Fiber optic sensing for ultrasonic NDE

An innovative approach to nondestructive evaluation (NDE) using noncontacting optical sensors has demonstrated. In this effort a single mode optical fiber interferometer (OFI) was used to sense the presence and form of Rayleigh waves traveling along the surface of a steel test bar at a velocity of nearly 3mm/µs. Acousto-optic time-domain data was successfully used to detect the presence and locate the position of a test flaw (a machined slot) in the bar, and spectrum analysis was used to estimate its geometry and size. This approach has many potential applications in the ultrasonic evaluation of real flaws in structures with complex geometries. Coupled with the authors' earlier work demonstrating the feasibility of generating acoustic waves in metals using laser light pulses transmitted through the fiber optic probes, this latest achievement points to the development of a fully noncontacting, fiber optic based thermal-acousto-photonic (TAP) NDE system, with potential applications to the reliability testing of many important structures where composition, scale, geometry, or restricted access preclude the use of conventional NDE techniques.     

Optimal deployment of multiple passive sensors in the sense of minimum concentration ellipse

In the sense of minimum concentration ellipse, the optimal deployment of multiple passive sensors in bearing-only location system is studied in terms of cut angle. It is found that the optimum cut angle depends only on l, the ratio of the distance between target and the baseline of the two sensors to the baseline length. When l is greater than 0.5, the optimum cut angle can be obtained if the deployment of the target and the two passive sensors is an isosceles triangle with the baseline length as its base side. When l is less than 0.5, the optimum cut angle can be obtained if the target and the two passive sensors are located on the circle with the baseline length as its diameter, that is, the deployment of the target and the two passive sensors being a right triangle. When l equals 0.5, the optimum cut angle can be obtained if the target and the two passive sensors are deployed on an isosceles right triangle. The global minimum value of concentration ellipse can be asymptotically reached by decreasing the ratio l to zero based on these properties of the optimum cut angle.     

Characterization of Capacitive Array for NDE Applications

Abstract

We have developed a capacitive array sensor which responds to the complex dielectric constant of an interrogated material. The sensor requires only single-sided access and operates in a differential mode for detection of discontinuities in the relative dielectric constant, ε _r , or in the absolute mode where the interest is in absolute quantities. The device in general is noncontacting but can be fabricated as an embedded sensor.

Various proof-of-concept studies have been performed to explore possible applications of the device. In the differential mode, small surface features (notches) were detected in a conductor; in dielectrics, both surface and subsurface features were detected. The probe was sensitive to change in impedance caused by a curing epoxy, viewed through a graphite-epoxy composite panel.

A study was performed to assess the use of the probe for noncontact characterization of sintering of ceramics. In this (absolute) mode, the effects of liftoff and dielectric constant must be separated. We propose a scheme based on the ability to multiplex the capacitive array probe to accomplish this. Preliminary investigation shows that the effect of parasitic capacitance between the probe and ground points in the environment must be suppressed.     

Characterization of capacitive array for NDE applications

We have developed a capacitive array sensor which responds to the complex dielectric constant of an interrogated material. The sensor requires only single-sided access and operates in a differential mode for detection of discontinuities in the relative dielectric constant, _r , or in the absolute mode where the interest is in absolute quantities. The device in general is noncontacting but can be fabricated as an embedded sensor.Various proof-of-concept studies have been performed to explore possible applications of the device. In the differential mode, small surface features (notches) were detected in a conductor; in dielectrics, both surface and subsurface features were detected. The probe was sensitive to change in impedance caused by a curing epoxy, viewed through a graphite-epoxy composite panel.A study was performed to assess the use of the probe for noncontact characterization of sintering of ceramics. In this (absolute) mode, the effects of liftoff and dielectric constant must be separated. We propose a scheme based on the ability to multiplex the capacitive array probe to accomplish this. Preliminary investigation shows that the effect of parasitic capacitance between the probe and ground points in the environment must be suppressed.Contribution of the National Institute of Standards and Technology (formerly National Bureau of Standards); not subject to copyright.     

