Slide 1
Etienne K.Ngoy, I. Campbell, R. Paskaramoorthy
School of Mechanical, Industrial, and Aeronautical Engineering
University of the Witwatersrand
© CSIR 2006 www.csir.co.za

OUTLINE

Introduction

This Analysis Contribution

Conclusion
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INTRODUCTION

What is the environmental degradation?

Motivation

Literature review

Objective
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What is the Environmental Degradation ?
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Environmental Degradation
Temperature and humidity, energetic radiations, chemicals
Wide spread use of FRP materials
Mechanical properties , colors, brittleness, cracks…
Large variety of service environments interaction
Change of Material properties
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Slide 6
1
3
2
Discoloration and flaking of a pipe surface by uv
(1), Inner of a pipe attacked by chemicals. The glass surface tissue hanging from the walls where the resin has been removed by the chemical (2),
Advanced corrosion on the surface of a pipe by
UV and humidity. The structural laminate becomes exposed, which looks like dry glass, with no resin bonding it together (3) (SASOL)
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Environmental Degradation
Temperature and humidity, energetic radiations, chemicals
Wide spread use of FRP materials
Mechanical properties , colors, brittleness, cracks…
Large variety of service environments interaction
Change of Material properties
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
In practice any change affecting the material properties relative to the initial desirable properties is called degradation
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Motivation
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Motivation
Rational utilization
Requests

Good understanding of the environmental degradation effects.

The availability of reliable method for quantification and prediction of environmental effects.
 Design optimization
 Economic assessment
 Safe utilization
 Equipment maintenance
Modeling
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Literature Review
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Literature Review

The Complexity of the environmental
Degradation process:
Interaction between many physical, chemical and mechanical processes not easy to model.
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Literature Review

No general or accurate predictive model has been available so far :
- modeling efforts focus on the characterization of effects and mechanism.
- Only partial models based on particular process and environment
- Accelerated prediction method based on Arrhenius law
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Literature Review

Exposure in the real service environment
- Standard lifetimes are determined based on statistical data resulting from long term exposure in real service environment.
- Implies that test lasts many years and must be conducted for each particular combination of environment and material
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Literature Review

Extended utilization slowed down in many fields.

Catastrophic failure reported in the industry.
“
However there have been a small but significant number of international failures witch have caused concerns. Cases of tanks containing demineralized water in particular at 700C failing catastrophically are reported.” (SASOL 2000).
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Objectives

Provide a comprehensive model of the environmental degradation of fiber reinforced plastics including the chemical degradation, the ultraviolet rays attack, the temperature and humidity effects, and the stress corrosion.

Provide a short term test method for environmental degradation of mechanical strength of FRP composites
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THE CONTRIBUTION

The theoretical approach.

Environmental degradation models.

Prediction method.

Simulation in laboratory.
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Theoretical Approach
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Basis of the Theoretical Approach
All FRP degradation results in one of the following effects :

Chemical: Chemical links density modification caused by either a chemical attack, a thermal attack or a ultra violet rays attack.

Physical: cohesion forces deterioration or plasticization caused by either moisture absorption or by temperature variation.

Mechanical: Stress state modification.
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Definitions

L d
: index of chemical linkage density degradation.

C f
: index of cohesion forces degradation .

 env
: index of environmental stresses.
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The Analysis of the Environmental
Degradation process and Modeling
Environmental causes
Degradation
Effects
Stiffness Matrix Stress state
Temperature T
Moisture,
 m
Chemicals C
0
UV Rays, I
UV
Rheology



C
E
L d d f
  env
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Modeling Process

Rheology = f(T,  m, C
0
, I
UV
,  E d
,  env
).
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Environmental Degradation Models
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Environmental Degradation Models

Partial model of uv rays caused degradation.

General model of stiffness matrix degradation.

General environmental degradation model involving stress corrosion.
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Model of ultraviolet rays caused degradation dL d dt

UV
 kI abs n
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Environmental Degradation Models

Partial model of uv rays caused degradation.

General model of stiffness matrix degradation.

General environmental degradation model involving stress corrosion.
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Environmental Degradation of the Material Stiffness.
General Model dE d dt
  e

0
TC
0 t

Where t = time,  and 
0 are constants depending on the material and environmental conditions.
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Environmental Degradation Models

Partial model of uv rays caused degradation.

General model of stiffness matrix degradation.

General environmental degradation model involving stress corrosion.
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Environmental Degradation. The
Stress Corrosion General Model.
Slide 29
 

 t
 env
 t
 t '
  t
 t '
   dt ' dt '

Where ε is the strain and t’ = time of strain application.

env (t) is the degradation function measuring environmental degradation history.
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Prediction Method in three stepladder
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Prediction Method

Exposure at constant environment.

Monitoring the chemical structures change or
Measurement of the stress relaxation time or creep rate.

Determination of the degradation parameters based on the mathematical model.
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Simulation in Laboratory
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Simulation in Laboratory

Chemical degradation of the Stiffness Matrix
The model shows good accuracy but the precision needs improvement due to instrumental methods used

Stress Corrosion
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Correlation Between the Model and
Experimental Values
Slide 34
100
90
80
70
60
50
40
30
20
10
0
0
R 2 =0.973
y = 0.0457x - 1.9291
R
2
= 0.9734
500 1000 1500
Ed (Raman Intensity)
2000
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2500

Simulation in Laboratory

Chemical degradation of the Stiffness Matrix
The model shows good accuracy but the precision needs improvement due to instrumental methods used

Stress Corrosion
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Relaxation under stresses only and under stress corrosion
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0 1000 relaxation under mechanical stress only relaxation under chemical degradation and mechanical stress
2000 3000
Relaxation time (secondes)
4000 5000 6000
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Environmental Degradation Factor on the
Stiffness Matrix.
 

 t
 env
 t
 t '
  t
 t '
   dt ' dt '
0.5
0.4
0.3
0.2
0.1
0
-0.1
0
-0.2
-0.3
-0.4
1000 2000 3000 4000 5000 6000
Degradation time (secondes)
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Simulation in Laboratory

Chemical degradation of the Stiffness Matrix
The model shows good accuracy but the precision needs improvement due to instrumental methods used

Stress Corrosion
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CONCLUSION

A theoretical analysis of the environmental degradation process based on the transformation of the material rheology has been suggested.

Two comprehensive mathematical models have been derived for the chemical degradation and for the stress corrosion.

The simulation of these models in laboratory showed good correlation with experimental data.
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Acknowledgement
We wish to acknowledge the support from:

Denel

DST/NRF Centre of Excellence in Strong Materials

ESKOM

THRIP

CSIR
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THANK YOU
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