Showing papers in "Journal of Nondestructive Evaluation in 1981"

Eddy current probe response to open and closed surface flaws

Abstract: A general theory of eddy current flaw response, appropriate to both standard low frequency and ferromagnetic resonance (FMR) probes, is developed for simple two-dimensional and three-dimensional open and closed surface flaw geometries. This analysis, based on the assumption of a uniform interrogating field applied to the flaw by the probe, shows that flaw opening responses increase with the operating frequency of the probe. Experimental results using both types of probe confirm this result for realistic practical geometries, where variations of crack mouth opening displacement under load provide useful information about crack dimensions.

Acoustic emission and inclusion fracture in 7075 aluminum alloys

Abstract: For slow crack growth (da/dn ≈0.1 µm per load cycle) in 7075-T6 aluminum, quantitative agreement was found between the amplitude distribution of burst acoustic emissions and the area size distribution of intermetallic inclusions ≈10 (µm)2 in area as measured on thepolished fracture face. This observation permits the prediction of the amplitude distribution of acoustic emissions due to crack growth in a particular sample of 7075-T6 aluminum directly from a simple, standard metallographic observation performed on the material in question. It was also found that a reduction of the yield stress from that of 7075-T6 aluminum (503 MPa) to that of 7075-0 aluminum (103 MPa) completely eliminated burst acoustic emission activity due to crack advance in the amplitude range studied.

Surface acoustic wave measurements of surface cracks in ceramics

Abstract: An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.

Potential and limits of holographic reconstruction algorithms

Abstract: For the purpose of ultrasonic nondestructive testing of materials, holography in connection with digital reconstruction algorithms has been proposed as a modern tool to extract crack sizes from ultrasonic scattering data. Defining the typical holographic reconstruction algorithm as the application of the scalar Kirchhoff diffraction theory to backward wave propagation, we demonstrate its general incapability of reconstructing equivalent sources, and hence, geometries of scattering bodies. Only the special case of a planar measurement recording surface, that is to say, a hologram plane, and a planar crack with perfectly rigid boundary conditions parallel to the hologram plane and perpendicular to the incident field yields a nearly perfect correlation between crack size and reconstructed image; the reconstruction algorithm is then referred to as the Rayleigh-Sommerfeld formula; it therefore represents the optimal case matched to that special geometrical situation and, hence, may be interpreted as a quasi-matched spatial filter. Using integral equation theory and physical optics, we compute synthetic holographic data for a linear cracklike scatterer for both plane and spherical wave incidence, the latter case simulating a synthetic aperture impulse echo situation, thus illustrating how the Rayleigh-Sommerfeld algorithm or its Fresnel approximation increasingly fail for cracks inclined to the hologram plane and excited nonperpendicularly. Furthermore, we point out how the physical data recording process may additionally influence the reconstruction accuracy, and, finally, guidelines for a careful and serious application of these holographic reconstruction algorithms are given. The theoretical results are supported by measurements.

A new digital correlation flaw detection system

Abstract: A new portable digital random signal flaw detection system is described which uses a digital delay line to replace the acoustic delay line of the original random signal system. Using this new system, a comparison was made between the two types of transmit signals which have been used in previous systems—m-sequences and random signals. This comparison has not been possible with these previous correlation flaw detection systems. Results indicated that for high-speed short code operation, the m-sequences produced slightly lower range sidelobes than typical samples of a clipped random signal. For normal long code operation, results indicated that system performance is essentially equivalent in resolution and signal-to-noise ratio using either m-sequences or clipped and sampled random signals. Further results also showed that for normal long code operation, the system produces outputs equivalent in resolution to pulse-echo systems, but with the added benefit of signal-to-noise ratio enhancement.

Static finite element stress intensity factors for annular cracks

Abstract: Results of finite element static stress intensity factor calculations for an annular crack around a spherical inclusion (void) are presented and compared with those from approximate analytical methods.

Spherical wave decomposition approach to ultrasonic field calculations

Abstract: A simple, flexible, accurate, and comprehensive numerical method is presented for theoretically analyzing the diffraction field of a continuous wave transducer of arbitrary size, shape, and frequency. Using the extensively studied circular transducer for comparison, numerical results are shown for an unfocused transducer with uniform velocity excitation as well as for a focused transducer with Gaussian velocity excitation. Data concerning the execution time, program size, and convergence of the method are also presented for its implementation as a design tool on a minicomputer system.

