The impact of fretting wear on structural dynamics: Experiment and simulation

This paper investigates the effects of fretting wear on frictional contacts. A high frequency friction rig is used to measure the evolution of hysteresis loops, friction coefficient and tangential contact stiffness over time. This evolution of the contact parameters is linked to significant changes in natural frequencies and damping of the rig. Hysteresis loops are replicated by using a Bouc-Wen modified formulation, which includes wear to simulate the evolution of contact parameters and to model the evolving dynamic behaviour of the rig. A comparison of the measured and predicted dynamic behaviour demonstrates the feasibility of the proposed approach and highlights the need to consider wear to accurately capture the dynamic response of a system with frictional joints over its lifetime

The impact of fretting wear on structural dynamics: Experiment and simulation

This paper investigates the effects of fretting wear on frictional contacts. A high frequency friction rig is used to measure the evolution of hysteresis loops, friction coefficient and tangential contact stiffness over time. This evolution of the contact parameters is linked to significant changes in natural frequencies and damping of the rig. Hysteresis loops are replicated by using a Bouc-Wen modified formulation, which includes wear to simulate the evolution of contact parameters and to model the evolving dynamic behaviour of the rig. A comparison of the measured and predicted dynamic behaviour demonstrates the feasibility of the proposed approach and highlights the need to consider wear to accurately capture the dynamic response of a system with frictional joints over its lifetime

Fretting wear of bolted joint interfaces

Under vibration loading, fretting wear between bolted joint interfaces may change the dynamic characteristics of structures. Even the reliability of long-lasting assembly structures could be affected. This paper focuses on an experimental study on the fretting wear behavior of bolted joint interfaces under tangential loading. A recently developed fretting test apparatus was used to measure the hysteresis loops and the bolt preload at different fretting wear cycles. Changes of tangential contact stiffness and friction coefficient were estimated from the measured hysteresis loops. Experimental results showed a large change in bolt preload, contact stiffness, and friction coefficient due to fretting wear. The effect of surface roughness on fretting wear behavior of bolted joint interfaces was discussed. A modified Iwan model, comprehensive of wear effects, was proposed to simulate the hysteresis loops. Comparison between simulations and experimental results was performed to validate the proposed method. Results achieved in this research can provide the basis for the dynamic analysis of long-lasting joint structures in which wear plays a fundamental role in modifying the contact parameters

On the Role of Contact and System Stiffness in the Measurement of Principal Variables in Fretting Wear Testing

In this work, the role of the contact stiffness in the measurement of principal variables in fretting wear tests is assessed. Several fretting wear tribometers found in the literature, including one developed by the authors, are analysed and modelled using numerical methods. The results show the importance of the tribosystem stiffness and tangential contact stiffness in the displacement sensor calibration and in the correct numerical modelling of fretting wear tests, especially for flat-to-flat contact configuration. The study highlights that, in most cases, direct comparisons between fretting results with severe wear obtained with different tribometers cannot be performed if the contact stiffness is not properly considered during the development of the experiments

Finite element modelling and experimental validation of fretting wear in thin steel wires

