Design Analysis Fabrication and Testing of Composite Leaf Spring

Composite Leaf Spring
The subject gives a brief look on the suitability of composite leaf spring on vehicles and their advantages. Efforts have been made to reduce the cost of composite leaf spring to that of steel leaf spring. The achievement of weight reduction with adequate improvement of mechanical properties has made composite a very replacement material for convectional steel. Material and manufacturing process are selected upon on the cost and strength factor. The design method is selected on the basis of mass production.

          From the comparative study, it is seen that the composite leaf spring are higher and more economical than convectional leaf spring.

In order to conserve natural resources and economize energy, weight reduction has been the main focus of automobile manufacturers in the present scenario. Weight reduction can be achieved primarily by the introduction of better material, design optimization and better manufacturing processes. The suspension leaf spring is one of the potential items for weight reduction in automobiles as it accounts for 10% - 20% of the unsprung weight. This achieves the vehicle with more fuel efficiency and improved riding qualities. The introduction of composite materials was made it possible to reduce the weight of leaf spring without any reduction on load carrying capacity and stiffness. Since, the composite materials have more elastic strain energy storage capacity and high strength to weight ratio as compared with those of steel, multi-leaf steel springs are being replaced by mono-leaf composite springs. The composite material offer opportunities for substantial weight saving but not always be cost-effective over their steel counterparts.


                Investigation of composite leaf spring in the early 60’s failed to yield the production facility because of inconsistent fatigue performance and absence of strong need for mass reduction. Researches in the area of automobile components have been receiving considerable attention now. Particularly the automobile manufacturers and parts makers have been attempting to reduce the weight of the vehicles in recent years. Emphasis of vehicles weight reduction in 1978 justified taking a new look at composite springs. Studies are made to demonstrate viability and potential of FRP in automotive structural application. The development of a liteflex suspension leaf spring is first achieved. Based on consideration of chipping resistance base part resistance and fatigue resistance, a carbon glass fiber hybrid laminated spring is constructed. A general discussion on analysis and design of constant width, variable thickness, composite leaf spring is presented. The fundamental characteristics of the double tapered FRP beam are evaluated for leaf spring application. Recent developments have been achieved in the field of materials improvement and quality assured for composite leaf springs based on microstructure mechanism. All these literature report that the cost of composite; leaf spring is higher than that of steel leaf spring. Hence an attempt has been made to fabricate the composite leaf spring with the same cost as that of steel leaf spring.

              Material properties and design of composite structures are reported in many literatures. Very little information are available in connection with finite element analysis of leaf spring in the literature, than too in 2D analysis of leaf spring. At the same time, the literature available regarding experimental stress analysis more. The experimental procedures are described in national and international standards. Recent emphasis on mass reduction and developments in materials synthesis and processing technology has led to proven production –worthy vehicle equipment.

                    Materials constitute nearly 60%-70% of the vehicle cost and contribute to the quality and the performance of the vehicle. Even a small amount in weight reduction of the vehicle, may have a wider economic impact. Composite materials are proved as suitable substitutes for steel in connection with weight reduction of the vehicle. Hence, the composite material have been selected for leaf spring design.

              The commonly used fibers are carbon, glass, keviar, etc.. Among these, the glass fiber has been selected based on the cost factor and strength. The types of glass fibers are C-glass,S-glass and E-glass. The C-glass fiber is designed to give improved surface finish.S-glass fiber is design to give very high modular, which is used particularly in aeronautic industries. The E-glass fiber is a high quality glass, which is used as standard reinforcement fiber for all the present systems well complying with mechanical property requirements. Thus, E-glass fiber was found appropriate for this application.

              In a FRP leaf spring , the inter laminar shear strengths is controlled by the matrix system used . since these are reinforcement fibers in the thickness direction , fiber do not influence inter laminar shear strength. Therefore, the matrix system should have good inter laminar shear strength characteristics compatibility to the selected reinforcement fiber. Many thermo set resins such as polyester, vinyl ester, azpoxy resin are being used for fiber reinforcement plastics(FRP) fabrication . Among these resin systems, epoxies show better inter laminar shear strength and good mechanical properties. Hence, epoxide is found to be the best resins that would suit this application. different grades of epoxy resins and hardener combinations are classifieds , based on the mechanical properties.

