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This paper presents the preliminary evaluation results of a magnetorheological elastomer (MRE) patch fabricated by oriented flake-type Fe–Ga alloy (Galfenol) particles bound in a silicone polymer matrix. A force/pressure sensor, based on the Galfenol MRE patch, was developed to verify the advantages of the MRE with magnetically aligned Galfenol flakes. Hall effect sensors embedded in the MRE sensing device detect the change of magnetic flux density due to the deformation of the Galfenol MRE patch induced by an externally applied force with non-magnetic disturbance. The Galfenol flakes are standing up and well aligned to the out-of-plane of the MRE patch using a 1 in diameter permanent magnet, parallel to the applied force direction to improve sensing performance and sensitivity. It was measured at $\sim 105$ mV in the sensor response to a thumb pressure, which exhibited five times higher sensing performance compared to that of another MRE patch with the Galfenol flakes aligned to the in-plane direction.

Extensive studies have been conducted to examine the use of piezoelectric transducers and other ultrasonic devices for structural health monitoring applications due to their high electroacoustic efficiency and simple operation principles. Most studies use guided Lamb wave inspection methods to detect, locate, and characterize damage in relatively simple plate-like structures. These studies have proven that this type of technique is useful for structural health monitoring in the simple structures, but little work has shown feasibility when using more realistic test articles. In our study, we present a sparse array technique using multiple piezoelectric transducers mounted to an external composite panel on a legacy F/A-18 wing section to investigate its capabilities for more realistic structural health monitoring applications. The panel is secured to the wing by a series of bolts surrounding the panel, and as the main method of simulating damage to the wing, these bolts are loosened by a specified torque in several different cases. The guided Lamb wave pitch-catch inspection method is used to identify such simulated damage by comparing baseline and damaged trials using differential analysis and a spatial mapping approach. This approach requires an estimate of the Lamb wave mode group velocities in the composite panel along with the geometry of the transducer array and the transducers location metrics. Despite the complexities of using a realistic test article, the sparse piezoelectric array combined with the Lamb wave inspection method shows promise in its ability to accurately detect, locate, and characterize the simulated damage to the airframe.

Background Rod fracture is a common complication of growing rods and can result in loss of correction, patient discomfort, and unplanned revision surgery. The ability to quantitate rod integrity at each lengthening would be advantageous to avoid this complication. We investigate the feasibility of applying structural health monitoring to evaluate the integrity of growing rods in vitro. Methods Single-rod titanium 4.5-mm growing rod constructs (n = 9), one screw proximally and one distally connected by in-line connectors, were assembled with pedicle screws fixed in polyethylene blocks. Proximal and distal ends were loaded and constructs subjected to cyclic axial compression (0–100 N at 1 Hz), with incrementally increasing maximum compressive loads of 10 N every 9k cycles until failure. Four piezoceramic transducers (PZTs) were mounted along the length the constructs to interrogate the integrity of the rods with an ultrasonic, guided lamb wave approach. Every 9k cycles, an 80 V excitatory voltage was applied to a PZT to generate high-frequency vibrations, which, after propagating through the construct, was detected by the remaining PZTs. Amplitude differences between pre- and postload waveform signals were calculated until rod failure. Results Average construct lifetime was 88,991 ± 13,398 cycles. All constructs failed due to rod fracture within 21 mm (mean = 15 ± 4.5 mm) of a screw or connector. Amplitude differences between pre- and postload increased in a stepwise fashion as constructs were cycled. Compared to baseline, we found a 1.8 ± 0.6-fold increase in amplitude 18k cycles before failure, a 2.2 ± 1.0-fold increase in amplitude 9k cycles before failure, and a 2.75 ± 1.5-fold increase in amplitude immediately before rod fracture. Conclusion We describe a potential method for assessing the structural integrity of growing rods using ultrasonic structural health monitoring. These preliminary data demonstrate the ability of periodic rod assessment to detect structural changes in cycled growing rods, which appear to correspond to subclinical rod fatigue before rod fracture.

