New high-sensitivity solid-state magnetoresistive (MR) sensor technologies offer significant advantages in nondestructive evaluation (NDE) systems. A key advantage of MR sensors is a flat frequency response extending from dc to hundreds of MHz, making them particularly attractive for low-frequency and multi-frequency eddy current detection for deep-flaw detection and depth profiling. MR sensors are mass produced by thin film processing techniques similar to integrated circuit manufacturing, dramatically reducing the cost per sensor. The fabrication process is compatible with silicon circuit technology, allowing integration of sensors with on-chip signal processing. MR sensors can easily be produced in dense arrays for rapid, single-pass scanning of large areas. The small size and low power consumption of these solid-state magnetic sensors enable the assembly of compact arrays of sensors on a variety of substrates as well as on-chip sensor arrays. Arrays have been fabricated with sensor spacing as small as 5 μm. This paper presents a review of the state of the art in MR sensors and applications in NDE. The physical principles, manufacturing process, and performance characteristics of the three main types of MR devices, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) are discussed. Their performance is compared to other magnetic sensor technologies for NDE applications. Finally, we provide a comprehensive review of the literature on NDE applications of MR sensors.

This paper describes the use of MWM eddy current array sensor networks and IDED dielectrometer array sensor networks as well as hybrid MWM-IDED sensor networks for monitoring of absolute electrical properties for the purposes of detecting and monitoring damage, usage and precursor states within an Adaptive Damage Tolerance (ADT) framework. We present specific results from MWM-Array fatigue monitoring demonstrations, temperature measurement and dynamic stress monitoring, along with IDED methods for age degradation monitoring. We also describe the use of such sensor networks as part of an ADT framework, as well as for generation of real damage standards (e.g., real cracks without starter notches), and for prognostics model validation.

A compact and high integrated radar sensor working in the millimeterwave range can be applied for monitoring the injection moulding processes with and without gas assistance. The millimeterwaves are introduced into the cavity by a hollow waveguide and by a window transparent to millimeterwaves. In the frame of injection moulding experiments the process has been recorded simultaneously optically and by millimeterwave radar. When the melted plastics or the gas bubble are passing over the position of the sensor window, the signal of the sensor is abruptly changing both with regard to amplitude and phase thus indicating if the process is running correctly or not. The experimental investigations have been complemented by simulation calculations.

MR sensors offer a significant increase in sensitivity for inspection of subsurface features and flaws in metallic aircraft structure. This is especially true for deeper inspections with depths between 10 and 20 mm below the accessible surface. I show C-scan images of deep flaws, with dimensions down to one-tenth of the thickness of the covering metal. Issues involved in developing and operating an imaging scanner using these sensors are discussed.

The economic efficiency and competitiveness of environment-friendly rail transportation depends on safety, availability and maintenance of single highly loaded structure components. Until now these components have been changed in fixed maintenance intervals irrespective of any usage related conditions. With the knowledge and evaluation of the component conditions, life cycle costs can be reduced by means of optimized maintenance and/or “fit for purpose” design. For example, rail-bound vehicle wheel sets are among the most highly stressed travelling gear components of the bogie. if such a component fails, a serious accident may occur. For this reason, a health monitoring system based on the interpretation of ultrasonic sound signatures has been developed. First, the ultrasonic waves generated by an artificial defect on the outer wheel tread of a railroad wheel towards an acoustic sensor, placed inside the hollow shaft of the railroad axis were simulated with a EFIT (Elastodynamic Finite Integration Technique). The results achieved proved that relevant signals can be found in a frequency range up to 300 kHz. Based on this a diagnostic unit was designed and built for application under rotation conditions, which consists of a piezo-electric sensor, primary electronics, an analog-to-digital converter, a digital signal processor, a trigger unit, and a telemetric transmitter. This diagnostic unit was integrated in the hollow shaft of a railroad wheel axis, a component of a special laboratory test rig. Algorithms which allow for the rotation-synchronized processing of acoustic signals were implemented into the rotating diagnostic unit. After successfully completing a campaign for this test rig, a second test was performed inside the wheel/railroad simulation test rig of the Deutsche Bahn AG under railroad-like conditions. The data generated inside the hollow shaft of the railroad wheel axis by the diagnostic unit were telemetrically transmitted to an industrial computer. The detection of artificial defects of different sizes is shown in correlation with theoretical assumptions.

