Monitoring of delamination is indispensable for CFRP structures. It is, however, very difficult to detect a delamination visually. This demands a new structural health monitoring method. For aerospace structures, it is required to monitor a delamination before flight, and this means the monitoring system must detect the delamination without loading. In authors' previous studies, the delamination can be monitored with the electric resistance change method. The method provided excellent performance of estimations. The method, however, requires complicated electric circuits and uses a two-probe method: two-prove method includes effects of the electric resistance change at the electrodes. To resolve these problems, an electrical potential method is employed here. In the previous paper, the electrical potential method showed poor performance of estimations for delamination cracks located near the center of the specimen. The practical zigzag crack has large effect on the performance of estimation when the delamination locates at the center segment of the specimen. This problem is overcome by means of a two-stage monitoring method. The new method shows excellent performance of estimations on the basis of FEM analyses. In this paper, experiments are conducted to identify the delamination using this method. The applicability of this method is examined using the experimental data.

Nondestructive evaluation of airspace structures can be performed using ultrasonic spectroscopy utilizing the information in the frequency domain obtained due to the constructive and destructive interference of elastic waves. The application field of ultrasonic resonance spectroscopy (URS) is likely to increase rapidly with the growing application of layered structures in modern aircraft. The aim of this paper is to enlighten the potential and the limitations of the URS techniques. We start from explaining principles of URS applied to multi-layered structures and then we present a theoretical model that enables predicting the modal shapes and resonance frequencies of the thickness mode resonances occurring in multi-layered structures. The model also includes the piezoelectric transducer used for sensing the structure resonances. Presentation of the narrowband URS technique is illustrated with results of measurements performed using specially designed resonance transducers in carefully selected narrow frequency bands. We also present a novel method for sensing transducer's resonance based on the use of a phase locked loop (PLL).

A novel 3D sensing system with real-time 3D processing has been developed which is capable of scanning an object at very high speeds (greater than 500,000 3D points/second) and creating high-resolution 3D surface maps. Laser triangulation is used in conjunction with a high-resolution camera, a laser diode, and processing electronics all incorporated into a small sensor package that traverses linearly or rotates from a fixed position to scan an object. Processing is done on board the instrument and the resultant 3D data is transmitted to a PC. This results in rapid scans, with 3D images produced as the instrument is scanning. The sensing system was developed for the NASA Mars rover program and for the inspection of Shuttle Thermal Protection System (tiles), radiator, and structures.

Laser ultrasonic imaging of surface acoustic waves on a material surface provides a non-contact and sensitive method for detecting and characterizing defects and anomalies in aerospace and industrial materials. It has recently been shown that the surface acoustic wave interactions with sharp material discontinuities such as surface-breaking cracks provides an additional benefit of an intensification of the displacement field in the immediate vicinity of the crack site. This near-field intensification can be exploited by scanning, point-by-point, a laser-ultrasonic system to create detailed microscopic images of the surface breaking cracks. In this effort, a number of laser ultrasonic system parameters were studied to better understand the optimized conditions to imaging surface breaking cracks using ultrasonic generation in the thermoelastic regime, and laser detection of ultrasound fields using heterodyne interferometer. In particular, laser beam sizes, separation distances, and motives were varied. In addition, several different time-gating analysis methods were studied, which had a significant effect on both the characteristics as well as the quality of the resultant crack images.

Optical interference techniques such as electronic speckle pattern interferometry (ESPI) and Digital Shearography have been shown to be of great value for the Non-destructive Evaluation of composite materials. The recent signing of a contract to purchase numerous Airbus aircraft for commercial use in South Africa as well as the purchase of the Grippen military aircraft has resulted in the capabilities of optical interference techniques for NDE purposes receiving increased attention. The NDT Laboratory in the Department of Mechanical Engineering at the University of Cape Town has for a number of years been involved with the research, development, and applications of the optical NDE techniques of ESPI and Digital Shearography. This has led to the development of a portable inspection unit, based on Digital Shearography and the latest addition of a portable ESPI prototype. In order to compare the capabilities of the developed prototypes, industry acceptable test specimens of composite aircraft components are subjected to tests using both systems. The results are presented and comparisons are drawn highlighting the advantages and disadvantages of these two optical NDE techniques.

