Preload Influence on the Dynamic Properties of a Polyurethane Elastomeric Foam.

Polymeric foams are widely used in engineering applications for vibration attenuation. The foams usually work preloaded and it is known that the dynamic properties and attenuation ability of these polymers depend on the preload. In this paper, experimental characterization of a polyurethane elastomeric foam is performed in a frequency range between 1 and 60 Hz, a temperature range between -60 and 30 °C and a preload range between 2 and 12 N, using a Dynamic Mechanical Analyzer. When going from the minimum to the maximum preload, results show the linear viscoelastic range increases 57%. In the frequency sweeps, the storage modulus increases 58% on average, while the loss factor remains unaffected by preload. Moreover, the glassy transition temperature of the material decreases for greater preloads. From the curve-fitting of a four-parameter fractional derivative model using the experimental data, a seven-parameter mathematical model is developed, reducing the number of parameters needed to describe the influence of frequency and preload on the dynamic properties of the material. Hence, it has been established that the relaxation time, relaxed modulus and unrelaxed modulus depend on the exponential of the squared prestress. In contrast, the fractional parameter does not depend on preload for the range under study.

A Robust Technique for Polymer Damping Identification Using Experimental Transmissibility Data.

This paper presents a robust method to estimate polymers' damping, based on modal identification methods on frequency functions. The proposed method presents great advantages compared to other traditional methods such as the HPB method for polymeric materials where high damping or noise levels can limit their use. Specifically, this new method is applied on an experimental transmissibility function measured in a composite cantilever beam and the complex modulus is determined as a function of frequency. From this, a regenerated function is obtained based on the Euler-Bernoulli beam theory, and it is compared with experimental data. It can be concluded that the best way to apply the curve-fitting method for further testing of polymeric materials is when it is used with the whole frequency range by means of the MDOF method considering the residuals. In addition, this has the added advantage that the number of experimental tests to be carried out is much lower compared to using the SDOF method.