Various damping sources of the selected leaf spring material were investigated using free, forced vibration and dynamic mechanical analysis. Static performance tests were evaluated for determining energy storage capacity and strain rate sensitivity of molded leaf spring.
LIST OF TABLES
200 8.23 Morphology of failure of the bearing surface of the test joint during fatigue. a) matrix fibrillation together with wear remnants of the SFLS joint at 19.7 MPa alternating voltage (b) excessive matrix fibrillation of the SFLS joint at 26.3 MPa alternating voltage (c-d) absence of matrix elongation in LFLS at 24.5 MPa alternating voltage (e) failure area at 45% UTS of SFLS tensile specimen (f) failure area at. SAE Society of Automotive Engineering SFLS Short fiber reinforced leaf spring SFPP Short glass reinforced polypropylene.
NOTATIONS
English Symbols
Le Effective spring length Ln Number average fiber length Lw Weight average fiber length m Fatigue sensitivity parameter m1 Mass of sample before testing m2 Mass of sample before after testing.
Greek Symbols
INTRODUCTION
- OVERVIEW ON COMPOSITE LEAF SPRINGS
- MOTIVATION
- OBJECTIVE
- Major Steps Accomplished to Achieve Objectives
- ORGANISATION OF THE THESIS
Since the hinged leaf springs are subjected to a time-varying load, the evaluation of the performance of the leaf springs at different deformation rates becomes essential. The presence of discontinuous fibers can change the performance of the material and the leaf spring, i.e. the static performance of the leaf spring ie.
LITERATURE SURVEY
INTRODUCTION
- Adhesive Wear Characteristics
- Abrasive Wear Characteristics
- Influence of Fiber Reinforcement on Abrasive Wear
- Damping Characteristics of Thermoplastic Composites
- Damping Evaluation through Free and Forced Vibrations
- Damping Evaluation through Dynamic Mechanical Analysis
- Fatigue Performance of Thermoplastic Composites
- Hysteretic Heating of Thermoplastic Composites
- Creep Performance of Thermoplastic Composites
- Creep Models
1999) observed the increase in temperature during fatigue testing of glass fiber reinforced polypropylene and related to the deteriorated stiffness of the composite. Dynamic mechanical analysis (DMA) was used to investigate the viscoelasticity of injection molded nylon 6/6 material reinforced with short and long glass fibers by Sepe (1994) and reported an increase in creep resistance for reinforced nylon composites with long glass fibers.
METALLIC LEAF SPRING
The effect of shot peening on full-scale tests of leaf springs was carried out by Aggarwal et al. 2006) and developed a fatigue model based on the surface modification of the leaf spring. The model addressed the influence of different fouling conditions on leaf spring performance and reported the influence of friction fatigue between mating leaves in order to decrease the fatigue life.
COMPOSITE LEAF SPRING DESIGN
Rajendran and Vijayarangan (2001) used a genetic algorithm to determine the width and thickness at the axle seat for a designed fiberglass leaf spring. Later, the equation developed for the glass epoxy leaf spring was used to determine the thickness at any section along the length of the leaf spring.
DEVELOPMENT AND PERFORMANCE OF COMPOSITE LEAF SPRINGS SPRINGS
Yu and Kim (1988) used the filament winding technique followed by a compression molding process to fabricate fiberglass epoxy leaf springs. Theren and Lundin (1990) replaced three steel leaf springs with filament-wound fiberglass-epoxy leaf springs and performed static and fatigue tests.
BOLTED JOINT PERFORMANCE OF COMPOSITE MATERIALS
- Composite Joint Design
- Influence of Clamping, Clearance and Notch on Composite joint
1982) used glass fiber reinforced polyester material for multibolted joints and reported pitch as an important design parameter in the bolted joint performance. Stockdale and Matthews (1976) investigated the effect of clamping pressure on bolt bearing loads for glass fiber epoxy polymer parts.
SUMMARY
Composite material properties play an important role in determining shear strength performance. 1989) confirmed higher level sensitivity in graphite polyetherketone composite than graphite epoxy composite and emphasized the importance of stress relief mechanism in reducing level sensitivity. It has also been reported that the higher sensitivity of the thermoset level than the thermoplastic material provides a lower tensile strength for the composite for the same amount of reinforced graphite fibers. 1997) confirmed that stress concentration was more evident at static and low cycle fatigue for fiber reinforced polypropylene material.
MATERIALS FOR THERMOPLASTIC LEAF SPRINGS
- INTRODUCTION
- MATERIALS AND PROCESSING CONDITIONS
- FIBER LENGTH DISTRIBUTION
- SUMMARY
The processing conditions used to prepare the test samples and determine the fiber length are also presented in this chapter. The materials used for the development of leaf springs and test specimens were highlighted in this chapter.
