A review on structural analysis and experimental investigation of fiber reinforced composite leaf spring

Abstract

A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Weight reduction is the major problem faced by many automobile industries. Weight reduction can be achieved by designing new materials and sophisticated manufacturing processes. Due to increasing competition and innovation in recent decades, automobile industries show interest in replacing conventional steel leaf spring with fiber-reinforced composite leaf spring which has advantages such as higher strength to weight ratio, higher stiffness, high impact energy absorption, and lesser stresses. Selection of constituents for the composites is based on the type of application, availability, strength required, and cost of material. This paper gives an overview about the research carried out for the part of two decades on selection of material, different fabrication processes, experimental investigation, design and analysis using CAD tools.

Keywords

Glass fiber epoxy natural fiber FEA leaf spring

Introduction

In order to conserve natural resources and economize energy, weight reduction has been the main focus of automobile manufacturer in the present scenario. The suspension of leaf spring is one of the potential items for weight reduction in automobile as it accounts for 10–20% of the unsprung weight. The introduction of composite materials has made it possible to reduce the weight of the leaf spring without any reduction in the load-carrying capacity and stiffness.¹ Energy conservation is one of the most important objectives in any vehicle design and reduction of weight is one of the most effective measures for energy conservation as it reduces overall fuel consumption of the vehicle.² Composite materials have high elastic strain energy storage capacity and high strength-to-weight ratio compared with those of steel. FRP springs also have excellent fatigue resistance and durability. But the weight reduction of the leaf spring is achieved not only by material replacement but also by design optimization.^3,4 The leaf spring should absorb vertical vibrations, shocks and bump loads by means of spring deflection so that the potential energy is stored in the leaf spring as strain energy and then released slowly. Thus elastic strain energy storage capacity is an important criterion while selecting the material for leaf spring.^5,6 The specific elastic strain energy is inversely proportional to the density and Young’s modulus. The automobile industry has shown increased interest in the replacement of steel leaf spring with fiber glass composite leaf spring because FRP composites possess lower young’s modulus, lower density and lesser weight as compared to steel.^7,8 Recently, natural fibers have been receiving considerable attention as substitutes for synthetic fiber reinforcements such as glass in plastics due to their low cost, low density, acceptable specific strength, fairly good mechanical properties, and eco-friendly and biodegradability characteristics.⁹ Fibers resulting from plants are considered as a budding substitute for non-renewable synthetic fibers such as glass and carbon fibers. There are various natural fibers suitable for various structural and automotive applications such as jute, flax, banana, coir, sisal, etc.¹⁰ Jute and E-glass woven roving mats are used as reinforcements and epoxy resin LY556 is used as the matrix material. Results show that the hybrid composite leaf spring is found to have lesser weight, lesser cost, lesser stresses, and higher stiffness than the conventional steel leaf spring.¹¹

Constituents of fiber reinforced composites

In general, a composite is a material mixture created by a synthetic/natural assembly of two or more physically and chemically distinct components. The first component is a selected filler or reinforcing agent (discontinuous phase) whilst the other component is a compatible matrix binder (continuous phase). These two components are combined in order to achieve specific characteristics and properties. Different types of fibers, matrix, and processing techniques are used for composite fabrication.¹²

Fiber

Fibers are the principal constituents in a fiber-reinforced composite material.

They occupy the largest volume fraction in a composite laminate and share the major portion of the load acting on a composite structure. Proper selection of the fiber type, fiber volume fraction, fiber length, and fiber orientation is very important.¹³

Glass fiber

Glass fibers are the most common of all reinforcing fibers for polymeric matrix composites (PMC). The principal advantages of glass fibers are low cost, high tensile strength, high chemical resistance, and excellent insulating properties. The disadvantages are relatively low tensile modulus and high density, sensitivity to abrasion during handling, relatively low fatigue resistance, and high hardness. The two types of glass fibers commonly used in the fiber-reinforced plastics (FRP) industry are E-glass and S-glass. Another type, known as C-glass, is used in chemical applications requiring greater corrosion resistance to acids than is provided by E-glass. E-glass has the lowest cost of all commercially available reinforcing fibers, which is the reason for its widespread use in the FRP industry.¹³