Parasitic voltages induced by artificial flaws when measured using the ac field technique

Abstract: The development of a successful and accurate instrument for measuring surface-breaking cracks in metals using the ac field technique has raised several interesting theoretical problems. Measurements with the instrument, known as the Crack Microgauge, do not rely on any prior calibration against an artificial flaw such as a saw-cut in a test block, but some users accustomed to such a calibration from other devices have nevertheless wished to use the instrument in that fashion and have in some instances reported erroneous results. In this paper, we develop a simple theory to explain and quantify this phenomenon. We calculate the parasitic voltages induced in the instrument probe due to the finite opening possessed by an artificial flaw, and we use these results to reinterpret the instrument readings. Controlled experimental measurements on artificial flaws of rectangular cross-section made in aluminum and in steel are found to be in good agreement with the theory. It is shown, however, that application of the theory requires additional information about the internal phase shift associated with the instrument. To enhance the accuracy of the theory, the effect of the corners of the artificial flaws was also considered, although it was not very influential in this work.

Strain Measurement by Optical Correlation

Abstract: The application of optical correlation measurements to evaluating elastic strain and plastic strain in metals is discussed in this paper. A selected area of the metal surface is illuminated by coherent light. The optical correlation intensity is then measured by transmitting light scattered from the surface through a holographic filter, in which information about the topography of the surface at an earlier time is stored. Changes in surface topography arising from rigid-body displacement, elastic strain, or plastic strain, respectively, cause corresponding changes in optical correlation intensity. Correlation changes arising from surface translation or rotation can be compensated for. An analysis of the process of holographic reconstruction from an elastically strained surface gives good agreement with the experimental results. The correlation technique is sensitive to elastic strains of the order of 10−5 and to monotonic plastic strains of the order of 10−4; the change in correlation intensity is essentially linear with increasing plastic strain, up to a maximum strain of about 10−3.

Propagation of elastic waves in honeycomb panels for application to rapid inspection techniques

Abstract: This paper considers the properties of elastic waves guided by an aluminum plate bonded to a honeycomb core for application to rapid inspection of honeycomb panels. Current acoustic inspection techniques involve the transmission of a signal between a pair of small transducers located on opposite sides of the panel. Scanning the transducers in raster fashion results in a high resolution inspection of the panel, but is very time consuming. An alternative technique would simultaneously inspect all points along a line between two widely spaced transducers located on the same side of the panel. Scanning the pair once over the panel permits rapid inspection, although with decreased resolution. Studies presented here indicate that such a method of inspection is feasible and that the flexural mode is probably the most useful.

Ill-posed and well-posed problems in inverse elastodynamic scattering for nondestructive evaluation

Abstract: The practical significance of ill-posedness in a data reduction problem is reviewed. Inverse elastodynamic scattering is shown to be ill-posed in general, although suitably restricted problems may be well-posed. These results underscore the need to analyze carefully the errors of data reduction problems in NDE, and to focus attention on final results of an NDE exercise, rather than on intermediate steps.

Detection of strength limiting defects in cellular glasses by dielectric measurements

Abstract: The foamed glasses that are being considered for structural support in solar thermal energy system silvered glass concentrator mirrors are assessed for strength-limiting defects by a nondestructive technique which measures the capacitance of well defined regions. The feasibility of locating large defects is demonstrated for the case of one type of cellular mass. Observed capacitance variations are believed to be due to small voids or other irregularities in the material. If the defects are equal to or greater than approximately 20 times the glass matrix pore size, they can be resolved with sufficient accuracy to delineate their spatial extent.

Digital ultrasonic image construction using electronic ordered dither techniques

Abstract: The use of a digital microprocessor based system for the acquisition of ultrasonic C-scan information is investigated. The C-scan information is displayed on a binary display device using electronic ordered dither techniques to represent gray levels. The digital system with a binary display increases system flexibility and yields better reproducibility and constant image quality independent of the display medium. Images may be stored on magnetic tape or disk for later retrieval and image processing. The techniques which are described allow for image magnification and a reduction in scan time by replacing the mechanical linkage between the scanner and the display with digital signals. A detailed comparison is made between two dither signals, and the advantages of each are discussed. The combination of digital signal processing and imaging techniques produces results which utilize the capabilities of ultrasonic inspection to the point where the transducer becomes the limiting factor.

