Astm stp 1105 1991

STP 1105 Tribological Modeling for Mechanical Designers Kenneth C Ludema and Raymond G Bayer, editors As M 1916 Race Street Philadelphia, PA 19103 Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Library of Congress Cataloging-In-Publication Data Tribo]ogica] m o d e l i n g f o r m e c h a n i c a l d e s i g n e r s / K e n n e t h C Ludema and Raymond G B a y e r , e d i t o r s (STP ; 1105) P a p e r s from a symposium h e l d in San F r a n c i s c o on May , 1990, and s p o n s o r e d by ASTM Committee G-2 on Near and E r o s i o n Inc]udes bibllograohica] r e f e r e n c e s and i n d e x e s ISBN - - - I Ludema, K C T r i b o ] o g y - - R a t h e m a t i c a l models Congresses II B a y e r , R G (Raymond G e o r g e ) , 1935III ASTM C o m m i t t e e G-2 on E r o s i o n and Wear I V S e r i e s : ASTH s p e c i a l t e c h n i c a l publication ; 1105 TJ1075.A2T725 1991 91-8238 821.8'9'015118 dc20 CIP Copyright 1991 by the American Society for Testing and Materials All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication Peer Review Policy Each paper published in this volume was evaluated by three peer reviewers The authors addressed all of the reviewers' comments to the satisfaction of both the technical editor(s) and the ASTM Committee on Publications The quality of the papers in this publication reflects not only the obvious efforts of the authors and the technical editor(s), but also the work of these peer reviewers The ASTM Committee on Publications acknowledges with appreciation their dedication and contribution of time and effort on behalf of ASTM Printedin Baltimore October1991 CopyrightbyASTMInt'l(allrightsreserved);ThuDec3113:22:02EST2015 Downloaded/printedby UniversityofWashington(UniversityofWashington)pursuanttoLicenseAgreement.Nofurtherreproductionsauthorized Foreword This publication, Tribological Modeling for Mechanical Designers, contains papers presented at the symposium of the same name held in San Francisco, CA on 23 May 1990 The symposium was sponsored by ASTM Committee G-2 on Wear and Erosion Professor Kenneth C Ludema of the University of Michigan in Ann Arbor, MI and Raymond G Bayer of IBM in Endicott, NY presided as symposium chairmen and are editors of the resulting publication Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions aut Contents Overview K c LUDEMA AND R G BAYER vii WHAT MECHANICAL DESIGNERS NEED IN TRIBOLOGICAL MODELING Comments on Engineering Needs and Wear Models R G BAYER Design of Plain Bearings for Heavy M a c h i n e r y - - w A GLAESER t2 WHAT IS AVAILABLEIN TRIBOLOGICAL MODELS (MOSTLY FOR WEAR) The Structure of Erosive Wear M o d e l s - - s BAHADUR 33 Success and Failure of Simple Models for Abrasive W e a r - - J LARSEN-BASSE 51 Wear by Chemical Reactions in Friction Contacts J L LAUER 63 Tribological Models for Solid/Solid Contact: Missing Li n k s- - s L RICE AND F A MOSLEHY 77 DATA BASE AND SIMULATION ISSUES FOR TRIBOLOGICALMODELING Friction in Machine Design K G BUDINSKI 89 Considerations on Data Requirements for Tribological Modeling A w RUFF 127 Classification of Metallic Materials from a Viewpoint of Their Antiwear Behavior T SASADA 143 Wear Transition Surfaces for Long-Term Wear E f f e c t s - - c s YUST 153 PRINCIPLES OF MODEL MAKING AND USE IS Modeling in Tribology a Useful Activity? J R BARBER 165 Wear Modeling: How Far Can We Get With Principles? M GODET, Y BERTHIER, J LANCASTER,AND L VINCENT 173 Cultural Impediments for Practical Modeling of Wear Rates K C LUDEMA 180 Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Overview Purpose The symposium for which the following papers were written was organized out of the recognition that those tens of thousands of mechanical designers who design consumer products need far better information than they now have when they design mechanical components for wear life They have equations (tables, graphs, guidelines, etc.) for the analysis of stresses, for vibration modes and natural frequencies, for rates of heating and cooling, and for most other phenomena - but very little for the wear life of products The needs of designers may best be seen in the dichotomy between the mechanical sophistication of machines and devices, and the fact that these devices are most often discarded because of mechanical wear Tribological adequacy seems to be one of the last considerations in the design process if it is explored at all, and probably for good reason - it is very complicated Tribological design requires knowledge of materials (including lubricants), surface making processes, running-in procedures and assembly procedures The designer is handicapped because neither friction nor wear are intrinsic properties of material in any form, but rather are highly dependent on the mechanical system and how it is run Most designers have been caught in attempting to upgrade products only to find that the new product fails too often Some then attempt a test program, only to find that there is no correlation between test results and the functioning of production items Proaress in wear modeling The impetus for developing useful information on the wear properties of material comes mostly from those in research, referred to here as research tribologists Their first priority Is to maintain research activities and write scholarly papers By the nature of their work tribologists select relatively impractical materials