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materials
Article
Bending Performance and Reinforcement of Rocker
Panel Components with Unidirectional Carbon
Fiber Composite
Huili Yu 1,2,3,*, Hui Zhao 2,3 and Fangyuan Shi 2,3
1School of Automotive Engineering, Chongqing University, Chongqing 400044, China
2Crash Safety Department, Chongqing Changan Automobile Company, Ltd., Chongqing 401120, China;
zhaohui@changan.com.cn (H.Z.); shify@changan.com.cn (F.S.)
3
Crash Safety Department, State Key Laboratory of Vehicle NVH and Safety Technology, Chongqing 401120,
China
*Correspondence: yuhl@changan.com.cn; Tel.: +86-138-8319-0959
Received: 26 August 2019; Accepted: 25 September 2019; Published: 27 September 2019
Abstract:
Unidirectional carbon fiber composite material is one of the most common types of
composites employed in vehicles, and its bending performance plays an important role in crash safety,
especially in side pole impact. This study aimed to redesign one of the most important components of
the side structure of a vehicle, the rocker panel, with unidirectional carbon fiber composite material.
Our results show that it is not easy to acquire the same bending performance as that of a steel rocker
panel by merely replacing it with carbon fiber material and increasing the wall thickness. Therefore,
reinforcements were employed to improve the bending performance of the carbon fiber rocker panel,
and a polypropylene reinforcement method achieved a weight reduction of 40.7% compared with
high-strength steel.
Keywords: bending performance; rocker panel; unidirectional carbon fiber; simulation
1. Introduction
Carbon fiber is regarded as one of the most promising materials in the automotive industry due
to its high strength and low density. Nowadays, it is utilized in the mass production of vehicles,
especially for electric vehicles like the BMW i3 or NIO es6. Because it provides weight reduction while
maintaining high strength of the body structure, it is possible to increase the driving mileage of electric
vehicles significantly.
Although carbon fiber material has excellent mechanical characteristics, there are some obstacles
in the way of its broad application, one of which is the simulation approach. It seems that no perfect
material model exists that could characterize the strain rate effect and post-failure behavior of the
material and simulate braided fabric composites. However, an appropriate material model can still
possibly be chosen to simulate a given carbon fiber material and achieve the required accuracy.
Some scholars have studied different kinds of carbon fiber composite materials through different
material models and different modeling approaches. Aleksandr Cherniaev [
1
] investigated three
different LS-DYNA material models for modeling the axial crush of a CFRP energy absorber and
identified the advantages and disadvantages of those models. XC. Sun [
2
] studied three different
LS-DYNA finite element analysis (FEA) models in simulating low-velocity impact damage for two
carbon fiber materials, and all the models provided reliable predictions. Post-failure behavior modeling
is the key to composite damage prediction, so researchers like P. F. Liu et al. [
3
–
8
] explored different
methods to predict the intraluminal and interlinear damage. In studying the carbon fiber component
energy absorption performance, axial crushing is one of the most popular methods used by many
Materials 2019,12, 3164; doi:10.3390/ma12193164 www.mdpi.com/journal/materials
Materials 2019,12, 3164 2 of 11
experts [
9
–
16
]. Zheyi Zhang [
17
] studied the influence of geometry on the statistical energy analysis
(SEA) of tubes in four different materials. The literature also shows that the modeling of composites
has been applied in actual structures [
18
–
20
]. Ahad Torkestani [
21
] studied pedestrian head injury
caused by engine hoods made of four different materials, and the results showed that the stacking
sequence of composites had a great influence on the head injury criterion (HIC) value. Sha Yin [
22
]
used a pyramidal lattice core to optimize the pedestrian protection performance of a composite engine
hood and achieved a 25% weight reduction.
Because of the mechanical performance of carbon fiber materials in the material direction,
most investigations are focused on their energy absorption capability in axial crushing. However,
for those components located in the occupant compartment zone, small deformations with large
energy absorption would be preferred, which means that those components require good bending
performance. Unidirectional carbon fiber composite material is one of the most common types of
composites employed in vehicles, and its bending performance plays an important role in crash safety,
especially in the case of pole impact. So, in this paper, unidirectional carbon fiber composite material
was chosen to redesign the rocker panel, one of the most important components in the side structure of
a vehicle. Its bending performance was studied and optimized.
