ABSTRACT

Glass mat reinforcement, which can be easily shattered, is widely used across the world in military, automobile, civil, railway and electronic engineering among others. This research investigated the fracture mechanics of reinforced polyester composites on exposure to sudden impact force, using experimental and analytical methods based on impact and Linear Elastic Fracture Mechanics (LEFM) test procedures, to study the stress distribution around crack tip and zone. Plies of randomly varied combination of E-glass fibre in forms of woven roving, hard and soft mat was used as reinforcement in polyester resin matrix to manufacture test specimens. Fourteen (14) test samples with geometry 210 mm x 150 mm were fabricated using hand lay-up method. Hence, were cut and tested in accordance with ASTM standards for composite polymeric material tests under mode I and Charpy impact test conditions, using the compact tension and the Charpy impact test specimens respectively. From the experiment, the fibre volume fraction, the mode I, KI and mode II, KIIstress intensity factors, critical stress, σC,shear stress, τntthe impact energy, E and impact strength, U were determined for each specimen. The mode I fracture toughness, KIC was found to be 4.97 MPa.m1/2 at a critical stress of 13.53MPa while the mode II fracture toughness, KIIC was 1.31 MPa.m1/2 at a shear stress, τntof 3.71MPa and also, the effective thickness was found to be in the range of 80-100mm atfibre volume fraction, Vf of within 0.35-0.50. From the results, the specimens containing woven roving reinforcement were found to possess higher fracture toughness and resistance to both fracture and impact damage. This was largely found to be as a result of fibre bridging and crack arrest mechanisms. This mechanism prevented crack growth direction in specimens containing woven roving not to propagate along the original direction, but change the direction to an inclined path till failure with the exception of those containing soft and hard mat in which the crack grew in the original crack direction as the stress intensity increased. During the impact test, fibre stacking sequence played a vital role, thereby making specimens containing woven roving to resist impact damage and failure, and this resulted in fibre pull-out during fracture. The increase in fibre volume fraction was also found to improve the impact strength of the laminates. From the experiments, ways of improving composite performance were recommended, to ensure optimum impact and fracture resistance.

TABLE OF CONTENTS

Contents Page

Title Page i

Certification ii

Dedication iii

Acknowledgement iv

Abstract v

Table of Contents vi

List of Tables vii

List of Figures viii

Nomenclature ix

CHAPTER ONE INTRODUCTION

1.1 Background of the Study 1

1.2 Why Reinforced Composites are Prone to Impact Damage? 2

1.3 Failure Mechanism in Composite Material Exposed to Sudden Impact Force 4

1.4 Failure Criteria of Composite Materials 5

1.5 Statement of the Problem 7

1.6 Aim and Objectives of the Research 7

1.7 Significance of the Research 8

CHAPTER TWO LITERATURE REVIEW 9

CHAPTER THREE RESEARCH METHODOLOGY

3.1 Materials 13

3.1.1. Reinforced Composite Specimen Manufacture and Preparation 13

3.1.2. Fracture Mechanics Assumptions in Reinforced Composite Analysis 16

3.1.3. Fibre Areal Weight, AW and Volume Fraction Vf Analysis 17

3.2 Test Machines 18

3.2.1 The Universal Testing Machine 18

3.2.2 The Charpy Impact Testing Machine 19

3.3 Description of the Study 19

3.4 Experimental Procedures and Analysis 20

3.4.1 Linear Elastic Fracture Mechanics 20

3.4.2 Test Procedures for the Determination of Mode I Stress Intensity Factor, KI 21

3.4.3 Analysis of Load-Displacement Records and Mathematical Analysis of the SIFs 22

3.4.4 Analysis of Stress Field near the Crack Tip 26

3.5 The Charpy Impact Test 27

3.5.1 Experimental Procedure for Impact Strength Determination 27

3.6 Primary Experimental Data Analysis: Load, P – Displacement, δ diagrams 28

CHAPTER FOUR RESULT AND DISCUSSIONS

Results and Discussions 38

CHAPTER FIVE CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 63

5.2 Recommendations 65

REFERENCES 67