Carbon fiber

Carbon fibers are commercially available with a variety of tensile modulus values ranging from 207 GPa (303106 psi) on the low side to 1035 GPa (1503106 psi) on the high side. In general, the low-modulus fibers have lower density, lower cost, higher tensile and compressive strengths, and higher tensile strains-to-failure than the high-modulus fibers. Among the advantages of carbon fibers are their exceptionally high tensile strength–weight ratios as well as tensile modulus–weight ratios, very low coefficient of linear thermal expansion (which provides dimensional stability in such applications as space antennas), high fatigue strengths, and high thermal conductivity (which is even higher than that of copper). The disadvantages are their low strain-to-failure, low impact resistance, and high electrical conductivity, which may cause “shorting” in unprotected electrical machinery. Their high cost has so far excluded them from widespread commercial applications. They are used mostly in the aerospace industry, where weight saving is considered more critical than cost.¹³

Natural fibers

Examples of natural fibers are jute, flax, hemp, sisal, coconut fiber (coir), and banana fiber (abaca). All these fibers are grown as agricultural plants in various parts of the world and are commonly used for making ropes, carpet backing, bags, and so on. The components of natural fibers are cellulose microfibrils dispersed in an amorphous matrix of lignin and hemi cellulose. Depending on the type of the natural fiber, the cellulose content is in the range of 60–80 wt% and the lignin content is in the range of 5–20 wt%. In addition, the moisture content in natural fibers can be up to 20 wt%.¹³

Matrix

A matrix is a binder material that is used to hold fibers in position and transfer external loads to internal reinforcements. In natural fiber-reinforced polymer composites, both thermoset and thermoplastic matrices such as unsaturated polyesters, epoxies and phenolics, and polypropylenes, polyethylenes and elastomers, respectively, are widely used for composite applications.^14,15 These matrices have different chemical structures and undergo different reactivities with the surface molecules of fibers in composites.

Methodology

Fabrication of composites is usually carried out on hand lay-up technique which is the simplest method of composite processing and also the infrastructural requirement for this method is also minimal. In some cases, vacuum bag molding is used to carry out fabrication process which uses a flexible film outside air. In this process, the composite materials are subjected to vacuum to remove air bubbles from the laminate. Post this stage, the material may be subjected to atmospheric pressure while it undergoes curing process in an oven.¹³ But this method is not widely used to fabricate the leaf spring. The reason behind this is it involves infrastructure requirement in addition to the hand lay-up technique which may increase fabrication cost, some of which are a vacuum pump and vacuum bag which are made of strong rubber-coated fabric or a polymer film, and highly skilled labor is required for the fabrication process and the pressure generated on the laminate removes excess resin which is a clear waste of money and resources. Due to these inherent reasons, it is not widely used. The templates (mould die) were made from wood and plywood according to the desired profile obtained from the computer algorithm. The glass fibers were cut to the desired lengths, so that they can be deposited on the template layer by layer during fabrication. In the conventional hand lay-up technique, a releasing agent (gel/wax) was applied uniformly to the mould which had good surface finish. This is followed by the uniform application of epoxy resin over glass fiber. Another layer is layered and epoxy resin is applied and a roller is used to remove all the trapped air. This process is continued till the required dimensions were obtained. Care must be taken during the individual lay-up of the layers to eliminate the fiber distortion, which could result in lowering the strength and rigidity of the spring as a whole. The duration of the process may take up to 20 min. The mould is allowed to cure for about 24 h at room temperature.¹ While fabricating hybrid composites at first, the glass fiber and jute fiber mats of required size are cut so that they can be deposited on the template layer by layer during fabrication. Next, the epoxy resin with hardener in a weight ratio of 10:1 is stirred slowly for about 10–15 minutes. Two OHP sheets are used at the top and bottom of the mold to give a smooth surface finish. Now some amount of resin is poured into the mold, and care is taken to avoid formation of air bubbles during pouring. Ten layers of fiber mats are placed one over another with resin layer in between. Brush and roller are used to impregnate fiber mats and also to avoid air entrapped. The fiber weight fraction is maintained at 40–45% since optimum mechanical properties are achieved at this fiber weight fraction.¹⁶ In hand lay-up vacuum bagging method of composite fabrication, the fibers are laid on the mould by hand and matrix material is applied by brushing or spraying. Staking of the layers is done to get the required thickness. Simultaneously, the deposited layers are densified with rollers. Then using the vacuum bagging method, the atmospheric pressure is used to hold the matrix or resin-coated components of a lamination in place until the adhesive cures.¹⁷

