Showing papers in "Journal of Nondestructive Evaluation in 1997"

Dependence of Ultrasonic Velocity on Porosity and Pore Shape in Sintered Materials

Abstract: A new approach to predict the longitudinal and transverse ultrasonic velocities in porous materials is presented. The model is based on a previously derived Young's modulus-porosity correlation assuming spheroidal geometry of the pores. It is also assumed that the Poisson's ratio of porous materials does not change significantly with porosity. The longitudinal and transverse ultrasonic velocities are given as functions of the Young's modulus, Poisson's ratio, density of the pore-free material and of the porosity and axial ratio (z/x) of the spheroidal pores. Experimental data drawn from the literature on different porous sintered materials including SiC, Al2O3, YBa2Cu3O7−x , porcelain, sintered iron, Si3N4, and sintered tungsten, were used to verify the model. A strong relationship between pore shape and the slope of the ultrasonic velocity–porosity curve was confirmed. In general, the calculated values are in fairly good agreement with the experimental data. When the actual shape (axial ratio) of the pores was known, the approach was shown to predict the experimental data better than a similar model derived by Phani. It is suggested that the present approach, coupled with the measurement of the ultrasonic velocity, may constitute a simple nondestructive technique to gain knowledge of the morphology of the porosity in sintered materials.

Acoustic harmonic generation due to thermal embrittlement of inconel 718

Abstract: This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time.

Acoustic harmonic generation at diffusion bonds

Abstract: The distortion of a sinusoidal acoustic wave at unbonded interfaces has been determined in terms of the first and second harmonic amplitudes. The results demonstrate for the first time that the second harmonic can reach the theoretically predicted maximum value. As also predicted, the harmonic generation efficiency at unbonded interfaces first increases and then decreases with an externally applied compressive load. The technique has been applied to diffusion bonded specimens in an attempt to quantify their achieved strength. As already demonstrated earlier, the energy reflected from such diffusion bonds is also useful to characterize their strength. Indications are that a combination of reflected energy and harmonic generation data could be a powerful tool to quantify the strength of diffusion bonds, particularly those of nearly perfect strength. A strength determination of diffusion bonds by nondestructive evaluation is a necessity for the qualification of such bonds in critical applications.

Improving the Dynamic Range of Real-Time X-Ray Imaging Systems via Bayesian Fusion

Abstract: The performance and reliability of detecting flaws using X-ray techniques are largely conditioned by the dynamic range of the real-time X-ray imaging systems. This paper proposes a software solution to the problem of dynamic range improvement. The idea is to acquire two images of the same object under two different acquisition conditions, and to integrate these two images in order to obtain a more accurate range measurement of signal levels. To do this, a data fusion technique is developed that is based on the Bayesian theory. The Bayesian fusion method is illustrated with the aid of both simulations and examples on real images. The study demonstrates the possibility of improving significantly the dynamic range of real-time X-ray imaging systems using data fusion techniques.

A 3-D Image Detection Method of a Surface Opening Crack in Concrete Using Ultrasonic Transducer Arrays

Abstract: The detection of the size and the location of existing three dimensional cracks in a concrete structure is an important topic in civil engineering In this paper, a multisource, multireceiver method that considers the travel times diffracted by a crack tip is introduced, to backcalculate a 3-D image of the crack tip of a surface opening crack The possible location of the crack tip front is on the surface of an ellipsoid, which is constructed by a fixed travel time length measured from the source to the receiver, by letting the source and receiver points be the foci of the corresponding ellipsoid If the locations of the source and the receiver, together with the associated measured travel time of the diffracted echo between each source-receiver pair are known, the image of the tip can be determined by counting the number of intersections of the ellipsoidal surfaces in an image construction cellular structure The backcalculated crack tip image, as seen from experimental data, match the dimensions of the real crack very well, demonstrating the capability and accuracy of this newly proposed multisource, multireceiver method for concrete NDE

Advanced Image-Processing Techniques for Automatic Nondestructive Evaluation of Adhesively-Bonded Structures Using Speckle Interferometry

Abstract: In conventional optical nondestructive evaluation (NDE) of structures using shearography or electronic speckle pattern interferometry (ESPI), results are typically provided in the form of fringe patterns or deformation contour plots However, in order to fully automate the process of defect detection, it is desirable to obtain simpler results which are easier to interpret We present here one such optical system based on additive–subtractive shearography/ESPI This system processes additive–subtractive fringe patterns and provides the sizes and locations of defects such as disbonds in adhesively-bonded composite structures This is achieved by exciting the structure under inspection using an acoustic stressing mechanism which sweeps a range of vibration frequencies of the structure Since the defective areas of the structure have different mechanical properties from their neighboring regions, varying and complex fringe patterns are obtained at different stressing frequencies We propose an algorithm which enables the automatic identification and selection of relevant additive–subtractive fringe patterns that pertain only to localized deformations associated with defects, and which excludes images that pertain to any overall modes of the entire structure The algorithm also includes a pixel-by-pixel adjustable thresholding scheme which compensates for intensity variations due to nonuniform reflectivity from unpainted and dirty test objects Morphological processing is then performed to extract the shapes of the defect from the processed fringe clusters Various structures, from simple aluminum specimens with simulated defects to a complex honeycomb-based aviation repair patch specimen, have been successfully evaluated using this system

