REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Volume 19

The new face of innovation
View Description Hide DescriptionThe rapid changes in technology, the changes in the national and global economy and the emergence of many new nations that acquire an increasing competence to innovate is presenting us with new issues and opportunities. In particular, it affects the innovation system of the country, namely the scientific and technological infrastructure, the workforce and the policy environment in which government, industry, and academia operates. From a sequential or serial model we are moving or have moved to a dynamic, interactive one that encompasses more stakeholders in a realtime way. Of late, the work of the Council on Competitiveness has focused on the capacity for innovation as a prerequisite for national competitiveness. This talk will discuss the results from its report “Going Global: The New Shape of American Innovation” and its “Findings from the Innovation Index” and assess the forces that affect the future. The main conclusion will be that the changes we have and will be facing are irreversible and require the active and positive participation of the technical professional and technical institutions. It also requires new relationships between the main participants of the innovation system.

Changes needed in engineering education—The demands of globalization
View Description Hide DescriptionDevelopments in engineering education for the last five decades are briefly reviewed. Particular attention is given to the period for 1985 to the present, which was a period of unprecedented study of engineering education with many indepth reports issued. These reports tend to have a common theme focusing significantly on the global engineering enterprise. Because of increasing globalization of industry, engineering education in the United States is undergoing major reform. Pressure from industry to revisit certain aspects of engineering education has been a major factor in changes currently underway in the U.S. engineering education program accreditation process. I am not a futurist, but I dare to make some projections of the challenges and changes for engineering education during the period from the year 2000 onward, focusing on the next two to three decades. One dominant factor driving these predicted changes is the fast growing developments in information technology and the rate that information can be transmitted globally. Another among the several I mention is that much of engineering will move more and more toward the molecular level. As a result, engineering education must include biology along with the traditional chemistry and physics as the basis for the practice of engineering. Finally, I enumerate some of the challenges that the engineering professors will face in the next century. We will see unrelenting pressure for changes in engineering education and if professors are to be successful they will have to adopt a culture of rapid change—something rather difficult to imagine in the past.

Improved nuclear safety through international standards
View Description Hide DescriptionIn this shrinking world, what happens in an industry in one country can significantly affect the same industry elsewhere around the globe. In April 1986, a nuclear accident destroyed Unit 4 of Ukraine’s Chernobyl nuclear power plant, focusing worldwide attention on the dozens of Sovietdesigned reactors still in operation. The Chemobyl accident led to public concerns about all operating nuclear power plants and, in some countries (e.g., Italy and Sweden), to proposals for nationwide plant closures or moratoriums on new construction. However, for most former Soviet Union countries, plant shutdown was and still is not a viable option—the plants produce a significant percentage of each nation’s total electricity, and those countries do not have sufficient economic resources to develop alternative power sources. In cooperation with similar programs initiated in Canada, Japan, and western European countries, the U.S. Department of Energy (DOE) is conducting a comprehensive effort to reduce risks at Sovietdesigned nuclear power plants until they can be shut down or brought into compliance with international standards. This paper describes DOEsupported initiatives with participating host countries to: • improve reliability and accuracy of nondestructive evaluation equipment used for inservice inspection • transfer technologies and infrastructure to improve inservice inspections • enhance management systems for training and certifying hostcountry inservice inspection personnel The goal of these initiatives to enhance the use of international standards (ASME, ASNT, European Standards EN, and ISO) in operating and regulatory practices at Sovietdesigned nuclear power plants.

Ultrasonic modeling of an embedded elliptic crack
View Description Hide DescriptionExperiments indicate that the radiating near zone of a compressional circular transducer directly coupled to a homogeneous and isotropic solid has the following structure: there are geometrical zones where one can distinguish a plane compressional wave and toroidal waves, both compressional and shear, radiated by the transducer rim. As has been shown previously the modern diffraction theory allows to describe these explicitly. It also gives explicit asymptotic description of waves present in the transition zones. In case of a normal incidence of a plane compressional wave the explicit expressions have been obtained by Achenbach and coauthors for the fields diffracted by a pennyshaped crack. We build on the above work by applying the uniform GTD to model an oblique incidence of a plane compressional wave on an elliptical crack. We compare our asymptotic results with numerical results based on the boundary integral code as developed by Glushkovs, Krasnodar University, Russia. The asymptotic formulas form a basis of a code for highfrequency simulation of ultrasonic scattering by elliptical cracks situated in the vicinity of a compressional circular transducer, currently under development at our Center.