2. Material Tests and Simulation Methods
The unidirectional carbon fiber composite material used to replace the steel rocker panel was
manufactured through the vacuum-assisted resin infusion method. The density of this composite is
1.53
×
10
−6
kg/mm
3
. The carbon fiber was produced by Zhongfu Shenying Carbon Fiber Co., Ltd
Suzhou, China) and contains 12 k carbon fiber in the axial direction. The matrix is an epoxy vinyl ester
resin produced by Guangdong Broadwin Advanced Materials Co., Ltd (Guangdong, China).
2.1. Coupon-Level Test
To determine the properties of this unidirectional carbon fiber composite material, material tests
including of tension and compression at 0
◦
and 90
◦
in-plane shear were conducted according to ASTM
(American Society of Testing Material) standards. The material properties are listed in Table 1.
Table 1. Material properties.
Property LS-DYNA Parameter Value
Modulus in longitudinal direction EA 118.5 GPa
Modulus in transverse direction EB 8.3 Gpa
Major Poisson’s ratio PRBA 0.026
In-plane shear modulus GAB 3.8 Gpa
Strain at longitudinal compressive strength E11C 0.006
Strain at longitudinal tensile strength E11T 0.014
Strain at transverse compressive strength E22C 0.008
Strain at transverse tensile strength E22T 0.005
Shear strength GMS 0.05
Compressive strength in longitudinal direction XC 0.619 Gpa
Tensile strength in longitudinal direction XT 1.763 Gpa
Compressive strength in transverse direction YC 0.144 Gpa
Tensile strength in transverse direction YT 0.033 Gpa
Shear strength SC 0.055 Gpa
The out-of-plane shear modulus, denoted GCA, was measured by three-point bending of a short
beam flex sample [
23
,
24
]. The test specimen was 24
×
7
×
3.5 mm in size and consisted of 12 plies
of 0
◦
unidirectional layers. The average shear modulus calculated from the linear portion of the
apparent shear stress–shear angle (Figure 1) was about 1.3 Gpa. The transverse shear modulus was not
measured, and it was assumed to be equal to GCA.
Materials 2019,12, 3164 3 of 11
Materials 2019, 12, x FOR PEER REVIEW 3 of 11
apparent shear stress–shear angle (Figure 1) was about 1.3 GPa. The transverse shear modulus was
not measured, and it was assumed to be equal to GCA.
Figure 1. Shear stress vs. shear angle curve.
2.2. Component-Level Test
The rocker panel of one electric vehicle was selected to be redesigned in the unidirectional
carbon fiber composite material. The component section with a thickness different from that of the
steel version is shown in Figure 2a. It was composed of two parts, a flat plate and a beam with a bowl
profile section, which were connected by structural adhesive. Their stacking sequence was
−45/45/0/0/0/45/−45 with a wall thickness of 2 mm. The whole component was 1200 mm in length.
(a) (b)
Figure 2. Rocker panel component in carbon fiber. (a) Cross section of the rocker panel. (b) Test
specimens.
The rocker panel component was subjected to a three-point bending test, which is an effective
way to evaluate the bending performance. It was placed on two supports with 600 mm span and was
crushed by an impactor moving at 2 mm/min. The top surfaces of the supports and impactor were
cylindrical with diameters of 40 mm and 304.8 mm, respectively. The test diagram is shown in Figure
3a.
Tests were repeated three times, and the reacting force and displacement of the impactor were
recorded. The results are shown in Figure 3b. It was found definitely that the force–displacement
curve of each test could be divided into four stages. The first was the stage of elastic deformation.
Here, the curves went up linearly before the impactor moved downward 5 mm. The second was the
plasticity-like deformation stage. As the load increased, the reacting force continued to rise until it
reached a peak. However, in this period, the curves went up nonlinearly and dispersedly. At the
beginning of this period, local buckling on the top surface of the bowl profile beam was initiated in
the contact area and spread to the side surfaces of the rocker panel component, which may be the
reason for the nonlinear rise of the curve. Internal differences caused by the manufacturing method
would be the main reason for the three scattered curves. The third was the failure stage. Although
each test component reached a different peak force, they all experienced a sudden load drop at the
Figure 1. Shear stress vs. shear angle curve.
