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Energy Absorption Characteristics of Crash Box of New Honeycomb Core

Structure with Foam-Filled

Article in International Journal of Automotive Technology · February 2021

DOI: 10.1007/s12239-021-0022-6

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International Journal of Automotive Technology, Vol. 22, No. 1, pp. 221–230 (2021) Copyright © 2021 KSAE/ 119–22
DOI 10.1007/s12239–021–0022–6 pISSN 1229–9138/ eISSN 1976–3832

ENERGY ABSORPTION CHARATERISTICS OF CRASH BOX OF

NEW HONEYCOMB CORE STRUCTURE WITH FOAM-FILLED

Xiang Li1, 2)*, Yanmiao Wang2), Xingxing Xu2), Xiangbin Cao2) and Rui Li2)
1)
Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University,
Yichang, Hubei 443002, China
2)
College of Mechanical and Power Engineering, China Three Gorges University, Yichang, Hubei 443002, China

(Received 4 October 2019; Revised 12 March 2020; Accepted 15 May 2020)

ABSTRACT–By combining honeycomb core with the traditional crash box, an innovative foam-filled honeycomb core
structure crash box is proposed to improve safety performances. Three kinds of honeycomb core crash boxes were obtained
by filling the inside of the traditional crash box with the quasi-honeycomb core, the quasi-square honeycomb core and
hexagonal honeycomb core. It is deduced that the energy absorption of the quasi-honeycomb core crash box increased by
42.2 %, the peak of the collision force reduced by 8.9 %. The hexagonal honeycomb core crash box and the quasi-square
honeycomb core crash box also enhance the energy absorption characteristics, but the peak impact force increased by 5.6 %
and 9.6 %. After filling of the Expanded Polypropylene (EPP) foam, the peak collision force of the EPP hexagonal
honeycomb core crash box and the EPP quasi-square honeycomb core crash box increased by 10 %, while Fmax of the EPP
quasi-honeycomb core crash box reduced by 2.3 % and the energy absorption increased by 54.6 %. Six structures were
compared to highlight the energy absorption characteristics of the filling the inside of the traditional crash box with the light
in weight honeycomb core and EPP foam. The research results provide a new idea for the design of crash box.

KEY WORDS : Crash box, Low-speed collision, Energy absorption characteristics, Honeycomb core

1. INTRODUCTION mainly focus on the optimization design of the cross-

section shape (Shin et al., 2008; Tanlak and Sonmez,
With the increasing attention is paid for the safety of the 2014) and the dimension parameter. Omkar et al. and Liu
automotive, crashworthiness has been the research focus and Ding analyzed the crash boxes with different cross-
of the vehicle design. In the event of impact due to frontal section shapes and studied their energy absorption
collision, the bumper system absorbs impact energy by characteristics and physical deformations under dynamic
collapsing so that there is minimum damage to the and static loads (Omkar et al., 2018; Liu and Ding, 2016).
vehicle parts and the passengers inside the vehicle are Zhou et al. and Wang et al. studied a parameterized
safe. Thus, it is of great significance to explore the energy model of the NPR crash box, which integrates the design
absorption performance of the bumper system in the parameters of the basic NPR cell structure, the structure
process of the collision so as to promote the safety of of the box was optimized using multi-objective genetic
automobile. As one of the key energy absorbing algorithm and the energy absorption behavior was
components, the crash box can not only absorb energy improved efficiently (Zhou et al., 2016; Wang et al.,
through its own deformation but also make the remaining 2018). These researches above made great contributions
collision force continue to transfer along the longitudinal to the design of the crash box, the performances of the
direction (Cho et al., 2012; Jamian et al., 2015). crash box were improved significantly and the
Therefore, the design of the crash box has significant crashworthiness of the vehicle was enhanced greatly.
influence on the energy absorption performance and the However, due to the limitation of design space, it is hard
living space of the passengers after the collision. To to further improve the performance of the crash box
improve the behavior of the energy absorption, a lot of efficiently by the traditional structural optimization
studies on the design method of the crash box were method. With the increasing requirements of the
conducted. Nowadays, researches on the crash box or the crashworthiness, the application of material structure
structure with good energy absorption characteristics integration is a promising option for further enhancing
the performance of the crash box.
Because composite International Journal of
*Corresponding author. e-mail: lixiangcfy@ctgu.edu.cn Crashworthiness materials and sandwich structure have

