In-plane modal testing of a free isotropic rectangular plate

In-plane vibration modes of an aluminum panel were experimentally identified from frequency response tests. Responses were measured on the panel edges and at selected locations on the panel surface. The measurements on the surface were made by attaching accelerometers oriented parallel to the panel plane. Resonance frequencies, relative damping ratios and mode shapes were established for the lowest 12 in-plane modes found in the frequency range between 1600 and 7000 Hz. A damping ratio of less than 0.05 percent of critical damping is proved to be valid for the aluminum panel. A finite element software was used to calculate 12 corresponding theoretical in-plane eigenfrequencies and mode shapes. An outline for a nondestructive procedure is suggested to estimate the input data for the elastic constants of an isotropic plate model. Two of the modes were used in analogy with the flexural vibration of beams and plates. The modes illustrate the deformation pattern including shear deformations, through the thickness, for the bending modes of thick beams or plates. The Rayleigh-Timoshenko theory also was used for the calculation of these two eigenfrequencies.