Modeling and analysis with experimental investigation

A parabolic leaf spring of EN45 has been taken for his work. The CAD modeling of parabolic leaf spring has been done in CATIA V5 and analysis was done by ANSYS 11.0. The finite element analysis (FEA) of the leaf spring was carried out initially discretizing the model into finite number of elements and the nodes by applying the boundary conditions. A correlation of CAE analysis with experimental results taken at industrial laboratory has been provided with the work. The results depicts the total deflection as 56.806 mm in the parabolic leaf spring at full load, i.e. the deflection obtained from FEA results with 16% deviation from experimental value. The corresponding equivalent von Mises stress developed in the leaf spring at same full load is 1083.2 MPa i.e. the equivalent von Mises stress is observed to be well below the yield stress indicating that the design is safe. By comparing the FEA with experimental, it gives very close results providing the validation of FEA model as well as the work.¹⁸ Mono-leaf composite leaf spring with varying width and varying thickness is designed and manufactured. The 3-D modeling of both steel and composite leaf spring is done and analyzed using ANSYS. A comparative study has been made between composite and steel leaf spring with respect to weight, cost and strength. The analytical results were compared with FEA and the results show good agreement with experimental results. From the results, it was observed that the composite leaf spring is lighter and more economical than the conventional steel spring with similar design specifications. Composite leaf springs exhibit less deflection 105 mm when compared with steel leaf spring 107.5 mm and also it produces less stress of 473 MPa than the conventional one which has 503.3 MPa, and also a considerable amount of weight reduction is achieved with composite mono leaf spring such as 85% for E-glass/epoxy, 91% for graphite/epoxy, and 90% for carbon/epoxy over conventional leaf spring.¹ In the comparison of conventional steel leaf spring with E-glass mono composite leaf spring, there was the weight reduction from 23 kg to 3.59 kg, i.e. 84.40% of weight reduction for the same level of performance. The experimental results showed that natural frequency of composite material is 102.14 Hz which is higher than the steel leaf spring of 93.56 Hz.¹⁹ A composite mono leaf spring with Carbon/Epoxy composite materials is modeled and subjected to the same load as that of a steel spring. The design constraints were stresses and deflections. The composite mono leaf springs have been modeled by considering varying cross-section, with unidirectional fiber orientation angle for each lamina of a laminate. Static analysis of a 3-D model has been performed using ANSYS 12.0. Comparative analysis of carbon/epoxy composite leaf spring and steel leaf spring is done by analytical, FEA using ANSYS 12. The result of FEA is also experimentally verified. Compared to the mono steel leaf spring, the laminated composite mono leaf spring is found to have lesser stresses, i.e. 391.95 MPa when compared with steel spring stress of 674.31 MPa for the maximum load of 3400 N and weight reduction of 22.15% is achieved.²⁰ E-glass fiber-reinforced epoxy composite leaf spring is fabricated by using hand lay-up vacuum bagging technique to replace metallic leaf spring of light passenger vehicle. Experimental tests are performed to compare the load-carrying capacity and stiffness of composite leaf spring with metallic one and also the fabricated composite leaf spring is fitted to the vehicle and its performance under actual working conditions is studied. The results showed that composite leaf spring weighing 2.78 kg has given 57.23% weight reduction compared to the existing conventional metallic leaf spring which weighed 6.5 kg.¹⁷ The mechanical properties of glass fiber-reinforced plastic (GFRP) by varying volume percentage are evaluated for the application of leaf spring. The mechanical properties involved in the study are tensile strength, bending strength, and impact strength. The results are compared for different loading conditions and a suitable composition is selected for the fabrication of mono composite leaf spring. For the compositions of 50:50%, 40:60%, 30:70% by volume of E-glass/epoxy, 40:60% composition yielded maximum tensile strength, impact strength, and flexural strength. Then the entire fabrication of composite leaf spring was done with 40:60% of E-glass/epoxy composition. A comparative study made between composite and steel leaf springs with respect to weight which gives 3.25 kg for steel and 1.8 kg for composite leaf spring and strength and showed that composite leaf spring has more load-carrying capacity as 1010.4 N when compared to Steel leaf spring load-carrying capacity as 804.4 N.²¹ A single leaf with constant cross-sectional area similar to that of conventional leaf spring(CLS) in each case such as bidirectional glass fiber-reinforced plastic (GFRP), bidirectional carbon fiber reinforced plastic (CFRP), bidirectional carbon-glass reinforced plastic (C-GFRP) and bidirectional glass-carbon reinforced plastic (G-CFRP) was fabricated by hand lay-up technique and tested by universal testing machine. By using universal testing machine, load per deflection and maximum load that a leaf spring can withstand were measured. The cyclic loading with specific duration was given to the aforementioned composite leaf springs by using a laboratory designed loading set up through milling machine. From the experimented results, it was observed that if conventional leaf springs are replaced by composite leaf springs, an appropriate amount of weight reduction as GFRP – 0.690 kg, CFRP – 0.720 kg, G-CFRP – 0.782 kg, C-GFRP – 0.740 kg can be achieved when compared with CLS – 2.38 kg and also observed that the composite leaf springs can take more amount of load than the conventional leaf spring for constant specified deflection. Also, among the composite leaf springs, the glass–carbon hybrid composite leaf spring can take up more amount of load than others, and thereby improved vehicle performance could be achieved.