221
222 Xiang Li et al.

superior mechanical properties and specific rigidity and

strength, the need for composite sandwich structure has
continued to grow (Lee et al., 2011; Sun et al., 2016). The
honeycomb core has been the research focus for the recent
years due to the unique properties. To improve the
performance of the crash box, a novel foam-filled
honeycomb core structure crash box suggests in this work.
Unlike the traditional materials, the main idea of this paper:
considering the creatures in nature, including bees, they can
build a hexagon honeycomb, and have obtained the structure
with excellent mechanical properties in the course of long-
term evolution, which can achieve stronger function with
lower weight. The filling of the honeycomb core brings a
small change in the peak value of the collision force. The
peak value of the collision force of the quasi-square
honeycomb core crash box and the hexagonal honeycomb
Figure 1. Schematic diagram of three kinds of honeycomb
core crash box is enhanced by 5.6 % and 9.6 % respectively
core structures.
compared with the conventional crash box, while the quasi-
honeycomb core crash box decreases by 8.9 %. After further
filling of the EPP foam, The peak value of the collision force
is increased, but the EPP quasi-honeycomb core crash box
has a lower peak value than the conventional crash box. The
average collision force and total energy absorption are
gradually increased with the sequential filling of the
honeycomb core and the EPP foam, and the average collision
force and total energy absorption of the EPP hexagonal
honeycomb core crash box are the largest. In addition, the
specific energy absorption of the quasi-honeycomb core
crash box is equal to that of the conventional crash box, the Figure 2. Three kinds of honeycomb core crash box sections.
specific energy absorption of the other five crash boxes is
larger than that of the conventional crash box, and the
hexagonal honeycomb core crash box has the largest energy
absorption. Through the design of the overall structure, it is
to improve the energy absorption effect of the new crash box,
and to further enhance the crashworthiness and passive safety
of vehicles.
To address the above-mentioned problems, the rest of this
paper is organized as follows: The model of the new crash
box is established in Section 2.Section 3 gives crash box
performance evaluation parameters. Performance analysis of Figure 3. Sectional views of three EPP foam filled honeycomb
energy absorption characteristics is described in Section 4 core crash boxes.
and conclusions are given in Section 5.The presented new
method also serves as a good example for other application
and optimization of honeycomb structure. Table 1. Mass parameters of crash box.
Core Foam Total
2. MODELING OF THE CRASH BOX Type
mass (g) mass (g) mass (g)
Quasi-square honeycomb 60 11 209
2.1. Innovative Configuration of Crash Box Hexagonal honeycomb 60 12 210
The shell of the new crash box is designed with quadrilateral
Quasi-honeycomb 58 13 209
cross section. The unit cell of the inner core of the new crash
box is the quasi-honeycomb, the quasi-square honeycomb and
hexagonal honeycomb structure respectively, as shown in
Figure 1 (a) ~ (c), which obtain three new honeycomb core Figure 3. The length, width and the wall thickness of the crash
crash boxes, as shown in Figure 2. This work uses EPP foam box of square cross section are 110, 55 and 2 mm respectively.
as the internal filling material of the inner core, as shown in The wall thickness of the honeycomb sandwich is 0.02 mm. It
 ENERGY ABSORPTION CHARATERISTICS OF CRASH BOX OF NEW HONEYCOMB CORE STRUCTURE 223

