Composite Materials in Leaf Springs: A Review

Composite Materials in Leaf Springs: A Review

Abstract— This review aims to research and compile data on the feasibility of the use of composites such as, Glass Fiber Reinforced Plastic (GFRP), Graphite/epoxy, Carbon/epoxy in leaf springs as a replacement to Spring Steel. For the purpose of comparison stresses in critical regions are investigated such as the spring eye and the springs are investigated for fatigue failure. A background on the design and analysis of Composite leaf springs is important to fully understand the areas that undergo shock or fatigue loading. The spring constant or the spring stiffness is used as a measure to check if composites provide comparable spring characteristics. The investigations are carried out in various ways such as analysis in FEA software, on road tests and use of multibody dynamics to simulate actual loading conditions. The investigation reveals that use of traditional composites is ideal for low load application since using composites drops the stiffness of the springs and the load bearing capacity, but the use of composites also leads to an improvement in ride quality.

Keywords—Strain Energy, Delamination, hybridization, Multibody, Spring eye.

I.     Introduction

Leaf springs are a type of spring used commonly in automobiles, it consists of a laminated piece of steel with rectangular cross-section which forms an arc like shape. It is usually fixed at the center of the arc to the axle and absorbs loads using its compressive and tensile stresses to absorb loads along its arc. Leaf springs have many ways to be attached to the automobile, it is sometimes attached to from both ends to the frame and sometimes attached to the frame from one end and shackled at other end to make a swinging arm like structure.

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There are many setups to the leaf spring the most popularly used today is the elliptical leaf spring with the semi elliptical part. The semi elliptical part has the thickest part of the leaves joined together to form an absorption system. There is another used setup the parabolic leaf spring that is used for better ride quality but has a lower load bearing capacity. This property is since the friction between the leaves is eliminated using spacers. This reduction in load bearing capacity brings the need for more dampers and washers.

Fig. 1. Semi-elliptical leaf spring

The leaf spring has a further advantage of simplifying the suspension setup by eliminating the need for a separate  linkage system for the axle. Most leaf springs are not known for their good ride quality due their higher than traditional stiffness. Leaf springs have some room for improvement as they are extremely heavy for their load absorption capacity, it is worthwhile to investigate possibilities for weight reduction in leaf springs while trying to keep their load absorption capabilities. For this purpose, using fiber composites in leaf springs as a replacement for spring steel is a popular choice.

II.    Literature review

H.A. Al-Qureshi [1]

This paper aims to determine feasibility of use of composite variable thickness leaf spring in automobiles. The leaf spring

Table 1. Comparison between steel and GFRP springs

selected is a variable thickness leaf spring with its curvature radius at 94 cm and the thickness being a maximum of 4.5cm at center and 2.5 cm at ends. Though replacement of material seems straightforward it brings with Bending strength is 640 MPa and shear strength of 31 MPa. The thickness varies as a function of distance from spring center. refer fig. 2

FEA analysis cannot completely simulate the loading that takes place on the composite springs so there is a need to test the springs real time, which is why the composite spring is manufactured and tested on a hydraulic loading machine. The test is used to mirror a 3-point loading test. The specifications for loading test can be seen in table 1.

Fig. 2 Load Deflection curve of steel and composite spring

Pankaj Saini et al. [2]

This paper aims to investigate the possibility of using composites in leaf spring to increase the strain energy absorption capability of spring to increase the ride quality while decreasing weight. composites have low density. Leaf springs though known for their strain absorption capabilities are notorious for their heavy weight, one solution that can show a decent strain absorption capability while showing a decent weight reduction over steel leaf springs is the composite leaf spring. The equation for strain energy shows that a low density and modulus of elasticity favor high strain energy. Composite materials seem a perfect fit for this, there is still a need for experimental investigations before using composite in leaf springs can be justified. A proper research plan is devised in which three different cross-sections for mono leaf spring are considered namely,

• constant thickness and varying width
• varying thickness and varying width
• constant thickness and constant thickness

the constant cross section design is selected for its superior manufacturing capability.

