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Adaptive finite element modeling of oblique ultrasonic NDE waves
N DT Abstracts This paper discussesa nonlinear diffractive inversion of the Helmholtz equation for multiscatteringconfigurations,where the scattersare embedded in an inhomogeneousbackground medium. Using the finite element model to iteratively compute the scatteredfield in conjunctionwith a novel discrete cosine transform (DCT) representation of the object function permits the development of an efficient nonlinear inversion algorithm. The object function expansion is obtained by applying the DCT to sampling points which are chosen at the zeros of the Chebyshev polynomials, and achieves an accuracy comparable to the more popular sine basis with far fewer expansion terms. After the inverse scatteringformulation is convertedinto a nonlinear parameter estimation problem, the final matrix equation is linearized and solved by a standard least-squaresalgorithm. 57177 Nouailhas, B.; Canas, R. 3D ultrasonic modelling (In French, English abstract) 6th European Conference on Non Destructive Testing, Nice (France), 24 28 Oct. 1994. Vol. 1. pp. 601605. ECNDT (1994) We present here an approachto solve the inverse problem. this approach is based on an iterative resolution of the direct problem with an optimisation of input parameters. First we have developed a three dimensional finite element code for propagationand wave scattering in the solid coupled with an integral method to calculate the propagation in the fluid. Then using the “hogeneisation”technique, well known in mechanical studies of composite materials, to adjust simultaneous the 9 elastics constantsfor variation of parameters.
56758 Issa, C.A.; Balasuhramaniam, K.; Srirengan, K. Adaptive finite element modeling of oblique ultrasonic NDE waves Review of Progress in Quantitative Nondestruction Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13A, pp.85-92. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN 0- 306-4473 l-2 In our study, a self-adaptive grid refinement techniquecoupled with the p-version of the finite element method is investigated.This type of approach is called the hp version of the finite element. In the modeling process,the approximate solution to the exact differential equation achieves convergence by applying two distinct solution enrichment strategies. One strategy is to solve the problem using a very coarse grid and to enrich the quality of the approximation by increasing the degree p of the interpolating polynomial shape functions for thoseelements where it is needed.The other strategy involves local grid h refinements for those elements where it is needed.
56706 Nigam, H.; Scarton, HA.; Das, P.; Kaveh, M.; Murray, F.S.; Mitchell, J.R. Experimental study of the mechanical behavior of superconducting coils Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 169 1- 1698. Edited by D.O. Thompson and D.E. Chime& Plenum Press ( 1994) ISBN o-306-4473 l-2 Mechanicaltesting of coil sections is performedunder room temperature conditions to obtain some of the stiffnesscoefficients which are needed for finite element modeling of tire composite behavior. Some of these composites are also determirssd using ultrasonic techniques. Ultrasonic methods are expected to be the preferred means of in-situ low temperature measurementsof elastic constants, and as a part of this study are being evaluated for their suitability. Acoustic emission measurementshave been made on a monotonically loaded coil section to determine the origin and nature of micro-cracking that occurs in the material.
56686 Gosz, M.; Achenbach, J.D.; Chu, Y.C.; Rokhlin. XI. Comparison of finite element and analytical determination of fiber/matrix interface stiffnesses from ultrasonic data Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 1469- 1475. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN o-306-4473 l-2 We will compare finite element and analytical methodsof inversion for determination of the interphase stiffnesses from the moduli of the composite. The effect of possible errors in measurement of composite moduli on the accuracy of the calculated interphase stiffnesses is also discussed.
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56685 Rokhlin, S.I.; Chu, Y.C. Determination of interphasial elastic properties in fiber composites from ultrasonic bulk wave velocities Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 1461- 1468. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press ( 1994) ISBN o-306-4473 l-2 Starting with the generalized self-consistent model @SCM) we introduce for composites with nonhomogeneousfibers the concept of an equivalent fiber. While four elastic moduli for the equivalent fiber can be found using analytical models, we obtain the transverse shear modulus of the equivalent fiber using finite element calculations. We briefly describe the method to determine the interphasial moduli in fiber composites focusing on the role of the equivalent fiber. The applicability of the method to different composite systemsis also discussed. 56639
Kishoni, D.; Ta’asan, S.