2.2. Component-Level Test
The rocker panel of one electric vehicle was selected to be redesigned in the unidirectional carbon
fiber composite material. The component section with a thickness different from that of the steel version
is shown in Figure 2a. It was composed of two parts, a flat plate and a beam with a bowl profile section,
which were connected by structural adhesive. Their stacking sequence was
−
45/45/0/0/0/45/
−
45 with a
wall thickness of 2 mm. The whole component was 1200 mm in length.
Materials 2019, 12, x FOR PEER REVIEW 3 of 11
apparent shear stress–shear angle (Figure 1) was about 1.3 GPa. The transverse shear modulus was
not measured, and it was assumed to be equal to GCA.
Figure 1. Shear stress vs. shear angle curve.
2.2. Component-Level Test
The rocker panel of one electric vehicle was selected to be redesigned in the unidirectional
carbon fiber composite material. The component section with a thickness different from that of the
steel version is shown in Figure 2a. It was composed of two parts, a flat plate and a beam with a bowl
profile section, which were connected by structural adhesive. Their stacking sequence was
−45/45/0/0/0/45/−45 with a wall thickness of 2 mm. The whole component was 1200 mm in length.
(a) (b)
Figure 2. Rocker panel component in carbon fiber. (a) Cross section of the rocker panel. (b) Test
specimens.
The rocker panel component was subjected to a three-point bending test, which is an effective
way to evaluate the bending performance. It was placed on two supports with 600 mm span and was
crushed by an impactor moving at 2 mm/min. The top surfaces of the supports and impactor were
cylindrical with diameters of 40 mm and 304.8 mm, respectively. The test diagram is shown in Figure
3a.
Tests were repeated three times, and the reacting force and displacement of the impactor were
recorded. The results are shown in Figure 3b. It was found definitely that the force–displacement
curve of each test could be divided into four stages. The first was the stage of elastic deformation.
Here, the curves went up linearly before the impactor moved downward 5 mm. The second was the
plasticity-like deformation stage. As the load increased, the reacting force continued to rise until it
reached a peak. However, in this period, the curves went up nonlinearly and dispersedly. At the
beginning of this period, local buckling on the top surface of the bowl profile beam was initiated in
the contact area and spread to the side surfaces of the rocker panel component, which may be the
reason for the nonlinear rise of the curve. Internal differences caused by the manufacturing method
would be the main reason for the three scattered curves. The third was the failure stage. Although
each test component reached a different peak force, they all experienced a sudden load drop at the
Figure 2.
Rocker panel component in carbon fiber. (
a
) Cross section of the rocker panel. (
b
)
Test specimens.
The rocker panel component was subjected to a three-point bending test, which is an effective
way to evaluate the bending performance. It was placed on two supports with 600 mm span and
was crushed by an impactor moving at 2 mm/min. The top surfaces of the supports and impactor
were cylindrical with diameters of 40 mm and 304.8 mm, respectively. The test diagram is shown in
Figure 3a.
Tests were repeated three times, and the reacting force and displacement of the impactor were
recorded. The results are shown in Figure 3b. It was found definitely that the force–displacement
curve of each test could be divided into four stages. The first was the stage of elastic deformation.
Here, the curves went up linearly before the impactor moved downward 5 mm. The second was the
plasticity-like deformation stage. As the load increased, the reacting force continued to rise until it
reached a peak. However, in this period, the curves went up nonlinearly and dispersedly. At the
beginning of this period, local buckling on the top surface of the bowl profile beam was initiated in
the contact area and spread to the side surfaces of the rocker panel component, which may be the
reason for the nonlinear rise of the curve. Internal differences caused by the manufacturing method
would be the main reason for the three scattered curves. The third was the failure stage. Although
each test component reached a different peak force, they all experienced a sudden load drop at the
same impactor displacement of around 20 mm when fracture was observed on the side surface of the
Materials 2019,12, 3164 4 of 11
bowl profile beam (Figure 3c). The last was the load keeping stage. The rocker panel component still
carried a load of about 2–4 kN after passing the peak.
Materials 2019, 12, x FOR PEER REVIEW 4 of 11
same impactor displacement of around 20 mm when fracture was observed on the side surface of the
bowl profile beam (Figure 3c). The last was the load keeping stage. The rocker panel component still
carried a load of about 2–4 kN after passing the peak.
