Showing papers in "Journal of Nondestructive Evaluation in 1984"

Ultrasonic scattering from imperfect interfaces: A quasi-static model

Abstract: A quasi-static model for the ultrasonic transmission and reflection at imperfect interfaces is developed. The interface is represented by a distributed spring, determined by the change in static compliance of the medium with respect to one with a perfect interface, and a distributed mass, representing excess mass at the interface. Comparison of the model predictions to exact solutions for two simple cases illustrates its accuracy at low frequencies. The spring stiffnesses can be derived from existing solutions for the elastic displacement of materials containing cracks and inclusions under static load. Results for a variety of cases are reviewed. Applications of the model to study the characteristics of partially contacting surfaces in several problem areas of current interest are discussed.

The effects of focusing and refraction on Gaussian ultrasonic beams

Abstract: A scalar theory of the propagation of Gaussian ultrasonic beams through lenses and interfaces is presented. For radiation into a fluid, the Fresnel approximation is employed to derive the laws of propagation of Guassian beams (previously employed in the analysis of coherent optical systems). These are then generalized to situations commonly found in nondestructive evaluation by treating the effects of propagation through lenses and through curved interfaces at oblique incidence. A numerical example illustrates the ease with which insight into diffraction phenomena for complex geometries can be gained by this approach. The limitations imposed on the theory by aberrations and the scalar assumption are discussed, and the relationship of the Gaussian theory to the radiation of piston transducers is explored.

The interaction of ultrasound with contacting asperities: Applications to crack closure and fatigue crack growth

Abstract: The partial contact of two rough fatigue crack surfaces leads to transmission, reflection, diffraction, and mode conversion of an acoustic signal at those contacts. This paper reviews recent experimental and theoretical efforts to understand and quantify such contact on actual fatigue cracks in greater detail. It is shown that the size and density of individual contacts, or asperities, can be estimated from acoustic measurements. Furthermore, it is shown that this information is useful to provide the static stress across a partially closed crack as well as the “effective” stress intensity range which activates fatigue crack propagation.

Numerical modelling of the scattering of elastic waves in plates

Abstract: This paper describes the application of finite difference methods to the calculation of the scattering of elastic waves. The emphasis is on cracklike defects in plates, and it is shown that a common numerical technique can span a range of wavelengths from Lamb waves to ultrasonic waves with many reflections from the surfaces of the plate. Quantitative results are given for the scattering of Lamb waves and ultrasonic shear waves from surface-breaking cracks.

Scattering of ultrasound in solids as visualized by the photoelastic technique

Abstract: Our efforts in the past few years to visualize the scattering of ultrasound in solids by the photoelastic technique are briefly reviewed. Photoelastic photographs are presented showing the dynamic processes of scattering in glass of plane longitudinal or transverse waves by a cylindrical cavity or cavities as well as by two-dimensional surface-breaking or internal cracks. Phenomena like creeping and multiple scattering are clearly seen, in addition to some details which are not predicted by known theories.

Distributed strain measurement in a rectangular plate using an array of optical fiber sensors

Abstract: Single mode optical fiber waveguide has been used to determine the two-dimensional strain distribution on a simply supported rectangular plate. Each of the fifty individual fibers in the rectangular grid array attached to one surface of the plate yields a measurement of the strain integrated along the length of that fiber on the specimen. By using similar sensor information from all of the fibers, both the functional form and the amplitude of the distribution may be determined. Limits on the dynamic range and spatial resolution are indicated. Applications in the measurement of internal strain and the monitoring of physically small critical-structural components are suggested.

An inverse problem technique for detecting irregular cavities in circular cylinders by infrared scanning

Abstract: An inverse problem technique has been developed for detecting irregular cavities in circular cylinders. In this method, the cavity is considered a part of the unknown geometry of the investigated system, and the evaluated temperature is used to locate this geometry. An auxiliary problem is introduced in the solution of this problem; and in the solution, the cavity wall is located by forcing the temperature to satisfy the condition imposed at the cavity. The new methodology is validated by an experiment presented in this paper, and the test results indicate that this method is highly successful in locating cavities. The accuracy of the method is closely related to the accuracy of the temperature that can be measured at the surface. A small error in the surface temperature results in a slight cavity error for deep cavities, while a shallow cavity is not severely affected by a surface temperature error. This method is particularly attractive in detecting shallow cavities in nondestructive evaluation.

