This paper will discuss opportunities and future trends in non-destructive evaluation (NDE) and health monitoring based on new sensor principles and advanced microelectronics. Conventional NDE is based on single probe approaches, where systems are portable and used for local inspection, or on multi probe techniques with the support personnel in workshops using complex NDE systems with multiple sensors. Miniaturization of electronics and new sensor approaches allow realizing local probe techniques. This denotes that the probe and data acquisition are located at the structure to be monitored or inspected. Hence, this approach can be used for continuous monitoring as well as for periodic inspections and will increase the efficiency of inspection procedures and reduce inspection time.

The external coating systems of nearly all military aircraft are stripped to bare metal during programmed depot maintenance cycles. This paint stripping process has become cost prohibitive in recent years, and is expected to continue to be a major and escalating problem for the sustainment of an aging Air Force fleet. Although a number of competing factors come into play, the key reason behind current paint stripping practices is centered on requirements for visual inspection of the aircraft structure to determine if corrosion and/or fatigue damage is present. In recent years, a number of advancements have been made in the area of nondestructive evaluation (NDE) that provide new inspection capabilities for aircraft skins without the requirement for protective coating removal. In this effort, several advanced imaging methods are evaluated for hidden damage detection and quantification through typical aircraft coating systems. A number of measurement examples are provided for engineered and realistic aircraft reference standards with variations in coating type, coating thickness, hidden damage type, and component complexity being considered. A comparison of measurement sensitivity, resolution, area coverage, ease-of-use, quantitative assessment, data processing requirements, and inspection speed are also made. It is anticipated that the use of one or more of these advanced NDE methods for thru-paint inspections will provide an enabling capability for long-life coating systems and condition based maintenance practices resulting in significant reductions in hazardous waste generation, dramatic cost savings, and enhanced readiness levels for a wide variety of Air Force systems.

Acoustic thermography is a relatively new NDE method based on thermographic detection of dissipated ultrasonic energy. The system presented uses an uncooled high-speed camera and low power ultrasonic excitation. This technology is characterized by low system costs, since no nitrogen or stirling cooling is necessary. Low power applications are facilitated by efficient ultrasonic coupling, hence taking care of sensitive objects. A camera has been developed which uses a 384 x 288 pixels micro-bolometer array as sensor and may record up to 100 frames per second. The advantages of this system are demonstrated on a set of carbon fiber reinforced plastic (CFRP) plates damaged by impacts of various strengths. The impact flaws consisting of fiber fractures and delaminations can be detected while transmitting low ultrasonic energy of only about 1.3 W to the specimen.

Photonic crystals (PC) are artificially fabricated crystals with a periodicity in the dielectric function. The spectral signature of such crystals is intricately tied to their underlying crystal lattice structural parameters. A result of this is that significant spectral changes can occur if damage is induced in the photonic crystal. In this work we present preliminary experimental results that demonstrate the possible use of photonic crystals as sensors for the detection, quantification and diagnosis of sub-micron damage. The experimentally observed variation in the reflection spectra of the photonic crystals is related to the damage induced in the material. A novel damage metric, based on principles of fuzzy pattern recognition, is introduced and is used to identify and quantify micro-damage in the photonic crystal. The corresponding damage metric is also presented and discussed. The detailed fabrication steps, as well as the advantages and limitations of this new approach are also addressed. It is concluded that photonic crystals can be successfully used for micro-damage quantification.

Terahertz (THz) impulse ranging is used to examine ceramic ball bearings for fractures. THz radiation is demonstrated to transmit through both unfired and fired ceramic targets. The electromagnetic scattering signature of commercial aluminum oxide bearings is measured and compared to identical bearings damaged by thermal stress. Two separate methods are used to determine the presence of fractures: late time impulse response and time domain angular modulation. Late time impulse response detects changes in temporally shifted scattering mechanisms, while time domain angular modulation allows rapid detection of fractures. This evaluation technique is non-contact, nondestructive, requires no liquid medium, and is insensitive to ambient temperatures.

