Resistively shunted piezocomposites for passive damping

The goal of this work was to theoretically model, fabricate and characterize 3-3 coupled piezocomposite structures, using shunted piezoelectric fibers with integrated resistive shunting. Advantages and disadvantages of the different shunted piezocomposite configurations were examined from both modeling and fabrication points of view. Two configurations of practical interest were chosen for detailed study: (1) piezoelectric whiskers in a resistive matrix, and (2) piezoelectric whiskers in a layer of epoxy and shunted with a thin film resistive coating.; Two different models were developed: a “smeared” dynamic model and a finite element model. Composite beam theory was modified to include shunted piezoelectric behavior and governing equations and boundary conditions were formulated. For the finite element model, a shunted piezoelectric element was formulated and the discretized governing equations were converted to state-space form. Modeling results demonstrated the possibility of modal loss factors as high as 10% in a single mode, for an effective piezoelectric volume fraction of 25%, as well the ability to tailor achievable levels of damping. Non-linear potential variation and local effects were successfully modeled. The critical parameters affecting modal damping were piezoelectric whisker volume fraction, shunt resistance, structural geometry and the location of whiskers.; Experimental efforts involved fabrication of shunted piezocomposites for both configurations. Using chopped continuous poled PZT-5H fibers; a whisker/resistive matrix composite was successfully fabricated. For the resistive matrix case, matrix conductivity was a function of filler volume fraction and showed the percolation effect. However, the dielectric constant of the matrix also increased significantly (factor of 1000) at the design filler volume fraction, which drastically altered the electrical behavior of the piezocomposite from the designed case. The change in dielectric constant appeared to be due to percolation.; For the thin film case, piezocomposite specimens of dimensions 1.1 cm x 0.6 cm x 200 μm, with volume fraction between 30–48%, were successfully fabricated and characterized. d₃₃ values ranged from 80 to 300 × 10–12 strain/V/m. Higher values of d₃₃ were obtained for specimens fabricated using unpoled whiskers with poling being performed after cure. Capacitance values of the specimens ranged from 0.9 to 1.2 pF, primarily because of the geometry. SiC films of thicknesses between 1200 and 1700 A° were deposited and resistivities in the range 104 to 105 ohm.m were obtained.; The fabricated 3-3 piezocomposite thin film specimens were externally bonded to an aluminum beam and dynamic tests performed. Measurements showed the presence of stray capacitances higher than that of the specimen (4.6 pF compared to 1.2 pF). The finite element model accurately predicted the effective “short-circuit” due to stray capacitances and resultant change in optimal shunt resistance and reduced damping.