and experimental parameters, and interact mostly with others who the same However, some tribological concepts have diffused into general design practice The most common are equations for designing fluid film bearings Further mature concepts have made their way into the design of rolling element bearings, belts, gears, pumps, etc such that predictions can easily be made of functional product life Whereas many mechanical devises can be built up with such components, many consumer products can not, because they must sell at the lowest cost The majority of designers are connected with consumer products This is the third symposium on modeling for wear resistance, each with different sponsors The first was held at Columbia University in New York City, December 17-19, 1986 (1) and the second was held at Argonne National Laboratories (2) These were attended mostly by researchers, and by invitation These were serious efforts and much good information was exchanged It may be seen from the proceedings of these symposia that each of the specialties in tribology communicates in very different and esoteric language, compared with the needs of designers vii Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized The third symposium, the ASTM Symposium of May 23,1990 reported here, sought to meet the needs of designers Authors were invited to show how the great chasm between research language and designers needs could be bridged Perhaps the extent of the chasm may be seen in that only two authors from industry submitted papers The great majority of the papers were written by research tribologists The latter were written from the perspective of a physicist, a chemist, several in materials engineering, on specialist in solid mechanics and five mechanical engineers The latter are "near" the design process, but not often design consumer products Overview of the papers of the symposium To a great extent the authors report that they have a long way to reach in order to reach designers Designers have an equally long reach, but they have no better idea than research tribologists which direction to reach Our authors made a valiant effort to propel us toward sensible wear models Most authors agreed on the nature of the problem and some offered specific improvements in the understanding of wear In particular, some of the points made were the following, with editorial comment: Research papers in tribology contain information that is rarely applicable to practical problems The reasons may include: a Terminology is a major point of confusion in the field This is probably a consequence of the presence of several very different academic disciplines in the field b Research papers focus on very few of the operating variables and phenomena in real machines that control wear Research papers range from the "near applied" to the fundamental, the latter often from the point of view of the atomic and molecular structure through the sliding interphase region c Attempts to harmonize the methods in the various specialized areas in tribology are largely philosophical and not well directed d Research results as presented seem to imply that the dual phenomena of friction and wear are uncoupled from each other and from considerations of the mechanical properties of the machinery holding the sliding pairs e Research results rarely provide information on the changes that occur at interfaces (debris formation and migration, eg.) over time The greatest advances in tribology have been made in capital products and machinery that are expected to last for a long time The design of consumer products involves minimum cost for material and manufacture, which involves variables (surface roughness, materials variation, etc.) that have been inadequately studied Several wear models exist but these are extremely limited in scope and applicability Unfortunately, the limits of applicability of these models are rarely published In fact, the literature would suggest, by virtue of the lack of comment, that the available models are universal in application This is particularly misleading in designing weal" tests when only those variables that appear in simple models are thought to be the controlling variables in all sliding systems viii Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Designing for wear resistance outside of the scope of the current limited models should be done primarily by empirical methods in the next decade This is so because it is not reasonable to expect the many relevant and disparate variables in wear to be rationalized in the next decade, whether in the form of broadly applicable equations, models, algorithms or handbook entries The same applies to wear tests as well as to the selection of materials The empirical method includes: a Gathering data from practical sliding elements over a reasonable range of controllable variables The entire system, including the machinery surrounding the sliding surfaces, must be thoroughly characterized b A data base of research results, for equally well characterized laboratory systems should be (and is being) gathered c Bench wear tests should be done but only after the results of the bench test are known to correlate very well with the results from the practical system being simulated d Special attention should be paid to wear debris and other residue the manner in which sliding systems retain or flush out debris, which will depend on, among