²² GFRP and the polyester resin can be used which was more economical and this will reduce the total cost of composite leaf spring. A spring with constant width and thickness was fabricated by hand lay-up technique which was very simple and economical. The experiments were conducted on UTM and numerical analysis was done via FEA using ANSYS software. Stresses and deflection results were verified for analytical and experimental results. Result shows that, the composite spring has stresses around 220.18 MPa much lower than steel leaf spring which has 743.10 MPa and weight reduction is achieved from 13.4 kg to 2.365 kg, i.e. weight of composite spring was nearly reduced up to 85%.²³ With the materials as SAE 9260 and glass fiber-reinforced epoxy considered for comparative study for multi-leaf spring, a finite element approach for analysis of a multi-leaf springs using ANSYS software is carried out. The leaf spring model is generated by using Pro/E and imported in ANSYS. Harmonic analysis for vibration due to road irregularities and static analysis for gross vehicle mass load analysis are carried out for both materials, and comparative behaviors are observed such as stress and deflection of the multi-leaf spring. The results were concluded that glass fiber-reinforced epoxy has a lower stress 2364 N/mm² and strain 0.010552 mm compared with SAE 9260’s stress and strain of 2477 N/mm² and 0.012389 mm, respectively and it is also observed that deformation reduces up to 14.6%, stress reduced up to 4.67%, and strain reduced up to 16.01%. Reducing the deformation, stress and strain make leaf spring life longer against loads.²⁴ In a comparative study between multi-steel leaf spring with E-glass epoxy, carbon epoxy, and graphite epoxy using FEA analysis, the results were observed that maximum displacement of 10.16 mm in the steel leaf spring and the corresponding displacements in E-glass/epoxy, graphite/epoxy, and carbon/epoxy are 15 mm, 15.75 mm, and 16.21 mm and from the static analysis results, and the von Mises stress in the steel is 453.92 MPa. And the von Mises stress in E-glass/epoxy, graphite/epoxy, and carbon/epoxy is 163.22 MPa, 653.68 MPa, and 300.3 MPa, respectively. Among the three composite leaf springs, only graphite/epoxy composite leaf spring has higher stresses than the steel leaf spring.²⁵ Fatigue analysis of multi-leaf springs is carried out for steel leaf springs (65Si7), and static analysis for steel multi-leaf springs, composite leaf springs, and hybrid leaf springs. In that models are generated by Solidworks and imported in ANSYS to carry out analysis process. The results showed that stresses in E-glass/epoxy composite leaf springs are less as compared to the conventional steel leaf springs, also a new combination of steel and composite leaf springs (hybrid leaf springs) is given the same static loading and is found to have values of stresses in between that of steel and composite leaf springs. Conventional 65Si7 steel leaf springs were found to weigh about 58.757 kg, while the composite leaf springs weighed only 19.461 kg, and the hybrid leaf springs weighed 41.14 kg for the same specifications.²⁶ In the study of an analysis and behavior of a mono leaf spring made of hybrid composite materials, i.e. carbon and E-glass fibers, it is observed that the natural frequency of a hybrid composite leaf spring is 236.8 Hz which is twice the frequency of a conventional leaf spring 134.3 Hz, particularly in the vertical direction, which means that the occurrences of resonance will be less. Also, it is observed that the stress produced in composite leaf spring (100 MPa) lower than that in a conventional steel leaf spring (350 MPa).²⁷ Jute and E-glass woven roving mats are used as reinforcements and epoxy resin LY556 is used as the matrix material. The CAD models of Leaf spring are prepared in Unigraphics NX6 and imported in static structural analysis workbench of ANSYS 14.5 where FEA is performed. This study provides a comparative analysis between steel leaf spring (65Si7) and Jute/E glass-reinforced Epoxy leaf spring. While comparing the results between steel, composite and hybrid composite leaf spring, it is found that on the application of 5000 N load, the maximum von Mises stress for steel leaf spring is 383.88 MPa which is higher than the stresses induced in composite and hybrid composite leaf spring. Simultaneously for the composite leaf spring, maximum stress induced is 145.36 MPa which is below 205 MPa, i.e. yield strength of E-glass/epoxy composite material. For hybrid composite leaf spring, maximum stress induced is 163.36 MPa which is below 185 MPa, i.e. yield strength of jute/E-glass/epoxy hybrid composite material. The maximum value of deflection for steel leaf spring is 15.16 mm, for composite leaf spring is 5.34 mm, and for hybrid composite leaf spring is 6.12 mm, which are below the camber length of leaf spring, i.e. 125 mm. This shows that stiffness of composite and hybrid composite leaf spring is higher than the stiffness of steel leaf spring. The maximum value of strain energy stored for steel leaf spring is 29.72 MJ, for composite leaf spring is 55.16 MJ, and for hybrid composite leaf spring is 63.64 MJ. This shows that elastic strain energy storage capacity for hybrid composite leaf spring is higher as compared to both steel and composite leaf springs. The conventional steel leaf spring weighs about 4.5 kg whereas E-glass/epoxy leaf spring weighs 2.8 kg and jute/E-glass/epoxy leaf spring weighs 2 kg. Thus the weight reduction of 37% is achieved while using composite leaf spring and further if we use hybrid composite leaf spring in place of steel leaf, weight reduction of 55% is achieved.¹¹ Using natural fibers such as jute and sisal, hybrid composites (NFRC) are fabricated with E-glass fiber (GFRC) and they are compared with mono E-glass composite leaf spring. In that the analytical analysis was carried out by considering leaf spring as cantilever beam which is subjected to bending stresses, longitudinal and transverse shear stress. Calculated results were compared with FEA and the results show good agreement with the stiffness in NFRC (86.155 N/mm for JFRC and 103.531 N/mm for SFRC) and for GFRC 80.61 N/mm which are much higher.¹⁰