Non-reflecting regions for finite difference methods in modeling of elastic wave propagation in plates Review of Progress in Quantitative Nondestruction Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13A, pp. 195- 202. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN o-306-4473 l-2 Solution of the wave equation using techniquessuch as finite difference or finite element methodscan model elastic wave propagationin solids. This requiresmapping the physical geometry into a computationaldomain whose size is governed by the size of the physical domain of interest and by the required resolution. This computational domain, in turn, dictates the computer memory requirementsas well as the calculation time. Quite often, the physical region of interest is only a part of the whole physical body, and does not necessarily include all the physical boundaries. Reduction of the calculation domain requires positioning an artificial boundary or region where a physical boundary does not exist. It is important however that such a boundary, or region, will not affect the internal domain, i.e., it should not cause reflections that propagate back into the material. This paper concentrateson the issueof constructingsuch a boundary region.
56250 Lee, Y C.; Kim, J 0.; Achenbach, J.D. Measurement of stresses by line-focus acoustic microscopy Ultrasonics, Vol. 32, No. 5, pp. 359-366 (Sep. 1994) In this paper, line-focus acousticmicroscopy has been used to determine local near-surfacestressesin isotropic materials. Two surface wave modes, namely a leaky Rayleigh wave and a leaky surface- skimming longitudinal wave, have been excited by the acoustic microscope. It has been observed that the changes of the wave velocities are. linearly proportional to the applied stresses,as predicted by acoustoelastic theory. The non-uniform stressfield in a loaded specimen has been determined from wave velocity measurements by the use of acoustoelastic constants obtained from a calibration test. The measured stressesare in good agreement with the results calculated by a finite element method. A self-calibrating method, which determines the stress profile directly from velocity measutements without a calibration test, is proposed and the results ate compared with experimental data.
56200 Nor&has, B.; Canas, R. 3D ultrasonic modelling Ultrasonics lntemational 93 Conference Proceedings, Vienna (Austria), 6-8 Jul. 1993. pp. 339-342. But&worth-Heinemann Ltd (1993) ISBN 0 750618779 We presenta 3D finite element model for propagation and diffraction of elastic waves on defect in anisotropicmedia, We usedthis model to solve an inverse problem with an iterative method. By comparison between experimental and numerical signals we can optimise the parametersof the model to converge to the solution. We present the case of an underclad crack. 56021
Bennett, J.; Hamilton, R.; Hayward, G.
Finite element modelling of 1-3 composite transducers for underwater applications IEEE 1993 Ultrasonics Symposium, Baltimore, Maryland (United States), 3 1 Oct. - 3 Nov. 1993. Vol. 2, pp. 1113-l 116. Edited by M. Levy and B.R. McAvoy. IEEE (1993) ISSN 105 l-01 17 Finite element mcdelling was employed in two separate applications involving 1-3 connectivity composite transducers: low volume fraction transducersfor hydrophoneapplications;and the influence of the electrode pattern on monolithic and composite arrays. Specifically, the composite microstructureis investigatedwith respect to the shape and distribution of the ceramic pillar, in addition to the polymer filler material. Particular
56758 Issa, C.A.; Balasuhramaniam, K.; Srirengan, K. Adaptive finite element modeling of oblique ultrasonic NDE waves Review of Progress in Quantitative Nondestruction Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13A, pp.85-92. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN 0- 306-4473 l-2 In our study, a self-adaptive grid refinement techniquecoupled with the p-version of the finite element method is investigated.This type of approach is called the hp version of the finite element. In the modeling process,the approximate solution to the exact differential equation achieves convergence by applying two distinct solution enrichment strategies. One strategy is to solve the problem using a very coarse grid and to enrich the quality of the approximation by increasing the degree p of the interpolating polynomial shape functions for thoseelements where it is needed.The other strategy involves local grid h refinements for those elements where it is needed.
56706 Nigam, H.; Scarton, HA.; Das, P.; Kaveh, M.; Murray, F.S.; Mitchell, J.R. Experimental study of the mechanical behavior of superconducting coils Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 169 1- 1698. Edited by D.O. Thompson and D.E. Chime& Plenum Press ( 1994) ISBN o-306-4473 l-2 Mechanicaltesting of coil sections is performedunder room temperature conditions to obtain some of the stiffnesscoefficients which are needed for finite element modeling of tire composite behavior. Some of these composites are also determirssd using ultrasonic techniques. Ultrasonic methods are expected to be the preferred means of in-situ low temperature measurementsof elastic constants, and as a part of this study are being evaluated for their suitability. Acoustic emission measurementshave been made on a monotonically loaded coil section to determine the origin and nature of micro-cracking that occurs in the material.
