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August 31 - September 2, 1998
The University Park Hotel, Salt Lake City, Utah
To view an abstract, please click the asterisk placed at the end of each abstract title.
Distinguished Keynote Paper
Session 1: Keynote Papers
Session 2: Fretting Fatigue Parameter Effects
Session 3: Environmental Effects
Session 4: Fretting Fatigue Crack Nucleation
Session 5: Material and Microstructural Effects
Session 6: Fretting Damage Analysis
Session 7: Life Prediction
Session 8: Experimental Studies
Session 9: Surface Treatments
Session 10: Applications
Distinguished Keynote Paper
Plastic Deformation in Fretting Processes
(top of page)R.B. Waterhouse
Department of Materials Engineering and Materials Design
University of Nottingham, U.K.
In recent analytical treatments of contacting surfaces and resultant fretting, the initiations and early propagation of fatigue cracks have been the subject of elastic stress analysis. However, direct observations of fretting damage in the optical and scanning electron microscopes indicates that plastic deformation of the contacting surfaces is usually an important feature. In this respect it has some similarity with other surface deformation processes such as shot peening and surface rolling, in that residual stresses are developed or existing stresses are modified. Surface films which are there as a result of oxidation or applied as an anti-fretting palliative can be seriously disrupted by plastic deformation of the substrate resulting in a "tribologically transformed layer" or third body interventions. Consideration of these factors can play a role in the development of methods to counteract the effect of fretting.
Fretting Fatigue in Practice; History and Practical Significance
(top of page)D.W. Hoeppner
Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84112.
In the last symposium on fretting fatigue an extensive review of the history of fretting fatigue was presented by the author. This paper will update the review with emphasis on engineering issues connected with fretting fatigue. The issues to be discussed include the occurrence of fretting fatigue, engineering approaches to design for fretting fatigue, means of characterizing fretting fatigue, and the development of alleviation systems for fretting fatigue.
One of the greatest challenges in dealing with fretting fatigue has been in the ways that it is or is not characterized when failed components are subjected to a failure analysis. Thus the issue of characterizing fretting fatigue on failed components also will be discussed from a fractographic point of view. The importance of understanding the identifying characteristics of fretting fatigue will be presented. The paper also will present areas where the fretting fatigue research community could focus efforts to assist the engineering community in designing to prevent and control fretting fatigue.
A New Approach to the Prediction of Fretting Fatigue Life
Considering Shift of Contact Edge by Wear (top of page)
T. Hattori
Mech. Eng. Res. Lab., Hitachi Ltd., Japan.
In fretting fatigue, it has been known that the contact edge will shift inward because of wear. The effect of the contact edge shift on crack propagation behavior is considered here to predict accurate fretting fatigue life. The stress intensity factor (K1) for fretting fatigue crack was calculated by using the contact pressure and frictional stress distributions. They were analyzed by finite element method. The S-N curves of fretting fatigue were predicted by using the relationship between the calculated stress intensity factor range (D K1) and the crack propagation rate (da/dN) obtained from CT specimens. Fretting fatigue tests were performed on Ni-Mo-V steel specimens. The S-N curves of experimental results were in good agreement with the analytical results obtained by considering the contact edge shift.
On the Standardization of Fretting Fatigue Test Method - Modeling Issues
(top of page)M.H. Attia
Adjunct Professor, Mechanical Engineering Dept., Concordia University
Montreal, Quebec, Canada
Predictions of the contact temperature, the state of stresses and the extent of the partial slip zone in fretting fatigue processes are critically important for proper design and testing of tribological systems. The temperature field around the micro-contact asperity affects significantly the material properties, its micro-structure, the oxidation process and thermoelastic stresses in the contact zone. Therefore, analytical models are required to predict contact temperatures during the fretting action. These models should take into consideration the partitioning of the frictional heat, the co-presence of adhesion and micro-slip zones, and the nonuniformity of friction and the micro-slip amplitude over the slip region. The paper presents a methodology for estimating the thermal micro- and macro- constriction resistance and the maximum contact temperatures under given design and test conditions.
The paper discusses also the modelling issues related to prediction of the distribution of the contact stresses and the extent of the micro-slip region. This information is required for standardization of test methods and for application of fracture mechanics principles to estimate the fretting fatigue strength and the fatigue life. The accuracy of the prediction is directly related to the accuracy of the mathematical description, and computer simulation of the physical phenomena which take place at the interface. In this paper, a three-dimensional interface element, which is formulated to model the material and geometric nonlinearities of the fretting fatigue process is presented. The integration of the interface element with a linear finite element code is demonstrated by analyzing the contact problem in conventional fretting fatigue testing.
Fretting-Wear and Fretting-Fatigue Relation through mapping concept (top of page)
S. Fouvry*, Ph. Kapsa*, L. Vincent**
Ecole Centrale de Lyon, CNRS, BP 163, 69131 ECULLY CEDEX, France
*LTDS (UMR 5513), ** IFoS (UMR 5621)
Developments in wear mapping recently described frontiers between partial and gross slip and between the main damages i.e. cracking and particle detachment both from experiences and from theoretical analysis. These maps known as running condition fretting maps (RCFM) and material response fretting maps (MRFM) are powerful tools to analyze the contact and to predict failures. Specially MRFM is a major contribution for designers and engineers. Mapping concept enables a new approach of fretting-fatigue as it considers crack nucleation for ranges of contact conditions. Effects of displacement and normal load on cracking are now well admitted under fretting wear conditions. The question is still related to the effect of the cyclic external loading superimposed the contact. Theoretical and experimental results related to homogeneous metallic contacts are discussed to attempt an objective analysis on the interest of fretting fatigue tests.
High Temperature Fretting Fatigue Behavior of an XDTM
g --base TiAl (top of page)T. Hannson*, M. Kamaraj**, Y. Mutoh*, and B. Pettersson#
*Department of Mechanical Engineering, Nagaoka University of Technology, Nagaokashi, 940-2188 Japan., **Institut Fur Werkstoffe, Ruhr-Universitat Bochum, D-44801 Bochum, Germany, #Materials Technology, Volvo Aero Corporation, S-461 81 Trollhattan, Sweden.
The plain and fretting fatigue behavior was studied of a g -base TiAl intermetallics at 675° C in air. The TiAl material was produced by the XDä process and had a nominal composition of Ti-47Al-2Nb-2Mn+0.8vol%TiB2. The contact material was Inconel 718. The reduction of fatigue strength by fretting was very small (around 20%) compared to metallic materials (30-60%). The plain and fretting fatigue S-N curves were flat, which resembled the behavior of ceramic materials. The flat S-N curves are corresponding to the very steep crack growth curve reported for TiAl-based materials containing TiB2. A fretting fatigue crack initiated in the very late stage of fatigue life (less than few percent of Nf) tor Steels. The fatigue lifetimes of this material were suggested to be determined by the cycles needed for initiation of a crack rather than by crack propagation.
Applications of Fracture Mechanics in the Assessment of Fretting Fatigue
(top of page)A.E.Giannakopoulos, T.C.Lindley and S. Suresh
A number of independent research studies have shown that fretting fatigue cracks can be initiated at a very early stage (<5-10%) of fretting fatigue life. The cracks propagate obliquely to the specimen surface under the combined action of tangential fretting forces and the cyclic body stresses. When the crack reaches a certain depth, which depends on the applied cyclic and mean body stresses, as well as the prevailing contact conditions (contact pressure, friction coefficient, relative slip amplitude), further crack propagation occurs perpendicularly to the applied stress since the frictional forces will then be insignificant. It is well established that as well as accelerating crack initiation, fretting can also cause an increase in the rate of crack growth. Additionally, a crack which might be dormant under pure fatigue loading might resume propagation under fretting fatigue conditions. Since under fretting, the crack initiation phase of life can be greatly reduced, we need to accurately predict crack growth and methodologies based on fracture mechanics provide an obvious staring point.
