INTRODUCTION
1.2 MOTIVATION
In the recent years, long fiber reinforced thermoplastic material are being used extensively in mass transit industrial components which includes; dashboard carriers, front ends, seat shells, battery trays, spare wheel dwells, running boards (Krause et.
al., 2003; Bartus et. al., 2006; Thattaiparthasarathy et. al., 2008). In this direction, due to its mass production capability and good endurance strength, the present investigation attempted to recognize and evaluate injection mouldable discontinuous fiber reinforced thermoplastic material for suspension leaf spring application.
Utilization of new materials and manufacturing processes for the functionally critical machine elements necessitates complete systematic understanding of required relevant mechanical properties of the new materials under the service condition. Due to the inadequate available data of the discontinuous fiber reinforced thermoplastics for load bearing application, critical properties for the leaf spring application were investigated for the chosen materials.
In the automobile suspension system, spring ends are bent to form eye through which leaf spring is fixed to the automobile body by the bolts. Since leaf springs experience contact load while in service condition, understanding friction and wear behavior of the leaf spring material under adhesive and abrasive mode with steel material becomes important. Predominant wear occurs through adhesive and abrasive modes (ASM Handbook, 1988; Karger-Kocsis et al., 2010). In adhesive wear mode, the surface energy and shear strength significantly influences the wear mechanism,
while in an abrasive wear mode, the toughness and hardness plays a major role (Bayer, 1994).
The inherent damping characteristic of thermoplastic materials aids to be an excellent alternative material over steel and thermoset material for leaf spring application. However, incorporation of discontinuous fibers in the thermoplastic material may significantly influence the damping characteristics of material. Increase in fiber volume fraction increases the storage and loss modulus and reduces the damping behavior of thermoplastic composites (Crema et al., 1989; Wray et. al., 1990; Rezaei et al., 2009). As the main function of leaf spring in the automobile is to absorb shock and vibration, damping performance of the chosen leaf spring materials was investigated in detail. Due to the viscoelastic nature of the chosen thermoplastic leaf spring material, comprehensive understanding of time and temperature dependent mechanical properties is of practical importance.
Some of the major design parameters that decide energy storage capacity of the leaf springs are material and shape (Yu and Kim, 1988). Various forms of leaf springs as suggested by SAE standards were considered and their structural behaviors were predicted using commercial finite element analysis tool. Thermoplastic composite material properties are time dependent due to the viscoelastic behavior (Rosato et. al., 2000). Since suspension leaf springs are subjected to time varying load, performance evaluation of leaf spring at various strain rates becomes crucial.
Less hysteresis is generally preferred for leaf springs at service conditions (Yu and Kim, 1988). Presence of discontinuous fibers may alter the material and leaf spring performance, thus static performance of leaf springs viz. load carrying capability, strain rate sensitivity and hysteretic behavior need to be investigated in details.
Suspension leaf springs are subjected to repetitive cyclic loads while the vehicle is in
operation, thus identifying safe operating regime under fatigue condition is one of the major requirements. Apart from the composite endurance strength, accumulated damage in the form of stiffness loss and energy dissipation should also be addressed (Orth et al., 1993). Presence of discontinuous fibers in a matrix contributes to the microscopic stress concentrations at fiber ends and fiber/matrix interface which significantly alters the fatigue behavior (Bureau and Denault, 2004; Bernasconi et al., 2007). Hence the failure mechanism and damage progression for the developed product is to be clearly understood.
In the service conditions, suspension leaf springs are subjected to constant stress for long duration as the chassis weight is taken by the suspension system.
Besides, leaf springs are also subjected to various stress levels due to the different payload conditions. Thus, creep response investigation of leaf springs made of new material and process is important.
Successful utilization of composite leaf spring lies equally important on the joint performance. Leaf spring should have appropriate joints so that it can be fixed to the axle and the vehicle body to provide a reliable suspension system. The joint strength should have sufficient strength in view of the design load of leaf spring (Yu and Kim, 1988). Bolted joints are preferred for effective end joint over adhesive joint owing to low cost and simplicity in manufacturing (Shokreih and Rezaei, 2003).
However, the potential site of stress concentration due to drilling in the joint region determines the strength of a structure. Hence there is a need to identify the joint strength and identify the safe operating regime under fatigue loading conditions.
In a summary an ideal leaf spring should possess the following requirements
Leaf springs are subjected to contact loads at the ends as well as middle portion, hence wear resistance under adhesive and abrasive condition is one the requirement of leaf spring material.
Amount of energy storage and release would determine the quality of the suspension system; hence the damping performance of leaf spring material is important.
Spring rate and static deflection of leaf springs significantly affect the suspension quality; hence load deflection characteristics need to be evaluated.
Suspension leaf springs are subjected to repetitive fluctuating loads under service conditions hence developed leaf springs must have fatigue resistance.
Leaf springs are subjected to constant stress for long duration as the chassis weight is taken by the suspension system and hence an ideal leaf spring must have creep resistance.
Developed leaf springs should be fastened with automobile body, hence joint performance under static and fatigue conditions are important.