Structural health monitoring is the implementation of a damage detection strategy for aerospace, civil and mechanical engineering infrastructure. Typical damage experienced by this infrastructure might be the development of fatigue cracks, degradation of structural connections, or bearing wear in rotating machinery. The goal of the research effort reported herein is to develop a robust and cost-effective structural health monitoring solution by integrating and extending technologies from various engineering and information technology disciplines. It is the author's opinion that all structural health monitoring systems must be application specific. Therefore, a specific application, monitoring welded moment resisting steel frame connections in structures subjected to seismic excitation, is described along with the motivation for choosing this application. The structural health monitoring solution for this application will integrate structural dynamics, wireless data acquisition, local actuation, micro-electromechanical systems (MEMS) technology, and statistical pattern recognition algorithms. The proposed system is based on an assessment of the deficiencies associated with many current structural health monitoring technologies including past efforts by the authors. This paper provides an example of the integrated approach to structural health monitoring being undertaken at Los Alamos National Laboratory and summarizes progress to date on various aspects of the technology development.

Today's commercial and military aircraft require significant manpower resources to provide operational readiness and safety of flight. Aging aircraft fleets are much in need of new and innovative health-monitoring methods to prevent catastrophic failure and reduce life-cycle costs. The key items to be addressed are describing in situ structural integrity characteristics of corrosion and barely visible impact damage (BVID) to determine the 'damage susceptibility.' This paper presents a new concept for performing onboard real-time monitoring using conductive polymer sensor array technology.

Surface mountable eddy current sensors are a revolutionary new concept in nondestructive inspection. These eddy current sensors can be mounted, like a strain gage, at critical locations for detection of crack initiation and monitoring of crack growth. This can be accomplished on a fatigue test article, as well as on in-service aircraft or other structures (patents pending). The mountable periodic field eddy current sensors, described in this paper, can be used as a replacement for standard eddy-current sensors without introducing new requirements. This is not the case with other proposed health monitoring sensors. For critical structures, substantially reduced inspection costs and life extension is possible with permanently mounted eddy current sensors. This is particularly true for difficult-to-access locations that require surface preparation (e.g., sealant or insulation removal) and disassembly when conventional eddy current testing is performed. By enabling eddy current testing in areas currently not accessible to conventional inspection, such as locations deep in an aircraft structure, damage tolerance can be achieved with low cost inspections. Embedded versions might even be mounted between layers, such as in a lapjoint. Surface mountable eddy current sensors are suitable for on-line monitoring and in-service inspections. This paper provides an introduction to surface mountable eddy current sensors, presents specific results from fatigue coupon tests and describes upcoming full-scale aircraft fatigue tests. Also, ongoing efforts to implement this technology on commercial and military aircraft are described. This research has been funded in part by the U.S. Navy, U.S. Air Force, JENTEK Sensors, Inc., and Lockheed Martin Aeronautics Company. The goal of this paper is to provide a basic understanding of surface mounted eddy current sensor capabilities and potential, and to promote their broader use in fatigue testing, aircraft health monitoring as well as for health monitoring of non-aerospace structures.

Two homemade FBG (Fiber Brag Grating) sensors were installed on specific locations of a small-scale three-story structure model. The model was excited by various kinds of vibrations and the dynamic strain data were measured. In order to make a full comparison, the traditional RSG sensors were also installed on the same location of the FBG sensors. The accelerometers and LVDTíªs were also used to offer detailed dynamic information about the structure. Analysis on the test results showed good agreements between the FBG and the RSG sensors and the advantages of using FBG sensors. The fundamental frequency of the structure was also obtained from analyzing the measured data. The result shows that the FBG sensors can be a good alternative for the RSG sensors in monitoring the structure behavior. In addition, experimental study of plates with cracks was carried out using the FBG sensors. Four homemade FBG (Fiber Brag Grating) sensors were installed on specific locations of an aluminum plate. Four traditional RSG sensors were also installed on the same location of the FBG sensors. Test results shows that the FBG sensors can be used to locate the position of cracks. This study shows that the FBG sensors are very good candidates in structural health monitoring and could be widely used in the near future.

Polycrystalline mercuric iodide (HgI2) photoconductor material was directly deposited on flat panel amorphous silicon (a-Si) thin film transistor (TFT) pixel arrays in order to test their application as direct x-ray conversion detectors. The 4' x 4' and 2' x 2' detector plates were fabricated either by Physical Vapor Deposition (PVD) or the Screen-Print (SP) method. Although developed for medical radiological imaging, they can also be used for nondestructive test imaging. The present HgI2 arrays have 100 μm x 100 μm pixels on the 2' x 2' detector and 139μm x 139μm on the 4' x 4' imager. The initial results are very promising and show high x-ray sensitivity and low leakage current. The advantage of these detectors is that they can be directly deposited on the pixellated arrays containing the TFTs and other electronic read out circuits and can be fabricated in large sizes. These polycrystalline PVD-HgI₂ thick film detectors have now been fabricated up to 1,800μm thick, which makes them also useful for higher-energy X-ray applications. Imaging results obtained by both PVD- and SP-HgI₂ will be shown. The effect of the crystallite size on the imaging properties will be demonstrated and the difference in sensitivity applying positive or negative bias on the top electrode will be discussed. Comparison of x-ray sensitivity to other photoconductor materials such a-Se and PbI₂ will also be presented.

