Hindawi Publishing Corporation Smart Materials Research Volume 2012, Article ID 621364, 8 pages doi:10.1155/2012/621364 Research Article Preliminary Study of Optimum Piezoelectric Cross-Ply Composites for Energy Harvesting David N. Betts, H. Alicia Kim, and Christopher R. Bowen Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK Correspondence should be addressed to H. Alicia Kim, h.a.kim@bath.ac.uk Received 24 November 2011; Accepted 29 January 2012 Academic Editor: Sontipee Aimmanee Copyright © 2012 David N. Betts et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Energy harvesting devices based on a piezoelectric material attached to asymmetric bistable laminate plates have been shown to exhibit high levels of power extraction over a wide range of frequencies. This paper optimizes for the design of bistable composites combined with piezoelectrics for energy harvesting applications. The electrical energy generated during state-change, or “snap- through,” is maximized through variation in ply thicknesses and rectangular laminate edge lengths. The design is constrained by a bistability constraint and limits on both the magnitude of deflection and the force required for the reversible actuation. Optimum solutions are obtained for differing numbers of plies and the numerical investigation results are discussed. 1. Introduction Energy harvesting which converts ambient mechanical vibra- tions into electrical energy is an area of considerable re- search interest and has received extensive attention in the past decade. A variety of methods have been considered in- cluding inductive, capacitive, and piezoelectric materials [1– 3]. In many cases harvesting devices have been designed to operate at resonance to optimize the power generation, for example, simple linear cantilever beam configurations. How- ever, ambient vibrations generally exhibit multiple time-de- pendent frequencies which can include components at re- latively low frequencies. This can make typical linear sys- tems inefficient or unsuitable; particularly if the resonant frequency of the device is higher than the frequency range of the vibrations it is attempting to harvest. In order to improve the efficiency of vibrational energy harvesters, recent work has focused on exploiting nonlinearity for broadband energy harvesting. Encouraging results [2] have been obtained using nonlinear or bistable cantilevered beams. Stanton et al. [2] modeled and experimentally validated a non-linear energy harvester using a piezoelectric cantilever. An end magnet on the oscillating cantilever interacts with oppositely poled stationary magnets, which induces softening or hardening into the system and allows the resonance frequency to be tuned. This technique was shown to outperform linear systems when excited by varying frequencies. However, such a system would require an obtrusive arrangement of external magnets and could generate unwanted electromagnetic fields. An alternative method has been recently found where a piezoelectric element is attached to bistable laminate plates with 2n plies and a total (T) layup of [0 n / 90 n ] T to induce large amplitude oscillations [3]. Such harvesting structures have been shown to exhibit high levels of power extraction over a wide range of frequencies. This arrangement can be designed to occupy a smaller space and is potentially more convenient and portable for broadband energy harvesting. Bistable composites have been extensively studied for the development of morphing or adaptive structure concepts [4– 7]. When a composite laminate has an asymmetric stacking sequence the resulting mismatch in thermal expansion coeffi- cients between plies leads to a thermally induced strain. This leads to the laminate developing a curved deformation as it is cooled from its high temperature cure cycle. Under certain geometric conditions the thermal strain can lead to the development of two stable equilibrium states (“bistability”). Such structures are of interest for shape-change applications since the “snap-through” between stable-states results in a large deflection that does not require continuous energy input to be maintained. Figure 1 shows an example of this