Criminalistics, one of the many sub-divisions of forensic science, is an applied science in which items of evidence are analyzed to provide investigative information and scientific evidence to be used in courts of law. Laboratories associated with governmental public agencies are typically involved in criminal cases as opposed to civil cases, and those types of cases that fall within the jurisdiction of the particular agency. Common analytical divisions within criminalistics laboratories include blood alcohol testing, toxicology, narcotics, questioned documents, biology, firearms, latent fingerprints, physical and trace evidence sections. Specialized field investigative services may be provided in the areas of clandestine drug laboratories and major crimes (firearms, biology, trace, arson/explosives). Forensic science best practice requires the use of non-destructive testing whenever reasonably possible. Several technically difficult situations (bodies and evidence encased in cement and metal) are presented as a challenge to audience.

As a part of an aircraft inspection, characterization of fatigue cracks and corrosion spots is a vital task to ensure airworthiness of airplane structures. Conventional techniques for crack or corrosion characterization are based on either echo-amplitude measurements or time-of-flight evaluation. Application of more advanced imaging or scanning techniques significantly improves the accuracy and resolution of ultrasonic measurements. However, these techniques usually require high-cost instrumentation and training. A novel adjustable transducer array for an ultrasonic scanning using low-cost portable data acquisition units has been developed at Northwestern University. The array is utilized to detect and characterize small incipient fatigue cracks and corrosion spots, as well as to monitor the sealant quality in the multi-layered airplane structures. The arrays are suitable for rapid adjustments of elements with different parameters over a wide range of spatial orientations based on the configuration of the aircraft structure, type of the flaw to be detected, and the inspection procedure. The adjustable transducer arrays will replace currently used single element transducer assemblies that are custom-made for each particular application. The new arrays require minimal time to be readjusted from one application to another. The arrays are compatible with the commercially available portable ultrasonic units.

This paper discusses the potential for using Piezoceramic and Nanotube materials to develop an artificial neural system for structural health monitoring. An artificial neural system array was modeled using piezoceramic nerves and electronic components. The neural system was simulated using one hundred dual-output sensor nodes on a four-foot square composite panel. The nodal outputs were combined into twenty neuron firing signals, one row time signal, and one column time signal. This system was able to detect and locate acoustic waves and large strains in the panel. Also discussed, is the potential for using nanotubes for building the artificial neural system. In carbon nanotubes, an electrochemical process can be used to achieve low voltage actuation at high strain, but the process velocity is slow and a structural polymer electrolyte must be used for ion exchange. Carbon and boron nitride nanotubes can be piezoelectric, and piezonanotechnolgy may be useful for building high bandwidth neural systems. The operating temperature of boron nitride is high and the amount of material needed to build artificial nerves is small, but the piezoelectric coefficients appear to be small. Nanotube molecular electronics and the change in conductance of nanotubes might also be used to develop artificial nerves.

A built-in cost-effective diagnostic system is being developed to monitor fatigue crack growth in aircraft structures. The proposed system consists of a SMART Layer by Acellent Technologies with an embedded network of distributed piezoelectric sensors/actuators, a diagnostic unit, and software. Multi-riveted aluminum lap joints (936 x 462 mm) were tested for a constant amplitude fatigue test under tensile loading to monitor crack growth at rivet holes. A SMART Layer was designed as a strip and surface-mounted to the critical area of lap joints where crack initiation and growth were suspected. The SMART Layer installed between the two rivet rows was used as an actuator strip and the other layer located below the rivets was used as a sensor strip. Using the software, pre-selected diagnostic signals from a designated piezoelectric actuator to its neighboring sensors were generated by the diagnostic unit. A corresponding sensor signals were recorded and compared to a previously recorded baseline reference. The signal processing technique and interpretation algorithm were developed and implemented for extracting features related to crack growth condition at the time of measurement. The results of estimates were compared with non-destructive testing (NDT) data taken from the experiments, which were conducted during the course of the investigation. The probability of damage detection, compared to conventional NDT techniques, was evaluated.

Imagine an inspector conducting an NDE on an aircraft where you notice something is different about him - he is not real but rather he is a robot. Your first reaction would probably be to say 'it's unbelievable but he looks real' just as you would react to an artificial flower that is a good imitation. This science fiction scenario could become a reality at the trend in the development of biologically inspired technologies, and terms like artificial intelligence, artificial muscles, artificial vision and numerous others are increasingly becoming common engineering tools. For many years, the trend has been to automate processes in order to increase the efficiency of performing redundant tasks where various systems have been developed to deal with specific production line requirements. Realizing that some parts are too complex or delicate to handle in small quantities with a simple automatic system, robotic mechanisms were developed. Aircraft inspection has benefitted from this evolving technology where manipulators and crawlers are developed for rapid and reliable inspection. Advancement in robotics towards making them autonomous and possibly look like human, can potentially address the need to inspect structures that are beyond the capability of today's technology with configuration that are not predetermined. The operation of these robots may take place at harsh or hazardous environments that are too dangerous for human presence. Making such robots is becoming increasingly feasible and in this paper the state of the art will be reviewed.

Recent structural health monitoring techniques have focused on developing global sensor systems that can detect damage on large structures. The approach presented here uses a piezoelectric sensor array system that mimics the biological nervous system architecture to measure acoustic emissions and dynamic strains in structures. The advantage of this approach is that the number of channels of data acquisition used for an N-by-N sensor array can be reduced from N² to 2N. For large arrays the number of data acquisition channels is tremendously reduced. When transient damage events occur on the structure, the array output time histories can be recorded and the location of the excitation can be accurately determined using combinatorial logic. A trade-off is the difficulty of extracting individual sensor time histories from the array outputs without a neural network or a regressive technique. Only the sums of the sensor strains of each row and column can be exactly calculated using the voltage outputs of the array. The array approach allows efficient use of data acquisition instrumentation for structural health monitoring. Applications for the sensor array include crack and delamination detection, dynamic strain measurement, impact detection, and localization of damage on large complex structures.

