Measurement of Contact Parameters of Flat on Flat Contact Surfaces at High Temperature

2012 ◽  
Author(s):  
D. Botto ◽  
M. Lavella ◽  
M. M. Gola
Keyword(s):  
Friction Coefficient ◽  
Vibration Amplitude ◽  
Contact Stiffness ◽  
Relative Displacement ◽  
Wear Behaviour ◽  
Test Rig ◽  
Contact Parameter ◽  
Contact Surfaces ◽  
Contact Parameters ◽  
Flat Contact

In aircraft engines the blade resonant vibration amplitude is normally reduced by increasing the structural damping by using, for example, tip shrouds. These devices dissipate the energy generated at the contact surfaces between the relative motion and the friction force. Contact parameters, principally the friction coefficient and contact stiffness, are required to characterize the dynamics of shrouded blade system. Moreover, if at these contact surfaces severe wear occurs, a loss of interference takes place and the energy dissipated by the shroud decreases. Consequently the blade vibration amplitude increases and a catastrophic blade failure could take place. In this work a test rig for the contact parameter measurements and micro wear characterization of flat-on-flat contact surfaces has been developed. The test rig works at high temperatures of up to 1000 °C, by means of induction heating. One of the specimens was attached to the rig frame, basically an inertial mass and four springs, and subsequently excited by an electromagnetic shaker. The second specimen was allowed to approach the first specimen and to rotate in such a way than the geometric contact between the two surfaces occurred at three points. In this way a real “flat to flat” contact was obtained. The two surfaces were kept in contact by means of a constant normal load. The tangential contact force was measured by a force sensor while the relative displacements between the contact surfaces were measured by two laser vibrometers. The relative displacement was kept under control by acting on the shaker force. Tangential force and relative displacement were used to describe the hysteresis loop and, consequently, to obtain the friction coefficient and contact stiffness during the wear process. The temperature is feedback controlled by using two thermocouples placed within the specimens near the contact surfaces. The expected results are the contact parameters and the wear behaviour of real flat-on-flat contact surfaces. The aim of this work is to describe the design principle of the test rig and present the initial measurements.

Test Rig for Wear and Contact Parameters Extraction for Flat-on-Flat Contact Surfaces

2011 ◽  
Author(s):  
M. Lavella ◽  
D. Botto ◽  
M. M. Gola
Keyword(s):  
Friction Coefficient ◽  
Contact Stiffness ◽  
Inertial Mass ◽  
Aircraft Engine ◽  
Wear Behaviour ◽  
Test Rig ◽  
Contact Surfaces ◽  
Blade Failure ◽  
Contact Parameters ◽  
Flat Contact

In aircraft engine the blade resonant vibration amplitude is reduced by increasing the structural damping by using, for example, tip shrouds. These devices dissipate the energy generated at the contact surfaces between the relative motion and the friction force. Contact parameters as friction coefficient and contact stiffness are required to characterize the dynamics of the shrouded blade systems. Moreover if at the contact surfaces severe wear occurs a loss of interference takes place, the shroud is not longer able to dissipate energy and a catastrophic blade failure could take place. A test rig for contact parameters measurement and micro wear characterization of flat-on-flat contact surfaces has been developed. One of the specimens is attached to the rig frame, basically an inertial mass and four springs, excited by an electromagnetic shaker. The second specimen is allowed to approach the first specimen and to rotate in such a way the geometric contact between the two surfaces occurs on three points. In this way a real “flat-to-flat” contact has been obtained. The tangential contact force and the relative displacements between specimens are measured and the friction coefficient and contact stiffness, are determined. The expected results are the contact parameters and the wear behaviour of real flat-on-flat contact surfaces. The aim of this work is to describe the design principle of the test rig and show the first measurements.

Fretting Wear of CMSX4 at Blade Tip Interface With and Without Coating

2009 ◽  
Author(s):  
M. Lavella ◽  
D. Botto ◽  
M. M. Gola
Keyword(s):  
Flat Surface ◽  
Contact Stiffness ◽  
Three Dimensional ◽  
Large Change ◽  
Fretting Wear ◽  
Wear Behaviour ◽  
Wear Process ◽  
Blade Tip ◽  
Coated Surfaces ◽  
Contact Parameters

