Application of Carbon Fiber-Based Composite for Electric Vehicle

Miftahul Anwara, Sukmaji Indro Cb, Wijang Wisnu Rc, and Kuncoro Diharjod

Department of Mechanical Engineering, Faculty of Engineering, Sebelas Maret University Jl. Ir. Sutami 36A Surakarta, 57126 Central Java, Indonesia

amiftahwar@ft.uns.ac.id (corresponding author), bsukmaji@ft.uns.ac.id, cm_asyain@yahoo.com, dkuncorodiharjo@ft.uns.ac.id

Abstract. In the present work, we study how to improve mechanical properties of carbon fiber

reinforced plastics (CFRP) in order to increase crashworthiness probability. Experimentally, hybrid

carbon /glass fiber composite was made in order to get higher mechanical properties. As a result, with

increasing carbon fiber volume fraction (% vol.), tensile strength and flexural strength of the

composite are increased. Simulation of impact testing is also performed using data properties taken

from the experiment with variation of impact forces on front bumper structure. By varying external

load to the bumper, the result shows that higher thickness of hybrid carbon/glass fiber composite has

always smaller stress values than thinner one. On the other hand, the displacement of hybrid

carbon/glass car bumper increases linearly with increasing external load.

Introduction

Achieving vehicle safety in crash conditions involves a continuous iterative process that starts

with the definition of a structural material and design, and ends when all criteria are satisfied in order

to reduce potential injury to occupant which is also called as crashworthiness.[1,2] In automotive

industry, primary issues for producing the crashworthy vehicles are light weight but strong material

structure, and overall cost production. This issue leads toward nontraditional materials, i.e.,

composite,[3-5] in which capable of participating in the energy absorption process associated with

accident.

Recently, fiber reinforced plastics (in particular CFRP) are already an important material in

automotive industry due to its lightweight, strong mechanical properties and high energy absorption

structure.[6,7] However, massive use of CFRP in entire vehicle structures, e.g., bumper, trunk, hood

etc., resulting in high fabrication cost.[8,9] One technique to higher the mechanical properties without

exsessive cost increase is to incorporate carbon fiber with other material, known as hybrid

composite.[10]

Therefore, as a next step, it is essential to construct and manipulate new carbon-based composite

material with higher mechanical properties and to analyze crashworthiness probability for the

application in the prototype of Indonesia electric vehicle. For that purposes, in this work, we present

the experiment on hybrid carbon/glass fiber composite as well as the simulation of impact testing on

car front bumper, as the first step, toward future crashworthy vehicles.

Experiment on Hybrid Carbon/Glass Fiber Composite

Experiment on tensile strength and flexural strength of hybrid carbon/glass fiber composite was

performed in order to provide higher the mechanical properties and to lower the fabrication cost. To

make specimens (Fig. 1 (a)) of hybrid carbon-glass fiber/polyester composite (with 60/40 % vol.

ratio), carbon fiber and glass fiber are used as reinforcement of composite, while polyester is used as

matrix and methyl ethyl ketone peroxide (MEKPO) as a catalyst. Woven roving sheets of carbon fiber

(200 gr/m2) and glass fiber (300 gr/m

2) was arranged in order as shown in Fig. 1(b). Woven carbon

fiber sheets placed always at the top and bottom site. The middle site is varied according to the

expected volume fraction ratio of carbon/glass fiber.

These materials then are mixed by hand lay-up technique. Next, the composite was pressed using

vacuum bagging technique with pressure 〜5 Psi to remove the air void inside the composite, as the

porous can be formed by the trapped of many air void. To obtain optimum result, we varied

carbon/glass fiber volume fraction (20/80, 30/70 and 50/50 % vol.), while the matrix is kept constant.

Advanced Materials Research Vol. 896 (2014) pp 574-577© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.896.574

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 114.79.29.114-09/02/14,07:38:25)

Tensile and flexural strength measurement (according to ASTM D638 [11] and ASTM D790 [12])

was performed using tensile and flexural strength test machine provided in Material Laboratory,

Sebelas Maret University.

Figure 1. (a) Specimen sample for tensile measurement with variation of carbon/glass fiber volume

fraction [(1) 20/80, (2) 30/70 and (3) 50/50 % vol.]. (b) Optical microscope image showing

cross-sectional view of arrangement of carbon and glass fiber of hybrid composite. (c) and (d) are

tensile strength and flexural strength vs carbon fiber fraction of hybrid carbon/glass fiber reinforced

composite. The matrix (polyester) is kept constant at 60 % vol. Dashed line is the eye guide for

increasing trend of the hybrid composite.

