MATERIAL SELECTION PROCESS FOR LEAF SPRINGS

USED IN VEHICLE SUSPENSION SYSTEM

Term Paper Report

Submitted in partial fulfillment of the requirements for the award of the degree of

Master of Technology in

Machine Design

by

YALAMANCHILI RAJESH (Roll No.: M140250ME)

Department of Mechanical Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

April 2015

CERTIFICATE

This is to certify that the report entitled MATERIAL SELECTION

PROCESS FOR LEAF SPRINGS USED IN VEHICLE SUSPENSION SYSTEM

is a bonafide record of the Seminar presented by YALAMANCHILI RAJESH (Roll

No.: M140250ME), in partial fulfillment of the requirements for the award of the degree

of Master of Technology in Machine Design from National Institute of Technology

Calicut.

Faculty-in-charge

(ME6612 Material selection in mechanical design)

Dept. of Mechanical Engineering

Professor & Head

Dept. of Mechanical Engineering Place : NIT Calicut Date : 25 April 2015

ABSTRACT

The objective of this study is to select a suitable materials for leaf springs used in

vehicle suspension system. Leaf springs are crucial elements in automobile, necessary to

minimize the vertical vibrations, impacts and bumps due to road irregularities. In this

study first we derived the suitable materials for leaf springs. It is found that medium

carbon steels are suitable material for leaf springs as to increase strain energy.

In these days reducing vehicle's weight has been in practice in order to economize

energy. So we selected a material for light weight leaf spring by using CES software. It is

found that GFRP (Glass Fiber Reinforced Polymer) meeting the requirement of

minimizing the mass compare to conventional steel leaf springs.

1

CONTENTS

List of Abbreviations ii

List of Symbols iii

List of Figures iv

List of Tables v

1 Introduction 1

1.1 Introduction 1

1.2 Problem Definition 1

1.3 Outline of report 1

2 Material Selection for Leaf Spring 2

2.1 Material Performance Index for Leaf Spring 2

2.2 Limit selection using CES Software 3

3 Alternate Material for Leaf Springs 5

3.1 Material Performance Index 5

3.2 Limit selection using CES Software 8

4 Results and Discussion 10

5.1 Results 10

5.2 Analysis 10

6 Conclusions 11

References 12

2

LIST OF ABBREVIATIONS CES Cambridge Educational Software CFRP Carbon Fiber Reinforced Polymer GFRP Glass Fiber Reinforced Polymer

3

LIST OF SYMBOLS

Deflection of the spring

m Mass of the spring

E Modulus of elasticity

Strength

Density

U Strain Energy

L Length of the leaf spring

b Width of the leaf spring

t Thickness of the leaf spring

4

LIST OF FIGURES

2.1 Youngs modulus vs Strength Ashby chart 3

2.2 Fatigue strength/young modulus against density 4

3.1 Leaf spring 5

3.2 Specific Modulus vs Specific Strength 7

3.3 Fatigue strength vs Density 8

3.4 Fracture toughness vs Price 9

5

LIST OF TABLES

2.1 Candidate materials for leaf spring 3

3.1 Requirement Translation table 5

3.2 Candidate Materials for light leaf spring 9

6

CHAPTER 1

MATERIAL SELECTION FOR LEAF SPRING

1.1 INTRODUCTION

Leaf springs are crucial suspension elements in automobile, necessary to

minimize the vertical vibrations, impacts and bumps due to road irregularities. The

functions of the suspension springs for an automobile are to maintain a good control

stability and to improve riding comfort.

When a vehicle rides over rough grounds, the wheels will rise when rolling over

bump and will deflect the springs. The energy created due to this movement is

momentarily stored in the spring; it is then release again, due to the elasticity of the

spring material, and in expending this energy the spring will rebound.

1.2 PROBLEM DEFINITION

Leaf spring should have an ability to absorb the energy when vehicle is moving

on rough road or bumps. We have to select a suitable material for this purpose. In

addition to that we have to minimize the mass of the spring in order to economize energy.

1.3 OUTLINE OF THE REPORT

The first objective of the present study is to select a material for the leaf spring

which can store more elastic energy. After that minimizing the mass of the spring is our

main objective. In addition to that it should be cost effective. In this paper we used CES

software to do the limit selection of materials from candidate materials.

7

CHAPTER 2

MATERIAL SELECTION FOR LEAF SPRING

2.1 MATERIAL PERFORMANCE INDEX FOR LEAF SPRING

The primary function of a spring is that of storing elastic energy and releasing it

again when required. The elastic energy stored per unit volume in a block of material

stressed uniformly to a stress is:

Where E is the Young's modulus. It is this that we wish to maximize. The spring will be

damaged if the stress exceeds the yield stress or failure stress f. So the constraint is

f. The maximum energy density is therefore:

For leaf springs loaded in pure bending the maximum energy storage is

The best material for springs, regardless of the way in which they are loaded, is that with

the biggest value of

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2.2 LIMIT SELECTION USING CES SOFTWARE

Based on the material index M1 the candidate materials selected by using Ashby

chart is listed below:

Fig.2.1. Youngs modulus vs Strength Ashby chart

Table.2.1. Candidate Materials for Leaf Spring

9

A limit selection was conducted base on materials with high fatigue strength,

high fracture toughness and high yield strength. Under this constrain, the CES software

eliminated 98 materials and the possible materials for leaf spring were found to be:

o High carbon steel o Medium carbon steel o Low carbon steel o Nickel o Nickel base super alloys o Stainless steel o Tungsten alloy

Comparison of fatigue strength/young modulus against density and fracture

toughness/ density against price was further investigated and it is found out to be medium

carbon steel is best suitable material for automotive leaf spring.