Fretting wear is a complex phenomenon that occurs at component interfaces that are subjected to low amplitude oscillation under high contact pressure. In turbomachinery fretting occurs also at the blade tip interfaces where shrouds, that have the aim to reduce the blade resonant vibration amplitude, are machined. To diminish the fretting damage coatings are applied to the blade tips. The aim of this study is to compare the fretting wear behaviour of single crystal CMSX-4 superalloy interfaces with and without plasma sprayed T-800 coating. Experiments have been conducted with hemispherical surface in contact with a flat surface of the same materials at temperature of 800 °C. The hysteresis cycles have been measured through the experiment. The comparison of the hysteresis cycles shown that the tangential contact stiffness of the coated surfaces is greater then that of the surfaces without coating. At the end of wear process, the mating surfaces have been characterized by three-dimensional optical interferometry and SEM analysis. After 10×106 wear cycles, the uncoated surfaces show a large change in the contact parameters and fretting cracks on the flat surface. On the other hand, the coated surfaces do not shows a measurable change in the contact parameters while the coating damage on the flat surface leads to predict an incipient catastrophic wear.

Validation of Test Rig Measurements and Prediction Tools for Friction Interface Modelling

2010 ◽  
Author(s):  
C. W. Schwingshackl ◽  
E. P. Petrov ◽  
D. J. Ewins
Keyword(s):  
Good Accuracy ◽  
Contact Stiffness ◽  
Contact Interface ◽  
Friction Contact ◽  
Test Rig ◽  
Normal Stress Distribution ◽  
Nonlinear Interface ◽  
Two Parameters ◽  
Contact Parameters ◽  
Interface Behaviour

The modelling of friction contact interfaces in structural dynamics attracts much interest in the gas turbine industry. In order to obtain reliable predictions of typical friction interfaces, such as encountered in under platform dampers or blade roots, accurate characteristics of friction interfaces must be provided to the analysis. It must be ensured that a sufficient number of parameters are provided, characterising all aspects of the friction contact, that the values are measured accurately, and that the contact parameters are interpreted and used correctly in the numerical modelling of the contact interfaces. This investigation demonstrates that measured friction coefficient and tangential contact stiffness are sufficient to reproduce the experimental friction interface behaviour and that these two parameters can be measured reliably in the available test rig. In combination with fine nonlinear interface meshes and accurate contact pressure representations, the measured interface behaviour of stick, micro- and macroslip is reproduced with good accuracy. The capability of modelling the microslip behaviour for the contact interface by a multitude of friction contact elements is explored and the effect of the normal stress distribution over the contact area on the microslip is studied.

Test Rig for Extraction of the Contact Parameters for Plane on Plane Contact

2012 ◽  
Author(s):  
D. Botto ◽  
M. Lavella ◽  
M. M. Gola
Keyword(s):  
Contact Area ◽  
Contact Stiffness ◽  
Point Contact ◽  
Real Plane ◽  
Operational Environment ◽  
Test Rig ◽  
Friction Forces ◽  
Plane Contact ◽  
Wide Range ◽  
Contact Parameters

The modelling of the friction interfaces has received much attention in recent years from the aerospace industry. In order to obtain reliable prediction of the nonlinear dynamic behaviour of the disc and blades in the aerospace engine the friction forces at interfaces, such as in under-platform dampers, blade and fir tree roots or shrouds, must be modelled accurately. Two contact parameters, namely the contact stiffness and the coefficient of friction, are sufficient to model, with good accuracy, the friction contact. The contact parameters are obtained experimentally, and are of interest for the designer only if representative of the operational environment of the engine. To pursue this aim a test rig has been designed to perform experiments in a wide range of temperatures, with different combinations of normal and tangential load, frequencies and mating materials, representative of the real operating condition of the engine. Most of the rigs found in literature perform most likely point contact even if the two bodies have plane mating surfaces. The design of a real plane-on-plane contact test rig is not an easy task but despite the difficulty a solution was found in the design shown in this work. The core of the rig is a tilting mechanism enabling one surface to lies down on the other so that the plane-on-plane contact is achieved, at least within the flatness geometrical tolerance of the surfaces. The results of the experiments are the hysteresis loops, namely the tangential contact force against the relative displacement, from which the contact parameters can be calculated. Measurements of displacements are taken very close to the actual contact area and are performed by means of two laser interferometers. Localized heating is achieved by means of an induction heating machine while a thermocouple measures the temperature at points close to the contact area.

