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Full Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2011

Carbon Fiber Reinforced Composite Coil Springs First Dr. D. Abdul Budan 1, Second T.S. Manjunatha2 1

UBDT College of Engineering, Department of Studies in Mechanical Engineering, Davangere-577004, India abdul_budan@rediffmail.com 2 GM Institute of Technology, Department of Mechanical Engineering, Davangere-577006, India tsmdvg@rediffmail.com

Abstract—Three types of coil springs are fabricated using carbon fibers in unidirectional, 0/90 and + 45 degree orientations. Carbon fiber springs in + 45 degree orientations give better results. Stiffness obtained by these springs are 10% more than the unidirectional carbon fiber springs and 27 % more than the 0/90 degree orientation springs. Weights of the fabricated springs are 60% less than the steel springs.

The fabrication methods can be used for mass production.

Results compared with steel springs shows that steel springs can be replaced by high strength carbon fiber springs.

not popular due to manufacturing difficulties. This paper discusses about the use of carbon fibers in three different orientations for coil spring manufacturing. Three types of springs are fabricated using carbon fibers in unidirectional (CFR), 0/90 (CF0/90) and ± 45 (CF ± 45) degree orientations.

The results of these springs are compared with the steel spring.

Index Terms—Composite springs, coil springs, fiber springs

A. Specific strain energy The main factor to be considered in the design of a spring is the strain energy of a material used. Specific strain energy in the material can generally be expressed as

II. THE PRINCIPLE OF CYLINDRICAL HELICAL SPRINGS

I. INTRODUCTION The fuel efficiency and emission gas regulation of automobiles are two important issues in these days. The best way to increase the fuel efficiency is to reduce the weight of the automobile by employing composite materials in the structure of the automobiles. The principal advantages of fiber reinforced polymer matrix composites for automotive parts are weight savings, part consolidation, and improvement in Noise, Vibration, and Harshness (NVH) [12]. The randomly oriented chopped E-glass fiber reinforced polymer matrix composites which are not load bearing structural materials are mainly used in automotive industry due to their low cost. However there have been several attempts and some successful applications of fiber reinforced composite structural members to various parts of automobiles including load bearing structural parts [5-6]. Especially glass fiber reinforced polymer composites have been used for static and dynamic load bearing structures such as bumpers and leaf springs [4].

Springs are crucial suspension elements on automobiles which are necessary to minimize the vertical vibrations, impacts and bumps due to road irregularities and create a comfortable ride [3]. Coil springs are commonly used for automobiles suspension and industrial applications. Metal coils springs can be replaced by composite springs because of weight reduction and corrosion resistance. Composite coil springs can be manufactured using carbon/graphite/glass fibers and resin impregnation. Composite coil springs, compared to standard metal coil springs reduces weight from 45% to 25%, gives high natural frequency, Excellent NVH property, and corrosion free behavior [11]. Some researchers have used the E-glass fibers and carbon fibers and low cost resin for fabricating the coil spring [7]. The composite leaf springs are successfully used in the suspension of the light vehicle [10-13]. Many researchers [7-14] have investigated on leaf springs, elliptic springs, circular springs and springs of arbitrary shape. However the composite coil springs are © 2011 AMAE DOI: 02.AIPE.2011.01.508

U

2 E

(2.1)

This indicates that a material with lower young’s modulus (E) or density (ρ) will have higher specific strain energy under the same stress (σ) condition [12]. Thus the composite materials offer high strength and light weight.

B. The spring constant of a cylindrical helical spring As shown in Fig. 1, when a cylindrical helical spring with rectangular cross section is under the action of an applied compression force F, the primary reaction force on the coil is torsion and thus induces shear stresses on the cross section.