other things, specimen shape, vibration characteristics and duty cycles Efforts should be made to trace the chemical and mechanical "pathways" by which the debris and residue was formed, transformed or ejected A significant fraction the efforts of research tribologists should be devoted to such empirical work Overall, tribology is seen to be a very broad and complicated topic There is a major problem in communication across the field which should be addressed in the next decade Research tribologists should devote some of their efforts to making their results useful, but designers should indicate what they need from research tribologists Many more symposia on wear modeling must be held - Approaches to Modeling of Friction and Wear, Proc Workshop on the Use of Surface Deformation Models to Predict Tribology Behavior , Eds FF Ling and C.H.T Pan, Springer-Verlag Proc of the International Workshop on Wear Modeling, June 16 and 17, 1988, Eds, F.A Nichols, A.I Michaels and L Northcutt, DOE-Conf.-8806370, June 19,1989) Ken Ludema University of Michigan, G.G Brown Building Ann Arbor, MI 48109-2125 Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized What Mechanical Designers Need in Tribological Modeling Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Raymond G Bayer COMMENTS ON ENGINEERING NEEDS AND WEAR MODELS REFERENCE: Bayer, R G., "Comments on Engineering Needs and Wear Models, = Tribolo~ical Modelin~ for Mechanical Desis ASTM STP 1105, K C Ludema and R G Bayer, Eds., American Society for Testin B and Materials, Philadelphia, 1991 ABSTRACT: Engineering applications can involve complex tribological considerations other than the ranking and selection of materials These can include such aspects as assessment of the, significance of a variety of design parameters, determination of tolerances for these parameters, comparisons of designs, and life projectlons Several of these are illustrated in terms of actual situations encountered by the author, along with the methods used to address these aspects in those cases The typical needs of such applications of tribology are identified, and current approaches in wear modeling and wear testing are compared to these needs Inadequacies are identified and suggestions as to how to address these are made In addition, the differences between a material engineering and a more general design approach to engineering problems are identified, and the significance of these in terms of modeling ~nd testing are discussed KEYWORDS: wear models, wear tests, wear design, wear prediction, wear transitions INTRODUCTION The subject of wear models is one of current and perhaps perennial interest One can go back fifty or so years and find models for wear More recently there have been informal and formal meetings sponsored by various technical and governmental agencies of wear models (1,2,3) This volume on the proceedings of ASTM's Symposium on Tribological Modeling for Mechanical Designers reflects the current interest in this area In these meetings, and in discussions with tribologists and engineers regarding wear models, a variety of meanings for the term "wear model" can be found along with a variety of expectations for Raymond G Bayer is a senior engineer at International Business Machines Corporation, Systems Technology Division, Endicott, NY He is co-chairman for this year's Symposium on Tribological Modeling for Mechanical Designers Copyright by 9ASTM rights reserved); Thu EST 2015 Copyright 1991 byInt'l A S T(all M International w w w Dec a s t m31 o r13:22:02 g Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authoriz 172 TRIBOLOGICAL MODELING FOR MECHANICAL DESIGNERS with those of the initial simple case All plausible complicating factors should be examined this way separately so that we can assess which of them are the most potent in influencing the behaviour of the system The interaction of several such effects can then be investigated for the more potent parameters, the results always being compared with our 'guess', based on their separate effects C r i t e r i a for q u a l i t y o f p u b l i c a t i o n s The best scientific papers are those which:9 Demonstrate that a qualitatively new and previously unsuspected phenomenon can occur as a result of the interaction of well understood physical mechanisms Lead to simple expressions which permit the estimation of important physical quantities such as strength, force etc with a high degree of generality, even if the accuracy is moderate (say within 30%) Increase our understanding - - i.e enhance our ability to characterize relatively complex behaviour in simple terms Policy for the Scientific Community All the preceding suggestions are directed principally at the individual researcher, who must take primary responsibility for the parlous state of theoretical modeling in tribology However, the individual is himself or herself affected by the attitudes of his peers and the structure of the professional comnmnity and it is arguable that things are not going to change unless the system changes All the criteria discussed in the Section 'Some Constructive Suggestions' above can be interpreted as policy guidelines for those in positions of power such as funding agency program directors, journal editors and committees for promotion and tenure in universities However, we cannot afford to be too complacent on this score, since we are all of these things as