Figures 1.(c) and 1.(d) show tensile strength and flexural strength of hybrid carbon/glass

composite with variation of carbon fiber weight fraction i.e., 20, 30 and 50 % vol. For hybrid

carbon/glass composite, the increase of fiber fraction results in the increase of tensile strength of the

composite from 246 to 307 MPa, respectively. On the other hand, flexural strength of hybrid

carbon/glass composite is also increased linearly from 100 MPa to 190 MPa at fiber volume fraction

of 20 and 50 % vol., respectively. The increasing trends of tensile and flexural strength are most

probably due to the increasing of fiber-fiber interaction and fiber-matrix interaction [13] between

carbon fiber, glass fiber and polyester matrix.

Moreover, more uniform layers arrangement of carbon fiber as shown in Fig. 1.(b) results in the

strength in the tensile and flexural properties of hybrid composite. Woven shape of carbon and glass

fiber could also absorb external stress in x-y direction, while arranged sheet of fibers strengthen the

composite in z direction.

Simulation of impact testing

Simulation of impact testing with finite element method (FEM) was used to simulate the real

condition of car accident on the front bumper since the highest crash probability usually occurs at the

front of the car. The simple scenario of collision between two cars was simulated using Solidwork

simulator. One car moves and hit the second fixed car with variation of load i.e., 1, 3, 5, 8 and 10 kN,

which is associated to different mass and velocity of the first car. We measured stress distribution and

displacement of car front bumper after collision.

For stress distribution and displacement measurement, hybrid carbon/glass fiber composite was

used and carbon fiber/epoxy as a comparison. Highest tensile strength value (307 MPa) from the

experiment was used in the material properties input in the simulation. In order to find the optimum

stress and displacement value of front bumper, we varied the thickness of the composite i.e., 3 mm, 4

mm and 5 mm. During collision, crush load is uniformly distributed to entire car front bumper.

Figure 2 shows stress distribution images of car front bumper for hybrid carbon/glass fiber

composite [Fig. 2 (a) – (c)] and CFRP (Fig. 2.(d)) composite for comparison with thickness variation.

Even though the crush load is uniformly distributed, Fig. 2 suggests that maximum stress on the

Advanced Materials Research Vol. 896 575

bumper is observed at the center of lower band of the bumper. Lower band of bumper is not backed up

with crushing element compared with higher band of bumper, thus it is much weaker than other part

of the bumper.

The displacement is also observed in the car bumper as shown in Fig. 2. Displacement values are

calculated by subtracting crushed portion after and before (initial condition marked by dashed line in

Fig. 2) collision. Despite the accurate values of displacement are shown in transitional displacement

figure of car bumper (not shown), in Fig. 2 the displacements regions are observable at the place

where the maximum stress occur. In general, the displacement of car bumper decreases with

increasing composite thickness as shown Fig. 2(a) to (c).

In order to extract general information, the results of maximum stress and displacement have

been quantitatively plotted in the graph shown in Fig. 3 obtained for varied composite thickness 3, 4,

and 5 mm respectively. The results are also obtained for different composite, i.e., CFRP (black lines)

composite with tensile strength 600 MPa for comparison. For both result values (maximum stress and

displacement), the increase in composite thickness results in the decrease in values of maximum

stress and displacement. On the other hand, increasing external load increases maximum stress

linearly for 4 and 5 mm thickness. However, for 3 mm thickness maximum stress (up to 700 MPa)

and displacement (up to 120 mm)are also increased at almost exponentially, which can be most likely

due to less carbon fiber content in the composite leads to the decrease of material strength.

Figure 3. Maximum stress (a) and displacement (b) vs external load of front bumper for hybrid

carbon-glass composite with thickness variation simulated with commercial Solidwork simulator.

The inset of (a) and (b) show more detail differences for high external load of each thickness.

Moreover, comparing with CFRP (black), maximum stress and displacement values of hybrid

composite for 4 (blue) and 5 mm (green) thickness are significantly comparable as shown in Fig. 3

and Figs. 2 (b–d). At higher external load (8 kN and 10 kN), however, maximum stress and

displacement value of CFRP increase higher than that of hybrid composite by 20 MPa and 5 mm

different, respectively as shown in the inset of Fig. 3. It is reasonable to assume that for hybrid

composite, even though tensile strength value is smaller than that of CFRP, combination between

properties of carbon fiber and glass fiber will increase internal properties of hybrid composite.