Due to increased demand for design flexibility, in recent years engineers have progressively employed polymers in the design of electronics enclosures. As the circuits in these enclosures are miniaturized, dissipate more thermal energy and run at higher clock speeds, electromagnetic interference (EMI) and heat dissipation concerns become more apparent and are more problematic. The high thermal impedance of polymers slows their implementation in these situations. In addition, many electronics devices are subject to industrial and governmental regulations for EMI emission and isolation. To address these concerns it is becoming increasingly popular to apply conformal metallic films to polymer-based enclosures to increase EMI shielding and decrease thermal impedance through heat spreading. As with any coating, quality assurance of adhesion between the film and substrate is of immense importance. Without standardized testing procedures for assuring the quality of these processes, it is difficult to place them into practice. When new and alternative manufacturing processes are brought forth quality assurance is of paramount importance. The majority of existing commercial testing procedures for determination of adhesion quality for metallic films pertain to metallic substrates. This paper presents the application of a practical shear wave lens to overcome these issues. It will be demonstrated that the shear wave lens will provide sufficient resolution in this application to allow visualization of bond quality and determination of to what degree a conformal coating has been achieved.

Structural health monitoring (SHM) activities in civil engineering are increasing at a rapid pace in both research and field applications. This paper addresses the specific issue of incorporating internet technology into a structural health monitoring program. The issue of data volume versus communication speed is discussed along with a practical solution employed by ISIS Canada. The approach is illustrated through reference to several current case studies which include two bridges and a statue. It is seen that although the specifics of the projects and monitoring needs are different, the manner in which on-line monitoring can be conducted is very similar and easily allows for centralized monitoring. A general framework for website construction integrating sensing data and web camera options are presented. Issues related to simple real-time performance indices versus more comprehensive complex data analysis are discussed. Examples of on-line websites which allow visualization of new and historic data are presented. The paper also discusses future activities and research needs related to centralized remote structural monitoring and management of real-time data.

Distrbuted Brillouin intensity vs. temperature measurements along an optical fiber are studied theoretically and experimentally using counter-propagating laser beams interacting at a fixed beat frequency. By monitoring the Brillouin temperature spectrum without scanning the beat frequency, one can acquire distributed temperature measurements within seconds rather than minutes, making this acquisition method suitable for dynamic processes such as hot spots and fire detection. This technique requires knowledge of a threshold temperature condition, which is mathematically derived by considering the temperature dependence on Brillouin peak power and linewidth in the frequency domain. Temperature varying fiber regions are monitored using 0.2 and 2 m spatial resolutions along 2 and 11 km fiber lengths respectively.

There is a growing need for built-in monitoring systems for civil engineering infrastructures, due to problems such as increasing traffic loads and rising costs of maintenance and repair. Fibre optic sensors (FOS), capable of reading various parameters are promising candidates for life-long health monitoring of these structures. However, since FOS have only been introduced recently into the field of structural monitoring, their acceptance and widespread implementation will be conditioned by their durability under severe climatic and loading conditions. This paper reports on the performance of strain extrinsic FOS attached to carbon fibre reinforced polymer (CFRP) plates used to strengthen concrete structures. The specimens tested in this project are reinforced concrete (RC) beams with an additional external CFRP reinforcement. The FOS-instrumented beams were first subjected to fatigue loading for various numbers of cycles and load amplitudes. Then, they were tested monotonically to failure under four-point-bending. The test results provide an insight on the fatigue and post-fatigue behaviour of FOS used for monitoring reinforced concrete structures.

The arching action in concrete deck slabs for girder bridges is utilized fully in steel-free deck slabs. These concrete slabs, requiring no tensile reinforcement, are confined longitudinally by making them composite with the girders, and transversely by external steel straps connecting the top flanges of external girders. Between 1995 and 1999, five steel-free deck slabs without any tensile reinforcement were cast on Canadian bridges. All these slabs developed fairly wide full-depth cracks roughly midway between the girders. While extensive fatigue testing done in the past three years has confirmed that the presence of even wide cracks does not pose any danger to the safety of the structures, wide cracks are generally not acceptable to bridge engineers. The developers of the steel-free deck slabs have now conceded that these slabs should be reinforced with a crack-control mesh of nominal glass fibre reinforced polymer (GFRP) bars. Steel-free deck slabs with crack-control meshes are being referred as the second generation slabs. With the help of testing on full-scale models, it has been found that deck slabs with GFRP bars have the best fatigue resistance and those with steel bars the worst.