Laser excitation through fiber optics for NDE

This paper describes experiments designed to generate acoustic waves by using a laser pulse, transmitted through fiber optics, to thermally shock the surface of a steel specimen. The purpose of this effort was to explore the noncontacting generation of Rayleigh surface waves appropriate to the interrogation of structures for the detection of subcritical defects, with the ultimate goal of developing an efficient laser-based nondestructive evaluation technique utilizing flexible fiber optics.     

Amplitude-modulated lock-in vibrothermography for NDE of polymers and composites

We present a nondestructive testing method based on lock-in thermography with mechanical heat excitation. Stresses are generated in the sample by vibrating it with a mechanical shaker. The mechanical energy is converted to thermal energy due to the acoustical damping. The defected regions have a stronger damping and also a stress concentration next to them, both of which result in a higher temperature generation. Because of the changes of the thermal properties, the defects also affect the heat conduction. These phenomena result in thermal anomalies due to the defects. The high-frequency vibration used for excitation is amplitude-modulated with a low frequency. The magnitude and phase of the sample temperature with respect to the modulation are measured with an infrared camera and a software lock-in technique. The use of phase information increases the reliability of the defect detection, and the application of high vibration frequencies results in a good thermal signal even at low stress levels, which helps to keep the test truly nondestructive. The suitability of the method was proved with samples of CFRP and aramid composites, and different polymers. The measurements included detection of impact damages, inclusions, voids, and cracks, and the evaluation of stress level distributions, paint thicknesses, and quality of bondings.     

稀土超磁致伸缩换能器的研制及其在桥梁检测中的应用

稀土超磁致伸缩材料是在磁场的作用下能产生巨大伸缩变形的一种新型功能材料，它的应用越来越受到广泛的关注，针对目前大型工程质量检测对声波无损评价系统震源的新要求，研制了稀土超磁致伸缩换能器，该换能器以大功率稀土超磁致伸缩材料作能量转换部件，具有能量集中、发射声波传播距离远、发射频率适中，容易与采集系统集成而实现自动检测控制，显著提高工作效率．最后以某桥梁声波CT无损评价为例说明其广泛的应用。     

Stationary formulas for computing the response in eddy-current NDE

Stationary or variational formulations have been proven to be useful in a number of electromagnetic problems. This paper presents such a formulation for the response in the eddy-current NDE problem. It thus provides an approach to estimating the response due to arbitrary defects in cases where a reasonable first guess for the current distribution responsible for the scattered fields can be obtained.     

Performance evaluation of fiber array for NDE application

This paper evaluates the performance of an optical fiber phased array as a source of ultrasonic generation for nondestructive applications. The results of the shear and longitudinal wave directivity patterns generated with this technique are presented. The signal enhancements produced by increasing the number of fibers in the array are presented for both shear and longitudinal waves. The receiving transducers were two piezoelectric transducers.     

Some computational considerations in eddy-current NDE

Theoretical calculations of eddy-current phenomena often involve the numerical evaluation of various integral expressions. A discussion of some of the possible evaluation methods and of the factors to be considered in choosing a method is presented.     

Ultrasonic signal/image restoration for quantitative NDE

Our overall objective is to improve the spatial resolution of ultrasonic images used for quantitative non-destructive evaluation of materials. These images contain distortion due to the band-pass frequency responses of the associated transducers and propagation paths. Our specific objective is to estimate the impulse responses of flaws and interfaces in material samples. Thus, we are attacking the restoration problem, which is, in general, ill-posed and difficult to solve in practice. We have applied several techniques for regularizing the problem, including both time- and frequency-domain methods, and some constrained optimization algorithms. Our new results demonstrate that regularization techniques can allow successful, practical solutions to this ill-posed problem. Our work concentrates mainly upon the problem in which the impulse response is expected to be impulsive (consisting of a weighted sum of delayed impulses). Our results show that some problems of practical interest are tractable by using a two-step procedure involving first system identification and then spectrum extrapolation to obtain sharper spatial impulses. The paper presents the results of experiments using actual laboratory signals, in which we demonstrate improved spatial resolution for impulsive signals, allowing more meaningful interpretation of ultrasonic A- and B-scans.     

