Showing papers in "Journal of Nondestructive Evaluation in 1989"

Shearography: A novel and practical approach for nondestructive inspection

Abstract: This paper presents an optical technique referred to as shearography for nondestructive inspection. Shearography is an interferometric method which allows full-field observation of surface displacement derivatives. It reveals flaws in materials by looking for flaw-induced strain anomalies. Both surface and interior flaws can be detected. Because of its numerous advantages, shearography is more practical than holography. The simplicity and the alleviation of vibration isolation requirement make shearography a practical approach for nondestructive testing in field/plant environments.

Measurement of in-plane and out-of-plane ultrasonic displacements by optical heterodyne interferometry

Abstract: An heterodyne optical probe, which permits one to measure out-of-plane and in-plane displacements at the surface of a specimen excited by ultrasound is presented. The principles at the basis of the two modes of operation are explained and the sensitivities for in-plane and out-of-plane detection are analyzed. The optical layout of the probe and the schematic of its demodulation circuitry are presented. Its accuracy is tested with Rayleigh surface waves. Examples of application to laser-generated Rayleigh and Lamb waves are also presented.

Measurement of coating physical properties and detection of coating disbonds by time-resolved infrared radiometry

Abstract: This paper describes the principles and applications of time-resolved infrared radiometric (TRIR) imaging to characterization of coating systems. Examples are given of its application to the measurement of coating properties such as thickness and thermal diffusivity and to the detection of regions of coating disbond. Results are shown for coatings of different thicknesses, for test specimens containing artificial disbonds, and for thermal barrier coating specimens exhibiting real disbonds. A theoretical model describing the time development of the surface temperature of a coating during step heating is presented and the experimental results show good agreement with this model. Methods for applying the technique for inspection of large areas of coating as would be required in a process control or in service inspection environment are discussed and examples of parallel data acquisition using line heating sources are presented.

An electro-optic holography system with real-time arithmetic processing

Abstract: This paper reports the use of a real-time arithmetic image processor in an electro-optic holography system. A speckle interferometer is used to combine an image of an object, lit by laser light, with a mutually coherent reference beam. A CCD TV camera detects the interference pattern, and the phase of the reference beam is advanced by 90° between frames. An image is generated from each set of four sequential TV frames by subtracting alternate frames, squaring, and adding the two results. The result is improved picture quality compared with the use of binary pixels and compared with electronic speckle pattern interferometry. Experimental results are shown.

Porosity estimation using the frequency dependence of the ultrasonic attenuation

Abstract: A new technique is reported for estimating the volume fraction of porosity in structural materials. The estimate for the volume fraction is proportional to the slope of the ultrasonic attenuation when plotted as a function of frequency. Both theory and experiment are considered. The theory, appropriate for dilute porosity, uses the uncorrelated, single-scatter approximation. An “attenuation slope” algorithm is derived within this approximation and its limits of validity are tested by computer simulation. Experimental tests consist of three parts. First, the method is compared with other existing techniques through estimates from the published data on gas porosity in aluminum casts. For the second test, cylindrical porosity is simulated by parallel through-holes drilled in aluminum blocks. Finally, the attenuation in porous graphite-epoxy samples is measured and compared with results predicted from theory.

Recent improvements to the application of the volume integral method of eddy current modeling

Abstract: A previous attempt by Dunbar to model eddy current NDE data using the volume integral equation method produced qualitative agreement with experimental data, but true verification was not possible due to the fact that the results were not independent of the discretization of the flaw and there were numerical problems encountered in calculating the unperturbed electric field. We have overcome these problems by calculating the integrals of the Green's tensor more accurately, especially in the vicinity of any singularities, and by using a fast Hankel transform to calculate the incident electric field. Further improvements were made by considering a linear variation of the electric field, instead of taking it to be constant over each cell used in the discretization. Results obtained from rectangular flaws in a half-space and in a thin plate are presented.

Embedded optical fiber sensors for materials evaluation

Abstract: Optical fiber waveguides embedded within materials have been applied to the measurement of chemical changes, strain, and temperature inside materials. This paper reviews some of the research that has been performed in this area over the past 10 years and cites the theoretical and demonstrated performance of such sensors for material evaluation during its manufacturing, in service lifetime and degradation periods.

