Degradation of structural integrity is related to existing inclusions. This is attributed to irreversible temperature effects. Damping is an important property of a material subjected to vibratory loads. Among the various damping mechanisms [1], the thermodynamic theory of damping was used to find damping for a porous material in a homogeneous, isotropic, elastic rectangular bar with uniformly distributed cylindrical [2] or ellipsoidal [3] cavities under alternating axial stress. A similar procedure was presented in [4] to determine the damping for cracked beam under alternating axial stress. In these cases additional damping mechanisms were caused by irreversible heat generation and it was shown that thermodynamic damping can be used as a measure of material porosity.

In this work, the thermodynamic damping was used to characterize damping caused by porosity that gives additional damping due to heat generation and non-reversible heat flow for linear, isotropic and homogeneous thermo elastic porous rectangular plates including uniformly distributed cavities. Previous research findings suggested that damping changes due to the existence of porosity is remarkable. For verification, changes in modal damping were evaluated experimentally. Damping measurements were performed on five porous metallic rectangular plates with varying porosities. The data for each plate was inserted in ANSYS to achieve numerical solutions. The modal damping factor of each plate was obtained by applying the logarithmic decrement method [5]. From the experimental values of damping for each plate the change in damping is found by subtracting the damping value of each plate from the damping value of the plate with the lowest porosity. Thus, we find the change in damping which permits the elimination of the effect of all damping mechanisms, apart from the one related to porosity. This damping difference only accounts for the porosity of each plate.

The analytical eigenvalues and natural frequencies for each plate were found to be close enough to the relevant values from ANSYS for all modes of plates. It can be seen that the damping difference that accounts only for the porosity has a positive correlation with the voids, namely, damping increases with increasing porosity through a nearly linear relationship. Concluding, the change in the modal damping factor was proved to depend on porosity. Increasing voids leads to higher damping. The measured damping in this work can be used as an indicator of structural integrity. The proposed research leads to an analytical - arithmetic and experimental tool that can give a reliable and objective solution to applications such as monitoring for the production of materials sensitive to porosity and crack initiation, design of noise-reduction materials, design and preventive maintenance.

1

B.J. Lazan, Damping of Materials and Members in Structural Mechanics, Oxford: Pergamon Press, 1968.

2

S.D. Panteliou, A.D. Dimarogonas, "Thermodynamic Damping in Porous Materials with Ellipsoidal Cavities", Journal of Sound and Vibration, 201(5), 555-565, 1997. doi:10.1006/jsvi.1996.0784

3

S.D. Panteliou, A.D. Dimarogonas, "Thermodynamic Damping in Porous Materials with Spherical Cavities", Journal of Shock and Vibration, Vol. 4, No. 4, pp. 261-268, 1997.

4

S.D. Panteliou, T.G. Chondros, V.C. Argyrakis, A.D. Dimarogonas, "Damping Factor as an Indicator of Crack Severity", Journal of Sound and Vibration, 235-245, 241(2), 2001. doi:10.1006/jsvi.2000.3299

5

A. Dimarogonas, Vibration for Engineers, Englewood Cliffs, N.J.: Prentice-Hall 2nd edition, 1996.

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