Showing papers in "Journal of Nondestructive Evaluation in 1986"
TL;DR: In this paper, the authors apply geometrical ray theory to the calculation of the surface displacements generated by point force and thermoelastic sources of ultrasound in plates containing planar defects.
Abstract: This paper applies geometrical ray theory to the calculation of the surface displacements generated by point force and thermoelastic sources of ultrasound in plates containing planar defects. The calculation includes direct wave arrivals, waves undergoing back-wall reflection with or without mode conversion, and waves diffracted by the crack-tip. Ultrasonic B-scan data are also simulated so that comparison can be made with experimental data. It is shown that the thermoelastic source, which can be generated by a pulsed laser, is particularly well suited to defect detection by the ultrasonic time-of-flight technique.
20 citations
TL;DR: In this article, the effects of the impact load on attenuation and stress wave factor were measured after different number of impacts, and it was found that the reduction in fatigue life is proportional to sudden changes in attenuation.
Abstract: Fiber-reinforced composites, because of their superior specific strengths and stiffnesses, are used in many aircraft components. However, in this application these composites are subjected not only to fatigue loading, but to occasionally high velocity impact due to the bird injection, hail, dust, and rain. Thus, it is important to evaluate the residual life and degradation due to combined fatigue and impact loadings. Unidirectional graphite epoxy composites (MA8276-Tiger) which are used in the aerospace industry were impacted by a free falling weight at energy levels of 0.567j, 1.134j, and 1.571j [impact energy toughness (j/cm3); 0.12, 0.24, 0.34], respectively. The subsequent changes/degradation in elastic moduli, strength, toughness, and fatigue properties were measured after different number of impacts. It was found that for all energy levels these properties vary linearly with the number of impacts. Furthermore, attenuation changes is not a good ultrasonic parameter for degradation estimation, since it does not incorporate the micro- and macrocracks beyond the impact point. However, these micro- and macrocracks have significant effect on the mechanical properties. In contrast to the attenuation, the stress wave factor, which indicates the efficiency of wave propagation along the specimen, correlates very well with degradation, and it can be used effectively to measure the residual strength after impact. Ultrasonic characteristic on specimens subjected to combined fatigue and impact were also studied. Based on these experiments, it is concluded that the loss in fatigue residual life due to impact loads may be predicted by measuring the effects of the impact load on attenuation and stress wave factor. It was found that the reduction in fatigue life is proportional to sudden changes in attenuation and stress wave factor. Damage accumulation models based on Coffin-Manson equation, was suggested for impact and combined fatigue and impact. It was found that residual properties and fatigue life can be estimated from these models.
13 citations
TL;DR: In this paper, a modification of the null field approach is adapted to the study of scattering of elastic waves by fluid-filled nonplanar cracks, where a closed surface is formed by adding a fictitious surface, on which latter surface boundary conditions of welded contact are applied.
Abstract: A recent modification of the null field approach is adapted to the study of scattering of elastic waves by fluid-filled nonplanar cracks. The fluid-filled crack is modeled as a surface over which friction-free boundary conditions apply. A closed surface is formed by adding a fictitious surface, on which latter surface boundary conditions of welded contact are applied. The surface fields on the closed surface are expanded in vector spherical harmonics in a manner which takes the edge conditions into account. Some numerical results on farfield quantities, such as scattering cross sections and backscattering amplitudes (both in the frequency and time domains), are presented for rotationally symmetric cracks.
8 citations
TL;DR: In this paper, the scattering of ultrasonic Rayleigh waves incident normally on corners containing cracks is considered by using elastodynamic ray theory and detailed calculations are presented for vertical and horizontal cracks in right-angle corners in aluminium.
Abstract: The scattering of ultrasonic Rayleigh waves incident normally on corners containing cracks is considered by using elastodynamic ray theory. Detailed calculations are presented for vertical and horizontal cracks in right-angle corners in aluminium. It is shown that crack depth can be measured simply from the spacing of interference fringes in the high-frequency spectra of either the back- or forward-scattered Rayleigh waves, given only a knowledge of the Rayleigh wave speed. Use of the back-scattered wave is preferable because its fringes show stronger modulation, and because an experiment requires a single transducer and access to only one face of the specimen. The technique is applicable without modification to the more general case of a crack at any angle in a corner of any angle.
8 citations
TL;DR: In this article, an unfolding method for cracks in plane surfaces is extended to provide an approximate mathematical model for the perturbations produced by a symmetrical corner crack when interrogated by a surface electric field, disregarding the nonuniform nature of the current distribution upstream of the crack which is produced by the presence of the corner.
Abstract: An unfolding method, developed earlier for cracks in plane surfaces, is extended in this work to provide an approximate mathematical model for the perturbations produced by a symmetrical corner crack when interrogated by a surface electric field. The model disregards the nonuniform nature of the current distribution upstream of the crack which is produced by the presence of the corner, and this simplification leads to unfolded field problems of plane Laplacian form. The Schwarz-Christoffel transformation is applied here in their solution. Experimental measurements on slots cut on steel blocks to simulate corner cracks are found to be in good agreement with the predictions of the model.
4 citations
TL;DR: In this paper, an array of eight piezoelectric transducers was used to detect acoustic emission and to receive velocity measurement pulses; the use of the velocity data to locate the acoustic emissions was discussed.
Abstract: A technique for locating acoustic emission in brittle rock specimens is described. An array of eight piezoelectric transducers was used to detect acoustic emission and to receive velocity measurement pulses. Axial and transverse velocities were calculated from the transmission data; the use of the velocity data to locate the acoustic emissions is discussed. Waveforms were recorded for about 140 acoustic emissions during the deformation of the Westerly granite specimen and about 100 acoustic emissions were located. The effect of transducer size and incident angle on apparent arrival times is evaluated using a lead zirconium titinate transducer bonded to a small brass rider with a curved surface matched to the brass specimen. The relation between location accuracy in various regions of the specimen and the geometry of the receiver array is studied. It is observed that for incidence angles away from the normal, the velocity appears to be higher than for normal incidences.
3 citations
TL;DR: In this paper, the authors describe the design and construction of a concrete strength ultrasonic digital meter, which relies on the transmission and reception of two ultrasonic sinusoidal signals.
Abstract: This paper describes the design and construction of a concrete strength ultrasonic digital meter. The method depends on the transmission and reception of two ultrasonic sinusoidal signals. These two signals are so processed so as to yield a digital output number representative of the speed of propagation through the concrete specimen. This velocity figure can be related to the applications on finding the concrete crush strength, concrete crack depth, as well as the concrete behavior with electrical curing temperature. The electronic system employs Complementary-Oxide Semiconductor logic circuits and hybrid conversion circuits. It consumes such a little power, which makes such a design transportable and hence a battery operated device.
2 citations
TL;DR: In this article, a method based on a few simple hypotheses on the shape of the defect and the reaction of the probe which permits the extraction of information sufficient enough for classification was proposed.
Abstract: As a nondestructive testing, eddy currents are largely used by industry. Nevertheless, complete automatization of the devices are still unusual, especially those that are able to make a classification of the different kinds of defects likely to appear in the tested material. We propose a method based on a few simple hypotheses on the shape of the defect and the reaction of the probe which permits the extraction of information sufficient enough for classification. The analysis of the “image” of the defect which is obtained by the scanning of the tested piece involves two steps: (1) an analysis of the orientation and the shape of the defect and (2) the analysis of the relief of this image, which determines the position of significant features. The method has been tested on a given sample of population with a 80% rate of success.