Fretting wear is one of the main degradation mechanism produced in steel wire ropes. This phenomenon leads to produce wear scars between the contacting thin steel wires reducing considerably the fatigue life of these components. The complex assemblage of the wires in the wire rope systems make difficult the experimental characterization of this phenomenon, being economically costly and too much time consuming. With the aim to reduce these disadvantages and increases the knowledge of this phenomenon in complex components such as the wire rope systems, in this thesis a fretting wear simulation methodology for predicting wear scars in thin steel wires has been developed. The complex multicontact problem presented in wire rope systems, has been simplified with the development of an individual crossing cylinder model, in which the severity of the wear scars can be analysed under the range of the different geometrical and operational parameters appeared in each contact. In order to define the frictional and wear behaviour of the thin steel wires, in terms of the coefficient of friction and the wear coefficient, fretting wear tests have been carried out under different conditions (loads, strokes and number of cycles). Firstly in a more common tribological system of 90º crossed cylinder arrangements and secondly with different crossing angles, which are a more realistic configuration to that one presented in wire ropes. This study has reported the influence of the contact pressure and the wear debris in the wear behaviour. Thus two wear mechanisms have been derived from this type of tests, a more aggressive wear mechanism in the beginning of the test and a stable wear mechanism in the remaining part of the test. The developed wear simulation methodology is based on the Archard’s wear law applied in a local perspective. So that the wear simulation is an iterative process in which the Archard’s local equation is solved according to the contact pressures and slip distributions obtained by the finite element model, as many times as sliding increments needed to reach the desired sliding distance or, in the case of fretting wear, the desired number of cycles. Nevertheless this process requires high computational time, which increases in the case of the 3D problem presented in the thin steel wires. Therefore an optimized methodology for modelling any tribological system has been proposed, based on the optimization of the main parameters involved in the wear modelling (mesh size, sliding increment and speed factor). This methodology has been applied first in a cylinder on plane sliding wear 2D FE model and then was adapted to the 90º crossed cylinder 3D fretting wear FE model. Finally this approach has been implemented in the crossing angle 3D FE model, in which the parameterization of all the geometrical parameters and meshes that form the model has been introduced, in order to reduce the time required for modelling all the type of contacts presented in the wire rope systems. It has been demonstrated that following this methodology it can be defined the optimum parameters for modelling fretting wear of 3D problems in terms of the minimum computational time with the minimum difference in the final wear scar. The optimized wear simulation methodology has been finally validated following an exhaustive validation methodology. To this end the specific wear coefficients obtained in the fretting wear thin steel wires tests have been used. The validation consisted of three steps: wear scar dimensions, wear scar depth (profile) and wear scar volume comparison. This procedure has shown the accurate validation of the entire wear scar shape in both top and bottom specimens. The proposed methodology has been used then to study the severity of the wear scars under the different geometrical and operational parameters presented in the wire ropes contacts. The analysis of the severity of the wear scars has been done in terms of the percentage of resistant area loss and the stress concentration factor, concepts that are in relation with the catastrophic rupture of the wire and the fatigue life reduction of the wire, respectively. To develop this study the methodology of design of experiments (DOE) has been used. The great influence of the diameter and the crossing angle was derived from this study. Finally the effect of fretting wear on the fatigue life reduction of thin steel wires has been analysed. For this purpose the frictionally-induced multiaxial contact stresses obtained from the 90º thin steel wires FE model have been used. The fatigue life prediction model uses a critical-plane SWT (Smith-Watson-Topper) approach in a 3D crossed cylinder problem. A new damage accumulation methodology for the adaptive mesh simulation in 3D problems, based on the cyclic material removal, has been developed. The effect of the contact pressure and stroke has been analysed and contrasted with the results obtained experimentally. Guidelines for developing a more robust methodology were proposed in the end of this research work.Kable metalikoen barnean gertatzen den degradazio fenomeno garrantzitsuenetako bat da fretting-a. Gertaera honek dakar kablearen neke bizitza murriztea, hariz osatutako kableetan pitzadurak sortuaz. Altzairuzko harien muntai konplexuak fenomeno honen karakterizazio esperimentala oztopatzen du, batez ere ikerkuntzaren luzapenean eta koste ekonomikoan. Desabantaila hauek ahal diren einean gutxitzeko batetik, eta kable metalikoa bezalako sistema konplexuetan fenomeno honi buruzko ezagutza handitzeko bestetik, fretting fenomenoa sortutako pitzadurak iragartzeko higadura simulazio metodologia garatu da tesi honetan. Kableetan sortzen den harien arteko multi-kontaktu arazoa zaila suertatzen dela jakinda, bi zilindrok gurutzatzean sortzen duten kontaktu bakarreko modelo bat eraiki da. Modelo honek, kableetan agertzen diren kontaktu bakoitzeko pitzaduren larritasuna neurri geometriko edo funtzionamenduen ondorioz aztertzeko gaitasuna dauka. Fretting saiakuntza tribologikoak baldintza ezberdinetan egin dira (indar normala, deslizamendu anplitudea eta ziklo-kopurua) marruskadura eta higadura portaerak ezagutzeko. Saiakuntzak hasieran, 90ºko angeluz gurutzatuta dauden zilindroen konfigurazio arrunt batean egin dira, ondoren, kableetan agertzen diren benetazko angelu ezberdinetan izan direlarik. Azterketa hauetan, harien arteko presioa eta higaduraren ondorioz sortzen diren partikulen eraginak aztertu dira, bi higadura mekanismo nagusi bereiztuz: higadura oldarkorra (saiakuntzaren hasieran) eta higadura egonkorra. Tesi honetan garatu den higadura simulazioren metodologia, Archard higaduraren legea maila lokalean aplikatzean datza. Higadura-simulazioa prozesu iteratiboa izanik, Archarden ekuazio lokala ebazten da elementu mugatuen (FE) modelotik lortutako presio kontaktu eta deslizamendu erlatiboa erabiliz, behar diren deslizamendu erlatibo gehikuntza adina errepikatuz, distantzia osoa bete arte, Fretting-aren kasuan ziklo kopuru osoa bete arte. Tesi honetan aztertu den altzairuzko hariaren 3D simulazioak denbora konputazional handia eskatzen duela jakinik, edozein tribosisteman higadura simulatzeko metodologia optimizatua proposatu da, higadura simulazioan parte hartzen duten parametro garrantzitsuen optimizazioan oinarrituta (sare tamaina, irrista gehikuntza eta higadura bizkorgailua). Metodologia hau, zilindro-plano 2D FE modelo batean aplikatu da lehenik eta 90º zilindro gurutzatuta dauden 3D FE modelo batera egokitu da ondoren. Azkenik, metodologia hau angelu ezberdinez gurutzatzen diren zilindroen 3D FE modeloan inplementatu da. Modelo honek gainera parametro geometriko eta sarearen parametrizazioa dakar, kableetan agertzen diren kontaktu mota guztiak modelatzeko behar den denbora murrizteko. Metodologia honen erabilerarekin fretting higadura simulatzeko