Wear Modelling and FEA Simulation for Dry Sliding Contacts

147 Pages·2009·2.68 MB·English

University of South Australia

School of Electrical and Information Engineering
Division of Information Technology, Engineering and the Environment

Wear Modelling and FEA Simulation for Dry
Sliding Contacts

Muhammad Azeem Ashraf

Submitted for the award of the degree of Doctor of
Philosophy

March 2009
T C
ABLE OF ONTENTS
Table of Contents ..................................................................................................... i
Acknowledgments .................................................................................................. iv
Statement ................................................................................................................ vi
Abstract .................................................................................................................. vii
List of principal symbols ....................................................................................... ix
List of figures ........................................................................................................... x
List of tables .......................................................................................................... xii
1. Introduction ................................................................................................... 1
1.1. Background ............................................................................................ 3
1.2. Pilot study: Automotive rear view mirror ................................................. 9
1.3. Research questions .............................................................................. 14
1.4. Thesis outline ....................................................................................... 15
2. Wear analysis fundamentals ...................................................................... 17
2.1. Friction .................................................................................................. 20
2.2. Wear ..................................................................................................... 22
2.3. Wear analysis strategies ...................................................................... 25
2.3.1. Indirect wear analysis ................................................................. 27
2.3.1.1. Statistical useful life prediction ..................................... 27
2.3.1.2. ANN-based useful life prediction .................................. 28
2.3.2. Direct approaches to wear analysis ............................................ 28
2.3.2.1. Experimental wear testing ............................................ 29
2.3.2.2. Analytical wear modelling ............................................. 29
2.3.2.3. Finite element based wear modelling ........................... 29
2.4. Wear modelling .................................................................................... 30
2.4.1. Steady state wear models ........................................................... 31
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2.4.1.1. Archard’s wear model .................................................. 32
2.4.2. Dynamic wear models ................................................................. 34
2.4.2.1. Yang’s transient wear model ........................................ 35
2.5. Wear of injection moulded polymer ...................................................... 36
3. Wear analysis–A systematic approach ..................................................... 38
3.1. Initial experimentation ........................................................................... 40
3.1.1. Tribo-system examination ........................................................... 40
3.1.1.1. Tribometry .................................................................... 41
3.1.1.2. Wear scar examination ................................................ 42
3.1.2. Wear characterisation ................................................................. 43
3.1.3. Design of experiments ................................................................ 44
3.2. Analytical modelling .............................................................................. 44
3.2.1. Model forming and determination of ‘wear coefficients’ .............. 45
3.2.2. Wear extrapolation ...................................................................... 47
3.2.3. Translation of the wear volume to geometry ............................... 47
3.3. FEA wear modelling ............................................................................. 48
3.3.1. Application of FEA to geometric design ...................................... 48
3.3.2. Contact solution .......................................................................... 49
3.3.3. Nodal wear calculation ................................................................ 49
3.3.4. Geometry update ........................................................................ 49
4. Analytical wear modelling: A case ............................................................ 50
4.1. Wear scenario analysis ........................................................................ 53
4.2. Experimental setup ............................................................................... 54
4.3. Wear modelling – Wear coefficient determination ............................... 58
4.4. Useful life prediction of a sliding contact ............................................... 62
4.5. Discussion ............................................................................................ 64
5. FEA-based wear analysis ........................................................................... 67
5.1. Basics of contact mechanics ................................................................ 69
5.1.1. Types of contacts ........................................................................ 71
5.2. Analytical contact solution .................................................................... 72
5.3. Computational contact solution ............................................................ 75
5.3.1. Computational contact theory ..................................................... 76
5.3.2. Determination of contact status ................................................... 77
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5.3.3. Determination of contact constraints ........................................... 79
5.3.4. Finite Element Analysis (FEA) basics ......................................... 79
5.3.5. Solution of energy equilibrium equations .................................... 81
5.3.6. Lagrange multiplier method ........................................................ 82
5.3.7. Penalty method ........................................................................... 82
5.4. Formulation of contact problem in ANSYS© ......................................... 83
5.5. FEA wear algorithm .............................................................................. 85
5.6. Remaining useful life prediction of a contact – FEA ............................. 89
5.6.1. Sliding step size determination ................................................... 90
5.6.2. Numerical results ........................................................................ 93
5.6.2.1. Contact pressure analysis ............................................ 93
5.6.2.2. Wear calculation ........................................................... 95
5.6.2.3. Accumulated dimension loss ........................................ 96
6. Contact separation (Schallamach) wave ................................................ 100
6.1. Schallamach (contact separation) wave phenomenon ....................... 100
6.2. Contact separation travelling wave in FEA results ............................. 102
7. Conclusions and recommendations ....................................................... 106
References ........................................................................................................... 110
Appendix A .......................................................................................................... 118
A-1 Programming code for FEA wear algorithm using ANSYS ................. 118
Appendix B .......................................................................................................... 134
B-1 List of publications .............................................................................. 134
B-2 Selected publications .......................................................................... 136