This paper presents the research and development of an Acoustic Health Monitoring (AHM) system that uses Guided Lamb Wave (GLW) technology to determine the thickness of railroad tank car shells for identification of wall loss due to corrosion. In recent regulatory changes, the emphasis has shifted from the traditional hydrotest to more modern methods for assuring tank car integrity. The new generation of maintenance programs will rely heavily on nondestructive testing, and will use damage tolerance concepts and risk analysis to establish inspection frequencies and items to inspect. It is the responsibility of the owners to set up experience-based maintenance programs that are suitable for the working conditions of their own particular fleets. Development of an ideal AHM system for railroad cars would be an instrument that incorporates Acoustic Emission (AE) and GLW technology. The combination of active and passive acoustic technologies integrated into a single system would be a highly efficient means of determining the structural integrity of tank cars. The integration of the GLW technology will allow identification of corrosion wall loss in a zone between two sensors, rather than at a single point (traditional ultrasonic thickness measurements). Thus, a much larger area of the structure can be inspected for approximately the same inspection cost. With a suitable integration of this new technology into the overall inspection and corrosion management program, the fleet can be more efficiently maintained and the risk of accidental release through progressive corrosion damage can be significantly reduced.

A key question that needs to be addressed and answered with regard to successfully implementing Structural Health Monitoring technologies in Air Force systems involves the long-term operability, durability, and survivability of integrated sensor systems and their associated hardware. Whether a sensor system is fully integrated within a structural material, or surface-bonded to the structure, a number of environmental and system level influences will tend to degrade the sensor system’s performance and durability over time. In this effort, an initial sensor durability study was undertaken to better understand the performance and degradation of piezo wafer active sensor (PWAS) systems under adverse mechanical, temperature, and moisture conditions. A novel displacement-field imaging approach was utilized to understand the vibration characteristics of PWAS transducers placed in accelerated vibration, temperature-cycling, and moisture-cycling conditions. The results showed damage in the form of PWAS sensor cracking events, bond degradation and failure, as well as indications of performance variation and reduction due to the accelerated exposure levels. Future activities will focus on identifying critical durability and survivability issues through advanced sensor modeling and additional accelerated testing efforts, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.

A copolymer incorporating polyaniline was used as a sensing medium in the construction of a resistance based humidity sensor. Aniline monomer was polymerised in the presence of poly (butyl acrylate / vinyl acetate) and a copolymer containing polyaniline emeraldine salt was obtained. The sensing medium was then developed by redissolving 1-2 w/w% of the resulting polymer residue in dichloromethane to produce a processable polymer blend solution. Some of this polymer residue was also de-doped in a solution of ammonia, and then washed with distilled water until the waste water had a neutral pH. This residue was then redissolved at 1-2 w/w% in dichloromethane to produce a second processable polymer blend this time containing polyaniline emeraldine base. The final sensor design utilised 125μm polyester insulated platinum wire as conducting electrodes that were dip coated in the emeraldine salt copolymer solution and allowed to dry in a desiccator. The sensor was then dip-coated in a protective barrier layer of the emeraldine base copolymer to prevent over-oxidation and/or de-protonation of the emeraldine salt sensing medium under this coating. The sensors had an overall final thickness of less than 150μm and showed high sensitivity to humidity, low resistance, and good reversibility without hysteresis. Sensors were monitored for 2-probe resistance changes when in contact with water. Calibration curves for each sensor were produced to convert the resistance reading to mass uptake of water. Individual sensors were embedded within Aluminium 5083 / Araldite 2015 adhesive joints to monitor mass uptake of water when exposed to marine environments. Correlations between mass uptake of water and joint strength were made. There are various advantages of such a sensor design. Polymer based thin film humidity sensors have the advantage that the high processability of the material allows for simple fabrication of a range of geometries including smaller sensor designs. The ease of processing gives a low cost sensor, whilst the small size and good mechanical properties gives a robust sensor which has the flexibility to be able to be used in applications where dynamic stresses and strains are encountered. Such sensors may find uses in a number of areas including electronic textiles, food/ electronics packaging and corrosion detection.

