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Školitel: Jan Tippner
Instituce: Mendel University in Brno, Faculty of Forestry and Wood Technology
Obor: Wood Technology
O mém projektu
The knowledge of dynamic properties of wood is crucial for mechanical analysis and acoustic applications in wood science and technology. To understood the influence of factors on dynamic properties of wood (that include also acoustic properties), there are two different approaches. The first is the experimental one and the second one is theoretical analysis. As we know, wood is a natural compound and it has a hierarchical structure: from microscopic to macroscopic scale. Wood is characterized by anisotropy: depending by the orientation of our observation physical properties change. Factors of wood: -Microstructure: wood cells may be treated as “tubes” of cellulosic crystalline substance embedded into an amorphous matrix - lignin. The longitudinal orientation of the tubes is slightly disturbed by horizontal elements, the medullary rays. In the longitudinal direction the dissipation of acoustical energy takes place at the edges of the tubes. Accordingly, the longitudinal axes which are constructed from long elements provide high values of velocities and relatively small values of attenuation. The highest attenuation is expected in the transversal direction in which no continuous structural elements exist (Bucur, 1980). Latewood, with more density, shows high velocities of sound compared to earlywood. Diameter of cellular elements influence acoustic of wood: the wider the cellular lumen the more muffled and obtuse the sound will be (Burckle and Grissino- Mayer, 2003). The microfibrillar angle must be minimal, but the angular difference between early and late wood must also be small (Buksnowit, 2006). Moreover, the length axial xylem cells, like fiber and vessel elements in hardwoods or tracheids in softwoods, are considered the most significant parameter at the microscopic level (Baar and Tippner, 2016). -Then, other factors influence the acoustic properties: for example moisture and temperature. Analysis: -Experimental will include acoustic analysis and vibrometry- non destructive evaluations. A. Stress-wave method (impact) is used to define the Modulus of Elasticity reproducing resonance vibration: this approach was developed by Sobue (1986a,b,c) and involves tapping the specimen with a hammer and receiving the signal with a wireless microphone. A computer connected to the system permitted the instantaneous treatment of data (Bucur, 2006) through an FFT analyzer. B. Ultrasonic technique could be used for wood material. The technique is very simple and the time measurements very accurate with error less than 1% (Bucur, 2006). It consists in a generator of impulses and two piezoelectric transducers. In this way we record the signal's time taken to cross the sample, then we derive the velocity of sound. -Theoretical approach will involve the numerical simulation. Modelling is a simplification of reality, a simulation of physical systems' behavior through computer. In the field of Wood Technology, numerical simulation is made to understand the complex relationship between wood and sound. The most effective and wide-spread method is note like FEM: Finite Element Method. The FE models of vibro-acoustic behaviour of wood will be build-up and used for sensitivity analyses of influence of selected factors.