parametro optimoak identifikatu daitezkeela frogatu da, bai 2D modeloetan, baita 3D modeloetan, denbora konputazional txikiena eta pitzaduran diferentzia txikiena erabiliz. Higadura simulatzeko metodologia optimizatua balioztatu da, balioztatzeko prozesu zehatz bat jarraitu ondoren. Hau lortzeko hariekin egindako fretting saiakuntzen ondorioz lortutako higidura koefizienteak erabili dira. Balioztapen hau, pitzadurekin erlazionatuta dauden hiru alderdietan oinarritu da: pitzaduraren dimentsioak, pitzaduraren sakonera (erdiko profila) eta bolumena. Pitzaduren larritasuna aztertzeko proposatu den higadura simulatzeko metodologia, kableetan agertzen diren parametro geometriko eta funtzionalen pean erabili da. Hariaren larritasunaren azterketa, azalera erresistenteren portzentai galera eta tentsio kontzentrazioa baliatuz aztertu da. Bi parametro hauek hariaren apurtzeko ahalmenarekin eta nekearen bizitza murrizteko ahalmenarekin loturik daude. Beraz, azterketa hau garatzeko esperimentu diseinuaren metodologia (DOE) erabili da. Azterketa honetan diametro eta angelu gurutzatuen bizitza murrizteko ahalmena ikusi da. Azkenik, Fretting fenomenoak altzairuzko harien neke bizitza murriztean duen efektua aztertu da, horretarako, 90º-ko higadura simulatzeko modelotik, marruskaduraren ondorioz agertzen diren kontaktuzko tentsio multiaxialak erabili dira. Neke-bizitza aurreikusteko metodologia, planu kritiko eta SWT (Smith-Watson-Topper) neke eredua 3Dko zilindro gurutzatu modeloaren inplementazioan datza. Horretaz gain, 3D modeloen higadura ziklikoa kontutan hartzeko, kaltea pilatzeko modelo berri bat gehitu zaio. Azkenik, kontaktu presioaren eta deslizamendu anplitudearen eragina aztertu da, fretting saiakuntzan lortutako emaitzekin alderatuz. Tesi honen amaieran fretting fenomenoaren eraginez altzairuzko hariz osaturik dauden kable metalikoen neke bizitza aurreikusteko metodologia sendoagoa garatzeko jarraibideak proposatu dira.El fretting es uno de los principales mecanismos de degradación presente en el interior de los cables metálicos. Este fenómeno conlleva la aparición de huellas de desgaste entre los hilos en contacto reduciendo considerablemente la vida a fatiga de estos componentes. El complejo ensamblaje de los hilos de acero en los cables metálicos dificulta la caracterización experimental de dicho fenómeno, resultando una tarea costosa tanto en tiempo como dinero. Con el fin de reducir estas desventajas y aumentar el conocimiento de este fenómeno en sistemas complejos como los cables metálicos, en esta tesis se ha desarrollado una metodología de simulación del desgaste para predecir las huellas de desgaste producidas por el fenómeno de fretting. El complejo problema de contacto múltiple presente en los cables, se ha simplificado por medio del desarrollo de un modelo individual de cilindros cruzados, en el cual se puede analizar la severidad de las huellas de desgaste bajo el rango de los parámetros, tanto operacionales como geométricos, presentes en cada uno de los contactos del cable metálico. Con el fin de definir el comportamiento a rozamiento y desgaste de los hilos, en términos de coeficiente de rozamiento y desgaste, se han realizado ensayos tribológicos de fretting bajo diferentes condiciones (fuerza normal, carrera y número de ciclos). Primeramente con una configuración mas común de cilindros cruzados a 90º y a continuación con una configuración mas realista a la presente en los cables metálicos de ángulos cruzados. En dicho estudio se ha mostrado la influencia de la presión de contacto y las partículas de desgaste en el comportamiento ha desgaste de los hilos, observándose dos mecanismos principales de desgaste: un mecanismo de desgaste agresivo en el inicio del ensayo y un mecanismo de desgaste mas estable en la restante parte del ensayo. La metodología de simulación de desgaste desarrollada en esta tesis, se basa en la ley de desgaste de Archard aplicada a nivel local. Por lo tanto la simulación del desgaste consiste en un proceso iterativo en el cual la ecuación de Archard local es resuelta por medio de las presiones de contacto y distribución de deslizamientos obtenidas por el modelo de elementos finitos (EF), tantas veces como incrementos de deslizamientos sean necesarios para completar al deslizamiento requerido o, en el caso de fretting, para completar el número de ciclos. Sin embargo este proceso requiere un alto coste computacional, el cual es mayor en el caso del problema 3D presentado en los hilos. Por lo tanto se ha propuesto una metodología de simulación de desgaste optimizada para cualquier tribosistema, basada en la optimización de los principales parámetros involucrados en la simulación del desgaste (tamaño de la malla, incremento de deslizamiento y acelerador de desgaste). Esta metodología se ha aplicado primeramente en un modelo 2D de EF, el cual representa un ensayo de deslizamiento continuo de cilindro-plano, y a continuación se ha adaptado al modelo 3D de EF de cilindros cruzados a 90º sometido a fretting. Finalmente dicha metodología se ha implementado en el modelo 3D de EF de cilindros cruzados, en el cual se ha incluido la parametrización de todos los parámetros tanto geométricos como de malla, con el fin de reducir el tiempo requerido para el modelado de todos los tipos de contactos presentes en los cables. Se ha demostrado que mediante la utilización de esta metodología se pueden identificar los parámetros óptimos para la simulación del desgaste en problemas 3D de fretting en términos de mínimo tiempo computacional y minima diferencia en la huella de desgaste final obtenida. La metodología de simulación de desgaste optimizada ha sido finalmente validada por medio de un exhaustivo procedimiento de validación. Para ello se han utilizado los coeficientes de desgaste específicos obtenidos tras los ensayos de fretting con hilos. Dicha validación se ha basado en tres aspectos relacionados con las huellas de desgaste: dimensiones de la huella, profundidad de la huella de desgaste (perfil central) y volumen de desgaste de la huella. Este procedimiento ha demostrado la precisa validación tanto de la huella del cuerpo superior como del inferior. La metodología propuesta ha sido utilizada a continuación para el estudio de la severidad de las huellas de desgaste bajo los diferentes parámetros tanto operacionales como geométricos presentes en los cables. El estudio de la severidad se ha realizado en términos de porcentaje de área resistente perdida y factor de concentración de tensiones, conceptos que están relacionados con la rotura catastrófica del hilo y con la reducción de la vida a fatiga del mismo, respectivamente. Para su desarrollo se ha utilizado la metodología del diseño de experimentos (DOE). Dicho estudio ha demostrado la influencia tanto del diámetro como del ángulo de cruce en la reducción de vida de este tipo de componentes. Finalmente se ha estudiado el efecto del fretting en la reducción de la vida a fatiga de los hilos. Para ello, a partir del modelo de simulación de desgaste de cilindros cruzados a 90º, se han utilizado las tensiones multiaxiales de contacto producidas por el rozamiento. El modelo de predicción de la vida a fatiga consiste en la implementación de la metodología del plano critico y el modelo de fatiga SWT (Smith-Watson-Topper) en el problema 3D de cilindros cruzados. La metodología propuesta incorpora además un nuevo modelo de acumulación de daño que tiene en cuenta el desgate cíclico y consiguiente remallado adaptativo del material en problemas 3D. El efecto de la presión de contacto y la carrera han sido analizados y contrastados con los resultados obtenidos experimentalmente. Al final de este tesis se han presentado las directrices para el desarrollo de una metodología mas robusta para la predicción de la vida a fatiga debido al fenómeno de fretting en los hilos del cable