Measurement of Tangential Contact Hysteresis During Microslip

2004 ◽  
Vol 126 (3) ◽  
pp. 482-489 ◽  
Author(s):  
Sergio Filippi ◽  
Adnan Akay ◽  
Muzio M. Gola
Keyword(s):  
Measurement System ◽  
Measurement Errors ◽  
Small Amplitude ◽  
Contact Stiffness ◽  
Tangential Force ◽  
Systematic Errors ◽  
Relative Displacement ◽  
Tangential Contact ◽  
Measurement Artifacts ◽  
Contact Parameters

This paper describes a measurement system designed to determine the hysteresis that develops between two surfaces as a result of small-amplitude tangential relative motion. Hysteresis is determined by measuring the tangential force and relative displacement of the contacting surfaces as they oscillate. These measurements also produce values of contact parameters such as friction coefficient and tangential contact stiffness. Although these parameters depend on the tribological properties, most of them also exhibit strong sensitivity to measurement errors. The measurement system described here avoids or at least reduces many of the measurement artifacts. This paper validates the measurement system by analyzing and estimating potential errors and describes corrections to systematic errors where possible.

scholarly journals Estimation accuracy vs. engineering significance of contact parameters for solid dampers

2017 ◽  
Vol 1 ◽  
pp. VLXC9F ◽  
Author(s):  
Chiara Gastaldi ◽  
Muzio M. Gola
Keyword(s):  
Numerical Models ◽  
Contact Stiffness ◽  
Point Of View ◽  
Contact Forces ◽  
Estimation Accuracy ◽  
Contact Friction ◽  
Experimental Frequency ◽  
Contact Parameter ◽  
Friction Parameters ◽  
Contact Parameters

AbstractAll numerical models of friction-damped bladed arrays require knowledge or information of contact-friction parameters. In the literature, these parameters are typically tuned so that the experimental Frequency Response Function (FRF) of a damped blade matches its numerical counterpart. It is well known that there exist multiple combinations of contact parameters capable of satisfying a given experimental-numerical FRF match. A better approach towards a finer tuning could be based on directly measuring contact forces transmitted between blade platforms through the damper: in this case friction coefficients are estimated through tangential over normal force components during those hysteresis segments which are safely identified as being in a slip condition. This has been applied by these authors to rigid bar (solid) dampers. Unfortunately, the four contact stiffness values (left and right damper-platform contact, normal and tangential) are more than the measurements available in the technique presented by these authors. Therefore, the problem is underdetermined. The purpose of this paper is twofold,i.e., to propose an alternative way to estimate contact stiffness values (i.e.thus solving the under-determinacy mentioned above) and to check the effective significance of such estimates from a practical engineering point of view. The contact parameter estimation technique proposed by these authors produces, for each contact parameter, a best-fit value and an uncertainty band. It will be shown that the uncertainty affecting each contact parameter results in an uncertainty on the equivalent damping and stiffness indicators at blade level which is lower than 5%.

Evaluation of Friction Damping in Dovetail Root Joints Based on Dissipation Energy on Contact Surfaces

2009 ◽  
Author(s):  
Kunio Asai ◽  
Shigeo Sakurai ◽  
Takeshi Kudo ◽  
Norihiko Ozawa ◽  
Taizo Ikeda
Keyword(s):  
Contact Force ◽  
Contact Stiffness ◽  
Relative Displacement ◽  
Contact Angles ◽  
Forced Response ◽  
Contact Forces ◽  
Friction Damping ◽  
Normal Contact ◽  
Tangential Contact ◽  
Contact Surfaces

It is necessary to increase and estimate friction damping at contact interfaces to reduce vibratory stresses in turbines. The hysteresis behavior between tangential contact force and relative displacement should be precisely estimated to improve the accuracy of fiction-damping estimates. There is a difficulty in establishing a general model of hysteresis because tangential contact stiffness depends on many parameters, such as normal contact force, contact geometry, surface roughness, and wear status. We discuss a procedure to empirically calculate friction damping in dovetail root joints using the tangential contact stiffness estimated from measured natural frequencies and the micro-slip model whose coefficients were experimentally obtained from special fretting tests. Instead of the multi-harmonic balance methods, we calculated the friction damping on the basis of the energy dissipation at contact surfaces to discuss the effects of the tangential contact stiffness on several physical values, i.e., tangential and normal contact forces, natural frequency, and micro-slip. In our model, the linear forced response analysis was conducted by taking into consideration the non-linearity between the tangential contact force and the relative displacement by defining the actual and imaginary tangential contact stiffness. We confirmed that the numerically calculated damping ratios are quantitatively in very good agreement with the measured ones under different contact angles, input gravity levels, and contact forces. This indicates that if the tangential contact stiffness is accurately estimated, friction damping with our method can be precisely estimated under different test conditions. We also showed that the estimated tangential contact stiffness for dovetail root joints are smaller than those obtained by the fretting tests at high input gravity. This is probably because the contact interface partially separates during a cyclic loading in the former case; this results in the decrease of the contact area and contact stiffness.