Ultrasonic crack diffraction in metals using lasergenerated ultrasonic shear waves and broadband EMAT detection
View Description Hide DescriptionUnderstanding the nature of ultrasonic propagation, scattering effects and mode conversation for crack diffraction in materials is critical to the effectiveness of ultrasonic characterization and sizing cracks and defects in structures. Thus far, a complete solution for the diffraction of an ultrasonic pulse of arbitrary shape at a crack of arbitrary shape has not yet been achieved. In this work, diffraction of a thermoelastic lasergenerated ultrasonic line source at a semiinfinite halfplane is examined using rigorous diffraction theory. Directivity patterns are calculated for a laser line source, for diffraction of a plane wave at a semiinfinite halfplane and for diffraction of an ultrasonic shear wave generated by a laser line. Experimental validation of the theory is performed using lasergenerated ultrasonic diffraction of a shear wave at a slot and its subsequent detection with an EMAT receiver sensitive to shear displacements. Results from calculations and experiments show that the directivity of a laser line source is identical to a laser point source in two dimensions if thermal conduction and subsurface optical penetration effects are neglected. Calculations of the directivity pattern for diffraction of an ultrasonic shear wave at a semiinfinite halfplane show that the majority of the diffracted energy propagates in the direction of the incident plane wave with a smaller portion diffracted at all angles. Diffraction of a lasergenerated line source at a semiinfinite halfplane shows similar results. Both models indicate that there will not be a sharp shadow boundary for ultrasonic diffraction at a crack. Experiments performed on an aluminum halfcylinder milled with an EDM slot along the radius verify that ultrasonic signals appear in the shadow zone of the crack. Surface scans using the hybrid laser/EMAT system on aluminum plates milled with slots of different sizes show that crack diffraction is a frequency dependent process. Lower frequencies are found to dominate in the shadow zone. These results indicate that crack sizing may be aided by filtering the signals or similar signal processing methods.

Modeling of ultrasonic detection of a rectangular crack
View Description Hide DescriptionA complete ultrasonic nondestructive testing situation involving a transmitting transducer, scattering from a defect and a receiving transducer is modeled. The defect is taken to be a smooth rectangular crack in an otherwise homogeneous, isotropic, elastic solid. The crack may be located close to a free surface. To solve the scattering problem, an integral equation method is employed. Starting from an integral representation of the displacement field an integral equation for the crack opening displacement is derived. To solve the equation, the crack opening displacement is expanded in a conveniently chosen set of functions with the correct squareroot behavior at the crack tips. The transmitting probe is modeled by prescribing the traction on the area of contact. To model the receiver an electromechanical reciprocity argument is utilized which determines the change in signal response due to the presence of the defect. Some numerical results for the signal response are presented for various combinations of the relevant parameters (transducer type, frequency, crack location and orientation). The results are compared to previously obtained results for scattering from a striplike crack.

Reflection and scattering analysis of shwave using a combined method of BEM and ray theory
View Description Hide DescriptionA Boundary Element Method (BEM) is one of the most frequently used techniques for numerical simulation of ultrasonic nondestructive testing. In simulating a pulseecho flaw detection test by a BEM, it is desirable that both transmission and reception points of ultrasound are included in the analysis model. However, numerical simulation with that model takes enormous computational time because the scale of the problem is characterized by the distance between these two points compared to the wave length and it is very large in this case. The objective of this study is to develop an efficient numerical method to carry out the simulation which covers the whole process of pulseecho method concerning mechanical wave propagation. The approach adopted here is to use elastodynamic ray theory in addition to the conventional BEM. In this method, the reflection and propagation of waves in a defectfree region are evaluated by the ray theory and the scattered wave induced by defects by BEM. In this study, the combined method of BEM and ray theory is applied to reflection and scattering problems of 2D time harmonic SH waves in a thick plate model. Both BEM and ray analysis are conducted in the frequency domain and the results obtained are transformed into timedomain solutions when it is needed. Numerical examples show that the combined method can give as accurate solutions as the conventional BEM with less number of boundary elements and the proposed method is very efficient.