(a) (b)
(c)
Figure 3. Three-point bending test. (a) Three-point test diagram. (b) Force–displacement curve. (c)
Onset of fracture.
2.3. Simulation
The LS-DYNA MAT58 material model (*MAT_LAMINATED_COMPOSITE_FABRIC) was
selected for modeling this unidirectional carbon fiber composite material. It assumes a nonlinear pre-
peak and post-peak response, and the slope of the pre-and post-peak response can be changed via
parameter εm (εm = E11T, E11C, E22T, E22C, GMS). εm is the corresponding strain at strength. This
continuum damage mechanics model provided a smooth increase in the stress to strength and then
a smooth decrease to the stress decided by scale factor n (n = SLIMT1, SLIMC1, SLIMT2, SLIMC2,
and SLIMS). The typical stress–strain curve for this material model is schematically shown in Figure
4.
Figure 3.
Three-point bending test. (
a
) Three-point test diagram. (
b
) Force–displacement curve. (
c
)
Onset of fracture.
2.3. Simulation
The LS-DYNA MAT58 material model (*MAT_LAMINATED_COMPOSITE_FABRIC) was selected
for modeling this unidirectional carbon fiber composite material. It assumes a nonlinear pre-peak and
post-peak response, and the slope of the pre-and post-peak response can be changed via parameter
εm
(
εm
=E11T, E11C, E22T, E22C, GMS).
εm
is the corresponding strain at strength. This continuum
damage mechanics model provided a smooth increase in the stress to strength and then a smooth
decrease to the stress decided by scale factor n (n =SLIMT1, SLIMC1, SLIMT2, SLIMC2, and SLIMS).
The typical stress–strain curve for this material model is schematically shown in Figure 4.
In plane stress conditions, the four failure criteria for composite materials can be simplified into
the following forms:
For the tensile fiber mode, σ11 ≥0,
e2=(σ11/Xt)2−1≥0. (1a)
For the compressive fiber mode, σ11 <0,
e2=(σ11/Xc)2−1≥0. (1b)
Materials 2019,12, 3164 5 of 11
For the tensile matrix mode, σ22 ≥0,
e2=(σ22/Yt)2+(τ/Sc)2−1≥0. (1c)
For the compressive matrix mode, σ22 <0,
e2=(σ22/Yt)2+(τ/Sc)2−1≥0. (1d)
A detailed description of this model can be found elsewhere [25].
The material parameters listed in Table 1were adopted for the physical parameters of MAT58,
and the strain rate effect was not considered according to the tensile test results (Figure 1) at different
strain rates. Calibrations on the coupon level were done in the following. The nonphysical parameters
of MAT58 were determined through the component-level calibration.
Materials 2019, 12, x FOR PEER REVIEW 5 of 11
Figure 4. Shear stress vs. shear angle curve.
In plane stress conditions, the four failure criteria for composite materials can be simplified into
the following forms:
For the tensile fiber mode, σ11 ≥ 0,
e=σ X
⁄
−1≥ 0. (1a)
For the compressive fiber mode, σ11 ＜ 0,
e=σ X
⁄
−1≥0. (1b)
For the tensile matrix mode, σ22 ≥ 0,
e=σ
Y
⁄
+τS
⁄
−1≥ 0. (1c)
For the compressive matrix mode, σ22 ＜ 0,
e=σ
Y
⁄
+τS
⁄
−1≥ 0. (1d)
A detailed description of this model can be found elsewhere [25].
The material parameters listed in Table 1 were adopted for the physical parameters of MAT58,
and the strain rate effect was not considered according to the tensile test results (Figure 1) at different
strain rates. Calibrations on the coupon level were done in the following. The nonphysical parameters
of MAT58 were determined through the component-level calibration.
2.3.1. Coupon-Level Calibration
The multilayered composite test samples were modeled by one-layer shell elements. The
*PART_COMPOSITE keyword was used to model the composite and material, where the thickness
and orientation properties of each ply could be defined separately. For a tensile test specimen
consisting of 8 plies of 0° unidirectional layers, the thickness was about 2.4 mm. Each ply had the
same material, same thickness (0.3 mm), and same fiber orientation (0°). However, the orientations
of each element should be adjusted to match the reference direction of the layup through the Beta
angle in *ELEMENT_SHELL_BETA. The element size of the finite element (FE) model for the coupon-
level calibration was 3 mm. The Belytschko–Tsay element formulation was employed for the FE
model. The stress–strain curves of coupon tests and simulations are shown in Figure 5a–e, showing
that the FE model matched well with test results.
