This PDF file contains the front matter associated with SPIE Proceedings Volume 8694, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Based on the micromechanical approaches, high frequency elastic waves are generated in carbon fiber/polymer composites due to three main damage sources as fiber breakage, matrix crack or delamination. The occurrence of damage mode depends on the ratios of energy release rates. Simultaneous generation of damage modes such as multiple fiber breakage called as fiber fragmentation influences the characteristics of propagating elastic waves which are used for detecting, locating and understanding damage modes for acoustic emission method. Understanding the wave characteristics via experimental methods is difficult as the control of damage mode sequence is a challenge. In this paper, wave propagations due to single or multiple fiber breakages positioned at various locations along the laminate and through thickness in different composite lay-ups are studied using dynamic finite element models. Fiber breakage is defined as a point load with the rise time of 1 μsec. The load amplitude is identified using stress-strain curve of carbon fiber. The amplitude of matrix cracking is defined as crack opening displacement as a boundary load to the finite element model. The positions and amplitudes of damage modes are varied to understand the characteristics of waveform signatures. Each lamina is defined as a different plate and orthotropic material properties are entered as input to the model. The paper shows that multi-mode simultaneous damage in composites causes complex waveform generation, which makes pattern recognition based on amplitude and frequency real time a challenging task.

The use of Lamb waves in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) is gaining popularity due to their ability to travel long distances without significant attenuation, therefore offering large area inspections with a small number of sensors. The design of a Lamb-wave-based NDE/SHM system for composite materials is more complicated than for metallic materials due to the directional dependence of Lamb wave propagation characteristics such as dispersion and group velocity. Propagation parameters can be theoretically predicted from known material properties, specifically the stiffness matrix and density. However, in practice it is difficult to obtain the stiffness matrix of a particular material or structure with high accuracy, hence introducing errors in theoretical predictions and inaccuracies in the resulting propagation parameters. Measured Lamb wave phase velocities can be used to infer the stiffness matrix, but the measurements are limited to the principal directions due to the steering effect (different propagation directions of phase and corresponding group velocities). This paper proposes determination of the stiffness matrix from the measured group velocities, which can be unambiguously measured in any direction. A highly anisotropic carbon-fibre-reinforced polymer plate is chosen for the study. The influence of different stiffness matrix elements on the directional group velocity profile is investigated. Thermodynamic Simulated Annealing (TSA) is used as a tool for inverse, multi variable inference of the stiffness matrix. A good estimation is achieved for particular matrix elements.

An experimental and numerical investigation was carried out for the feasibility of determining damage development in carbon/epoxy composite laminates with a circular hole. The study involved flash thermography to map the damaged region in these specimens. First, thermographic response of implanted delaminations in composite laminates was examined using both numerical and experimental approaches. Next, damage development around a circular hole in a composite specimen was examined using thermography. The specimen was cyclically loaded until fracture to generate different levels of damage near the center of the specimen. Thermographic images of damage development at different fractions of fatigue life were recorded. Numerical models of thermal response of the region with evolving damage were also examined to assess the effectiveness of flash thermography under different conditions. The carbon/epoxy composite laminate was pulsed and examined using laser vibrometry in order to characterize the modes of stress wave propagation. Numerical simulations of a similarly excited carbon/epoxy panel were carried out as well.

Residual stress can affect the performance of thin-film micromachined structures and lead to curling in cantilevers as well as distortion in the frequency of resonant devices. As the origin of residual stress is dependent on the fabrication processes, a nondestructive method for characterization of residual stress independent of processes conditions is desirable for supporting the design of microcantilever-based microsystems. In this paper we present a nondestructive characterization of the residual stress within composite microcantilever beams providing valuable insights toward predicting their deflection profile after mechanical releasing from the substrate. The approach relies on the assumption of a linear gradient stress and a quadratic deflection profile across a composite microcantilever.

It is very difficult to detect chloride-induced corrosion early, because it is not possible to confirm the degradation of concrete until cracks induced by corrosion expansion of re-bars appear on the concrete surface. Thus, development of truly non-destructive tests to estimate the chloride content in concrete could make it possible to study the changes in chloride concentration over time, without having to physically approach the structure, or cause any damage to it. Such methods would greatly improve our ability to foresee the possibility of reinforcement corrosion at early stages, and enable us to take the required corrective action at an appropriate time. In this report, in order to evaluate the applicability of non-destructive method which is possible to estimate the chloride content included in cover concrete, the results of theoretical and experimental attempts to estimate the chloride content in cover concrete of the RC structure using the electromagnetic waves are reported.

The depassivation and corrosion of bonded prestressing steel strands in concrete bridge members may lead to major damage or collapse before visual inspections uncover evident signs of damage, and well before the end of the design life. Recognizing corrosion in its early stage is desirable to plan and prioritize remediation strategies. The Acoustic Emission (AE) technique is a rational means to develop structural health monitoring and prognosis systems for the early detection and location of corrosion in concrete. Compelling features are the sensitivity to events related to micro- and macrodamage, non-intrusiveness, and suitability for remote and wireless applications. There is little understanding of the correlation between AE and the morphology and extent of early damage on the steel surface. In this paper, the evidence collected from prestressed concrete (PC) specimens that are exposed to salt water is discussed vis-à-vis AE data from continuous monitoring. The specimens consist of PC strips that are subjected to wet/dry salt water cycles, representing portions of bridge piles that are exposed to tidal action. Evidence collected from the specimens includes: (a) values of half-cell potential and linear polarization resistance to recognize active corrosion in its early stage; and (b) scanning electron microscopy micrographs of steel areas from two specimens that were decommissioned once the electrochemical measurements indicated a high probability of active corrosion. These results are used to evaluate the AE activity resulting from early corrosion.

Pavement condition surveys are typically done manually by inspection teams. They can be expensive and can take several months to complete. This paper considers a vehicle-mounted pavement inspection system that aims to be more affordable and faster than manual surveys. The system is comprised of a vehicle outfitted with a multi-modal sensor array that includes accelerometers, microphones, radar, tire pressure, and video. In a first step, each channel is processed individually to extract parameters related to friction, roughness, and distress. The second step applies data fusion concepts to combine all sensor data into a single pavement condition value. This paper elaborates on the system and describes a real-world field test. The primary contribution is a proof-of-concept for the system by comparing field test results to those from a conventional manual survey.