Fig.2.2. fatigue strength/young modulus against density.

10

CHAPTER 3

ALTERNATE MATERIAL FOR LEAF SPRING

3.1 MATERIAL PERFORMANCE INDEX

In vehicle suspension design it is desirable to minimize the mass of all

components. Here we are going to select a material which minimizes the mass of the leaf

springs. Consider that the existing leaf spring is a beam as shown in Fig.3.1.

The following tables will gives the requirement for minimizing the mass of leaf spring

Fig.3.1. Leaf spring

Table.3.1. Requirements translation table

11

The objective function is to minimize the mass of the spring m is given by:

Stiffness of the leaf spring is given by

Maximum allowable deflection is given by

By substituting for b and t in mass equation then

The material performance index is given by

Materials selection for light leaf springs is shown in Fig.3.2. This Ashby chart gives us

the candidate materials for light leaf spring. The candidate materials are listed in

Table3.2.

12

Fig.3.2. Specific modulus vs Specific strength

Table.3.2. candidate materials for light leaf spring

13

3.2 LIMIT SELECTION USING CES SOFTWARE

The leaf spring flexes through thousands of compression and expansion cycles without

breaking and still retains their original shape. So it should have enough fatigue strength.

In selecting a suitable advance material that will be of light weight, cost effective and

better service efficiency, selection was performed using the CES EduPack 2011 software

using the following parameters: Density-100 kg/m^3, Young Modulus=20Gpa, Fatigue

strength=70Mpa, Fracture toughness=15Mpa.m^ 0.5 and Yield strength=50Mpa. The

materials selected among these are

o GFRP, epoxy matrix (isotropic) o CFRP, epoxy matrix (isotropic) o Cast magnesium alloy o Aluminum/silicon carbide composite

Fig.3.3. Fatigue strength vs Density

14

From the investigation conducted, GFRP (Glass Fiber Reinforced Polymer) were

found to be the best suitable advanced material for the automotive leaf spring maintaining

good advantage over others.

Fig.3.4. Fracture toughness vs Price

15

CHAPTER 5

RESULTS AND DISCUSSION

5.1 RESULTS

We have found out the materials for leaf spring which can store the maximum

elastic strain energy. Finally we found out that medium carbon steel is suitable material

for the leaf spring by taking into consideration of fatigue strength and fracture toughness.

For minimizing the mass of the leaf spring we found out that GFRP is best

suitable material among all materials by CES limit selection.

5.2 ANALYSIS

We have selected GFRP as best suitable alternating material for minimizing the

mass of the leaf spring. Low E- modulus approximately to be 7-times lower in

comparison with the steel, together with internal damping and low dynamic stiffness in

comparison with static stiffness, makes GFRP composite suitable for the manufacturing

of road friendly leaf spring and suspension.

Low specific weight- it has been augured that a 60% reduction of the whole

suspension weight can be obtained by replacing steel spring with GFRP springs with the

same functions. This is particularly important because springs represent unstrung mass. It

has excellent fatigue resistance. It is also free from corrosion.

16

CHAPTER 6

CONCLUSIONS

The movement from the normal conventional material for leaf spring to the

development of an alternative composite leaf spring having constant cross sectional area,

where the stress level at any station in the leaf spring is considered constant due to the

parabolic type of the thickness of the spring, has proved to be very effective. The

research demonstrated that composites can be used for leaf springs for light weight

vehicles and meet the requirements, together with substantial weight savings. The

selection decision of both steel and composite leaf spring is done and analyzed using

Cambridge educational software CES EduPark 2013. A comparative study has been made

between composite and steel leaf spring with respect to weight, cost and strength. From

the results, it is observed that the composite leaf spring is lighter and more economical

than the conventional steel spring with similar design specifications. Composite leaf

spring reduces the weight by 85 % for E-Glass/Epoxy, over conventional leaf spring.

17

REFERENCES

[1] AL Qureshi., H. A (2001) Automotive Leaf Spring from Composite Material. Journal of Materials Processing Technology Vol.118 pp 56-61[

[2] Stephen.A.Takim, Performance Characteristics and Evaluation of Alternate Materials for Automobile Advanced Leaf Springs Volume 11, Issue 4 Ver. IV

[3] Michael F. Ashby, Third Edition, Material Selection in Mechanical Design.