well as being individual researchers! I offer one concrete suggestion on the organizational level, which is that we might press for the formation of a journal devoted to papers meeting the criteria of the Section 'Criteria for quality of publications' above, using the very highest critical standards Acceptance of a paper in this journal would acquire high prestige and might also ultimately become an influential factor in funding and tenure/promotion decisions) Such a journal would explicitly refuse papers characterized by the qualities listed in the Section 'Some common feature' above It would specificMly target papers which are intelligible and useful to the design community in the sense of the preceding discussion It may therefore be appropriate in some cases for an author to submit a 'summary type' paper describing qualitatively interesting results of an analysis, the rigorous formulation of which is published elsewhere for the archives Rejection of a paper by the journal therefore would not imply that the material was not worthy of publication, but rather that it was inappropriate for this particular purpose Unfortunately, if the critical standards are maintained, the number of publishable papers of this kind might only warrant a modest number of issues and journal pages and the project might therefore not be very attractive to a technical publisher However, it is crucial to the success of such an enterprise that critical standards be not eroded by commercial pressures Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions autho Maurice cent WEAR Godet, MODELING Yves : HOW Berthier, FAR John CAN WE Lancaster GET WITH FIRST and L6o Vin- PRINCIPLES REFERENCE : Godet, M., B e r t h i e r , Y., L a n c a s t e r , and V i n c e n t , L., "Wear M o d e l i n g : H o w F a r C a n W e w i t h First P r i n c i p l e s ?" T r i b o l o q i c a l M o d e l i n g for chanical Designers, A S T M STP 1105, K.C L u d e m a R.G Bayer, Eds., A m e r i c a n S o c i e t y for T e s t i n g and terials, Philadelphia, 1991 ? J., Get Meand Ma- ABSTRACT : A brief description of w h a t is m e a n t by "wear m o d e l i n g for design" is given The t h r e e contributors or triplet that m u s t be c o n s i d e r e d in any w e a r m o d e l i n g attempt are identified T h e s e are : - mechanisms : i.e all s y s t e m s in w h i c h one or m o r e c o n t a c t s are e s t a b l i s h e d Quasi-static liaisons such as g a s k e t s , h i g h s p e e d b e a r i n g s and gears are examples of mechanisms - f i r s t - b o d i e s : i.e the r u b b i n g e l e m e n t s of e a c h mec h a n i s m ; g e a r teeth, balls, r a c e s a n d c a b l e strands, are examples of first-bodies - third-bodies : i.e a n y m a t e r i a l , w h e t h e r n a t u r a l or a r t i f i c i a l , which separates the rubbing firstbodies S o l i d and l i q u i d l u b r i c a n t s a n d d e b r i s beds are examples of third-bodies The diversity of conditions (loads, speeds, temperatures, environments) encountered in d i f f e r e n t mechanisms, the d i f f e r e n c e s n o t e d in s t r e s s and temper a t u r e fields found in first-bodies, the v a r i a t i o n s in c o m p o s i t i o n and r h e o l o g y o b s e r v e d in t h i r d - b o d i e s are highlighted KEYWORDS: wear, modeling, v e l o c i t y trapolation, p a r t i c l e detachment accommodation, ex- M a u r i c e G O D E T is P r o f e s s o r of M e c h a n i c s and directs the L a b o r a t o i r e de M ~ c a n i q u e d e s C o n t a c t s (CNRS U R A 856) Institut National des s c i e n c e s Appliqu~es de Lyon, 69621 V i l l e u r b a n n e Cedex FRANCE Yves BERTHIER is " C h a r g ~ de R e c h e r c h e s " in t h e Laboratoire de M~canique des Contacts and is c o d i r e c t o r of the General T r i b o l o g y group John LANCASTER is V i s i t i n g Professor at the L a b o r a t o i r e de M ~ c a n i q u e des Contacts L ~ o V I N C E N T is P r o f e s s o r of M a t e r i a l s c i e n c e at the D ~ p a r t e m e n t de M a t ~ r i a u x - M ~ c a n i q u e P h y s i q u e (CNRS U R A 447) E c o l e C e n t r a l e de Lyon, E c u l l y C e d e x FRANCE 173 Copyright 1991 by ASTM l~crn~ional www.astm.o~ Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions 174 TRIBOLOGICAL MODELING FOR MECHANICAL DESIGNERS The steps followed to create a w e a r particle, from particle detachment to d e b r i s elimination f r o m the t r i b o l o g y circuit are identified Some of t h e s e steps have been m o d e l e d individually but no a t t e m p t has been m a d e to date to assemble what exists and signal the m i s s i n g links The notion of t h i r d - b o d y flow is introduced with first-bodies acting as s o u r c e s and the wear track exits acting as sinks In conclusion, wear m o d e l i n g from first p r i n c i p l e s is impossible today, as the necessary information is not available or even sought after Finally practical ways of obtaining empirical wear laws are discussed INTRODUCTION Yesterday, designers produced equipment that worked T o d a y they must design equipment and m e c h a n i s m s that last D u r a b i l i t y has joined p e r f o r m a n c e at the top of the list of e n g i n e e r i n g requirements D e s i g n e r s n e e d to predict durability, but instead they try to predict wear Durability and wear are not identical Wear is material loss It is e x p r e