Luminescent coatings applications have been increased dramatically over the last decade as imaging capacities have advanced. These coatings have been used to monitor surface temperature and air pressure (oxygen sensing) in testing facilities around the world. Through the commercial suppliers of these coatings, custom assembled hardware systems and especially data reduction and analysis software, the use of smart luminescent coatings are starting to find their way in to inspection monitoring and nondestructive evaluation testing. The use of a temperature sensitive paint for example, can be a potential replacement for infrared imaging where IR techniques are limited due to access, reflections and complex geometries. Detection of the luminescent signal can use simple intensity ratio methods with synchronized pulsing systems to capture frequency responses in imaging applications. Time or frequency methods allow signals to be detected in the presence of high background noise that allow measurements that were previously unobtainable. This paper describes general luminescent sensors, detection methods and examples of coatings that are applied over test examples or embedded in materials to measure or monitor the health of a specimen.

For the last decade a tremendous development in the field of imaging radiation detectors has taken place. Conventional X-ray film has been replaced by digital X-ray imaging systems in a number of ways. Such systems mainly consist of silicon charge coupled devices (CCDs) where incident photons create electron-hole pairs in the thin silicon absorption layer near the surface. In contrast to visible light, which is absorbed within a 2 μm layer of silicon, the penetration of X-ray is much deeper due to higher photon energy. This disadvantage is often circumvented by the use of a scintillator absorption layer. Due to scattering of the low energy fluorescence photons, resolution and contrast of the X-ray images decrease. In order to eliminate these disadvantages, hybrid detectors consisting of direct converting semiconductors and readout electronics parts are fabricated. For this configuration, it is advantageous that both parts can be optimized separately and different materials can be used. Because of the well developed technology, the readout chip is fabricated out of silicon. As absorbing material, silicon is less suitable. In a silicon substrate of 500 μm thickness, only 15% of a 30 keV radiation is absorbed and converted into charges. In order to increase the absorption, materials with a higher atomic mass have to be used. Several compound semiconductors can be used for this purpose. One of them is GaAs, which is available as high quality semiinsulating wafer material. For detector optimization, GaAs wafers from several manufacturers with different properties were investigated. Test structures with Schottky and PIN diodes were fabricated. The I/V curves of the diodes, the spectral response from 5 up to 150 keV, the carrier concentration, and the carrier mobility were measured and compared. A survey of the results and the criteria for material selection resulting from these measurements will be provided in the paper.

Critical components of propulsion systems frequently operate at high stress levels for long periods of time. The integrity of these parts must be proven by non-destructive evaluation (NDE) during various manufacturing steps and also during systematic overhaul inspections. Conventional NDE methods, however, have unacceptable limits. Some of these techniques are time-consuming and inconvenient for service aircraft testing. Impedance-based structural-health-monitoring (SHM) uses piezoelectric (PZT) patches that are bonded onto or embedded in a structure; each individual patch both actuates the surrounding structural area and senses the resulting structural response. The size of the excited area varies with the geometry and material composition of the structure. A series of experiments on simple geometry specimens (thin-gage aluminum square plates) was conducted for assessing the potential of E/M impedance method for structural damage detection. Based on the results of this preliminary study, further testing was conducted on a subscale disk specimen. Based on the results it can be concluded that the E/M impedance method has the potential to be used for damage detection of structures. The experimental method, signal processing, and damage detection algorithm should be tuned to the specific method used for structural interrogation.

Evaluating the elastic properties of rubber is important for improving tire performance. Here, new ultrasonic techniques and results are reported for both soft rubber and real tire materials. First, on soft rubber, immersion C-Scan images revealed high attenuation and non-uniform grain size distribution. Through the application of new broadband, high power, high resolution transducers, air-coupled ultrasound succeeded in traveling through the soft rubber showing the efficiency of the new air coupled technique for imaging and evaluation of rubber materials. Secondly, sections of three tires were tested: (1) new, (2) 3 year-20,000 miles, (3) a 5 year-40,000 miles. Each tire section consists of three layers made of different rubber materials, separated by wire mesh. Because of the complexity of the tire’s structure and its high attenuation, evaluation of all three layers, but especially the middle layer, is difficult. High power tone-bursts at 1 MHz were applied to a high impedance immersion transducer. Layer reflections could be separated such that the middle layer and the wire belts at the interfaces could be interrogated. This report will detail our new techniques and provide examples for the results obtained.