The extensive use of deicing salts in Canada during winter times is identified as the main reason behind the deterioration of highway bridges and parking garages. To fight this infrastructure crisis, Fibre Reinforced Polymers (FRP) has become a very attractive alternative to traditional reinforcing steel due to their non-corrosive nature and light weight. The replacement of steel with Glass FRP bars in bridge deck slabs has been extensively researched in the last few years. This paper presents the first efforts to implement these bars in two highway bridges in Quebec, Canada, and Vermont, USA. These projects are aimed to prove the feasibility of using GFRP bars in bridge construction. GFRP bars were used as reinforcement for parts of the deck slabs in the two bridges while traditional steel was used in the remaining parts. Fibre Optic Sensors (FOS) were used to measure strains in the concrete, reinforcing bars and steel girders. The sensors were surface mounted on the bars or steel girders using standard glue, or embedded in concrete. Static and dynamic testing of the bridges was done using loaded trucks placed for maximum stresses. The design, construction, testing, and results obtained from the bridges are briefly outlined in this paper. The results indicated the accuracy of the sensors and their feasibility for bridge construction and remote monitoring.

Non destructive evaluation (NDE) is a critical technology for improving the quality of a component in a cost-sparing production environment. NDE detects variations in a material or a component without altering or damaging the test piece. Using these techniques to improve the production process requires characterization of the faults and their influence on the component performance. This task depends on the material properties and on the complexity of the component geometry. Hence, the NDE technique is applied to study the structural durability of ceramic matrix composite materials used in gas turbine engine applications. Matrix voids are common anomalies generated during the melt infiltration process. The effects of these matrix porosities are usually associated with a reduction in the initial overall composite stiffness and an increase in the thermal conductivity of the component. Furthermore, since the role of the matrix as well as the coating is to protect the fibers from the harsh engine environments, the current design approach is to limit the design stress level of CMC components to always be below the first matrix cracking stress. In this study, the effect of matrix porosity on the matrix cracking stress is evaluated using a combined fatigue tensile testing, NDE, and 3 D image processing approach. Computed Tomography (CT) is utilized as the NDE technique to characterize the initial matrix porosity’s locations and sizes in various CMC test specimens. The three dimensional volume rendering approach is exercised to construct the 3 D volume of the specimen based on the geometric modeling of the specimen's CT results using image analysis and geometric modeling software. The same scanned specimens are then fatigue tested to various maximum loads and temperatures to depict the matrix cracking locations in relation to the initial damage. The specimen are then re-scanned and checked for further anomalies and obvious changes in the damage state. Finally, rendered volumes of the gauge region of the specimen is generated and observed to check damage progression with increasing cycles. Observations and critical findings related to this material are reported.

A limiting factor for many current structural health monitoring methods is that in order to locate damage, modeling of the structural response is required. The structural model itself can introduce significant errors, in addition to sensor noise, both through limitations in the assumptions applied and manufacturing variations. This work presents a method of localizing damage that eliminates the requirement for an independent structural model. The method is based on the flexibility parameters of the structure in pre- and post- damage states. An technique has been developed that allows the required information to be constructed from only sensor and actuator data. From the sensor data, a set of damage location vectors are determined. These vectors are shown to localize damage via two methods: The first analysis reapplies each set damage location vector as applied forces to the structure. The second, more applicable to real-time health monitoring, locates lowest values of the damage location vectors themselves. Simulations on a plate are performed for two sensor meshes (eight and thirty-two locations). The results demonstrate excellent damage localization, and some indication of damage severity. Finally an experimental demonstration of the method utilizing eight sensors surface mounted to an aluminum plate is presented.

A model for the Scanning Laser Source (SLS) technique is presented. The SLS is a novel laser based inspection method for the ultrasonic detection of small surface-breaking cracks. The generated ultrasonic signal is monitored as a line-focused laser is scanned over the defect. Characteristic changes in the amplitude and the frequency content are observed. The modelling approach is based on the decomposition of the field generated by the laser in a cracked two-dimensional half-space, by virtue of linear superposition, into the incident and the scattered fields. The incident field is that generated by laser illumination of a defect-free half-space. A thermoelastic model has been used which takes account of the effect of thermal diffusion, as well as the finite width and duration of the laser source. The scattered field incorporates the interactions of the incident field with the surface-breaking crack. It has been analyzed numerically by a direct frequency domain boundary element method. A comparison with an experiment for a large defect shows that the model captures the observed phenomena.

