Thin unsymmetric composite laminates possess more than one equilibrium position when cooled to room temperature due to the difference in thermal expansion of the plies. Bistable cross-ply laminates have cylindrical shapes at room temperature provided appropriate side-length-to-thickness ratio is used. The mechanical bistability makes the laminate able to snap from one stable equilibrium to the other when displacement reaches a critical value. This snapthrough motion is highly nonlinear and associated with large-amplitude vibrations. This property opens chances for bistable laminates in morphing and energy harvesting applications.
The snapthrough and free vibration response of bistable cross-ply [90n/0n] composite laminates is investigated using a simplified Rayleigh-Ritz model that depends on only four time-dependent parameters for the general dynamic response. The laminate is modelled according to the classical lamination plate theory taking into account the von Karman geometric nonlinearity. The simplified model is validated against experimental and finite element results and an acceptable agreement is obtained. The laminate’s length-to-thickness ratio is key to assess the existence of bistability. The model is used to investigate the snapthrough response of an 8-ply [904/04] laminate that is subjected to three loading schemes: concentrated moments, normal forces, and tangential forces.
An experimental study of the single-well and twin-well, also referred to as intra-well and inter-well, respectively, nonlinear dynamics of a four-ply [0/90/0/90] carbon-fiber bistable laminate. An electromechanical shaker is used to excite the laminate at its center by a controlled amplitude and frequency excitation. The dynamic response of the laminate is measured using a Polytec laser vibrometer. The laminate’s natural frequencies and damping under small-amplitude excitations that would match the natural frequencies of an underlying linear system are experimentally identified. A primary-resonance excitation of the first bending mode is carried out using amplitude sweep and frequency sweep. Single-well and twin-well periodic and chaotic responses are identified. It is shown that the period-doubling cascade is responsible for the escape from the single-well to the twin-well response.
This study opens chances for bistable laminates as a potential morphing structure. For instance, there is ongoing research on the dynamic control of piezoelectrically actuated bistable composite wings of small aircraft under aerodynamic loading. The bistability of the laminate is exploited to achieve self-equilibrated positions while aerodynamically loaded. The dynamic control is based on the concept of the reduced stiffness of the structure near resonance which makes the PZT patches attached to the wing able to induce snapthrough and reverse snapthrough as needed.
Dr. Samir Emam received his Ph.D. in Engineering Mechanics from Virginia Polytechnic Institute and State University (Virginia Tech), USA, in 2002. Following his graduation, he worked for Virginia Tech as a postdoctoral associate for about one year before he went back home to Egypt to work as an Assistant Professor for three years for Zagazig University. In 2006, he joined the Department of Mechanical Engineering, UAE University, Al Ain, as an Assistant Professor where he got promoted to the rank of Associate Professor in 2012. In January 2017, he joined the Department of Mechanical Engineering, the American University of Sharjah, UAE, as an Associate Professor where he is currently serving. Samir Emam’s areas of expertise include nonlinear vibrations, mechanics of composite structures, buckling and post-buckling of elastic structures, vibration energy harvesting/vibration control and nanoscale mechanics.
The Google Scholar webpage of Dr. Emam can be reached here