is on the design general standards premise that the mass of the the crash box is fixed. Nonlinear finite element solver
three different EPP honeycomb core crash boxes should be the Hypermesh software is used to simulate the collision process,
same as possible. The size and mass parameters are shown in and the contact mode between the honeycomb core and the
Table 1. The shell mass is 138 g. The honeycomb core and shell of the crash box, the foam and the honeycomb core are
shell of the crash box materials are aluminum alloy. all defined as single surface contact algorithm. Two rigid walls
were made at the upper and lower ends of the crash box. The
2.2. Numerical Method rigid upper wall, which has 4.4 m/s initial velocity collided
The FEM model of the honeycomb core filled the crash box, with the traditional crash box, the honeycomb core crash box
which was imported into Hypermesh for mesh division, is and the foam-filled honeycomb core crash box during 30 ms
established based on the solidworks model. There are finite to simulate a frontal collision between a vehicle and the rigid
element model of the crash boxes, as shown in Figure 4 ~ 5. wall. Static and dynamic friction factors are set at 0.3 and 0.2
There is the main hypothesis for establishing the computation respectively. In order to prevent the hourglass can be higher
model for structural analysis: the thickness of the shell of the than 5 %, the time step is set at -3e-007. The output K file is
crash box and the honeycomb core is small enough the feature imported into the LS-DYNA solver for analysis.
sizes. Based on this hypothesis, the mechanical properties can
be considered as constant along the thickness direction. 2.3. Model Validation
Therefore, a two dimensional sandwich panel model can be In order to prove finite element simulation model we take a
adopted to replace the actual three dimensional model to correlation was performed by comparing experimental data of
simplify the problem. The shells of the six crash boxes are all Tarigopula et al. (2006) with results of finite element
shell element. The mesh size of the honeycomb core and the simulation. The purpose of the experimental data was to be
crash box shell is 0.6 mm, and the mesh size of the foam is considered only for comparison and correlation with the
1 mm. The number of elements and nodes were shown in simulation. The finite element analysis was similar to the
Table 2. They are all divided by a quadrilateral mesh. In the method used in experiment of Tarigopula et al. (2006). For the
actual processing and manufacturing, the honeycomb core reason of equivalence, the finite element model was built to
combines with the inner cavity of the traditional crash box, replicate the experimental specimen, with dimensions of the
which is generally firmly combined by means of adhesive or crash box specimen as 309 mm length, cross section as 58.8 ≤
welding. Nonlinear finite element solver Hypermesh software 57.0 mm and thickness 1.18 mm with an impact velocity of
is used to simulate the collision process, and the contact mode 5.1 m/s. The weight of the rigid upper wall was 600 kg.
between the honeycomb core and the shell of the crash box, Figures 6 ~ 7 shows the axial force vs. displacement curve and
the foam and the honeycomb core are all defined as single deformation modes of comparison of the test result with the
SingleSuface contact algorithm. In this way, the shell of the finite element simulation. As shown in the Table 2, we
crash box and the honeycomb core as a whole are compression compared force parameter between simulation and
deformation under low-speed collision. The type is to be experimental result of the crash box. The finite element
Eroding. The shell material of the crash box is 6063 aluminum simulation outputs are significantly correlated with the
alloys and the foam material is EPP. Table 3 is the main experimental results. The error of Fmax between experiment
parameters. In this section, the reliability and accuracy of the and finite element model is 4.2 % and the error of mean force
model are verified by simulation according to the low-speed between experiment and FEA is 11.9 %. Little error still exists
collision requirement of RCAR legislation (Zhu et al., 2011; between the finite element model and experiment and it is
Kang, 2016). In the simulation, the bottom end of the shell of acceptable. A good agreement between simulation results and

Figure 4. Finite element model of traditional crash box.

224 Xiang Li et al.

Figure 5. Finite element model of the crash boxes filled with six kinds of EPP honeycomb cores.

Table 2. The number of elements and nodes.

Type Shell Core Foam
Quasi-square honeycomb 72168 and 72532 340100 and 328322 318780 and 441786
Hexagonal honeycomb 29340 and 29548 135888 and 127950 236775 and 341322
Quasi-honeycomb 27289 and 27485 103284 and 98834 380435 and 498570

Table 3. Main parameters of crash box materials.