In this paper, we describe a model based on a spatial distribution of point sources, called 'DPSM' (Distributed Point Sources Method), applied to magnetic and ultrasonic sensors modelling. Magnetic and acoustic fields are theoretically generated for two types of sensors. The sensor surface is discretized into a finite number of elemental surfaces. A point source is placed at the centroid position of every elemental surface. Point source strength is proportional to the elemental surface area for acoustic sensors and it is obtained by inverting a matrix to satisfy the equipotential boundary conditions for magnetic sensors. Total field is computed at a given point by adding fields generated by all sources. The main difference between the magnetic and acoustic field modelling is that for a magnetic sensor the magnetic potential remains constant on the sensor surface and the magnetic flux varies from point to point, while for the acoustic sensor the particle velocity remains constant on the sensor surface and the acoustic pressure varies. This difference causes an additional matrix inversion in the magnetic field modeling, which is not necessary for the acoustic field modeling. Like other numerical modeling schemes, accuracy of the computation depends on the sensor surface discretization or mesh generation. Effect of the spacing between two neighboring point sources on the accuracy of the field computation is studied and the optimum spacing for accurate numerical computation is given. For accurately modelling acoustic fields the spacing between two neighboring sources should be less than the acoustic wavelength. Flat sensors with circular and rectangular cross-sections as well as point focused concave sensors have been modelled by this technique.

The increased use of polymer adhesives and polymer-based composites in primary structural applications has stimulated the need for a minimally invasive, structurally compatible means for assessing the in-situ health of these materials during the service life of the structure or component. While there are several approaches, the present research has implicitly included structural compatibility as a constraint in the design, fabrication, and installation of such embedded 'structural health' monitoring sensors. A non-contact method of extracting information from the embedded sensors has also been developed, and a working prototype has been devised. The issues associated with manufacturing stable, minimally invasive sensors will be presented, together with a series of potential applications including rotary wing systems such as the Comanche and ground vehicles such as the Crusader and the Future Combat Systems. The advantages of the proposed embedded sensor concept include low cost, ease of installation, unitized construction, compatibility with the host polymer matrix, and a wireless means of retrieving data from the embedded sensor element. The need for an embedded power source has also been eliminated, allowing the sensors to assume a low profile and dimensional stability.

Health diagnostics is an area where major improvements have been identified for potential implementation into the design of new reusable launch vehicles in order to reduce life cycle costs, to increase safety margins, and to improve mission reliability. NASA Ames is leading the effort to develop inspection and health management technologies for thermal protection systems. This paper summarizes a joint project between NASA Ames and SRI International to develop SensorTags, radio-frequency identification devices coupled with event-recording sensors, that can be embedded in the thermal protection system to monitor temperature or other quantities of interest. Two prototype SensorTag designs containing thermal fuses to indicate a temperature overlimit are presented and discussed.

This paper presents a model-independent boundary effect evaluation (BEE) method for pinpointing crack locations and estimating crack sizes using only operational deflection shapes (ODSs) measured by a scanning laser vibrometer. An ODS consists of central and boundary solutions. Central solutions are periodic functions, and boundary solutions are exponentially decaying functions due to boundary constraints. The BEE method uses a sliding-window curve-fitting technique to extract boundary solutions from an experimental ODS. Because cracks introduce localized small boundaries to a structure, boundary solutions exist around cracks as well as structural boundaries. Since crack-induced boundary solutions show characteristics different from those of actual boundaries, cracks can be easily located. A local strain energy method is derived for estimating crack sizes. In the method, the crack-induced strain energy extracted from an ODS is compared with the one calculated using fracture mechanics to estimate the crack size. To verify the capability and accuracy of this BEE method, experiments are performed on six 22' X 1' X 0.25' 2024-T4 aluminum beams each having a through-the-width Mode I crack at its midpoint. These cracks are slots having a width of 0.039' and depths of 0.0625' (25% of the beam thickness), 0.05' (20%), 0.0375' (15%), 0.025' (10%), 0.0125' (5%), and 0.005' (2%), respectively. Results show that this BEE method is capable of locating and estimating small cracks.

Military and commercial aircraft structures are being fielded well beyond their designed life cycle, resulting in escalating maintenance costs. The principle driver behind these costs is the need to nondestructively interrogate large areas to detect and quantify anomalies such as corrosion, cracks, and delaminations. Manual ultrasonic techniques are routinely applied to inspect aircraft structures, but these techniques are time consuming, laborious, and are prone to errors such as operator fatigue and subjectivity. Automated ultrasonic systems require costly, complex scanning systems that are often difficult to adapt to complex shaped structures. Acoustography can provide full-field ultrasonic images in near real-time, making it a suitable method for high-speed, wide area inspection applications. This paper will report on progress being made toward developing acoustography for NDE of aging aircraft structures.

Statistics released in the fall 1989 show that 238,357 (41%) of the nation's 577,710 bridges are either structurally deficient or functionally obsolete. New materials are being explored for use in bridge systems to solve this problem. These materials are less affected by corrosive environmental conditions than conventional civil engineering materials and thus, require less maintenance and potentially provide a longer life span. A material being considered for these applications is glass fiber reinforced vinyl ester matrix composites. Fiber reinforced plastic (FRP) composite deck systems made of this material are favorable potential replacements for deteriorating conventional bridge decks. The decreased specific weight of the FRP greatly reduces the dead load of the superstructure helping avoid load posting of bridges. However there is a lack of long-term durability data concerning this material system in typical bridge environments. Thus, an efficient and effective method must be devised to monitor the health of an FRP structure in-situ. This paper will discuss the use of Infrared Thermography as a means of detecting structural imperfections -- delaminations, disbonds, voids -- caused by conditions encountered both in fabrication and in the field. As forced convection hot air is circulated through the bridge deck, delaminations and disbonds in the top of the deck appear cold while defects in the bottom of the deck give rise to areas with higher temperatures. The discontinuities in thermal propagation patterns are detected with a thermal imaging system and indicate present and possible future structural deficiencies. Laboratory results revealing fabrication/installation problems and those from field tests will be presented.