Ultrasonic guided wave (GW) techniques have been used widely for structural health monitoring applications in various engineering structures such as rods, tubes, pipes and plates [1]. Magneto-strictive sensors/transducers [2] provide an attractive alternative to piezoelectric transducers for generating and detecting GWs because there are some advantages such as cost-effectiveness, flexibility, durability, no depolarization effect, and non-contact sensing. Results from our previous work validated that a magnetostrictive patch transducer (MPT) composed of a highly textured Fe-Ga alloy (Galfenol) sheet with a preferred orientation was capable of directional sensing of incoming guided Lamb waves (GLWs) in thin aluminum plates [3]. The sensing feature of the proposed MPT resulted from the unique anisotropic magnetostriction nature of the textured Galfenol patch. The Galfenol patch exhibited high magnetostriction values of ∼270 ppm along the orientation, approaching to around 80% of the single crystal magnetostriction performance at the same composition [4]. The use of the magnetostrictive component in the Galfenol-based MPT was recently extended to a polymer-bonded composite with Galfenol flakes for substituting the bulk Galfenol patch [5]. The metal-polymer composite offers useful band width at high frequencies due to high electrical resistivity and a non-conducting binder [6]. The prior work demonstrated the effect of particle shape on sensing performance and directional sensitivity of three different MPTs composed of spherical nickel, granular and flake-type Galfenol powders, respectively [5]. The use of Galfenol flakes in the composite-based MPT showed the best sensing response among those three kinds of powders. It was because the demagnetizing field caused by shape anisotropy was possibly minimized in the flake-type particles along the in-plane direction [7]. In addition to the small demagnetizing field effect, the packing density of composites can be enhanced if the particles are well aligned. Thus, in order to make the composite compact, the processing parameters such as particle size and epoxy content should be optimized for maximizing sensing performance.

This article presents the magnetic shape anisotropy effect on sensing performance and directional sensitivity in a directional magnetostrictive patch transducer using a uniaxial comb-shaped nickel patch, for the use of ultrasonic guided Lamb wave technology in metallic planar structure. The uniaxial comb patch was modified from a disc-shaped patch made of polycrystalline nickel, so that its comb fingers were aligned to a specific radial direction to promote the magnetic shape anisotropy in the nickel patch. Previously, it was found that a magnetostrictive patch transducer using a nickel disc patch appeared to have omnidirectional sensitivity for sensing guided Lamb waves in an aluminum plate due to the isotropic magnetostrictive nature of polycrystalline nickel. In this work, we initially investigated a theoretical approach to determine demagnetization factor change caused by the geometrical shape modification of a nickel patch, which strongly correlated with a specific magnetization characteristic of the patch. Then, experimental guided Lamb wave analysis was followed to demonstrate the directional guided Lamb wave sensitivity of magnetostrictive patch transducers using nickel patches with three different geometrical configurations. We verified from the experimental guided Lamb wave testing that the proposed magnetostrictive patch transducer using the uniaxial comb-shaped nickel patch exhibited excellent directional sensitivity along the comb fingers’ orientation in the aluminum plate.

This paper presents the development of an ultrasonic guided Lamb wave (GLW)-based magnetostrictive phased array sensor (MPAS) using a circular comb-shaped nickel disc patch with 1” in diameter. And its damage detection capability to identify loosened joint bolts is experimentally demonstrated. The compact sized MPAS was comprised of the nickel disc patch and a detachable magnetic circuit device. The disc patch was machined with 24 comb fingers along its radial direction and the magnetic circuit device contained 6 sensing coils and cylindrical biasing magnets. The individual sensing coils appear to have distinct directional sensing preferences designated by the normal direction of coil winding. The directional sensing feature of the developed MPAS is offered by the combined effect of the magnetic shape anisotropy of comb finger formation in the nickel patch and the sensing directionality of the coil sensor. The MPAS detects the strain-induced magnetic property change on the nickel comb patch due to the mechanical interaction between the patch and GLWs. Although the MPAS holds only the 6 physical coil sensors, the array sensor enables to acquire additional GLW signal data from different sensing sections within the nickel patch, by simply altering the rotational orientation of the magnetic circuit device. Such signal data additions allow to provide a higher resolution damage detection scheme for the advanced phased array signal processing technique. The MPAS apparatus and its damage detection capability were experimentally validated by GLW inspection testing with a thin aluminum plate installed with numerous joint bolts.

