Showing papers in "Journal of Nondestructive Evaluation in 1990"

Developments in ultrasonic modeling with finite element analysis

Abstract: An overview is given of finite element analysis and its application to the modeling of ultrasonic nondestructive evaluation phenomena. Following a discussion of the underlying weighted residual methodology, a mass-lumping technique is described which results in an efficient computer implementation for 2D geometries. Code predictions are compared with both analytical and experimental results, and data from studies of attenuation, anisotropy, defect interactions, and surface waves are given. Initial results from a full 3D formulation are also shown.

A system model for the ultrasonic inspection of smooth planar cracks

Abstract: This paper describes a system model for the ultrasonic inspection of smooth planar cracks in ferritic steel, using pulse-echo probes. The model predicts the echo amplitudes and ranges as functions of the probe position. It is applied to problems of procedure design, assessment, and technical justification on power station plant. The model is implemented as a suite of versatile and user-friendly computer codes, suitable for use by practical NDT engineers, and is supported by a comprehensive user manual. The paper describes the principles of the model and gives examples of its application to power plant problems. Illustrations are also given of the extensive validation which the model has undergone through comparison with experiment.

Finite element studies of the impact-echo response of plates containing thin layers and voids

Abstract: Quantitative nondestructive evaluation based on the use of transient stress waves generated by point impact is hindered by the fact that the governing partial differential equations admit closed-form solutions for only the most trivial cases, an infinite half-space and an infinite plate. In previous studies carried out by the authors, the finite element method has been shown to provide useful numerical solutions for a variety of cases involving bounded solids containing flaws. Numerical results have been verified with carefully controlled experiments. Currently, the method is being used to establish the basis for a new nondestructive evaluation technique for civil engineering structures. This technique is called impact-echo, and it is based on the use of low frequency, transient stress waves generated by elastic point impact. In this paper, the impact-echo response of plates containing thin layers is studied using finite element models. The purpose of these studies was to determine the applicability of using the method for detecting voids in layered civil engineering structures. Results of the numerical studies show that it is feasible to use the impact-echo method for this application.

Modulated laser array sources for generation of narrowband and directed ultrasound

Abstract: Patterned illuminating sources and appropriate time modulation may be used to enhance certain features of laser-generated acoustic waves in time or frequency as a function of direction. Steerable, narrowband, “toneburst” ultrasonic signals have been generated from a single laser pulse which was spatially modulated by transmission through a lenticular array. In addition, narrowband toneburst ultrasonic waves have been generated from a mode-locked laser pulse train providing spectral narrowing of the laser-ultrasonic signal. Both temporal and spatial modulation of the laser pulse improve the signal-to-noise ratio for laser ultrasonics.

Ultrasonic Reflection and Guided Waves in Fluid-Coupled Composite Laminates

Abstract: In this article, we review the present authors' own approach to elastic wave modeling and experimental measurements in fibrous composites. The materials and structural problem addressed here concerns the propagation of guided elastic leaky waves in continuous-fiber composite plates. The guided wavevector may be oriented along the fibers or in an arbitrary azimuthal direction. These plates can be structured as single-layer or multilayer media, where each successive layer contains fibers in different directions. Further, the multiaxial plates are loaded by a fluid or by different fluids on each boundary. Each of these cases has been investigated both experimentally and theoretically. It is found that some reasonable approximations lead to significant simplifications in treating these complicated structures and yet preserve the accuracy needed to make useful predictions of realistic sound wave behavior. Comparisons of the results of model calculations and experimental measurements of ultrasonic reflection show very good agreement over a wide range of experimental parameters and types of composites. Suggestions are offered at the end of the article for extensions of the modeling to account for non-ideal behavior of the materials and the chosen means of interrogation.

Application of the boundary element method to elastic wave scattering by irregular defects

Abstract: A time-harmonic boundary element formulation for elastic wave scattering in 3D is adapted to ultrasonic NDE. Defect classes addressed are volumetric voids and inclusions, and crack-like elliptical voids. For axisymmetric flaws, comparisons are made with method of optimal truncation (MOOT) and transition-matrix calculations. Comparison to experiment is made for more general shapes. For crack-like voids, comparisons are made with the Kirchhoff, geometric theory of diffraction (GTD), and quasistatic asymptotic approximations. The efficiency and usefulness of the boundary element method (BEM) in finding the bounds of applicability of these approximate theories are demonstrated. An example of a flaw characterization technique based on intermediate frequency scattering data simulated by BEM is given. The ability of BEM to handle nonplanar incident fields, as described by a transducer beam model, is shown. Other computational and modeling efficiencies of the BEM are noted.