This paper presents damage detection tests of five-story steel frame with simulated damages. We discuss pre-analytical study and results of experiments. Fiber brag grating (FBG) sensors, accelerometers, strain gauges and laser displacement meters are installed in this test frame. We assume damages by removing studs from only one story, loosening bolts of beams, cutting part of beams and extracting braces from only one story. From the results of pre-analytical study, we can estimate which story is damaged from the change of natural period and mode shape to some extent. We applied flexibility method which is one of a damage identification methods using modal properties. We also apply flexibility method to results of experiments. In some cases we can estimate which story is damaged, and in other cases we cannot. We also applied a method using multiple natural frequency shifts. Making use of the change in five natural frequencies due to damage, the location of damaged stories can be pinpointed. In both methods, we cannot identify damaged story in some cases. Some methods other than methods using modal properties have to be tried to apply in such cases.

Paint stripping from large steel ships and other metallic surfaces is a major environmental safety, cost, and operational challenge in effectively and efficiently maintaining and refurbishing large structures. Environmental concerns are greatly limiting the possible options. As a result, a hybrid system composed of a waterjet with water recycling and robotic mobile manipulators with scanning bridges has become the leading form of paint stripping and was constructed by various manufacturers to address this need. The application of such scanning bridges is slow and their access is constrained by the complex shape of the ship hull and various features on the surface. To overcome these limitations, a robotic system that is called UltraStrip (UltraStrip Systems, Inc., Stuart, FL) is developed. This system uses magnetic wheels to attach the stripper to the structure and travel on it while performing paint stripping. To assure efficient paint stripping feedback data is required to control the travel speed by monitoring the paint thickness before and during the stripping process. Efforts at JPL are currently underway to develop the required feedback capability to assure effective paint stripping. Various possible sensors were considered and issues that can affect the sensitivity, reliability and applicability of the sensors are being investigated with emphasis on measuring the initial conditions of the paint. Issues that affect the sensory data in dynamic conditions are addressed while providing real-time real feedback for the control of the paint stripper speed of travel.

This paper describes work towards the development of a Lamb wave scanning method for the detection of defects in thin plates. The approach requires the generation of an ultrasonic S₀-Mode Lamb wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. A pair of receiving transducers, with a fixed relative separation, remotely scans line sections of the thin plate. The global position of the receiver pair is moved to cover a large plate area. The arrival time information coming from incident and reflected waves contain information associated with the location of reflection surfaces or potential flaws. The cross-correlation between the excitation signal and the receivers' waveforms is obtained and subsequently demodulated using a quadrature amplitude method in order to facilitate the determination of arrival times. Distances from the source, to the reflection surface and to the receivers are found from the arrival times of the reflected waves and the Lamb wave phase velocity. The distances and the source and receiver locations are incorporated in an elliptical solution to find coordinates of the reflection points. In a line scanning the set of predicted reflection points define the extent of the defect. The Lamb wave scanning approach is tested using 1.6 mm-thick Aluminum plates with notches of various lengths and orientations from 0, 22.5 and 45 degrees with respect to the far edge of the plates. The results are summarized with defect maps that compare favorably to the actual notch locations.

As a nondestructive evaluation technology, the EM impedance method allows us to identify the structural dynamics directly through in-situ active piezoelectric sensors. Previous work performed on 1-D steel beams structures shown through both theoretical analysis and experimental results that E/M impedance (or admittance) spectrum is a direct identifier of structural dynamics. The scope of presented work was to extend the positive results obtained for 1-D structure onto 2-D structures. Experiment analysis of 1-D and2-D structures has shown that E/M impedance (or admittance) spectrum accurately identifies the natural frequency spectrum of the specimens. Theoretical analysis was performed for particular boundary conditions to model the experimental set-up. Experiments were conducted on simple specimens in support of the theoretical investigation, and on thin-gauge aluminum plates to illustrate the method's potential. The number of specimens was sufficient to form a statistical data set. The aging aircraft panel was instrumented with piezoelectric active sensors and the spectrum of natural frequencies was measured at high frequency range. The changing of the spectrum due to presence of local small crack was noticed.

An ultrasonic pulse/echo technique is used to measure tensile preload on the umbilical flange joint bolts of the space shuttle orbiter. The ultrasonic instrument measures the round trip pulse travel time through the length of bolt. The pulse travel time increases with the applied load. The umbilical bolts experience bending loads in addition to the tensile load. The bending load affects the preload readings. The paper provides simplified theoretical derivations to explain the effect of bending on ultrasonic measurements. The bending loads cause rotation and translation of the ultrasonic pulse reflecting face. The bending also causes stress gradient in bolts. These effects cause phase gradient and physical shift in the received ultrasonic beam across the face of transducer, distorting the ultrasonic signal and introducing errors in the preload readings. A number of experiments were performed on the bolt to study the effect of bending combined with variation in transducers, bolt end designs and configurations. The bolt end design of the operational bolts was modified to reduce error in the readings. The paper provides explanation of the effect of bending in ultrasonic preload measurements and suggests approaches to minimize their effect.