Photonic bandgap materials (PBM) are synthetic materials that artificially manufactured at the nano-scale to control light propagation. These crystals have the ability to control light propagation in three dimensions by opening a frequency gap in which light is forbidden to propagate. When light is reflected by a nano photonic (NP) crystal a spectral signature that is directly related to its crystalline structure periodicity can be observed. It is suggested here that microscale damage in a substrate attached to the NP sensor might result in a significant change in the spectral signature of the NP sensor, hence allowing for micro-scale damage detection and quantification. To demonstrate the use of sensors for microdamage detection in structural materials an integrated numerical modelling approach was used. The approach augments two numerical methods to simulate the effect of microdamage in the material substrate on the spectrum signature of NPC sensors. First, the finite element method (FEM) was used to simulate structural response of the NP sensor under strain induced in the substrate with and without substrate damage. Second, the results of the finite element analysis were used as inputs to simulate the optical response of the NP sensors using the finite difference time domain method (FDTD). The integrated numerical approach was applied to a wood pile NP sensor attached to a silicon substrate. The numerical analysis showed promising results. Changes in the NP spectral signatures due microdamage in the silicon substrate were successfully identified.

Presently there exists no way for direct measurement of strain at high temperature in engine components such as combustion chamber, exhaust nozzle, propellant lines and turbine blades and shaft. Existing fatigue and life prediction studies for high temperature zones in propulsion systems depend on strain/stress values computed from indirect measurements of temperature, flow velocity, pressure, et al. Thermomechanical fatigue (TMF) prediction, which is a critical element for blade design, is a strong function of the temperature and strain profiles. Major uncertainties arise from the inability of current instrumentation to measure temperature and strain at the critical locations. This prevents the structural designer from optimizing the blade design high temperature environment, which is a significant challenging problem in engine design. Ability to measure directly strains in different high temperature zones would deeply enhance the effectiveness of aircraft propulsion systems for fatigue damage assessment and life prediction. State of the art for harsh environment high temperature sensors has improved considerably for the past few years. This paper lays down specifications for high temperature sensors and does the technological assessment of these new sensing technologies. This paper presents a review of the recent advances made in harsh environment sensing systems and takes a peek at the future of such technologies.

A technique has been developed whereby the thickness and elastic modulus of a coating applied to a substrate can be calculated from measurement of the resonant bulk wave ultrasonic modes of the combined substrate and coating. The density of the coating and the material properties of the substrate are required for this method. We have investigated the difference between models that take account of material attenuation and simpler models that do not and have found that there is little difference in the predicted resonant frequencies of the system for modes that can be observed in the experimental data. We have applied the technique to explain the experimental measurements for a 100μm thick epoxy resin coating curing on a 1mm thick aluminium substrate using wideband radially polarised SH shear waves. In this dynamic system the elastic properties of the coating change and particular resonant modes not only shift but can disappear or appear in the experimental data. A model is used to explain this behaviour and show that the technique has potential for coating thickness measurement in other areas. We have taken samples of 220μm thick aluminium sheet with a fully cured epoxy coating of nominal thickness 11μm, and have used ultrasonic measurements to calculate the thickness of the epoxy coating layer.

In this paper we present the results on the design of a unique two-dimensional phased array with low channel applications for imaging defects on a metal surface. First, basic transducer calculations will be shown. Followed by the results of important phased array variables, such as focusing, and angle beam sweeping ability, The final design will be given. Next the computer simulation results will be discussed. These results will indicate the performance of the actual array. The second half of the paper will be devoted to a discussion on the phased array testing results with a demonstration phased array.

Guided waves have been widely used for the long-range non-destructive damage detection of cylindrical waveguides such as pipes. Noting that the non-dispersive torsional wave mode is the most preferred in long-range inspection, this work in concerned with the generation and measurement of the torsional wave by a new magnetostrictive transducer. The magnetostrictive effect represents the coupling phenomena between magnetic field and mechanical deformation of magnetostrictive materials such as nickel. Because earlier magnetostrictive transducers require circumferential pre-magnetization of the magnetostrictive patch before actual experimentation, they are not so desirable for long-term on-line monitoring. To avoid the pre-magnetization process, we have recently developed a new transducer that employs slender rectangular nickel patches bonded at 45 degrees to the pipe/tube axial direction. Though this transducer does not require pre-magnetization, its performance can be substantially improved if patch shape is altered. This paper presents a new patch shape yielding higher output voltage and better signal-to-noise ratio. The key idea was to use yoke to concentrate the magnetic flux density at the slender rectangular patch. When the yokes are attached at both ends of the rectangular patch, the patch looks like an alphabet character Z. Several sets of experiments were conducted to check the transduction efficiency of the proposed transducer. When the Z-shaped patches were employed in actual experiments, the magnitude of the measured wave signal was 13 times larger than that by the yokeless transducer. Other experimental findings are also reported in this work.