Rayleigh wave propagation in a solid with a cold-worked surface layer

Abstract: The propagation of the Rayleigh surface wave is experimentally studied along the top surface of used railroad rail under conditions where ultrasonic pulses have carrier frequencies ranging from 0.4 to 3.0 MHz and approximately 10 µs duration. The generation of the first higher (M21 or Sezawa) mode as well as the fundamental (M11) mode and their dispersion properties are observed. These phenomena are attributable to the presence of the cold-worked surface layer caused by the wheel passage. It is shown that a theoretical model of a single layer overlying a half space, whose elastic constants are determined by a destructive method, yields results which agree with the dispersion curves obtained experimentally. On the basis of this one-layered model, an inversion method to estimate the layer thickness and its elastic constants is discussed.

Resonance testing of cast iron

Abstract: An outline is given of the existing nondestructive vibration tests for cast irons and their drawbacks in terms of the dimensional accuracy of the components. A proposal is made for a new test based on the change of resonant frequency with vibration amplitude exhibited by cast irons. This proposal is evaluated using material data obtained from dynamic tests of a wide range of cast iron specimens. A test rig is described suitable for testing cylindrical cast bars, and typical results are presented in relation to material strength. There is also a brief description of preliminary testing on cast products.

Love wave propagation in a solid with a cold-worked surface layer

Abstract: In this paper, experiments show Love wave generation along the top surface of used railroad rail, where the shear wave velocity has been slightly reduced by the cold-working of wheel passage for years. The rf pulses used in the experiments have about 10 µs duration and a relatively narrow frequency spectrum. The group velocity of the Love wave is found to have a strong dependence on the carrier frequency over the tested range of 0.45–3.1 MHz. Application of the seismological one-layered model to the experimental measurements yields an NDE technique for the elastic properties and the thickness of the cold-worked surface layer. The results are interpreted on the basis of a destructive observation by micro-Vickers hardness testing.

Note on ultrasonic tomography

Abstract: Ultrasonic tomography is being explored as a potential tool to map residual stresses in solids. The angular scanning required in tomography imposed limitations on the stress configuration adaptable to such a treatment. These are discussed in the following note.

Effects of sinusoidal crack-face perturbations on scattering of elastic waves

Abstract: The scattering of a plane longitudinal wave from a two-dimensional crack, with a sinusoidal surface perturbation whose amplitude and wavelength are much smaller than the length of the crack, is investigated. The amplitude of the cylindrical body waves in the far field are calculated from a Kirchhoff approximation that utilizes the solution to the reflection from the sinusoidal surface profile of a semiinfinite solid. The results are compared to those for a flat crack, and conditions for significant differences of the amplitude as a function of the angle of observation are discussed. Characteristic changes in the scattered field produced by profiles with different amplitudes and periods are explained.

Treatment of false calls in evaluating nondestructive inspection proficiency

Abstract: A test designed to evaluate nondestructive flaw detection proficiency by ultrasonic and eddy current methods has been conducted. Test data for each participant are reduced to contingency table format, and analysis of these tables via traditional measures of association is used to rank individual performances. The presence of false calls in an inspection record introduces a random inflation in flaw detection results. Measures are required which compensate for that fact. Proficiency rankings based on two such measures, Somers'd coefficient and the mean square contingency coefficient, show reasonable consistency with intuition-based rankings by experienced observers. Finally, a weighted generalization of Somers'sd is suggested, although optimal selection of weights remains an open problem.

SOL-X: An on-line nondestructive method for measuring grain size with x-rays

Abstract: It is shown that the grain size of commercial materials can be estimated by x-rays with little or no sample preparation. The method is readily automated and can be employed on-line, for example, in a rolling mill.

Computerized evaluation of acoustic images utilizing a method of averaging

Abstract: A computerized technique is presented which has been developed for analyzing acoustic images of an inhomogeneous material. The acoustic imaging system is briefly described with emphasis on its adaptation to billets of graphite for atmospheric reentry body use. Ultrasonic images of such inhomogeneous material contain a nonuniform distribution of anomalies and few, if any, geometrically regular features which can be assessed to establish material integrity. Results obtained with our new algorithms show, for certain discriminants, a high degree of correlation between the numerical accept/reject values obtained from acoustic image processing and the time-to-failure of the graphitic material in simulated atmospheric reentry tests.