In fracture mechanics models, the stress intensity factor at the tip of a crack growing beneath a fretting contact will arise not only from the body stresses but also from contributions arising from the tangential and vertical forces due to the fretting contact. This composite stress intensity factor can be evaluated by several distinct methods (1) finite element stress analysis which might be necessary for the highly complex geometries found in practice (2) by using stress intensity factors arising from the tangential and normal forces at the fretting position using Green's flinctions (3) by the use of distributed dislocation methods. In these fracture mechanics models of fretting fatigue, it is necessary to have a knowledge of frictional forces which arise from the specimen or component contacts.
Crack propagation lives can then be estimated by integrating a fretting fatigue crack growth law which incorporates the composite D Kapp. In this manner, Edwards and his co-workers were able to successfully predict experimentally derived fretting S-N curves, thereby validating their fracture mechanics model of fretting fatigue. Alternatively, Lindley and his co-workers and independently Hattori compared the composite with the experimentally determined threshold in order to predict the balance between crack growth or arrest. These analyses will require a knowledge of both long and short crack behavior and the valid application of linear elastic fracture mechanics will need to be demonstrated.
By identifying certain aspects of equivalence between contact mechanica and fracture mechanics, the present authors [1] have recently suggested a "Crack Analogue" model for fretting fatigue. Here, an analogy is invoked between the geometry of the near-tip regions of cracked specimens and that between the sharp-edged contact region between two contacting surfaces. It can be shown that the asymptotic elastic stress and strain fields around the periphery of the contact region (as derived from classical contact mechanics) are identical to those given by linear elastic fracture mechanics for analogous geometries. In this model, the geometry of the contact pad/substrate system naturally introduces a fictitious crack length, thereby providing a physical basis for the analysis of crack initiation and growth in contact fatigue. Based on this approach, it has been possible to develop a new life prediction methodology for fretting fatigue.
[1] A.E.Giannakopoulos, T.C.Lindley and S.Suresh, Aspects of Equivalence between Contact Mechanics and Fracture Mechanics: Theoretical Connections and a Life Prediction Methodology for Fretting Fatigue, Acta Materialia, In the press.
Session 2: Fretting Fatigue Parameter Effects
(top of page)D.L. Anton*, M.J. Lutian#, L.H. Favrow*, D. Logane*, K. Brown^, and B. Annigeri*
* United Technologies Res. Ctr., E. Hartfbrd, CT 06108
#
Sikorsky Aircmft, Stratford, CT 06497^
Toiland, CT 06084A comprehensive evaluation1 of fretting fatigue variables was conducted on shot peened Ti-6A-4V forging material in the b -STOA condition in contact with 17-4PH pins, a material couple representative of helicopter dynamic component interfaces. Utilizing test equipment incorporating independent fatigue stress and fretting slip displacement control (as described elsewhere in this symposium), a test matrix spanning slip distances, d , of 25 < d < 75 m m and contact stresses, s f, of 50 <s f < 200 MPa. Fatigue stresses were used which resulted in cycle lives ranging from run out, >107, to 103. A flat against flat contact geometry was used with the contact area covering 10 mm2. The coefficient of friction was continuously monitored during cycling as well as the fretting energy expended per cycle. Representative fretting scars through out the test matrix were examined via serial section analysis and the crack number, density, location, length and flank angle noted through the scar volume. A 3-D elastic-plastic FEM analysis was performed at all test conditions to determine maximum principal and shear stresses generated in the near surface fretting region. Fretting fatigue endurance maps of contact stress vs. slip distance show that slip amplitude dominates mean fretting fatigue strength at 107 cycles under the tested conditions with contact stress playing only a moderate role. The fretting surface could be characterized as moderately pitted with dense third body debris. The debris was determined to be TiO2 with traces of Fe and the other alloying elements present. The crack nucleation mechanism generated many 10-20 mm cracks nonuniformly distributed on the fretting scar face. Critical cracks formed through a linking of smaller cracks across the fretting scar as evidenced by a number of nucleation sites on the fracture surface. The coefficient of friction, COF, was observed to increase form its initial value of 0.3 to a stable 0.75 through the first one to ten thousand cycles. The FEM analysis predicted maximum stresses to occur at the edge of the fretting contact, but outside of the contact. These results did not match the observed crack nucleation sites, indicating that modeling of simple fiat on fiat contact (without consideration of surface aspirates and/or debris accumulation) may not be sufficient as a design tool to predict fretting fatigue lives.
1This work was jointly funded by the Rotorcraft Industry Technology Association and the National Research Technology Center, and executed by Sikorsky Aircraft and United Technologies Research Center.
Length Scale Considerations in Fretting Fatigue
(top of page)D. Nowell, D.A. Hills, and R. Moobola
University of Oxford, Department of Engineering Science
Parks Road, Oxford, U.K., OX1 3PJ
The process of fretting fatigue may conveniently be divided into three distinct regimes: first the initiation (or nucleation) phase in which damage accumulates at the fretting interface and an embryo crack is formed; secondly there is normally a phase of short crack growth where the embryo crack grows in a manner which is influenced by the microstructure of the material and or microscopic conditions at the contact; thirdly there is a phase of long crack propagation where the crack growth may conveniently be described using the standard techniques of LEFM or EPFM. Of these three phases the third is the easiest to analyze. The crack tip is remote from the contact and standard techniques may be employed to predict the rate of crack growth from a knowledge of the applied stress field. Crack propagation often takes place in a rather complex stress field, with variable R-ratio and mixed-mode loading, but in principle, at least, it should be possible to predict growth rates in this regime, given appropriate standard plain fatigue crack growth data.
Analysis of the initiation process and the subsequent short crack propagation is much more demanding, yet it is in these regimes that the majority of life may be consumed and which may govern whether the life is finite or infinite. These issues are of practical importance to engineering designers. In attempting to analyze component behavior in these regimes it is important to understand a number of length scales which together control the material response and which define the appropriate form of analytical approach. The characteristic dimensions are (i) The overall size of the contact
(ii) The amplitude of slip
(iii) The size of individual asperity contacts
(iv) The average spacing of asperity contacts
(v) The grain size of the material
(vi) The threshold crack size for the long crack regime
By determining and comparing these length scales it is possible to highlight the important factors in any particular configuration. Our paper will explain the importance of each of the length scales and how they may be determined for practical geometries. For example, if the grain size is extremely small compared with the contact patch size, the short-crack regime is likely to be brief; if the maximum amplitude of relative slip displacement is several times the characteristic asperity spacing, we know that there are likely to be several load reversals per bulk cycle, at the asperity level; if the overall contact size is significant compared with the length at which the long crack regime is entered we infer that the contact stress field will have an important effect throughout the nucleation and short crack phases. Many other characteristics will be explored in the full paper. We will use our own experimental results to illustrate both the effect of the different scales on fretting behavior and the most appropriate analytical approaches to be used in representative cases.
Investigation of Friction Force in Fretting Fatigue
(top of page)W. Switek
Universidad de las Americas - Puebla, Departmento de Ingenieria Mecanica, Mexico
Fretting fatigue limit of a mechanical junction is effected by various parameters but most important of them are friction force, relative slip and clamping pressure which appear in the area of contacting surfaces. Unfortunately these three parameters are influencing each other and their individual effect is not easy to be established. In these paper a stress model of fretting fatigue is presented according to which fretting fatigue limit depends only on the stress distribution around the fatigue cracks developing in fretting fatigue process. That hypothesis have been proved by investigation carried out on carbon steel and stainless steel in the ambient conditions and in an oxygen-free environment. For the last research a special chamber has been built in which the fretting junction has been investigated in argon environment. The friction force and the relative slip between the specimen and the fretting pads have been measured by means of strain gauges. The results of investigation are in agreement with the theoretical formula developed by the author in previous works and prove predominate influence of friction force on the fretting fatigue limit of a mechanical junction.