An experimental methodology intended to characterize the autorotation efficiency and vehicle dynamics of samara-like micro-air vehicles is presented. The scale inherent in these vehicles precludes the use of most conventional in situ sensors, so data collection is limited to external observation. The studies presented herein demonstrate a new technique for collecting flight data from a vision-based, motion-capture system. A test apparatus has been constructed to yield repeatable initial conditions, minimizing uncertainty of the samara's dynamic state prior to data collection. The test specimen is constructed of an ultraviolet-cured liquid polymer, which is dispensed and cured via the use of a rapid prototyping machine. The mechanical samaras in this study are compared on the basis of equal wing loading. The results of these experiments identify a geometry for a minimal descent rate as well as producing a quantitative Fourier series representation of the roll and pitch. In addition, a new relationship has been established, which elucidates trends in samara descent velocity not captured by wing loading or disk loading. This information will aid in the development and validation of an inflow model for the mechanical samara.

A new and robust 2D phased array technique with multiple distributed actuators is experimentally studied for damage detection in a thin isotropic panel. This array technique is combined with guided Lamb wave (GLW) interrogation. A 2D phased array technique using a single actuator, located near the center of the array, is unable to detect a linear crack whose orientation is normal to the wavefront of the GLW excited from the actuator. To overcome this limitation, the array is coupled with multiple actuators positioned at four different locations on a test panel, while the array is mounted at the center of the panel. A piezoceramic-based 2D spiral phased array is used as a sensor array. The associated array signal processing is used to produce array responses and detect two types of damages, such as a rubber patch and linear crack. An innovative GLW propagation and reflection analysis method is implemented to evaluate the damage locations within the panel geometry. Experimental results demonstrate that the spiral array damage detection technique, using multiple distributed actuators, can provide more robust damage detection scheme, in thin isotropic panels, than the array technique with a single actuator collocated at the center of the array.

Pulsars are unique celestial sources that produce distinctive, periodic signals. Since their discovery, they have been considered as potential navigation aids and timing beacons to form a reference coordinate system and universal time scale. Further study of these astronomical objects has shown that their pulse timing can achieve sub-microsecond level performance, and interest and research into these sources has grown to conceive a fully autonomous navigation and timing system for deep space vehicles. Sources that radiate within the X-ray band of the electromagnetic spectrum have high potential for creating a practical spacecraft navigation solution, which includes accurate time and position determination onboard a space vehicle using the precisely periodic sources, and precise attitude determination using both pulsating and steady sources. This paper provides an overview of spacecraft navigation methods and various research and results that have evolved since the concept of X-ray pulsar-based navigation was first proposed.

There is currently a large effort underway to understand the flight dynamics of avian-based flapping-wing vehicles, or ornithopters, as they represent a critical intersection between existing biological flyers and the need for small aerial robots to conduct a variety ofmission scenarios. Efforts tomodel the flight dynamics of these vehicles for feedback control have been complicated by a number of factors including nonlinear flight motions, unsteady aerodynamics at lowReynolds numbers, and limited sensor payload capacity. This paper presents data for a 0.45 kg ornithopter research platform, flown in straight and level mean flight. A visual tracking system was employed to follow retroreflective markers on the ornithopter and reconstruct state measurements. A multibody model of the flight dynamics was used to investigate the spatial distribution of kinematic variables over the duration of a wing stroke, and system identification techniqueswere employed to extractmodels for the lift, thrust, andpitchingmoment coefficients. Two methods of parameter estimation showed good results for relatively simple aerodynamic models that can be used for feedback control.

This paper details the flight dynamics and control of a prototype mono-wing rotorcraft that mimics the passive transit of the species of samara (winged seed), Acer diabolicum Blume. The asymmetric and all-rotating platform requires the development of a novel sensing and control framework. The general rigid body dynamics are separated into rotor dynamics and particle navigation, which are derived for a coordinated helical turn flight path. The equations of motion are used to calculate the forces necessary for flight along a trajectory recorded with a visual motion capture system. The result is a framework for state estimation and control, applicable to scaled versions of the robotic samara.

Damage detection in composite laminated panels using Lamb waves is demonstrated with an innovative use of a sensor array and processing algorithm. Two models were developed to characterize the Lamb wave propagation properties of orthotropic panels. Predictions of the dispersion relations were made for a fiber-reinforced composite laminate. Experiments were conducted to empirically characterize the wave propagation behavior in a manufactured laminate. Piezoelectric patches were used as sensors and actuators in the experiments. Comparisons were made between analytical predictions and experimental results, which demonstrate that the higher order model captured essential wave propagation behavior at frequencies of interest. Sensor arrays and associated processing were used for wavenumber decomposition and filtering of the Lamb wave modes. Composite laminates were manufactured with an embedded defect to simulate inter-ply delamination. Experiments were conducted to detect the presence of delamination damage in a composite laminate.