Scattering of a Rayleigh wave by a surfacebreaking crack under compressive stress
View Description Hide DescriptionThis paper presents a theoretical investigation into the scattering of a Rayleigh wave by a surfacebreaking crack subjected to a compressive stress. The boundary conditions at the partially contacting crack surfaces are modelled according to the quasistatic approximation. Following the results of recent investigations on the subject, the ratio between in transverse and the normal stiffness constants of two rough surfaces in contact is assumed to be 0.55. The model allows for arbitrary distributions of the interfacial stiffness constants in order to simulate the effect of applied compressive stress fields having arbitrary shape along the crack faces. Results are presented to illustrate the behavior of the reflection and transmission coefficients, as well as the dependence of the total energy radiated into the bulk, as functions of a) the ratio between the crack depth and the Rayleigh wavelength, and b) the interfacial stiffness constants. At high frequency, the model predicts an surprising initial increase of the reflection coefficient as the interface stiffness increases, followed by the expected decrease below the value typical of an open crack. It is also shown that the relative variation of reflection coefficient from cracks having the same interfacial stiffness depends on the depthtowavelength ratio.—This work was funded by the 5^{th} Framework Program of the European Commission.

Scatter of toroidal elastic waves from a plane
View Description Hide DescriptionExperiments indicate that the radiating near zone of a compressional circular transducer directly coupled to a homogeneous and isotropic solid has the following structures: there are geometrical zones where one can distinguish a plane compressional wave and toroidal waves, both compressional and shear, radiated by the transducer rim. As we have shown previously, the modern diffraction theory allows to describe these explicitly. It also gives explicit asymptotic description of waves present in the transition zones. A lot of work has been done by other authors in calculating the fields obtained as the result of a plane wave incident on a plane crack. Here, we present calculations which have been carried out in the framework of Geometrical Elastodynamics and pertain to scattering of toroidal waves by an infinite plane crack. We demonstrate that when the incident wave is toroidal shear there are gaps in the portion of the crack which at a given moment in time contribute to the scattered compressional waves. Using the uniform Keller’s GTD our results may be extended to modeling diffraction of the toroidal waves by elliptic cracks and included in the code for highfrequency simulation of ultrasonic scattering in such cracks situated in the vicinity of a compressional circular transducer, currently under development at our Center.

Nondestructive evaluation of a transversely isotropic cylinder encased in a solid elastic medium
View Description Hide DescriptionA mathematical model is developed to describe the scattering of a plane wave incident at an arbitrary angle on a transversely isotropic cylinder imbedded in a solid elastic matrix. The model is based on the normal mode expansion method, and accommodates both compression and shear waves, polarized in any direction. The geometry is a good model for a composite material featuring a single reinforcing fiber encased in an elastic matrix. The sensitivity of the system to perturbations in the cylinder’s elastic constants in explored, as revealed by changes in the scattered wave spectrum. The results indicate several system resonances, each sensitive to perturbations in certain elastic constants of the cylinder. Also, dispersive pseudoStoneley waves that travel on the matrixcylinder interface lead to resonances in the scattered wave spectrum that would be good indicators of deficiencies in the integrity of the interface.

FEM analysis of bulk wave scattering at an inclusion
View Description Hide DescriptionUltrasonic wave scattering at interface of a cylindrical solder embedded in an aluminum block is analyzed by finite element method under plane strain. For nondestructive characterization of inclusion, the analysis of ultrasonic wave scattering at heterogeneous, anisotropic and irregular shaped inclusions is necessary. Theoretical analysis is limited for only idealized geometry and material properties, therefore only numerical analysis is applicable for inclusions of irregular shape and real material properties. As a first step, FEM analysis is applied for an aluminum block with an inclusion of cylindrical solder. Longitudinal short pulse is given over a width larger than the diameter of the cylinder on one face of the aluminum block perpendicular to the axis of the cylinder. The refracted longitudinal wave at aluminum/solder interface is focused within the solder, and this wave partially reflected at another solder/aluminum boundary. The amplitude of the reflected wave is comparable to those reflected at the aluminum/solder interface. The shear wave which have excited by mode change at the aluminum/solder interface are backscattered in the directions of +.50 degree and superposed incoming shear wave. The numerical results are close to the surface wave propagation measured by laser ultrasonic method.