Three popular materials are selected for this investigation, Carbon epoxy, Graphite epoxy, E-glass epoxy. The leaf spring is modelled in auto CAD 2012 and exported to ANSYS 9.0, since the model is symmetric about its loading axis only half of the model is exported. For the FEA analysis Mahindra Commander 650 direct injection is considered. The leaf spring specifications are listed in table 2, and table 3. The specifications for the vehicle are as follows,

• Vehicle weight: 2150 kg
• Un sprung mass: 250 kg
• Sprung mass: 1910 kg
• FOS (factor of safety): 1.4
• Acceleration due to gravity is assumed to be 10 m/s^2

Table 2 Leaf Spring specifications

Table 3 Mechanical Properties

Static load analysis results are compared for three composites steel being the reference material. E-glass has lowest stress for similar deflection values. Performing only static load analysis will fail to give a complete idea of all loads experienced by the springs additionally failure mode analysis needs to be performed. E-glass epoxy has lowest weight reduction over steel carbon epoxy gives highest reduction. But stress introduced in e glass is lowest too, so it is a decision based on use or requirement. 

J.P. Hou et al. [3]

Though the research doesn’t align with automobiles at first glance there is a lot we can take away from this research that we can use to better design a composite leaf spring. This research aims to obtain the best design for freight rail application, it aims to do so by reduction of a phenomenon called delamination, three eye designs are tested for delamination one after the other the designs are improved to and results are used to make the next design. Fig. 3 shows delamination in composite leaf springs.

Two eye end designs, and an open-end eye design  are the three designs. The material properties of composite used are as listed in the table below.

Even though the first design does survive a load of 150KN it produces less than satisfactory results since there is evidence of delamination. Even though spring stiffness is not negatively influenced by delamination, it occurs at a relatively low load.

Fig. 3. Delamination

An FEA analysis is performed to determine the reason for delamination even though there is no failure at the point where delamination starts. The FEA results show that there is a stress concentration at the point where failure occurs. Next design is aimed to resist delamination in the area of delamination. Solutions proposed are to make a wrapping of GRP around the area of stress concentration. This solution did reduce the propagation of delamination to the eye and reduced the max shear stress value, but there was still delamination, a better design was necessary to completely stop delamination. Fig. 4 shows comparison graph for FEA to measured values.

Fig. 4 comparison of predicted FEA values to measured values

Considering FEA analysis had presented realistic results comparable to real world tests. The third test returned promising results by showing absence of delamination in the eye region and the results needed to verify by fabricating a prototype, under static loading there is no delamination or stress concentration. The most relevant point about this research is it investigates an approach rarely focused upon and could present a previously unknown region for improvement that could boost leaf spring capabilities even further.

Vinkel Arora et al. [4]

Investigate the possibility of modifying eye design for leaf spring to increase the strain energy absorption. Selection of material and manufacturing process also impact the capabilities of leaf spring; therefore, it is critical to understand the selection criteria for materials. Table 4. Lists material chemical composition.

Table 4. Chemical Composition

The joint selection is important for boundary conditions angles at which loads will act need to be accurately represented. Considering a camber angle of 162 degree at no load revolute joint at 18 degree is introduced inside the eye. A zero-displacement support is introduced at the other end. Loading is through the center of the spring. Fig 5. Lists all the boundary conditions.

Fig. 5. Boundary conditions

Two designs are used for analysis the standard eye design and casted eye design. The reduction in shear stress is minimal (2%) and simultaneously the bending stress increases substantially (19%) which is not at all desirable.

Table 5. Results

The research has not been performed in a robust manner as the materials used is structural steel this may introduce unnecessary variable into the equation. The idea behind the research is relevant and can be properly pursued upon to explore possibilities of improving performance of leaf springs further. Refer table 5 for the results.