A surface wave on a liquid/solid interface is well-known to radiate acoustic energy into the liquid and is therefore rapidly attenuated. In this work, we have been able to show by experiments and calculations that the proximity of another surface (layer 1 to layer 3 and layer 3 to layer 1) sustains the surface wave through long distances for layers of both plates and concentric tubes. In addition, even when the surface wave is reflected from a distant edge, the returning wave is sustained in the multi-layer system and can be easily detected. This is apparently one of the first observations of leaky surface waves traveling over large distances, in this case over a thousand wavelengths. The effect is modeled on the basis of a cooperative phenomenon between two interfaces separated by a water layer. The effect represents a valuable result in the wave propagation of acoustic surface waves and opens the door to many applications.

A recently-developed magnetostrictive transducer consisting of a circular nickel patch and a set of permanent magnets and a figure-of-eight coil has been shown to successfully generate and measure guided waves in non-ferromagnetic plate structures. In this work, various transduction characteristics of the transducer are investigated and the experimental findings are reported. This transducer uses a thin circular nickel patch bonded to a non-ferromagnetic test plate. If alternating current is supplied to the coil, the magnetic field by the coil causes dynamic deformation of a patch mainly in the magnetic field direction; the deformation is due to the magnetostrictive effect of nickel. The patch deformation will then generate elastic waves in the plate. Since the coil and the magnets are enclosed inside a plastic bobbin which is placed on top of the patch, the flux direction can be freely adjusted. Therefore, we can generate and measure waves in any direction by simply changing the flux direction of the transducer without patch detaching and re-bonding. In this study, the directivity and frequency characteristics of generated Lamb and SH (shear-horizontal) waves by the transducer will be investigated. In this report, we first demonstrate that there exist specific magnetic flux directions in which only the Lamb or SH waves can be picked up. These directivity characteristics were experimentally investigated for various alignment angles and the observed results were explained accurately by our theoretical analysis. Additionally, the relation between the frequency characteristics of the transducer and the patch size was also investigated.

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates ultrasonic waves in a carbon fiber reinforced plastic (CFRP) laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor using a newly developed high-speed optical wavelength interrogation system. In this research, this system was applied to the evaluation of debonding progress in CFRP bonded structures. At first, small-diameter FBG sensors, whose cladding diameter is about 1/3 of common optical fibers, were embedded in an adhesive layer of a double-lap type coupon specimen consisting of CFRP quasi-isotropic laminates, and the ultrasonic wave was propagated through the debonded region. After that, the wavelet transform was applied to the received waveforms and the results showed clear difference depending on the debonding length. Hence, a new damage index was proposed, which could be obtained from the difference in the distribution of the wavelet transform coefficient. As a result, the damage index increased with an increase in the debonded area. Furthermore this system was applied to the skin/stringer structural element of airplanes made of CFRP laminates. Both of the waves received by a bonded FBG and by an embedded FBG changed sensitively to the debonding progress. Also, the damage index could evaluate the length of the debonding between the skin and the stringer.

It is important to conduct early age strength monitoring of concrete structures because it will help speed up construction. Piezoelectric-based strength monitoring method provides an innovative experimental approach to conduct concrete strength monitoring at early ages. In the presented paper, piezoelectric transducers were embedded into the concrete specimen during casting. The development of strength of concrete structures was monitored by observing the development of harmonic amplitude from the embedded piezoelectric sensor at early ages. From the experimental results, the amplitude of the harmonic response decreased with the increment of the concrete strength. The amplitude of the harmonic response from piezoelectric sensor dropped rapidly for the first week and continued to drop slowly as hydration proceeded which matches the development of the concrete strength at early ages. Concrete is heterogeneous and anisotropic which makes it difficult to be mathematically analyzed. Fuzzy logic has the advantage to conduct analysis without requiring a mathematical model. In this paper, a fuzzy logic system was trained to correlate the harmonic amplitude with the concrete strength based on the experimental data. The concrete strength estimated by the trained fuzzy correlation system matches the experimental strength data. The proposed piezoelectric-based monitoring method has the potential to be applied to strength monitoring of concrete structures at early ages.