Energy and Thermodynamically Based Approaches for Analysis of Damage in Contact Problems

A novel methodology for analysis of damage in contact problems in presented. Two important categories of contact damage are studied, namely crack nucleation and adhesive wear. The proposed methodology is primarily based on the laws of energy and thermodynamics, and as such offers great advantages by unifying the analysis of damage in a variety of contact configurations. Crack nucleation is the first damage type analyzed. For this purpose, the line-contact fretting configuration is chosen. A thermodynamically-based continuum damage mechanics (CDM) approach is employed. Intense stress gradients in the contact region are found to be highly influential in the crack nucleation process. A methodology is proposed that identified the averaging zone as a function of the contact and loading parameters. The predicted crack nucleation lives are verified by comparing against the published experimental data for two different alloys. The utility of the proposed methodology is also investigated for the case of rough surface contact. The deterministic approach is employed to investigate the effect of roughness on the surface tractions and contact stresses. A special averaging technique, proposed for the case of smooth surface, is adopted. The predictions of the crack nucleation life for different roughness values are compared with relevant experimental data in literature, and confirm the validity of the analysis. Adhesive wear is another type of contact damage investigated. The study is carried out for three different contact conditions. These include disk-on-disk unidirectional dry sliding, pin-on-flat reciprocating dry sliding and pin-bushing grease-lubricated oscillatory sliding. It is shown that the wear rate is linearly related to the power dissipation as well as the entropy generation rate. The linear correlations are verified experimentally. Degradation coefficient is obtained and a simple approach is proposed for prediction of wear in dry sliding configuration. The proposed technique can be employed for prediction of wear in circumstances where the direct measurement of power dissipation is encumbered by practical limitations. Also investigated are the relationship between the system’s wear rate, power dissipation and thermal response. The wear-energy dissipation coefficient (WED) is identified as an important property of tribo-systems. The methodology relies on measurement of temperature rise in the sliding system. It is shown that the correlation between the frictional power dissipation and temperature rise can be obtained through thermal analysis of the system. The proposed methodology is shown to be capable of predicting the wear rate under a wide range of loading conditions