For homogeneous and isotropic materials, both the spring constant, K, and the induced shear stress under torsion, τ, for rectangular wire spring can be approximately derived as [3]:

K

F Gt 2 b 2    nD 3

 

(2.2)

PD tb tb

(2.3)

Where and in Fig. 1, F is the applied load on spring (kgf); D, the mean coil diameter (mm); L, the free length (mm); α, the helical angle (degree); δ, the deflection (mm); G, the modulus of rigidity (N/mm2); n, the no of active coils in the spring; t, the side parallel to axis of spring, b the side perpendicular to axis of spring and c is spring index (D/b). β is a factor depend upon b/t and γ is a factor depend spring index.

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Full Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2011 For the fabrication of fiber springs in 0/90 degree bi-directional fiber mats in 0/90 degree orientations are wound on the mandrel after dipping in the resin.

For the fabrication of fiber springs in + 45 degree orientations, fiber mats are cut in the form of 45 degree orientation tapes or cross ply tapes and then wound on the mandrel in the same way as explained above. The dimensions of the fabricated springs are D=47 mm, b=10mm, t=8mm, L=200mm. Three specimens were fabricated in each type of spring. The fabricated springs are shown in Fig.2.

Figure 1. Schematic diagram of cylindrical helical spring of rectangular cross section

III. MATERIALS Reinforcement materials; PAN based high strength carbon fiber roving of 215 GSM, PAN based high strength carbon fiber plain weave fabric of 204 GSM. Matrix material; L-12/K-6 epoxy resin.

Figure 2. Fabricated springs

V. EXPERIMENTATION

TABLE I. PROPERTIES OF MATRIX AND REINFORCEMENT MATERIALS

A. Fiber volume fraction The fiber volume fractions are determined as per ASTM D3171 [15]. The fiber volume fractions of the three types of springs are given in Table II. B. Load and deflections of springs (load vs deflection) The fabricated composite coil springs were tested for the load and deflections on a spring testing machine as per JIS B 2704 [16]. In this study a digital spring testing machine is used which automatically records the data of applied load and the corresponding compression in each regular interval.

The load-deflection curve can be derived for each spring from these values. The spring constant and maximum compression can then be evaluated.

C Stiffness of composite coil springs According to JIS B2704, the stiffness or spring constant or spring rate of a helical spring can be determined at two measured points, corresponding to deflection at both 30% and 70% of the full loading, on the load deflection curve from the compression test of a helical spring.

IV. FABRICATION In the present investigation hand lay up and filament winding technique is used for the fabrication of the spring. A mandrel is prepared having the profile of the spring. The material used for the mandrel is cast iron. Springs were fabricated with a fiber volume fraction of 60:40. For the fabrication of unidirectional roving springs, the measured quantity of epoxy resin is taken. The fiber roving is cut into the required length. Mandrel is fixed in between the centers of the lathe or chuck. A mould release agent is applied on the mandrel to remove the spring easily after curing. The fiber roving after dipping in the resin is wound in the grooves of the mandrel. The fibers are wound till it completely fills the gap on the mandrel. A shrink tape is wound on the mandrel.

Pressure is applied by winding the rubber tape on the mandrel.

Mandrel along with the fiber and resin in the same condition is kept for curing in ambient temperature for 24 hours. Rubber tape and shrink tapes are removed after curing. Spring is removed from the mandrel by winding in the reverse direction. Š 2011 AMAE DOI: 02.AIPE.2011.01.508

D Stress acting on the spring Stresses acting on the springs are determined by knowing the loads and dimensions of the spring by using equation 2.3. The loads are recorded in the compression testing of the springs. The maximum load acting on the springs are used for determining the stresses acting on the springs.