Classifications of road conditions are crucial because officials prioritize road maintenance decisions based on them. Pavement condition index (PCI) surveys are performed manually and used by many cities in the U.S. to evaluate road surface conditions. In this research, a more efficient method is used to detect road surface conditions. This method applies a probabilistic analysis to acoustic pressure data collected from a vehicle-mounted microphone. The data is collected while the driving and processed in real time. Acoustic pressure data contains information on road surface conditions because acoustic pressures change when the tire impacts different road surfaces. This change is audible to human ears, for example, a driver transitions from a normal road to a bridge. The acoustic pressure data used in this research was collected from roads with known PCI values that are used as a reference. To reveal the dominant common features and neglect trivial differences within a certain length of road, a probabilistic method is used to evaluate road surface conditions. This approach uses the Weibull probability density function (PDF) to evaluate road surface conditions. This distribution was chosen because it is closest to the actual PDF among other distributions such as the normal distribution and lognormal distribution. A key finding of this paper is that the Weibull PDF shows the largest change between roads with different PCI values. Another finding is that the Weibull pdf changes when the van hits road defects such as cracks and patches.

Damages in composite components of wind turbine blades and large-scale structures can lead to increase in maintenance and repair costs, inoperability, and structural failure. The vast majority of condition assessment of composite structures is conducted by visual inspection and non-destructive evaluation (NDE) techniques. NDE techniques are temporally limited, and may be further impeded by the anisotropy of the composite materials, conductivity of the fibers, and the insulating properties of the matrix. In previous work, the authors have proposed a novel soft elastomeric capacitor (SEC) sensor for monitoring of large surfaces, applicable to composite materials. This soft capacitor is fabricated using a highly sensitive elastomer sandwiched between electrodes. It transduces strain into changes in capacitance. Here, we present a fabrication method for fabricating the SEC. Different surface treatment techniques for the nanoparticles are investigated and the effects on the mechanical and the electrical properties of the produced film are studied. Results show that using melt mixing fabrication method was successful at dispersing the nanoparticles without using any surface treatment, including coating the particles with PDMS oil or the use of Si-69 coupling agent. Yet, treating the surface would result in increasing the stiffness of the matrix as well as improving the interaction between the filler particles and the matrix.

The offshore wind farm industry has substantially grown in recent years. There is a substantial number of wind farms in the waters offshore Europe with several others proposed for regions in North America and Asia. A vast majority of the existing offshore wind turbines are supported on monopile foundations. The presence of a monopile in a flowing body of water causes the water to accelerate around the monopile, which can result in seafloor scour. Scour can significantly reduce the soil support thereby reducing the monopile stability and strength. Scour can also reduce the fundamental natural frequency of the turbine structure. If the natural frequency of the support structure approaches the turbine rotor frequency, a resonant condition will develop that can damage the turbine or its support structure. To avoid this situation, a monitoring system consisting of two strategically located bidirectional accelerometers was installed on the towers of four turbines of an existing offshore wind farm. The objective of the monitoring campaign was to correlate frequency measurements with the overall structural performance of the monopile foundations. The developed approach was based on operational modal analysis and system identification techniques using acceleration response data and calibrated computer models. Identified frequencies were correlated to specific structural performance limit states including; vibrating resonance of the tower with the turbine rotor, lack of foundation stability, and yielding of the monopile foundation material. This paper presents the findings of this work, which can assist owners and operators who may benefit from similar structural health monitoring systems.

We have applied dicyclopentadiene (DCPD) resin for reinforcing pothole patch materials due to its unique properties – low cost, low viscosity at beginning and ultra-toughness after curing, chemical compatibility with tar, tunable curing profile through catalyst design. In this paper, we have designed a two layer structure – well compacted base layer and DCPD reinforced 1-1.5" top layer – for pothole repair. By choosing two graded asphalt mixes, a porous top layer and fully compacted base layer was prepared after compaction and ready for DCPD resin infiltration. The DCPD curing and infiltration profile within this porous top layer was measured with thermocouples. The rutting resistance was tested with home-made wheel rutter. The cage effect due to the p-DCPD wrapping was characterized with wheel penetration test. The results showed that this two layer structure pothole repair has greatly improved properties and can be used for pothole repair to increase the service life.

The nondestructive evaluation (NDE) inspection for building and bridge structures has attracted a lot of attentions for the fundamental research and the sensor system development. In this paper, development of a distant subsurface imaging radar is reported. Theoretical background of subsurface radar imaging is first provided. Experimental laboratory measurements using radar signals in the frequency range of 1-18 GHz were conducted. From the theoretical analysis and the initial experimental results on a laboratory reinforced concrete specimens, it is proved that the proposed subsurface imaging radar system can detect the location of steel reinforcement inside a concrete cylinder and identify the material property of panel specimens. Range-dependent attenuation of radar signals is experimentally studied using different materials. Findings are reported in the summary.

Accurate detection of rebar location using Ground Penetrating Radar (GPR) proves to be very useful in assessing roadway/bridge deck concrete structure. Due to large signal processing resource needed to locate and image rebar on a whole road structure, more efficient signal processing method is needed. This paper proposes to combine two-dimensional (2D) entropy algorithm to narrow down the scope of interested data from large radar data set and Short Time Fourier Transform (STFT) algorithm to perform further feature characterization. To validate the analysis, experiments with different rebar setups are conducted.

Ground penetrating radar (GPR) has developed into a primary investigation tool for the non-destructive evaluation of reinforced concrete bridge deck. The collected GPR signals are generally composed of direct wave, rebar reflections, bottom reflection and noise. Before making GPR-based predictions, rebar reflections need to be extracted first. One obstacle is that the first rebar layer is in the shallow subsurface in some cases and the target signals may be overlapped with the direct waves. Another problem associated is that the testing surfaces may be rough in real field testing environments, which lead to strong uneven direct waves in the collected signals. This kind of direct wave cannot be removed by the commonly adopted average removing method. In this paper, Matching pursuits equipped with a customized dictionary are applied to deal with the above mentioned two problems. The dictionary is built based on the excitation function and the electromagnetic scattering mechanisms. Compared to the commonly used Gabor dictionary, the proposed dictionary is more adapted to the GPR wavefronts and each target signal can be reconstructed by just one atom. In addition, there is no need for a reference signal that is not always available. The performance of the proposed method is evaluated by simulation data in this paper.