The current investigation focuses on the development of a low-cost computational methodology to design and optimize hovering rotors for small-scale vehicles using circular arc airfoils. Rotors for vehicles having a representative Re between 5000 and 60,000 were considered. A detailed experimental study of rectangular and tapered blades generated the data necessary to identify main performance trends and the effect of planform modifications. A blade element momentum theory model coupled with a table lookup scheme was implemented. The database was interpolated along three dimensions (Re, camber, and angle of attack) to obtain the local aerodynamic coefficients used in the calculation of the nonlinear inflow distribution along the blade span. Two methodologies were used in the calculation of the database. First, a purely numerical approach using the two-dimensional flow solver INS2D was evaluated. Second, a reverse method that used the experimental rotor data to refine the original previously generated database was investigated. Validation showed that the model predictive capabilities improved with the empirical corrections. An optimization algorithm was implemented to perform a grid search using power loading as the hover efficiency metric. The methodology proposed is able to optimize blade geometry and operating conditions following imposed constraints within a defined design space.

Miniature helicopters provide a unique design choice for performing missions autonomously, vital to which is knowledge of a dynamic model to enable the development of control and state estimation algorithms. Toward these goals, a linear model for a miniature electric helicopter in hovering flight was identified from data obtained from a visual positioning system. Model structure determination was performed using first principles, previous work, and stepwise regression. Model parameters were computed using a two-step equation-error/output-error process in both the time and frequency domains, resulting in two complete sets of parameter estimates. Modal characteristics and predictive capability of the identified models were compared with those of a model previously identified for this aircraft using the frequency-response method.

This paper presents a methodology for detecting and diagnosing gear faults in the planetary stage of a helicopter transmission. This diagnostic technique is based on the constrained adaptive lifting (CAL) algorithm, an adaptive manifestation of the lifting scheme. Lifting is a time domain, prediction-error realization of the wavelet transform that allows for greater flexibility in the construction of wavelet bases. Adaptivity is desirable for gear diagnostics as it allows the technique to tailor itself to a specific transmission by selecting a set of wavelets that best represent vibration signals obtained while the gearbox is operating under healthy-state conditions. However, constraints on certain basis characteristics are necessary to enhance the detection of local wave-form changes caused by certain types of gear damage. The proposed methodology analyzes individual tooth-mesh waveforms from a healthy-state gearbox vibration signal that was generated using the vibration separation synchronous signal-averaging algorithm. Each waveform is separated into analysis domains using zeros of its slope and curvature. The bases selected in each analysis domain are chosen to minimize the prediction error, and constrained to have approximately the same-sign local slope and curvature as the original signal. The resulting set of bases is used to analyze future-state vibration signals and the lifting prediction error is inspected. The constraints allow the transform to effectively adapt to global amplitude changes, yielding small prediction errors. However, local waveform changes associated with certain types of gear damage are poorly adapted, causing a significant change in the prediction error. A diagnostic metric based on the lifting prediction error vector termed CAL4 is developed. The CAL diagnostic algorithm is validated using data collected from the University of Maryland Transmission Test Rig and the CAL4 metric is compared with the classic metric FM4.

This paper discusses the design, development, and testing of an unmanned aerial vehicle pneumatic telescopic wing that permits a change in the wingspan, while simultaneously supporting structural wing loads. The key element of the wing is a pressurized telescopic spar able to undergo large-scale spanwise changes while supporting wing loadings in excess of 15 lb/ft 2 . The wing cross section is maintained by NACA 0013 rib sections fixed at the end of each element of the telescopic spar. Hollow fiberglass shells are used to preserve the spanwise airfoil geometry and ensure compact storage and deployment of the telescopic wing. A full-scale telescopic wing assembly was built and tested in the Glenn L. Martin Wind Tunnel at the University of Maryland. These tests included aerodynamic measurements at a variety of Reynolds numbers. The telescopic wing was tested in three different configurations and experimental results are compared with finite wing theory and results obtained on a rigid fixed-wing counterpart Preliminary aerodynamic results were promising for the variable wingspan telescopic wing. As expected, the telescopic wing at maximum deployment incurred a slightly larger drag penalty and a reduced lift-to-drag ratio when compared to its solid fixed-wing counterpart. However, the penalty was minimal and thus the development of an unmanned aerial vehicle with a pneumatic variable span wing is feasible.