E Maximum compression, load bearing capacity, failure load and critical axis load of springs The maximum compressions of the springs are recorded on the spring testing machine when the load is applied on the springs till all the coils touch each other. The load at this point is the maximum load that can withstand by the springs due to its spring action. Any load applied on the springs 18

Full Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2011 TABLE III. LOAD AND DEFLECTIONS OF CFR , CF 0/90, CF + 45 SPRINGS

beyond this is with stand by the spring materials and not due to the spring action. Spring acts like a solid material and the compression tests are continued till the yield point and the failure load of the materials. Due to the limitations of the spring testing machine in order to apply the loads which may damage the load cells, compression tests are continued on the universal testing machine. The load is applied on the composite spring beyond its maximum compression on the universal testing machine. At the yield point a small cracking sound is heard. Spring starts to fail at this point. Loads are still applied until the fiber starts delaminating. The load at this point is called the failure load VI. RESULTS AND DISCUSSIONS A Fiber volume fraction The fiber volume fractions of all three types of composite coil springs are given in Table II. It is evident from the results that there is a variation in the fiber volume fraction of the fabricated springs. The springs were fabricated with a fiber volume fraction of 60:40. However due to manufacturing difficulties and variation in the fabrication process the exact fiber volume fraction could not be achieved. The exact fiber volume fraction can be achieved by precisely controlling the manufacturing variables. Since the maximum variation in the fiber volume fraction is less than 10%, the results obtained are not much affected.

C Stiffness of the spring The stiffness or spring constant or spring rate is the force required to compress a spring by unit length. Spring rates depend on the rigidity modulus, no of coils, shape and dimensions of the spring. Rigidity modulus plays a major role in the spring rates. The spring rates were determined from the load deflection curves and are evidently very stable and almost constant. Spring rates were determined at 30% and 70% of the full loading as per JIS B 2704. The spring rates are shown in Table IV and the corresponding bar charts are shown in Fig 4. As observed from the results the highest spring rates are achieved from the springs made in + 45 degree orientation compared to other two orientations. Since the forces acting on the helical springs are shear, the springs made in + 45 degree orientation withstands more loads and gives maximum stiffness. Carbon fiber springs gives more stiffness due to its superior mechanical properties and higher rigidity modulus. The next highest spring rates are obtained with the springs made using unidirectional roving. Springs fabricated in 0/90 degree orientation gives very less spring rates compared to other two orientations. The spring rates of carbon fiber springs in + 45 degree orientation is 10% more than carbon fiber roving springs and 27% more than the carbon fiber springs in 0/90 degree orientation. Spring rates were also calculated analytically by using rigidity modulus.

Experimental results were compared with these results and the maximum variations observed are between 15 to 17%.

This variation is observed in the beginning of the fabrication process. Once the fabrication process standardized these variations can be reduced. Using carbon fibers increases the cost of springs however one can go for a hybrid spring made of glass fiber/carbon fibers which also gives better results to reduce the costs of springs.

TABLE II. FIBER VOLUME FRACTIONS OF COMPOSITE COIL SPRINGS

B Load deflection characteristics of the springs The measured average values of the load deflections of the carbon fiber springs in unidirectional, 0/90 degree and + 45 degree orientation are given in Table III The corresponding load deflection curves are shown in Fig 3. As it is evident from the results that the carbon fiber springs in + 45 degree orientation with stand more load than the other two types of springs. A linear curve is obtained for all the nine types of the springs. A small variation was observed in the curves of the specimens of springs. This variation is due to the dimensional variations in the fabrication process. This variation can be reduced by standardizing the fabrication methods. As it is seen from the curves carbon fiber springs in + 45 degree orientation gives better results compared to other two types.

TABLE IV. STIFFNESS OF COMPOSITE COIL SPRINGS

Figure 3. Load vs. deflections of all three types of springs

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Full Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2011 the maximum compression and load bearing capacity of the springs depends upon the orientation and characteristics of the fibers.

TABLE VI. MAXIMUM COMPRESSION OF COMPOSITE COIL SPRINGS

Figure 4. Stiffness of composite coil springs

D Stress acting on springs Two types of shear stresses acting on the spring one is torsional shear stress and the other is direct shear stress.

The combined maximum shear stresses are determined by knowing the load acting on the spring and the dimensions of the spring using the equation 2.3. Stresses acting on the composite coil springs are given in Table V. Stresses acting on all the springs are less than the allowable shear stresses.

