Smart Composite Laminates

Smart Composite Laminates

Research Project

Smart materials such as piezoelectrics and shape memory alloys (SMA) are receiving increasing attention due to their possible application in actuators technology, shape morphing structures, energy harvesters, and vibration control. However, their practical diffusion is limited due to restrictions associated with scarce mechanical properties, low electro-mechanical conversion rates, or difficulties in the modulation of their morphed shape while actuated. Overarching objective of this project is developing and characterizing innovative smart structures that can either serve as conductors, energy harvesters, or selectively modulate their shape (shape morphing) by combining innovative piezoelectric materials with SMAs to form a new class of smart structural composites. Final effort of this project is not only the development of innovative smart composite materials, but also the development of prototypal energy harvester and shape morphing structures to assess their effective smart capabilities. The proper development of such a technology involves a broad range of expertise. First, the development, optimization, and characterisation of such smart composite materials. Second, the formulation of tools capable of predicting the complex thermo-electro-mechanical behaviour of the envisioned structures to aid the optimization of their design. Third, the development of mechatronic techniques for the autonomous implementation of the morphing process, which passes through the creation of a robust control policy capable of selectively actuate the morphing structure as a function of its application. To tackle such a challenging process, we here envisage developing smart structures by utilizing both SMAs and innovative piezoelectric nanofibers. In particular, members of the proposed research team have recently developed the piezoelectric polymeric nanofibers production technology. These offer the twofold advantage of significantly increase the electromechanical conversion rate with respect to traditional piezoelectric materials, whereby their morphology allows their introduction into composite laminates at the production stage, resulting into a piezoelectric structural material. Similarly, SMA fibers will be utilized as reinforce for the composite. These allow for higher actuation loads and larger deformations, extending the application ranges. Analytical and numerical models of the thermo-electro-mechanical response will be developed and utilised for the optimisation of the active structures. Results from the proposed research will be finally applied to specific case studies, e.g. a micro-actuator, a energy harvester from a broadband excitation, and shells with shape morphing capabilities under selective control. The potential impact and importance of these goals on materials science, and for a wide spectrum of industrial applications, high-tech industry, and finally in actuating and sensing technology is indeed of extreme interest.

Research activities

1) Development and characterization of active structures for vibration control and energy harvesting. One of the final aims of the present proposal is the development of active composite structures with vibration-control and energy harvesting capabilities. Energy harvesting is associated to piezoelectric-based composites only, whereby an electric load is scavenged from the structural vibrations through the piezoelectric effect. While scavenging the electrical load, structural vibrations are passively mitigated. Active piezoelectric composites will be assembled following the design optimized through the analytical and numerical simulations, then experimentally characterized to assess the effective capability to harvest energy from structural vibrations. The energy harvesting capability will be assessed by applying a broadband excitation to a prototypal structure and parametrically varying the shunting resistance. Adaptive vibration control will be attained selectively modifying the stiffness and the damping characteristics of the SMA-composite through its actuation. This will modify the dynamical response of the structure. By adjusting the stiffness, control will be gained on natural frequencies and mode shapes. By adjusting the damping coefficient, control will be gained on the amplitude of the oscillations lead by a given vibrations pattern. The adaptive vibration control will be experimentally studied by developing a mechatronic experimental device capable to induce both the SIM and the TIM phase transformation and characterizing the dynamical response of the active structure. Enhancing the magnetic permeability of the SMA wires, parasitic currents will be here utilized to apply the desired thermal cycle. This will allow to heat the SMA wires while preventing the matrix to overheat. Selectively varying the natural frequencies can be useful for instance to prevent the stationing of a machine at critical speeds, while the control of adaptive damping coefficient can be usefully applied to the problems of vibration isolation.

2) Development of active composite structures with morphing capabilities. Morphing structures will be here designed as bi-stable composite shells. This part of the project aims characterizing the snap-through phenomenon as a function of multiple variables, such as structural shape, stiffness, and boundary conditions. On one side, peculiarity of the snap-through phenomenon is that there is a high-energy rate involved with it. A high-energy content can thus be scavenged from cyclic switching between two stable configurations. We will thus develop a prototypal bi-stable structure where the structural configuration is modulated by the external load to demonstrate and characterize its energy harvesting potential. On the other side, we will focus on the actuating capabilities of the developed piezo- and SMA-composite materials. The characterization of the actuating capabilities of the piezoelectric structures will be assessed evaluating the minimum voltage supply to be applied for the snap-through initiation. The characterization of the actuating capabilities of the SMA structure will be instead assessed evaluating the thermal cycle to be applied for the snap-through initiation. By exploiting the full potential of the active materials, it is further possible to envision an energetic self-sufficient morphing structure. In this vision, the piezoelectric should harvest energy during the snap-through caused by an external load. As the load is over, the piezoelectric should switch from harvester to actuator, to morph the structure back utilizing the energy scavenged before. Two electrical circuits are thus necessary for the piezoelectric layer (one to harvest energy and one to actuate the snap-through) and a third to actuate the SMA layer. Further, to automate the morphing cycle, it is necessary to have a robust control for the smart switching of the two circuits and a reliable switching policy. In our vision, it is possible to perform the control of the switching policy based on the knowledge of the structural shape. To this aim, we will generalise a methodology for the live-monitoring of the structural deformation field based on FGB sensing technology and a modal decomposition method recently proposed by [Panciroli (1,2)]. FBGs are chosen as these are insensitive to magnetic fields (needed for the SMA-actuation) and can be embedded into the composite at the production stage. For the practical application of such methodology it is needed to generalise its formulation to efficiently account for a multitude of complicating factors, e.g. thermal effects; orthotropic materials; combined effect of membrane, flexural, and shear stresses. Assessment of the influence of the accuracy of the modal base will be a fundamental step within this research topic. Further, we will assess the reliability of the results to variations of number and locations of the strain sensors. Results from this analysis will allow formulating guidelines for the proper design of bistable structures both for energy harvesting and for actuation purposes. Auxiliary outcome will be the development of a structural health monitoring technique based on FBG sensing.