Following the explosion of Delta 241 (IIR-1) on January 17th, 1997, the failure investigation board concluded that the Graphite Epoxy Motorcases (GEM's) should be inspected for damage just prior to launch. Subsequent investigations and feedback from industry led to an Aerospace Corporation proposal to instrument the entire fleet of GEM's with a continuous health monitoring system. The period of monitoring would extend from the initial acceptance testing through final erection on the launch pad. As this proposal demonstrates, (along with the increasing use of advanced composite materials in aircraft, automobiles, military hardware, and aerospace components such as rocket motorcases) a sizable need for composite health assessment measures exist. Particularly where continuous monitoring is required for the detection of damage from impacts and other sources of high mechanical and thermal stresses. Even low-momentum impacts can lead to barely visible impact damage (BVID), corresponding to a significant weakening of the composite. This damage, undetectable by visual inspection, can in turn lead to sudden and catastrophic failure when the material is subjected to a normal operating load. There is perhaps no system with as much potential for truly catastrophic failure as a rocket motor. We will present an update on our ongoing efforts with the United States Air Force Delta II Program Office, and The Aerospace Corporation. This will cover the development of a local, portable, surface-mounted, fiberoptic sensor based impact damage monitor designed to operate on a Delta II GEM during transport, storage, and handling. This system is designed to continuously monitor the GEMs, to communicate wirelessly with base stations and maintenance personnel, to operate autonomously for extended periods, and to fit unobtrusively on the GEM itself.

An experimental investigation was conducted for the determination of defects at aircraft components and for the composite patches evaluation. Cracked aluminum panels, untreated and repaired with carbon and boron composite patches, were inspected. The non-destructive techniques used in the assessment of these aircraft materials were infrared thermography and fiber optics microscopy. Infrared thermography is used for the localization of defects on aluminum panels, as well as on repaired ones with composite patches. Furthermore, the detection of defects on repaired aluminum panels that have undergone to fatigue testing, is attempted. Fiber optics microscopy is employed in order to examine the surface morphology of both carbon and boron composite patches. The results of this laboratory research work can lead to the development of an integrated non- destructive method for in field inspections of aircraft components.

Genetic algorithms (GA) have been widely used in many optimization and system identification problems. Nevertheless, largely because of the stochastic nature, the approach of using GA has been found to be not efficient in fine-tuning in terms of convergence to the optimal solution from its neighborhood. In this study, the GA approach as a global search tool is combined with a local search (LS) method which is developed on the basis of the classical univariate method, so as to expedite the search process by perturbations near the optimal solution. This LS method, herein named the modified univariate method, does not search all the unknown variables of the problem but only a few selected ones. Furthermore, the initial step size of LS is adjusted according to the average efficiency of each LS operator. For comparison, the Solis-and-Wets LS method is also considered. A numerical example of 10-DOF structure is presented to compare the accuracy and efficiency of the various methods considered.

In many engineering materials micro-cracks develop when the materials are subjected to repeated loading or fatigue loading. These micro-cracks increase the material attenuation and alter the ultrasonic wave speed. Careful experiments show that as a material is subjected to a greater number of fatigue cycles, its higher order (also known as the nonlinear) acoustic properties change significantly. The percentage change in the nonlinear acoustic properties is greater than the percentage changes in the wave speed and attenuation. However, experimentally it is very difficult to measure these nonlinear acoustic properties. Materials like concrete develop a large number of micro-cracks under fatigue loading and show measurable changes in its attenuation. However, some other materials, such as Plexiglas, apparently do not develop micro- cracks under fatigue loading. They look same, equally transparent, before and after the repeated loading up to the point of its failure. Its longitudinal wave speed and attenuation also do not change by any measurable amount due to the fatigue loading. Changes in the nonlinear ultrasonic properties are too difficult to measure. Can there be any relatively robust ultrasonic measurement to capture the property changes in such materials due to fatigue loading? It is investigated in this paper. It is found that some guided wave propagation characteristics change with fatigue loading. These changes are strong enough to be detected by ordinary ultrasonic measurements without taking help of any highly precise ultrasonic measuring instrument in the experimental setup.

A primary concern with composite repair patches is the potential degradation of load transfer capabilities due to aging and environmental effects. The development of low profile, distributed, embeddable, real-time, optical fiber sensors capable of detecting the onset of patch delamination on repaired regions of the aircraft would eliminate a significant portion of the related maintenance costs as well as improve confidence levels in the technology. The presented sensing system is comprised of optical fiber long period gratings (LPGs) for chemical measurement and Bragg gratings for strain measurement. The sensors can be multiplexed together to monitor the structural health of the patch system and status of any remaining damage in the parent structure. The LPG sensors operate based on specially designed sensing coatings which cause a measurable change in the refractive index 'seen' by the LPG in the presence of target molecules. In this configuration, LPGs can be used to detect moisture infiltration and other chemical changes within a localized environment. Complementary to the long period grating, Bragg grating strain sensors can be fabricated on the same optical fiber to measure load transfer and composite delamination in patches used to repair cracks that occur in aging aircraft.

This paper considers the problem of wave propagation in a nonlinear elastic medium with a quadratic stress-strain relationship. The paper is limited to one-dimensional wave propagation. Under these conditions, the initial value problem is formulated into a hyperbolic system of conservation laws. The Riemann problem due to an initial step function excitation is considered first. Analytical solutions to the Riemann problem are obtained by solving the corresponding eigenvalue problem. In addition, a computer program is developed based on the high-resolution central scheme of Kurganov and Tadmor. The accuracy of this numerical procedure is verified by comparing the numerical results with the exact solutions. The second part of the paper considers several different types of initial excitations in order to determine special characteristics of the wave propagation due to material nonlinearity.