Integrated Planar HTS SQUID Gradiometer for NDE Application

The performance of a SQUID NDE probe with integrated planar SQUID gradiometers was compared to that with an electronic axial SQUID gradiometer. A circular excitation coil was used for the SQUID NDE probe with the planar gradiometer, whereas a differential excitation coil was adopted for the one with the axial gradiometer. These NDE systems are used to detect a buried flaw in aluminum plates, and the phase-depth relations of the buried flaw were found to show similar linear dependences. Overall, the SQUID NDE system with the planar gradiometer is relatively simple and practical for unshielded operations.     

High-power high-resolution pulser for air-coupled ultrasonic NDE applications

This paper describes the design and development of a high-power high-resolution pulser designed especially for air-coupled ultrasonic nondestructive evaluation applications requiring high material penetration capability. The pulser can deliver up to 800 V in a spike excitation type pulse to appropriate low/medium frequency transducers. Moreover, the pulser operates in three different modes: single pulse, double pulse (pulse cancellation technique), and external reference signal, where the last two operating modes are intended to enhance the axial resolution of ultrasonic inspections. Experimental results show that the received pulse duration can be significantly reduced, and reduction factors of 2 or 3 in the pulse duration can be easily achieved.     

Functional composites for sensors,actuators and transducers

Following the trend in structural applications, composite structures are being used more commonly in transducer applications to improve acoustic, mechanical and electrical performance of piezoelectric devices. Functional composite transducers for actuators and sensors generally consist of an active ceramic phase incorporated with a passive polymer phase, each of which has a phase transition associated with it. In this paper, several polymer–piezoelectric ceramic composite transducers, mostly designed for sensing hydrostatic waves, are discussed based on the connectivity of the constituent phases. Also discussed are some recent examples of metal–ceramic composites, and single element ceramic transducers with modified shapes for improved performance. A comparison of these designs is given based on their hydrophone figure of merit (d_h·g_h).     

Pulse-echo microwave imaging for NDE of civil structures: Image reconstruction,enhancement, and object recognition

This article presents an overview of the application of microwave imaging in nondestructive evaluation (NDE) of civil structures. The presentation includes the image formation algorithm operating in monostatic multiple-frequency mode, resolution enhancement by wavefield statistics, and object recognition procedures. Results from theoretical analysis, laboratory experiments, and full-scale field tests are discussed sequentially. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 407–412, 1997     

Configurable electrodes for capacitive-type sensors and chemical sensors

This paper describes a novel design for capacitive sensors or chemical sensors, which features configurable interdigitated electrodes: The electrode spacing can be varied by means of switches on the CMOS chip. This new design allows for performing two capacitive measurements with one single-sensor capacitor so that the number of sensors required to acquire a certain amount of information can be significantly reduced. The use of the same sensor and the same polymer layer for two measurements at a different electrode periodicity provides a better signal quality for the difference signal since detrimental influences, such as humidity and sensor drift, are similar for both electrode configurations and are strongly correlated. Such high signal quality is required for, e.g., the successful recognition of n-octane in the presence of tenfold larger background signals of humidity or, in general, for the determination of low analyte concentrations in humid air. The baseline drift in the concentration predictions based on the differential signal from the two electrode configurations was an order of magnitude lower than that for uncorrelated signals produced by two separate interdigitated capacitors on the same chip. Since the number of required sensors is reduced and, owing to the differential readout of two electrode configurations, reference capacitors are no longer necessary, the overall chip size and/or the number of sensor chips and, consequently, costs can be considerably reduced.     

High-Resolution deconvolution techniques and their applications in ultrasonic NDE

The technique of ultrasonic pulse echo measurement is a very cost-effective method of non-destructive evaluation (NDE). These measurements are, however, usually masked by the characteristics of the measuring equipment and the propagation paths taken by the ultrasonic pulse. With a proper modeling of the pulse echo, these effects can be reduced by deconvolution. In this paper, several deconvolution algorithms that have been popularly used in other areas such as seismic explorations are revised and adapted to ultrasonic NDE applications. Simulation results are presented to support the feasibility of using these algorithms to extract impulse responses from the ultrasonic pulse echoes for use in defect classifications. The performances and computational complexities of these algorithms are analyzed and compared. In addition, application of these deconvolution techniques to B-scan enhancement is also discussed.