Bismaleimide (BMI) composites are used in applications that require good mechanical properties at high temperatures. In this paper, a Non-destructive inspection technique for BMI composites which can be used at high temperatures is presented. Cavity based External Fabry-Perot Interferometer (EFPI) optical sensors have been developed and embedded in the laminates. These sensors are capable of operating in temperatures up to 800°C. The embedded sensors are used to perform real time cure monitoring of a BMI composite. The composite is cured using an out-of-autoclave (OOA) process. Once the composite is cured, the same sensors are used to measure mechanical performance of the laminate. The performance of the embedded sensor is investigated under tensile loading at room temperature as well as elevated temperatures.

We present a fiber Bragg grating (FBG) interrogation method using a micro-controller board and optical filter that achieves high strain sensitivity and high dynamic range. This interrogation method allows high sensitivity detection of ultrasonic waves superimposed on low-frequency (on the order of 100Hz) vibrations of arbitrary magnitude. One possible application is in-situ structural health monitoring of windmill blades exposed to strong winds by using FBG sensors for detection of ultrasonic waves. Interrogator operation is based on the edge filtering method using a broadband source, fiber Fabry-Perot filter and a micro-controller board which acts as a control feedback loop that locks the filter wavelength to the mid-reflection point on the FBG spectrum. Wavelength locking method allows high sensitivity for edge filtering of high-frequency waves, while the feedback signal is the measurement of low-frequency vibration with high dynamic range. The concept of the interrogator operation and different implementations are described and discussed with experimental results.

This paper is a summary of the preliminary experimental results regarding the use of optical fiber Bragg grating (OFBG) sensing equipment to monitor the local strain about the circumference of un-axially restricted, thick walled, unreinforced concrete vessels, developed under the moniker Simulated Carbon Ash Retention Cylinder (SCARC), subject to internal expansion pressure from ash carbon concrete (ACC). Internal pressure developed following the introduction of varying combinations of cement, fly-ash, aluminum powder, carbon-dioxide, and water to the voided region of the SCARC specimens. Seven specimens were created, and monitored, so that cracking patterns, material property variables, and OFBG strain irregularities could be investigated to begin formulating a crack location prediction and detection system.

This paper presents a nondestructive ultrasound test method for characterizing resonant frequencies of polydimethylsiloxane (PDMS) thin films by using a miniature fiber optic photoacoustic (PA) probe. The PA probe was fabricated with an optical fiber and the synthesized gold nanocomposite. During the experiment, a PDMS film with thickness of 25 μm was cured and immersed into water media within a designed holder to clamp the film. An acoustic pulse was generated from the PA probe, propagated in the water media and excited the clamped film. A fiber optic pressure sensor based on Fabry-Perot (FP) principle was applied to collect excited acoustic signals on the other side of the film. The sensed response of the acoustic pulse was used to compute the resonant frequencies of the PDMS thin film based on de-convolution method.

Recently, many advanced ultrasound applications require wide bandwidth and compact ultrasound generators to achieve better resolution as well as the capability of being operated in a compact space. Generating ultrasound signals through photoacoustic principle is a promising way to generate wide bandwidth ultrasound signals by the optical approach. Meanwhile, optical fibers are ideal candidates for applications where compact size is required. Therefore, fiber optic photoacoustic generators, which put advantages of the photoacoustic principle and optical fibers together, lead to novel ultrasound generation devices which can meet the most advanced ultrasound applications requirements. This paper firstly reports using the gold nanocomposite to achieve the fiber optic photoacoustic ultrasound generator. The gold nanocomposite was synthesized by directly mixing the gold salt in polydimethylsiloxane. The gold nanocomposite showed high optical energy absorption capability and the high coefficient of thermal expansion. The photoacoustic generation efficiency was increased by applying such material. The synthesis protocol of the gold nanocomposite was presented in this paper. The optical fiber was coated with the gold nanocomposite to generate ultrasound signals. Experimental results have demonstrated that ultrasound signals can be generated by this approach and the fiber optic ultrasound generator can be used in ultrasound applications.

A neutron and X-ray combined computed tomography system (NXCT) has been developed1 at the Missouri University of Science & Technology. It is believed that it will provide a superior method for non-destructive testing and evaluation. The system is housed within the Missouri University of Science & Technology Reactor (MSTR) and is the first such imaging platform and synthesis method to be developed. The system utilizes neutrons obtained directly from the reactor core and X-rays an X-ray generator. Characterization of the newly developed digital imaging system is imperative to the performance evaluation, as well as for describing the associated parameters. The preliminary evaluation of the NXCT system was performed in terms of image uniformity, linearity and spatial resolution. Additionally, the correlation between the applied beam intensity, the resulting image quality, and the system sensitivity was investigated. The combined neutron/X-ray digital imaging system was evaluated in terms of performance parameters and results are detailed. The MTF of the X-ray imaging module was calculated using the Edge method. The spatial frequency at 10% of the MTF was found to be 8 lp/mm, which is in agreement with the value of 8.5 lp/mm determined from the square wave response method. The highest detective quantum efficiency of the X-ray imaging module was found to be 0.13. Furthermore, the NPS spectrum for the neutron imaging module was also evaluated in a similar way as the X-ray imaging module. In order to improve the image quality of the neutron imaging module, a pin-hole mask phantom was used to correct the geometrical non-linearity of the delay line anode readout. The non-linearity correction of the delay line anode readout has been shown through the corrected images of perforated cadmium strip and electroformed phantom.

Portland Cement Concrete (PCC) and concrete containing Reclaimed Asphalt Pavement (RAP) as aggregate were evaluated by the Superpave Indirect Tensile (IDT) strength test in conjunction of X-ray Computed Tomography (CT). Each concrete specimen for Superpave IDT strength test was subject to several procedures of preparation: concrete slicing, air drying, surface cleaning, gage points mounting, and LVDTs aligning. The concrete specimen was administrated a specified load level for ten seconds and then unloaded. A pre-sitting load of ten to fifteen pounds was constantly applied in order to hold still the concrete specimen. The load level was repeated with approximately 10% increment of peak stress (eg., 10%, 20%, 30%...etc.), until when the fracture of concrete occurred. The X-ray CT scans were performed: before the load was initially applied and after the load was released. It was to identify subtle changes in concrete before and after the repeated load levels were administrated. An image-processing technique was developed to assess the air voids distribution for each load level. The maximum variation of air voids measured near the central area of the specimen was within ±0.3%. The volume of air voids in the concrete was observed to increase significantly when the concrete specimen was loaded to fracture. The significance of this study is a better understanding of microstructural property in terms of air voids by using X-ray CT to assess the microdamage in concrete before fracture.