A boundary element model for acoustic-elastic interaction with applications in ultrasonic NDE

Abstract: The boundary integral equation method is applied to a class of time-harmonic acoustic scattering problems where the bounded elastic scatterer is submerged in a fluid. An exact mathematical model is presented for a finite scatterer with a closed surface, where the surface integral equations are exclusively used to represent the fluid-solid or acoustic-elastic interaction of the scattering process. The numerical procedure involves application of point collocation with quadratic isoparametric approximations that reduce the integral equations to a discrete set of complex linear algebraic equations. Examples emphasize the potential of the method to solve three-dimensional problems of practical interest. Limitations of the formulations and the extension to the case of a semi-infinite plane and curved fluid-solid interface are discussed in the latter part of the paper.

Nondestructive testing of microstructures by picosecond ultrasonics

Abstract: We describe a new method we have developed which can be used to make ultrasonic measurements with picosecond time resolution, and give examples of its application to the study of the acoustic and mechanical properties of microstructures.

Computer simulation of ultrasonics and its applications

Abstract: Numerical calculations have been carried out for about 20 years, and the results are demonstrated graphically by vector or lattice representations. These results are now being used mainly for clarifying the mechanisms of ultrasonic interaction with material and defects and checking and improving the experimental results. Parts of this paper review results, many involving cracktip diffraction in various geometries, which have been published mainly in conference proceedings and have not appeared in journals. Other parts, dealing with focused fields, focal lenses, and radiation in an anisotropic medium are new, and have not appeared even as conference papers.

Modeling ultrasonic inspection of austenitic welds

Abstract: The strength of austenitic welds will be considerably reduced if cracks are present. To guard against this possibility good workmanship is essential; quality is monitored by inspecting the welds after manufacture. Ultrasound is the favored method of inspection but there are difficulties associated with this in the coarse-grained columnar weld structures often associated with austenitic materials. We present two different approaches to modeling such inspections: ray tracing and full numerical solution of the governing wave equation. We show that the two approaches, although very different in physical nature and numerical method, give similar and consistent results.

Estimating the depth and width of arbitrarily-oriented disbonds in laminates using shearography

Abstract: Disbonds in a laminated plate are readily revealed from anomalies in the fringe pattern of a shearogram. The shearographic fringes represent loci of constant displacement derivatives of the deformed surface of the plate when subjected to a load increment such as vacuum stressing. In this investigation, a simple method is developed for easy estimation of the size and depth of arbitrarily-oriented square disbonds in laminated plates from a shearogram. The theory and experimental method presented may also be extended to the assessment of disbonds of arbitrary shapes.

Ultrasonic velocity and attenuation determination by laser-ultrasonics

Abstract: Materials are often characterized by measuring the velocity and attenuation of ultrasonic waves. Laser-ultrasonics, which uses lasers for generation and detection of ultrasound, has several advantages compared to the classical piezoelectric techniques, but the use of lasers is often associated with ill-defined source and receiver characteristics making diffraction effects hard to evaluate. We have identified two regimes which, in practice, allow the measurement of velocity and attenuation: the point source/point receiver and the large uniform source/large uniform receiver regimes. These approaches are discussed and illustrated with several examples of application. Limitations caused by misalignment between the generating and detecting laser spots are also analyzed.

Reflection and transmission of ultrasound by a region of damaged material

Abstract: Reflection and transmission of ultrasonic waves by a layer-like region of distributed microcracks in a bulk material has been investigated. It has been assumed that for sufficiently low frequencies and far away from the damaged layer, the reflected and transmitted waves are plane waves. By applying the Betti reciprocal theorem to a cell containingN cracks, and by choosing one elastodynamic state as the actual wave state and the other as a suitably chosen auxiliary wave state, the reflection and transmission coefficients have been expressed in terms of integrals over theN cracks. Simple expressions have been obtained for the case that all cracks are identical and parallel to each other. For the case that the cracks do not interact with each other, numerical results for the reflection and transmission coefficients are presented for a distribution of penny-shaped cracks. The variation of these coefficients with frequency, relative layer thickness and angle of incidence has been displayed in graphs.

