Journal of Intelligent Material Systems and Structures

Active composites based on a hollow fiber topology have advantages over solid piezoelectric fiber composites in that they require lower voltages for activation and are not limited to electrically non-conductive matrix materials. One critical issue for hollow fiber composites is the fiber aspect ratio (ratio of wall thickness to fiber radius). In this paper analytical and finite element models, which include electric field variations, were developed to determine the “effective d ₃₁”of an individual piezoelectric hollow fiber. The effective fiber properties were used to derive a single-row lamina strain/electric field model from classical composite theory. The models were validated with a series of deflection/voltage experiments conducted with hollow piezoelectric fibers fabricated utilizing microfabrication by coextrusion (MFCX) techniques. To determine the feasibility of MFCX hollow fiber composites, the models and experimental results were employed to study the effect of the fiber aspect ratio on a variety of fiber/composite design issues: fiber/lamina performance, fiber strength, matrix material, and electric field effects (incomplete poling and field concentrations).

Design and testing of a mesoscale actuator device is presented in this paper. The device uses frequency rectification concepts to amplify the displacements while retaining comparable force outputs to current piezoelectric actuators. The design is based on an inchworm motor with the exception that microridges are used in the clamping system. A FEM model is used to design a device that produces a peak to peak amplitude of 900 N at 11 mm/sec with a weight of 100 g (i.e., specific power of 99 W/kg). Experimental data for piezoelectric stacks and adhesives are used in the design analysis. A prototype device is fabricated and tested with excellent agreement between analysis and testing results. Stress and modal analysis are used to demonstrate that the device has an infinite fatigue life and a first modal frequency at 1777 Hz.

Displacement-amplifying mechanisms can be systematically designed using topology optimization. Due to the special need for large displacement amplification of piezoelectric stacked actuators, suitable objective functions should be formulated. Among various possible formulations, this work considers maximum output stroke, magnification factor and mechanical efficiency as objective functions. As the formulation of maximum magnification factor performs better numerically than the rest, it is recommended as a suitable design goal for displacement amplifiers undergoing undamped harmonic response. The design problem is solved as a 2D material distribution problem using Method of Moving Asymptotes (MMA). Plane stress solid is assumed for the design domain. Stiffness of the actuator and the workpiece is included in the analysis. To show the viability of this design methodology, two examples of magnification mechanisms for printer heads driven at different excitation frequencies were presented.

This paper reports on an experimental study into the application of surface-mounted piezotransducer elements to the detection of remote crack growth under loading conditions promoting severe closure. Crack growth is inferred from measurements of the acoustic power transmitted through lamb modes across wave paths bridging the crack. It is shown that the proposed arrangement is able to detect sub-millimetre crack growth despite the presence of closure, although variations in the degree of closure were found to cause large sporadic fluctuations in the transmitted acoustic power. Although informative of crack-state, these episodes could impair the proper interpretation of the sensor response data, highlighting the need for a cautious and robust approach to the application of the method. In the present implementation, the piezoceramic elements are placed in regions exposed to cyclic stress, raising the issue of element durability. More importantly, no evidence of piezoceramic degradation was found in the course of extended testing at working stress levels, bolstering confidence with respect to the application of these elements to long-term in situ health monitoring.

The optimal dynamic response control of adaptive thin-walled beam cantilevers carrying externally mounted stores and exposed to blast pulses is investigated. The structure under consideration is modeled as a thin-walled beam of closed cross-section contour, encompassing a number of non-classical features such as transverse shear, secondary warping, and anisotropy of the constituent materials. The structure can carry along its span heavy concentrated masses and/or tip stores. The vibration control is achieved via the use of actuating and sensing capabilities provided by piezoelectric devices which are bonded or embedded into the host structure. The cases of piezoactuators spread over the entire span of the structure, or in the form of a patch are considered, and issues related to the influence of patch location and size upon the vibration control efficiency are highlighted. Other issues related to the minimization of the required input power and of that generated by the inclusion/discard in the quadratic performance index of time-dependent external excitations are also addressed.

This paper presents the use of piezoceramic transducers, Lead Zirconia Titanate (PZT) actuators and Polyvinylidene Fluoride (PVDF) sensors, for experimental modal testing of a simply supported plate. A series of rectangular-shaped PVDF films are evenly distributed over the plate and act as the sensing devices instead of traditional acceleration sensors. Pairs of rectangular PZT patches are bounded to opposite sides of the plate to form actuators and when they are excited by 180° out-of-phase stimulation, they will cause pure bending. The theoretical formulation of frequency response functions (FRFs) of the piezoceramic transducers is developed. The mode shape functions of the PZT actuator and the PVDF sensor are identified, respectively. Experiments are performed to obtain a column of FRF matrix. The structural modal parameters, including natural frequencies, modal damping ratios and mode shapes, can then be extracted by a modal parameter extraction method. Results show the modal parameters can be properly obtained and physically interpreted. This paper presents the concepts of smart structural testing (SST) with the use of piezoceramic transducers and leads to the applications of smart structures to health monitoring of structural systems.

This paper demonstrates the concept of sensitivity enhancing control (SEC) to aid in damage detection in smart structures through both experimental and simulation evaluation. Methods of implementing state estimate feedback using point measurements of strain along the structure are described, and an initial proof-of-concept laboratory experiment demonstrating enhancement of modal frequency shifts due to tip mass damage in a cantilevered beam is reported. Simulation results focus on applying state feedback control to finite-element models of a cantilevered structure with slot, through-surface crack, or surface crack damage. The simulation analysis ascertains the ability to enhance sensitivity of modal frequency shifts due to realistic damage cases that are difficult to evaluate experimentally. The simulation also ascertains the potential for using the same sensors and actuators for implementing both sensitivity enhancing control laws and vibration damping control laws that are insensitive to damage. In the control model with which SEC laws are designed, damage consists of simple reductions in thickness over a small area of the structure. Finite-element models to which control laws are applied are developed using commercial software (ABAQUS^‘) that more accurately models stiffness damage by releasing element connections or by using line spring elements to model fatigue cracks. Experimental results show that enhancement in sensitivity of modal frequencies to damage can be achieved using a single piezoceramic actuator and multiple piezoelectric strain sensors along the beam. Simulation results indicate that feedback control laws can be designed for either sensitivity enhancement or vibration suppression using identical hardware, providing for multifunctional smart structures. In addition, analysis demonstrates that commercial finite-element software is useful for model-based simulation of damaged controlled structures.