A multiaxial fatigue analysis of fretting contact taking into account the size effect and the surface roughness
(top of page)S. Fouvry*, Ph. Kapsa*, L. Vincent**
Ecole Centrale de Lyon, CNRS, BP 163, 69131 ECULLY CEDEX, France
*LTDS (UMR 5513), ** IFoS (UMR 5621)
Cracks in contacts induced by fretting are uneasy to detect and can lead to catastrophic damages [1,2]. However, recent developments have shown that crack appearance in fretting contacts can be predicted by applying multiaxial fatigue criteria [3,4]. In fact the quantitative prediction is barely altered by the sharp gradient of stress which characterizes the contact loading. Based on classical bending and shear fatigue variables, the Dang Van multiaxial fatigue analysis of the partial Slip sphere/plane contact is compared with experiments realized on a well defined low alloyed steel displaying a very low surface roughness. It is shown that the crack nucleation prediction is achieved if the stress contact analysis is averaged on an elementary volume of about 5 to 6 m m edge. Such a critical elementary volume is analyzed in terms of probability to activate a micro-defect into the tempered microstructure. Fretting tests have been also realized for different surface roughnesses. It is shown that local stress discontinuities which are induced by the surface asperities strongly decrease the critical loading of first crack nucleation. The surface roughness effect on crack initiation is there more easily quantified through the Dang Van modeling.
References
[1] R.B. Waterhouse, Fretting Fatigue, 1981.
[2] D.W. Hoeppner, Mechanisms of fretting fatigue, ASTM, STP 1139, 1992, 23-32 pp.
[3) S. Fouviy, Ph. Kapsa, L. Vincent, K. Dang Van, Wear 195, 1996, 21-34 pp.
[4] K. Dang Van, ASTM STP 1191,1993, 120-130 pp.
The effects of load spectrum on the fretting fatigue behavior of Ti-6A1-4V
(top of page)S.E Kinyon*, D.W. Hoeppner**
*MTS Systems Corporation, 14000 Technology Drive, Eden Prairie, MN 55344-2290. **Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84112.
An investigation was performed to determine the effects of spectrum loading on the fretting fatigue life of a Titanium alloy to test the hypothesis that the alloy would follow Miner's Rule in Fretting Fatigue. The alloy was Titanium 6-aluminum 4-vanadium. The spectrum load cycle consisted of 6 conditions of interest. All tests were performed at 10 Hz. Baseline tests were performed at 3 stress levels, s1, s2, and s3, using a haversine wave form. Tests were performed using a spectrum load wave form consisting of 2 cycles, one at a high stress level, followed by another at a low stress level, and repeated to failure. Additional tests were performed using s1, and s2. The final condition of interest was investigated using a s1/s2/s3 wave form. It was found that some tests did not fail due to fretting, some specimens fretted at each stress level, while others did not. Miners rule was applied to each of the spectrum load cases to test It was found that one condition of interest, s1/s2, failed within 23% of prediction from Miner's Rule, based on average number of cycles to failure. The second spectrum load case failed within 30% of the predicted number of cycles. The final condition sl/s2/s3 was within 64%. It was hypothesized that Miners Rule may work for simple spectrum loading, but is insufficient when more complex wave forms are used.
Interaction of High Cycle and Low Cycle Fatigue on Fretting Behavior of Ti-6-4
(top of page)Rebecca Cortez1 and Shankar Mall2
1Air Force Institute of Technology, Department of Aeronautics and Astronautics
2
Air Force Research Laboratory, Materials & Manufacturing DirectorateWright-Patterson AFB, OH 45433
The fretting behavior of Ti-6Al-4V when subjected to combined high cycle and low cycle fatigue was investigated. Constant amplitude fretting fatigue tests were first conducted at room temperature under both 1 Hz and 200 Hz frequencies. The fretting fatigue life was smaller for the high frequency condition than the low one. Then, fretting tests under a complex axial loading combining the interaction of both low and high cycle fatigue were conducted which yielded slightly lower fretting fatigue lives than the constant amplitude tests. Microscopic examination of the fretting surfaces provided insights into the damage mechanisms prevalent under these different loading conditions. The presence of additional fretting wear interfaces were noted on the specimens tested under the variable amplitude loadings, thus, leading to an increase in the number of possible crack initiation sites, and slightly lower fretting fatigue lives.
(top of page)Sung-Keun Lee*, K. Nakazawa**, M. Sumita** and N. Maruyama**
*
Dong-A University, Korea., **National Research Institute for Metals, 1-2-1, Sengen,Tsukuba, Ibaraki 305, Japan.
Fretting fatigue of high strength steel was studied at various contact loads 'rains bridge type fretting cylinder pads of various contact curvatures. The span lengths of the bridge were 8-12 mm. The contact length of the cylinder pad was 6 mm. The contact curvature radii were 15, 30 and 60 mm. The contact load was maintained below 2 kN. The fretting fatigue life exhibited a minimum at a certain contact load irrespective of the curvature radius of pad. Under a given contact load, the fretting fatigue life showed a tendency to increase with an increase in curvature radius. These results were discussed with various mechanisms proposed so far.
An Experimental Investigation of Fretting Fatigue with Spherical Contact in 7075-T6 Aluminum Alloy
(top of page)B. Wittkowsky, P. Birch, L Dominguez and S. Suresh
Department of Materials science and Engineering, Massachusetts institute of Technology,
Cambridge, MA 02139, USA.
A new fretting fatigue loading system which facilitates continuous monitoring and control of such parameters as normal and tangential contact loads, and cyclic displacements m addition to the fatigue loading Parameters as the cyclic stress amplitude, frequency and load ratio has been designed and built.
While the device can, in principle, accommodate different contact geometries, systematic and controlled experiments have been carried out first for the case of a spherical-tip contact pad in contact with a flat surface. The pad and specimen material chosen for the initial investigation has been 7075-T6 aluminum alloy. For this pad geometry, an analysis of the mechanics of the fretting test has been carried out, as well as a study on the of the influence of a number of fretting parameters on the fatigue life time has been experimentally investigated.
The geometry of fretting scars and the stick-slip annuli have been established for the different contact conditions employed. An approach has been developed to determine the eccentricity produced by the axial load on the stick zone, which correlates very well with the experimental results. Metallographic and fractographic examinations have been done, and the experimental results have been critically examined in the light of a variety of known multiaxial fatigue criteria. An attempt has been made to rationalize, in a quantitative manner, the mechanistic origins of fretting fatigue evolution in the commercial alloy.
Session 3: Environmental Effects
Fretting Fatigue of Inconel Alloys at High Temperature
(top of page)M.H. Attia
Principal Research Engineer, Ontario Hydro Technologies, Toronto, Ontario, Canada
Fretting damage can accelerate crack initiation and propagation processes and may lead to substantial decrease in the fatigue strength of the material. This is primarily due to the addition of alternating frictional stresses, and the continuous destruction of the surface oxide film. Data pertaining to the fretting fatigue characteristics of Ni alloys that are commonly used in the nuclear industry is very limited. In the present paper, plain and fretting fatigue tests were conducted to establish the fretting fatigue characteristics of Inconel 600 against carbon steel and Incoloy 800 against 41 OS stainless steel in steam environment at 265° C.
The results indicated that the reduction factor in the fatigue strength of Inconel 600 alloy due to fretting is in the range of 2.2 to 3.5, depending on the loading conditions. The results showed also that Incoloy 800 is more susceptible to fretting fatigue than Inconel 600 by approximately a factor of 1.5. The effect of the main process parameters, namely, contact and mean stresses on fretting fatigue strength has been established. A model for extrapolating the test results to other conditions that have not been tested is also presented.