The stress-strain curve and residual stresses in thin diaphragms used in microelectronic applications are determined by means of a nondestructive approach. The method is based on the measurement of the deflection of the diaphragm near its center when it is subjected to an externally applied normal pressure and inversion of the deflection data to determine the quantities of interest. The measurements are carried out by means of a Twyman-Green laser interferometer. The relationship between the applied pressure and the deformation profile of the diaphragm is derived based on membrane mechanics. It is shown that if the diaphragm deflection in the neighborhood of its center can be measured accurately, then the strain and stress in this region can be determined through data inversion based on this relationship. It follows that, given the deformation profiles of a diaphragm for a range of pressurer, the biaxial modulus and the residual stress in the diaphragm can be extracted. By utilizing this local bulge testing method, many complex diaphragm shapes can be analyzed, without resorting to complicated numerical modeling. The method is applied to a nitride membrane with initial tensile stress and also to a silicon composite membrane with initial compressive stress, with reasonable results.

Several nondestructive testing methods can be used to estimate the extents of damage in a concrete structure. Pulse-velocity and amplitude attenuation, are very common in nondestructive ultrasonic evaluation. Velocity of propagation is not very sensitive to the degrees of damage unless a great deal of micro-damage having evolving into localized macro-damage. Amplitude attenuation is potentially more sensitive than pulse-velocity. However, this method depends strongly on the coupling conditions between transducers and concrete, hence unreliable. A new active modulation approach, Nonlinear Active Wave Modulation Spectroscopy, is adopted in our study. In this procedure, a probe wave will be passed through the system in a similar fashion to regular acoustics. Simultaneously, a second, low frequency modulating wave will be applied to the system to effectively change the size and stiffness of flaws microscopically and cyclically, thereby causing the frequency modulation to change cyclically as well. The resulting amplified modulations will be correlated to the extents of damage with the effect that even slight damage should become quantifiable. This study unveils the potential of nonlinear frequency analysis methods for micro-damage detection and evaluation using actively modulated acoustic signals. This method can interrogate materials exaggerating the nonlinearly that exists due to microcracking and deterioration.

Acoustic microscopy provides not only the morphology, but also the biomechanical properties of the biological soft tissues. The biomechanics of atherosclerosis is important because the pathophysiology of atherosclerosis is closely related with mechanical properties and mechanical stress. Rupture of the fibrous cap of atheromatous plaque is the initial event in acute coronary syndrome such as acute myocardial infarction or unstable angina. In addition to extrinsic physical stresses to the plaque, the intrinsic biomechanical property of the plaque is important for assessing the mechanism of the rupture. Two sets of SAMs operating in 100 to 200 MHz and in 800 MHz to 1.3 GHz were equipped to measure the acoustic properties of atherosclerosis of human or mouse arteries. The values of attenuation and sound speed in the tissue components of atherosclerosis were measured by analyzing the frequency dependent characteristics of the amplitude and phase signals. Both values were highest in calcification and lowest in lipid pool. Although attenuation and sound speed were relatively high in intimal fibrosis, the inhomogeneity of acoustic parameters was found within the fibrous cap. Polarized microscopy for the collagen stained with Picrosirius red showed that the attenuation of ultrasound was significantly higher in type I collagen with orange polarized color compared to type III collagen with green color. SAM has shown the possibility to detect the plaque vulnerability and it might improve our understanding of the sudden rupture from micro-mechanical point of view.

The physical properties of cytoplasm are primarily determined by the state of cytoskeletal element, i.e. their polymerisation, crosslinking and supramolecular interactions with other molecules. These interactions are involved in signal transduction processes as well as in morphogenesis. Scanning acoustic microscopy proved to be a powerful tool to determine the mechanical properties of living cells. The interpretation of the sound propagation parameters, however, has to be based on investigation of in vitro models. Therefore polymerisation of actin and tubulin have been followed using a novel oscillating rod rheometer which allows for synchronous determination of sound velocity, sound attenuation and viscosity. Sound velocity measures the elastic propterties of cytogels, attenuation the supramolecular associations. All these parameters are evaluated with minimal strain, in the range of 1- 100 nm actin with glycolytic enzymes not only modulated polymerisation in a specific, and substrate dependent manner, but also the stiffness of the fibrils was altered, e.g. by hexokinase in the presence of high ATP, this enzyme exhibited actin severing properties and reduced stiffness. Differences in polymerisation kinetics were observed comparing pyrene-labeled actin fluorimetry and oscillating rod viscosimetry. This comparison led to the detection of pseudocrystalline structures produced by g-actin and aldolase (in the absence of fructose-bisphophate, the substrate of aldolase). Elastic stiffness of actin filaments can be modulated by ATP/ADP and by actin binding proteins (e.g. the glycolytic enzyme hexokinase) as well. The in vitro observations support the interpretation of SAM data calculated for living cells.