Shaped-field eddy current Meandering Winding Magnetometer (MWM) sensors and MWM-Arrays, designed to fit physical models provide new inspection capabilities for materials characterization, quality control, and damage detection in aerospace structures. Accurate modeling is enabled by designing primary winding distributions that create either a spatially periodic magnetic field or a single period shaped- field. Accurate modeling of the sensor response permits absolute property measurements with minimal calibration, e.g., calibration in air without a reference standard. This paper will provide an overview of shaped-field eddy- current sensors and their use in several aerospace applications. In one group of applications, the sensors are permanently mounted on test components and can be mounted on actual structures for on-line fatigue damage monitoring. This supports the damage tolerance and retirement for cause methods for life extension and safe operation of numerous commercial and military aircraft, particularly in locations where the high cost of inspection is associated with disassembly and surface preparation. This capability can also be used to make damage standards having known flaws, including representative crack clusters. In a second group of applications, scanning of inductive element arrays permit high-resolution wide-area imaging of the properties revealing quality, damage state, or spatial variations of properties of conductive and magnetic materials. Model-based inversion methods convert each sensing element response into property measurements and permit independent property and lift-off measurements with each element. Furthermore, new MWM sensors incorporating giant magnetoresistive sensors allow low frequency measurements, even down to dc. This permits inspection for hidden cracks or hidden corrosion in thick multilayer structures.

Aging wiring has become a critical issue to DoD, NASA, FAA, and Industry. The problem is that insulation on environmentally aged wire becomes brittle and cracks. This exposes the underlying conductive wire to the potential for short circuits and fire. The difficulty is that techniques to monitor aging wire problems focus on applying electrical sensing techniques that are not very sensitive to the wire insulation. Thus, the development of methods to quantify and monitor aging wire insulation is highly warranted. Measurement of wire insulation stiffness by ultrasonic guided waves is being examined. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. Experimental measurements showed that the lowest order axisymmetric mode may be sensitive to stiffness changes in the wire insulation. To test this theory, mil-spec wire samples MIL-W-81381, MIL-W-22759/34, and MIL-W-22759/87 (typically found in aircraft) were heat-damaged in an oven, in a range of heating conditions. The samples were 12, 16, and 20 gauge and the heat-damage introduced material changes in the wire-insulation that made the originally flexible insulation brittle and darker in color. Axisymmetric mode phase-velocity increased for the samples that were exposed to heat for longer duration. For example, the phase velocity in the 20-gauge MIL-W-22759/34 wire changed from a baseline value of 2790m/s to 3280m/s and 3530m/s for one-hour exposures to 349 degree(s)C and 399 degree(s)C, respectively. Although the heat-damage conditions are not the same as environmental aging, we believe that with further development and refinements, the ultrasonic guided waves can be used to inspect wire-insulation for detrimental environmental aging conditions.

Wear resistant cermet coatings are used in aircraft landing gears and thermal barrier coatings (TBC) are applied to hot- section components of turbine engines. A series of experiments have been conducted to characterize cermet and TBC coatings using NDE techniques. A cermet coating is tested using conventional ultrasonic and eddy current methods as well as an ultrasonic leaky surface wave technique. The results demonstrate the ability of these techniques to detect the presence of defects on the surface or beneath the surface of the coating and at the coating- substrate interface. Ultrasonic time-of-flight and eddy current quadrature measurements also show the ability to detect minute changes in the thickness of cermet coatings. Knowing the coating thickness, the density of the coating is estimated by comparison of the theoretical and the experimental transfer functions of the ultrasonic signals. NDE techniques were also used to inspect thermal barrier coatings. In particular, eddy current technique was used to measure the thickness of plasma-sprayed TBC specimens, and knowing the thickness, ultrasonic techniques were applied to obtain an estimate of the porosity content.

Widespread acceptance of Condition Based Monitoring (CBM) systems has been hampered by, among other things, high costs and inaccurate diagnostics. The advent of new methods for signal processing, local wireless networks, and an industry standard architecture is an opportunity to develop low cost, reliable, practical health monitoring systems. We will discuss the signal processing issues that contribute to poor performance and how new algorithms can provide near optimum detection and recognition of broad-band signals in nonideal, changing, noise. We will then discuss the ONR sponsored Open Systems Architecture (OSA) and how that provides a common operating protocol for health monitoring systems. The heart of the OSA/CBM system is protocols for communication among the hardware and software components of a generalized CBM system which allows rapid and easy integration of specialty components. Finally, we will discuss the impact of several new technologies, including local wireless networks. For example, in many potential installations, almost 90% of the cost of installation is the wiring from the sensors to the processing units. By processing the raw data at the sensor and using a local wireless network to move data and monitor the CBM system itself, the cost of health monitoring can be dramatically reduced.

