Enhancement of the Dynamic Buckling Load and Analysis of Active Constrained Layer Damping with Extension and Shear Mode Piezoceramic Actuators
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We consider geometric and material nonlinearities when studying numerically, by the finite element method, transient three-dimensional electroelastic deformations of a graphite-epoxy square plate sandwiched between two piezoceramic (PZT) layers. Points on the four edges of the bottom surface of the plate are restrained from moving vertically. The two opposite edges of the plate are loaded by equal in-plane compressive loads that increase linearly with time and the other two edges are kept traction free. The plate material is modeled as orthotropic and neoHookean. For the transversely isotropic PZT the second Piola-Kirchhoff stress tensor and the electric displacement are expressed as second degree polynomials in the Green-St. Venant strain tensor and the electric field. Both direct and converse piezoelectric effects are accounted for in the PZT. The plate is taken to have buckled when its centroidal deflection equals three times the plate thickness. The dynamic buckling load for the plate is found to strongly depend upon the rate of rise of the applied tractions. With the maximum electric field limited to 1kV/mm, the buckling load is enhanced by 18.3$\%$ when the PZT elements are activated. For a peak electric field of 30kV/mm, the buckling load increased by 58.5$\%$. When more than 60$\%$ of the surface area of the top and the bottom surfaces of the plate are covered by the PZT layers, then square PZT elements placed symmetrically about the plate centroid provide a larger enhancement in the buckling load than rectangular shaped or cross-shaped PZT elements. An increase in the plate thickness relative to that of the PZT actuators decreases the effectiveness of the PZT in enhancing the buckling load for the plate. The finite element code was modified to also analyze, in time domain, transient deformations of a viscoelastic material for which the second Piola-Kirchhoff stress tensor is expressed as a linear functional of the strain history of the Green-St. Venant strain tensor. It was used to analyze three-dimensional deformations of a thick laminated plate with layers made of aluminum, a viscoelastic material and a PZT. The following two arrangements of layers are considered. In one case a central PZT layer is surrounded on both sides by viscoelastic layers and aluminum layers are on the outside surfaces. The PZT is poled in the longitudinal direction and an electric field is applied in the thickness direction. Thus shearing deformations of the PZT layer are dominant. In the second arrangement, the aluminum layer is in the middle and the PZT layers are on the outside. The poling direction and the electric field are in the thickness direction; thus its extensional deformations are predominant. Three indices are used to gauge the damping of motion of plate particles, and the effectiveness of PZT actuators in enhancing this damping. It is found that the optimum thickness of the viscoelastic layers for maximum total energy dissipation is the same for each set-up. Also, the total thickness of the PZT layers which results in the maximum value of one of these indices of energy dissipation is the same for the two set-ups. Both arrangements give the largest value of this index for a plate of aspect ratio 10. Buckling behavior of a sandwich plate containing a soft core is also studied. The effects of the ratio of the elastic moduli of the outer layers to those of the core, and of the core thickness on the buckling load are analyzed. The top and the bottom layers are connected by very stiff blocks on two opposite edges where in-plane compressive time-dependent tractions are applied.
- Doctoral Dissertations