Active periodic structures exhibit unique dynamic characteristics that make them act as tunable mechanical filters for wave propagation. As a result, waves can propagate along the periodic structures only within specific frequency bands called the 'pass bands' and wave propagation is completely blocked within other frequency bands called the 'stop bands'. The spectral width of these bands can be tuned according to the nature of the external excitation. In this paper, the emphasis is placed on developing a new class of these periodic structures called active periodic struts (APS) which can be used to support gearbox systems on the airframes of helicopters. When designed properly the APS can stop the propagation of vibration from the gearbox to the airframe within critical frequency bands and consequently minimize the effects of transmission of undesirable vibration and sound radiation to the helicopter cabin. The theory governing the operation of this class of APS is presented and their tunable filtering characteristics are demonstrated experimentally as a function of their design parameters. The presented concept of the APS can be easily employed in many applications to control the wave propagation and the force transmission in an attempt to stop/confine the propagation of undesirable disturbances.

The use of pulsars for spacecraft position determination has been considered since their discovery. These celestial sources provide unique signals that can be detected by sensors placed onboard spacecraft. Upon sufficient detection and processing, these signals can be used to generate range measurements with respect to an inertial ref- erence location. Multiple measurements can be used to maintain an accurate navigation solution and enhance autonomous vehicle operation. This paper provides a description of blending pulsar-derived range measurements within a Kalman filter for continuous determination of Earth-orbiting spacecraft position and velocity. Several examples at different orbital altitudes are presented to establish the expected performance using recursive meas- urements obtained from models of pulsed X-ray signals.

This paper discusses a new signal processing tool involving the use of empirical mode decomposition and its application to health monitoring of structures. Empirical mode decomposition is a time-series analysis method that extracts a custom set of basis functions to describe the vibratory response of a system. In conjunction with the Hilbert Transform, the empirical mode decomposition method provides some unique information about the nature of the vibratory response. In this paper, the method is used to process time-series data from a variety of 1-D structures with and without structural damage. Empirically derived basis functions are processed through the Hilbert–Huang Transform to obtain magnitude, phase, and damping information. This magnitude, phase, and damping information is later processed to extract the underlying incident energy propagating through the structure. This incident energy is also referred to as the dereverberated response of a structure. Using simple physics-based models of 1-D structures, it is possible to determine the location and extent of damage by tracking phase properties between successive degrees of freedom. This paper also presents experimental validation of this approach using a civil building model. Results illustrate that this new time-series method is a powerful signal processing tool that tracks unique features in the vibratory response of structures.

Nomenclature Ar = rotor disk area CD = sectional drag coefficient CD0 = zero-lift drag coefficient Clα = lift-curve slope CP = power coefficient CPi = induced power coefficient CP0 = profile power coefficient CT = thrust coefficient c = chord length D = drag force D.L . = disk loading L = lift force m = mass P.L . = power loading SF = separated flow T = rotor thrust V = local wind velocity perceived by flap W = weight W f = final weight Wo = gross takeoff weight α = blade section angle of attack η = efficiency μ = dynamic viscosity ρ = air density σ = rotor solidity = flapping amplitude (peak to peak)

The feasibility of determining spacecraft time and position using x-ray pulsars is explored. Pulsars are rapidly rotating neutron stars that generate pulsed electromagnetic radiation. A detailed analysis of eight x-ray pulsars is presented to quantify expected spacecraft position accuracy based on described pulsar properties, detector parameters, and pulsar observation times. In addition, a time transformation equation is developed to provide comparisons of measured and predicted pulse time of arrival for accurate time and position determination. This model is used in a new pulsar navigation approach that provides corrections to estimated spacecraft position. This approach is evaluated using recorded flight data obtained from the unconventional stellar aspect x-ray timing experiment. Results from these data provide first demonstration of position determination using the Crab pulsar.