Some examples of nondestructive flaw detection by shearography

Abstract: This paper presents some examples of nondestructive flaw detection using an optical method based on speckle shearing interferometry called shearography. In the method, a structure under study is illuminated by laser and imaged by a special image-shearing camera. After suitable processing, a fringe pattern which represents loci of surface displacement derivatives, i.e., strains, is observed in the image. Since defects in structures usually induce strain concentrations around them and since strain concentrations usually cause perturbations on the surface of structures, shearography reveals defects from anomalies in the recorded fringe pattern. In this work, the technique has been applied to the nondestructive detection of various flaws in plain and welded pipes, composite plates, and other engineering components. A simple model was also developed for the estimation of the depth of disbonds in glassfiber reinforced plastic sheets. Results obtained are good and demonstrate the usefulness of the method as a complement to other conventional NDT techniques.

A Numerical Integration Green's Function Model for Ultrasonic Field Profiles in Mildly Anisotropic Media

Abstract: A numerical integration technique utilizing a point source Green's function is introduced to analyze the wave behavior in transversely isotropic-type anisotropic media allowing us to make fast and accurate computations of the acoustic field. The centrifugally cast stainless steel (CCSS) used in nuclear power plants is chosen as a sample medium because of its columnar grain character leading to material anisotropy. A representative number of field profiles are computed and plotted to illustrate the quasi-longitudinal, quasi-transverse, and horizontally-polarized shear wave propagation in a transversely-isotropic medium. Phenomena such as beam skewing, beam splitting, beam focusing, unsymmetrical beams, and other anisotropic effects, some of which are already known from earlier experimental observations, emerge as a computational result of the introduced technique.

Synchronous thermal wave IR video imaging for nondestructive evaluation

Abstract: We describe a system for real-time processing of infrared video images in synchronism with the time-dependence of the target object's temperature. The system can either be used either with periodic or pulsed heating of the target. With periodic heating, the system operates as if it were a collection of lock-in amplifiers, one for each of the quarter of the million pixels of the image. With pulsed heating, it operates as if it consisted of a similar number of box-car averagers. In both cases, the signal-to-noise ratio and temperature sensitivity of the infrared camera are improved. The technique lends itself to a wide spectrum of NDE applications.

Calculation of ultrasonic surface waves from an extended thermoelastic laser source

Abstract: A method is developed to calculate ultrasonic surface waveforms generated by an extended laser source, operating in the thermoelastic regime of laser-pulse energy density. This approach integrates over a suitably weighted distribution of point surface centers of expansion, for observation to within 1 mm of the edge of the source. Power spectra as well as both horizontal and vertical displacements are presented and discussed for ultrasonic waveforms on an aluminium surface, for incident laser pulses having Gaussian lateral profiles of various sizes. Far from the source, the waveform is dominated by a dipolar Rayleigh (R) wave, whose amplitude and spectral content depend on laser spot size. Weak, monopolar pulses also occur at the intersection of bulk pressure and shear wavefronts with the surface (denoted assP andsS, respectively). Close to the source, thesP wave amplitude approaches that for theR wave, and overlaps theR wave for large source sizes. The fall-off with distance for bothsP andR waves is given. Finally, the changes in pulse shape and amplitude are calculated when anR wave from an extended thermoelastic source is reflected or transmitted by a right-angled corner of an aluminium block.

Statistical Pattern Recognition Techniques: A Sample Problem of Ultrasonic Determination of Interfacial Weakness in Adhesive Joints

Abstract: A pattern recognition approach was applied to the analysis of ultrasonic echo signals from two classes of aluminum-to-aluminum adhesive bonds. The two classes differed in the surface preparation of the adherends prior to bonding, resulting in different interfacial properties of the joints. These properties have a crucial effect on the long-term adhesive properties of the specimens. Application of advanced signal processing and pattern recognition techniques enabled the classification of the joints according to the surface preparation of the adherends, based on features extracted from the ultrasonic signals. The statistics yielded an upper bound for the probability of mis-classification of the specimens. The sensitivity of certain features, extracted from the ultrasonic signal, to the interfacial characteristics of the specimens is explained by means of the natural frequencies of a joint's components and surface condition of the adherends. This leads to a method for selecting the optimal probe frequency for carrying out the ultrasonic inspection.

Use of magnetically-saturated regions in remote field eddy current tools

Abstract: The remote field eddy current (RFEC) technique is of potential interest for inspecting pipelines for stress corrosion cracking. Magnetic saturation techniques would allow higher operating frequencies and scanning speeds to be used. The use of selectively-saturated regions (windows) near the exciter and detector offers additional advantages. Finite element, analytic, and experimental measurements are presented in this paper. They show that, while saturation techniques are helpful, the effects are less than initially estimated from simple skin depth approximations.