Fretting fatigue of 8090-T7 and 7075-T651 aluminum alloys in vacuum and air environments
(top of page)C.B. Elliott* and A.M. Georgeson**
*Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84112.
**
The Boeing Company, Seattle, WA 98124-2207.Personnel from the Quality and Integrity Design Engineering Center (QDEC) at the University of Utah, investigated the fretting fatigue characteristics of 8090-T7 and 7075-T651 aluminum alloys. The purpose of this investigation was to determine the relative effects of wear and environmental degradation from laboratory air on the fretting fatigue process. Alcoa provided funding and the 8090 alloy used in this research.
The fretting fatigue test system used in this experimentation was developed within QDEC to enable testing either in air or a scanning electron microscope (SEM) vacuum environment. This allows investigation of fretted Surfaces of specimens tested in the SEM without the SEM vacuum being broken. Therefore, fretting fatigue damage in these tests can be considered as resulting from wear only, whereas damage from tests in air result from the concurrent wear and oxidation mechanisms. A paper on this test system was reported at the April 1993 International Conference on Fretting Fatigue in Sheffield, England. Based on this experimentation, it was concluded that:
·
Degradation of 8090-T7 fatigue life in air due to fretting is caused primarily by wear mechanisms.·
Degradation of 7075-T651 fatigue life in air due to fretting is caused primarily by environmental mechanisms.·
Fretting fatigue behavior is material dependent.Session 4: Fretting Fatigue Crack Nucleation
Nucleation In Fretting Fatigue
(top of page)J. Woodtli, O. von Trzebiatowski, M. Roth
EMPA, Swiss Federal Laboratories for Materials and Testing, 8600 Dubendorf, Switzerland.
Fretting damage often participates as a nucleation process in failures caused by multiple site damage (MSD). Fretting itself is extremely complex degradation process consisting of a combination of mechanical and chemical attacks. In order to avoid damage it is essential to understand which dominating parameter influences the crack nucleation.
The topography of the fracture surface, as well as changes in microstructure provide information about the dominant damage mechanism. By microscopic and metallographic investigations an exact differentiation of the predominant damage mechanism involved can be established.
Two service failures in shafts for ski lifts, which suffered fretting fatigue, are discussed. Though the macroscopic findings are very similar, the microscopic investigation indicates a completely different crack nucleation mechanism.
Experimental Study of Fretting Crack Nucleation in Aerospace Alloys with Emphasis on Life Prediction
(top of page)M. P. Szolwinski, G. Harish, P. A. McVeigh and T. N. Farris
School of Aeronautics & Astronautics, Purdue University, USA.
Fretting, a synergistic damage mechanism experienced at clamped, contacting surfaces in a wide variety of mechanical systems subjected to oscillatory loading, plays a particularly critical role in the nucleation of fatigue cracks in aging aircraft systems. From riveted aluminum lap joints in the fuselage to titanium dovetail/disc assemblies in turbine engines, the localized near-surface stresses, strains and surface microslip associated with fretting contact lead to a detrimental tripartite combination of corrosion, wear and fatigue phenomena. An understanding of the mechanics of fretting crack nucleation is crucial to those interested in not only airframe structural design but also technologies for life prediction and maintenance of aging aircraft Systems, both civilian and military.
A statistically-designed experimental program engendered to validate an analytical approach for the prediction of fretting crack nucleation in 2024-T35 I aluminum alloy has been completed. Details of the analytical method [1], experimental setup [2] and potential application to riveted structural joints [3] are available elsewhere. The test results indicate that the cyclic fretting contact stress and strain field can be juxtaposed with a multiaxial fatigue model relying on uniaxial strain-life constants to predict crack nucleation for a wide range of load intensities and conditions representative of those experienced in riveted joints. Companion to these results will be a tribological assessment of the fretting damage, including quantification of the evolution of friction coefficient over the life of the tests and inspection of the wear and corrosion damage at the contact surface.
In light of these encouraging results, efforts are underway currently to extend this life prediction tool to fretting and high-cycle fatigue in titanium alloys subjected to loading
conditions more representative of aircraft engine dovetail/disc assemblies. Included in this approach are efforts to model the tribological conditions at the blade/disc interface and determine stress intensity factors associated with near-surface cracks. The paper concludes with a synopsis of both numerical and experimental results from this effort.
References
[1] Szolwinski, M. P. and Farris, T. N., "Mechanics of Fretting Crack Nucleation," Wear (198). 1996, pp. 93-l07.
[2] Szolwinski, M. P., Harish, G., McVcigh. P. A. and Farris, T. N., "The Role of Fretting in the Onset of Widespread Fatigue Damage: Analysis and Experiments," in Proceedings of the FAA -NASA-Symposium on the Continued Airworthiness of Aircraft Structures, Atlanta, Georgia, August 1996, pp 585-596.
[3] Harish, G. and Farris, T. N., "Modeling of Skin/Rivet Contact: Application to Fretting Fatigue." in Proceedings of the 38th AIAA/ASME/ASCE/AHS/ASC, Structures. Structural Dynamics, and Materials Conference", Orlando, Florida, April 1997 (in press).
Crack Behavior in the Early Stage of Fretting Fatigue Fracture
(top of page)K. Kondoh and Y. Mutoh
Department of Mechanical Engineering, Nagaoka University of Technology, Japan.
In the ultra high cycle fatigue regime (>107 cycles), fretting fatigue cracks initiated under stresses lower than "the conventional fretting fatigue limit", which was determined at 107 cycles. They could propagate without arresting at the lower stress levels. However, the estimated mode I stress intensity factor, where the tangential force due to fretting action was taken into connsideration, was lower than the mode I threshold stress intensity factor. It is well known that the short crack modification of threshold value gives a reasonable estimation for fatigue strength mix-mode crack growth should be also taken into account in the early stage crack growth under fretting fatigue condition. The FEM analysis indicated that the mode II component of stress intensity factor under fretting condition increased with reducing crack length and with increasing crack angle. The crack growth behavior observed at the stresses lower than the fatigue limit could be successfully estimated by combining the short crack effect and the mix-mode effect.
Session 5: Material and Microstructural Effects
Influence of Microstructure on Fretting Fatigue Behavior of Near-
a Titanium Alloy (top of page)T. Satoh
Technical Research & Development Institute, Japan Defense Agency, Tokyo, 190 Japan.
Fretting fatigue tests using near-a titanium alloy for compressor blade and disk for jet engine were carried out. In order to investigate the influence of the microstructure on fretting fatigue behavior, titanium alloys which have two kinds of microstructure such as equiaxed alpha + fine lamellar and acicular alpha were used. The fretting fatigue crack initiation sites and fatigue crack paths were observed by means of scanning electron microscope in detail. Effect of contact pressure on fretting fatigue life was also investigated. Fretting fatigue lives for the material had acicular alpha microstructure are longer than those of the material had equiaxed alpha + lamellar microstructure. Fretting fatigue lives decreased with increase of the contact pressure.
(top of page)Alisha L. Hutson1 and Ted Nicholas
Metals, Ceramics & NDE Division, Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRLIMLLN), Wright-Patterson AFB, OH.
1
University of Dayton Research Institute, Dayton, OH.A two phase study was conducted to evaluate fretting fatigue damage in a flat on flat contact at room temperature. First, the axial stresses necessary to fail specimens at 10 million cycles for different contact radii, applied normal stresses, and stress ratios were evaluated. Then, the effect of fretting on unfretted specimen life was quantified by conducting interrupted fretting tests for various load ratios and normal stresses followed by residual strength uniaxial fatigue tests. Ti-6A1-4V was used throughout the study. Stress concentrations at the edge of contact were quantified using FEA. Metallography and fractography were used to qualify fretting damage.