Figure 4. Shear stress vs. shear angle curve.
2.3.1. Coupon-Level Calibration
The multilayered composite test samples were modeled by one-layer shell elements. The
*PART_COMPOSITE keyword was used to model the composite and material, where the thickness and
orientation properties of each ply could be defined separately. For a tensile test specimen consisting
of 8 plies of 0
◦
unidirectional layers, the thickness was about 2.4 mm. Each ply had the same
material, same thickness (0.3 mm), and same fiber orientation (0
◦
). However, the orientations of each
element should be adjusted to match the reference direction of the layup through the Beta angle in
*ELEMENT_SHELL_BETA. The element size of the finite element (FE) model for the coupon-level
calibration was 3 mm. The Belytschko–Tsay element formulation was employed for the FE model.
The stress–strain curves of coupon tests and simulations are shown in Figure 5a–e, showing that the FE
model matched well with test results.
Materials 2019,12, 3164 6 of 11
Materials 2019, 12, x FOR PEER REVIEW 6 of 11
(a) (b)
(c) (d)
(e)
Figure 5. Comparison between test and simulation at the coupon level. (a) 0° tension. (b) 90° tension.
(c) 0° compression. (d) 90° compression. (e) Shear.
2.3.2. Component-Level Calibration
The multilayered composite component was also modeled by one-layer shell elements, and each
layer was modeled with the *PART_COMPOSITE keyword. The element size of the FE model for the
component-level correlation was 5 mm, which was close to the mesh size of the vehicle crash
simulation. The Belytschko–Tsay element formulation was employed. Adhesive was modeled by LS-
DYNA MAT240 and was connected to parts by *CONTACT_TIE_NODES_TO_SURFACE. In order
to save running time, the simulation loading velocity was 0.5 mm/ms, which was far beyond the
testing velocity.
The results of the three-point bending from the simulation and tests on the component level are
shown in Figure 6. It seemed impossible to match the test results without modifying ERODS, and the
force–displacement curve of the simulation would rise after the impactor moved downward 4 mm.
However, a quick improvement was achieved after ERODS was adjusted to 0.4. The maximum load
and energy absorption obtained by the simulation and tests are compared in Table 2. The errors
Figure 5.
Comparison between test and simulation at the coupon level. (
a
) 0
◦
tension. (
b
) 90
◦
tension.
(c) 0◦compression. (d) 90◦compression. (e) Shear.
2.3.2. Component-Level Calibration
The multilayered composite component was also modeled by one-layer shell elements, and each
layer was modeled with the *PART_COMPOSITE keyword. The element size of the FE model for
the component-level correlation was 5 mm, which was close to the mesh size of the vehicle crash
simulation. The Belytschko–Tsay element formulation was employed. Adhesive was modeled by
LS-DYNA MAT240 and was connected to parts by *CONTACT_TIE_NODES_TO_SURFACE. In order
to save running time, the simulation loading velocity was 0.5 mm/ms, which was far beyond the
testing velocity.
The results of the three-point bending from the simulation and tests on the component level are
shown in Figure 6. It seemed impossible to match the test results without modifying ERODS, and the
force–displacement curve of the simulation would rise after the impactor moved downward 4 mm.
Materials 2019,12, 3164 7 of 11
However, a quick improvement was achieved after ERODS was adjusted to 0.4. The maximum load and
energy absorption obtained by the simulation and tests are compared in Table 2. The errors between
those values were about 5.45% and 2.22%, respectively, which could meet the required precision for
engineering applications.
Materials 2019, 12, x FOR PEER REVIEW 7 of 11
(a) (b)
Figure 6. Comparison between test and simulation at the component level. (a) Force–displacement
curve. (b) Energy absorption–displacement curve
Table 2. Maximum load and energy absorption from the tests and simulation.