Elastic wave scattering by various defects in anisotropic materials
View Description Hide DescriptionSince wave speeds in anisotropic materials depend on the direction of propagation, the characteristics of scattering and propagation of elastic waves are different from those in isotropic materials. It is therefore necessary to solve scattering problems of elastic waves to establish a quantitative ultrasonic nondestructive method for anisotropic materials. In this study, a time domain boundary element method is developed with use of fundamental solutions for a general anisotropic solid obtained by Wang and Achenbach [Geophysical J. Int., Vol. 118, pp. 384–392, 1994]. The boundary element method is applied to scattering problems of elastic waves by various defects such as a cavity and an inclusion in an infinite anisotropic solid. It is shown that the amplitudes and phases of backscattered waves are quite different from those in isotropic materials and the scattering characteristics are very important factors in the nondestructive ultrasonic evaluation of anisotropic materials.

Finitedifference modelling of pulseecho scattering using a watercoupled transducer
View Description Hide DescriptionWe report the development of new techniques for modeling pulseecho scattering using a watercoupled transducer, and their success in accurately predicting both the size and shape of the echoes from small solid defects. Our new methods use separate grids to model the solid and the liquid, and these are only related along the interface using pseudonode formulations based on the boundary conditions. The twin grid formulation does not have any problems with discontinuities along the interface. It has also been shown to have excellent stability properties. When we compare our finite difference predictions with experimental data from known defects, we take care to subtract measurements made without the defect (with all else unchanged) from the measurements with the defect, in order to avoid spurious interface echoes. We do this for both the experimental and the finitedifference “data.” The finite difference predictions match the experimental measurements to high accuracy. Having validated the finite difference techniques, we can use them to monitor and visualize the various scattered and reflected waves, giving valuable insights into their physical origins.

Numerical evaluation of fundamental solution for doubly periodic structures
View Description Hide DescriptionAn integral equation is formulated for the scattering problem by the planar periodic array of scatterers. In this case, the fundamental solution is in a form of double series and the series may converge very slowly in a certain configuration of periodic scatterers. In this paper, a practical numerical integration based on Ewald method is investigated for the fundamental solution with the double infinite series. In more detail, the fundamental solution is converted into the integral form with rapidly convergent integrand and the Gauss’ numerical integration is performed here to evaluate the integral. The convergence and the efficiency of the method are demonstrated numerically for the integral equation by the planar array of scatterers. The reflection and transmission coefficients are calculated as an application of the present numerical evaluation for the fundamental solution.

Low frequency acoustic response of a periodic layer of spherical inclusions in an elastic solid to a normally incident plane longitudinal wave
View Description Hide DescriptionThe influence of particle mass density on the reflection and transmission spectra of a plane longitudinal wave normally incident on a periodic (square) array of identical spherical particles in a polyester matrix are measured at wavelengths which are comparable to the particle radius and the interparticle distance. The spectra are characterized by several resonances whose frequencies are close to the cutoff frequencies for the shear wave modes, which are analogs of spectral orders in diffraction gratings. Arrays of heavy particles (lead and steel) exhibit a pronounced resonance anomaly which occurs when the lattice resonant frequency is close to the frequency of the rigid body translation (dipole) resonance of an isolated sphere in an unbounded matrix. An approximate low frequency theory is developed which takes into account the multiple scattering effect. The theory shows good comparison with the experimental data for arrays with particle area fractions as high as 32%.

Two dimensional wave propagation from a point load in an anisotropic material
View Description Hide DescriptionIt may be shown formally that one possible approach to evaluating the dynamic response of a multiply fiber composite plate to an impulsive point load is by integrating the response to a line load over all orientations of the load line. This is a computationally intensive exercise and as a preliminary step, it is instructive to consider the simpler generic problem of the dynamic response of a twodimensional elastic medium to a point load. For a twodimensional isotropic material, integration of the line load solutions may be performed analytically and leads, as is well known, to the Bessel function solution. For an isotropic material, analytic integration is not possible and it is necessary to carry out the integration numerically. The paper will report the results of the numerical integration for both the isotropic and anisotropic media, showing some of the contributions at a fixed location arising from the individual line loads. For any orientation of the line load, the contribution to the overall response is noncausal but complete integration leads to a properly casual solution.