Fretting Fatigue Strengths of Forged and Cast Al-Si Aluminum Alloys
(top of page)T. Nishida*, Y. Mutoh** and O. Ebihara#
*Department of Mechanical Engineering, Numazu College of Technology, 3600 Ooka, Numazu, 410 Japan., **Department of Mechanical Engineering, Nagaoka University of Technology, Nagaokashi, 940-2188 Japan., #Topy Industries Limited, Toyohashi 441, Japan.
Plain fatigue and fretting fatigue tests were carried out using cast Al-Si aluminum alloy JIS AC4CH-T6 as well as forged aluminum alloy JIS 6061-T6. Plain fatigue strength of the cast aluminum alloy was lower than that of the forged one. The lower fatigue strength was attributed to porosity in the cast material, which enhanced fatigue crack initiation. On the other hand, fretting fatigue strength of the cast aluminum alloy almost coincided with that of the forged one. No difference of tangential force coefficient and then tangential force in fretting fatigue was observed between two materials. From fatigue crack growth tests, fatigue crack growth curves for both materials were also found to coincide with each other. According to the results of fretting fatigue tests interrupted at various fatigue cycles, fretting fatigue cracks initiated in the very early stage of fatigue life at the edge of fretting region for both materials and porosity in the cast material had no influence on fretting fatigue crack initiation process. These phenomena and properties result in the agreement of fretting fatigue strength between the cast and forged materials.
Session 6: Fretting Damage Analysis
Analysis of Fretting Damage Using Confocal Microscope
(top of page)V. Chandrasekaran, Young In Yoon and D.W. Hoeppner
Department of Mechanical Engineering, University of Utah, Salt lake City, Utah, 84112.
Fretting fatigue experiments were conducted on 7075-T6 and 2024-T3 Aluminum alloy specimens. The primary objective of this study was to quantitatively characterize fretting damage that resulted on the fatigue specimens. Fretting fatigue experiments were performed in laboratory air at various maximum fatigue stress levels at a constant normal pressure. The hypothesis of this study was that the intensity and the nature of fretting damage would vary depending upon the applied maximum fatigue stress and the three dimensional nature of the damage that would result from fretting could be quantified. Fretting fatigue experiments were interrupted at a predetermined number of cycles to analyze the damage on the fatigue specimens. Confocal microscopy was used to analyze and quantify fretting damage. Digitized images of fretting damage were obtained from the confocal microscope, using a pixel counting software package which also allowed length measurement of fretting induced cracks on the faying surface of the fatigue specimen. In addition, fretting damage was quantified in terms of material removal by characterizing the depth as well as the geometry of fretting-generated pits on the faying surface of the specimen. Pit size in terms of pit depth (Pd), pit area (PA), and pit dimension perpendicular (PDy) as well as parallel (PDx) to the applied load also were quantified. From the confocal microscopy analysis of fretting damage, it was observed that fretting-generated multiple cracks on the faying surface could be responsible for the fracture of 7075-T6 aluminum alloy specimens where as the fracture of 2024-T3 aluminum alloy specimen could be attributed to fretting-generated multiple pits on the faying surface. From the results, it is proposed that fretting nucleates damages of different nature depending on the material microstructure as well as its composition and the methods to alleviate fretting should consider issues pertaining to a specific material.
Analysis of Fretting Damage in Polymers by Means of Fretting Maps
(top of page)Chateauminois*, M. Kharrat** and A. Krichen**
*Laboratoire d'Ingenierie et Fonctionnalisation des Surfaces, UMR 5621, Ecole Centrale de Lyon, BP 163-69131, Ecully, France., **Laboratoire de Physique et Mecanique des Materiaux, Ecole Nationale d'Ingenieurs de Sfax, BP 3038, Sfax, Tunisie.
Fretting maps concepts have been widely used to predict fretting wear and fretting fatigue damage in metallic materials. They have proved to be an useful tool to predict the main initial damage (crack nucleation, wear) in relation to the local contact loading and the material properties.
The various problems related to the development of fretting maps in the context of polymer fretting are addressed in this paper. In-situ monitoring of the contact zone kinematics and the fretting damage was carried out using model systems consisting in thermoplastic and thermoset polymers rubbing against glass counterfaces. Depending on the polymer properties, the main damage was related either to cracking (epoxy thermosets) or particle detachment (PMMA). Data regarding the contact loading and the damage were synthesized in maps. The effects of viscoelasticity, adhesion, bulk properties and contact zone kinematics on the boundaries of the various domains of these maps are discussed in the light of numerical simulations of the fretting test.
Practical Optical Characterization of Fretting Damage in Turbine Engine Fan Disk
(top of page)S. C. Gustafson
Visiting Scientist, Air Force Research Laboratory, Materials and Manufacturing Directorate AFRL/ML, Wright-Patterson AFB, OH 45433.
Fretted surfaces may develop on the dovetail joints that secure blades in turbine engine fan disks. These surfaces may form sites for crack nucleation and subsequent disk and engine failure. Thus a practical and reliable method for the characterization of these surfaces is needed. In this paper straightforward optical techniques are considered for identifying and quantifying fretted regions. For example, digital images of candidate surfaces may be processed to separate fretted from unfretted regions and to evaluate the relative degree of fretting. Ideally, this optical characterization should be rapid, convenient, inexpensive, reliable, and insensitive to the details of lighting and image capture.
(top of page)R.W. Neu and J.A. Pape
Georgia Institute of Technology, George W. Woodruff School of Mechanical Engineering, Atlanta, Georgia 30332-0405
Within the last 20 years, models for predicting either the nucleation or propagation of fretting fatigue cracks have been developed and verified. The simplest nucleation model is the Ruiz Criterion (or also known as the Fretting Fatigue Damage Parameter). Fracture mechanics methods have been used to predict fretting fatigue crack growth. This paper describes methodologies that could potentially be used to bridge these approaches. Multiaxial fatigue criteria based on critical plane approaches are viable alternatives to the Ruiz criterion for predicting fretting fatigue crack nucleation. These approaches have a better potential for describing the physical nature of the damage including the orientation and growth direction of the crack nuclei. These criteria may be further linked to microstructural fracture mechanics approaches that could be used to predict propagation especially during the early stages of crack propagation.
Session 8: Experimental Studies
Fretting Fatigue Testing Methodology Incorporating Independent Slip and Fatigue Stress Control
(top of page)L.H. Favrow*, D. Werner**, K. Brown#, M.J. Lutian^, B. Annigeri* and D.L. Anton*, *United Technologies Research Center, 411 Silver LaneE. Hartford, CT 06108., **Epsillon Inc., Jackson Hole Wyoming, ^Sikorsky Aircraft, Stratford, CT 06497., # Tolland, CT 06084.
To address practical limitations on separation of critical variables for fretting fatigue research, an advanced fretting fatigue apparatus was designed, built and tested1 incorporating unique independent computer control of fretting fatigue slip distance and fatigue stress. This was accomplished through utilization of two coaxial servo-hydraulic test actuators controlled in real time by computer. The central hydraulic actuator applies the fatigue load to the test specimen, while the outer hydraulic actuator moves the fretting pin carrier apparatus. Independent control of slip displacement is achieved with the use of a capacitance displacement gage attached to the specimen and fret pin carrier in such a manner that relative displacements of ~5 um can be controlled. The fatigue specimen is typically sinusoidally loaded in load control, while the fretting slip displacement is similarly programmed to be sinusoidal and in phase with the fatigue loading. Capacitance gage measurements indicate the relative motion of the fatigue specimen surface caused by loading with respect to the fret pin. The fret pin carrier is subsequently moved to accommodate this motion plus its own programmed motion. Load cells are provided both above and below the fatigue specimen allowing for measurement, by difference, of the forces applied by the fretting pins. These forces can be used to calculate the dynamic coefficient of friction during test operation. Readings of specimen loading, fret pin motion, fret pin loading and ram displacements are continuously recorded at 60 Hz and stored for future analysis. Finally, a 3-D finite element analysis model was constructed of the fatigue specimen and fret pin to determine analytically the slip occurring at the fatigue specimen surface within the bounds of test operation.