Terms Max. Load/kN Energy Absorption/J
Test01 6.37 187.8
Test02 8.22 248.22
Test03 7.05 205.78
Test_average 7.21 213.93
Simulation 6.82 218.69
Error 5.45% 2.22%
3. Result
3.1. Comparison of Bending Performance
Safety performance has always acted as a contradictory element in lightweight design. Assessing
the bending performance is an important means to evaluate the safety performance of this carbon
fiber rocker panel. Therefore, its bending performance was compared with that of the steel version.
Two types of steel material commonly used for rocker panels were studied via the same
simulation method as the carbon fiber rocker panel. One was high-strength steel (HSS), and the other
was advanced high-strength steel (AHSS). The yield stresses of those two steel materials are 447 MPa
and 1080 MPa, respectively, and their tensile stresses are 644 MPa and 1290 MPa, respectively. The
thickness chosen for the two parts of the rocker panel component was 1.2 mm. A material card for
the steel was calibrated through the coupon-level test, and the material parameters related to the
strain rate were not defined.
The simulation results are shown in Figure 7. The reacting force of the steel rocker panel
component was higher than that of the carbon fiber component from the beginning of deformation
to the end. The peak reacting force values for carbon fiber, HSS, and AHSS were 6.82 kN, 10.54 kN,
and 20.98 kN, respectively. The energy absorption values during crushing for those three materials
were 218.69 J, 443.09 J, and 778.42 J, respectively. Therefore, unless the wall thickness is greatly
increased, this kind of carbon fiber material cannot achieve bending performance as good as that of
the steel material, although its tensile stress in the longitudinal direction was 1763 MPa.
Figure 6.
Comparison between test and simulation at the component level. (
a
) Force–displacement
curve. (b) Energy absorption–displacement curve
Table 2. Maximum load and energy absorption from the tests and simulation.
Terms Max. Load/kN Energy Absorption/J
Test01 6.37 187.8
Test02 8.22 248.22
Test03 7.05 205.78
Test_average 7.21 213.93
Simulation 6.82 218.69
Error 5.45% 2.22%
3. Result
3.1. Comparison of Bending Performance
Safety performance has always acted as a contradictory element in lightweight design. Assessing
the bending performance is an important means to evaluate the safety performance of this carbon fiber
rocker panel. Therefore, its bending performance was compared with that of the steel version.
Two types of steel material commonly used for rocker panels were studied via the same simulation
method as the carbon fiber rocker panel. One was high-strength steel (HSS), and the other was
advanced high-strength steel (AHSS). The yield stresses of those two steel materials are 447 MPa and
1080 MPa, respectively, and their tensile stresses are 644 MPa and 1290 MPa, respectively. The thickness
chosen for the two parts of the rocker panel component was 1.2 mm. A material card for the steel was
calibrated through the coupon-level test, and the material parameters related to the strain rate were
not defined.
The simulation results are shown in Figure 7. The reacting force of the steelrocker panel component
was higher than that of the carbon fiber component from the beginning of deformation to the end.
The peak reacting force values for carbon fiber, HSS, and AHSS were 6.82 kN, 10.54 kN, and 20.98 kN,
respectively. The energy absorption values during crushing for those three materials were 218.69 J,
443.09 J, and 778.42 J, respectively. Therefore, unless the wall thickness is greatly increased, this kind
of carbon fiber material cannot achieve bending performance as good as that of the steel material,
although its tensile stress in the longitudinal direction was 1763 MPa.
Materials 2019,12, 3164 8 of 11
Materials 2019, 12, x FOR PEER REVIEW 8 of 11
(a) (b)
Figure 7. Three-point bending simulation results of steel and carbon fiber. (a) Force–displacement
curve. (b) Energy absorption–displacement curve.
3.2. Improvement of Bending Performance
Two approaches, foam core filling (Figure 8a) and polypropylene (PP) reinforcement (Figure
8b), were employed to improve the bending performance of the carbon fiber rocker panel component
so as to not influence its exterior profile. The wall thickness of the PP reinforcement was 3 mm. The
densities of those two materials are 5 × 10−7 kg/mm3 and 1.07 × 10−6 kg/mm3, and their elastic moduli
are 1.6 GPa and 2.7 GPa, respectively.
The material models for modeling the foam core and PP reinforcement were
LOW_DENSITY_FOAM and SAMP-1 in LS-DYNA. The foam material model was provided by
Henkel, and the PP material parameters were identified by testing. The stress–strain curves of the PP
material are presented in Figure 9.
