Variations in Mechanical Properties of Wood Plates Due Fluctuations on Relative Humidity of Air
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Variations in Mechanical Properties of Wood Plates Due Fluctuations on Relative Humidity of Air J. A. Torres1, M. De Icaza1, R. R. Boullosa2 1
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México Campus Juriquilla, Querétaro, México CP76230. E-mail: [email protected] 2 Laboratorio de Acústica Aplicada y Vibraciones, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Cd. Universitaria México CP 04370.
ABSTRACT Often, mechanical properties of wood plates are accurately approximated through orthotropic characterization. However, the numerical values may show considerable variations in the case of real wood plates immersed in fluctuant environmental conditions, especially if the relative humidity of air (RH) is changing. Problems arising from the natural variations in RH have been reported on making musical instruments. In this work, the influence of changing RH on the mechanical properties of a guitar’s top plate (clamped on its contour) has been measured. Vibratory responses of the plate were experimentally obtained, through forced vibration, while data were recorded for several RH values, without any alteration in the experimental set-up devices. The six lowest natural frequencies of the structure were extracted from peaks detected in the responses. Natural frequencies depend on mass distribution and mechanical properties of the structure. For the highest sensed RH (67 %), each natural frequency of the plate decreased at least in 11 Hz compared with reference values measured at the lowest sensed RH (53 %). To determine the connection between the shift in natural frequencies and the changes in the wood elastic and shear modulus, a finite element model of the plate was performed. It was useful calculating natural frequencies through simulated modal analyses. Elastic and shear modulus were handled to match simulated results with experimental natural frequencies recorded for a RH=53% level. These elastic moduli must be decreased around 10 % to obtain a similar frequency decrease in the experimental natural frequencies obtained under a RH=67% level. The experimental results show quantitative information on an interaction scarcely researched, that is, the modification on mechanical properties of wood plates induced by RH variations. Sensed changes due this wood-air interaction are responsible of a modification in the behavior of musical instruments, as guitars and violins. Keywords: Acoustic, Elastic properties, Structural. INTRODUCTION Intuitively, we know that leaves are more flexible when they are fresh than when they are dry. This is due to their moisture content, which are dramatically different. Analogously, mechanical properties of wood plates are influenced by moisture content of the structure. Tuning and acoustical-mechanical properties of string instruments and woodwinds depend critically on the environment: room temperature, relative humidity of air (RH) and moisture. Continuous changes in temperature and RH are import
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