The prediction of the remaining life of a structure can be assisted by the characterization of the current cracking mode. Usually tensile phenomena precede shear fracture. Due to the different movement of the crack sides according to the dominant mode, the emitted elastic energy possesses waveforms with different characteristics. These are captured by acoustic emission sensors and analyzed for their frequency content and waveform parameters. In this study fracture experiments on structural materials are conducted. The goal is to check the typical acoustic signals emitted by different modes as well as to estimate the effect of microstructure in the emitted wave as it propagates from the source to the receivers. The dominant fracture mode is controlled by modification of the setup and acoustic emission is monitored by two sensors at fixed locations. Signals belonging to tensile events acquire higher frequency and shorter duration than shear ones. The influence of heterogeneity is also obvious since waveforms of the same source event acquired at different distances exhibit shifted characteristics due to damping and scattering. The materials tested were cement mortar, as a material with microstructure, and granite as representative of more homogeneous materials. Results show that in most cases, AE leads to characterization of the dominant fracture mode using a simple analysis of few AE descriptors. This offers the potential for in-situ application provided that care is taken for the distortion of the signal, which increases with the propagation distance and can seriously mask the results in an actual case.

Composite materials are profuse emitters of acoustic emissions (AE) during the occurrence of damage. In some cases, the signals measured are due to growth of noncritical damage. To have a more accurate SHM technique, distinguishing between critical and non-critical damage is necessary. This work focuses on monitoring damage growth and relating AE signals of critical growth to the reduction in structural integrity. Specifically, AE signals were experimentally measured in carbon fiber reinforced polymers (CFRP) specimens. Once the specimens were prepared, they were placed under quasi-static and fatigue loading until failure. Attenuation of AE signals were studied and mode-I delamination tests were conducted as well.

Relative motion between surfaces of mechanical parts causes surface wear and damage. The degradation to the surfaces has been monitored using vibration characteristics as well as acoustic emissions generated during the relative surface movements. In particular, acoustic emission signals were found to be sensitive to some of the microscopic processes occurring at the frictional interface. In this study, friction between two surfaces was monitored experimentally under controlled conditions. Relative velocity, contact pressure, and surface roughness values were varied in the experiments. Friction related acoustic emission signals were recorded and analyzed to understand the relationship between the signals generated and the physical processes giving rise to these signals. Information related to the stick-slip movements during cyclic motion, in the experiments, was observed from the signals. Features of the waveforms were found to reveal the conditions existing at the friction interface. In particular, the changes in the surface roughness and contact pressure were readily observed from the acoustic emission signals.

Fiber-reinforced polymer (FRP) has become increasingly popular in the application of strengthening and retrofitting existing concrete elements, such as beams, columns, slabs, and bridge decks. In view of the maintenance and the safety issues against these retrofitted systems, development of a robust and reliable nondestructive testing (NDT) technique that provides an accurate and remote assessment of interfacial properties in the FRP-bonded concrete system is required. In this paper, a one dimensional predictive model of interfacial stiffness in the FRP-bonded concrete system based on an acoustic-laser technique is proposed. This model is constructed based on a traditional beam theory in which an infinitely long beam sits on top of a series of springs, which can be regarded as a soft foundation. It is noticed that the resonance frequency can indicate the interfacial stiffness when the size of detected defect is small enough. The result from this predictive model is compared with the existing literature on the interfacial stiffness in epoxy-silica system and a good agreement is observed.

Acoustic emission (AE) has been recognized for its unique capabilities as an NDT method. However, there is untapped potential for the practical application of AE to structural health monitoring and prognosis. As part of the development of a wireless sensor network for structural bridge health monitoring, this study aims to provide a framework for the estimation of fatigue damage and remaining life of steel bridge components through AE monitoring. Fourteen compact tension (CT) specimens and nine cruciform fillet welded joints were used in AE monitored fatigue tests to investigate the correlation of AE features with crack growth in base materials and weldments. The material (structural steel A572 Grade 50) and the welding procedures are representative of those used in actual bridge construction. Based on the balance between AE signal energy and the energy release due to crack growth, deterministic models are presented to predict crack extension and remaining fatigue life for stable and unstable crack stages. The effect of weld length and fatigue load ratio on the AE activity is evaluated. The presence of noise is inevitable in the application of AE monitoring. The efficiency of data filtering and reduction algorithms is key to minimize the power and data storage demand of the wireless sensing system. AE data filtering protocols based on load pattern, source location, waveform feature analysis, and pattern recognition are proposed to minimize noise-induced AE and false indications due to wave reflections.

Fretting is a wear phenomenon that occurs when cyclic loading causes two surfaces in intimate contact to undergo small oscillatory motions with respect to each other. During fretting, high points or asperities of the mating surfaces adhere to each other and small particles are pulled out, leaving minute, shallow pits and powdery debris. Sometimes these surface conditions are neglected, but they are important in some application such as the aerospace industry. In this research work, non-contacting and contacting thermoelectric power techniques are performed in fretted 7075-T6 and Ti-6Al-4V samples. It has been found that the contacting and non-contacting thermoelectric power measurements are associated directly with the subtle material variations such as work hardening and residual stresses due to plastic deformation produced in the fretting zone but surface topography. Therefore, both techniques could be used for a global characterization of the most relevant fretting induced effects. Potential of these techniques to monitor subsurface changes in other severe surface plastic deformation processes are clearly envisaged.

Some problems of the electromechanical impedance (EMI), especially its applications for structural health monitoring of aircraft bolt-joints and innovated approach of EMI prediction at loosening of bolt-joints are objectives of research. The full-scale test was performed using the Mi-8 helicopter tail beam. Effect of bolt-joint loosening was investigated by electromechanical impedance (EMI) method. It was shown that loosening of bolt-joint affects significant and stable changes of EMI parameters. It was assumed that both as the weak shift of resonance frequencies or EMI magnitude and resistance change is caused mainly by damping variation. The developed 2D model of constrained PZT based on modal decomposition approach can be used for analysis of influence of the elastic properties and geometrical parameters to properties of constrained piezoelectric transducer. This model has some advantages and can also be useful for structural health monitoring of aircraft components with different possible damages including bolt-joint loosening.

Cyclically periodic structures, such as blade-disk assembly in turbo-machinery, are widely used in engineering practice. While these structures are generally designed to be periodic with identical substructures, it is well-known that small random uncertainties exist among substructures which in certain cases may cause drastic change in the dynamic responses, a phenomenon known as vibration localization. Previous studies have illustrated that the introduction of small design modifications, i.e., intentional mistuning, may alleviate such vibration localization. The design objective here thus is to identify proper deign modification that can reduce the response variation under uncertainties. In this research, we first develop a perturbation-based approach to efficiently quantify the variation of forced response of a periodic structure, without and with the design modification, under uncertainties. We then propose a Gaussian process emulation which enables us to evaluate the objective function over the design space by using only a small number of design candidates. The combination of these algorithms allows us to perform effective design modification to minimize the response variation in nearly periodic structures.