1
The fretting fatigue testing/modeling portion of this work was jointly funded by the Rotorcraft Industry Technology Association and the National Research Technology Center, and executed by Sikorsky Aircraft and United Technologies Research Center.
An Analysis of Rotating Bending Fretting Fatigue Tests using 'Bridge' Specimens
(top of page)G. Demello, M. Ciavarella, D.A. Hills'
Diparimento di Progrettazione e Produzione Industriale, Politecnico di Bari, Italy.
'Department of Engineering Science, Oxford University, Parks Road, Oxford, OXI 3PJ. UK.
The 'bridge' style of fretting fatigue test remains popular as it is possible to conduct tests very quickly. The most troublesome part of the experiment is analyzing the contact conditions and associated stress state. The 'pads' on the bridges are conventionally made flat, and hence the contact with the main specimen is complete. Further, a half-p1ane formulation cannot be employed for the pad, as there is no supporting material either side the contact itself, and most idealizations give rise to the result that the contact is either in full adhesion or full slip (sliding). The problem is further compounded by the influence of bending of the specimen itself, which means that the contact pressure is increased on one side of the pad and may be depleted slightly on the other.
Hitherto, only a limited number of numerical solutions, based on the boundary element method, have been published on this problem. In this paper we re-examine several aspects of the problem, using an analytical formulation. The pad is assumed to consist of a central flat portion, with a blend radius at each end, and this means that the contact is incomplete, and hence capable of being attacked using a half-plane formulation. This permits a closed form solution to be generated for the contact pressure, the internal stress state, and the corresponding partial slip state, whose stability in considered. Also, the influence of a rotation of the beam specimen (due to bending) with respect to the pad geometry is taken into account, and the effect this has on making the contact pressure asymmetric is accurately represented.
Turning to the distribution of interfacial shear, because the transition from full stick to full slip is quite abrupt, with only a relatively small range of shearing force over which partial slip occurs, it is important to know the relationship between the shearing force generated and the stiffnesses of the elements composing the apparatus, viz. pad, bridge, contact and specimen, and this question is addressed in detail. We therefore present results for all the stiffness contributions needed, and hence a design procedure for the apparatus is generated.
Lastly, with the internal stress state known, it is relatively straightforward to solve for stress intensity factors for cracks developing from the contact, using a distributed dislocation approach, based on a kernel for a dislocation in a strip, and hence solutions are also provided for this. It is shown that with an indenter of the shape described the possibility of crack self arrest, as the crack tip grows out of the region controlled by the contact stress field, is much greater than it is for a Hertzian contact, and hence this type of test provides a much better opportunity for investigating this phenomenon.
Evaluation of Fretting Stresses Through Full-Field Temperature Measurements
(top of page)G. Harish*, M.P. Szolwinski*, T. Sakagami** and T.N. Farris*
*School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907-1282., **Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565 Japan.
The near-surface stress field in fretting has long eluded experimental characterization due in large part to the fact that the friction coefficient in the slip zones associated with partial slip contacts cannot be evaluated from measured forces. Attempts made at circumventing this issue through measurements of the level of microslip or extent of the slip zones have proven inconclusive. However, newly-available infrared technology is capable of resolving finely, both spatially and temporally, subsurface temperatures near the fretting contact. These temperatures are induced by both frictional heating due to the microslip and the coupled thermoelastic effect arising from strains in the material.
The temperature histories collected to date for contacts between flat and cylindrical aluminum surfaces show clearly the transition from global sliding to partial slip under constant load amplitude as the friction coefficient increases with the number of loading cycles. During gross sliding, the thermographic sequences reveal a patch of heating throughout the contact length whose temperature magnitude can be correlated with moving heat source theory using measured loads, sliding velocity and the Hertzian contact length. As the friction coefficient increases and the contact transitions to a partial slip regime, the temperature rises are more clearly associated with strain through the coupled thermoelastic effect.
We have developed a finite element model for fretting that includes the heat generation due to both sliding and partial slip, and the coupled thermoelastic effect. The finite element model includes the coupled heat conduction and thermal deformation problem. The correlation between temperature rise measured from the infrared radiation and the temperature changes calculated by finite elements is remarkable. This comparison fully validates both the finite element calculations, which include the compliance of the experimental rig, as well as ancillary experiments that estimate the evolution of friction coefficient with history. These finite element calculations then contain fully validated values for fretting induced stresses and microslip.
The talk will emphasize both the development of the technique for using measured temperatures to infer stress as well as its detailed application to fretting. The technique has proved very successful for fretting of aluminum and efforts are underway to apply it to fretting of titanium. It is possible that the technique can be used to resolve the peak in contact pressure at the edges of nominally flat contacts.
(top of page)M. Dubourg and V. Lamacq
Laboratoire de Mecanique des Contacts, Villeurbanne Cedex, France.
Fretting damage is a reel preoccupation for various industrial situations. The most damageable phenomenon linked to this contact loading is a great decrease in component life. This decrease is the consequence of the early initiation and propagation of fatigue cracks under cumulative effects of contact and fatigue loadings. The understanding of crack initiation and propagation under these loading conditions is thus a necessary step to propose palliatives against this damage.
Crack propagation during stage I and branching mechanisms under fretting fatigue condition have been investigated in previous papers by the authors (Lamacq et al., 1997). Crack location and initial growth directions during stage I were thus predicted. Two major crack initiation mechanisms were experimentally observed and theoretically identified. Further conditions governing transition from stage I to stage II, revealed by a branching towards a new propagation direction, were identified through a subsurface layer stress analysis. These predictions are in very good agreement with experimental observations.
This study aims now at determining the direction and the mode of propagation of a network of cracks during stage II under fretting fatigue loading. This loading induces non proportional conditions at crack tips. Very little attention in the literature is focused to that problem and most of existing criteria, except an approach proposed by Hourlier, are adapted to proportional loading conditions and further inadequate to predict crack growth direction under these conditions. To answer this problem, tests were conducted on three aeronautical aluminum alloys. A new set of experiments were then carried out with epoxy transparent photo elastic specimen (Lamacq et al, 1996). The latter was submitted to similar fretting fatigue conditions.
Parallel to these experimental studies, the behavior of a crack network was simulated under fretting fatigue conditions in an aluminum alloy. The fretting contact problem was solved as a unilateral frictional contact problem, at the specimen surface. Each loading cycle was incrementally described. The stick-slip zone repartition and pressure distribution were then determined in the contact area. Dubourg fatigue crack model (Dubourg et al., 1992a,b) was used to describe the cracked body response under this fretting fatigue loading. Contact conditions at crack faces were determined, again through a unilateral contact analysis with friction. Stress intensity factors KI, KII at crack tips and stress fields along crack interfaces were deduced. Propagation directions during stage II are determined according to D s q q *, the effective amplitude of the tractive tension perpendicular to the crack trajectory. Similar directions are obtained by using the Hourlier's approach. Both are in good agreement with experimental data. A main crack propagates thus along the direction optimizing its opening and leading to the maximum amplitude of the opening. The mode I is therefore dominant while the mode II influence is minimized. The stress field analysis shows that the crack trajectory borders the tractive-compressive and the tractive zones existing around the crack tip. The conditions governing stage II propagation are therefore by continuity, the conditions, that we already identified, leading to stage I-stage II transition.