Current issues in vibration-based fault diagnostics and prognostics are: (a) fault/damage identification; (b) localization; and (c) monitoring. The fault/damage identification and localization are needed for diagnostics. The monitoring allows observing the fault/damage progression in time. Based on monitoring, the prognostics of the moment of failure, and hence of the remaining life could be performed. Critical in both the diagnostics and the prognostics is the existence of a reliable damage metric. Several methods for achieving this, from single parameter classification, through overall-statistics classification of the entire spectrum, to probabilistic neural network (PNN) method are presented. These methods are illustrated on: (i) simulated data, (ii) experimental data taken on simple- geometry calibration specimens; and (c) experimental data recorded on aging aircraft panels. The experimental data was obtained with the electro-mechanical (E/M) impedance technique using small and unobtrusive piezoelectric-wafer active sensors (PWAS) bonded to the structure. This experimental data was in the form of very high frequency (hundreds of kHz) spectrum representing the structural dynamics in sensor's neighborhood. The overall statistics approach, which is simplest to apply, could correctly classify near-field spectra using the dereverberated response and the correlation coefficient difference (CCD) damage metric, but it could not classify medium field spectra. The features-based PNN could correctly identify both medium-field and near-field data, but it requires more elaborate, two-stage, data processing that involves first features extraction and then classification.

We present a new methodology for vibration based damage detection derived from the characterization of changes in the geometric properties of the time domain response of a structure. Many new features present themselves when the geometry of attracting objects in phase space are considered. The most promising avenue of study are metrics that describe changes to the attractor shape or dimension. In particular, the utility of a feature consisting of the ratio of average local variance (or spatial dispersion) of the input to the average local variance of the response is assessed. Presenting the results of the geometric time domain method in a statistical framework highlights the method's increased sensitivity to subtle damage-inflicted changes to the structure when compared to more traditional modal based methods. In addition the geometric method demonstrates a more robust handling of changes due to ambient environmental fluctuation. Results are presented from a finite element model of a thin plate with weld line damage implemented through a relaxation of a boundary condition.

All structures, natural and man-made, accumulate damage over their lifetime. The concern is: when does the level of damage interfere with safe performance. In the extreme, catastrophic failure is a clear indication of unsafe levels of damage. One of the goals of health monitoring systems is to identify damage long before this final level of criticality is obtained. Algorithms are required that identify damage prior to the degradation of structural parameters. In the current work several identification algorithms are examined for use on plate structures. Plates are of interest because they are basic building blocks for many structures. Even simple flat plates exhibit complex structural response such as anticlastic bending. Damage is included in the computational study presented in the form of damage to the connecting joints. Trends in local and global response are evaluated. Natural frequency, Fourier, power spectrum and wavelet interrogations are evaluated. Dynamic loading conditions consisting of impulse and chaotic oscillations are examined. Interrogations are performed on displacement and strain histories. Sensitivity and potential uses of these interrogators for damage identification are discussed.

Many different vibration-based dynamic input-output and output only data features have been used to identify structural damage and assess structural integrity. Since structural damage introduces linear or nonlinear variations into all of these features, all of them might give positive indications of damage but may not distinguish between linear or nonlinear types of damage. This information can sometimes be used to more reliably diagnose damage by first, helping to distinguish between damage, which is inherently nonlinear, and healthy nonlinearities in a baseline structure; and second, serving as an absolute damage prognosis indicator which, together with prior information about the structural mechanics, determined the degree to which a structure is damaged. A set of potential features that distinguish between linear and nonlinear damage are discussed here. These features are auto-regressive exogenous dynamic transmissiblity model coefficients in the frequency domain. The auto-regressive coefficients are used to characterize the nonlinear nature of damage states and the exogenous coefficients are used to characterize the linear nature of such states. After reviewing the theoretical development of this data model, experimental measurements from a three-story test structure are analyzed using these model coefficients and statistical features are extracted from the coefficients. By using two complementary features, a better indication of the severity of damage is obtained.

An instrument called the ultrasonic extensometer is used to measure tensile preload in bolts used in structural joints. The instrument can also be used to measure bending in bolts used in shear joints. The bolt bending creates a bending stress in the bolt. The bending stress causes the ultrasonic beam to bend slightly. The paper develops theory governing the path of ultrasonic beam passing through the bending stress field. The theoretical differential equations are solved to provide a numerical solution to trace the path of ultrasonic beam. A numerical example is provided to illustrate the solution. In addition, the paper discusses the effect of beam bending on the ultrasonic measurement of bending loads on a bolt.

DPSM (Distributed Point Source Method) is a computational technique that can be used to model the pressure field generated by ultrasonic acoustic transducers. This technique involves discretization of the transducer face of any geometrical shape, into a number of elemental surface areas. Point sources are placed at the centroids of the elemental surface areas. The strengths of the point sources are proportional to the surface areas. Pressure field at a given point is the cumulative effect of the pressure fields generated by all point sources. The accuracy of the computational technique depends on the sensor surface discretization. In this paper, circular transducers are modeled using the DPSM technique. This technique is applied to calculate the pressure field distribution in non- homogeneous fluids with interface. The non-homogeneous fluid is composed of two fluid half-spaced with the interface in front of the transducer face parallel or inclined to the transducer face. Pressure fields in both fluids for normal and angular incidence of the ultrasonic beam have been calculated using DPSM technique.

The project described in this paper was undertaken to demonstrate that finite element analysis can be used to define the parameters of structural health monitoring sensing system. Such an approach to sensor system property definition is necessary because in actual applications one will almost never have a damaged structure available to test. Instead, one will have to postulate the anticipated damage, simulate that damage with a numerical model and then analyze the computational data to determine the necessary properties of the sensing system (number, type, location, sensitivity, bandwidth). To demonstrate this process, a finite element model was constructed of a simulated three-story building used for damage identification experiments. Damage was introduced by loosening bolts in the connections of the columns to plates representing the floors the structure. Of particular interest was modeling slip in joints as a function of bolt torque and predicting the smallest change of torque that could be detected experimentally. After being validated with modal analysis results from the physical structure, the model was used to produce data to test the capabilities of damage identification algorithms and define the sensing system properties. This paper describes the finite element model that was constructed, the results obtained, and proposed future uses for such model as part of an integrated approach to structural health monitoring.