Micro machined micro sensors for gas or flow detection based on physical behaviour of a special layer of a membrane have to fulfil high quality and reliability requirements especially in safety or security applications. Up to now, most of the research studies neglected mechanical issues related to reliability of these structures. In this sense, the study and characterization of the stress distribution on the membranes after fabrication and during their operative life is required. Thin films used in micromachined structures exhibit residual mechanical stress strongly dependent on the layer composition and the deposition process parameters. Often, a deposition of a multilayer is required, and this adds factors like abrupt transitions in thermal, elastic and plastic mismatch across the interfaces that have a direct effect on the resultant stress. Moreover, in operating conditions, a thermal stress originated due to the difference in the thermal expansion coefficient (CTE) of the membrane materials adds to the residual stress of the membrane. The resultant stresses can induce excessive deformation, fracture, delamination and microstructural changes in the material that can lead to the breaking of the structure during the fabrication stage or affect the behaviour of the final device. In the present work, the 2D deformation of a gas and a flow sensor membrane under different thermo-mechanical load states will be analysed by means of the digital image correlation (DIC) techniques based on scanning probe microscopy (SPM) data. With this technique which is introduced as the nanoDAC method (nano Deformation Analysis by Correlation) deformation fields can be determined with nanometer-accuracy. In addition ion milling by focused ion beam (FIB) technique is demonstrated at membrane specimens with residual stresses. Object deformations fields nearby the milling area are measured by fibDAC allowing the evaluation of very local residual stresses. Some principal experiments illustrate the feasibility of the chosen approach.

For aerospace applications, the successful transition and use of integrated structural health monitoring systems will require durable sensors that can perform in their intended environment for many years. For legacy aircraft the primary means of implementing a sensor system will be through surface mounting or bonding of the sensors to the structure. Previous work has shown that the performance of surface-bonded piezo sensors can degrade due to environmental effects such as vibrations, temperature fluctuations, and substrate flexure motions. This performance degradation included sensor cracking, disbonding, and general loss of efficiency over time. In this activity, the bond and piezo material characteristics of a typical surface-bonded piezo sensor system were studied to understand and improve the long-term durability and survivability of the sensor system. Analytic and computational models were developed and used to understand stress-strain relationships for the bonded sensor system, with a special emphasis being place on coefficient of thermal expansion issues. Accelerated environmental testing was accomplished for simple bonded piezo sensor systems, where a displacement-field imaging technique was used to understand the piezo sensor performance. Future activities will focus on identifying the optimal bond conditions and piezo material type, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.