Measurement of acoustic propagation speed (C) and attenuation (a) in biological tissues serves to enhance our understanding of how tissue composition and structure affects organ function. We applied the V(z)-technique to measurement of C in embedded cancellous bone at 1 GHz using an acoustic microscope and succeeded in recording the Cs of both longitudinal lateral waves (C_L) and Rayleigh waves (C_R). The former ranged between 2.33 and 4.33 km/s (mean ±SD: 3.37 ±0.61 km/s) and the latter between 1.93 and 2.07 km/s (2.00 ± 0.06 km/s), which is in the range expected on the basis of known properties of bone and acoustic field theory. With respect to soft tissue sections, the V(z)-technique is practically impossible to use, and therefore we applied the V(f)-technique to sections of chordae tendineae. Initial measurements of C and a of aortic tissue, which is well characterized, showed a C of 1.59 ± 0.04km/s and α of 0.230 ±0.001 dB/μm at signal frequencies 0.95 to 1.02 GHz. These results were in agreement with those of others and the chordae subsequently revealed a mean C of 1.79 ±0.18 km/s and α of 0.220 ±0.010 dB/micrometers . The distribution of the properties across the chordal sections showed a regularly undulating pattern which followed the undulating pattern of the collagen fiber arrangement. We conclude that the V(z)- and V(f)-technique are both valuable techniques for microelastic characterization of biological tissues.

In the middle of the 90s, the image processing and analysing method subtraction scanning acoustic microscopy (SubSAM) was developed to display and quantify cell surface motility. Since then, the approach of this method supported already existing models and provided new insights into the regulation of cell surface motility. The investigations with SubSAM revealed that cell surface motility is modulated by tension and is regulated by known F-actin reorganising signal transduction pathways via the small GTP-binding protein Rac and protein kinase C. Beyond that, the in vitro cell surface motility as revealed by SubSAM corelates with cell transformation respectively the invasive or metastatic potential. The enhancement of cell surface motility is always accompanied by characteristic actin reorganisation: loss of stress fibers and the formation of cortical F-actin knots or larger aggregates. From these observations a model has been developed for the regulation of cell surface motility and an addtitional mechanism for cell surface deformation based on myosin-dependent F-actin aggregation is proposed.

Ultrasound scan of liver is performed and a total of 22 2-D images are obtained using mechanical jig each at every 10 degree interval. Each image slice is the projection of the liver in a vertical plane. These images are digitized into bitmaps and are averaged using a spatial filter to make them noiseless. To enhance the noiseless image, contrast stretching is also performed using Histogram Equalization. The Detection and Localization is done with a Sobel Compass Operator. The Sobel Compass Operator detects edges of the individual noiseless and enhanced image slices. Subsequently, Tissue Characterization is done using the property of Differential Absorption of Ultrasound waves. Thus the attributes of the image are extracted. Finally defects are localized and stored for future reference. The stored 2-D images are then interpolated to develop 3-D images. The default position of the viewer of the 3-D image is the center of the liver, which is movable according to viewer's interest. The goal of this paradigm is to provide explicit view of the liver at any mentioned depth from the center, thus providing a distinctive advantage over the existing methods. The 3D images are made more comprehensive by pseudocoloring and Perspective Zooming.

Acoustic wave devices based on waveguide modes with shear-horizontal polarisation, i. e. Love modes, are very promising for sensor application, especially in liquid environments. They can be used for determination of liquid density and viscosity as well as for chemical sensors. Up to now, several systems have been reported based on standard ST-quartz. Those devices lack temperature stability which is essential for field application. However, appropriate combinations of crystal cut angle and SiO₂ overlay thickness should provide temperature compensation. Thus, different systems based on Y-rot quartz and lithium tantalate plates with SiO₂ waveguiding films have been investigated. Temperature behavior as well as relevant acoustic properties are compiled. It has been proven that numerical calculations yield a very accurate description of the physical device properties. For both cases of material combinations it has been shown that systems are available combining temperature compensation with a high device sensitivity.

Corrosion has been recognized as a serious problem in the maintenance of aging aircraft. The Industrial Materials Institute (IMI) has explored the use of laser-ultrasonics for the detection of hidden corrosion in metallic lap joint structures. For inspection with painted surfaces, IMI has shown that a resonance spectroscopy approach using a simple two-layer model can be used to determine the thickness of the paint layer and of the top metal skin. Validation of the model has been made using a test sample with a broad range of paint thickness. Once combined with a numerical inversion method, the model is used to produce a thickness map of the top metal skin from measured resonance frequencies. Results from standard samples with flat-bottom holes showed that the laser-ultrasonic technique could detect metal loss below 1%. The reliability of the method was also demonstrated on accelerated corrosion samples. Comparison to X-ray images showed that the laser-ultrasonic method presented a thickness map that had the same accuracy as the X-ray system without the need for dismantling the sample. These results indicated that laser-ultrasonics could be a useful tool not only to inspect aircraft during routine maintenance but also to provide valuable data in the study of corrosion inception and growth in lap joint structures.

This paper is dealing with the ultrasonic imaging techniques and instrumentation that are used for the inspection of aircraft and bridge structures. A concept of a modular inspection system with interchangeable components will be presented. The modular concept provides greater flexibility in the integration of various commercially available sensors and data acquisition units deployed by operators. Miniature scanning and interface modules were developed as components of the integrated system for wide area inspections using ultrasonic sensors. A novel ultrasonic imaging technique has been utilized for characterization of internal fatigue cracks. The technique is based on a linear scan acquisition and processing. Portable ultrasonic flaw detectors or thickness gages can be used to acquire the data. An image-processing module was developed to manipulate the acquired images and to display the inspection results. Several rapid scanning techniques have been developed to satisfy flaw detection and characterization criteria as well as the scanning and data acquisition module capabilities. The techniques and instrumentation were successfully tested on various aircraft and steel bridge structures.