A novel test rig to study the effect of fretting wear on the forced response dynamics with a friction contact

This paper presents a novel test rig to study the effect of fretting wear and of the contact surface evolution on the forced response of systems with dry friction contact. This rig allows simulating contacts similar to the type of contacts present between the shrouds at the blade tip. Several research groups have been studying how fretting wear affects the dynamic response of mechanical systems, developing numerical prediction tools that consider dry friction contact and nonlinearity. The aim of this work is to experimentally study the evolution of contact interfaces and how this evolution affects the system dynamics. Experimental results will aid to validate the numerical predictions. The test rig developed for this activity is made of a cantilever beam fixed at one end and with a friction contact at the free end. The contact couple is made of two replaceable specimens. The contact is loaded via a lifting mechanism through a screw with fine thread. Fretting wear test was performed at a constant frequency and force amplitude, exciting the beam with an electromagnetic shaker. To emphasize the change of the dynamic response, frequency sweeps were performed at various intervals during the wear test. The full range test with ‘changing preload’ due to progressing wear was performed until a full loss of contact. This paper describes the test rig design, intent, set-up, instrumentation, test plan and results. Results include the frequency response curves for unworn contact, wear profiles at multiple intervals and the effect of wear on the frequency response. Though energy dissipation per cycle is quite small, wear leads to material loss at the contact with a sufficiently large number of cumulative cycles and substantially affects the dynamic response. Results collected in this research activity are of particular importance to validate numerical tool that aim to simulate the dynamic behaviour of systems with dry friction contacts that undergo material loss caused by wear

On the effects of roughness on the nonlinear dynamics of a bolted joint: a multiscale analysis

Accurate prediction of the vibration response of friction joints is of great importance when estimating both the performance and the life of build-up structures. The contact conditions at the joint interface, including local normal load distribution and contact stiffness, play a critical role in the nonlinear dynamic response. These parameters strongly depend on the mating surfaces, where the surface roughness is well known to have a significant impact on the contact conditions in the static case. In contrast, its effects on the global and local nonlinear dynamic response of a build-up structure is not as well understood due to the complexity of the involved mechanisms. To obtain a better understanding of the dependence of the nonlinear dynamic response on surface roughness, a newly proposed multiscale approach has been developed. It links the surface roughness to the contact pressure and contact stiffness, and in combination with a multiharmonic balance solver, allows to compute the nonlinear dynamic response for different interface roughness. An application of the technique to a single bolted lap joint highlighted a strong impact of larger roughness values on the pressure distribution and local contact stiffness and in turn on the nonlinear dynamic response