Acoustography is a full-field ultrasonic imaging process where a high resolution 2D acousto-optic sensor based on liquid crystal technology is employed to directly convert the ultrasound into a visual image in near real time. Unprocessed acoustography images typically suffer from non-uniformity due to spatial variations in the optical brightness response of the acousto-optic sensor field to ultrasonic intensity. Additionally, dynamic range of the acousto-optic sensor is limited to approximately 20 to 30 db. The nonuniformity and dynamic range limitation can result in difficulty in acoustography image interpretation, impracticality for large field application, and difficulty for use on samples having a wide range of attenuation. The approach of this ongoing study is to apply various methodologies that address these limitations in hopes of extending the usefulness and applications of acoustoography for nondestructive testing. This article shows initial results of methodologies developed to correct for image non-uniformity and explains the proposed approach to extend the dynamic range of acoustography images.

As per the recent advances in remote in situ monitoring of industrial equipment using long wire waveguides (~10m), novel applications of existing wave generation techniques and new acoustic modeling software have been used to advance waveguide technology. The amount of attainable information from an acoustic signal in such a system is limited by transmission through the waveguide along with frequency content of the generated waves. Magnetostrictive, and Electromagnetic generation techniques were investigated in order to maximize acoustic transmission along the waveguide and broaden the range of usable frequencies. Commercial EMAT, Magnetostrictive and piezoelectric disc transducers (through the innovative use of an acoustic horn) were utilized to generate waves in the wire waveguide. Insertion loss, frequency bandwidth and frequency range were examined for each technique. Electromagnetic techniques are shown to allow for higher frequency wave generation. This increases accessibility of dispersion curves providing further versatility in the selection of guided wave modes, thus increasing the sensitivity to physical characteristics of the specimen. Both electromagnetic and magnetostrictive transducers require the use of a ferromagnetic waveguide, typically coupled to a steel wire when considering long transmission lines (>2m). The interface between these wires introduces an acoustic transmission loss. Coupling designs were examined with acoustic finite element software (Coupled-Acoustic Piezoelectric Analysis). Simulations along with experimental results aided in the design of a novel joint which minimizes transmission loss. These advances result in the increased capability of remote sensing using wire waveguides.

This paper presents the latest results obtained from Acoustic Emission (AE) monitoring and detection of cracks and/or damage in diaphragm couplings, which are used in some aircraft and engine drive systems. Early detection of mechanical failure in aircraft drive train components is a key safety and economical issue with both military and civil sectors of aviation. One of these components is the diaphragm-type coupling, which has been evaluated as the ideal drive coupling for many application requirements such as high speed, high torque, and non-lubrication. Its flexible axial and angular displacement capabilities have made it indispensable for aircraft drive systems. However, diaphragm-type couplings may develop cracks during their operation. The ability to monitor, detect, identify, and isolate coupling cracks on an operational aircraft system is required in order to provide sufficient advance warning to preclude catastrophic failure. It is known that metallic structures generate characteristic Acoustic Emission (AE) during crack growth/propagation cycles. This phenomenon makes AE very attractive among various monitoring techniques for fault detection in diaphragm-type couplings. However, commercially available systems capable of automatic discrimination between signals from crack growth and normal mechanical noise are not readily available. Positive classification of signals requires experienced personnel and post-test data analysis, which tend to be a time-consuming, laborious, and expensive process. With further development of automated classifiers, AE can become a fully autonomous fault detection technique requiring no human intervention after implementation. AE has the potential to be fully integrated with automated query and response mechanisms for system/process monitoring and control.