An ultrasonic extensometer is used to measure tensile preload in fasteners such as bolts and studs. Raymond Boltgage and StressTel Boltmike are two of the most commonly used extensometers. It is necessary to know the accuracy of extensometer measurements. Many factors affect the accuracy of extensometers. Characteristics of the fasteners affect the accuracy. Moreover, the variability in transducer coupling also affects the accuracy. Variation in the grip length of fastener also affects the accuracy. Therefore, the extensometer doesn't have a fixed amount or percentage of measurement accuracy. A detailed procedure to estimate the accuracy is currently not available in the manufacturer's manuals. The paper discusses all sources of errors and provides a procedure to compute the preload measurement accuracy for the two instruments. The paper also provides relationships between the corresponding parameters of the two instruments. Moreover, it provides equations to program the two instruments so that the difference between preload measurements by the two instruments is minimized.

We conducted theoretical and experimental approaches for applying Brillouin optical time domain reflectometer (BOTDR) to aircraft and spacecraft structure health monitoring system. Firstly, distributed strain was measured by BOTDR under 3-point bending test and a spatial resolution was enhanced up to 0.5m using Brillouin spectrum analysis and processing though the device used in this experiment had a spatial resolution of 2m normally. Secondly, dynamic strain measurement was executed under cyclic loading conditions. Brillouin spectrum measured under dynamic conditions is equivalent to superposed spectrum using many spectra measured under static loading conditions. As the measured spectrum was decomposed into many spectra in static loading state, the strain amplitude and its ratio could be estimated. Thirdly, strain and temperature could be measured independently using combined system of BOTDR and fiber Bragg grating (FBG) with wavelength division multiplexing (WDM). Additionally, the application of BOTDR sensing system was shown for a prototype carbon fiber reinforced plastic (CFRP) liquid hydrogen (LH₂) tank under cryogenic condition.

In this paper we describe a laser ultrasonic system for real-time monitoring of the degree of cure of a graphite-epoxy composite part during manufacturing. The system is integrated with a Resin-Transfer Molding (RTM) machine, and contains (i) a fiberized laser ultrasonic source, and (ii) an embedded ultrasonic sensor based on an intrinsic fiber optic Sagnac interferometer. Bulk ultrasonic waves generated by the laser source are transmitted into the composite structure and are subsequently detected by the embedded ultrasonic sensor. The degree of cure can be obtained from measurements of ultrasonic velocity and attenuation in the composite part. The use of an optical switch in the fiber optic delivery system of the laser ultrasonic source allows ultrasonic generation at several locations of the composite part. In this paper we discuss the design of the laser ultrasonic source and the sensor optimized for cure monitoring applications, and their integration with the RTM mold. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. Our results show that laser ultrasonics offer distinct advantages for manufacturing of modern composite structures including the ability to operate in a high temperature and high pressure environment and provide distributed sensing that can cover critical areas of a component.

Carbon fiber reinforced plastic (CFRP) plate tends to suffer large and complicate internal damages at being impacted. The damage significantly reduces the residual compressive strength, and is needed to be detected by an advanced nondestructive method. Both the real-time source location and non-contact damage inspection systems are important for transportation equipment. We first develop a new source location method of impact position by using the arrival time difference of zero-th order anti-symmetric Lamb waves (A₀- Lamb waves) at selected frequency. AE monitoring system was utilized for this study. The arrival times were automatically determined from the peak arrivals of time-transient wavelet coefficients of four AE transducer signals. This system enables us to determine the impact location within two times of plate thickness within 1 second. Next, we propose a non- contact laser ultrasonic system for detecting the damage location and shape in cross-ply CFRP plate impacted by flying steel balls. Delamination shapes were revealed by comparing the cross-correlation and/or amplitude difference of laser Lamb waves over sound and damaged zone. Here, A₀-Lamb waves were monitored by scanning both a line focused pulse YAG laser and a probe laser of heterodyne-type laser interferometer at 5 to 10 mm step. Distance between the incident and probe laser was changed from 10 to 100 mm. Double-tree shaped delamination with 25 mm long was revealed in ball hit CFRP plate.

Shape Memory Alloys (SMA) are increasingly used as smart devices ina erospace applicatons.Their priry advnatage over other smart materials z9piezo-ceramics adnpiezo-polymers) is in their ability to undergo large strains and dispalcements that enables the devlopment of smart mechanissm. The specific objective of this research was toinvestigate the use of embedded SMA for active shape correctionof flexibl emirror substrates.Active shape correction is a possiblemeans of mitigating thrmally induced distortions in space based optical iaging systems inorder to understand and quantify the designvalraibles that lead to that goal, embedded SMA wires we used toa cctuate a series of composite bmeasm.Themoemvent induced byactutionw as monitored with theMoire inteferietery method,and the rsutls wer compared witha n anlaytical model.The repeatabiltya nd rlaibltiyof a possible acutation systmeand the propertis of the SMa wires was also studied by testing the stress-strian anthe stress reo ery behavior under controlled conditons.110

Many versions of dynamic atomic force microscopy have been proposed for imaging specimens. All of these methods rely on the relative motion between the atomic force microscope (AFM) tip and the specimen surface. These techniques are used to extract quantitative information about the surface stiffness with high resolution. These techniques utilize the dynamic response of the cantilever, specifically in terms of the higher-order cantilever modes. These techniques rely on tip-sample mechanics models in order to determine material properties. The implications of the different models on the interpretation of AFM images is discussed. In particular, the effect of adhesion on these measurements is discussed.