(a) (b)
Figure 8. Reinforcement approaches. (a) Foam core filling. (b) Polypropylene reinforcement.
(a) (b)
Figure 7.
Three-point bending simulation results of steel and carbon fiber. (
a
) Force–displacement
curve. (b) Energy absorption–displacement curve.
3.2. Improvement of Bending Performance
Two approaches, foam core filling (Figure 8a) and polypropylene (PP) reinforcement (Figure 8b),
were employed to improve the bending performance of the carbon fiber rocker panel component so as
to not influence its exterior profile. The wall thickness of the PP reinforcement was 3 mm. The densities
of those two materials are 5
×
10
−7
kg/mm
3
and 1.07
×
10
−6
kg/mm
3
, and their elastic moduli are
1.6 GPa and 2.7 GPa, respectively.
Materials 2019, 12, x FOR PEER REVIEW 8 of 11
(a) (b)
Figure 7. Three-point bending simulation results of steel and carbon fiber. (a) Force–displacement
curve. (b) Energy absorption–displacement curve.
3.2. Improvement of Bending Performance
Two approaches, foam core filling (Figure 8a) and polypropylene (PP) reinforcement (Figure
8b), were employed to improve the bending performance of the carbon fiber rocker panel component
so as to not influence its exterior profile. The wall thickness of the PP reinforcement was 3 mm. The
densities of those two materials are 5 × 10−7 kg/mm3 and 1.07 × 10−6 kg/mm3, and their elastic moduli
are 1.6 GPa and 2.7 GPa, respectively.
The material models for modeling the foam core and PP reinforcement were
LOW_DENSITY_FOAM and SAMP-1 in LS-DYNA. The foam material model was provided
by
(a) (b)
Figure 8. Reinforcement approaches. (a) Foam core filling. (b) Polypropylene reinforcement.
(a) (b)
Figure 8. Reinforcement approaches. (a) Foam core filling. (b) Polypropylene reinforcement.
The material models for modeling the foam core and PP reinforcement were
LOW_DENSITY_FOAM and SAMP-1 in LS-DYNA. The foam material model was provided by
Henkel, and the PP material parameters were identified by testing. The stress–strain curves of the PP
material are presented in Figure 9.
Materials 2019,12, 3164 9 of 11
Materials 2019, 12, x FOR PEER REVIEW 8 of 11
(a) (b)
Figure 7. Three-point bending simulation results of steel and carbon fiber. (a) Force–displacement
curve. (b) Energy absorption–displacement curve.
3.2. Improvement of Bending Performance
Two approaches, foam core filling (Figure 8a) and polypropylene (PP) reinforcement (Figure
8b), were employed to improve the bending performance of the carbon fiber rocker panel component
so as to not influence its exterior profile. The wall thickness of the PP reinforcement was 3 mm. The
densities of those two materials are 5 × 10−7 kg/mm3 and 1.07 × 10−6 kg/mm3, and their elastic moduli
are 1.6 GPa and 2.7 GPa, respectively.
The material models for modeling the foam core and PP reinforcement were
LOW_DENSITY_FOAM and SAMP-1 in LS-DYNA. The foam material model was provided by
Henkel, and the PP material parameters were identified by testing. The stress–strain curves of the PP
material are presented in Figure 9.
(a) (b)
Figure 8. Reinforcement approaches. (a) Foam core filling. (b) Polypropylene reinforcement.
(a) (b)
Figure 9. True stress–true strain curves of polypropylene material. (a) Tension. (b) Compression.
The reinforced FE models experienced the same loading conditions as the carbon and steel
component models. The simulation results are shown in Figure 10. Six parameters were acquired
from the simulation results: component weight, maximum load, related displacement when reaching
maximum load, energy absorption when the displacement of the impactor reached 50 mm, maximum
load per unit weight of component, and energy absorption per unit weight of component. All of these
results are shown in Figure 10c.
Materials 2019, 12, x FOR PEER REVIEW 9 of 11
Figure 9. True stress–true strain curves of polypropylene material. (a) Tension. (b) Compression.
The reinforced FE models experienced the same loading conditions as the carbon and steel
component models. The simulation results are shown in Figure 10. Six parameters were acquired
from the simulation results: component weight, maximum load, related displacement when reaching
maximum load, energy absorption when the displacement of the impactor reached 50 mm, maximum
load per unit weight of component, and energy absorption per unit weight of component. All of these
results are shown in Figure 10c.