In this paper, a two-stage time-variant structural system identification method is proposed based on incomplete structural acceleration responses. In the first stage, an external excitation identification method is developed for time-variant structural system. The unknown structural response could be re-constructed in state space and regularization method. In the second stage structural parameter is identified and updated by Extended Kalman Filter. The re-constructed structural response and identified external excitation are used in the second stage. The proposed method is validated numerically with the simulation of a fifteen-story shearing structure subject to earthquake excitation. A model of a fourteen-storey concrete shear wall building was studied experimentally with shaking table tests to further validate the proposed method. The shear wall structure has a two-storey steel frame on top with base isolation. Results from both numerical simulations and laboratory tests indicate that the proposed method can be used to identify structural parameter and external excitations effectively based on the structural acceleration responses. The proposed method is capable to identify the dynamic load and structural parameters fairly accurately with measurement noise, numerical model error and environmental disturbances.

We describe a stochastic ltering approach for tracking progressive fatigue damage in structures, wherein physically based damage evolution information is combined with active sensing guided wave measurements. The input waveform used to excite dispersive modes within the structure is adaptively con gured at each time step in order to maximize the damage estimation performance. The damage evolution model is based on Paris Law, and hidden Markov modeling of time-frequency features obtained from received signals is used to de ne the measurement model. Damage state estimation is performed using a particle lter. Results are presented for fatigue crack estimation in an aluminum specimen.

This study applied the AE techniques with statistical analysis to investigate the damage process of singleedge notched beam (SEB) tests with normal strength concrete specimens. The SEB tests with the labprepared NSC specimens were conducted by employing a clip-gauge controlled servo-hydraulic testing system and an AE damage detection system. It was found that the cumulative AE events with respect to the crack mouth opening displacement (CMOD) or the crack tip opening displacement (CTOD) correlate to the mechanical loading of the specimens. A Weibull rupture probability distribution was proposed to quantitatively describe the mechanical damage behavior under the SEB test. A bi-logarithmic regression analysis was conducted to calibrate the Weibull damage distribution with detected AE signals and to predict the damage process as a function of the crack opening displacements. The calibrated Weibull damage functions were compared among concrete specimens. The comparison results indicate the Weibull function calibrated with AE signals can describe the damage behavior of concrete beam specimens.

Current routine inspection practices for bridge health monitoring are not sufficient for timely identification of areas of concern or are incapable of providing enough information to bridge owners to make valuable informed decisions for maintenance prioritization. Continuous monitoring is needed for long-term evaluation from an integrated sensing system that would act as a monitoring and early warning alarm system that is able to effectively communicate the information from the bridge directly to the bridge owners for potential and immediate response. Due to the variety of deterioration sources and locations, currently there is no single NDE method that can detect and address the potential sources globally. To address the need of this urgent highway bridge health monitoring, a joint venture research has been initiated under the NIST Technology Innovation Program by incorporating a novel and promising sensing approach based on piezoelectricity together with energy harvesting to reduce the dramatic uncertainty inherent into any inspection and maintenance plan. One approach to damage detection and classification has been focused on the use of piezoelectric sensors (PES) at both active (ultrasonic NDE) mode and passive (acoustic emission) mode on steel bridge. The acoustic emission (AE) method has been shown the best potential for global bridge health monitoring while active sensing will provide additional quantification process. Two types of the PES have been studied to provide fundamental principles for their applications to steel bridges. Extensive laboratory investigation was performed supported by a theoretical modeling analysis.

Structural Health Monitoring of large-scale bridges requires collection, storage and processing of large amounts of data, and must provide distributed concurrent access. In this paper we report on the progress of the design and implementation of a cyber-infrastructure system that is currently being field-tested on a long-span bridge in California and a short-span bridge in Michigan. This system provides remote access for sensor data acquisition systems, data analysis modules, and human operators. The implementation is based on an object-oriented data model description and makes extensive use of code generation to allow for the rapid development and continued improvement of the system. Currently the system provides storage of raw and processed sensor data, finite element models, traffic data, links to PONTIS data, reliability modeling data, and model-based analysis results. Apart from the ubiquitous read/write access, the system also includes an event system that allows data consumers to be triggered by the arrival of new data. In addition to essential backup/restore facilities, the system also includes import/export tools that can migrate data between versions, which is very useful in keeping pace with the continuous improvements that are being made in the design and implementation of the cyber-infrastructure system. The system also provides introspection, as the data model is made available by means of an inspector client interface, which allows the development of generic client tools that can dynamically discover the data model, and present a corresponding interface to the user. Currently available user-interfaces include an editor GUI application, and a read-only web application.

In this paper, a methodology for integrating a monitor and its warning system with results from both truckload tests and structural analysis is presented for the Jeremiah Morrow Bridge. Wireless data collection system, data cleansing and archiving procedures, Eigen vector prediction model, and capacity rating based upon truckload test results for the instrumented members are detailed. The truckload test and monitored responses document the “normal” or expected behavior of the structure to traffic and environment, respectively. A warning system is then designed upon a threshold technique which minimizes the probabilities of false alarms and the missed detection of critical events based upon the capacity rating. Past results of the implemented system for the Jeremiah Morrow and other bridges are discussed. Alarm scenarios are reviewed based upon the collected historical data from the monitors and generated warnings.

This paper presents a study on the seismic response trends evaluation and finite element model updating of a reinforced concrete building monitored for a period of more than two years. The three story reinforced concrete building is instrumented with five tri-axial accelerometers and a free-field tri-axial accelerometer. The time domain N4SID system identification technique was used to obtain the frequencies and damping ratios considering flexible base models taking into account the soil-structure-interaction using 50 earthquakes. Trends of variation of seismic response were developed by correlating the peak response acceleration at the roof level with identified frequencies and damping ratios. A general trend of decreasing frequencies was observed with increased level of shaking. To simulate the varying behavior of the building with response levels, a series of three dimensional finite element models were calibrated considering several points on the developed frequency-response amplitude trend lines as targets for updating. To incorporate real in-situ conditions, soil underneath the foundation and around the building was modeled using spring elements and nonstructural components (claddings and partitions) were also included. Sensitivity based model updating technique was applied taking into account concrete, soil and cladding stiffness as updating parameters. It was concluded from the investigation that knowledge of the variation of seismic response of buildings is necessary to better understand their behavior during earthquakes, and also that the participation of soil and non-structural components is significant towards the seismic response of the building and these should be considered in models to simulate the real behavior.