In this work, a newly proposed NDE method named Tapping Sound Analysis was verified and demonstrated. The numerically simulated reference data for the detection of the subsurface defects of laminated composites were verified experimentally. The closeness of numerical results and experimental results were checked by using the concept of feature index. According to the comparison of numerical results and experimental results, sound print (the reference data of Tapping Sound Analysis) could be successfully obtained through present numerical procedures. In order to show the performance of Tapping Sound Analysis as a NDE method, detection of delamination inside laminated composite was demonstrated. Using numerically simulated reference data and the concept of feature index, delaminated region was successfully detected. To improve the practicality of Tapping Sound Analysis, more large laminated composites were inspected. A two-dimensional image of feature indices was drawn to visualize the damaged area.

The transient vibration response of a cracked flexible rotor passing through its critical speed is analyzed for crack detection and monitoring. The effects of different factors such as various crack depths, acceleration, damping, torque, unbalance eccentricity, and rotor weight on the rotor vibrational response are studied. The breathing types of cracks are analyzed using simple hinge model in a case of shallow cracks, and the cosine function is employed in the case of deep cracks. The developed strategy enables the analysis of cracked rotor vibrational response with and without weight dominance, taking into account also the nonsynchronous rotor whirl. In addition, the local cross-flexibility for deep cracks is taken into account. Lastly, the effect of the crack depth on “stalling” of the rotor due to the limited driving torque is investigated.

This paper presents an active monitoring method based on Lamb wave and wavelet transform to determine damage locations. The method compares damage signal with reference signal, namely undamage signal, and takes their algebraic difference as a signal caused by damage. In fact, by the analysis of Lamb wave signals recorded before and after damage, the differential signal is a reflect signal from damage which can be regarded as an acoustic emission signal. It is showed that the wavelet transform using the Gabor wavelet effectively decomposes the differential signal into its time-frequency components, and the peaks of the time-frequency distribution near the center frequency of exciting signal indicate the arrival times of waves. By calculation of time delays between the arrival of the differential and exciting signals, the damage localization points can be obtained. Our experimental results on fiber-glass materials which suffer a delamination defect prove that the damage location is reliable.

As the design and construction of civil structures continue to evolve, it is becoming imperative that these structures be monitored for their health. In order to meet this need, the discipline of Civionics has emerged. It involves the applications to civil structures and aims to assist engineers in realizing the full benefits of structural health monitoring (SHM). Therefore, the goal of the specification outlined in this work is to ensure that correct installation and operating of fiber optic sensors, such as bridges, will be discussed that motivated the writing of these specifications. The main reason for the failure of FOS based monitoring systems can be traced directly to the installation of the fiber sensor itself. Therefore, by creating a standard procedure for SHM, several ambiguities are eliminated such as fiber sensor specifications and the types of cables required. As a result, these specifications will help ensure that the sensors will survive the installation process and eventually prove their value over years of monitoring the health of the structure. The Civionics FOS specifications include the requirements for fiber sensors, specifically Bragg grating sensors, and their corresponding readout unit. It also includes specifications on the cables, conduits, junction boxes, cable termination and the environmental.

This paper explores the use of unsupervised neural networks and frequency sensitive competitive learning for novel event identification in structural health monitoring (SHM) systems. Our approach assigns a novelty metric based upon the output states of an SHM system. The technique can be applied in data decimation schemes, to enhance the monitoring of such systems, and as an aide to SHM data analysis. Learning units provide a means of characterizing an SHM system, and are subsequently used to assign a novelty metric to new SHM data. The system has been evaluated using data from the Taylor Bridge and Golden Boy statue in Winnipeg, Canada and the Portage Creek bridge in Victoria, Canada. The system is capable of analyzing SHM data from a 14-channel system, recording data at 32 Hz, using 32 learning units at approximately 30 times real-time on an AMD AthlonXP 2500+ based computer. The event identification system is most sensitive to SHM data which exhibits unusual power spectra, including data which shows abrupt changes in sensor outputs. The system may be cascaded in order to perform basic classification of events after identification.

A fiber optic system intended to perform Structural Health Monitoring (SHM) of composite motor cases has been investigated. The method described here allows for commercial-of-the-shelf (COTS) optical fiber to be integrated into a cylindrical composite motor case prior to cure. The fiber requires no pre-processing before it can be placed inside the composite material. This allows the fiber to act as a distributed sensor not a point sensor as is the case with Bragg gratings and etalons. The distributed nature of the sensor also allows the output data to be naturally multiplexed without the need for complex software or hardware interfaces. After cure the optical fiber can be interrogated to determine a base-line scan of the motor. Subsequent scans can be taken of the motor to determine if damage of a sufficient nature has occurred that would require further investigation or retiring of the motor. In this study optimum wind patterns and proper placement of the optical fiber was investigated. In addition cost reductions of the instrumentation and the practicality of optical fiber egress options were undertaken.