Volumetric inspection of moderately thick austenitic stainless steels by multifrequency eddy currents

Abstract: This paper describes the initial phase of a project to develop eddy-current methods to inspect welds joining sections of austenitic stainless steel pipe having walls up to 13 mm (0.5 in.) thick. The objective of this phase was to demonstrate the feasibility of detecting and characterizing flaws in austenitic stainless steel base metals. These materials and welds present challenging eddy-current problems because of their relatively large thickness and ferromagnetism. Multiparameter analysis shows that a reflection coil probe operated with three discrete driver frequencies and phase detection can locate and size a cracklike defect in a single conductor in the presence of variations in conductor resistivity, permeability, and thickness and in the probe-conductor spacing (liftoff). Experiments were performed with a modular three-frequency instrument. Flat-plate specimens of types 304L and 347 stainless steel machined to 12.7 to 15.9 mm thickness simulated pipe walls; saw-cut slots 10 to 30% of nominal specimen thickness simulated cracklike defects. The same slots were used in duplicate experiments as near-side (directly under the test probe) or far-side (in the face opposite the probe) defects. Flaw detection and characterization capability was demonstrated by a series of experimental measurements fitted to specimen properties by least squares techniques. The quality of the fit determined the expected accuracy of measurement. Comparison of accuracy estimates determined the best choice of operating frequencies. From the 1,2,5 sequence of frequencies between 0.5 and 20 kHz, the optimum set of operating frequencies was selected to be 0.5, 2, and 10 kHz. Estimates of measurement accuracy for combined near- and far-side defect cases were: plate thickness, 0.74 mm; probe liftoff, 0.03 mm; defect location (depth of material above defect), 3.48 mm; and defect size (vertical slot depth), 1.09 mm. A few property values were back-calculated from instrument readings; the errors in these values were somewhat larger than the measurement accuracy estimates because of instrument drift and the absence of calibration circuits.

Nondestructive testing of materials using 14 MeV neutrons and a pulse-shape analyzer system

Abstract: A 14 MeV neutron generator and fast neutron spectrometer consisting of an organic scintillation detector and a pulse-shape analyzer are used for the nondestructive testing of materials. The 14.6 MeV peak (in the 12.7–15.7 MeV region) is observed to change as a function of both the kind of testing material and the defect condition inside the material. The number of neutron counts under the 14.6 MeV peak for lighter testing materials (e.g., plastic, brass, and aluminum) with a defect will increase, as compared to the number of neutron counts under the same peak region of an identical testing material but with no defect. For heavier testing materials (e.g., steel and lead), the inverse is observed. The method is feasible even with defects of size 0.25 cm3 (i.e., a cylindrical cavity 0.4 cm in diameter and 2.0 cm in length), although quantitative evaluation of the size of the defect as a function of the change of the number of neutron counts under the 14.6 MeV peak region is difficult due to instability of the 14 MeV neutron output.

Interior imaging using spatial frequency sampling

Abstract: A new acoustic synthetic aperture geometry is demonstrated in which the image field is sampled in the spatial frequency domain. This means the sampled field is recorded in a particularly convenient form for presentation to the back propagation algorithms used to reconstruct the field at the object plane. The method has applications in interior visualization. It is difficult to image the interior of solid objects using lens based imaging systems because a different lens geometry must be used for each distinct object material. The system presented here overcomes such problems since both the angular variation of the transmission coefficient at the object-water interface and the aberrations introduced by the velocity mismatch at the object surface may be readily compensated for in the back propagation routine. Experimental results are presented illustrating the detection of four half wavelength diameter defects, spaced by three wavelengths, at a depth of eight wavelengths below the surface of an aluminum block.

Application of metal wedges to wave propagation

Abstract: Techniques for propagating Rayleigh waves with frequencies varying from <1 to 270 MHz were developed for the purpose of nondestructive evaluation of composition or defect gradients near the surface of nonpiezoelectric solids. The surface waves with frequencies greater than 15 MHz were obtained from a novel technique using single crystal wedges, whereas the low frequency waves were produced by a previously used high voltage impulse. Measurements of Rayleigh wave velocities on single crystals of Ge and Al were performed as a test of accuracy and precision and are compared with calculated values.