Fatigue performance of macro-fiber piezoelectric composite actuator with respect to variable beam geometry

Abstract

This study is an investigation into the reliability and performance over the lifetime of the piezoelectric fiber composite, macro fiber composite (MFC), with respect to variable beam geometry. MFC's are a class of smart structure utilizing the piezoelectric effect. The MFC is a thin flexible composite system that can be laminated to surfaces or embedded in classic composite structures for actuation and sensing. These piezocomposite structures are rectangular patches made of Lead-Zirconium-Titinate (PZT) piezoceramic fibers, copper-clad polyimide film, and epoxy. MFC's were originally developed at NASA Langley Research Center and are now commercially available from a single manufacturer. In this study, lifespan and performance were characterized by using the MFC as an actuator to impart deflection in a substrate. This structure is referred to as a Unimorph. The beam geometry affects the bending stiffness of the beam, and thus affects the reaction of the MFC. The only free geometrical dimension in this study was beam height. The unimorph was actuated cyclically by an electrical field of 3E+6 volts per micron at a frequency of 3750 Hz. Expected cycles to failure was 10 9 cycles. The test specimens consisted of cantilevered A2 tool steel beams, with six discrete beam heights, and an MFC patch laminated to one surface by a two-part epoxy. Beam tip displacement measurements were taken using a laser displacement sensor as an indication of cyclical performance over time. The beams were cycled until failure or 10 9 cycles for all beam geometries. The results of the experiment indicate a severe drop off in life with an increase of work energy out of the system. This is a function of the ratio of beam stiffness to MFC stiffness. After a break-in period of less than 250E+6 cycles, no significant degradation in operational performance was indicated by the recorded tip displacement despite an immense amount of crack propagation in the piezoceramic fibers. The results of this testing can be used in designing piezoelectric actuators and as a basis for further study of MFC's.