DESIGN, DEVELOPMENT AND PERFORMANCE OF THERMOPLASTIC COMPOSITE LEAF SPRINGS
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In recent years, thermoplastic composites are widening their applications due to their mass production ability, superior strength to weight ratio and mechanical strength. However, utilization of thermoplastic composite material in load bearing engineering applications is limited due to the inadequate understanding of heterogeneous material behavior under service condition. In this work, an attempt was made to investigate the potential of discontinuous fiber reinforced thermoplastic material for suspension leaf spring application. Prior to the product development, relevant mechanical properties for leaf spring applications; adhesive wear, abrasive wear and damping characteristics were investigated. Unreinforced polypropylene, 20% short glass fiber reinforced polypropylene and 20% long glass fiber reinforced polypropylene materials were considered for leaf spring materials and injection molding was chosen as manufacturing technique. Developed leaf springs were evaluated for static, fatigue as well as short term creep performance, besides joint strength performance were evaluated under static and fatigue condition. Adhesive and abrasive wear characteristics of leaf spring materials were investigated with the help of pin on disc configuration. Fiber length, plastic deformation energy and material crystallinity were identified as the major contributing factors to the wear performance. Various damping sources of the chosen leaf spring materials were investigated with the aid of free, forced vibration and dynamic mechanical analysis. Fiber end density was found to influence the damping behavior of composites significantly. Design of leaf spring and injection molding die were carried out with commercial finite element analysis and injection molding simulation tools. In-house molded leaf springs were evaluated for its performance with the aid of developed fixture integrated with the servo hydraulic fatigue testing machine (Instron 8801). Static performance tests were evaluated for determining energy storage capability and strain rate sensitiveness of molded leaf spring. Fatigue leaf spring performance was evaluated at fixed frequency, with various loads till 2 × 105 cycles or failure (fracture or 10 % drop in spring rate) whichever was earlier. During fatigue testing, cyclic load-deflection of test leaf spring of each and every cycle was measured; energy dissipation ratio and spring rate of the test leaf springs were reported as an index for the accumulated damage at various stages of life. Short glass fiber reinforced and unreinforced polypropylene leaf springs exhibited drop in spring rate; whereas long glass fiber reinforced polypropylene exhibited fracture as leaf spring failure. Short-term flexural creep tests were performed on molded leaf springs at various stress levels. Experimental creep performance of molded leaf springs for 2 h was utilized to predict the creep performance with the aid of four parameter HRZ model and compared with 24 h experimental creep data. Test results confirmed the suitability of long fiber reinforced thermoplastic material for creep application over other considered materials. Joint for the developed leaf spring was designed and compared with its static performance. Test joints were subjected to completely reversed fatigue loads, long fiber reinforced leaf spring joint exhibited superior performance at high cycle fatigue conditions and poor performance at low cycle among chosen material, due to its high notch sensitivity characteristics. Load-deflection hysteresis plot of the joints were used to monitor the bearing damage. Failure morphology of joint exhibited net-tension and shear out failures besides bearing damages....
Supervisor: S. Senthilvelan