Safety and availability of ageing infrastructures require periodic or continuous monitoring of the structure’s integrity. Innovative design criteria for new infrastructure components may allow material and energy conservation if components are continuously monitored by using advanced sensor systems. This concept for recurring Structural Health Monitoring will replace a significant part of conventional NDE by new maintenance concepts. The goal consists in sensor networks based on advanced principles of testing technology with integrated signal/data processing and data communication. NDE modeling is required for the quantification of measurement results. Finally, a decision on the integrity of the structure based on sensor results requires detailed knowledge about material behavior and modeling capacity for materials and components. IZFP has developed sensor concepts for complex solutions applicable to Structural Health Monitoring for different applications. These applications include railroad inspection, aircraft inspection, inspection of wind energy systems, power electric switches and micro gas valves. Basic concepts and applications of sensor networks will be presented.

Acoustic monitoring of technological processes requires methods that eliminate noise as much as possible. Sensor-near signal evaluation can contribute substantially. Frequently, a further necessity exists to integrate the measuring technique in the monitored structure. The solution described contains components for analog preprocessing of acoustic signals, their digitization, algorithms for data reduction, and digital communication. The core component is a digital signal processor (DSP). Digital signal processors perform the algorithms necessary for filtering, down sampling, FFT computation and correlation of spectral components particularly effective. A compact, sensor-near signal processing structure was realized. It meets the Match-X standard, which as specified by the German Association for Mechanical and Plant Engineering (VDMA) for development of micro-technical modules, which can be combined to applicaiton specific systems. The solution is based on AL₂O₃ ceramic components including different signal processing modules as ADC, as well as memory and power supply. An arbitrary waveform generator has been developed and combined with a power amplifier for piezoelectric transducers in a special module. A further module interfaces to these transducers. It contains a multi-channel preamplifier, some high-pass filters for analog signal processing and an ADC-driver. A Bluetooth communication chip for wireless data transmission and a DiscOnChip module are under construction. As a first application, the combustion behavior of safety-relevant contacts is monitored. A special waveform up to 5MHz is produced and sent to the monitored object. The resulting signal form is evaluated with special algorithms, which extract significant parameters of the signal, and transmitted via CAN-bus.

With ongoing miniaturization from micro electronic mechanical systems (MEMS) towards nano electronic mechanical systems (NEMS), there is a need for new reliability concepts making use of meso-type (micro to nano) or fully nanomechanical approaches. For the development of theoretical descriptions and their numerical implementation on the basis of simulation tools experimental verification will be of major interest. Therefore, there is a need for measurement techniques with capabilities of determination and evaluation of strain fields with very local (nanoscale)resolution. Following this challenge the authors developed the nanoDAC method (nano Deformation Analysis by Correlation) which enables the extraction of nanoscale displacement fields from scanning probe microscopy (SPM) images. Components of interest are thermomechanically loaded under the SPM and topography scans of the critical areas are taken at specific load states. The obtained images are analyzed by digital image correlation resulting into full-field displacement and strain fields. Due to the application of SPM equipment deformations in the micro-, nanometer range can be easily detected. The method can be performed on bulk materials, thin films and on devices i.e microelectronic components, sensors or MEMS/NEMS. Furthermore, the characterization and evaluation of micro- and nanocracks or defects in bulk materials, thin layers and at material interfaces can be carried out. In combination with finite element simulations the application of the described experimental method to sensor elements is a promising approach for reliability analysis of newly designed sensor architectures.

Global Positioning System (GPS) is an emerging tool for measuring and monitoring both static and dynamic displacement responses of long span cable-supported bridges under gust winds. Since vertical vibration of a long span cable-supported bridge is more significant than its lateral vibration, this paper focuses on the assessment of dynamic displacement measurement accuracy of GPS in vertical direction. In the first phase of work, the accuracy of GPS in measuring vertical sinusoidal displacement motions is examined by using a motion simulation table. The comparison of the table motion recorded by the GPS with the original motion generated by the table shows that the GPS can measure accurately sinusoidal dynamic displacements within certain frequency and amplitude ranges. In the second phase of work, the capability of the GPS in tracking the measured wind-induced bridge deck motion of Tsing Ma Bridge during Typhoon Victor is examined. To reduce multi-path effects on GPS measurements, an adaptive filter based on the recursive least-squares (RLS) algorithm is used to enhance the measurement accuracy of GPS. The comparative results demonstrate that the GPS can trace wind-induced dynamic response of long span cable-supported bridges satisfactorily.