Y.S. Kong et al. [5]

Perhaps one of the most important researches on leaf springs       

due to the unique approach take to try and replicate the real-life loading experienced by leaf springs instead of investigating static loading on springs in theoretical scenarios. Poly beam elements are a segment of the beam which connect bodies refer

Fig. 6 Poly-beam Elements

These elements are used to represent the spring. The spring is modelled as a lumped mass. To replicate real ground loading conditions multibody dynamics modeling is proposed. Even though this research is aimed at preventing failure in extreme loading conditions, but this information can still be used to determine more accurate values for factors of safety. Loading conditions such as spring windup are investigated which are experienced in spring eyes during dynamic loading. investigate ability of leaf springs to resist failure under various loading conditions.

For a lateral acceleration of 0.8g the multibody simulation returns values of normal load of 1.62g and 0.32g at wheel center. These loads obtained from multibody analysis was applied to FEA analysis and used to obtain stress values.

For these loads the vertical stiffness of spring is 343N/mm and windup stiffness is 2661 N/mm. A graph is plotted for comparison of the results endured by the spring.

Table 6. Results of multibody simulation

Fig. 7. displacement for different loads

For the spring thickness of 17, 16, 15, 13 mm stresses generated are 1383, 1483, 1503, and 2131 MPa. The lower thicknesses yield for their loading and therefore fail the design.

Gullur Sidaramanna et al. [6]

This paper is important not for its research but for its approach it is of utmost importance to understand the importance of analytically calculating the for static loading even though it we have means such as FEA at our disposal. Analytically calculating beforehand gives us an idea for a starting point when deciding boundary conditions in more complex systems.

The parameters of spring and its material properties are listed in the

Table 7. Mechanical Properties

The tediousness of hand calculations cannot be denied, more so in iterative calculations therefore use of software like C or MATLAB are necessary for iterative calculations.

Before programming the flow, chart is necessary to have a complete idea of the flow of the program and to understand what the requirement is.

Table 8. Results

Bonded end joint mono composite leaf spring is investigated for failure and fabricated. A C program is used to design the spring, CAE analysis is performed in ANSYS these results are then used to compare to on field tests.

It is concluded that Bonded joints prevent delamination of leaf spring. There needs to be further research on the effect of delamination on the strain absorption capabilities of leaf springs.

Conclusion

1. Leaf spring strain energy absorption is inversely proportional to the density of the spring material and its modulus of elasticity. Therefore, composites are ideal for this purpose as a replacement to Spring Steel.
2. Delamination is an important indication of stress concentration at the spring eye though it does not adversely affect the fatigue life of the spring, this phenomenon and its effect on spring life need to be investigated in further detail.
3. Spring eye design is an important factor in prevention of fatigue failure and stress concentration. The open-end design shows promising results for prevention of stress concentration.
4. Hybridization of composites gives similar spring mechanical properties to that of spring steel.

References

[1]      H.A. Al-Qureshi, “Automobile leaf springs from composite materials” Journal of Material Processing technology, vol. 118, pp. 58–61, 2001.

[2]      Pankaj Saini, Ashish Goel and Dushyant Kumar, “Design and Analysis of  composite leaf spring for light vehicles” International journal of innovative research in science, Engineering and Technology, vol. 2, issue 5, May 2013.

[3]      J.P. Hou, J.Y. Cherruault, I. Nairne, G. Jeronimidis and R.M. Mayer “Evolution of the eye-end design of a composite leaf spring for heavy axle loads” Composite Structures, vol. 78,  2007, pp. 351–358.

[4]      Vinkel Arora, Gian Bhushan and M.L. Aggarwal, “Eye design analysis of single leaf spring in automotive vehicles using cae tools” International Journal of Applied Engineering and TechnologyVol. 1,  2011, pp.88-97.

[5]      Y.S.Kong, S.Abdullah, M.Z.Omar and S.M.Haris, “Failure assessment of a leaf spring eye design under various load cases” Engineering Failure Analysis, Volume 63,  May 2016, pp. 146-159.

[6]      Gulur Siddaramanna, Shiva Shankar and Sambagam Vijayarangan “Mono composite leaf spring for light weight vehicle–design, end joint analysis and testing” Materials Science, Vol. 12,  2006.