The concept of a dual stiffness sensor that measures material degradation at the micromechanics scale and introduces damage variables to reflect average material degradation at a macromechanics scale is developed. The idea of a mechanical multifunctional dual stiffness sensor for in situ real-time stiffness and energy density measurements is motivated by Eshelby’s equivalent inclusion theorem. A complete analytical derivation is presented and the use of a dual stiffness/energy sensor to diagnose damage and to determine the remaining life of structures is evaluated. A numerical verification using finite element analysis is presented, and the results indicate that the sensor can be used for diagnostics and prognostics of structures. A distributed array of sensors is used to determine the onset and location of damage. Experimental results presented for uniaxial experimental tests under monotonic quasi-static loading conditions indicate that the concept can easily be implemented. For a sensor pair of known geometry and material properties, the Eshelby tensors are independent of the host: A methodology was thus developed to detect material properties of different hosts and was used to predict accurately elastic material properties of diverse materials.

Passive periodic structures exhibit unique dynamic characteristics that make them act as mechanical filters for wave propagation. As a result, waves can propagate along the periodic structures only within specific frequency bands called “pass bands” and wave propagation is completely blocked within other frequency bands called “stop bands”. In this paper, the emphasis is placed on developing a new class of these periodic structures called passive periodic struts, which can be used to support gearbox systems on the airframes of helicopters. When designed properly, the passive periodic strut can stop the propagation of vibration from the gearbox to the airframe within critical frequency bands, consequently minimizing the effects of transmission of undesirable vibration and sound radiation to the helicopter cabin. The theory governing the operation of this class of passive periodic struts is introduced and their filtering characteristics are demonstrated experimentally as a function of their design parameters. The presented concept of the passive periodic strut can be easily used in many applications to control the wave propagation and the force transmission in both the spectral and spatial domains in an attempt to stop/confine the propagation of undesirable disturbances.

Over the past 25 years, much research has been devoted to the development of Health and Usage Monitoring (HUM) systems for rotorcraft gearbox and drivetrain components. The promise of HUM systems is the ability to provide accurate information regarding the condition of various flight critical components. This paper reviews the state of the art in vibration-based helicopter transmission diagnostics. The development of various damage detection techniques is discussed from a historical perspective, and the ability of these techniques to detect damage in helicopter transmissions is reviewed. Emerging research trends suggest that improvements in signal processing, sensor development and individual-tooth mesh waveform modelling could improve the performance of current and future helicopter transmission diagnostics.

Fo ra spacecraft to make scientific measurements in the upper atmosphere, it might be necessary for the spacecraft to “dip” into the upper atmosphere. This aeroassist maneuver presents stability challenges that are compounded if the spacecraft contains long flexible booms. Several stability aspects that are unique to this flexible class of spacecraft undertaking an aeroassist are examined. Using a detailed, linear model, simple analytic equations are developed to predict the stability of spacecraft with two or four flexible booms in symmetric configurations. In general, rearward-positioned and rearward-oriented booms help stabilize the spacecraft, but the spacecraft becomes less stable as boom flexibility increases, particularly for forward oriented booms. Also, depending on the spacecraft body and boom parameters, there are some configurations where the booms actually bend forward into the flow and not backwards as might be expected.

Phased array filters using piezoelectric sensors are presented as an approach to detect damage in isotropic plates. Plate dynamics can be described in terms of wave propagation. Boundaries and other discontinuities, such as damage, produce reflections from incident wavefronts. Phased arrays, acting as a directional filter, can be used along with a wave propagation approach to look in different directions on a plate. Damage to the plate can be inferred if there is a significant change in the transient response of the plate. The location of this damaged area can be determined using the phased sensor array response. This paper presents results from simulated damage on an isotropic plate. A piezoelectric sensor array is used to actively interrogate the plate to determine the presence and location of damage using low frequency Lamb waves.

This paper develops an integrated model of periodic 1D structures with piezoelectric actuators for complete active/passive control. The approach utilizes the property of periodic structural elements that create stop and pass band regions in the frequency spectra, predominantly in the higher frequency range. This basic property of periodic structures is enhanced by the application of periodically placed piezoelectric actuators, with piezo-forces as a function of displacement. Using collocated feedback control, the piezoelectric actuators can introduce the proper force to reduce wave propagation, both in the high and low frequency ranges. An analytical model is developed to predict the performance of the periodic rods and beams with piezoelectric actuators acting as controllers. For the purpose of this research, only geometric periodicity is considered and every cell is assumed to be identical.