Conclusion

This paper provides a brief review over the recent researches carried out on leaf spring in the view of weight reduction in which each paper consists of material selection, processing techniques, modelling, Numerical analysis, FEA analysis and experimental investigation also. From this review, it was understood that the major disadvantages of composite leaf spring are chipping resistance. The matrix material is likely to chip off when it is subjected to a poor road environments (that is, if some stone hit the composite leaf spring then it may produce chipping) which may break some fibers in the lower portion of the spring. This may result in a loss of capability to share flexural stiffness. But this depends on the condition of the road. In normal road condition, this type of problem will not be there. Apart from these demerits, composite leaf spring exhibits accountable advantages as more than 50% of weight reduction can be achieved with composite leaf spring. Stresses produced in composite leaf springs are also 20–30% less than conventional steel leaf spring which also depends on type of constituents used. Frequency analysis also gives better results compared with steel leaf spring, i.e. natural frequency of composite leaf spring higher than steel leaf spring, due to which resonance occurrence can be avoided. This paper may also provide an intension on natural fiber-based hybrid composite over synthetic fiber-based mono composites due to their cost, recyclable, availability, and also considerable weight reduction. Overall, by using composite leaf spring, we can achieve better fuel economy, riding quality, and also we can use our natural resources properly.

Footnotes

Conflict of interest

None declared.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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