A domain-extension method for quantitative detection of cavities by infrared scanning

Abstract: A domain-extension method for quantitative detection of irregular-shape cavities inside irregular-shape bodies is presented. An auxiliary problem is introduced in the solution of the cavities. In the auxiliary problem, the original body domain at the cavity side is extended so that the original cavity walls become interior points. The position of the cavities can then be found by solving the temperature field in the extended domain and matching the temperatures and heat fluxes at the interior points to the conditions imposed on the cavities. A boundary-element method is used for the solution of the auxiliary problem, and by means of four examples, the accuracy of the domain extension method is established. The paper provides the details for the numerical solution of the cavities. Limitation of the method in the detection of multiple cavities is also explored. The domain-extension method has shown to be highly effective in quantitative detection of cavities in single-cavity bodies.

Image-enhanced backlighting: A new method of NDE for translucent composites

Abstract: A quick and inexpensive method of nondestructive evaluation is described based on imagenhanced backlighting. The method is capable of being applied to translucent composite materials such as Kevlar/epoxy and glass/epoxy to detect delamination with high resolution. The use of an enhancing dye also makes the method suitable for monitoring surface cracking. The technique has also been demonstrated to be an especially convenient method of observingin situ the growth of cracking and delamination damage as a composite is loaded, and is capable of monitoring thedynamic growth of damage in impact-loading situations. It has been shown to be appropriate for detecting barely visible damage resulting from either impact or static loading on thick laminated structures that even include honeycomb reinforcement.

Acoustic emission from crack growth in an advanced zirconia refractory under thermal shock

Abstract: Crack initiation and growth during the thermal shock tests of a partially stabilized zirconia advanced refractory were investigated by the analysis of acoustic emission (AE) amplitudes. The growth of cracks that were detected by AE was systematically monitored by SEM observations as increasingly severe thermal shocks were applied. The measurements of strength loss after thermal cycling in the ribbon test with various applied temperature differentials correlated with continuous monitoring by acoustic emission and confirmed the effects of microcrack growth on the resistance to thermal shock damage.

Nondestructive low-cycle fatigue characterization of multi-layer thin film structures

Abstract: A multi-layered thin film structure (namely, electrodeposited Cu/sputtered Cr/Kapton substrate/sputtered Cr/electrodeposited Cu), utilized as a flexible component for computers, has been exposed to fatigue. Although a standardized testing method for fatigue ductility is available for a solid monolayer of electrodeposited foil, there is no method available for examining such a multi-layered thin film structure. In this study, four different methods were employed to characterize the low-cycle fatigue damage: (1) DC resistance measurement, (2) residual stress development by x-ray diffraction, (3) dislocation density calculation by using obtained x-ray diffraction line profiles, and (4) microscopic observations. Low-cycle fatigue was conducted at eight levels of applied total strain, i.e., δe T =13.95%, 7.69%, 5.83%, 4.69%, 3.37%, 2.37%, 1.59%, and 1.19%. The number of fatigue cycles, when the crack was first observed on the outer Cu layer, was identical to that observed with the onset of increased resistance. This cycle number is thus designated as the number of cycles-to-fatigue crack initiation,N c . AtN c , the residual stresses also show a noticeable relaxation, and the dislocation density shows a remarkable increase. IfN c is plotted against the applied total strain amplitudes, a Manson-Coffin's relationship is obtained with an exponent of 0.39. It is recommended that monitoring the continuous changes in DC resistance could provide a reliable nondestructive evaluation of low-cycle fatigue life of a multi-layered thin film structure.

An iterative calculation of eddy-current responses due to three-dimensional defects

Abstract: An iterative technique utilizing a volume integral formulation and a stationary expression for the eddy-current response is discussed and applied to defects of spherical and cylindrical geometry in a conducting half-space. The computations converge in one to four iterations and give known results for small spherical defects. The technique is computationally stable and will efficiently handle defect dimensions at least up to several skin depths. The air-metal interface is considered by means of image theory.

Automatic ultrasonic defect locating and sizing by a probabilistic approach with high accuracy

Abstract: Based on dynamic Time-Of-Flight Measurement with scanning transducers, a probabilistic approach has been proposed for automatic defect locating and sizing. This approach significantly reduces the inconsistent effects of human factors on results. The probabilistic approach suppresses the effects of random noises as well as measurement errors and makes possible the identification and elimination of spurious indications from mode-converted waves. No prior knowledge of an unknown defect is required to obtain a sizing image for the distribution of the unknown defects under a scan line. The probabilistic approach has been applied to the inspection of three testblocks, which include different types of defects, with the location and sizing accuracy as expected.