Development of an impact noise reduction method by the adding of a small thickness elastomeric material
Abstract
The starting point of this Ph.D. is the industrial issue submitted to the ETS by the company Bombardier Recreational Products (BRP) of the noise reduction of the tracked drive mechanism of snowmobiles. The overall goal of is to develop a method to predict the impact noise reduction obtained by the adding of an elastomeric layer specimen of small thickness between the impacting body and the impacted structure which is a complex structure (i.e. a structure whose geometry is complex and whose composition involves several materials). To reach this overall goal, three specific goals have been fixed: (1) characterize the behavior under impact of different small thickness elastomeric layers; (2) predict the impact force generated when an elastomeric layer is added on a complex vibrating structure; and (3) validate experimentally the whole method by applying it to the impact noise reduction of a bar of the snowmobile track. To reach the first specific goal (characterize the behavior under impact of different small thickness elastomeric layers), a specific experimental characterization method has been developed. Firstly, an experimental device has been realized to submit the elastomeric layer specimens to the reproducible impact conditions of an impact hammer. The measurement of the penetration depth of the hammer into the elastomeric layer is achieved by recording its motion with a high-speed camera and by detecting its position by further analysis on the individual images. Secondly, the experimental curves obtained are analyzed to point out their main characteristics and choose an appropriate impact model. Thirdly, the contact force parameters are estimated from the experimental results and from the impact model. Using this method, eight impacted elastomeric specimens have been characterized. The results show that a more precise characterization than hardness is obtained. To reach the second specific goal (predict the impact force generated when an elastomeric layer is added on a complex vibrating structure), a simulation model of the impact on a structure whose vibrations are due to bending waves has been used. The physical model developed by the European project "Sounding Object" (Rocchesso et Fontana, 2003) has been chosen. From an analogy between the theory used in this model and the modal formulation used in vibration studies, some first modifications of the original program (MATLAB impact_modal.m script) have been made to simulate physically the impact of a mass on a vibrating structure. Some other modifications of the original program have been made in order to simulate the rigid body motion of the structure in the case of free boundary conditions (because the structure used for the validation of the method has free boundary conditions). To reach the third specific goal (validate experimentally the whole method by applying it to the impact noise reduction of a bar of the snowmobile track), the first step has been the measurement of the force and the acoustic pressure in two configurations: WITH and WITHOUT the elastomeric layer in the contact zone. The second step has been the simulation of the configuration WITH the elastomeric layer by applying the impact model of a mass on a vibrating structure (presented in Chapter 2). In order to estimate the value of the model parameters describing the track bar, the modal parameters of the six first bending modes of the bar have been measured using experimental modal analysis. Finally, validation of the method has been performed firstly by checking experimentally the hypothesis of linearity by comparisons between the reductions of force spectra obtained thanks to the adding of the elastomeric specimen and the reductions of noise spectra. Secondly, validations of the method in time and frequency domains have been performed by comparisons between simulated and measured force signals. These comparisons show that the discrepancies may be high enough for some specimens (especially because the rigid motion of the structure is more complicated than a pure translation) but that the order of magnitude of simulated time and frequency signals is satisfactory. (Abstract shortened by UMI.)
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2010
- Bibcode:
- 2010PhDT........11A
- Keywords:
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- Applied Mechanics;Engineering, Industrial;Physics, Acoustics