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A
CKNOWLEDGMENTS
I would like to start off my words here by giving my humble thanks to Almighty
Allah for bestowing His blessings in the form of a supporting family, strength,
knowledge and a very supportive group of friends and supervisors, which have
enabled the completion of this research. I take this opportunity to express my
appreciation to the special people who have shaped my career and a fulfilling
research candidature at the University of South Australia.
Foremost, I wish to express my profound gratitude to my supervisors Dr Bijan
Sobhi-Najafabadi and Professor Özdemir Göl. Their invaluable support, advice and
periodic reviews on day-to-day activities enhanced my confidence to accomplish this
research. I am indebted for their encouragement and inspirational advice received
throughout the association I had with them.
I would also like to extend my thanks to Dr Ross Bensley and Mr Michael Ellis of
Visiocorp Australia Pty Ltd (formerly Schefenacker Vision Systems Australia Pty
Ltd), who despite their very busy schedules, supported this research by providing
access to their test laboratories, product prototypes and shared their technical
experiences promptly whenever requested.
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A special thanks to Dr. Rick Morgans of CMIT, CSIRO (now with department of
mechanical engineering at University of Adelaide) for his support and coaching for
programming in ANSYS APDL environment.
I would like to take this opportunity to extend thanks also to my friends Dr. Zahid
Qureshi, Yousef Amer and Sugumar Dharmalingam for their advice, help and
encouragement during the past four years.
The technical and logistic support from the staff at the School of Electrical and
Information Engineering and CAMR at University of South Australia should not go
unacknowledged, as without their support this thesis and research would still be
crawling.
On a personal front, first and foremost I would like to acknowledge the
continuous support and encouragement from my wife Nazish, which no words can
express my gratitude for. The same goes for my parents as it is largely their support
and efforts I have been able to reach this far.

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S
TATEMENT

I declare that this thesis does not incorporate, without acknowledgement, any
material previously submitted for a degree or diploma in any university. To the best
of my knowledge it does not contain any materials previously published or written by
another person except where due reference is made in the text.

Muhammad Azeem Ashraf

Adelaide, Australia
1st March 2009

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A
BSTRACT
Wear is one of the critical factors, which influence the reliability and useful life of
mechanical components subject to mutual sliding motion. Wear resulting out of such
sliding motion is dependent on a number of factors, which include component
design, materials involved and operating conditions. As a consequence of wear, the
wearing components lose their dimensional accuracy and may render the overall
product unfit for use and, in some cases, may even lead to catastrophic failure.
Therefore, the ability to predict wear at the development stage enables the
designers to come up with a better design, longer useful life and more reliable
products. Moreover, the prediction of time-to-failure for a component will lead to
better maintenance helping to avoid catastrophic effects of unexpected failures.
Study of wear has been a complex task and is often accomplished with over-
simplified experimental methods like pin-on-disc or scratch tests. Such experiments
result only in a qualitative assessment of the suitability of material composition and
are not sufficient for the prediction of wear of a mechanical component and
eventually the useful life of the overall product.
The research detailed in this thesis presents a systematic wear modelling
methodology that can applied to any wear scenario regardless of wearing mode,
thereby providing step-by-step guiding tool to designers for wear experiments and
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modelling. The major stages of the approach are: tribo-system examination,
experimentation, experimental wear coefficient determination and model formation.
The thesis presents a Finite Element Analysis (FEA) based wear modelling
algorithm devised in the course of the reported research activity. FEA is used as a
tool to calculate nodal pressures at the contact region for small sliding steps. These
pressures are then inputted to a customised wear calculating routine. The routine
uses averaged wear coefficients (wear rates) obtained from custom designed
experiments. The FEA contact geometry is modified after each sliding step to
account for the contact height decay, thus determining the volume loss due to wear
over usage time, thus predicting the worn geometry. Consequently, the designer
gains invaluable insight into the extent of wear-caused component deformation
along with the number of usage cycles lapsed prior to such deformation.
The methodology solicited in this thesis was implemented to model the wear
phenomenon of a polymer-to-polymer dry sliding contact. Model verification was
accomplished by comparing the simulated results with the actual test results, which
were found in conclusive agreement.
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L
IST OF PRINCIPAL SYMBOLS
Symbol Parameter
F Friction force, N
μ , μ Coefficients of static and kinetic friction
s k
V Wear volume, mm3

P Normal force, N
d Sliding distance, mm
H Brinell hardness, N/mm2
K Steady-state wear coefficient
s
h Wear depth, mm
k Dimensional wear coefficient, mm3/mm
V Transient wear volume, mm3
t
L Transient sliding distance, mm
t
E Young’s modulus, N/mm2
ν Poisson’s ratio
a Maximum contact radius, mm
П Summation of internal energy as work , J
int
П Summation of external energy as work, J
ext
П Potential energy of contact, J
p
u Nodal displacement, mm
λ , λ Lagrange’s normal & tangential multipliers
N T
, normal & tangential penalty parameters
N T
g Penetration function
N

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