Modal Minimum Rank Perturbation Theory (MRPT) computes perturbation matrices approximating structural changes from one linear state to another due to damage. The number of measured flexible modes determines the rank of the computed perturbation and is usually low in comparison with model dimension. This work extends a time-domain MRPT (TD-MRPT) for detection of state-dependent stiffness damage. TD-MRPT computes an instantaneous stiffness perturbation with the force balance residual arising in successive time steps. Recursive rank-1 updating of the current stiffness estimate results in a cumulating stiffness matrix that converges to the unknown stiffness regardless of the rank difference between the nominal and perturbed matrices. As a result, damage is detected at onset and is exactly tracked in the case of a rank-1 nonlinearity. Time variation of higher-rank nonlinearities will be approximated provided the rate of change is slow compared with the algorithmic convergence rate. Numerical integration is used to render dependency solely on acceleration response measurement and a time-dependent elemental subspace recognition procedure was employed for detecting damage location and extent in the case of incomplete spatial measurements. The algorithm is evaluated with a nonlinear three degree-of-freedom oscillator and virtual experimental data from a 96-degree-of-freedom structure corrupted with artificial measurement noise.

One of the nondestructive thermography inspection techniques uses photographic flash lamps. The flash lamps provide a short duration (about 0.005 sec) heat pulse. The short burst of energy results in a momentary rise in the surface temperature of part. The temperature rise may be detrimental to the top layer of part. Therefore, it is necessary to ensure the nondestructive nature of the technique. Heat damage is dependent upon the magnitude of temperature rise. A direct method for the temperature measurement uses an infrared pyrometer or camera with a response time shorter than the flash duration. In this paper, an alternative technique is given using the irreversible temperature indicators. This is an indirect technique and it measures the temperature rise on the irreversible temperature indicators and computes the incident flux. Once the flux is known, the temperature rise on part can be computed. A wedge shaped irreversible temperature indicator for measuring the flux is proposed. A procedure is given to use the wedge indicator.

A transmission-line matrix (TLM) model was developed to simulate the ultrasound propagation in the multi-layer structures. The spatial resolution of the proposed model is better than tenth wavelength. The numerical modeling is carried-out for frequencies that are usually used in ultrasound imagery (3.5 - 25MHz). The acoustic impedance profile of multi-layer structures considered are similar to those found in nondestructive evaluation and in medical imaging. The structures modeled are: brazed joints, stomach and colon walls. Structures with artificial flaws are also modeled. A comparison between real images and numerical generated ones is provided for each considered structure. Both frequency and time domain responses are obtained from the structures under investigation. Both single and array transducer techniques are modeled and their performances are evaluated for the proposed structures. Different shapes for the incident pulse are considered in numerically generated images.

Many recent studies have indicated that externally bonded Fiber Reinforced Plastic (FRP) composite sheets can be used effectively to strengthen concrete structures. Composite action suggests that the individual parts of a composite work together as one. Stresses must be transferred from one constituent to the other through some interface. If there is little or no bond at the interface then there is little or no transfer of stress and therefore poor composite action. In this study, we intend to develop a NDE tool for evaluating such interfacial bond conditions. An innovative active modulation approach, Nonlinear Active Wave Modulation Spectroscopy (NAWMS), is adopted in our study. In this procedure, a probe wave will be passed through the system. Simultaneously, a second, modulating wave will be applied to the system. Using three-inch thick concrete slabs we have prepared several FRP laminated samples with artificial flaws (one-inch square) designed into the interface. The sender is on one side of the flaw and the receiver on the other. Maintaining the separation of this pair of transducers, we progressively move them away from the flaw location. The resulting frequency modulations will be analyzed, and will be correlated to the presence of the flaws.

The scattering of elastic waves in a medium with damage from microcracking is discussed. A generalized tensor-based approach is used such that the results are coordinate free. The influence of damage from penny-shaped microcracks within a homogeneous medium is considered. The microcracks are assumed to be randomly oriented and uniformly distributed. Explicit expressions are derived for the attenuation of longitudinal and shear elastic waves in terms of the statistics damage parameter and the effective elastic moduli of the medium. The derivation is based upon diagrammatic methods. The problem is formulated in terms of the Dyson equation, which is solved for the mean field response within the limits of the first-order smoothing approximation. The attenuations are given here in a direct way. The longitudinal and shear attenuations are discussed in terms of their frequency dependence and damage dependence. In particular, the effective elastic stiffness of statistical distribution of microcracks and the example results are discussed. The attenuations are shown to scale with the square of the damage parameter for low frequency.

We present a technique of remote non-destructive inspection of concrete structures and location of inner defects. The technique is based on detection of ultrasonic vibration in concrete with the use of laser interferometer. The interferometer uses principles of dynamic holography via two-wave mixing in photorefractive crystal and homodyne detection. The use of the dynamic hologram of the inspected surface excludes possibility of wave front mismatch between interfering signals, one of which (probing signal) has speckle structure due to the roughness of the inspected surface. Bismuth Silicon Oxide (BSO) photorefractive crystal is used as a nonlinear medium for the recording of the dynamic hologram. CW YAG:Nd laser radiation of 532 nm wavelength is used for detection. Results of detection are verified by piezoelectric sensor. Vibration in concrete is initiated by hammering or laser impact. For completely remote operation, vibration should be initiated by laser impact. In purpose of reliable detection, we analyze both shape of propagating ultrasonic wave and its spectrum. Spectrum of low frequency (1-10 kHz) vibration provides information of the presence or absence of inner defects. Analysis of propagating ultrasonic wave of higher frequency (10-100 kHz) allows to locate the defect more precisely.