Laser-based ultrasonics on gr/epoxy composite:

Abstract: Critical issues are examined in the application of laser generation and detection of ultrasound to the inspection of large area air-frame composites. Among these issues are surface roughness, signal-to-noise ratio, insensitivity to the path length between the part and detector, and wide band vs. narrow-band generation. Demonstrated is the feasibility of transmission C-scans of 150 ply Gr/epoxy panels containing simulated delaminations. Waveforms having a signal-to-noise ratio of greater than 40 dB in 15-mm thick Gr/epoxy were obtained with generating laser powers well below the ablation limit for the graphite epoxy. Detection was shot-noise limited with a detector noise figure of about 5–10 dB.

Directional Laser Generation and Detection of Ultrasound with Arrays of Optical Fibers

Abstract: The generation and detection of ultrasound with laser beams has become a viable technique in nondestructive testing of materials [1,2]. The main advantage of the technique is its intrinsic non-contact nature. Its main limitation seems to be its fairly low efficiency as compared with that of other standard NDE techniques. An interesting recent development [3–5] consists in using optical fibers to guide the laser light and illuminate the sample under investigation in virtually any desired source configuration. Another inherent advantage of the use of optical fibers is that the optical bench is completely decoupled from the sample under investigation, thus rendering the technique practical for in-situ measurements. The objective of the present study is (1) to present some preliminary experimental results complementary of those of Vogel [4] on the generation of ultrasound with an array of optical fibers; (2) to discuss the possibility of generating directional surface waves in a very narrow frequency band, thus increasing the signal-to-noise ratio; and (3), to discuss the feasibility of the directional detection of ultrasound by using an array of optical fibers as a receiver, also with the goal of increasing the signal-to-noise ratio.

Ultrasonic modeling and experiments: An industrial case: Bimetallic weld in nuclear power plant

Abstract: To improve the ultrasonic inspection of the safe ends connecting the nuclear vessel to the piping, we are conducting modeling studies using two models: PROMANIS, a ray tracing code, and ULTSON, a 2-D finite element code. The study has shown a great interest in combining modeling and experiments to improve the codes and react to the experimental conditions.

Model of the acoustic microscope response to scattering by a near-surface void

Abstract: The response of the scanning acoustic microscope to scattering by a near-surface void is formulated rigorously. The transmission and reception responses of the acoustic microscope transducer-lens assembly are represented as Fourier spatial frequency spectra. Likewise, the interaction of the focused wavefield with the specimen fluid-solid interface is formulated via Fourier integrals. The scattering of the focused beam by a void positioned within the solid near the fluid-solid interface is formulated as a boundary integral equation employing the Green function for contacting fluid-solid half-spaces. The problem is evaluated numerically to demonstrate the scattering response dependence on void depth, transducer defocus, and void diameter.

Ultrasonic NDE using a concentric laser/EMAT system

Abstract: A pulsed laser source, together with a concentric annular electromagnet acoustic transducer (EMAT) as detector, have been investigated for use as a noncontact ultrasonic inspection system. The characteristics of this hybrid transduction method, and its application to various problems in nondestructive evaluation have been studied. Results are presented for applications in imaging, the testing of diffusion and adhesive bonding, and thickness estimation.

A novel method of processing pulse echo data in adhesive bond inspection

Abstract: A novel method for evaluating nondestructively the quality of bonded assemblies is presented. It is shown that a combination of HOC (higher order crossing) analysis and another frequency domain parameter can distinguish between different surface treatments, as well as predicting cohesive or adhesive failure of bonds. It is believed that this method will open the way to predict the durability of bonded structures at the manufacturing or inservice stages.

On the calculation of laser-generated ultrasound pulses

Abstract: The generation of ultrasound pulses with pulsed lasers has received considerable attention in recent years. Promising applications are to nondestructive evaluation and to materials characterization, where it is convenient to have a broad-band source requiring no contact with the sample. Quantitative applications in nondestructive evaluation require theoretical calculations incorporating realistic sample and source properties. This has, until recently, dictated approaches which were very computation-intensive. After the results obtained through these approaches are reviewed, we will describe and illustrate a very recent formulation which enables such calculations to be accomplished more easily. Numerical results will be presented to demonstrate the advantages of this approach, with emphasis on the effects of finite source dimensions, and on the initial waveform. We also show that this approach may be used to investigate surface-wave generation.

A hybrid finite difference/helmholtz integral scheme for ultrasonic scattering

Abstract: Two techniques for analyzing the scattering of elastic waves by flaws are combined in a hybrid model. One is a finite difference scheme for handling the details of the interaction with the flaw, the other is a Helmholtz integral scheme for extending the results into the far field. Results are given for the diffraction coefficients for a semi-infinite thin crack, and for a 270° corner.