Laser generation and detection of ultrasound in concrete

Abstract: Laser ultrasonic techniques are used to examine the propagation of ultrasonic waves in concrete. This optical methodology provides a repeatable, broad band generation source and an absolute detection system that does not interfere with the process being monitored. The presence of aggregate, in addition to voids and flaws, can cause wave scattering in concrete. Fast Fourier Transform techniques are used to determine the effect of aggregate size and propagation distance on the frequency content of both surface and body waves. This paper examines the scattering of ultrasonic waves in undamaged concrete and establishes the fundamentals for the application of laser ultrasonics for the material characterization and nondestructive evaluation of concrete.

Experimental and numerical investigation of the interaction of Rayleigh surface waves with corners

Abstract: This paper presents the results of an experimental and numerical investigation of the interaction of Rayleigh surface waves with corners. The experimental portion uses a dual probe laser interferometer to measure the interaction of a normal incident Rayleigh wave with corners of varying angles. These incident Rayleigh waves are optically generated with a pulse laser. The numerical procedure uses the frequency domain approach of the boundary element method (BEM) combined with an inverse Fourier transformation to calculate surface displacements. The experimental and numerical results examine the scattered wave fields as a function of corner angle. In addition, the experimentally obtained values are compared with the numerically predicted results and good agreement is obtained.

Laser phased array generated ultrasound for nondestructive evaluation of ceramic materials

Abstract: This paper describes the experimental application of a phased optical fiber array to transmit pulsed laser light to the free surface of Carborundum CS-3 ceramic samples for ultrasonic nondestructive evaluation (NDE). The pulsed laser beams heat the surface of the ceramic rapidly, and can thermoelastically generate longitudinal, shear, and surface (Rayleigh) waves. Without the optical fiber array all three wave modes are generated simultaneously, and no control over directivity or beam-width is possible. The experimental results presented here indicate that the wave modes, beam-width, and directivity of the generated ultrasound can be controlled by using the phased optical fiber array. Experimental directivity patterns for the laser phased array are compared to a simple theoretical model based on Rose’s point source theory. The model predicts shear wave directivities better than either longitudinal or surface waves. This work demonstrates some of the capabilities and limitations of using laser-phased arrays on ceramics.

Using a single transducer ultrasonic imaging method to eliminate the effect of thickness variation in the images of ceramic and composite plates

Abstract: This article describes a single transducer ultrasonic imaging method based on ultrasonic velocity measurement that eliminates the effect of thickness variation in the images of ceramic and composite plate samples. The method is based on using a reflector located behind the sample and acquiring echoes off the sample and reflector surfaces in two scans. As a result of being thickness-independent, the method isolates ultrasonic variations due to material microstructure. Its use can result in significant cost savings because the ultrasonic image can be interpreted correctly without the need for precision thickness machining during nondestructive evaluation stages of material development. Velocity images obtained using the thickness-independent methodology are compared with apparent velocity maps and c-scan echo peak amplitude images for monolithic ceramic (silicon nitride), metal matrix composite and polymer matrix composite materials having thickness and microstructural variations. It was found that the thickness-independent ultrasonic images reveal and quantify correctly areas of global microstructural (pore and fiber volume fraction) variation due to the elimination of thickness effects. A major goal achieved in this study was to move the thickness-independent imaging technology out of the lab prototype environment and into the commercial arena so that it would be available to users worldwide.

Measurement by LASER-generated ultrasound of four stiffness coefficients of an anisotropic material at elevated temperatures

Abstract: Ultrasonic waves are generated through a composite material by means of a noncontact technique It uses a Nd:Yag LASER for the generation and an interferometric probe for the detection of acoustic waveforms From a suitable set of experimental data, an inversion scheme is used for the recovering of four stiffness coefficients They characterize the elasticity in a principal plane of symmetry of the material which exhibits an orthorhombic symmetry The measurements are performed at various temperatures, elevated by steps up to 300°C for two specimen The sensitivity of the method appears convenient to measure the temperature induced stiffness changes The anisotropic degradation of the material properties are then pointed out

Ultrasonic 2-D SH crack detection in anisotropic solids

Abstract: A 2-D SH flaw scattering model in anisotropic media is developed in this paper. The situation to be simulated is the scattering of waves generated by a probe incident upon an infinite strip-like flaw between two different homogeneous anisotropic solids. The scattering problem is solved by combining a 2-D SH probe model with a 2-D SH scattering model for an incoming plane wave. The probe model is based upon the assumption that the traction is known at the probe interface and the flaw is modeled by the so called spring boundary conditions. The solution is obtained by means of Fourier methods. In addition to the displacement field, a probe signal response which is directly proportional to the voltage output in a pulse-echo experiment is calculated. Numerical examples are presented both in the frequency and the time domain for the total displacement field and in the time domain for the probe signal response for two fictitious material configurations. An example where the flaw is a surface-breaking crack is also given.

