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Reaz A Chaudhuri - One of the best experts on this subject based on the ideXlab platform.

  • three dimensional asymptotic stress field in the vicinity of the circumference of a bimaterial penny shaped interfacial Discontinuity
    International Journal of Fracture, 2006
    Co-Authors: Reaz A Chaudhuri
    Abstract:

    An eigenfunction expansion method is presented to obtain three-dimensional asymptotic stress fields in the vicinity of the circumference of a bimaterial penny-shaped interfacial Discontinuity, e.g., crack, anticrack (infinitely rigid lamella), etc., located at the center, edge or corner, and subjected to the far-field torsion (mode III), extension/bending (mode I), and sliding shear/twisting (mode II) loadings. Five different Discontinuity-Surface boundary conditions are considered: (1) bimaterial penny-shaped interface anticrack or perfectly bonded thin rigid inclusion, (2) bimaterial penny-shaped interfacial jammed contact, (3) bimaterial penny-shaped interface crack, (4) bimaterial penny-shaped interface crack with partial axisymmetric frictionless slip, and (5) bimaterial penny-shaped interface thin rigid inclusion alongside penny-shaped crack. Solutions to these cases except (3) are hitherto unavailable in the literature. Closed-form expressions for stress intensity factors subjected to various far-field loadings are also presented. Numerical results presented include the effect of the ratio of the shear moduli of the layer materials, and also Poisson’s ratios on the computed lowest real parts of eigenvalues for the case (5). Interesting and physically meaningful conclusions are also presented, especially with regard to cases (1) and (2).

  • three dimensional asymptotic stress field in the vicinity of the circumference of a penny shaped Discontinuity
    International Journal of Solids and Structures, 2003
    Co-Authors: Reaz A Chaudhuri
    Abstract:

    Abstract An eigenfunction expansion method is presented to obtain three-dimensional asymptotic stress fields in the vicinity of the front of a penny shaped Discontinuity, e.g., crack, anticrack (infinitely rigid lamella), etc., subjected to the far-field torsion (mode III), extension/bending (mode I) and sliding shear/twisting (mode II) loadings. Five different Discontinuity-Surface boundary conditions are considered: (i) penny shaped crack, (ii) penny shaped anticrack or perfectly bonded thin rigid inclusion, (iii) penny shaped thin transversely rigid inclusion (frictionless planar slip permitted), (iv) penny shaped thin rigid inclusion in part perfectly bonded, the remainder with frictionless slip, and (v) penny shaped thin rigid inclusion alongside penny shaped crack. The computed stress singularity for a penny shaped anticrack is the same as that of the corresponding crack. The main difference is, however, that all the stress components at the circular tip of an anticrack depend on Poisson’s ratio under modes I and II.

Thorsten Koch - One of the best experts on this subject based on the ideXlab platform.

  • instability of a tangential Discontinuity Surface in a three dimensional compressible medium
    Physics of Fluids, 2021
    Co-Authors: Thorsten Koch
    Abstract:

    Compressible flows appear in many natural and technological processes, for instance, the flow of natural gases in a pipe system. Thus, a detailed study of the stability of tangential velocity Discontinuity in compressible media is relevant and necessary. The first early investigation in two-dimensional (2D) media was given more than 70 years ago. In this article, we continue investigating the stability in three-dimensional (3D) media. The idealized statement of this problem in an infinite spatial space was studied by Syrovatskii in 1954. However, the omission of the absolute sign of cos θ with θ being the angle between vectors of velocity and wave number in a certain inequality produced the inaccurate conclusion that the flow is always unstable for entire values of the Mach number M. First, we revisit this case to arrive at the correct conclusion, namely that the Discontinuity Surface is stabilized for a large Mach number with a given value of the angle θ. Next, we introduce a real finite spatial system such that it is bounded by solid walls along the flow direction. We show that the Discontinuity Surface is stable if and only if the dispersion relation equation has only real roots, with a large value of the Mach number; otherwise, the Surface is always unstable. In particular, we show that a smaller critical value of the Mach number is required to make the flow in a narrow channel stable.

Chongmin Song - One of the best experts on this subject based on the ideXlab platform.