FRICTION AND WEAR CHARACTERISTICS OF THERMOPLASTIC LEAF SPRING MATERIAL
INTRODUCTION
ADHESIVE FRICTION WEAR
- Transient Friction Mechanism
- Influence of Fiber Length on Friction
- Influence of Loading Conditions on Coefficient of Friction and Specific Wear Rate Wear Rate
- Wear Mechanisms of Test Specimens
This behavior was due to the low sliding resistance provided by the asperities of the sample under higher loading conditions. The increase in the specific rate of consumption was due to the higher normal. load acting on the specimen.
ABRASIVE WEAR TEST
- Effect of Fiber Length on Abrasive Wear
- Effect of Load and Grit Size on the Abrasive Wear Performance
- Wear Topography
The influence of fiber length on the abrasive wear performance of leaf spring materials is shown in Figure 4.14. Briscoe and Evans (1989) reported that abrasive wear resistance increased with improved matrix crystallinity.
SUMMARY
DAMPING CHARACTERISTICS OF THERMOPLASTIC LEAF SPRING MATERIAL
INTRODUCTION
Several works have been carried out in the past to understand the damping mechanism of discontinuous fiber reinforced thermoplastic materials (Crema et al., 1989; Wray et al., 1990; Kultural and Eryurek 2007), but the influence of fiber length on damping performance of the thermoplastic compositions were not investigated. The influence of fiber reinforcement and the length of fiber reinforcement on the internal heat generation of discontinuous fiber reinforced thermoplastic material during fatigue was also reported.
HYSTERESIS DAMPING
Fig.5.2 Schematic representation of fibers in a matrix for the same volume fraction (a) short fiber reinforced polymer and (b) long fiber reinforced polymer. For the solid volume fraction of the reinforcement, due to the higher fiber ends and fiber matrix interfacial area (Figure 5.2), reinforced material with short fibers showed superior hysteretic damping properties than that of reinforced material with long fibers.
DAMPING DUE TO FIBER-MATRIX INTERFACE
Among the reinforcement materials, SFPP showed a higher hysteresis area than LFPP due to the presence of more fiber ends/fiber matrix interface area.
DAMPING UNDER FREE VIBRATION
Transient time decay curves are shown in the figure, showing the reduction of the logarithmic decay and damping factor with the increase of the fiber volume fraction of glass fibers in the composite. The damping performance of the selected leaf spring materials showed a similar trend as indicated by the hysteresis region.
DAMPING UNDER FORCED VIBRATION
The phase shift obtained from the forced vibration experiment for all test specimens considered is shown in Figures 5.8 (a-c). The damping factor and phase lag obtained from the forced vibration tests showed a similar trend to the hysteresis area and logarithmic decrement obtained from the free vibration tests.
DYNAMIC MECHANICAL ANALYSIS
For example, the storage modulus of the test materials was found to increase with increasing frequency (Figure 5.11). It is clear from Figures 5.12 (a-b) that the loss factor decreased with the increase in fiber length.
HYSTERETIC HEATING
Temperature rise due to the material's hysteretic heating is highly dependent on the material's damping properties (ASM International, 1988; Kultrural and Eryurek, 2007). The temperature increase for unreinforced PP was due to the hysteretic heating behavior of the polypropylene matrix material, and the measured surface temperature was less than reinforced PP.
SUMMARY
Logarithmic decay and damping factor were found to decrease with the increase in fiber length.
DESIGN, MANUFACTURE AND STATIC PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
INTRODUCTION
This chapter also presents the evaluation of the static performance of leaf springs to understand the load deformation and strain rate sensitivity.
LEAF SPRING DESIGN
- Material Design
- Leaf Spring Geometry Design
- Mono Leaf Spring Geometry
The shape of the leaf spring must provide uniform bending stress along the length of the spring under vertical forces. Fig.6.2 Optimization results for (a) leaf spring width and center thickness (b) leaf spring weight.
COMPUTER AIDED SIMULATION OF LEAF SPRING MOLDING
- Details of Leaf spring Model and Molding Conditions
- Mold Shrinkage of Test Leaf Springs
- Warpage Analysis of Test Leaf Springs
Figures 6.12 (a-b) show the volumetric shrinkage of unreinforced and glass fiber reinforced polypropylene leaf springs. However, with fiberglass leaf springs, no uniform deflection is observed due to anisotropic shrinkage due to the presence of fiberglass in the matrix.