Analytical solution for impedance change due to flaws in eddy current testing

Abstract: Green's function is used in order to derive the analytical solution for the change in impedance due to a presence of the flaws in a conductor. This solution is applied to a cylindrical flaw and a spherical flaw whose radii are much smaller than the radius of the test coil. For both cases, the change in impedance is obtained within Born's limit.

Ultrasonic reflection from a stressed interface

Abstract: In seeking to develop an NDE technique for verifying the serviceability of NiTi couplers for plumbing unions, it was found that the reflection coefficient for a normally incident ultrasonic wave correlated with the stress level at the NiTi-tubing interface. Current theory pertaining to interface reflection is not complete but, even at present status, there is semiquantitative accord between theoretical predictions and normalized data for reflection coefficient versus stress level. These results are at fixed frequency and with comparable surface roughness. Variation of either frequency or surface roughness further affects the reflection coefficient. Current studies are being pursued to define this effect.

Separation and characterization of stress levels and texture in metal sheet and plate: I. Principles and preliminary experiment

Abstract: An approach is proposed for obtaining separate measures of stress anisotropy and texture in metal sheet or plate. The approach is based on the propagation characteristics ofSH n modes of ultrasonic waves. Experimental work has established that differences betweenSH 0 modes with orthogonal directions of propagation provide a measure of stress anisotropy that is insensitive to texture, with the restriction that the pseudosymmetry of the polycrystalline matrix of the sheet or plate is orthorhombic or higher. In the present work, it is argued that the differences between elastic constants associated with orthogonal directions of propagation of higher order modes should provide a meaningful measure of texture. The only requirement is that the texture be sufficiently well developed so that the following inequality is obeyed: ∣C′44−C′55∣>0.002 $$\bar C$$ with $$\bar C$$ =1/2 (C′44+C′55). HereC′44 andC′55 are the effective shear moduli for planes normal to the sheet or plate surface and at right angles to each other. They are measurable by observing the differences between the phase velocities of theSH 0 and theSH n>0 modes in the principal directions. By using Electromagnetic Acoustic Transducers (EMATs), the phase velocities can be deduced from the frequencies at which an SH mode of fixed wavelength is optimally excited. This approach has the advantage of being independent of specimen geometry and dimensions other than thickness.

The radiographic edge scattering distortion

Abstract: Previously described scattering based edge distortions in optical density scans obtained from a neutron radiographic knife-edge image have been here confirmed using analog Monte Carlo simulation. In an attempt to estimate the effects of additional object scatter on such artifacts, the effect on the edge response of diffusely incident neutrons has been determined both empirically and by Monte Carlo simulation. The diffuse neutron incidence has been found to diminish the relative magnitude of the distortion, offering explanation for the infrequent observation of the corresponding edge scattering halo artifacts. The potential of this edge distortion as a diagnostic indicator of the buildup factor in radiography is suggested and examined.

Acousto-optic method for nondestructive testing

Abstract: An alternative method for nondestructive testing based on light diffraction by ultrasonic waves is presented. It enables us to make more accurate measurements of intensity and phase of reflected waves, and therefore detailed information about the reflecting system can be obtained. Applications are numerous but special attention is payed to examination of the quality of coupling systems.

Transform techniques for analysis of thermal wave propagation in anisotropic composite materials

Abstract: A general formulation for solving the three-dimensional thermal diffusion equation in anisotropic media is presented. The method is based on two-dimensional Fourier transform techniques and can provide a physical insight into the problem. The analysis can easily be adapted to take into account arbitrary spatial variations of the excitation beam (i.e., a laser or an electron beam). Results obtained from propagation of thermal waves in composites are presented and followed by simulations for cases where the source function is defined. Propagation through an anisotropic slab is formulated and applied to specific cases.