Lamacq V., Dubourg M.-C, Vincent L.. 1997, "A theoretical model for the prediction of fretting fatigue crack initial growth angles and sites", Tribology International,
Lamacq V., Dubourg M.-C, Villechaise B., 1996, "Fretting fatigue crack growth analysis an experimental photoelastic method combined with a numerical model", ECF 11, Mechanisms and Mechanics of Damage and failure, Poitiers 3-6 Sepptembre, 1996, p 1387-1392.
Dubourg M.-C., Villechaise B., 1992a, "Analysis of multiple cracks - Part I: Theory", ASME, Jnl of Tribology, Vol.114, pp 455461.
Dubourg M.-C., Godet M., Villechaise B., 1992b, "Analysis of multiple cracks - Part II:
Theory", ASME, Jnl of Tribology, Vol.114, pp 462468.
Development of a High-Temperature-Steam Fretting Wear Test Apparatus
(top of page)M. P. Blinn* and J. Lipkin**
*M.C.L. Corporations Park - Bldg. 704 Scotia, New York 12302.,
**
General Electric Power Generation, Schenectady, New York.
Power generation requires machinery that can withstand the effects of an aggressive operating environment. In particular, the effects of wear on moving part is always a concern in power generating equipment, especially if the wear leads to down-time of the equipment. Obtaining realistic information on the wear characteristics of candidate materials used in power generating equipment is vital for selecting the best materials for wear resistance. Overall, research and development of wear resistant materials for power generating equipment can reduce operating and maintenance time, leading to more cost efficient systems.
A prototypical, multi-specimen fretting wear test apparatus has been developed and verified for use at elevated temperature of up to 595° C (1100° F). In addition to high temperature capabilities, a unique feature of this fretting wear test apparatus is the ability to test in a super-heated steam environment Contact stresses of up to 6.9 MPa (1 ksi) can be applied to wear samples using this test apparatus. Finally, double amplitude motions of up to .25 mm (0.01 in) can be applied to wear samples using this tail apparatus.
In general, the high temperature steam fretting wear test apparatus is a stainless steel test chamber, with all openings to the test chamber sealed mechanically, or by the use of gaskets. Side doors, with high temperature glass inspection ports, are provided for access inside the test chamber. Heat to the test apparatus is supplied by the use of cartridge and strip heater units, mounted internal and external to the test chamber. The test temperature is maintained by the use of a temperature controller, servo-controlled by the use of thermocouples inside of the test chamber. Steam is generated external to the test chamber by the use of a specially designed system, with steam entering through an inlet tube, and existing to a condensing tube at the opposite end of the test chamber.
A table, supported on four sides by the use of linear bearing external to the test chamber, is used to hold the two moving samples that are tested. Welded metal bellows, internal to the test chamber, are used to enclose the table arms that exit the steam chamber. Horizontal actuation of the table is by the use of a 6.7 kN (1500 lb.) electro-magnetic shaker, external to the test chamber. Two static samples, coupled with the moving samples1 are mounted on separate fixtures. Two static sample fixtures are supported laterally, but can move vertically above the table, and are mounted directly to 4.5 kN (1000 lb.) pneumatic cylinders. Thus, sliding motion is supplied by the table-actuator, whereas the normal force is generated by separate fixtures-pneumatic cylinders. The normal arid friction forces are monitored by the use of load cells, mounted external to test chamber.
Application of the Method of Caustics to Fretting Fatigue
(top of page)K. Sato and K. Kounosu
Department of Mechanical Engineering, Chiba University, Japan.
This paper describes the modeling and theory for applying the method of caustics to contact analysis in fretting fatigue. The conventional Hertzian contact model and Mandolin's contact conditions are combined with the theory of caustics. Caustic images on a reference screen created by light rays passed through points around the contact interface between a specimen and a fretting pad has been simulated by this approach. By using calibrated relationships between a caustic image parameter Dx and normal contact load P, and between another parameter Dx and tangential contact load Q, we can evaluate P and Q during fretting fatigue tests. This can also evaluate the ratio of stick width to contact width relating to fretting crack initiation.
Fretting Fatigue Behavior of TiN Coated Steel
M. Okane, K. Shiozawa and T. Ishikura
Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama, 930 Japan.
Fretting fatigue tests of carbon steel coated with titanium nitride (TiN) by PYD method were carried out to discuss the effect of ceramic coating on fretting fatigue behavior. Fretting fatigue strength of the present carbon steel significantly increased by TiN coating onto the specimen. The fretting fatigue fracture process in TiN coated steel was as follows. Fretting cracks initiate at a certain stage of fatigue life from the flaws in TiN film initiated by fretting action, and propagate until final unstable fracture occurs. The improvement of fretting fatigue strength by TiN coating resulted mainly from the retardation of fretting fatigue crack initiation due to existence of hard TiN film on the contact surface of the specimen.
(top of page)M. Kubota*, K. Hirakawa* and T. Makino**
*Department of Mechanical Science and Engineering, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki Higasi-ku, Fukuoka, 812 Japan., **Integrated Research Labs., Sumitomo Metal Industries, Ltd., 6-18 Fusoh-tyoh, Amagasaki 660 Japan.
A push-pull fatigue loading is given to a tensile specimen with a flat part. A bridge pad which contacts with the specimen surface give a fretting conditions. In conventional fretting fatigue test, the shape of the contact edge of the pad has been paid little attention and the contact edged failure of the specimen has been reported. The objective of the present paper is to clarify the effect of the shape of the contact pad on fretting fatigue strength. Round and sharp edge pad is used in the fretting fatigue testing, and the effect on the fatigue strength and the type of cracks are discussed from the viewpoint of stress states. The effect of molybdenum coating on the fatigue strength is also discussed.
Influence of Surface Treatments on Fretting Fatigue of Titanium Alloys at Elevated Temperatures
(top of page)S. Chakravarty*, J.P. Dyer*, J.C. Conway Jr. **, and P.C. Patnaik*
*Orenda Aerospace Corporation, Advanced Materials and Energy Systems, 1420 Blair Place, Suite 608, Gloucester, Ontario, Canada K1J 9L8., **The Pennsylvania State University, U.S.A.
Where fretting fatigue of titanium alloy gas turbine engine components is concerned, the effect of temperature on fretting process can be quite significant. Service exposed third stage compressor blades, which operates at 496° C or 925° F, showed severe signs of fretting, wear, Cu-Ni-In coating delamination, oxidation and pitting on the dovetail pressure surfaces. Under cyclic loading conditions fretting fatigue cracks initiated in the damages area, can cause dramatic reduction in fatigue life of these components leading to potential catastrophic failure. This paper discusses the influence of various surface treatments on the fretting fatigue life of titanium alloys at elevated temperatures. Fretting fatigue and high temperature wear test results will be presented. Significant contrast has been observed between high temperature fretting fatigue and wear characteristics of surface treated alloy. Finally, an attempt has been made to develop a correlation between the microstructure of the surface treated layer and the fretting fatigue characteristics of titanium alloys to obtain an insight of the effects of various surface treatments on the fretting fatigue mechanism at high temperature.
Fracture Mechanics Approach to the Fretting Fatigue Strength of Axle Assemblies
(top of page)T. Makino*, M. Yamamoto* and K. Hirakawa**
*Applied Mechanics and Systems Research Department, Sumitomo Metal Industries, Ltd., 1-8 Fuso-cho, Amagasaki, Hyogo, Japan 660., **Department of Material Science and Engineering, Kyushu University, 6-10-1 Hakozaki Higasi-ku, Fukuoka, 812 Japan.