PROCESSING OF THERMOPLASTIC LEAF SPRINGS
Gate
Leaf spring cavity Leaf spring core
Locking plate
Leaf spring cavity
Leaf spring core
Long fiber leaf spring
Short fiber leaf spring
THERMOPLASTIC LEAF SPRING INSPECTION
Injected unreinforced and fiberglass-reinforced polypropylene leaf springs were examined with a 3-D scanner (Roland Picza, LPX-250) to assess volumetric shrinkage. The slope of the height direction and the width direction were kept at 0.2 mm during scanning to capture the smallest features of the injected leaf spring geometry.
FIXTURE FOR LEAF SPRING PERFORMANCE EVALUATION
When the load at the center of the leaf spring is through the bottom plate of the testing machine, the leaf spring flattens. To accommodate the increase in length during deflection, the ends of the leaf springs slide over the linear guide.
Fixture
The linear guideway was attached to the bottom plate of the fatigue testing machine. Two retaining blocks were attached over the sliding blocks of the guideways, with the leaf spring ends placed in the retaining blocks.
Linear guide way
With this configuration, cyclic loading of sinusoidal, triangular and square waveforms can be applied in the frequency range 0.1 - 10 Hz up to a maximum load of 100 kN (load cell capacity) with a maximum stroke length of 75 mm .
Hydraulic gripper controller
Load cell
Linear guide ways
STATIC PERFORMANCE EVALUATION OF LEAF SPRINGS
Cast test leaf springs were loaded to the design deflection of 12 mm and released according to SAE standard J1528 (SAE,1996). When a polymer component is subjected to an external force, part of the work is elastically stored, and the rest is irreversibly discharged.
Fixed Upper platen
The relative size of the elastic and viscous components depends on the rate of deformation of the component.
Movable block
Movable lower platen Leaf spring
- Load Deflection Behavior of Leaf Springs
- Influence of Strain Rate on Load Deflection Behavior of Leaf Springs
- Energy Storage Capacity of Leaf Springs
- Hysteretic Characteristics of Composite Leaf Springs
- Load Relaxation Characteristics of Composite Leaf Springs
- SUMMARY
Therefore, it is necessary to understand the energy storage capacity of the formed leaf springs. The load drop for maintaining the required displacement showed load relaxation properties of the formed leaf springs.
FATIGUE AND CREEP PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
INTRODUCTION
In addition, leaf springs are also subjected to various stress levels due to the different payload conditions. This chapter reported the fatigue and short-term creep performance of discontinuous fiber-reinforced thermoplastic leaf springs at various stress levels.
FATIGUE PERFORMANCE EVALUATION METHODOLOGY
Cyclic load-deflection curve of cast leaf springs was used as an index to quantify the accumulated leaf spring damage during fatigue testing and a safe operating regime was identified. Failure of cast leaf springs was considered when the spring rate was reduced by 10% or breakage.
FATIGUE PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
- Energy Dissipation of Test Leaf Springs
- Spring Rate of Test Leaf Springs
The relative spring rate of test leaf springs at a finite number of cycles at different loads was obtained from the cyclic load-deflection curve. Surface morphology of the LFLS exhibited cracks on the tensile surface of the leaf spring as shown in Figure 7.7.
Brittle Ductile
Neutral axis
Ductile Brittle (a)
CREEP PERFORMANCE
- BACKGROUND ON CREEP MODELS
- Findley’s Power Law Model
- HRZ Model
- CREEP PERFORMANCE EVALUATION METHODOLOGY
- Experimental Creep Performance
- Spring Rate of Thermoplastic Composite Leaf Spring
- Empirical Model for Predicting Short Term Creep Behavior
- Influence of Material Crystallinity on Power Law Coefficient
Since the power coefficient (ε't) and power exponent (n) are sensitive to stress level, the methodology adopted by Hadid et al. The obtained creep data results were found to fit a power law function.
SUMMARY
From the crystallite size and power law coefficient, it was confirmed that an increase in crystallinity decreased the power law coefficient. The evaluated power law coefficient of cross-layer laminates and unidirectional composites was found to be 0.1831 and 0.0729, respectively.
JOINT PERFORMANCE OF THERMOPLASTIC LEAF SPRINGS
INTRODUCTION
In this chapter, experimental static and fatigue performance of the selected composite leaf spring connections are reported. In this chapter, the failure morphology of the leaf spring joints was also presented to understand the influence of fiber length on joint performance.
DESIGN OF JOINT CONFIGURATION
- Joint Geometry and Design Methodology
- Parametric Study using Finite Element Analysis
A quasi-static failure analysis was performed on the selected joint geometry using commercial finite element software, Ansys10. The panel grid is similar to that used by Ireman (1993) , but the grid density is greater in the vicinity.