Air-coupled ultrasonic testing is gaining increasing attention in a variety of NDE applications that can benefit from non-contact probing and ease of transducer positioning. The main focus of this paper is the assessment of an air- coupled ultrasonic method for the inspection of railroad tracks focusing on the detection of internal defects. It is shown that resonant standing waves can be successfully generated in the rail cross-section to alleviate the challenges associated with the large acoustic impedance mismatch between air and steel. The effectiveness of the resonance concept is demonstrated in through-transmission laboratory testing of the rail head, web and head flange aimed at the detection of longitudinal defects. A pitch- catch configuration potentially effective for the detection of transverse defects in the head flange is also demonstrated. The suitability of the air-coupled ultrasonic method for field use is discussed with particular attention to the achievable inspection speed. Feasibility results are also presented from the measurement of flexural (guided) vibrations in rails. These vibrations propagate long distances and could therefore be well-suited for long-range testing.

This paper describes health monitoring of concrete aerial structures for a light-rail train system. The purpose of the study is to perform structural diagnostics and in-situ inspection for 8 line sections under various climate conditions, and with incremental static loading during construction, and dynamic loading due to moving trains. The field diagnostics include instrumentation on piers, pier caps, bridge girders and decks with electrical strain gages and tilt meters. Initial gage-installation efforts are focused on in-situ temperature developed in bridge decks and rail pads. Some tilt plates are installed early prior to deck construction on piers and beams to study the effects of deck placement such as changes in camber and pier rotation. Temperature gages to measure surface temperatures are installed next to every gage type. A non-contact infrared sensor is also being used to measure surface temperature. Air and wet bulb temperature are recorded with other measurements along with time and weather conditions. At this time, the in-situ temperature gages provide two datums: information about cement heat of hydration, and information about diurnal deck temperatures. Tilt-meter measurements record de-camber of pre-stressed concrete I-beams. Graphs of these measurements show miscellaneous events such as beam replacements on pads, pouring of decks, diaphragms, etc. Material properties and structural behavior are discussed for several aerial structural components.

A model of electromagnetic behavior of composite materials such as carbon epoxy or glass epoxy structures has been developed. Based on this model, an electromagnetic method to evaluate the electric conductivity and the electric polarization of this type of material, by measurement of magnetic and electric components of an incident electromagnetic field crossing through the material, has been also developed. A Health Monitoring System (HMS) derived from this technique is presented, which allows to detect and characterize a wide variety of defects inside composite structures. The HMS is constituted of emitting and receiving networks sensitive to magnetic or electric fields, integrated into the composite structure. This system delivers electric images in which the damages can be detected and localized. Various electric images obtained and related to structures including various damages such as impact delamination, fiber breaking, local burning and liquid ingress, are presented and compared to images resulting from classical ultrasonic NDE. The complementary of the present technique with acousto-ultrasonic technique based on Lamb waves propagation, and the interest of combining them in a unique integrated system is evoked.

A flat magnetostrictive sensor for active health monitoring of a large area of a structure has been developed. The sensor consists of a thin nickel foil and a coil placed over the nickel and, much like a strain gauge, is permanently bonded to the surface of a structure under monitoring. When activated, the sensor generates guided waves in the structure for interrogation and detects signals that are reflected back from the structural geometries and defects in the structure. Since guided waves can travel a long distance in the structure, a large area of the structure can be interrogated and monitored by using the sensor. By periodically acquiring the data and comparing it with the baseline data established at the time of sensor installation, structural changes occurred over that time can be quickly determined for suitable structural management decision. In addition to the ability to actively inspect and monitor a large area of the structure, the sensor is also rugged and inexpensive and therefore has high potential for practical use. As an example of its applicability to aircraft structure, data showing the monitoring of defect growth in fastener holes in a wing structure are presented.

This paper describes the use of the electro-mechanical (E/M) impedance method for health monitoring of aging aerospace structures. As a nondestructive evaluation technology, the E/M impedance method allows us to identify the structural dynamics directly by obtaining the E/M impedance signatures of attached piezoelectric wafer active sensors (PWAS). The theoretical model for 2-D structures, which predicts the E/M impedance response at PWAS terminals, was developed and validated. The model accounts for axial and flexural vibrations of a host structure and considers both, structural dynamics and dynamics of the sensor. The study of sensor's sensitivity to structural damage is presented. The presence of damage modifies the E/M impedance spectrum causing frequency shifts, peak splitting and appearance of new harmonics. The overall-statistics damage metrics and probabilistic neural network (PNN) were used to classify data according to damage severity. When installed on the aging aircraft panel, the sensors response features: (a) in the near field, spectral baseline change; (b) in the medium field, changes in harmonics distribution. These effects were successfully captured with overall-statistics damage metrics (correlation coefficient deviation) and PNN respectively. The health monitoring of aging aerospace specimens shows that unobtrusive permanently attached PWAS in conjunction with E/M impedance method can be successfully used to assess the presence of incipient damage through the examination and classification of the E/M impedance spectra.