Understanding the propagating elastic wave characteristics in materials is the foundation for quantitative Nondestructive Testing methods based on wave propagation such as guided wave ultrasonic and acoustic emission. The conventional finite element formulation requires very fine meshing and small time steps to prevent the dispersion pollution at high frequencies. The spectral finite elements reduce the required degrees of freedoms and the computation of time integration for dynamic finite element models via using high order orthogonal polynomials to define the locations of nodal coordinates. In this study, the advantage of spectral elements over conventional finite elements for frequencies up to 400 kHz is demonstrated on plane stress model of a structural steel plate. The excitation frequency is varied from 60 kHz to 400 kHz. The Legendre orthogonal polynomials with the orders of 3, 4 and 5 are selected. The required h refinements (i.e. element size) to eliminate the numerical error for three polynomial orders are identified. The results provide a guide for selecting the element sizes for different polynomial orders. The validity of the spectral element formulation is demonstrated via comparison with conventional finite element results.

As asphalt pavements age and deteriorate, recurring pothole repair failures and propagating alligator cracks in the asphalt pavements have become a serious issue to our daily life and resulted in high repairing costs for pavement and vehicles. To solve this urgent issue, pothole repair materials with superior durability and long service life are needed. In the present work, revolutionary pothole patching materials with high toughness, high fatigue resistance that are reinforced with nano-molecular resins have been developed to enhance their resistance to traffic loads and service life of repaired potholes. In particular, DCPD resin (dicyclopentadiene, C₁₀H₁₂) with a Rhuthinium-based catalyst is employed to develop controlled properties that are compatible with aggregates and asphalt binders. In this paper, a multi-level numerical micromechanics-based model is developed to predict the viscoelastic properties and dynamic moduli of these innovative nano-molecular resin reinforced pothole patching materials. Irregular coarse aggregates in the finite element analysis are modeled as randomly-dispersed multi-layers coated particles. The effective properties of asphalt mastic, which consists of fine aggregates, tar, cured DCPD and air voids are theoretically estimated by the homogenization technique of micromechanics in conjunction with the elastic-viscoelastic correspondence principle. Numerical predictions of homogenized viscoelastic properties and dynamic moduli are demonstrated.

A structural change quantification methodology is explored in which the magnitude and location of a structural alteration is identified in a rotor system. The proposed structural alterations may be interpreted as physical damage to a structure, in efforts of advancing structural health monitoring activities. The structural change quantification strategy involves the use of resonance and antiresonance frequencies which are collected from several transfer functions calculated from a finite element rotor model. These values are collected and included in an objective function which outputs an error value that is subsequently minimized. The resulting objective contains sufficient information to identify the dynamic characteristics of the rotor in both the frequency and spatial domains. A finite element model with carefully selected tunable parameters is iteratively adjusted using a numerical optimization algorithm to determine the source of the structural change. The numerical studies presented in this work utilize a generic rotor model with features such as a hollow shaft, two ball bearings, several disks, and multiple material layers. The method used for structural excitation is assumed to utilize magnetic actuators for nonintrusive operations. First, the investigations optimize the objective function using a hybrid optimization approach which applies both the NSGA-II genetic and the Nelder-Mead optimization algorithms. The objective function is optimized to maximize the sensitivity of the rotor’s finite elements to detect structural change. Second, a simulated local structural change is implemented in which the detection methodology is employed to locate. An investigation of the effect of error in the simulated data on the prediction’s accuracy is addressed.

In this work, a slotted patch antenna is employed as a wireless sensor for monitoring structural strain and fatigue crack. Using antenna miniaturization techniques to increase the current path length, the footprint of the slotted patch antenna can be reduced to one quarter of a previously presented folded patch antenna. Electromagnetic simulations show that the antenna resonance frequency varies when the antenna is under strain. The resonance frequency variation can be wirelessly interrogated and recorded by a radiofrequency identification (RFID) reader, and can be used to derive strain/deformation. The slotted patch antenna sensor is entirely passive (battery-free), by exploiting an inexpensive offthe- shelf RFID chip that receives power from the wireless interrogation by the reader.

We conduct a comparative study of approaches for optimal sensor placement from the fields of control engineering and civil engineering. The widely used civil engineering approaches such as EI and MAC have limitations due to the negligence of damping parameters. On the other hand, control engineering approaches do not consider the key parameters of interest in civil structures. We study the maximization of estimation performance of key structural characteristics from impulse response data, using approaches from both fields to unveil the pros and cons of each.

Wireless sensors have emerged to offer low-cost sensors with impressive functionality (e.g., data acquisition, computing, and communication) and modular installations. Such advantages enable higher nodal densities than tethered systems resulting in increased spatial resolution of the monitoring system. However, high nodal density comes at a cost as huge amounts of data are generated, weighing heavy on power sources, transmission bandwidth, and data management requirements, often making data compression necessary. The traditional compression paradigm consists of high rate (>Nyquist) uniform sampling and storage of the entire target signal followed by some desired compression scheme prior to transmission. The recently proposed compressed sensing (CS) framework combines the acquisition and compression stage together, thus removing the need for storage and operation of the full target signal prior to transmission. The effectiveness of the CS approach hinges on the presence of a sparse representation of the target signal in a known basis, similarly exploited by several traditional compressive sensing applications today (e.g., imaging, MRI). Field implementations of CS schemes in wireless SHM systems have been challenging due to the lack of commercially available sensing units capable of sampling methods (e.g., random) consistent with the compressed sensing framework, often moving evaluation of CS techniques to simulation and post-processing. The research presented here describes implementation of a CS sampling scheme to the Narada wireless sensing node and the energy efficiencies observed in the deployed sensors. Of interest in this study is the compressibility of acceleration response signals collected from a multi-girder steel-concrete composite bridge. The study shows the benefit of CS in reducing data requirements while ensuring data analysis on compressed data remain accurate.