  • representation of singular fields without asymptotic enrichment in the extended finite element method
    International Journal for Numerical Methods in Engineering, 2013
    Co-Authors: Sundararajan Natarajan, Chongmin Song
    Abstract:

    SUMMARY In this paper, we replace the asymptotic enrichments around the crack tip in the extended finite element method (XFEM) with the semi-analytical solution obtained by the scaled boundary finite element method (SBFEM). The proposed method does not require special numerical integration technique to compute the stiffness matrix, and it improves the capability of the XFEM to model cracks in homogeneous and/or heterogeneous materials without a priori knowledge of the asymptotic solutions. A Heaviside enrichment is used to represent the jump across the Discontinuity Surface. We call the method as the extended SBFEM. Numerical results presented for a few benchmark problems in the context of linear elastic fracture mechanics show that the proposed method yields accurate results with improved condition number. A simple code is annexed to compute the terms in the stiffness matrix, which can easily be integrated in any existing FEM/XFEM code. Copyright © 2013 John Wiley & Sons, Ltd.

  • representation of singular fields without asymptotic enrichment in the extended finite element method
    arXiv: Numerical Analysis, 2012
    Co-Authors: Sundararajan Natarajan, Chongmin Song
    Abstract:

    In this paper, we replace the asymptotic enrichments around the crack tip in the extended finite element method (XFEM) with the semi-analytical solution obtained by the scaled boundary finite element method (SBFEM). The proposed method does not require special numerical integration technique to compute the stiffness matrix and it improves the capability of the XFEM to model cracks in homogeneous and/or heterogeneous materials without a priori knowledge of the asymptotic solutions. A heaviside enrichment is used to represent the jump across the Discontinuity Surface. We call the method as the extended scaled boundary finite element method (xSBFEM). Numerical results presented for a few benchmark problems in the context of linear elastic fracture mechanics show that the proposed method yields accurate results with improved condition number. A simple MATLAB code is annexed to compute the terms in the stiffness matrix, which can easily be integrated in any existing FEM/XFEM code.

Giovanni Mastrorocco - One of the best experts on this subject based on the ideXlab platform.

  • Evaluation of the Use of UAV Photogrammetry for Rock Discontinuity Roughness Characterization
    Rock Mechanics and Rock Engineering, 2020
    Co-Authors: Riccardo Salvini, Claudio Vanneschi, John S. Coggan, Giovanni Mastrorocco
    Abstract:

    This paper describes the results of a field investigation with the objective of evaluating the possibility to produce drone-derived 3D digital point clouds sufficiently dense and accurate to determine Discontinuity Surface roughness characteristics. A discontinuous rock mass in Italy was chosen as the investigation site and Structure from Motion and Multi-View Stereo techniques adopted for producing three-dimensional point clouds from the two-dimensional image sequences. Since the roughness of discontinuities depends on direction, scale and resolution of the sampling, data were always collected along the maximum slope gradient. The scale effect was evaluated by analysing Discontinuity profiles of different lengths (10 cm, 30 cm, 60 cm and 100 cm), with measurements taken from drone flights flown at different distances from the rocky slopes (10 m, 20 m and 30 m). The accuracy of the derived joint roughness coefficients was evaluated by direct comparison with Discontinuity profiles measured during fieldwork using conventional techniques and from contemporaneous terrestrial laser scanning. Results from this research show that 3D digital point clouds, derived from the processing of drone-flight images, were successfully used for reliable representation of Discontinuity roughness for profiles longer than 60 cm, whereas less reliable results were achieved for shorter profile lengths. This, even if strictly related to this case study since several factors can affect the minimum profile length, represents a significant contribution to improve the knowledge on the use of remotely captured data for characterising the discontinuities in natural or man-made rock outcrops, particularly where access difficulties do not allow conventional engineering-geological surveys to be undertaken.