The problem of microdamage evolution in advanced composites is addressed in this paper. A combination of transient waveform classification and multiparametric filtering is used to extract the histories of damage evolution in a unidirectional graphite/epoxy composite loaded in the fiber and transverse directions. Three characteristic AE waveforms with different amplitudes, durations, and frequency spectra are identified based on the transient analysis. The waveforms are associated with matrix cracks, fiber breaks, and longitudinal splitting in the composite. Parametric regions for the characteristic waveforms are identified in the amplitude-risetime parametric space. Multiparametric filtering is applied to extract the microdamage evolution histories. The method is expected to be advantageous for the real-time damage and fracture monitoring of composite structures.

This work present a new design development of transducers with a Dry Point Contact (DPC) waveguiding extensions, which may serve for ultrasonic laboratory and field testing of materials. Their design allows to apply them for a number of cases where ultrasonic testing is believed to be impossible of inaccurate. Mathematical models based on solutions of the partial wave equation for different types of the DPC waveguiding extensions are applied for optimization of their design. Their amplitude-frequency characteristics are computed. Practical aspects of application of the DPC transducers and their advantages are described in this work.

This paper reports on a new methodology for detecting surface cracks in metallic structures by combining a microwave resonant cavity with infrared imaging. The underlying principle is based on crack induced disruptions of microwave wall currents creating localized concentrations of microwave energy. These regions of concentrated energy may, in turn, produce a localized heating in a thin layer of dielectric material placed adjacent to the surface being inspected. Detection of local hot spots via infrared imaging may then be used to infer the presence of a crack or other discontinuity in the surface. This study utilized a numerical simulation of electromagnetic fields within the resonant cavity and the resulting dielectric heating. The objective of the numerical study was to gain insight into fundamental electromagnetic and thermophysical processes on which this NDT scheme was based. The transient 3D Maxwell's equations were solved numerically using the method of Finite Difference-Time Domain to determine electromagnetic field distributions. The energy equation was then solved in order to determine thermal energy deposition and temperature fields in the dielectric layer. The sample numerical simulations indicate that combining microwave heating with thermographic imaging could lead to a viable non- destructive testing instrument for crack detection.

In this paper, a two-stage damage identification method is proposed using the data obtained from limited piezoelectric sensors. In the first stage, a first-order approximate technique is proposed for numerically obtaining the transient response of the change in electrical potential on sensors caused by damages. To identify damages, both numerical and experimental data in the time domain are transformed into the frequency domain using the FFT technique. The damage locations, then, can be detected by matching the numerical data and the experimental data in the frequency domain through a proposed detection technique, which can eliminate the effect of damage extents effectively and keep the influence of damage location only. After obtaining the possible damage locations, in the second stage, an iterative estimation scheme for solving nonlinear optimization programming problems, based on the quadratic programming technique, is proposed to predict damage extents. A beam example is employed to illustrate the effectiveness of the present algorithm. Furthermore, various investigations, such as the accuracy of the proposed first- order approximate technique, the influences of the excitation frequency of external forces, modeling errors and measurement noises and window methods used in the FFT have been carried out.

Optical fiber sensors are rapidly emerging as viable alternatives to piezoelectric devices as effective means of detecting and quantifying acoustic emission (AE). Compared to traditional piezoelectric-based sensors, optical fiber sensors offer much smaller size, reduced weight, ability to operate at temperatures up to 2000 degree(s)C, immunity to electromagnetic interference, resistance to corrosive environments, inherent safety within flammable environments, and the ability to multiplex multiple sensors on a single fiber. The authors have investigated low-profile fiber optic-based AE sensors for non-destructive evaluation (NDE) systems. In particular, broadband and resonant type optical fiber sensors were developed for monitoring acoustic emission for NDE of pressurized composite vessels and commercial airframe structures. The authors developed an in-plane, broadband sensor design based on optical strain gage technology. In addition, an out-of-plane, resonant sensor was developed using micromachining techniques. The sensors have been evaluated for performance using swept frequency and impulse excitation techniques and compared to conventional piezoelectric transducers. Further, application experiments were conducted using these sensors on both aluminum lap-joints and composite fracture coupons, with collocated piezoelectric transducers. The results indicate that optical fiber AE sensors can be used as transducers sensitive to acoustic events and the indication of imminent failure of a structure, making these sensors useful in many applications where conventional piezoelectric transducers are not well suited.

The propagation of guided elastic waves in an orthotropic compressible layer imbedded in a difference orthotropic compressible material is examined. The strain energy of the two materials is otherwise arbitrary with the material constants satisfying the strong ellipticity conditions. To extract the propagation characteristics, propagation is considered along a material axis of symmetry which lies in the interfacial plane. For the two materials having common axes of symmetry, the dispersion equation is derived in explicit form. Analysis of the dispersion equation reveals the propagation characteristics and their dependence on frequency and material parameters. Low and high-frequency asymptotes are defined and their existence is discussed, the high frequency asymptote corresponding to a Stoneley wave. For low frequencies, the interfacial wave speed is derived in explicit form yielding a simple material parameter condition for the existence of interfacial waves.

License plate automatic recognition is an important section of traffic management. In this paper, the procedure of license plate recognition is analyzed. A novel compact hybrid opto- electrical correlator and the optimum design of spatial matched filter (MSFs) are illustrated in details. The compact correlator, combining parallelism of optics and flexibility of electronics, plays a promising part in the field of image recognition. The optimal binary phase-only MSFs, based on Synthetic Discriminate Function (SDF), by means of NN Clipping and Monte Carlo learning algorithm, can greatly improve the accuracy of image recognition. The result is presented finally.