This paper uses a damage detection approach based on dereverberated transfer functions. In one-dimensional structures, conventional reverberated transfer functions are formed because of the interactions between incident and reflected waves in structures. By applying virtual controllers to eliminate wave reflections, dereverberated transfer functions can be obtained. Dereverberated transfer functions provide good representations of a structure's local dynamics. Because local dynamics are more sensitive to parameter changes, the dereverberated transfer function appears to be suitable to infer structural damage. In this study, a three-story building model is tested under seismic wave excitation. Experimental results show that this approach can be used to locate damage, determine damage type, and quantify damage extent.

This paper investigates the feasibility of sensing damage emanating from rotating drivetrain elements such as bearings, gear teeth, and drive shafts via airborne paths. A planar phased acoustic array is evaluated as a potential fault detection scheme for detecting spatially filtered acoustic signatures radiating from gearbox components. Specifically, the use of beam focusing and steering to monitor individual tooth mesh dynamics is analyzed taking into consideration the constraints of the array/gearbox geometry and the spectral content of typical gear noise. Experimental results for a linear array are presented to illustrate the concepts of adaptive beam steering and spatial acoustic filtering. This feasibility study indicates that the planar array can be used to track the acoustic signatures at higher harmonics of the gear mesh frequency.

This paper develops an analytical model to simulate the gear mesh contact for a spur gear pair with and without tooth damage. Three common gear tooth faults are simulated including pitting, wear and root cracks. The effect of tooth face width on detection sensitivity for pitting and the effect of crack width on detection sensitivity for crack are investigated. Using static performance measures, such as transmission error, results suggest that basic gear design parameters, such as diametral pitch, pressure angle and number of teeth, may have a significant effect on damage detection sensitivity. It appears that a decrease in diametral pitch will enhance damage detection sensitivity for all the three types of damage. An increase in pressure angle or number of teeth will enhance detection sensitivity for pitting damage, but tends to decrease the sensitivity to crack or wear damage.

This paper gives an overview of ongoing research and development in the field of structural health monitoring technologies in the US, with application to long-span bridges. Specifically, this paper attempts to review various key structural health monitoring technologies, including sensor development, data processing, damage detection algorithms, data analysis and information processing. Several examples are cited from the aerospace, civil and mechanical communities. Monitoring of constructed systems are of considerable interest since the consequences of failure can have a significant effect on the society at large. For instance, consider the 1100 major long-span bridges in the USA (those with spans of 100 m or longer), many are over 50 years old, and several notable ones are over 100 years old. These bridges fall outside the Standard Specifications issued by AASHTO (1998), and there is little generic experience related to maintaining their performance, especially after they age and/or following any damage. More than 800 of the long-span bridges in the National Bridge Inventory are classified as fracture-critical. It follows that structural health monitoring techniques may prove to be useful for maintaining and preserving this population of aging civil infrastructure. It is hoped that the following will stimulate additional discussion regarding the importance of structural health monitoring as an emerging research area for a variety of aerospace, civil and mechanical applications.

In this paper, the method of pulse phase tracking for X-ray pulsar based spacecraft navigation is expanded to accommodate signals arriving from pulsars with lower flux than the Crab Pulsar. Spacecraft position and velocity estimates can be calculated using phase and frequency estimates of an observed pulsar signal. This allows for nearly continuous updates of vehicle motion estimates over short time frames and eliminates the need to time transfer all of the arriving photons to an inertial reference at the solar system barycenter. Previous work in pulse phase tracking uses a maximum likelihood estimator (MLE) for initial phase and a second-order digital phase-locked loop (DPLL) with a one second block size. This technique accurately tracks phase and Doppler frequency using simulated photons modeling the output of the Crab Pulsar. This method breaks down when considering pulsars that have a lower flux. The main difficulty is balancing the need to keep DPLL block sizes short with the fact that very few photons arrive in any given block of the phase tracking algorithm. Most X-ray pulsars have fluxes that are multiple orders of magnitude lower than the Crab Pulsar. Two phase tracking methods for low flux X-ray pulsars are hypothesized and tested. The first involves removing the dynamics from the signal each block using estimates of the detectors velocity and acceleration. This parabolic phase model is used to compute an MLE for initial phase. The output is fed into a third-order DPLL to give estimates for the next block. The second method uses a three parameter MLE that is derived to estimate the phase, frequency, and frequency derivative of a pulsar signal. These estimates are sent to a frequency locked-loop assisted phase locked-loop. Both methods allow for longer blocks which is essential unless the detector area or background rejection is improved. The two methods are validated and compared on their ability to track detectors undergoing constant acceleration along the line-of-sight to a pulsar and to track a portion of the Cassini cruise trajectory using PSR B1821-24 and PSR B1937+21.