Monitoring the continued health of aircraft subsystems and identifying problems before they affect airworthiness has been a long-term goal of the aviation industry. Because in-service conditions and failure modes experienced by structures are generally complex and unknown, conservative calendar-based or usage-based scheduled maintenance practices are overly time-consuming, labor-intensive and expensive. Metal structures such as helicopters and other transportation systems are likely to develop fatigue cracks under cyclic loads and corrosive service environments. Early detection of cracks is a key element to prevent catastrophic failure and prolong structural life. Furthermore, as structures age, maintenance service frequency and costs increase while performance and availability decrease. Current non-destructive inspection (NDI) techniques that can potentially be used for this purpose typically involve complex, time-intensive procedures, which are labor-intensive and expensive. Most techniques require access to the damaged area on at least one side, and sometimes on both sides. This can be very difficult for monitoring of certain inaccessible regions. In those cases, inspection may require removal of access panels or even structural disassembly. Once access has been obtained, automated inspection techniques likely will not be practical due to the bulk of the required equipment. Results obtained from these techniques may also be sensitive to the sweep speed, tool orientation, and downward pressure. This can be especially problematic for hand-held inspection tools where none of these parameters is mechanically controlled. As a result, data can vary drastically from one inspection to the next, from one technician to the next, and even from one sweep to the next. Structural health monitoring (SHM) offers the promise of a paradigm shift from schedule-driven maintenance to condition-based maintenance (CBM) of assets. Sensors embedded permanently in aircraft safety critical structures that can monitor damage can provide for improved reliability and streamlining of aircraft maintenance. Early detection of damage such as fatigue crack initiation can improve personnel safety and prolong service life. This paper presents the testing of an acousto-ultrasonic piezoelectric sensor based structural health monitoring system for real-time monitoring of fatigue cracks and disbonds in bonded repairs. The system utilizes a network of distributed miniature piezoelectric sensors/actuators embedded on a thin dielectric carrier film, to query, monitor and evaluate the condition of a structure. The sensor layers are extremely flexible and can be integrated with any type of metal or composite structure. Diagnostic signals obtained from a structure during structural monitoring are processed by a portable diagnostic unit. With appropriate diagnostic software, the signals can be analyzed to ascertain the integrity of the structure being monitored. Details on the system, its integration and examples of detection of fatigue crack and disbond growth and quantification for bonded repairs will be presented here.

This paper describes some experimental results on a fundamental phenomenon of the single pulse discharge. The single pulse discharge phenomenon was measured by using the optical fiber sensor, piezoelectric sensor and current transducer. In order to investigate the generation factor and propagation characteristic of acoustic emission (AE) by single pulse discharge in oil, the voltage across the gap, the discharge current and the displacement velocity were measured. Detected the signals using the optical fiber sensor and piezoelectric sensor have been compared with each other. As the results, optical fiber sensor was more resist than piezoelectric sensor in an electromagnetic noise. It is likely that the optical fiber sensor was more useful than piezoelectric sensor for AE detection with electromagnetic field. In order to investigate the relationships between the single pulse discharge phenomena and the detected AE signals, numerical analysis by using finite element method (FEM) was adapted. The numerical analysis result using the FEM and the experiment result along with the single pulse discharge was compared. As the result, it was estimated that the generation factor of the AE wave was the 1μs duration force. The signal detected by the optical fiber sensor has a good agreement with the signal from the FEM one. It is found that optical fiber sensor can detect the displacement velocity with wide band. Moreover, it was considered that frequency response of the optical fiber sensor is approximately flat. The optical fiber sensor was effective in analysis of AE source.

Fiber Bragg gratings have been demonstrated as a versatile sensor for structural health monitoring. We present an efficient and cost effective multiplexing method for fiber Bragg grating and fiber Fabry-Perot sensors based on a broadband mode-locked fiber laser source and interferometric interrogation. The broadband, pulsed laser source permits time and wavelength division multiplexing to be employed to achieve very high sensor counts. Interferometric interrogation also permits high strain resolutions over large frequency ranges to be achieved. The proposed system has the capability to interrogate several hundred fiber Bragg gratings or fiber Fabry-Perot sensors on a single fiber, whilst achieving sub-microstrain resolution over bandwidths greater than 100 kHz. Strain resolutions of 30nε /Hz^1/2 and 2 nε/Hz^1/2 are demonstrated with the fiber Bragg grating and fiber Fabry-Perot sensor respectively. The fiber Fabry-Perot sensor provides an increase in the strain resolution over the fiber Bragg grating sensor of greater than a factor of 10. The fiber Bragg gratings are low reflectivity and could be fabricated during the fiber draw process providing a cost effective method for array fabrication. This system would find applications in several health monitoring applications where large sensor counts are necessary, in particular acoustic emission.

Optic fiber Bragg grating sensor is a new type of sensor, which plays an important role in structural healthy monitoring. When the optic fiber Bragg grating sensor is embedded in the structure or adhered to the structure, there is a strain transferring course between structure and sensor for the existence of interlayer. Important factors that affect the strain transferring are conducted on the base of existent theory in this paper. The influencing parameters are the length of the sensor, the thickness, Young's modulus and Poisson's ratio of interlayer. At the same time, different influencing factors on strain transfer rate are discussed in detail. Some useful conclusions are developed, which provide a theoretical basis for future researches and designs.