Crash box component material The elastic modulus/GPa The Poisson’s ration Density/kg•m-3 Yield stress/MPa
Shell 71 0.33 2700 180
Honeycomb 71 0.33 2700 180
Foam 4.5×10-3 - 40 -
 ENERGY ABSORPTION CHARATERISTICS OF CRASH BOX OF NEW HONEYCOMB CORE STRUCTURE 225

process, and the formula is:

D
E =  F( s )ds (1)
0

The compression displacement D is the displacement of the

crash box in the collision process. F(s) is collision force of the
crash box.
Specific energy absorption SEA refers to the energy absorbed
by per unit mass, which is defined as the ratio of the total
energy E to mass M, that is:
E
SEA = ----- (2)
M
Under the same collision conditions, excellent energy
absorption characteristics should be both E and SEA as large
as possible, so as to reduce impact damage to the important
Figure 6. Comparison of experiment (Tarigopula et al., 2006) structures and damage to members.
and simulation of collision force-displacement curves.
3.2. Cushioning Capacity
Cushioning capacity of the crash box is generally
characterized by the relevant index of impact force, and the
main indexes are as follows:
One is the average impact force Fm = E / d, which reflects the
average energy absorption level of the crash box. In order to
make the crash box absorb the energy as much as possible, the
Fm is expected to increase as much as possible within a
reasonable range.
The other is the peak impact force Fmax, which is the
maximum collision force of the crash box in the collision
process. The transfer process of the impact force is gradually
attenuated, and the maximum impact force Fmax of the front
structure is always the upper limit of the impact force of the
rear beam and passenger compartments. Therefore, it must
Figure 7. Comparison of experiment (Tarigopula et al., 2006) take control of the peak impact force Fmax, and make it less
and simulation of deformation modes of the crash box. than the permissible value. In a collision, the peak of collision
force generally appears in two positions: one is the moment of
Table 4. Force values comparison between simulation and contact between the collision object and the crash box, and the
experiment (Tarigopula V et al., 2006) for the crash box. other is that the peak value of the collision force Fmax will
suddenly increase when the entire crash box is fully
Parameter Experiment (KN) Simulation (KN) compressed. For low-speed collisions, the initial peak force
Fmax 95 91 should be paid more attention to, and the structural
Mean force 36.9 32.5 compression changes are closely related to it.

3.3. Main Deformation Modes of Crash Box

experimental data of Tarigopula et al. shows the correctness of Symmetric, asymmetric and transitional modes are the main
the simulation method of the present investigation. deformation modes of the crash box. Symmetrical collapse
mode is characterized by uniform deformation process and
3. PERFORMANCE EVALUATION PARAMETERS OF stable energy absorption. There are two types of asymmetric
CRASH BOX stacking modes. One is the unsteady deformation starting from
the middle of the crash box. One is the unsteady deformation
3.1. Energy Absorption Index starting from the tail of the crash box. This deformation mode
Energy absorption characteristics of the crash box are usually is not conducive to energy control and should not be adopted.
characterized by total energy E or specific energy absorption Transitional collapse mode is a comprehensive state of
SEA.E refers to the total energy absorbed by the crash box symmetric and asymmetric collapse mode. This deformation
through its own deformation during the whole collision mode is relatively common, which also has a definite impact
226 Xiang Li et al.

on the stability of energy absorption.

4. ANALYSIS OF ENERGY ABSORPTION

CHARACTERISTICS OF CRASH BOX

4.1. Deformation Modes

Figure 8 (a) ~ (g) are the deformation modes of traditional
crash boxes, three kinds of honeycomb core crash boxes and
three kinds of EPP honeycomb core crash boxes respectively.
It can be observed in Figure 8 (a) ~ (d) that the traditional crash
box is a transition folding deformation mode, and the quasi-
square honeycomb core crash box has a tendency to transition
folding mode, but no obvious transition folding mode
phenomenon. The deformation modes of the hexagonal
honeycomb core crash box and the quasi-honeycomb core
crash box are both symmetric and superimposed, and the
compression process is uniform from top to bottom, and the
internal honeycomb core is also compressed from top to
bottom. It can be seen from Figure 8 (d) ~ (g) that the
deformation mode of EPP quasi-honeycomb core crash box is
symmetrical folding mode, and the crash box is uniformly
compressed from top to bottom. The folding mode of EPP type
quasi-square honeycomb core crash box is relatively neutral. At
the beginning, there is a tendency of transitional folding mode,
and the latter deformation is symmetric folding mode. However,
EPP hexagonal honeycomb core crash box showed a very
noticeable transition and superposition mode, and the crash
box collapsed from the tail all the time. In addition, it can be
seen that the collapse deformation of the crash box follows the
deformation mode of its internal core. According to Figure 8
(b) ~ (g), only EPP hexagonal honeycomb core crash box has
the collapse deformation from bottom to top, and this crash box
has a very obvious bottom-up transition collapse.