This paper presents the development of a Galfenol-based directional magnetostrictive patch transducer (MPT) and the results of experimental investigations with the developed MPT for guided Lamb wave (GLW) applications. We used three magnetostrictive materials such as nickel, polycrystalline Galfenol, and single-crystal-like Galfenol (i.e. a highly textured Galfenol) with a 1-inch-diameter thin disc form to design the proposed MPT used to evaluate the directionality for sensing the GLW propagation in thin aluminum plates. Prior to the GLW inspection using the MPTs, the magnetostriction of the magnetostrictive patches was experimentally measured by a strain gauge, and electron backscatter diffraction (EBSD) patterns were captured and analyzed to explore grain configurations and crystal orientations of the patches. The material level analysis validated that the highly textured Galfenol behaves like a single crystal with large magnetostriction along a orientation. Therefore, the developed MPT using the single-crystallike Galfenol was expected to exhibit high directional sensitivity of the GLWs traveling in the metallic plate, corresponding to the preferred orientation of the Galfenol patch. The experimental study of the MPT-based GLW approach demonstrated that the MPT using the single-crystal-like Galfenol exhibits excellent sensitivity to the incoming GLWs along the preferred direction, while the nickel-based MPT has omnidirectional GLW sensitivity in the plate specimen. In addition, the MPT using the polycrystalline Galfenol showed two preferred directions for sensing the GLWs, corresponding to orientations.

The vibratory response of a structure can be represented by the superposition of traveling waves propagating along a complex structural network. Wave dynamics generated at natural boundary conditions and subsequently reflected at geometric boundary conditions can lead to pole-zero characteristics of conventional reverberated transfer functions. However, by implementing model-based wave virtual controllers at each structural element, a dereverberated transfer function can be obtained from the measured reverberated transfer function response. Because the dereverberated transfer function represents the direct path of energy transmission across a structure, it can be used to infer damage in a structure by tracking how variations in local element properties affect the propagation of incident energy through the structure. A methodology is presented for obtaining the dereverberated transfer function response for four types of structural elements, including symmetric and asymmetric discrete spring mass elements and spectral rod and beam finite elements for continuous structures. The dereverberated transfer function response is obtained by attaching virtual controllers at the terminals of each structural element. A phase damage index method is proposed based on the relative phase propagation error from element to element to quantify the type and amount of damage. Analytical results on several simulated examples confirm that the dereverberated response can be used as a method for locating and quantifying damage in structures.

A normalized energy metric is used to classify seeded faults of the OH-58A main transmission. This gearbox comprises a two-stage transmission with an overall reduction of 17.44:1. Loaded gearbox test runs are used to evaluate the sensitivity of a non-stationary fault metric for early fault detection and classification. The non-stationary fault metric consists of a simple normalized energy index developed to account for a redistribution of sideband energy of the dominant mesh frequency and its harmonics in the presence of actual gearbox faults. This index is used to qualitatively assess the presence, type and location of gearbox faults. In this work, elements of the normalized energy metric are assembled into a feature vector to serve as input into a self-organizing Kohonen neural network classifier. This classifier maps vibration features onto a two-dimensional grid. A feedforward back propagation neural network is then used to classify different faults according to how they cluster on the two-dimensional self-organizing map. Gearbox faults of OH-58A main transmission considered in this study include sun gear spalling and spiral bevel gear scoring. Results from the classification suggest that the normalized energy metric is reasonably robust against false alarms for certain geartrain faults.

An approximate analysis to estimate the stagnation-point heat e ux and heat load for hypersonic vehicles e ying transatmospheric periodic, glide, and steady-state cruise trajectories is presented. The analysis was developed by approximating density and velocity proe les for a hypersonic vehicle with aerodynamic characteristics that are assumed to be representative of current and future cone gurations. Results from a comparative investigation of relative heating rates and heating loads suggest that there exists a e ight corridor of mean altitudes and cruise velocities at which a periodic cruise trajectory achieves lower average heating rates when compared to glide or steady-state cruise trajectories.