Journal Articles

Nastic structures are synthetic constructs capable of controllable deformation and shape change similar to plant motility, designed to imitate the biological process of nastic movement found in plants. This paper considers the mechanics and bioenergetics of a prototype nastic structure system consisting of an array of cylindrical microhydraulic actuators embedded in a polymeric plate. Non-uniform expansion/contraction of the actuators in the array may yield an overall shape change resulting in structural morphing. Actuator expansion/contraction is achieved through pressure changes produced by active transport across a bilayer membrane. The active transport process relies on ion-channel proteins that pump sucrose and water molecules across a plasma membrane against the pressure gradient. The energy required by this process is supplied by the hydrolysis of adenosine triphosphate. After reviewing the biochemistry and bioenergetics of the active transport process, the paper presents an analysis of the microhydraulic actuator mechanics predicting the resulting displacement and output energy. Experimental demonstration of fluid transport through a protein transporter follows this discussion. The bilayer membrane is formed from 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt), 1-Palmitoyl-2-Oleoyl-sn-Glycero- 3-Phosphoethanolamine lipids to support the AtSUT4 H+ -sucrose cotransporter

Structural health monitoring is a means for drastically decreasing the maintenance and logistical cost associated with vehicular platforms especially aircraft. A system of small piezoelectric sensors distributed throughout the vehicle will be capable of acting passively or actively to monitor the changes within a structure that presage a component failure, and they will be able to detect and localize all impacts on the structure and evaluate any damage. Piezoelectric thin films were directly integrated with structural titanium utilizing a metal-organic chemical solution approach. The optimum integration strategy yielded a process easily performed without a cleanroom and semiconductor fabrication tools.

An innovative statistical methodology and generic expressions have been developed for the ratio of the dynamic mean excitation to the transmitted mean load under the influence of the important gear characteristics. Simple expressions have been established for identifying cases in which the dynamic mean excitation is more than, less than, or equal to the transmitted mean load.

The capability of embedded piezoelectric wafer active sensors (PWAS) to excite and detect tuned Lamb waves for structural health monitoring is explored. First, a brief review of Lamb waves theory is presented. Second, the PWAS operating principles and their structural coupling through a thin adhesive layer are analyzed. Then, a model of the Lamb waves tuning mechanism with PWAS transducers is described. The model uses the space domain Fourier transform. The analysis is performed in the wavenumber space. The inverse Fourier transform is used to return into the physical space. The integrals are evaluated with the residues theorem. A general solution is obtained for a generic expression of the interface shear stress distribution. The general solution is reduced to a closed-form expression for the case of ideal bonding which admits a closed-form Fourier transform of the interfacial shear stress. It is shown that the strain wave response varies like sin a, whereas the displacement response varies like sinc a. Maximum coupling is achieved when the PWAS length equals the half wavelength of a particular Lamb wave mode. Since Lamb wave modes wavelengths vary with frequency, the tuning of certain modes at certain frequencies can thus be achieved. Tuning curves are derived and verified against experimental results. A particular S0 mode ‘sweet spot’ is found at 300 kHz for a 7-mm PWAS attached to a 1.6-mm aluminum plate. Crack detection via the pulse echo technique using the phased array principle and tuned S0 mode Lamb waves is demonstrated as an effective structural health monitoring method.

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destruc-tive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand inter-rogation of the structure to determine its current state of structural health. The enabling element of embedded ultra-sonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteris-tics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lamb-wave damage detection. In each case, the conventional ultra-sonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and dif-fuse damage in metallic and composite structures are exem-plified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

A project to enhance the mechatronics/microcontroller education of mechanical engineering students at the University of South Carolina is presented. First, the state of the art in Mechatronics education is presented and discussed. Then, focus is shifted to the Mechatronics education in the Department of Mechanical Engineering at the University of South Carolina. Subsequently, the paper examines the hardware and software used for mechatronics/microcontroller education. Examples are given of the MC68HC11 microcontroller and the different evaluation boards used for (a) code development; (b) embedded applications. Then, attention is given to the software used in the mechatronics/microcontroller education. The THRSim11 comprehensive simulation and interfacing software is described. Finally, the paper discusses the interfacing between the microcontroller and the various electro-mechanical sensing and actuation components used in a mechatronics project. The use of functional modules for teaching interfacing skills to mechanical engineering students is described. The paper finishes with conclusions and further work.