We present an alternative to visual inspection for detecting damage to civil infrastructure. We describe a real-time decision support system for nondestructive health monitoring. The system is instrumented by an integrated network of wireless sensors mounted on civil infrastructures such as bridges, highways, and commercial and industrial facilities. To address scalability and power consumption issues related to sensor networks, we propose a three-tier system that uses wavelets to adaptively reduce the streaming data spatially and temporally. At the sensor level, measurement data is temporally compressed before being sent upstream to intermediate communication nodes. There, correlated data from multiple sensors is combined and sent to the operation center for further reduction and interpretation. At each level, the compression ratio can be adaptively changed via wavelets. This multi-resolution approach is useful in optimizing total resources in the system. At the operation center, Support Vector Machines (SVMs) are used to detect the location of potential damage from the reduced data. We demonstrate that the SVM is a robust classifier in the presence of noise and that wavelet-based compression gracefully degrades its classification accuracy. We validate the effectiveness of our approach using a finite element model of the Humboldt Bay Bridge. We envision that our approach will prove novel and useful in the design of scalable nondestructive health monitoring systems.

This paper describes the design, fabrication and testing of tunable Fabry-Perot filters. The goal of this research is to develop novel tunable filter with an area of 5x5 mm² that will be used in infrared gas sensors. This exploits the fact that most gases have unique infrared absorption signatures in the 2-14 µm wavelength region. The filter consists of two thin silicon wafers coated with quarter wave dielectric layers to form wavelength dependent high reflection mirrors and separated by air gaps with an average height of 8, 5.1 and 3.5 μm. The mirrors are supported by four elastic polymer posts (springs) each with an area of 100×100 μm² made by using photo definable polydimethylsilxane (PDMS). An electrostatic voltage is used to compress the springs, change the airgap height and hence shift the transmission peaks to a shorter wavelength. A finesse of 12 with full width at half maximum (FWHM) of 70 nm, and a peak transmission of 63% were achieved by applying 100 volts on a device with 8 µm post height and wafer thickness of 125 µm. In addition, the measured tunability before and after hard baking of the device was 210 nm and 130 nm respectively. The tunability stayed constant after hard baking the devices and did not show any changes with time. The tunability was also measured on a thinner silicon mirror with 3.5 µm post height. In this case, the filter was tuned 180 nm by applying 10 volts. However, the filter finesse was 3, transmission peak was 40% and FWHM over 200 nm. An antireflection coating was deposited on one side of silicon wafers and a Fabry-Perot filter to study transmission enhancement and satisfactory results were achieved.

An optical-fiber-based gas sensor for CH₄ gas real-time monitoring has been developed and tested. A long-distance silica fiber link with compact single-path or multi-path gas cells has been employed in conjunction with a wavelength-tunable. InGaAsP DFB laser diode at 1.64μm (at the R(6) absorption peak of methane) to achieve highly sensitive remote interrogation of CH₄ with potential application in the mining complexes and residential areas. By wavelength modulation with the DFB laser diode, multi-path gas cell and a self-design processing circuit, sensitivities of less than 1% of lower explosive limit (LEL) had been achieved in the laboratory.

Optical fiber sensors have received increasing attention in the fields of aeronautic and civil engineering for their superior ability of explosion proof, immunity to electromagnetic interference and high accuracy, especially fitting for measurement applications in harsh environment. In this paper, a novel FBG (fiber Bragg grating) strain sensor, which was packaged in a 1.2mm stainless steel tube by epoxy resin, was developed. Experiments were conducted on the universal material testing machine to calibrate its strain transferring characteristics. The sensor has the advantages of small size, high precision and flexible use, and demonstrates promising potentials. Ten of tube-packaged strain FBG sensors were applied in the vibration experiment of submarine pipeline model. The strain measured by FBG sensor agrees well with the electric resistance strain sensor.