Magnetostrictive Damping to Reduce Noise and Vibrations
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devoted to the behaviour of thick coatings of Fe-Cr-X alloys prepared by using a vacuum plasma spraying technique. The objective of this work is to characterize the crystalline and magnetic structures of such alloys in relation with their damping capacity. The attempts are focused, in particular, on the damping behaviour at higher frequencies and elevated amplitudes typical of urban transportation systems. MATERIALS The materials tested were basically coatings of Fe-16Cr and Fe-16Cr-2A1 (wt.%) deposited onto ferritic stainless steel substrates using a vacuum plasma spraying (VPS) method. In order to provide a realistic comparison between the damping capacity of plasma sprayed coatings and bulk materials, cast alloys with the same chemical composition were also tested. As for coatings, a layer of 2 mm thickness of above alloys was coated on the plates of 130 x 100 x 20 (mm 3 ) using a VPS method from Sulzer Metco AG. The coatings produced by such a way exhibit stratified microstructures where each layer corresponds to one passing of the plasma beam. To prepare the cast samples, raw materials of 99.9% purity were melted with deoxidized elements in an alumina crucible using a high frequency furnace with flowing argon. The ingots of 20 x 100 x 10 (mm 3 ) dimension were obtained from which damping specimens were machined. EXPERIMENTAL PROCEDURE In order to measure the damping capacity of the samples over a wide spectrum of frequencies and amplitudes, three different laboratory devices based on free and forced vibration techniques were used. The torsion pendulum and the resonant bar were assigned to use the free vibration method, and measure the logarithmic decrement of successive amplitudes of the oscillatory system. Each of these devices is able to measure vibration in a particular range of experimental conditions [12]. In contrast, the cantilever uses forced vibrations to determine the loss factor of a beam specimen by measuring the response to excitation at modal frequencies. The beam specimen is excited by an electro-magnetic shaker using a sinusoidal signal which is first obtained by a signal generator and amplified by a power amplifier. This amplified signal is then used to drive a dynamic vibrometer to excite the samples. The system excitation inputs were measured by an accelerometer mounted just above the beam root. The outputs were recorded using a laser vibrometer adjusted at the specimen free tip. Both the input and output signals were compared by a two channel signal analyser to produce the transfer function frequency response from which the specific damping capacity of the beam material was calculated. The beam width and grip length were maintained fixed, but the vibrating length and thickness were left as variable, to obtain various resonance frequencies. By this technique, a wide range of frequencies (10 Hz to 10 kHz) and deformations (F = 10-2 to 10-6) can be tested. To observe magnetic domains which are considered to be at the origin of damping in Fe-Cr based alloys, an optical device was built based on t
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