive force irrespective of deformation speed.
3. From the results of several impact tests, the hydroformed bumper stay provides good impact resistance.
The full requirements can be achieved if the beam and stay assembly is optimally designed.
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Acknowledgements
This work was supported by the AAA Foundation.
References
[1] M. Oyama, N. Masuta, Automotive Bumper Stay Structure, United States Patent, (1995) US Patent
5441319.
[2] K. Kariatsumari, Kobe, Car Body Energy Absorber and Bumper Stay, United States Patent, (2002) US
Patent 6481690 B2.
[3] Motor Vehicle Safety Regulations, Standard 215, Consolidated Regulations of Canada, 1978.
[4] S.W. Lee, Study on the Forming Parameters of the Metal Bellows, Journal of Materials Processing
Technology, 130-131 (2002) 47-53.
[5] S.M .Sohn, M Y. Lee, B.J. Kim, Y.H .Moon. Development of Manufacturing Technology for Crash
Energy Absorption Bumper Stay with Hydroforming, in: Proceedings of TUBEHYDRO 2005, 10-11
November 2005, pp. 38-41.
[6] R. Hosseinzadeh, M.M. Shokrieh, L.B. Lessard, Parametric Study of Automotive Composite Bumper
Beams Subjected to Low-Velocity Impacts, Composite Structures, 68 (2005) 419-427.
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Figure Captions
Fig. 1
Modeling of a conventionally produced stay and a hydroformed stay.
Fig. 2
Experimental setup for the hydroforming of a bumper stay.
Fig. 3
Illustration of the overall bumper stay hydroforming process.
Fig. 4
Hydroformed bumper stay die for the folding step in the process.
Fig. 5
Compression test of hydroformed bumper stay.
Fig. 6 Load-stroke and energy-stroke curves obtained from a static compression test of a hydroformed
bumper stay.
Fig. 7
Impact test results of hydroformed bumper stay system.
Table Captions
Table 1.
Comparison of impact test results between conventional stay and hydroformed stay.
8
(a) Conventional bumper stay
(b) Hydroformed bumper stay
Fig. 1 Modeling of a conventionally produced stay and a hydroformed stay.
(a) Hydroformed bumper stay
(b) Schematic view of the hydroforming machine
9
Fig. 2 Experimental setup for the hydroforming of a bumper stay.
forming
(a) Forming sequence
folding
(b) Hydroformed bumper stay
Fig. 3 Illustration of the overall bumper stay hydroforming process.
(a) Die set up
(b) Forming sequence (hydroforming and folding)
Fig. 4 Hydroformed bumper stay die for the folding step in the process.
10
(a) Compression under various speeds
(b) Compressed shape of the hydroformed stay
Fig. 5 Compression test of hydroformed bumper stay.
6
Load (tonf)
5
4
3
2
10mm/min
20mm/min
40mm/min
60mm/min
1
0
0
10
20
30
40
50
Stroke (mm)
(a) Load- stroke curve
11
60
70
350
Energy (tonfmm)
300
250
200
150
100
10mm/min
20mm/min
40mm/min
60mm/min
50
0
-50
0
10
20
30
40
50
60
70
Stroke (mm)
(b) Energy-stroke curve
Fig. 6 Load-stroke and energy-stroke curves obtained from a static compression test of a hydroformed
bumper stay.
(a) Center pendulum test
(b) Barrier test (5 mph, 10 mph)
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2000
CENTER
OFFSET
CORNER RH
CORNER LH
1750
1500
Force (kgf)
1250
1000
750
500
250
0
0.00
0.05
0.10
Time(sec)
0.15
0.20
(c) Pendulum test results (5 mph)
(d) Folded bumper stay (barrier 10 mph)
Fig. 7 Impact test results of hydroformed bumper stay system.
Table 1. Comparison of impact test results between conventional stay and hydroformed stay.
13