Bearing Friction Torque in Threaded Fasteners With Non-Flat Underhead Contact

2018 ◽  
Author(s):  
Sayed A. Nassar ◽  
Marco Gerini Romagnoli ◽  
Joon Ha Lee
Keyword(s):  
Friction Coefficient ◽  
Contact Pressure ◽  
Friction Torque ◽  
Sliding Speed ◽  
Friction Coefficients ◽  
Threaded Fasteners ◽  
Tension Testing ◽  
Contact Surfaces ◽  
Flat Contact ◽  
Bearing Friction

This study provides experimentally validated formulation of underhead bearing friction torque component during tightening of threaded fasteners with non-flat contact with the joint. Motosh model is utilized for spherical and conical contact surfaces for various scenarios of contact pressure. For each pressure scenario, a single non-dimensional 3-D graph is generated for the corresponding values of an effective bearing friction radius. A rotating sliding speed-dependent friction coefficient model is also investigated for its impact of the results of bearing friction radius. Torque-Tension testing is used to measure the bearing friction torque and the corresponding bearing friction coefficients using Motosh model, in which the newly formulated bearing friction radius expressions are entered. Obtained bearing friction coefficient values are then compared with those published by the threaded fastener manufacturer.

scholarly journals On the choice of contact parameters for the forced response calculation of a bladed disk with underplatform dampers

2017 ◽  
Vol 1 ◽  
pp. 5D19RH ◽  
Author(s):  
Chiara Gastaldi ◽  
Emanuele Grossi ◽  
Teresa M. Berruti
Keyword(s):  
Friction Coefficient ◽  
Contact Stiffness ◽  
Turbine Blades ◽  
Tangential Direction ◽  
Forced Response ◽  
Point Of View ◽  
Analytical Models ◽  
Test Results ◽  
Bladed Disk ◽  
Contact Parameters

AbstractUnderplatform dampers (UPDs) are still in use to reduce the vibration amplitude of turbine blades and to shift the position of resonant frequencies. The dynamics of blades with UPDs is nonlinear and the analysis is challenging from both the experimental and the numerical point of view. A key point in obtaining a predictive numerical tool is the choice of the correct contact parameters (contact stiffness and friction coefficient) that are required as input to the contact model. The paper presents different approaches to choose these parameters: the contact stiffness in normal and tangential direction are both calculated and measured. The calculation is based on the analytical models in literature, the measurements are carried out on a dedicated test rig. The friction coefficient is also measured. Test results of the forced response of the same bladed disk with UPDs are available for each blade, they come from an experimental campaign under controlled excitation and centrifugal force. The forced response of the bladed disk is not used as a mean to tune the contact parameters, but rather as a validation tool: the effect of the different choices of contact parameters in the code is highlighted by the comparison of the calculated and experimental forced response of the bladed disk.

scholarly journals The Effect of Co-Deposition of SiC Sub-Micron Particles and Heat Treatment on Wear Behaviour of Ni–P Coatings

Coatings ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 180
Author(s):  
Donya Ahmadkhaniha ◽  
Lucia Lattanzi ◽  
Fabio Bonora ◽  
Annalisa Fortini ◽  
Mattia Merlin ◽  
...  
Keyword(s):  
Heat Treatment ◽  
Friction Coefficient ◽  
Wear Mechanisms ◽  
Composite Coatings ◽  
Wear Behaviour ◽  
Maximum Hardness ◽  
Sic Particles ◽  
Heat Treated ◽  
The Coefficient Of Friction ◽  
The Difference

The purpose of the study is to assess the influence of SiC particles and heat treatment on the wear behaviour of Ni–P coatings when in contact with a 100Cr6 steel. Addition of reinforcing particles and heat treatment are two common methods to increase Ni–P hardness. Ball-on-disc wear tests coupled with SEM investigations were used to compare as-plated and heat-treated coatings, both pure and composite ones, and to evaluate the wear mechanisms. In the as-plated coatings, the presence of SiC particles determined higher friction coefficient and wear rate than the pure Ni–P coatings, despite the limited increase in hardness, of about 15%. The effect of SiC particles was shown in combination with heat treatment. The maximum hardness in pure Ni–P coating was achieved by heating at 400 °C for 1 h while for composite coatings heating for 2 h at 360 °C was sufficient to obtain the maximum hardness. The difference between the friction coefficient of composite and pure coatings was disclosed by heating at 300 °C for 2 h. In other cases, the coefficient of friction (COF) stabilised at similar values. The wear mechanisms involved were mainly abrasion and tribo-oxidation, with the formation of lubricant Fe oxides produced at the counterpart.

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