Estimation of dynamic paramaters for viscoelastic materials by longitudinal vibration testing using pulse method
In this paper, two methods based on longitudinal vibration testing of a thin bar are proposed for measuring the dynamic parameters of the viscoelastic materials in a wide frequency region. First, an inverse method by FEM based on resonance measurement is presented, in which the measured response of longitudinal vibration is asymptotically approached by the forward calculated response from the finite element computation, then the dynamic parameters can be obtained. Second, an improved wave propagation measurements based on free end longitudinal vibration signal is described. In which the dynamic parameters in the frequency range of 1kHz to 5kHz are calculated from the vibration velocity ratio between the long bar and the short bar.
Complete elastic characterization of viscoelastic materials by dynamic measurements of the complex bulk and Young's moduli as a function of temperature and hydrostatic pressure
Abstract Two independent systems to measure the dynamic complex Young's and bulk moduli of viscoelastic materials as a function of temperature and hydrostatic pressure are described. In the Young's modulus system, a bar-shaped sample is adhered to a piezoelectric shaker and mounted vertically inside an air-filled pressure vessel. Data are obtained using both the traditional resonant approach and a wave-speed technique. In the bulk modulus system, the compressibility of a sample of arbitrary shape immersed in Castor oil and placed inside a pressure chamber is measured. Data can be obtained at frequencies typically ranging from 50 Hz to 5 kHz, at temperatures comprised between −2 and 50 °C and under hydrostatic pressures ranging from 0 to 2 MPa (Young's), or 6.5 MPa (bulk). Typical data obtained with both systems are presented, and it is shown how these data can be combined to completely characterize the elasticity of the material under investigation. In particular, they can be used to obtain experimental values of the complex Poisson's ratio, whose accurate measurement is otherwise quite challenging to perform directly. As an example, the magnitude and loss tangent of Poisson's ratio are presented for a nearly incompressible rubber.
Response of a multi-supported beam to a random pressure field
Abstract Normal mode techniques are used to study the displacement response spectral density of a beam of finite length resting on many simple supports and subjected either to subsonic boundary layer or far field jet noise pressure fluctuations. In the expression for the response spectral density terms arise which represent the acceptance of the pressure field by the structure. These terms are divided into two categories: first, those formed from one mode alone, and, second, those formed from two separate modes. The two types are referred to as joint and cross acceptances respectively. The nature of these acceptances is discussed, and it is shown that both the joint acceptances and those cross acceptances formed from either two symmetric or two antisymmetric modes are wholly real, and that the cross acceptances formed from one symmetric and one antisymmetric mode are wholly imaginary. The effect of these acceptances on the response spectral density is illustrated by a numerical example. In the situation where there is no matching between the modal and pressure cross correlation wavelengths the cross terms are shown to be negligible.