1,721,076 research outputs found
High Temperature Tribological Study of Cobalt-based Coatings Reinforced with Different Percentages of Alumina
No full textContact surfaces, subjected to high pressure coupled with low-amplitude oscillation, are used to a great extent in aerospace engine components to reduce vibrations. Relative displacement of contact surfaces coupled with friction force dissipates energy and damps the vibrations. Fretting wear is harmful on interference-fit contacts because the loss of material reduces the normal load and hence, reduces their effectiveness in damping vibrations. Coating is an important means to control friction and wear of contact surfaces. To address this issue an experimental layout was designed, and fretting wear tests were performed at high temperature (1000 °C). A representative sphere-flat contact was investigated in laboratory environment. Two types of metal-ceramic coatings, made of the same metal but with a different alumina percentage, were tested. Results were reported in terms of volume loss against number of wear cycles. It was found that increasing the alumina percentage was detrimental because the volume loss increased dramatically with high number of wear cycles. It was shown that wear results were strongly related to material properties, such as modulus of elasticity, while no correlation was found with contact parameters, friction coefficient and contact stiffnes
Numerical Model for Elastic Contact Simulation
No full textRecently, interest has been growing among the engineering community to develop predictive models for the effect of joints on the tribology of jointed structures. The ability to predict contact forces and force-displacement relations of joints is key in enabling simulations to predict forced response and wear of jointed structures. Only for a limited number of contact geometries has a solution in closed-form been found, and it is available in literature. The finite element method has been used to a great extent to solve problems of elastic bodies in contact, but the iterative solution of large models is very demanding. This work deals with the development of a numerical procedure that utilizes the stiffness matrices of the bodies in contact modeled with the finite element method. The matrices are reduced with a lossless static reduction, and their small dimensions make the iterative solution of the contact problem very fast. Results are compared with contact models found in literature and the sample results agree well with corresponding exact solution
A numerical method to solve the normal and tangential contact problem of elastic bodies
No full textRecently, interest has been growing among the engineering community to develop predictive models for the effect of joints on the tribology of jointed structures. The ability to predict contact forces and force-displacement relations of joints is key in enabling simulations to predict forced response and wear of jointed structures. Only for a limited number of contact geometries has a solution in closed-form been found, and it is available in the literature. The finite element method has been used to a great extent to solve problems of elastic bodies in contact, but the iterative solution of large models is very demanding. This work deals with the development of a numerical procedure that utilizes the stiffness matrices of the bodies in contact modeled with the finite element method. The matrices are reduced with a lossless static reduction, and their small dimensions make the iterative solution of the contact problem very fast. Results are compared with contact models found in the literature and the sample results agree well with corresponding analytical solutions
Measurement of contact parameters on under-platform dampers coupled with blade dynamics
No full textIn the field of turbomachinery, one of the prime design aspects is to avoid high cycle fatigue failure commonly caused by fluctuating forces on the blades. These fluctuating forces render a high amplitude vibration of the blades that can be mitigated by introducing devices such as under-platform dampers and shrouds in the blade assembly. These devices are quite efficient in limiting the vibration amplitude of the blade even at the high temperatures that are typical of turbines. However, nonlinear forces are developed at the contact and these nonlinear forces make numerical simulations challenging for the designers. Therefore, along with numerical computations, experimental evidences are necessary to understand the actual physics of the damper-blade interactions. This paper represents an experimental study of the effect of these nonlinear contact forces on the vibration amplitude of the blade The present investigation explores, for the first time, the contact forces and relative displacement between the damper-blade contact interface measured directly using a recently developed experimental setup. These measured forces and relative displacement are further post processed to compute the equivalent contact characteristics, namely equivalent contact stiffness and damping. The contact characteristics are then associated with the classically measured performance of the dampers defined by the variation in frequency response of the blade. The experimental work presented here provides one of the first investigations to relate the macro/global behavior of the blade-damper with the micro/local behavior of their contact
Macchina di prova ad accostamento libero ed alta temperatura per la microtribologia di superfici vibranti piane
No full textIn 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. A test rig for contact parameters measurement 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 determine
On-line calculation of thermal stresses by time integration
Full text linkGreen's function technique (GFT) is largely used for on-line calculation of thermal stresses in machines and plants; it allows directly turning parameters such as fluid temperatures, pressures and flow rates in thermal stresses. Recently the use of the GFT is extended by the authors to cases 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 for thermal stress calculation. In order to reduce computation time it is proposed to convert the convolution integrals which characterize the GFT into time integration of an equivalent system of uncoupled first order differential equations, whose coefficients are estimated fitting Green's functions with a sum of exponential term
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