Since first reported in 1955 the Fermi-Pasta-Ulam (FPU) recurrence phenomenon has been thoroughly studied. Being reproduced in nonlinear electric lines this intermittent physical process became a possible instrument for a structural analysis. The FPU electromagnetic spectrum generated in coupled nonlinear lines proved to cover a wide band of frequencies (10 Hz-10GHz). Generator and a receiver of this spectrum have been developed and tested in different structures including water, electrolytes, blood and biological tissues. The sample under examination was placed between the generator and receiver resonators of the FPU spectrum. Such a system was able to distinguish normal and tumor tissues, healthy and leukemia blood, different water samples. Future research is aimed at the computerization of the system.

This paper presents the current research on impedance-based structural health monitoring technique at the Center for Intelligent Material Systems and Structures. The basic principle behind this technique is to apply high frequency structural excitations (typically higher than 30 kHz) through the surface-bonded piezoelectric transducers, and measure the impedance of structures by monitoring the current and voltage applied to the piezoelectric transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Three examples, including a bolted joint, gas pipeline and composite structure, are presented to illustrate the effectiveness of this health monitoring technique to the wide variety of practical field applications. Although many proof-of-concept experiments have been performed using the impedance methods, the impedance-measuring device (HP4194A) is still bulky and expensive. Therefore, we have developed an operational amplifier-based turnkey device that can measure and record the electric impedance of a PZT. The performance of this miniaturized and portable device has been compared to our previous results and its effectiveness has been demonstrated. This paper summarizes the experimental setup, procedures, considerations needed to implement the device in field applications.

More than 14 million teeth receive endodontic (root canal) treatment annually. Before a clinician's inspection and diagnosis, destructive access preparation by removing teeth crown and dentin is usually needed. This paper presents a non-invasive method for accessing internal tooth geometry by building 3-D tooth model from 2-D radiographic and endoscopic images to be used for an automatic prescription system of computer-aided treatment procedure planning, and for the root canal preparation by an intelligent micro drilling machine with on-line monitoring. It covers the techniques specific for dental application in the radiographic images acquirement, image enhancement, image segmentation and feature recognition, distance measurement and calibration, merging 2D image into 3D mathematical model representation and display. Included also are the methods to form references for irregular teeth geometry and to do accurately measurement with self-calibration.

The obstructive sleep apnoea (OSA) is a sleep related breathing disorder caused by a relaxation of the upper airway structure during the sleep that leads to a complete closure of the upper airway. The most successful therapy is the nasal continuous positive airway pressure (nCPAP) treatment that keeps the airway opened. More recent devices use an automatic adaptation of the applied pressure. Either the forced oscillation technique (FOT) or the evaluation of the inspiration flow contour are used to evaluate the severity of obstructions. Both methods have disadvantages that may lead to wrong applied pressures. Based on the precise measurement of airflow and mask pressure during nCPAP with a Weinmann SOMNOsmart and additional polysomnography a new parameter set is presented that uses the advantage of both methods to detect the obstructive sleep apnoea. To evaluate the applicability of this parameter set to control Auto-nCPAP-devices a fuzzy-controller is designed under MATLAB/Simulink using an A/D-D/A-converter to control the blower of the SOMNOsmart during Auto-nCPAP-therapy. Obstructive events are detected and treated with a rise of nCPAP-pressure depending on the inspiratory flow requirement. The pressure is lowered after the end of flow limited phases. Although temporary low pressures no oxygen desaturation is recognized by the pulse oxymeter.

This work is motivated by possible medical applications of focused ultrasound in minimally invasive treatment of a variety of disorders. The mechanical and thermal effects caused by high-frequency ultrasound in different material systems are calculated using a finite element based program called PZflex. The pressure distributions generated by plane as well as focused ultrasound beams are presented. For the focused beam, the temperature distribution in the focal zone is also calculated. The results indicate that the heating efficiency of the ultrasound energy in the focal region depends on the exciting frequency and the geometry of the focal zone depends on the material being tested. At higher excitation energies, cavitation and nonlinear effects need to be included in the simulations. These effects are under current investigation.

A plurality of articles discussing combined effects of acoustic high-pressure (mechanical factor) and heat (thermal factor) caused by acoustic vibration on biological tissues and cells has been published. Herein, we contribute the preliminary results describing the behavior of living human skin cells when separately applying shock waves and thermal insult to them. First, we gradually increased temperature of a culturing medium from 37.5 to 52 degree(s)C using the heat plate with temperature controller, and carried out in-situ observation of the cells grown on a substrate via the medium using a scanning acoustic microscope. Second, we provided the pressure using high power ultrasonic pulses generated by a laser induced ultrasonic shock wave system to the cells, wherein the pressure caused by the pulses was measured by a hydrophone, and wherein temperature was monitored by thermocouples. The cells were observed just after giving the impact. The difference between phenomena indicating cellular insult and injury (e.g., shrinkage or lift-off) were clearly visualized by the scanning acoustic microscope with frequency at 1.0 GHz.

This paper presents a novel technique for the in vivo detection of single strut failures in Bjork-Shiley convexo-concave prosthetic heart valves. The method makes use of Lorentz forces to excite the resonant modes of the strut. The forces are generated by immersing the valve in a static magnetic field and inducing eddy currents in the valve using an auxiliary coil excited by a tone burst. The tone frequency is set at a value that is close to the resonant frequency of an Intact Outlet Strut. The state of the strut is assessed by measuring the acoustic field generated in response to the excitation, using a hydrophone placed on the chest of the patient. An intact outlet strut is indicated if the excitation is able to excite the resonant modes, which correspond to those of a healthy outlet strut. The paper describes work done in simulating the BSCC heart valve using finite element methods to estimate the modal frequencies. Simulation results together with experimental data supporting the predictions are presented. This work was supported by the Trustees of the Bowling-Pfizer Heart Valve Settlement Funds.