The objective of the present paper is to evaluate the fatigue crack growth behaviors in press-fitted axles by using a fracture mechanics approach and to enable the prediction of the fatigue strength regarding crack propagation (s w2). The relationship between nominal bending stress (s n) and non-propagating crack length in press fitted axles is also discussed. Rotating bending fatigue tests were conducted on the induction hardened and quench-tempered axles of 40 mm in diameter. The equation for D K was formulated from the result of FEM analyses in which micro profile at contact edge was taken into consideration. The threshold stress intensity factor range D Kth for small cracks was estimated from the crack size measured after the fatigue tests with a modified stress ratio effect at fully compression stress reversals due to high compression residual stress. s w2 and the relationship between s n and non-propagating crack length were predicted by using the above mentioned D K and D Kth. The predicted non-propagating crack length was in good agreement with the experimental value.Fretting in Aeronautical and Aerospace Structures and Materials
(top of page)T.N. Farris, M.P. Szolwinski and G. Harish
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907-1282.
Fretting, the deleterious and synergistic combination of wear, corrosion and fatigue phenomena driven by the partial slip of tribosurfaces, has been attributed to severe reductions in service lifetimes of a myriad of contacting components, including bearings, turbine blades and mechanically-fastened joints-both structural and biological. Historically, the community of those responsible for the design, inspection and maintenance of aeronautical and aerospace vehicles-complex systems comprised of an seemingly-innumerable number of mechanical components-have been cognizant of the potential fretting can have in driving system failures. Yet events from the past decade of air and space travel, including the well-publicized catastrophic in-flight disintegration of a passenger plane fuselage, have refocused efforts in understanding the nature of fretting damage in aerospace and aeronautical structures and materials. This paper provides an assessment of the state of research of fretting in this area, with hopes of identifying critical areas for future attention.
In particular, recent periodic teardown inspections and laboratory simulations of common riveted aluminum aircraft lap splice joints have revealed evidence of fretting crack nucleation near the rivet/skin interface and at the interfacial or faying surfaces of riveted joints. It is the Interaction of these small, ofien undetectable cracks that can lead to the sudden "unzipping" of riveted structural components, as evidenced in the aforementioned in-flight incident. Understanding the role of fretting in the nucleation of this damage requires intimate knowledge of the contact conditions at and around the rivet hole, conditions influenced dramatically by the interrelated residual stress field induced by the manufacturing process of riveting and the resulting mechanism of load transfer in the joint.
Another recent collaborative initiative among the United States Air Force and its domestic aircraft engine vendors has sparked interest in fretting fatigue in titanium and other advanced engine alloys. To be discussed are efforts underway currently to not only characterize the fretting contact conditions in the dovetail notch of the blade/disk pair of aircraft engines, but also the subsequent formation of near-surface damage under the influence of interfacial partial slip conditions and contact stresses and strains.
Final comments will address the transitioning of both basic and applied current research efforts to cost-effective improvements in present design approaches, manufacturing processes and inspection routines for increased performance of aeronautical and aerospace vehicles.
On A New Methodology for Quantitative Modelization of Fretting Fatigue
(top of page)K. Dang Van & M.H. Maitournam
Laboratoire de Mecanique des Solides, CNRS UMR 7649, Ecole Polytechnique,
91128 Palaiseau, cedex, France
Fretting is the surface damage induced by small amplitude oscillatory displacements between metal components in contact. Depending on the prescribed forces or the displacements amplitude, damage can either be wear or crack nucleation. Many experimental results have been obtained these last years. Vingsbo et al. [1], Vincent et al. [2] have established a test methodology based on fretting maps. These maps give the material response fretting map (MRFM) (no damage, crack nucleation or wear) according to the running condition fretting map (RCFM) (partial slip and gross regime). They are very useful for a qualitative understanding of damage phenomena. However, the results obtained cannot be applied for another set-up (with different solid geometries and material properties) and consequently they are not directly applicable to an industrial component.
Thus, the use of an intrinsic methodology to predict fretting is essential to be able to transpose laboratory tests results to real applications on mechanical structures. For this purpose, we propose an original approach based on the use of new computational methods and of multiaxial fatigue criteria.
Because of problems arising from contacts between solids, many difficulties must be overcome.
(i) The initial inputs are geometries and nominal loading of the structure. One must compute the local relevant thermomechanical parameters (temperature, stress, total and plastic strain cycles, their evolutions and eventually their stabilized states) in the regions where fretting cracks may occur. The use of classical finite elements methods is inadequate, because of
- the type of loading, which is either a moving contact loading or a contact fixed in space but varying in time
- plastic flow which generally occurs during the first cycle even in the case of apparent elastic regime (elastic shakedown)
- their flexibility and accuracy are poor despite of very high computational time
(ii) the local stress and strain fields are completely tridimensional with no fixed direction during their evolution. This necessitate the use of fatigue criteria which are able to deal with complex multiaxial loading situations.
To solve the first problem, we have developed a new efficient finite element method. This method, called the direct cyclic method, is based on an original scheme of integration. It is used to evaluate the inelastic state of structures subjected to a cyclic loading. The stabilized state (elastic or plastic shakedown) can directly be obtained.
We then use the multiaxial fatigue criteria proposed by Dang Van. Its main specificity is that it can be easily identified by classical fatigue uniaxial laboratory tests like repeated tension or torsion. This criterion is essentially based on elastic shakedown hypothesis at all scales of materials description.
To validate this proposal we simulate fretting fatigue tests on a particular experimental set-up considered as a structure.
It is found that wear in gross slip regime is associated with plastic shakedown and therefore to low cycle fatigue properties. Calculations in stick and mixed stick-slip regimes lead respectively to purely elastic and elastic shakedown. The application of the multiaxial fatigue criterion used to predict crack nucleation leads to good agreement with experimental observations.
Thus the numerical direct cyclic method combined with Dang Van multiaxial fatigue crack nucleation allows prediction of the fretting fatigue map in relation with material properties. The proposed approach, based on numerical calculations and multiaxial fatigue criterion, takes account of solids geometries in contact and material properties and thus can be transposed to structures undergoing small oscillating loads which can induce fretting damages.
Reference
A Practical Method of Predicting Fretting Fatigue Limits by Surface Tangential Stresses
(top of page)K.
Nagata*, K.Smto*, T. Matsuura** and J. Kaneko***Power and Industrial Systems Research and Development Center, Toshiba
Corporation, Yokohama, Japan.
* * Turbine Design Deparunent, Keihin Product Operations, Toshiba Corporation,
Yokohama, Japan.
Fretting fatigue test was conducted on various contact pressures and bridge typed lengths, and a practical method of predicting analytically fretting fatigue limits of complicated contact components was proposed based on the amplitude and mean of tangential stresses on fretting surface.
Fretting fatigue strengths of NiCrMoV-steel used in steam turbine rotors were examined at room temperature: the span of bridge typed pad was 15,30,50,80 and 120mm, and the nominal contact pressure was 98, 196, and 294 MPa. Small relative slip amplitude between specimen and pad edge and frictional forces developed in the pad were continuously measured.
It was found that fretting fatigue occurs even if the slip amplitude is very small: the slope in N-S curve and fatigue limit were remarkably reduced up to 2-3 m m in the slip amplitude, however the reduction ratio of the fatigue limits were slight when the slip amplitude was greater level than that.
Two dimensional contact-elastic-plastic stress analysis was carried out using finite element method to understand the relationship between tangential stresses on fretting surface and the fatigue limits, and to find a practical method of estimating the fatigue limits for complicated structure components. Local contact pressures and shear forces at contact edge were smoothly increased due to elastic-plastic deformation at the contact pad end, namely contact opening phenomenon occurred. Consequently the singularity of stress at the contact end disappeared. Fretting fatigue limits under various testing conditions were successfully estimated by modified Goodman diagram method used tangential and mean stresses on fretting surface calculated by FEM analysis as compared with the fatigue limit of smooth specimen obtained by plain fatigue test.
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