The use of the electro-mechanical (E/M) impedance method for health monitoring of thin plates and aerospace structures is described. As a nondestructive evaluation technology, the E/M impedance method allows us to identify the local dynamics of the structure directly through the E/M impedance signatures of piezoelectric wafer active sensors (PWAS) permanently mounted to the structure. An analytical model for 2-D thin-wall structures, which predicts the E/M impedance response at PWAS terminals, was developed and validated. The model accounts for axial and flexural vibrations of the structure and considers both the structural dynamics and the sensor dynamics. Calibration experiments performed on circular thin plates with centrally attached PWAS showed that the presence of damage modifies the high-frequency E/M impedance spectrum causing frequency shifts, peak splitting, and appearance of new harmonics. Overall-statistics damage metrics and probabilistic neural network (PNN) are used to classify the spectral data and identify damage severity. On thin-wall aircraft panels, the presence of damage influences the sensors E/M impedance spectrum. When crack damage is in the PWAS medium field, changes in the distribution of harmonics take place and when crack damage is in the PWAS near field, changes in both the harmonics and the dereverberated response are observed. These effects are successfully classified with PNN and overall-statistics metrics, respectively. This proves that permanently attached PWAS in conjunction with the E/M impedance method can be successfully used in structural health monitoring to detect the presence of incipient damage through the examination and classification of the high-frequency E/M impedance spectra.

Advanced signal processing techniques have been long introduced and widely used in structural health monitoring (SHM) and nondestructive evaluation (NDE). In our research, we applied several signal processing approaches for our embedded ultrasonic structural radar (EUSR) system to obtain improved damage detection results. The EUSR algorithm was developed to detect defects within a large area of a thin-plate specimen using a piezoelectric wafer active sensor (PWAS) array. In the EUSR, the discrete wavelet transform (DWT) was first applied for signal de-noising. Secondly, after constructing the EUSR data, the short-time Fourier transform (STFT) and continuous wavelet transform (CWT) were used for the time-frequency analysis. Then the results were compared thereafter. We eventually chose continuous wavelet transform to filter out from the original signal the component with the excitation signal's frequency. Third, cross correlation method and Hilbert transform were applied to A-scan signals to extract the time of flight (TOF) of the wave packets from the crack. Finally, the Hilbert transform was again applied to the EUSR data to extract the envelopes for final inspection result visualization. The EUSR system was implemented in LabVIEW. Several laboratory experiments have been conducted and have verified that, with the advanced signal processing approaches, the EUSR has enhanced damage detection ability.

A new generic approach to condition monitoring and diagnostic feature-extraction is advocated. This generic approach is optimal for the analysis of frequency domain data for health monitoring and damage detection. The approach consists of using simultaneously new diagnostic features; real and imaginary parts of the Fourier transform. Thus approach is more genetic than conventional approaches based on power spectral density, and it can be shown that the power spectral density approach is a particular case of the proposed new generic approach. Numerical examples are given based on the analysis of synthetic signals generated using the nonlinear model of a diagnosis object. The synthetic signals simulated the forced vibroacoustical oscillation of a cracked part of machinery under narrow band Gaussian excitation. Our new generic approach is used for detecting damage consisting of a fatigue crack of various relative sizes. The numerical examples show that the detection, based on Fisher's criterion, was definitely more effective when using our new diagnostic features than when using the conventional power spectral density feature. The proposed new generic approach to damage detection offers a clear effectiveness improvement over the conventional approach based on the power spectral density.

Embedded-ultrasonics structural radar (EUSR) is a new concept and methodology for in situ nondestructive evaluation (NDE) and structural health monitoring (SHM) of thin-wall structures. EUSR consists of: (a) an array of piezoelectric wafer active sensors (PWAS) embedded into the structure; and (b) electronic modules for signal transmission/reception, processing, and interpretation. EUSR utilizes guided elastic waves (Lamb waves) generated omnidirectionally into the thin-wall structure by surface-mounted permanently attached PWAS. The paper starts with the general concepts of the EUSR algorithm: transmission beamforming, reception beamforming, and time-of-fight (TOF) determination. Next, details of the Lamb wave generation with PWAS, verification of group-velocity dispersion curves, identification of optimal excitation frequency, and confirmation of wave front omnidirectionality are discussed. In the third part of the paper, the actual implementation of the EUSR method in a proof-of-concept demonstration is presented. The construction of the PWAS-phased array is described, and detection of cracks located broadside and offside of the PWAS array is illustrated. The method is shown to be easy to use through a visually interactive LabView™ interface. Very good detection accuracy is observed. The proof-of-concept experiments presented in this paper were illustrated on metallic structures; however, the EUSR concept may also work on composite and hybrid structures, although the range of detection may be reduced by the medium attenuation.