The article presents an experimental study that has been conducted to evaluate the impact loading damage within hybrid fabric laminates-carbon and Aramid fibers. The experiments have been undertaken on a series of interply hybrid specimens with different preprags stacking sequences. Impact damage was created using an air-gun like impact device propelling spherical steel balls with diameters of 5.0mm and 10.0mm and having velocities of 113m/s and 40m/s respectively. The resulting specimen surface and internal damage (e.g., micro-cracking and debonding) was visualized nondestructively by a scanning acoustic microscope (SAM) while further interrogation of specific internal damage was visualized using a scanning electron microscope (SEM) on cross-sectioned panels.

Evidence of a direct correlation between attenuation and porosity in graphite-epoxy composites can be found during a literature review of ultrasonic research. This paper presents the possibility of detecting and quantifying porosity in thin laminates using amplitude histogram data obtained from ultrasonic C-scans in the pulse-echo mode. Seventeen composite panels were manufactured under various curing pressures to generate different levels of porosity. Multiple C-scans of these panels and their associated amplitude histograms were generated. Trends were calculated for quantifying porosity with respect to mean histogram amplitude data. The data indicate an effect of the material properties on the trendline which mean that different materials will need to be calibrated for this technique. Repeatability of the method was verified. Finally the sensitivity of the method was checked by zooming in on sections of the C-scans that displayed elevated levels of porosity. Comparing mean values for the zoomed histograms to mean values for the bulk histograms showed an ability to detect small local changes in the porosity levels. This method shows good potential for quantifying porosity levels larger than 1%. More data are still needed for a wider range of porosities to build more confidence in the porosity estimation.

A guided wave scanning system was developed and is being refined at NASA Glenn Research Center. Instead of isolating a single Lamb wave mode, this guided wave scan system utilizes a multi-mode ultrasonic response consisting of multiple, overlapping wave modes. Various time and frequency related parameters are calculated from the time domain waveform at each scan location to create images. In order to optimize the performance of the guided wave scanning device, many experimental conditions need to be considered. In this study, the effects of the transducer contact force, dry couplant pad configuration, and scan step size on the repeatability of the guided wave parameters and the intensity and quality of the ultrasonic waveform were investigated. Based on the results, an optimal couplant configuration was recommended for future use with the scanning device.

This paper presents the results of a comparison study of three ultrasonic nondestructive evaluation (NDE) methods applied to polymer matrix composite (PMC) specimens subjected to impact damage. Samples mainly consisted of various thicknesses of graphite/epoxy coupon panels impacted with various energy levels. Traditional pulse-echo and through transmission ultrasonic c-scan techniques were applied to impacted samples and served as the basis for comparison. Specimens were then inspected using acoustography, a large field ultrasonic inspection technique that is analogous to real-time X-ray imaging. Acoustography utilizes a unique, wide area two-dimensional (2-D) detector, called an acousto-optic (AO) sensor, to directly convert ultrasound into visual images; much like an image intensifier in real-time radiography. Finally, a newly developed guided wave scanning system was utilized to inspect the same set of samples. This system uses two transducers in a pitch catch configuration to examine the total (multi-mode) ultrasonic response in its inspection analysis. Several time- and frequency-domain parameters are calculated from the ultrasonic guided wave signal at each scan location to form images. Results are presented for all of the methods demonstrating each technique's detection capabilities and highlighting their advantages and disadvantages.

Composite materials are widely applied in aerospace, mechanical and civil structures. Delamination of composite material happens due to aging, chemical corruption and mechanical vibration, among other factors. It is important to detect the delamination in the incipient stage before the delamination reaches a notable level. Piezoelectric material can act as both actuators and sensors. In this research, two composite plates are fabricated as test specimen, of which one has a small delamination and the other is healthy. Four PZT patches are bonded at four corners of each composite plate, and one PZT patch is bonded in the middle of the composite plate. Wavelet packet analysis is applied as the signal-processing tool to analyze the sensor data. A damage index is formed based on the wavelet packet analysis to show the existence and the severity of damage. The experiment results show the proposed method can detect the delamination. This sensitive method is suited for delamination detection of inaccessible composite structures without using additional excitation facility.