Performing full-scale structural testing is an important methodology for researchers and engineers in civil engineering industry. The full-scale testing helps the researchers understand civil structure’s loading scenarios, behaviors, and health conditions. It helps engineers verify, polish, and simplify the structural design and analysis theories. To conduct a fullscale structural testing, sensors are used for data acquisitions. Structural researchers and engineers use sensors to record various physical parameters of civil structure and to convert these parameters to analog or digital signals. The physical parameters most often used to depict the property, behavior, or health condition of civil structure include accelerations, velocities, displacements, positions, strains, and forces. In addition, temperatures, pressures, sound, flow rates, viscosities, optical radiations, and electromagnetic fields have been used as well. Based upon their application methodologies, sensors used for civil structure testing could be categorized into traditional contact sensors and remote (non-contact) sensors. Based upon their data communication strategies, sensors used for civil structure testing could be divided into traditional cabled sensors and wireless sensors. Wireless sensing technologies employ wireless communications. They can be wireless accelerometers, wireless strain sensors, wireless thermal sensors, and many other types of sensors with wireless communication functions. Compared to cabled sensors, wireless sensors can potentially significantly reduce implementation time and costs, thus facilitating deployment of a dense network of sensors for structural health monitoring (SHM). This paper reviews the wireless sensing technologies for full-scale structural testing applications, especially focusing on the Illinois WSSN (wireless smart sensor network) applications. It is expected to help structural researchers and engineers get familiar with the novel wireless sensing technologies and select the most effective testing tools.

Performing full-scale structural testing is an important methodology for researchers and engineers in civil engineering industry. To conduct a full-scale structural testing, sensors are used for data acquisitions. Based upon their data communication strategies, sensors used for civil-structure testing could be divided into traditional wired sensors and wireless sensors. Compared to wired sensors, sensors that employ the wireless communication technology can potentially reduce implementation time and expenses, thus facilitating deployment of a dense network of sensors for structural health monitoring (SHM). This paper experimentally evaluates the advantages and disadvantages of wireless sensors versus wired sensors. Accelerometers are selected for case studies since they are one of the most widely used sensors in civil structure testing, primarily in system identification and SHM. Three different accelerometers studied in this paper include the piezoelectric wired accelerometer, the force balance tri-axial accelerometers, and the wireless smart sensing network sensor board based on the MEMS technology. These sensors were compared during a series of tests, which include free vibration testing of a laboratory scale steel frame, field ambient vibration testing of a building connection bridge, and full scale field monitoring of a 65-m high wind turbine tower vibration. The complete testing procedure (instruments deployment, data acquisition, and data processing) shows the advantages and disadvantages of different types of sensors. It is expected that the results of this study can help structural researchers and engineers get familiar with the wired and wireless sensing technologies and select the most effective testing instruments.

A multilayer structured thin film system, such as a biomedical thin film, MEMS (Micro Electric Mechanical System)/NEMS (Nano Electric Mechanical System) devices, and semiconductors, is widely used in various fields of industries. To non-destructively evaluate the multilayer structured thin film system, a mechanical scanning acoustic reflection microscope has been well recognized as a useful tool in recent years. Especially, the V(z) curve method with the scanning acoustic microscope is used to characterize the very small area of the system. In this study, V(z) curve simulation software for simulating transducer output when we transmit an ultrasound wave into the specimen has been developed. In the software, the Thompson-Haskell transfer matrix method is applied to solve for the reflectance function. All input and output interfaces incorporated in a GUI interface for users’ convenience. Surface acoustic wave velocities are calculated from the simulated V(z) curves. For the precise calculation advanced signal processing techniques are utilized. The surface acoustic wave velocity is compared to that from an experiment with a bulk solid. We also tested the simulation’s thickness sensitivity by simulating models with different thickness in nanoscale. A series of experiments with multilayered solids are carried out and the results are compared with the simulation results. It was the first time a comparison of analytical versus experimental for V(z) curves for multilayered system were performed. For the multilayered specimen, silicon (100) is used as a substrate. Titanium (thickness: 10 nanometer) and platinum (thickness: 100 nanometer) are deposited respectively.

This study investigates the internal-frost damage due to ice crystallization pressure in capillary pores of concrete. The pore structures have significant impact on freeze-thaw durability of cement/concrete samples. The scanning electron microscope (SEM) techniques were applied to characterize freeze-thaw damage within pore structure. The digital sample was generated from SEM imaging processing. In the microscale pore system, the crystallization pressures at subcooling temperatures were calculated using interface energy balance with thermodynamic analysis. The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). One comparison study between model simulation and test results indicates that internalfrost damage model can reasonably predict the crack nucleation and propagation within multiphase cement microstructure.

Monitoring structural integrity of large planar structures that aims at detecting and localizing impact or damage at any point of the structure requires normally a relatively dense network of uniformly distributed ultrasonic sensors. 2-D ultrasonic phased arrays, due to their beam-steering capability and all azimuth angle coverage are a very promising tool for structural health monitoring (SHM) of plate-like structures using Lamb waves (LW). Linear phased arrays that have been proposed for that purpose, produce mirrored image characterized by azimuth dependent resolution, which prevents unequivocal damage localization. 2D arrays do not have this drawback and they are even capable of mode selectivity when generating and receiving LWs. Performance of 2D arrays depends on their topology as well as the number of elements (transducers) used and their spacing in terms of wavelength. In this paper we propose a consistent methodology for three-step: theoretical, numerical and experimental investigation of a diversity of 2D array topologies in SHM applications. In the first step, the theoretical evaluation is performed using frequency-dependent structure transfer function (STF). STF that defines linear propagation of different LWs modes through the dispersive medium enables theoretical investigation of the particular array performance for a predefined tone-burst excitation signal. A dedicated software tool has been developed for the numerical evaluation of 2D array directional characteristics (beampattern) in a specific structure. The simulations are performed using local interaction simulation approach (LISA), implemented using NVIDIA CUDA graphical computation unit (GPU), which enables time-efficient 3D simulations of LWs propagation. Beampatterns of a 2D array can be to some extend evaluated analytically and using numerical simulations; in most cases, however, they require experimental verification. Using scanning laser vibrometer is proposed for that purpose, in a setup where LWs, excited by PZT transmitters of the investigated array are sensed in multiple points corresponding to the locations of the 2D array receiving elements. A virtual receiving sub-array is created in this way and the performance of various array architectures in the reception mode can be evaluated experimentally without the need of physical prototype; a change of topology requires only straightforward modification of the measurement points distribution at the tested structure. For illustration, beampatterns of three symmetrical 2D topologies, i.e., circular, star-shaped and spiralshaped, will be examined in the paper and compared in terms of their beam-width and side-lobes level. The effect of apodization applied to the array elements will be also investigated.