4.2. Dynamic Response in Low-speed Collisions

As indicated in Figure 9, there is excellent agreement between
the collision force curves of the traditional crash box and the
hexagonal honeycomb core box, the quasi-honeycomb core
box and the quasi-square honeycomb core box. It can see the
impact of filling honeycomb core on the impact force of
traditional crash box. The filling of the traditional crash box by
the quasi-square honeycomb core and the hexagonal
honeycomb core makes Fmax higher, and the filling of the
traditional crash box by the quasi-honeycomb core causes the
Fmax of the crash box to be reduced. Compared with the
traditional crash box, the Fmax of the hexagonal honeycomb
core crash box has increased by 9.61 %, the quasi-square
honeycomb core crash box has increased by 5.63 %, and the
quasi-honeycomb core crash box has decreased by 8.93 %.
The collision force curves of EPP hexagonal honeycomb core
box, EPP quasi-honeycomb core crash box and EPP quasi-
square honeycomb core box are shown in Figure 10. Their
impact force variation rules are similar to those of a
honeycomb core box. In the compression process, the impact
force of EPP box with honeycomb core filling structure is Figure 8. Deformation mode diagram.
 ENERGY ABSORPTION CHARATERISTICS OF CRASH BOX OF NEW HONEYCOMB CORE STRUCTURE 227

Figure 11. Fmax of the seven different crash boxes.

Figure 9. Honeycomb core crash boxes collision force-

displacement curve.
4.3. Analysis of Energy Absorption Characteristics
4.3.1. Total energy absorption
The total energy absorption curve of the four different crash
boxes at the effective compression amount of 80 mm is shown
in Figure 12. In 80 mm collision stroke, total energy absorption
of the hexagonal honeycomb core crash box keeps at a high
level and it is much larger than that of the other three types of
crash boxes. The minimum total energy absorption is
traditional crash box. Among them, the total energy absorption
curve of the hexagonal honeycomb core crash box and the
quasi-square honeycomb core crash box are almost the same
before 35 mm. After 80 mm, the crash box is completely
compacted. Since the hourglass can be controlled below 5 %,
the energy is conserved during the entire collision process.
Finally, the kinetic energy of the rigid wall is completely
converted into the internal energy of the crash box, that is, the
energy absorbed by the crash box. Moreover, EPP foam
Figure 10. EPP honeycomb core crash boxes collision force- additions in the crash box do not significantly affect the total
displacement curve. weight, but the impact energy absorption of the system is
considerably improved, as shown in Figure 13.
By comparing energy absorption values of EPP honeycomb
core crash box and honeycomb core crash box under the
higher than that of traditional honeycomb core box, which is effective compression stroke of 80 mm, energy absorption
the same as that of a honeycomb core box. The Fmax of EPP capacity of seven different kinds of crash boxes was analyzed
hexagonal honeycomb core crash box and EPP quasi-square and evaluated, as shown in Figure 14. In comparison with the
honeycomb core was more than 10 % higher than that of quasi-honeycomb core crash box, the energy absorption
traditional crash box. In addition, peak load of EPP quasi- capability of the EPP quasi-honeycomb core crash boxes is
honeycomb core and quasi-honeycomb core crash box is lower substantially increased by 8.7 %. Meanwhile, it was 54.6
than that of traditional crash box, which benefits from the percent higher than the traditional box. The results indicate
structure of honeycomb core. From the perspective of that the energy absorption capacity of the EPP quasi-square
longitudinal comparison, as shown in Figure 11, Fmax was honeycomb core crash box is respectively 4.2 % and 74.7 %
changed after the honeycomb core or EPP honeycomb core higher than that of the quasi-square honeycomb core crash box
was filled in the traditional crash box, and the Fmax of the three and the traditional box. The low energy absorption capability
kinds of EPP honeycomb core crash boxes was higher than of the hexagonal honeycomb core crash box and the traditional
that of the three kinds of honeycomb core crash boxes. It is box, 3.9 % and 95.4 % respectively, compared to the EPP
interesting to note that the Fmax of quasi-honeycomb core crash hexagonal honeycomb core crash box designs, is evident from
box and EPP honeycomb core crash box is lower than the Figure 15. It can be observed that the largest energy absorption
traditional crash box. capacity is for EPP quasi-honeycomb core crash boxes.
228 Xiang Li et al.