As compared to steel springs stresses acting on fiber springs are less. Stresses acting on the springs depend upon the dimensions and the load acting on the springs. Stresses acting on the fiber springs in + 45 degree orientation is less compared to the other springs. This can be explained by the stress distribution in the structure of the helical spring subjected to an applied compressive loading. When a helical metal spring is under the action of a compressive force, a shear stress distribution will be induced on its coil cross section, and stress concentration will also occur on the inner rim of the spring coil due to its helical bending shape, and therefore the inner rim of the spring coil is under the action of the maximum shear stress considering the effect of curvature. Because of inter twinning of fibers in + 45 degree orientation fiber springs resists higher shear load.

Figure 5. Maximum compression of springs

Figure 6. Load at Maximum compression of springs

F Failure load of springs Tests were also conducted on the springs to know the failure loads of the springs. The loads were applied beyond the maximum compression till the breaking of the springs.

The failure loads of the springs are given in Table VII and the corresponding bar charts are given in Fig 7. As it is observed from the results the failure loads of the springs made in unidirectional is more compared to other types of springs.

Since these springs are made in unidirectional the application of the loads creates a tensile load on the fibers. Since the tensile strengths of individual fibers are very high the loads in these springs are also very high. Springs made in 0/90 degree orientation resists less load before failure. As it is observed from the results the fiber springs are never broken into two pieces like steel springs. But it gives an indication of the point at which it starts delaminating. It reduces the load bearing capacity gradually. So the fiber springs are safer compared to steel springs which break suddenly.

TABLE V. STRESSES ACTING ON COMPOSITE COIL SPRINGS

E Maximum compression of springs The maximum compression and the corresponding loads of all the three types of springs are shown in Table VI. The corresponding bar chart is shown in Fig 5. Fig 6 shows load at maximum compression. The carbon fiber spring in + 45° orientations resists more loads compared to other springs for the deflection of 80 mm. Again this is due to more load bearing capacity of springs in + 45° orientations and good mechanical properties of carbon fibers. The springs in 0/90 degree orientation resists fewer loads compared to other types of springs. The springs made of carbon fiber roving also gives better results which is almost near to springs made in + 45° orientations. This is due to the continuous fibers in unidirectional which withstands more loads because a tensile load is applied on the fibers. It is evident from these results © 2011 AMAE DOI: 02.AIPE.2011.01.508

TABLE VII FAILURE LOADS OF COMPOSITE COIL SPRINGS

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Full Paper Proc. of Int. Conf. on Advances in Industrial and Production Engineering 2011 [3] A. M. Wahl, Mechanical Springs, 2nd ed., Mc Graw Hill Book Company:1963.

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[15] ASTM D 3171-99. Standard test method for Constituent Content of Composite materials.

[16] JIS B 2704, Helical Compression and Extension SpringsRequirements for Design, Performance test method.

Figure 7. Failure loads of springs

CONCLUSIONS The feasibility of producing composite coil springs is discussed. The spring produced subjected to various tests exhibited satisfactory results. The manufacturing process can be automated for mass production to reduce the cost of the springs. The weights of the fabricated composite coil springs were compared with the steel spring and are 60% less than the steel springs. Springs manufactured in Âą 45 degree orientation gives excellent results. Stiffness obtained by these springs is compared with the steel springs of the same dimensions and it is 50 % less than the steel springs. However we can increase the dimensions of the springs to increase the stiffness of the springs. The dimensions of the side parallel to the axis of the springs can be increased without affecting the deflection and the allowable dimensions for fixing the springs. This will increase the stiffness equivalent to steel springs. With this also we can achieve 40% weight reduction.

REFERENCES [1] Autar K Kaw, Mechanics of Composite materials, CRC Press: 1997.

[2] M.M.Schwartz, Composite Materials Hand book, McGraw Hill Book Company: 1984.

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