The goal of this paper is to discuss different data interpretation concepts for structural health monitoring based on Lamb wave propagation. Surface-mounted piezoelectric transducers are used as wave emitters and receivers. The waves emitted interact with discontinuities and experience a change in their propagation characteristics when damage is generated. By comparing sensor signals collected before and after damage has been generated, the condition of the structure can be determined. Two approaches are proposed for characterizing the damage. The initial approach analyzes only the first wave packet reflected at the damage and is based on a simple time-of-flight analysis in conjunction with a geometric method. The size of the damage can be estimated with this technique by using several pairs of piezoelectric conversion processes. This technique offers the potential for enhancing measurement accuracy. An estimation of the size and location of the damage becomes possible with just three piezoelectric sensors. It has been found that, in practice, the superposition of wave packets represents one of the main problems in realizing this approach. The decomposition of superimposed wave packets is investigated using wavelet transform and the pulse compression technique. It is shown that noise and dispersion represent a major drawback, hence rendering signal analysis distinctly more difficult.

This paper investigates dynamic characteristics (mode shapes and natural frequencies) and defect detection of circular plates using a scanning laser vibrometer. Exact dynamic characteristics of a circular aluminum plate having a clamped inner rim and a free outer rim are obtained using two methods; one uses Bessel functions and the other uses a multiple shooting method. An in-house finite element code GESA is also used to analyze the circular plate using the DKT plate element. Numerical results show that some reports in the literature are incorrect and that high-frequency Operational Deflection Shapes (ODSs) are needed in order to locate small defects. Detection of two defects in the circular aluminum plate is experimentally studied using the distributions of RMS velocities under broadband periodic chirp excitations. RMS velocities of ODSs, symmetry breaking of ODSs, splitting of natural frequencies and ODSs, and a Boundary Effect Detection (BED) method. The BED method is non-destructive and model-independent; it processes experimental ODSs to reveal extra local boundary effects caused by defects to reveal locations of defects. Experimental results show that small defects in circular plates can be pinpointed by these approaches. Moreover, a new concept of using the balance of elastic and kinetic energies within a mode cell for detecting defects in two- dimensional structures of irregular shapes is proposed.

Surface Brillouin scattering (SBS) has been used to determine the elastic properties of thin hard submicron films cBN deposited on silicon. In case of cBN films, only the Rayleigh mode can be clearly identified in the Brillouin spectra. The elastic properties of the films have been determined by fitting experimental data to theoretical dispersion curves. A Green's function method was used to predict Brillouin scattering spectra of the acoustic excitation at the free surface. Our results demonstrate that the effect of thin hBN interlayer located between cBN film and Si substrate on the velocity of surface acoustic wave (SAW) does not exceed 2% for thin (16 nm film) and is negligible for cBN films thicker than 100 nm. The elastic properties of the cBN films are not softer than those of bulk cBN.

Understanding the elastic properties of the various types of rubber is important for many commercial and academic applications. A sample set consisting of generic elastomeric compounds was studied using non-destructive non-contact ultrasonic techniques. The longitudinal sound wave velocities in the sample and wave amplitude attenuation in the sample were measured using the Second Wave Inc. Non-Contact Analyzer 1000 (NCA1000). The Contact method was then used to corroborate the results obtained. The preliminary results suggest that the differences in attenuation are driven by polymer type and also to a lesser extent by the loading level of carbon black fillers.

This paper is concerned with the nondestructive materials and defects characterization of foams using ultrasonics. Propagation of ultrasonic waves in a thick plate made out of highly porous, open cell, carbon foam is studied in an effort to understand their relationship with the elastic properties of the material. The foam is assumed to have a tetrakaidecahedral structure and its geometric and overall elastic properties are determined from microscopic data and wave propagation experiments. A simple, one-dimensional model, based on a periodic spring-mass system is proposed for propagation in the through-thickness direction. Due to the difficulty in performing experiments under dry coupling conditions, ultrasonic experiments were carried out with the foam immersed in fluid in a test bed. The wave speed in the fluid filled foam is calculated after obtaining the average elastic moduli of the fluid saturated medium. The calculated wave speed is found to be in good agreement with that measured in the experiment. Significant discontinuities within the plate are also detected through ultrasonic and radiographic experiments. The present models can be employed effectively to characterize any open-cell foam.

An instrument called the ultrasonic extensometer is used to measure tensile preload in bolts used in structural joints. In this paper, the same instrument is used in a different way to measure change in the ultrasonic measurements due to bending in the bolts. A theory and numerical model for the ultrasonic measurements are developed. The bending load results in a rotation and translation of the ultrasonic pulse reflecting face. The load also creates a stress gradient in the bolt. This results in a phase variation (or gradient) in the received ultrasonic beam across the face of transducer. The load also results in a physical shift in the received beam relative to the ultrasonic transducer. The phase gradient and shift in the beam cause a change in the pulse transit time. A numerical example chooses a bolt whose experimental preload characteristic had been published previously by the author. The example displays simulated ultrasonic waveforms before and after subjecting the bolt to bending loads. The simulated preload characteristic was compared to the experimental characteristic. The two characteristics indicate some agreement.