s s e d in terms of weight loss per meter travelled and per newton load [i] Durability relates to function taken in terms of duty or life Wear is not an intrinsic material property [2] and extrapolation from l a b o r a t o r y benches to i n d u s t r i a l applications is hazardous Wear laws are by essence qeneral, w h i l e m e c h a n i s m s (gears ) are specific Thus t r y i n g to e s t a b l i s h wear laws for all mechanisms is paradoxical A given m e c h a n i s m can accept at c e r t a i n p o i n t s of its w o r k i n g p r o f i l e a lot of w e a r w i t h o u t loss of function and only very little wear at o t h e r more s t r a t e g i c locations Thus, even in a g i v e n mechanism, wear laws have to refer to specific contact areas Finally, wear laws can only be e s t a b l i s h e d for a given mechanism if all elements of t h e triplet: mechanism, first and third-bodies are identified THE TRIPLET Mechanisms The speeds, running conditions (configuration, loads, temperature, environment etc.) i m p o s e d by a particular mechanism on the rubbing first-bodies have to be d e t e r m i n e d in any m o d e l i n g attempt as r e s u l t s show that they govern wear and life Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized GODET ET AL ON WEAR MODELING 175 First-bodies Both mechanical and material aspects of first-bodies should be examined Stress and temperature fields imposed by the mechanisms' running c o n d i t i o n s on the m a c h i n e e l e m e n t s or first bodies have to be identified This amounts to r u n n i n g a full classical, first-body, thermo-mechanical "Strength of M a t e r i a l s " analysis Composition, c o m p o s i t i o n gradients, s t r u c t u r a l changes, and residual stresses are required, w h e r e possible, to complete the analysis Further, first-body surface and subsurface damage or modification m u s t be c a r e f u l l y n o t e d and i d e n t i f i e d wherever possible in all mechanisms concerned Third-bodies T h i r d - b o d i e s separate the r u b b i n g first-bodies As such they have load-carrying capacities, and t h e i r l o a d - c a r r y i n g properties should be d e t e r m i n e d if their role is to be modeled As this exercise is more difficult t h a n the m e c h a n i s m s and f i r s t - b o d y studies outlined above, it should only be tackled after the two other elements of the triplet have been fully worked out Note h o w e v e r that third-bodies have both b u l k and surface properties and that: - bulk p r o p e r t i e s depend on c o m p o s i t i o n and g o v e r n third-body rheology, surface properties also depend on composition and govern third to f i r s t - b o d y interaction The very t h i n layers of m a t e r i a l which impose surface properties are known as screens Conclusions M o d e l i n g from first p r i n c i p l e s implies that each phase of the w e a r p a r t i c l e life be i d e n t i f i e d and understood The wear particle will therefore be followed from its detachment from first-bodies to its elimination from the wear track PARTICLE Classical LIFE and interface triboloqy Classical tribology refers to wear m e c h a n i s m s and identifies adhesion, abrasion, fatigue, corrosion as wear mechanisms Interface or t h i r d - b o d y tribology observes that a l~m particle is necessarily trapped for some time in the contact and thus distinguishes between: - particle detachment - transition from to body contacts - interface kinematics or third-body life Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions auth 176 TRIBOLOGICALMODELINGFOR MECHANICALDESIGNERS - particle elimination t r a c k trapping from the contact and wear W e a r is a F L O W p r o b l e m [3] w i t h its S O U R C E S a n d SINKS It is g o v e r n e d by t h i r d - b o d y k i n e m a t i c s Clearly t h e i n t e g r a t i o n of p a r t i c l e d e t a c h m e n t , third-body life and elimination f r o m b o t h c o n t a c t a n d w e a r t r a c k in a m o d e l for w e a r is not w i t h i n r e a c h today W h a t c a n be done however is to try to p r e s e n t t h e s u m of t h e s e p r o b l e m s in s u c h a w a y t h a t a p a t h t o w a r d s g l o b a l w e a r m o d e l i n g be t r a c e d and s u g g e s t a l t e r n a t i v e s to m e e t the i n d u s t r i a l p r o b l e m s Let us a n a l y s e e a c h e p i s o d e in the life of a particle Particle detachment M o d e l s exist [4] They are u s u a l l y p r e s e n t e d as w e a r models T h e y call upon d i f f e r e n t p r o p e r t i e s for d i f f e r e n t materials W e a r maps [5] are a c t u a l l y p a r t i c l e d e t a c h m e n t m e c h a n i s m maps as they are limited to t w o - b o d y contacts As such t h e y are v a l u a b l e in d e f i n i n g the first p h a s e of w e a r m o d e l i n g p r o c e s s but are of limited u s e later on Transition from to body c o n t a c t s This step should describe particle life following its d e t a c h m e n t f r o m a f i r s t - b o d y as it p r o c e e d s f r o m a t w o to a t h r e e b o d y contact T h e c h o i c e of d e t a c h m e n t p r o c e s s is not important, but k n o w i n g h o w m a n y o p e r a t i o n s are n e e d e d and h o w m a n y h y p o t h e s e s are r e q u i r e d to reach steady-state c o n d i t i o n s for a g i v e n t y p e of d e t a c h m e n t w o u l d be m o s