A Novel Multi-Probe Resistivity Approach to Inspect Green- State Metal Powder Compacts

Abstract: This paper describes a new instrumentation approach to the nondestructive testing of green-state powdered metallurgy components. These samples are likely to generate surface-breaking and subsurface defects prior to sintering. Exploiting the principles of electric resistivity or potential drop measurements in solids, a system is configured which is capable of recording surface voltage distributions due to impressed current inputs. At the heart of this novel testing procedure is a multiple-pin sensor which allows for flexible measurement conditions in order to cover wide surface areas. Practical tests with production samples compare well with both analytical and numerical modeling techniques in predicting surface voltage distributions. Furthermore, initial studies of surface-breaking flaws exhibit excellent defect detectability.

Quantitative Characterization of Corrosion Under Insulation

Abstract: A method is described to quantitatively characterize the corrosion status of steel samples under insulation. The method uses backscatter X-rays to obtain a density vs. depth profile of the sample, from which the mass absorption coefficient, density, and thickness of the rust layer are evaluated. From these data, the iron content of the rust layer is computed, and the steel losses are expressed in either wall thickness or in the mass per unit area. Rust with a thickness of less than 1 mm can be detected but not quantified. The upper limit for quantitative expression of steel losses is approximately 6 mm when an X-ray tube operated at 160 KV is used.

Detection of utility pole rot damage by measuring the reflection coefficient

Abstract: This paper presents a novel nondestructive approach for determining the presence of rot in wood telephone/utility poles carrying telephone and power lines. The method requires determination of the reflection coefficient of the buried end of the pole using physical point measurements such as position, acceleration, or strain. Wave propagation analysis is used to characterize the one-dimensional structural dynamics of the wood pole. This approach permits directional wave components which propagate up and down the pole to be extracted using a previously developed wave filtering scheme.0.2>These sensed directional wave components can then be used to compute the local reflection coefficient of a structural discontinuity based on longitudinal, transverse, or torsional excitation. The reflection coefficient yields information about the location and extent of pole rot in wood utility poles. Temporal data of incident and reflected wave components also permits a visual characterization of the effect of pole rot on the structural response of the utility pole. Experimental results are obtained on two laboratory test specimen excited in the axial direction to confirm analytical predictions.

Optimized Eddy Current Detection of Small Cracks in Steam Generator Tubing

Abstract: A complete, computer based design methodology is described, aiming to develop an eddy current sensor with increased sensitivity to flaws, and reduced sensitivity to probe lift-off The first part of the paper contains an analysis performed in order to establish detailed criteria for an effective design Numerical investigations have been carried out and their results are discussed, regarding various problems of detectability and lift-off noise level Based on these results, in the second part two probe arrangements are proposed, and it is shown how their performance parameters could be further improved

Residual Stress Analysis using Multiparameter Tomographic Reconstruction

Abstract: In this work, the potential application of acoustic tomography to determine the distribution of residual stresses is discussed. Multiparameter reconstruction techniques are presented for both 2-D and 3-D residual stress states along with results from synthetic data. The effect of measurement errors on the accuracy of the reconstruction is also presented.

Magnetic pressure stressing of lap joints: Modeling and experimental verification

Abstract: A novel electromagnetic stressing/optical detection method has been developed in response to the need for better nondestructive evaluation techniques for the detection of disbonds in aging aircraft lap joints. This technique uses magnetic pressure to pull the top surface of a thin conductive bonded sheet and senses the out-of-plane displacement of the surface with an optical lever fiber bundle detector. This method of inspection has the advantages of being noncontacting, relatively inexpensive, and because it pulls on the top surface, is a promising candidate for the detection of “kissing” disbonds—a condition in which there is no material missing from the joint, but the bond has failed. A series of three models was developed and implemented to simulate system performance from the driving circuitry to the measured response of the sample. Using a computer model of the driving circuit, component value variations could be analyzed to optimize the current through the electromagnetic coil as a function of time. An analytical pressure model was developed to predict pressure on the sample as a function of time for a given current waveform input. The predicted pressure was then used as the driving function in a finite element structural model which predicted displacement of the sample surface. Laboratory experiments were conducted on simple bonded and unbonded samples, and the two cases exhibited large differences in amplitude, resonant frequency, and damping. Test results compared favorably to the predicted displacement data. The close correspondence between measured and predicted results indicates that the models are useful not only in the system design but also as a means to predict performance.