Ultrasonic wave propagation in reallife austenitic Vbutt welds: Numerical modeling and validation
View Description Hide DescriptionIn nondestructive testing the evaluation of austenitic steel welds with ultrasound is a commonly used method. But, since the wave propagation, scattering, and diffraction effects in such complicated media are hardly understood, computer simulations are very helpful to increase the knowledge of the physical phenomena in such samples. A particularly powerful numerical time domain modeling tool is the well established Elastodynamic Finite Integration Technique (EFIT). Recently, EFIT has been extended to simulate elastic waves in inhomogeneous anisotropic media. In this paper, the stepbystep evaluation of ultrasonic wave propagation in inhomogeneous anisotropic media will be described and the results will be validated against measurements. As a simplified model, a Vbutt weld with perpendicular grain structure is investigated. The coincidence between the B Scans of the simulation and the measurement of an idealized Vbutt weld is remarkable and even effects predicted by theory and simulation—the appearance of two coupled quasiSV waves—can be observed. As a next step, an improved and more realistic model of the grain orientation inside the Vbutt weld is introduced. This model has been implemented in the EFIT code and has been validated against measurements. For this verification, measured and simulated BScans for a reallife Vbutt weld have been compared and a significant coincidence has been observed. Furthermore, the main pulses in the BScans are interpreted by analyzing the snapshotmovies of the wavefronts.

Ultrasonic propagation in austenitic stainless steel welds approximate model and numerical methods results and comparison with experiments
View Description Hide DescriptionElectricité de France conducts a study on the effects of metallurgical structures of austenitic stainless steel welds on ultrasonic wave propagation for application to ultrasonic nondestructive testing. Two complementary methods are performed: a direct experimental study on various kinds of welds and a theoretical study using ultrasonic simulation tools. This simulation study makes use of two different models. The first, called ULTSON2D, is a finite element code developed by the EDF Research and Development Division. It is adapted to orthotropic media and heterogeneous structures and takes into account the scattering of waves from flaws such as cracks and holes. The second, called ChampSons is a 3D semianalytical code developed by the French Atomic Energy Commission (CEA) which allows to predict ultrasonic beam radiation into anisotropic and heterogeneous media. Both models deal with broadband excitation pulses since they are timedependent. The present study aims at evaluating the ability of the two models to predict typical beam distortion effects in complex anisotropic and heterogeneous weld structures as compared to experimental measurements. This study is based on preliminary results of weld structural characterization presented in a companion paper.

Calculation of ultrasonic reflection and transmission in anisotropic austenitic layered structures
View Description Hide DescriptionThe theory of plane wave propagation in layered structures has been applied in the formulation of Nayfeh in order to calculate scattering coefficients due to reflection and transmission at the grain boundaries in austenitic weld metal and cast material. Each layer is assumed to be a cubic homogeneous monocrystal. Lower symmetries of the layer down to the triclinic case may also be assumed, e.g., transverse isotropy of the columnar grained texture in austenitic weld metal. The layers are rigidly bonded and the multilayer package embedded in water or between solid substrates. Scattering coefficients are calculated by a transfer matrix approach. Ultrasonic properties of the single layers are algebraically linked together resulting in a simple operator for calculation of reflection and transmission coefficients at the multilayer package boundaries. Critical angle phenomena may cause failure of solution and are limiting the range of application of the transfer matrix approach, as they cause the matrix numerical condition to decrease down to singularity. This is handled by using complex algebra. The transfer matrix method has been applied to multilayer packages immersed in water. In case of solid substrates of the multilayer package transmission is occurring at a larger range of incidence angles.

The farfield of a point source in a transversely isotropic elastic solid
View Description Hide DescriptionThe elastic field of a timeharmonic point source acting in a transversely isotropic, homogeneous, linearly elastic solid is studied. First, the representation of the Green’s tensor as an integral over the unit sphere is obtained. It consists of three waves: quasilongitudinal (P), shearhorizontal (SH) and quasishear (SV). Then, an original exact analytical solution for the SH wave in terms of elementary functions is derived. The complete farfield asymptotic approximation of P and SV waves is obtained next, using the uniform stationary phase method. For the P wave it involves the leading term of the ray series since there is only one arrival of this wave. The wave surface for the SV wave contains conical points and cuspidal edges. The asymptotic description applicable near these singular directions is derived involving the Airy and Bessel functions. The directions close to the points of tangential contact of the SH and SV sheets of the wave surface are also treated. Numerical results in both frequency and time domain are presented. They show that the agreement between the outputs of the asymptotic and direct numerical codes is very good throughout all regions but the former can be orders of magnitude faster.