Flaw detection close to the interface using ultrasonic echography

Abstract: The detection of flaws by ultrasonic echography becomes difficult when the flaw is at a shallow depth. The time delay between the interface and flaw echos is less than the width of the impulse-response time of the transducer, and the flaw echo generally has an amplitude much smaller than the dominating interface echo. When the surface is not too rough, the interface echo can be assumed to be known, and its comparison with the signal interface and flaw echo permits the detection of the flaw. In this article it is shown that signal processing techniques allow small flaws to be detected very close to the surface (∼0.5 mm). Two methods are proposed, the first one is based on the minimization of the mean-square error, and the second on the spectral substraction of the two echos.

Electromagnetic crack detection in ferritic steel

Abstract: The detection and sizing of fatigue cracks in ferritic materials, especially in offshore structures, is of major importance. This paper describes an eddy current system to detect such cracks, operating in an offshore environment with a search probe to electronic instrument separation of up to 600 m. An analysis of the various factors influencing the search coil behavior is given in terms of a mutually coupled primary and secondary electrical circuit; the component values of which are influenced by the coil design, induced electrical currents in the material, and any defects it contains. It is further shown that, with proper coil parameters, the phase variation in the phase angle of the coil complex impedance can be kept almost constant compared with the rapid variation occurring during lift-off. This behavior can be exploited in the instrumentation to give a clear vector display indication of the crack presence on less than ideal surfaces.

Some effects of the shape of a small defect in eddy-current NDE

Abstract: Available theoretical models for predicting defect responses in eddy-current NDE apply only to defects of spheroidal shape. In this paper, the boundary-element integral-equation approach is extended to allow for defects of more irregular shape. Sample results for cylindrical and conical voids are presented and discussed.

Thermographic nondestructive evaluation of steel-polypropylene laminates

Abstract: The efficacy of passive thermography as a nondestructive evaluation technique for detection of voids and interlaminar flaws in steel-polypropylene laminates was investigated. Both analytical and experimental studies were conducted to assess the flaw resolution characteristics of the technique and to determine the effects of pertinent geometric and thermal parameters on resolution. The implications of the analytical and experimental results from the parametric studies are discussed in terms of their effects on flaw resolution and test technique optimization for development of on-line NDE for quality control monitoring.

Some computational considerations in eddy-current NDE

Abstract: 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.

Concrete crack depth meter using nonlinear ADC

Abstract: This paper introduces the design and construction of an electronic hybrid signal processing meter that develops an output voltage proportional to the depth of a concrete crack. The circuit employs multiplication, squaring, and square-rooting functions by means of a special nonlinear ADC and counting scheme.

Application of Ferromagnetic Resonance Probes to the Characterization of Flaws in Metals

Abstract: This article presents some results obtained in the characterization of surface flaws by means of probes using ferromagnetic resonance of yttrium iron garnets (FMR probes). These experiments on artificial flaws show that FMR probes operate like eddy current probes for nonmagnetic materials and like magnetic field sensors for magnetic ones. Consequently, the working distance is larger for magnetic materials (1000–1500 µm) than for nonmagnetic ones (100–300 µm). FMR probes have good sensitivity to narrow flaws, good spatial discrimination, and are sensitive to flaw width and depth. Vector analysis allows the separation of distance and flaw effect by phase analysis on nonmagnetic materials. On magnetic materials this phase separation does not exist and another procedure is suggested. These results, and in particular those obtained on ferromagnetic materials, point to the possibility of replacing some eddy current or magnetic particle inspections by tests with ferromagnetic resonance probes.

Eddy Current Solutions Applicable to the Problem of Nondestructive Inspection of Flat and Cylindrical Surface Layer

Abstract: Nondestructive inspection of flat and long cylindrical metallic-surface layers is conveniently dealt with using the results of wave propagations in a one-dimensionally stratified medium. Analytical solutions for some special profiles are obtained, and perturbation technique is used to obtain results for more frequently encountered profiles.