56686 Gosz, M.; Achenbach, J.D.; Chu, Y.C.; Rokhlin. XI. Comparison of finite element and analytical determination of fiber/matrix interface stiffnesses from ultrasonic data Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 1469- 1475. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN o-306-4473 l-2 We will compare finite element and analytical methodsof inversion for determination of the interphase stiffnesses from the moduli of the composite. The effect of possible errors in measurement of composite moduli on the accuracy of the calculated interphase stiffnesses is also discussed.
244
56685 Rokhlin, S.I.; Chu, Y.C. Determination of interphasial elastic properties in fiber composites from ultrasonic bulk wave velocities Review of Progress in Quantitative Nondestructive Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13B, pp. 1461- 1468. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press ( 1994) ISBN o-306-4473 l-2 Starting with the generalized self-consistent model @SCM) we introduce for composites with nonhomogeneousfibers the concept of an equivalent fiber. While four elastic moduli for the equivalent fiber can be found using analytical models, we obtain the transverse shear modulus of the equivalent fiber using finite element calculations. We briefly describe the method to determine the interphasial moduli in fiber composites focusing on the role of the equivalent fiber. The applicability of the method to different composite systemsis also discussed. 56639
Kishoni, D.; Ta’asan, S.
Non-reflecting regions for finite difference methods in modeling of elastic wave propagation in plates Review of Progress in Quantitative Nondestruction Evaluation, Brunswick, Maine (United States), l-6 Aug. 1993. Vol. 13A, pp. 195- 202. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1994) ISBN o-306-4473 l-2 Solution of the wave equation using techniquessuch as finite difference or finite element methodscan model elastic wave propagationin solids. This requiresmapping the physical geometry into a computationaldomain whose size is governed by the size of the physical domain of interest and by the required resolution. This computational domain, in turn, dictates the computer memory requirementsas well as the calculation time. Quite often, the physical region of interest is only a part of the whole physical body, and does not necessarily include all the physical boundaries. Reduction of the calculation domain requires positioning an artificial boundary or region where a physical boundary does not exist. It is important however that such a boundary, or region, will not affect the internal domain, i.e., it should not cause reflections that propagate back into the material. This paper concentrateson the issueof constructingsuch a boundary region.
56250 Lee, Y C.; Kim, J 0.; Achenbach, J.D. Measurement of stresses by line-focus acoustic microscopy Ultrasonics, Vol. 32, No. 5, pp. 359-366 (Sep. 1994) In this paper, line-focus acousticmicroscopy has been used to determine local near-surfacestressesin isotropic materials. Two surface wave modes, namely a leaky Rayleigh wave and a leaky surface- skimming longitudinal wave, have been excited by the acoustic microscope. It has been observed that the changes of the wave velocities are. linearly proportional to the applied stresses,as predicted by acoustoelastic theory. The non-uniform stressfield in a loaded specimen has been determined from wave velocity measurements by the use of acoustoelastic constants obtained from a calibration test. The measured stressesare in good agreement with the results calculated by a finite element method. A self-calibrating method, which determines the stress profile directly from velocity measutements without a calibration test, is proposed and the results ate compared with experimental data.
56200 Nor&has, B.; Canas, R. 3D ultrasonic modelling Ultrasonics lntemational 93 Conference Proceedings, Vienna (Austria), 6-8 Jul. 1993. pp. 339-342. But&worth-Heinemann Ltd (1993) ISBN 0 750618779 We presenta 3D finite element model for propagation and diffraction of elastic waves on defect in anisotropicmedia, We usedthis model to solve an inverse problem with an iterative method. By comparison between experimental and numerical signals we can optimise the parametersof the model to converge to the solution. We present the case of an underclad crack. 56021
Bennett, J.; Hamilton, R.; Hayward, G.
Finite element modelling of 1-3 composite transducers for underwater applications IEEE 1993 Ultrasonics Symposium, Baltimore, Maryland (United States), 3 1 Oct. - 3 Nov. 1993. Vol. 2, pp. 1113-l 116. Edited by M. Levy and B.R. McAvoy. IEEE (1993) ISSN 105 l-01 17 Finite element mcdelling was employed in two separate applications involving 1-3 connectivity composite transducers: low volume fraction transducersfor hydrophoneapplications;and the influence of the electrode pattern on monolithic and composite arrays. Specifically, the composite microstructureis investigatedwith respect to the shape and distribution of the ceramic pillar, in addition to the polymer filler material. Particular