Figure 14. Energy absorption of the seven different crash

boxes.
Figure 12. Total energy absorption-displacement curve of
honeycomb core crash boxes.
exhibits better mechanical and physical properties, which have
the characteristics of impact resistance and high energy
absorption. Quasi-honeycomb core crash boxes have the worst
performance. By contrast with the EPP hexagonal honeycomb
core crash box and the traditional box, Fm of EPP hexagonal
honeycomb core structure is almost twice as much as the
traditional crash box, which shows an excellent crashworthiness
performance of hexagonal honeycomb core crash box with
EPP foam. Longitudinally, no matter what kind of crash box,
performance of Fm of honeycomb core crash box filled with
EPP is better than that of unfilled honeycomb core. Except that
the SEA of honeycomb crash box is equal to that of traditional
crash box, the SEA of other crash boxes is higher than that of
traditional crash boxes. From the longitudinal comparison of
their SEA, EPP hexagonal honeycomb core crash box is 1.9 %
lower than hexagonal honeycomb core crash box, EPP quasi-
square honeycomb core crash box is 1.2 % lower than that of
Figure 13. Total energy absorption-displacement curve of the
quasi-square honeycomb core crash box, but EPP quasi-
EPP honeycomb core crash boxes.
honeycomb core crash box is 2.2 % higher than that of quasi-
honeycomb core crash box. Due to the small elastic modulus
of EPP foam material filled in this paper, which is only 4.5
Therefore, EPP honeycomb core crash boxes combine the MPa, the increase and decrease of the honeycomb core crash
advantages of the traditional box and the honeycomb core box filled with EPP is very small, or even not obvious,
crash boxes. And has a better performance. compared with that without filling.

4.3.2. Average collision force and specific energy absorption 5. CONCLUSION

Fm and the SEA of the three kinds of crash boxes are compared
in Table 5. Compared with honeycomb core crash box and EPP In this paper, the folding deformation mode and energy
honeycomb core crash box, hexagonal honeycomb core structure absorption characteristics of traditional crash box, honeycomb

Table 5. Average collision force and specific energy absorption of crash box.
Hexagonal Quasi-square
The crash Quasi- Traditional EPP hexagonal EPP quasi-square EPP quasi-
honeycomb honeycomb
box types honeycomb core crash box honeycomb core honeycomb core honeycomb core
core core
Fm(kN) 87.5 78 66.1 46.5 90.87 81.25 71.87
SEA(J/kg) 35.3×103 31.5×103 26.9×103 26.9×103 34.6×103 31.1×103 27.5×103
 ENERGY ABSORPTION CHARATERISTICS OF CRASH BOX OF NEW HONEYCOMB CORE STRUCTURE 229

core crash box and EPP foam-filled honeycomb core crash box ACKNOWLEDGEMENT–This research work was supported by
were studied under low-speed impact load. The main Yichang Key Laboratory of Robot and Intelligent System, China Three
conclusions are as follows: Gorges University, Yichang 443002, China (JXYC00015), the
Hydropower Machinery Equipment Design and Maintenance Hubei
After filling the honeycomb core, the folding deformation of Provincial Key Laboratory Open Fund Project (2017KJX04) and
the traditional crash box becomes more stable than before. It Sponsored by Research Fund for Excellent Dissertation of China
is shown that the deformation of the hexagonal honeycomb Three Gorges University (2020SSPY034). The authors would like to
core box and the quasi-honeycomb core crash box is express their appreciation for above fund supports.
completely symmetrical and superposition mode, and the
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