t enlightening, and m o d e l i n g of t h a t p a s s a g e c o u l d be j u s t i f i e d just on this ground Modelinq of interface kinematics T h i s is not w i t h i n r e a c h t o d a y as t h e p a r a m e t e r s w h i c h g o v e r n v e l o c i t y a c c o m m o d a t i o n sites and m o d e s [6,7] are not i d e n t i f i e d , and a l s o b e c a u s e l i t t l e is k n o w n of the way in w h i c h the composition and rheology of third-bodies bulk and screens are affected by the p r o p e r t i e s of first-bodies, by the c r u s h i n g and s h e a r i n g that goes on in the contact, or by environment A considerable research effort on third-body rheology, p o w d e r w a l l f r i c t i o n etc m u s t be u n d e r t a k e n before representative i n t e r f a c e k i n e m a t i c m o d e l s c a n be expected W e a r track trappinq If i n t e r f a c e k i n e m a t i c s are known, t h e f l o w o u t of the contact (or e x t e r n a l flow) can be calculated However, external f l o w is not wear, as s o m e of the p a r t i c l e s e l i m i n a t e d f r o m the c o n t a c t c a n be t r a p p e d in the w e a r track and recirculated Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions GODET ET AL ON WEAR MODELING 177 The effect of c o n t a c t configuration, debris c o m p o s i t i o n and r h e o l o g y under w o r k i n g c o n d i t i o n s , track a d h e s i o n etc on the d i f f e r e n c e b e t w e e n e x t e r n a l f l o w and w e a r should be looked into F L O W IN W E A R M O D E L I N G I d e n t i f y i n g flow as the g o v e r n i n g p h e n o m e n o n in w e a r m o d e l i n g is important as it centers the a r g u m e n t a r o u n d a concept which is b o t h b a s i c and w e l l understood by s u r f a c e physicists, m a t e r i a l s c i e n t i s t s r h e o l o g i s t s and m e c h a n i c a l engineers It d e f i n i t e l y h i g h l i g h t s the m u l t i d i s c i p l i n a r y nature of wear Thus w e a r m o d e l i n g based on first p r i n c i p l e s has to s y n t h e s i z e k n o w l e d g e coming from at least the four d i r e c t i o n s noted earlier: Wear p a r t i c l e detachment, t r a n s i t i o n from to body c o n t a c t s interface kinematics, w e a r track trapping is flow STATE OF THE ART M o d e l s only exist for p a r t i c l e d e t a c h m e n t , and a lot m o r e r e s e a r c h is n e e d e d b e f o r e t h e g o v e r n i n g p a r a m e t e r s of each of the three other s u b j e c t s are first i d e n t i f i e d and later quantified This means that no w e a r m o d e l s from f i r s t p r i n c i p l e s w i l l be a v a i l a b l e in c o m p u t e r c o d e s in t h e n e a r future The q u e s t i o n of m o d e l i n g m u s t h o w e v e r be raised, if o n l y to t a k e s t o c k of t h e l i m i t a t i o n s of t h e m o d e l s u s e d t o d a y and e n s u r e t h a t s o m e of t h e n e w w o r k takes m o d e l i n g in mind WHAT SHOULD BE DONE ? D e s i g n engineers cannot w a i t for d e s i g n rules p r o v e n f r o m first principles W h a t can be d o n e in the i n t e r i m ? Simulation S i m u l a t i o n tests can be c o n d u c t e d [8] B e i n g e x p e r i m e n t a l t h e y are costly, but r e a s o n a b l y s u r e ; t h e y are j u s t i f i e d on big p r o j e c t s or w h e r e safety is involved Enqineerinq Science Data Unit type a p p r o a c h T h e E n g i n e e r i n g S c i e n c e D a t a U n i t [9] h a s p r o v i d e d v e r y u s e f u l i n f o r m a t i o n on the d e s i g n a n d s e l e c t i o n of dry rubbing bearings Following a definition of t h e a p p l i c a t i o n r e q u i r e m e n t s (mechanisms) m a t e r i a l s are ident i f i e d w i t h the a p p r o p r i a t e thermal, m e c h a n i c a l , and environmental properties (first-body) T h e w e a r r a t e s of these materials, obtained experimentally for a t y p i c a l Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproduc 178 TRIBOLOGICAL MODELING FOR MECHANICAL DESIGNERS bearing arrangement under "standard" conditions where f r i c t i o n a l h e a t i n g is n e g l i g i b l e are t h e n c o r r e c t e d to take into account the special features of the a p p l i c a t i o n eg t y p e of motion, pressure, speed, temperature and counterface properties (type a n d r o u g h n e s s ) Materials w i t h the r e q u i r e d life can then be i d e n t i f i e d and the final c h o i c e m a d e on the b a s i s of c o s t a n d a v a i l a b i l i t y A l t h o u g h t h i s is o b v i o u s l y n o t a w e a r m o d e l p e r se, t h e end result in so far as the d e s i g n e r is c o n c e r n e d is the same The only serious l i m i t a t i o n lies in the a c c u r a c y of t h e w e a r c o r r e c t i o n factors, w h i c h b e c a u s e of t h e f a c t t h a t t h i r d - b o d y e f f e c t s on w e a r are s p e c i f i c to i n d i v i d u a l m a t e r i a l s and c o n f i g u r a t i o n s , c a n o n l y be a s s e s s e d a p p r o x i m a t e l y w h e n m a t e r i a l s are c o n s i d e r e d in groups On the other hand, where a p p l i c a b l e it has the c o n s i d e r a b l e a d v a n t a g e of d o i n g a w a y w i t h t h e c u m b e r s o m e simulation d i s c u s s e d a b o v e and of e x t e n d i n g t h e r a n g e of p r o b l e m s w h i c h can be solved from a