The use of phased array methods are commonplace in ultrasonic applications, where controlling the variation of the phase between the narrowband emitters in an array facilitates beam steering and focusing. An approach is presented here, whereby all emitters in a 1 dimensional array are pulsed simultaneously, with a controlled bandwidth to emit a 2 dimensional wave. The key result is that one can generate a smooth, continuous wave-front emitted from the array, over a large solid angle, whose frequency varies as a function of angle to the array. Analytic and finite element models created to simulate this phenomena have been validated with experimental results using ultrasonic waves in metal samples. This pulsed approach provides a rapid means of flooding a region of space with a wave-front, whereby any wave that scatters or reflects off a body to a detector will have a distinct arrival time and frequency. This is a general wave phenomena with potential applications in radar, sonar and ultrasound.

Light-weight carbon fiber composite pressure vessel with inner thin-wall aluminum alloy liner has main problem of local buckling during manufacture and working process. The approach of acoustic emission and Bragg grating are adapted to monitoring the light-weight composite vessel under water pressure. Two channels of acoustic emission (AE) were bonded to front dome and cylinder to monitoring the performance of the vessel withstanding maximum 4.5MPa water pressure during loading, maintaining and unloading. Meantime six fiber Bragg sensors (FBG)were attached to front dome and cylinder of the outer surface by hoop and meridian direction respectively in order to monitor the vessel behavior. Analysis indicated Bragg sensors can evaluate outer surface behavior of the vessel with pressure. AE character parameters analysis illustrated the local buckling of inner thin-wall liner.

Frequency modulated thermography is becoming a popular thermal non-destructive testing (TNDT) technique which like other active thermographic techniques, requires an external heating stimulus, preferably on a blackened surface. It is however, not immune to non-ideal situation like non-uniform heating and surface emissivity variation. The phase image helps to reduce the effect of surface emissivity variation to some extent, but is inadequate in case of large variations. Further, structural noise can significantly interfere with the process of defect detection, as in the case of carbon fibre composite materials. This paper proposes two image reconstruction algorithms for off-line reduction of structural noise. The first utilizes the periodic nature of structural noise if present, and removes it using 2-dimensional Fourier transformation and a spatial band-stop filter. The other uses time serial reconstruction algorithm to remove such noise. The latter was originally proposed by the authors to remove artifacts from surface emissivity variation on metallic samples. The performances of both algorithms are successfully demonstrated on a carbon fibre test piece, having 2mm, 4mm, and 6mm diameter back drilled holes at various depths ranging from 0.25mm to 2mm in steps of 0.25mm.

The magnetic domain and magnetic properties of heat resistant steels including 10CrMo910, P91 and 23CrMoNiWV88 are investigated in the present work. The magnetic properties characterized by magnetic hysteresis loop of the three materials under 500-600^°C are measured by vibrating sample magnetometer (VSM). The magnetic domain structure of as-received and crept specimens is observed by magnetic force microscope (MFM). The magnetic domain of ferrite phase change from initial stripe pattern to maze pattern during creep. The black and white fringes and stripe-like pattern have also been found in the P91 and 23CrMoNiWV88 specimens, respectively. The experimental results reveal that the magnetic domain structure is strongly influenced by microstructures with different distributions of the carbides. It is shown that the coercivity and remanence of each material although has a remarkable decrease at 500-600^°C especially for P91 almost 64% decrease, it’s still the same magnitude as the one at room temperature. All the short-term crept specimens with different creep damage have a linear increase in coercivity and remanence comparing to the as-received 10CrMo910 specimens. These results indicate that it should be possible to develop an in-situ monitoring technology for creep damage based on magnetism measurement.

In this paper we propose an original analytical modeling applied to electromagnetic (EM) systems, in the aim of performing an inversion scheme. The purpose consists in imaging cracks through eddy current sensors, the image being constituted by an estimation of the conductivity of the piece of metal under test, voxel by voxel. There are plenty of industrial applications in Non Destructive Evaluations for monitoring of cracks in structures: Aeronautics, Metallurgy, ships building and so on. For modeling, a relatively new developed technique, called ‘Distributed Points Source Method’ (DPSM) has been used. This original modeling was invented at Ecole Normale Superieure of Cachan in 2000, and since developed and patented in many kind of applications (Ultrasonics, Electrostatics…).

Magnetic Flux Leakage (MFL) technology has been used in non-destructive testing for more than three decades. There have been several publications in detecting and sizing defects on metal pipes using machine learning techniques. Most of these literature focus on isolated defects, which is far from the real scenario. This study is towards the generalization of interpretation of the leakage flux in the presence of multiple defects based on simulation models, together with data-driven inference methodologies, such as Gaussian Process (GP) models. A MFL device has been simulated using both COMSOL Multiphysics and ANSYS software followed by prototyping the same device for experimental validations. Multiple defects with different geometrical configurations were introduced on a cast iron pipe sample and both radial and axial components of the leakage field have been measured. It was observed that both axial and radial components differ with different defect configurations. We propose to use GP to solve the inverse model problem by capturing such behaviors, i.e. to recover the profile of a cluster of defects from the measurements of a MFL device. The data was used to learn the non-parametric GP model with squared exponential covariance function and automatic relevance determination to solve this regression problem. Extensive quantitative and qualitative evaluations are presented using simulated and experimental data that validate the success of the proposed non-parametric methodology for interpreting the profiling of clusters of defects with MFL technology.

DC potential difference method are simple and fast non destructive testing method. It measures voltage between two points on surface touched by each needle. In some kind of engineering structural member, neighborhood of iron surface is required to have higher hardness than normal iron ’s one. To meet this demand, quench hardening process is a popular process to make iron hard from surface and hardening depth is important parameter to be ensured in target member after process. Because quench iron increases its resistance by 20 %, D.C.potential method is able to be used for evaluating hardening depth. However output voltage was insensitive for change of deep hardening depth over 5.0mm, measurement would fail over 2.0 mm with improper needle array probe in reality. We developed a novel methodology to determine needle position to improve its applicability over 5.0 mm depth which is demanded in some member. In this methodology, needle position are optimized for cost function that is voltage gradient to hardening depth. Throughout this optimization, (1) an analytical expression of surface voltage which are obtained solve Laplace equation in three dimensional space. (2) CAS system (Maple / MAXIMA) . are invoked. Experimental data shows good coincidence to those numerical computation, and monotone increasing property of voltage to depth are kept up to 10 mm hardening depth. Proposed needle positions have made DC potential difference method possible to evaluate hardening depth over 5.0 mm. The algorithm is most likely to work well with only one response function for this type probe.