Design, Analysis, Fabrication And Testing Of A Composite Leaf Spring

Abstract

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.

Literature Review

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 Selection

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.

FIBRES SELECTION

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.

RESINS SELECTION

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

DESIGN SELECTION

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.

CONSTANT THICKNESS, VARYING WIDTH DESIGN

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.

CONSTANT WIDTH, VARYING THICKNESS DESIGN

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.

CONSTANT CROSS-SELECTION DESIGN

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.

SELECTION OF MANUFACTURING PROCESS

Apart from the selection of material and design procedure, the selection of manufacturng process also determines the quality and cost of the product. Hence, the composite leaf spring manufacturing process should fulfill the following criteria.
• The process should be amenable to mass production.
• The process should be capable of producing continuous reinforcement fiber.
Based on above requirements, filament-winding techniques is selected. In filament winding process, continious fibre under controlled tension are drawn from spools mounted on creel stands wetted with the resin by passing the fibers through a resin bath and wound onto the rotating mould. After achieving the desired thickness, the process is stopped and the mould is removed from the machine and kept for curing. This process doesn’t involve huge investment.