              Among these grades , the grade of epoxy resin selected is Dobeckot 520 F and the grade of hardener used for this application is 758. Dobeckot 520 F is a solvent less epoxy resin.

              Which in combination with hardener 758 cures into hard resin . Hardener 758 is a low viscosity polyamine . Dobeckot 520 F , hardener 758 combination is characterized  by

q  Good mechanical and electrical properties.
q  Faster curing  at room temperature.
q  Good chemical resistance properties .

               By considering the property variation in the tapered system improper bonding and improper curing, etc. some constant of property value are reduced from calculated values using equations. The material properties for E-glass / Epoxy composite for 60% of fiber volume is given below: -

                       PROPERTIES                                                         VALUES

Tensile modulus along X direction (Ex),MPa                                   14000 

Tensile modulus along Y direction (Ey),MPa                                     6030

Tensile modulus along Z direction (Ez),MPa                                      1530

Tensile strength of the material,MPa                                                     800

Compressive strength of the material,MPa                                            450

Shear modulus along XY direction(Gxy),MPa                                    2433

Shear modulus along YZ direction(Gyz),MPa                                    1600

Shear modulus along ZX direction(Gzx),MPa                                    2433

Flexural modulus of the material,MPa                                               40000

Flexural strength of the material,MPa                                                 1000

Poisson ratio along XY direction(NUxy)                                           0.217

Poisson ratio along YZ direction(NUyz)                                           0.366

Poisson ratio along ZX direction(NUzx)                                           0.217

Mass density of the material (ρ),Kg/mm³                                         2 * 10

                   The leaf spring behaves like a simply supported beam and the flexural analysis is done considering it as a simply supported beam. The simply supported beam is subjected to both bending stress and transverse shear stress. Flexural rigidity is an important parameter in the leaf spring design and test out to increase from two ends to the center.

              In this design the thickness is kept constant over the entire length of the leaf spring while the width varies from a minimum at the two ends to a maximum at the center.

              In this design the width is kept constant over the entire length of the leaf spring while the thickness varies from a minimum at the two ends to a maximum at the center.

              In this design both thickness and width are varied through out the leaf spring such that the cross-section area remains constant along the length of the leaf spring.
              Out of the above mentioned design concepts, the constant cross-section design method is selected due to the following reasons:-
¨      Due to its capability for mass production and accommodation of continuous              reinforcement of fibers.
¨      Since the cross-section area is constant through out the leaf spring, same quantity of  reinforcement fibre and resin can be fed continuously during manufacture.
¨      Also this is quite suitable for filament winding process.

              A computer algorithm using C language has been developed for the design  of constant cross-section leaf spring. The design requirements of composite leaf spring are given below.
            Parameter                                                              Values
     1. Static load acting on the spring, (W),N                   2500
     2. Static deflection of the spring, mm                            100
     3. Distance between eyes (2L), mm                            1153
     4. Camber, mm                                                             160           
     5. Spring rate of the leaf spring(K),N/mm                     25
              In constant cross-section on design both the thickness and width are varied through to the leaf spring such that the cross-section area remains constants. Considering the bearing strength of the steel plate, the calculated thickness of the leaf at the bolted ends(tc)is 7.5 mm.Based on the leaf interchangeability in mounting the rear axel, the width at the center (bm)is chosen as 30 mm and corresponding thickness (tm) 31 mm .Stress and deflections are calculated from the theory of bending. The output of computer algorithm is the dimensions(breath and thickness) of leaf spring at various distances from the center. The dimensional details can be utilized in developing models for finite element analysis and mould making process.


1.      The composite leaf spring is designed according to constant cross-section area method.
2.      The 3-D model of the composite leaf spring is analyzed using finite element analysis.
3.      Static test has been conducted to predict the stress and displacement at different locations for various load value.
4.      The results of the FEM analysis are verified with the test results.
5.      A comparative study has been made between composite and steel leaf springs with respect to weight, riding quality, cost and strength.

              From the study it is seen that the composite leaf spring are lighter and more economical than that of conventional steel leaf springs for similar performance. Hence, the composite leaf springs are the suitable replacements to the convectional leaf springs.

1 comment:

  1. PRAMOD.S,
    b-tech9mechnical engineering


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