A. Alberti - One of the best experts on this subject based on the ideXlab platform.

  • Rilievo in tempo reale di difetti superficiali su corpi in movimento a velocità elevata con ultrasuoni senza contatto
    Gruppo Italiano Frattura, 2012
    Co-Authors: A. Alberti, A. Mancuso, V. Nigrelli, D. Cerniglia
    Abstract:

    The detection of Surface defects during periodic inspection is important because, usually, the stresses are higher at the Surface, thus affecting the growth of the Discontinuity. Surface defects can be detected using some traditional non-destructive testing methods, such as eddy current, dye penetrant, magnetic particle and ultrasonic. Some of these methods can be used only in static conditions; the others have limitations for the dynamic inspection.The recent progress in the field of non-contact ultrasonic sensors has led us to develop a simple system for the real-time inspection of moving bodies at a high speed. The paper presents the possibility of using, with the developed system, two methodologies, based on non-contact ultrasound, to detect Surface defects on objects moving at 100 km/h, so that in service inspection is possible. A methodology, based on laser and air-coupled sensors, uses the advantages of laser-generated Surface waves; the analysis is done on the reflected wave, created by the interaction of the Surface wave with the defect. The inspected Surface layer can be selected from the wavelength of the Surface wave. The other methodology, based on air-coupled sensors, uses the disadvantages of the ultrasound transmission at the air/metal interface; the analysis is done on the diffraction of the wave reflected from the Surface. The execution of the inspection is simple with both techniques. The experimental results indicate a good efficiency of the two methodologies proposed for the real-time detection of Surface defects on objects moving at high speed

  • rilievo in tempo reale di difetti superficiali su corpi in movimento a velocita elevata con ultrasuoni senza contatto real time non contact ultrasonic detection of Surface defects on objects moving at high speed
    2012
    Co-Authors: D. Cerniglia, V. Nigrelli, A. Mancuso, A. Alberti
    Abstract:

    The detection of Surface defects during periodic inspection is important because, usually, the stresses are higher at the Surface, thus affecting the growth of the Discontinuity. Surface defects can be detected using some traditional non-destructive testing methods, such as eddy current, dye penetrant, magnetic particle and ultrasonic. Some of these methods can be used only in static conditions; the others have limitations for the dynamic inspection. The recent progress in the field of non-contact ultrasonic sensors has led us to develop a simple system for the real-time inspection of moving bodies at a high speed. The paper presents the possibility of using, with the developed system, two methodologies, based on non-contact ultrasound, to detect Surface defects on objects moving at 100 km/h, so that in service inspection is possible. A methodology, based on laser and air-coupled sensors, uses the advantages of laser-generated Surface waves; the analysis is done on the reflected wave, created by the interaction of the Surface wave with the defect. The inspected Surface layer can be selected from the wavelength of the Surface wave. The other methodology, based on air-coupled sensors, uses the disadvantages of the ultrasound transmission at the air/metal interface; the analysis is done on the diffraction of the wave reflected from the Surface. The execution of the inspection is simple with both techniques. The experimental results indicate a good efficiency of the two methodologies proposed for the real-time detection of Surface defects on objects moving at high speed. SOMMARIO. Il rilievo di difetti superficiali durante le ispezioni periodiche e importante poiche in genere le sollecitazioni sono maggiori in superficie e possono accelerare la crescita delle discontinuita. I difetti superficiali possono essere rilevati utilizzando alcuni metodi tradizionali di controllo non distruttivo, quali correnti indotte, liquidi penetranti, polveri magnetiche ed ultrasuoni. Alcuni tra questi metodi possono essere usati solo in condizioni statiche; gli altri presentano delle limitazioni per l'ispezione dinamica. I progressi recenti nel campo dei sensori ultrasonori senza contatto ci hanno permesso di sviluppare un sistema semplice per l'ispezione in tempo reale di corpi in movimento a velocita elevata. Nel lavoro viene presentata la possibilita di usare, col sistema sviluppato, due metodologie, basate sugli ultrasuoni generati e ricevuti senza contatto con la struttura, per rilevare difetti superficiali su corpi in movimento a 100 km/h, cosi da poter effettuare l'ispezione anche in servizio. Una metodologia, basata sull'uso di laser e trasduttori senza contatto, utilizza i vantaggi delle onde superficiali generate con il laser; l'analisi viene fatta sull'onda riflessa, creata dall'interazione dell'onda superficiale con il difetto. Lo spessore superficiale ispezionato e selezionabile dalla lunghezza d'onda dell'onda