Figure 2. Simulation of stress distribution of

front bumper for carbon fiber [(a) – (c)] and

glass fiber (d) as a comparison measured with

commercial Solidwork simulator. Thickness of

the composites is used as a parameter, from 3

mm to 5 mm. Stress distribution value is

described in color variation from blue

(minimum value) to red (maximum value) in

unit of MPa. The displacement value is taken

using dashed line as an initial condition of the

front bumper before and after collision.

576 Advanced Materials Science and Technology

Therefore, hybrid composite absorbs more kinetic energy than CFRP which is very important for

future crashworthy vehicles.

Additionally, factor of safety of bumper structure for hybrid composite was also analyzed during

simulation. Factor of safety of car bumper is 2.24 at 8 kN external load which is still acceptable and

safe for the occupant.[14]

Conclusions

The results shown in this paper indicate that hybrid carbon/glass fiber composite has higher

mechanical properties even compared with other materials i.e., CFRP. On the other hand, hybrid

carbon/glass fiber composite, with an appropriate mixed fiber/matrix fraction, can be one possible

choice to provide higher mechanical properties. Using the simulation of impact testing of font car

bumper, we also showed that crashworthiness probability can be increased by increasing thickness of

the bumper. Using hybrid carbon/glass composite for front bumper in the simulation of impact testing

provided smaller stress and displacement in car structure than that of CFRP at high external load.

These findings can be useful for the design of future vehicle structures in order to provide light, strong

and low cost Indonesia electric vehicles.

Acknowledgement

We thank Cornellius H. R., Heri S. and Yunanto A. P. for their contributions during simulation and

experiments. This work was partially supported by Grants-in-Aid for National Electric Car

(023.04.2.1.189882/2013) from the Ministry of Education and Culture of Republic Indonesia.

References [1] G. Steieglitz, Parallels between aviation and automotive safety research, in: Passenger car design and

highway safety, Association for the Aid of Crippled Children and Consumer Union of U.S. Inc, New

York, 1962, p. 194.

[2] S. Newstead, L. Watson, M. Cameron, Vehicle safety ratings estimated from police reported crash data:

2008 update. Australian and New Zealand crashes during 1987-2006, Monash University Accident

Research Center Report, Report No. 280, 2008.

[3] V. P. McConnell, Automotive composite: A design and manufacturing guide, Ray Publishing, Wheat

Ridge, 1997, pp. 6-7.

[4] R. Piellisch, Beyond gasoline: Autos remains elusive for carbon composite structures, but alternative

cars still hold a great market potential, Soc. for the Advancement of Material and Process Engineering.

Journal 32 (1997) 1165-1186.

[5] G. Belingardi, M. P. Cavatora, D. S. Paolino, Repeated impact response of hand lay-up and vacuum

infusion thick glass reinforced laminates, Int. J. Impact Eng. 35 (2008) 609-619.

[6] H. Wallentowitz, I. Leyer, and T. Pharr, Material for future automotive body structure, Business

Briefing: Global Automotive & Manufacturing, Reference Section, 2003, pp. 1-4.

[7] G. Lu, T. X. Yu, Energy Absorption of structure and material, Woodhead Publishing, Cambridge, 2003,

p. 317.

[8] J. L. James, Carbon Fibers, in: Chemical Insight & Forecasting: IHS Chemical, Process Economics

Program Report, Report No. 165, 1983.

[9] J. L. James, Carbon Fibers, in: Process Economics Program Report, Chemical Insight & Forecasting:

IHS Chemical, Report No. 165A, 1992.

[10] A. K. Kaw, Mechanics of Composite Materials, second ed., CRC Press, Taylor & Francis Group,

Tampa, 2005.

[11] ASTM Standard D638-02, Standard Test Method for Tensile Properties of Plastics, Annual Book of

Standards, ASTM International, West Conshohocken, PA, 2003, pp.46-58.

[12] ASTM Standard D 790-02, Standard Test Method for Flexural Properties of Unreinforced and

Reinforced Plastics and Electrical insulating materials, Annual Book of Standards, ASTM International,

West Conshohocken, PA, 2002, pp.146-154.

[13] S. Y. Fa, X. Hu, C. Y. Yue, Effect of fiber length and orientation distributions on the mechanical

properties of short-fiber-reinforced-polymers: A Review, Mater. Sci. Res. Int. 5 (1999) 74-83.

[14] K. S. Bernstein, R. Kujala, V. Fogt, P. Romine, Structural design requirements and factors of safety for

spaceflight hardware: For human spaceflight, JSC 65828 Rev. A, NASA JSC, Houston, TX, 2011.

Advanced Materials Research Vol. 896 577