given set of data CONCLUSION Wear modeling must be c o n d u c t e d * from first principles, - where : it f o c u s e s on: p a r t i c l e detachment, t r a n s i t i o n from to body c o n t a c t s interface kinematics, w e a r track trapping * along design requirements, where simulation and e d u c a t e d p r a g m a t i c a p p r o a c h e s such as the one developed by ESDU should be followed The break down of the w e a r p r o c e s s in t h e four steps s u g g e s t e d above is not yet a c c e p t e d by all w o r k e r s in the field The a n t i n o m y b e t w e e n this v i e w and t h a t p r e s e n t e d in w e a r m a p s m u s t be r e s o l v e d if s i g n i f i c a n t s t e p s in w e a r m o d e l i n g f r o m f i r s t p r i n c i p l e s a r e to be made The e f f e c t of r u n n i n g conditions, environment, materials, on the relative importance of e a c h of t h e s e four steps s h o u l d be d e t e r m i n e d and it m u s t be a c c e p t e d t h a t t h i s i m p o r t a n c e can v a r y s i g n i f i c a n t l y f r o m one c o n d i t i o n to the other Modeling from first principles requires c h a n g e in basic r e s e a r c h orientation a significant REFERENCES [i] [2] Lancaster, J., "Material, S p e c i f i c W e a r Mechanisms: r e l e v a n c e to w e a r m o d e l i n g " P r o c e e d i n g of t h e Int e r n a t i o n a l W o r k s h o p on W e a r M o d e l i n g A r g o n n e N a tional Laboratory, June 1988, C O N F - 8 , 1989 Godet, M., " T h i r d - b o d i e s in T r i b o l o g y " WEAR, vol 136, 1990, pp 29-45 Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authori GODET E'I" AL ON WEAR MODELING [3] [4] [5] [6] [7] [8] [9] 179 Godet, M., and Berthier, Y., "Continuity and dry friction : an Osborne Reynolds approach" in "Fluid film l u b r i c a t i o n - Osborne Reynolds Centenary" Ed.D.Dowson, C.M.Taylor, M.Godet, and D.Berthe, EI~evier Tribology Series ii, Amsterdam, 1987, pp 653-661 Archard, J.F., "Contact and rubbing of flat surfaces", Journal of Applied Physics., Vol 24, 1953, pp 981-988 Lin, S.C., and Ashby, M.F., "Wear m e c h a n i s m maps" Acta Metall., vol 35, 1987, pp 1-24 Berthier, Y., "M~canismes et Tribologie" Th~se Dr.,s Sciences, INSA/UCB Lyon, 1988 Berthier, Y., Brendl~, M., and Godet, M "Velocity accommodation in friction", STLE Triboloqy Transactions, Vol 32, n ~ 4, 1989, pp 490-496 Play, D., and Godet, M., "Design of high performance dry bearings", WEAR, vol.41, n~ 1977, pp 25-44 Engineering science Data Unit, Tribology, Vol 2, Section 87007, ESDU International plc, 27 Corsham Street, London N1 6UA, U.K Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Kenneth C Ludema CULTURAL RATES IMPEDIMENTS TO PRACTICAL MODELING OF WEAR REFERENCE: Ludema, K C , "Cultural Impediments to Practical Modeling of Wear Rates," T ~ b o l o ~ i c a l Mod~lin~ for Mechanical ~esi~ners, ASTM STP 1105, K C Ludema and R G Bayer, Eds., American Society for Testing and Materials, Philadelphia, 1991 ABSTRACT: A perspective is given on some of the difficulties in developing of practical equations for use by designers in predicting the wear life of mechanical components Several reasons are given, beginning with the presence of authors and researchers in the field who are from very different educational backgrounds from each other Secondly, authors write from very different perspectives ranging from the strictly empirical to the scientific Thirdly, there has not been a very broad effort to determine which of the material properties usually found in wear equations are intrinsic, relevent or unique The best approach to wear modeling is to develop an organized way to accumulate empirical results from tests that simulate practical systems and build modets from these data KEYWORDS: Wear, Modeling, Testing, Design, Tribology Intr~uction The "transfer of technology" from tribology experts to designers is very slow Few designers have received instruction in tribology in coilege and not many have the opportunity to learn more later In fact, it is very difficult even for the serious engineer to learn the basics of wear from tribologists! Probably the main reason is the very broad nature of the field It requires skills in mechanics, material science and surface chemistry, to name a few These are very diverse fields with very limited cross communication One spends or more years developing an exclusive vocabulary and mind+set in each field These fields are virtually different cultures, different from each other, and different from the field of mechanical design Each culture has developed different tools to work with and different instruments as well The chemist has developed a number of instruments that provide electromagnetic absorption spectra, mostly for liquids Materials scientists often use the vacuum enclosure in which electron and x-ray spectra are obtained mostly for solids The mechanics people have relied heavily on their mathematics, with the indispensible aid of the computer The designer has been trained in the use of mature equations for beam bending, vibration analysis and computer aids to design, among others There is common ground among many tribologists, however, namely, in that they are usually oriented to the use of algorithms, constitutive equations, and functional models in some part of their work If some of these were available in the area of wear, designers could develop design