The results showed that the most effective way to increase the bending performance was the
foam core filling method. It achieved not only the highest maximum load and energy absorption but
also the highest maximum load per unit weight and energy absorption per unit weight. Although
only 0.81 kg heavier than AHSS, the maximum load and energy absorption of the foam-core-filled
carbon fiber rocker panel increased to 88.09 kN and 3107.76 J, around 4 times those of AHSS.
The second-highest maximum load per unit weight and energy absorption per unit weight were
achieved by the PP-reinforced carbon fiber rocker panel, second to the carbon fiber rocker panel. If
weight reduction is the priority target, PP reinforcement is a considerably effective way to improve
the bending performance when ignoring processing cost. The weight of the PP structure was 1.39 kg,
and the maximum load and energy absorption increased by 143% and 132% compared with the
carbon fiber rocker panel component, respectively. The bending performance of PP reinforcement
(a) (b)
(c)
Figure 10. Comparison of the performance of different rocker panel components. (a) Force–
displacement curve. (b) Energy absorption–displacement curve. (c) Histograms of parameters
showing the bending performance of the rocker panels.
4. Conclusion
Figure 10.
Comparison of the performance of different rocker panel components. (
a
) Force–displacement
curve. (
b
) Energy absorption–displacement curve. (
c
) Histograms of parameters showing the bending
performance of the rocker panels.
Materials 2019,12, 3164 10 of 11
The results showed that the most effective way to increase the bending performance was the foam
core filling method. It achieved not only the highest maximum load and energy absorption but also
the highest maximum load per unit weight and energy absorption per unit weight. Although only 0.81
kg heavier than AHSS, the maximum load and energy absorption of the foam-core-filled carbon fiber
rocker panel increased to 88.09 kN and 3107.76 J, around 4 times those of AHSS.
The second-highest maximum load per unit weight and energy absorption per unit weight were
achieved by the PP-reinforced carbon fiber rocker panel, second to the carbon fiber rocker panel.
If weight reduction is the priority target, PP reinforcement is a considerably effective way to improve
the bending performance when ignoring processing cost. The weight of the PP structure was 1.39 kg,
and the maximum load and energy absorption increased by 143% and 132% compared with the carbon
fiber rocker panel component, respectively. The bending performance of PP reinforcement was more
excellent than that of HSS, but its weight was only 3.07 kg, a reduction by 40.7%. The performance of
PP reinforcement could be improved if its rib arrangement and wall thickness were optimized.
4. Conclusion
In this paper, a unidirectional carbon fiber composite material was selected to be used in
rocker panel components. The LS-DYNA MAT58 material model was employed in modeling this
unidirectional carbon fiber composite. It showed good agreement in the coupon-level correlation, and
a good component-level correlation was also achieved through further calibration on nonphysical
parameters by trial and error.
Although its tensile stress in the longitudinal direction was 1763 MPa, the bending performance of
the rocker panel component made of this kind of carbon fiber composite material could not be as good
as that of a steel one unless the wall thickness were increased substantially and its profile changed.
One of the most effective ways found to increase the bending performance of composite for rocker
panel components is the foam core filling method. It achieved not only the highest maximum load
and energy absorption but also the highest maximum load and energy absorption per unit weight.
PP reinforcement is also a considerably effective way to improve bending performance when ignoring
the processing cost. Its bending performance could be superior to that of HSS, and the weight could be
reduced by 40.7% compared to HSS.
Author Contributions:
H.Z. put forward the original idea for this manuscript. H.Y. set up the material tests and
simulation study. F.S. completed the test and data analysis.
Funding:
The research was funded by National Key Research and Development Program of China
(2016YFB0101700).
Acknowledgments:
All the tests were performed by Tsinghua University Suzhou Automotive Research Institute
and Ningbo Institute of Materials Technology & Engineering. The material models of adhesive and foam were
provided by Dow and Henkel. GAO Cong and Liu Jiancai also helped a lot in composite testing. The authors
would like to acknowledge all these people, institutes, and companies.
Conflicts of Interest:
The funders had no role in the design of the study; in the collection, analyses, or interpretation
of data; in the writing of the manuscript; or in the decision to publish the results.
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