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

Abstract All 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%

Proceedings of "Il Convegno di Torino - L'Assicurazione Interna della Qualità dei Corsi di Studio - AIQ dei CdS" - 23 giugno 2006

CLUT Torin

Smorzatori sotto-pala per turbomacchine: un nuovo metodo basato sul calcolo simultaneo delle forze statiche e dinamiche di contatto

Underplatform dampers are widely used to reduce vibration amplitude of turbine bladed disks by friction. They are metal devices placed under the blade platforms and held in contact with them by the centrifugal force acting during rotation. The common procedure used to calculate the static loads acting on underplatform dampers consists in decoupling the static and the dynamic balance of the damper. A preliminary static analysis of the contact is performed in order to compute the static pressure distribution over the damper/blade interfaces, assuming that it does not change when vibration occurs. In this paper a coupled approach is proposed to solve the static and the dynamic balance equations of the system. Numerical case studies are presented in order to show the capabilities of the method and the improvements with respect to the state of the ar

Faster on-line calculation of thermal stresses by time integration

The Green’s function technique (GFT) is largely used for on-line calculation of thermal stresses in machines and plants, because it allows turning parameters such as fluid temperatures, pressures and flow rates directly in thermal stresses. Recently the use of the GFT has been extended by the authors to thermo-mechanical models having variable convective coefficients. The novel methodology is made of two steps. First of all, boundary temperatures are evaluated by time integration of a reduced thermal model and then thermal stresses are calculated by means of the GFT using as inputs the boundary temperatures previously evaluated. The new approach implies a large number of convolution integrals to be solved for thermal stress calculation. In order to reduce computation time it is here proposed to replace the convolution integrals which characterise the GFT with a reduced model of uncoupled first order differential equations, whose coefficients are estimated fitting the Green’s functions of the thermo-mechanical model with a sum of exponential terms. Thermal stresses are obtained by time integration of the model

Investigation of under-platform damper kinematics and its interaction with contact parameters (nominal friction coefficient)

The novel approach proposed by the authors consists in directly measuring the forces transmitted between the two platforms through the damper, versus the relative motion of the platforms. In a previous paper we presented the design and calibration of a test rig where such measurements can be accomplished. In that paper some demonstration results were presented in order to show the capacity of the test rig. In this paper we add 1) a full reconstruction of translational and rotational damper motion, 2) comparisons with the results from a numerical model. The combination of the two allows to understand the contact conditions even in quite complex situations, and to explore the reasons for remarkable changes of the measured hysteresis cycles in operation. The damper used here is a „three point‟‟ damper. It has a statically determinate configuration, moreover its single line contact acts on the „fixed‟ platform, which is supported by the force sensors. This allows to fully determine forces, as the point of application in the force sensor plane is known. Experiments are performed under so called out-of-phase (OoP) and in-phase (IP) condition simulating two important motion types in the platform-damper mechanics. Experiments show that the friction coefficient can be very sensitive to the kinematics of the damper and can evolve in different ways in different tests under the same nominal outer parameters, especially for OoP condition. The combination of rolling and translation of the damper cause a complex influence on the friction coefficient at the three contact lines and vice versa; i.e., the damper kinematics and friction coefficient interact. For the long-run tests of this damper under OoP condition, there is a tendency of increasing the friction coefficient on both sides, which leads to micro-slip. The combination of rolling and translation of the damper contact is simulated by applying one macro-slip contact element at each contact point with normal and tangential stiffness through a numerical algorithm based on Newmark- beta method

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

Assicurare la qualità della formazione: perché, per chi, come

La Qualità della formazione richiede di essere focalizzata su temi essenziali quali le funzioni del laureato in un contesto di lavoro, i conseguenti obiettivi di apprendimento da prevedere nella formazione, i risultati di apprendimento da accertare, le risorse e i metodi che consentono di raggiungerli. Le agenzie europee più attive e innovative tracciano chiaramente la strada in questa direzione. Occorre superare il concetto, imperante nel recente passato, di Qualità e valutazione come forma di puro managerialismo. La qualità deve tradursi in processi sostenibili basati su un Modello Informativo continuamente mantenuto su rete informatica sia per scopi di valutazione sia per la comunicazione pubblica in senso lat