procedures for long-lived products Wear life equations and a strategy for developing them may be the best hope for drawing the cultures together into a common purpose However, even the exercise of developing models is confused by the different methods used by the several cultures in tribology Models will be discussed further Kenneth C Ludema is Professor of Mechanical Engineering at the University of Michigan, Ann Arbor, MI, 48109-2125 180 Copyright9 by ASTM International www.astm.org Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Wear Life Pro~tiction LUDEMA ON PRACTICAL MODELING OF WEAR RATES 181 The goal in tribological design is to select materials, geometries and lubrication methods so that a given product life can be achieved The difficulty of wear prediction for any product may be seen by analogy Imagine predicting the useful working life of, say, a carriage horse in tourist service, and doing so for a newly born colt There we see that pedigree, care and feeding, variations in the weather, thi~ nature of work to be done and circulating horse diseases are all important variables Serious infection and injury may suddenly redefine expected working life Eventually there is the judgement of the point at which the horse can no longer sustain full duty and should be moved to more mild service, or retired to pasture Few of these variables are quantified, and even an expert on horses could not provide one single and firm projection on the eventual fate of a colt A useful alternative is to regularly examine the horse They carry within them some indication of the progression of wear, which may be useful for periodically adjusting wear life predictions The same applies to mechanical devices No complete and aeneral equation is available for predicting product life, and may never be available That, in spite of the more than 500 research papers published in the field every year! There are some empirical equations available now, such as for bearing selection, gear design, brake material formulation and the wear life of a few specialized items These equations come in many forms, some of which will be discussed in the next section Wear Eauations The Current Tvoes The most common types are emoirical in nature The word empirical is defined as "known from experience or experiment, used or applied without science" These are often derived by a least square fit of curves on a graph of wear rate or wear life versus some variable For cutting tools in lathes and milling machines, there are equations for tool life, t, in which the usual variables are feed rate, f, the depth of cut, d, and cutting speed, V The equations are written in several forms, one being: Vt n = fadbC Invariably the equations contain a "constant of proportionality", C, the numerical value of which depends on two choices, namely, an arbitrary choice of the point in time at which the tool is considered to be failed, and a choice of the units of the variables in the equation Tests also show that the influence of variations in f, d, and V on tool life are not linear, reflected most often by fractional exponents, n, a, and b Only "operational variables" appear in this equation - there is no mention of materials Thus any one equation applies to one particular tool shape, one tool material, one type of material being cut, and one condition of cut, eg., dry or with coolant It is also a "life" equation and not a wear rate equation, another distinction among "wear' equations A ohenomen01oaical equation by contrast contains some expression that is derived from "a manifestation produced by the action of different forces upon matter" i Examples would be equations that include more than operational variables, adding other effects not necessarily scientifically manipulated, but perhaps selected on the basis of what is thouaht to be the relevent chemistry or physics of the event under study One such model for the wear of ceramic materials relates the wear volume, V, per unit of sliding distance, s, to the normal contact pressure, P, fracture toughness, Kc, Youngs Modulus, E, and hardness, H, as follows: V = p1.125Kc-~176176 The variables and exponents arise from the assumption that those quantities that are operative in indentation by a sharp indentor are also operative in sliding wear Another equation, for polymer wear4, developed by way of dimensionless groups of material properties gives the volume of wear loss: Copyright by ASTM Int'l (all rights reserved); Thu Dec 31 13:22:02 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized 82 TRIBOLOGICAL MODELING FOR MECHANICAL DESIGNERS V = K",(1.775pl-47L1.2s/E3.225 where y is the surface energy, p is the contact pressure, L is the sliding distance and E is Youngs Modulus Hardness is not in this equation, and note that E is in the denominator instead of the numerator as in the previous equation! Phenomenological model building can sometimes produce converging results For example, nine equations are available for relating erosion and abrasion resistance with material properties as listed in the table below: Table Data (in bold face) from "Rules of Thumb "5 : Erosion resistance is found to be related to material properties as follows: ER ,~ H (hardness) E R , * E (Youngs modulus) (but we know to a first approximation that E = H) ER ,~ coefficient of expansion x B x !MP - TR) where; MP = melting point, T R = 20 C, B = bulk modulus, and since T R