ABSTRACT
Residual thermal stresses are developed in polymer matrix material as it cures. The degree of residual stresses developed is a function of the cool down path taken and this in turn influences their water absorption/entrapment capacity. By use of three chosen cool-down path, water adsorption/entrapment capacity of various degrees of residual stresses developed in the presence of prevailing atmospheric humidity during cure process of polymer matrix composite was investigated. The theory of mixtures and the partial stress equations arising from Sullivan Model were used to obtain stress values for the polymer composite and the water component respectively. These calculated stress values are compared with experimental results and they ensured good agreement. It became evident from this work that during lamination and cure process of polymer composites, the adsorbed/entrapped water exerts a certain value of stress (partial stress component) within the laminate composite. The value of this stress is directly proportional to the quantity of water adsorbed/entrapped which in turn is influenced by the percentage of relative atmospheric humidity and cool-down path taken during the lamination and cure period.
TABLE OF CONTENTS
Title page ………………………………………………………………………………………..i
Approval Page …………………………………………………………………………………ii
Certification …………………………………………………………………………………….iii
Dedication………………………………………………………………………………………..iv
Acknowledgement……………………………………………………………………………..v
Table of Content…………………………………………………………………………… vi-vii
List of Tables………………………………………………………………………………..viii
List of Figures…………………………………………………………………………………vii
Abstract…………………………………………………………………………………………..ix
CHAPTER 1: INTRODUCTION
1.1 Aims and Objective……………………………………………………………………..3
1.2 Scope of the Project Research……………………………………………………….3
1.3 Assumption ………………………………………………………………………………..3
CHAPTER 2 : LITERATURE REVIEW
2.1 Background ………………………………………………………………………………..4
2.2 Laminating Process……………………………………………………………………..4
2.3 Residual thermal stresses……………………………………………………………….6
2.4 Moisture conditioning and Testing…………………………………………………8
2.5 Moisture Conditioning…………………………………………………………………..9
2.6 Moisture adsorption of Natural Fibers…………………………………………….12
2.7 Mechanism of moisture absorption/adsorption………………………………..13
CHAPTER 3: THERMOCHEMISTRY AND FORMULATION OF THEORY
3.1 Description of the Mixture………………………………………………………19
3.2 Conservation laws for a Binary Mixture……………………………………21
3.3 A Simple Thermomechanical Model………………………………………..22
3.4 An expression for the partial pressure of water………………………….24
CHAPTER 4: METHODOLOGY
4.1 Materials ……………………………………………………………………………..27
4.2 Procedure ………………………………. …………………………………………..27
4.3 Measurement of Residual Thermal Stresses………………………………29
CHAPTER 5: RESULTS
5.1 Discussion of Result……………………………………………………………….34
CONCLUSION ………………………………………………………………………,. .37
RECOMMENDATIONS…………………………………………………………….38
REFERENCE
CHAPTER ONE
INTRODUCTION
Composite materials have been extensively applied in several ways, particularly as a structural component due to their high specific stiffness and strength [I]. Fiber reinforced plastics (FRPs) therefore, are increasingly becoming the preferred structural materials for a variety of applications including ground-based, marine and air-borne vehicles, space structures and sporting goods. Structural strength of these composite laminates depend so much on the processes taken and conditions undergone during the period of lamination and cure.
It has been shown that residual thermal stresses develop in polymer matrix laminates due to differing thermal expansions or contractions of the fibre and matrix, Phase transformation with their accompanying volume change (shrinkage) and temperature excursion from room values up to a certain maximum and back again to room values during cool down [2]. Shrinkage and warpage are however, directly related to residual stresses which result from locally varying strain field that occur during the curing or solidification stage of a manufacturing process. Such strain gradients are caused by non-uniform thermomechanical properties and temperature variations inside the mold cavity. These are, in great part, a result of the transient thermodynamic processes that take place during the curing stage of the matrix material during molding. On the other hand, strain gradient are also created by process dependent phenomenon such as the fibre orientation that occurs during flow, poor thermal mold lay-out and by combing or sandwiching different types of materials such as when manufacturing copper clad epoxy/glass fibre composites widely used by the printed circuit board industry
Residual thermal stresses may be generated as a result of non-uniform (rapid) cooling. Residual stresses, which are typically compressive on the surface and tensile in the interior, are frozen in the material. This is an undesirable situation, as tensional strain at the surface enhances environmental stress cracking.
It was shown specifically, that the magnitude of thermal stresses developed during cure period (which depends on cool-down path taken) accounts for as much as 8-15% lowering in the strength of a composite laminate [3]. Apart from reduction in strength , one of the greatest consequences of development of these stresses during lamination/cure process of polymer composites is increased vulnerability of these stress openings or pores to admission of water either by adsorption or entrapment or both, a property which is not usually desirable when put into Engineering use[4]. The obvious effect is that the magnitude of the restraining stress is proportional to the initial moisture content in specimen. In addition, water is known to lower the glass transition temperature in glassy polymers [5].
Researchers have over the past few years attempted to model, in a definite manner, the thermo-structural response of polymer composites as they are heated to high temperatures. These attempts have been based on and derived from the porous Media theory. Although some have included the effect of water on the thermo-mechanical response, emphasis has always been to simulate the effect of thermal decomposition of the polymer on the structural behaviour of the composites. The success of these attempts is the demonstration of the direct dependence of the transient thermo-mechanical response on the diffusion process. However the shortcoming of the porous media approach is the inability to accurately relate the stresses in the polymer chains to the chemical state of the volatile species (water). Diffusion mechanics and non-Fickan transport investigated in addition to visual experiments involving the movement of water during the absorption process, gave credence to understanding of this concept [6].
In this work, the response, contribution and the mechanics involved in the thermo-chemistry of water and polymer (treated as mixtures) as water gets entrapped/adsorbed into the interstitial spaces available in the polymer composite during cure for various stress levels developed from different cool-down paths taken under different relative humidity conditions during the lamination process is investigated.
- AIMS AND OBJECTIVES :
This project aims at using the conventional method to prepare specimen sample subjecting it to different curing conditions noting the moisture adsorbed with consequent effect on inherent residual thermal stresses which influence the strength of laminate and their service life if used in actual part. Its result will serve as a guide for optimum lamination practice. These therefore, could be achieved through the objectives of this study which are:-
- To establish variation in stresses developed during cure due to cool –down path taken during lamination.
- Application of a Model (Sullivan Model) to explain the adsorption thermo-chemistry and to obtain partial stress values for each quantity of water adsorbed.
- Validation of experimental results obtained by comparing with values calculated using Sullivan model.
- SCOPE OF THE PROJECT RESEARCH
This research is based on Experimental approach. Analytical, Mathematical and Statistical tools are employed in comparing existing chosen Model and Test data results. Ageing effects, which may be prominent during the cool down of newly created laminate were not considered owing to complete lack of data. Transient and time dependent effects were not also considered as fixed-time technique was adopted (fixed-time conditioning, where a test specimen is exposed to a conditioning environment for a specified period of time – lamination period). Therefore, adsorption/ entrapment case was considered as there was no much time allowed for reasonable absorption for equilibrium/ saturation state to be attained. Allowable experimental errors and accuracy of measuring tools were noted.
1.3 ASSUMPTIONS
In this work and for the purpose of simplicity, the following basic assumption are made for binary mixture of polymer and water
- No chemical reactions would occur to cause the generation or consumption of any of the chemical species,
- Polymer behaves as an elastic solid,(though viscoelastic in actual sense)
- Deformations are considered insignificant,
- Only the water constituent is considered volatile
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