An instrument called the ultrasonic extensometer is used to measure tensile preload in bolts. Commercial ultrasonic extensometers are intended for bolts in a clearance fit joint and not the interference fit joint. This paper develops theory governing the ultrasonic preload measurements when a diametrical interference is present in the joint. The theoretical model accounts for a change in the joint temperature. The model accounts for two cases, one when the bolt is preloaded by turning the nut and the other when the bolt is loaded due to a direct pull on bolt. The paper provides an ultrasonic technique to estimate the interference pressure and diametrical interference without disassembling the joint. A special case of the model, when the diametrical interference is zero, provides a more precise theoretical model for the preload measurement on bolts under pure tensile preload. The theory is modeled in a computer program to provide the preload measurement characteristics. A numerical example computes preload characteristics of a sleeve bolt whose experimental characteristics have been previously published by the author. The experimental and theoretical characteristics show good agreement.

Non-destructive monitoring of structures may be achieved by system identification to evaluate key parameters. Unfortunately many system identification methods that work for small systems do not necessarily give convergence for large systems. In recent years, the use of genetic algorithms (GA) has shown promising potential for parameter identification of complex systems owing to its many inherent advantages. For large systems involving many degrees of freedom and unknown parameters, the computational effort required by the GA approach may still be prohibitive. The main bulk of computational time lies in the numerous forward analyses that need to be carried out. With rapid advances in computer hardware, especially networking technology, nevertheless, the feasibility of applying the GA approach to large system identification problems has become closer to reality even by using low-cost personal computers. Distributed computing can be easily employed to expedite the GA search, thanks to the high concurrency of the GA approach. In this study, a parallel version of a hybrid algorithm of GA and local search is developed for distributed computing. The implementation involves a manager computer running the main algorithm, which distributes data files to many worker computers connected on the network. Each worker computer carries out the forward analysis with the assigned parameter set and, when completed, sends the output file to the manager computer, Numerical examples are presented to show that this approach is generally workable and robust.

Polycrystalline Mercuric Iodide coated TFT arrays are now the best candidates for direct digital radiographic detectors for both static and dynamic (fluoroscopic) applications. Their high X-ray sensitivity, high resolution, low dark current, low voltage operation, and good lag characteristics meet the required imaging performance parameters. Small area (2'x2') mercuric iodide x-ray detectors were already reported to have good resolution and high x-ray sensitivity. The present paper reports results obtained with good quality large active area (4'x4') HgI2 imagers deposited on an a-Si TFT matrix containing 768 x 768 pixels with pitch dimensions of 139 micrometers . Coating thickness of the HgI₂ is between 150 micrometers and 250 micrometers and the imagers were tested in the 25 kVp-100 kVp x-ray energy range utilizing exposures typical for both fluoroscopic and radiographic imaging. The resolution of the imagers is practically limited only by the pixel size (139micrometers ). The imager works at 15 Hz fluoro rate and the image lag is very small. The imager can use exceptionally low dose-rate x-ray illumination because of the very high x-ray sensitivity, which exceeds any other known X-ray imager materials. The paper also discusses potential application fields of this new unique imager.

The necessity to nondestructively monitor concentration of carbon monoxide (CO), which is a colorless, tasteless and poisonous gas and is harmful to people, is disclosed. The portable device for monitoring concentration of CO plays an important role in health care and environment supervising for civil and industrial purposes. A basic circuit-based principle for the implementation of the device is presented with a detailed analysis for the key issues in designing. Specifically, the designing for the preamplifier is of great importance to the performance of the device. There is also introduced a method for getting standard voltage value from the micro-ampere current signal outputted from a carbon monoxide sensor and for restraining other gases to exert influence on the CO monitoring. Meanwhile, the paper teaches an anti-jamming technique for eliminating interference between analog and digital circuits within a very small system. In said device, a multi-function alarm circuit, which periodically performing its self-checking function, produces alarm with sound and light if the power of a battery is insufficient or the concentration of CO is detected to be over a set threshold. In addition, the major characteristics and applications for the device are also presented.

The effect of incompressibility of a layer on the propagation of guided elastic waves in anisotropic layered materials is examined. The layer is either overlying a half-space (host material) or is imbedded in an infinite host material, which is compressible. To gain an understanding of the propagation characteristics and their dependence on the material parameters the mathematically tractable case of material orthotropy is considered. The strain energy of the two materials is otherwise arbitrary. For propagation along a material axis of symmetry the dispersion equation is obtained in an explicit form when the axes of symmetry of the two materials coincide and one of them is normal to the plane separating the layer from the host material. Analysis of the dispersion equation reveals the propagation characteristics of interfacial waves and their dependence on the material parameters. Propagation occurs either in single or multiple modes depending on the material parameters of both the layer and the host material. For a thin layer the guided wave phase speed is obtained in explicit form in terms of the material parameters. Parameter conditions are also defined under which the generation of guided waves is not permitted by the structure.

Diabetes mellitus is a disease that impacts the lives of millions of people around the world. Lower limb complications associated with diabetes include the development of plantar ulcers that can lead to infection and subsequent amputation. Shear stress is thought to be a major contributing factor to ulcer development, but due in part to technical difficulties with transducing shear stress, there is no widely used shear measurement sensor. As such, we are currently developing a directionally sensitive pressure/shear sensor based on fiber optic technology. The pressure/shear sensor consists of an array of optical fibers lying in perpendicular rows and columns separated by elastomeric pads. A map of pressure and shear stress is constructed based on observed macro bending through the intensity attenuation from the physical deformation of two adjacent perpendicular fibers. The sensor has been shown to have low noise and responded linearly to applied loads. The smallest detectable force on each sensor element based on the current setup is ~0.1 lbs. (0.4N). The smallest area we have resolved in our mesh sensor is currently ~1 cm².