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  • 1.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Brismar, Torkel B.
    Karolinska University Hospital.
    Paradossi, Gaio
    Università di Roma Tor Vergata.
    Characterization of Acoustic Properties of PVA-Shelled Ultrasound Contrast Agents2010In: Ultrasound Contrast Agents: Targeting And Processing Methods For Theranostics / [ed] G. Paradosi, P. Pellegretti, A. Trucco, Italia: Springer-Verlag , 2010, p. 99-108Chapter in book (Other academic)
    Abstract [en]

    This work examines the acoustic behavior of ultrasound contrast agents made of poly (vinyl alcohol) (PVA) shelled microbubbles manufactured at three different pH and temperature conditions. Backscattering amplitude, attenuation coefficient and phase velocity of ultrasonic waves propagating through suspensions of PVA contrast agents were measured at temperature values ranging between 24 oC and 37 oC in a frequency range from 3 MHz to 13 MHz.  A significant enhancement of the backscattering amplitude and displaying a weak dependence on temperature were observed.  Attenuation and phase velocity, on the other hand, showed higher sensitivity to temperature variations.  The dependence on system parameters such as the number of cycles, frequency, and exposure of the peak negative pressure, Pthr, at which ultrasound contrast agents fracture was also investigated.  The effects of temperature, blood, and, wherever data are available, of the dimension of the microbubbles on Pthr are also considered.  The large shell thickness notwithstanding, the results of this investigation show that at room temperature PVA contrast agents fracture at negative peak pressure values within the recommended safety limit.  Furthermore, Pthr decreases with increasing temperature, radius of the microbubbles, and number of cycles of the incident wave.  In conclusion, these results suggest that PVA-shelled microbubbles may offer a potentially viable system to be employed for both imaging and therapeutic purposes.

  • 2.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Brismar, Torkel B.
    Paradossi, Gaio
    Characterization of acoustic properties of PVA-shelled ultrasound contrast agents: linear properties (Part I)2009In: Ultrasound in Medicine and Biology, ISSN 0301-5629, E-ISSN 1879-291X, Vol. 35, no 7, p. 1127-1138Article in journal (Refereed)
    Abstract [en]

    This work examines the linear acoustic behavior of ultrasound contrast agents made of three types of poly (vinyl alcohol) (PVA) shelled microbubbles manufactured at different pH and temperature conditions. Back-scattered power, attenuation coefficient and phase velocity of ultrasonic waves propagating through suspensions of PVA contrast agents were measured at temperature values ranging between 24 degrees C and 37 degrees C in a frequency range from 3 MHz to 13 MHz. Enhancement of the backscattered power higher than 20 dB and displaying a weak dependence on temperature was observed. Attenuation and phase velocity, on the other hand, showed higher sensitivity to temperature variations. A modified version of the Church model, which accounts for the dispersion of the dynamic modulus of the PVA shells, was developed to simultaneously fit the attenuation and phase velocity data at 24 degrees C. The frequency dependence of the storage modulus was found to be that of semiflexible polymeric networks. On the other hand, the frequency dependence of the dynamic loss modulus suggests that additional mechanisms, which may be related to the finite dimensions of the shell and/or to its inhomogeneity, may play a significant role in the dissipation of the acoustic energy. For the microbubbles of interest, this model predicts frequency dependent resonance frequency higher than 100 MHz.

  • 3.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Brismar, Torkel B.
    Paradossi, Gaio
    Characterization of acoustic properties of PVA-shelled ultrasound contrast agents: ultrasound-induced fracture (Part II)2009In: Ultrasound in Medicine and Biology, ISSN 0301-5629, E-ISSN 1879-291X, Vol. 35, no 7, p. 1139-1147Article in journal (Refereed)
    Abstract [en]

    Knowledge of the magnitude of the peak negative pressure, P-thr, at which ultrasound contrast agents fracture is relevant for using these microbubbles both as devices for contrast enhancement purposes, as well as carriers of drugs to be delivered locally. In the second part of this communication, the acoustic properties of three types of microbubbles stabilized by poly (vinyl alcohol) (PVA) shells are further investigated. In particular, the dependence of P-thr on system parameters such as the number of cycles, frequency and exposure is examined. The effects of temperature, blood and, wherever data are available, of the dimension of the microbubbles on P-thr are also considered. The large shelf thickness notwithstanding, the results of this investigation show that at room temperature, PVA contrast agents fracture at negative peak pressure values within the recommended safety limit. Furthermore, P-thr decreases with increasing temperature, radius of the microbubbles and number of cycles of the incident wave. Fatigue seems to be a physical mechanism playing a dominant role in the fracture process. The effect of blood on P-thr varies according to condition under which the microbubbles have been synthesized, although stiffening of the shell is observed in most cases. In conclusion, these results suggest that PVA-shelled microbubbles may offer a potentially viable system to be employed for both imaging and therapeutic purposes.

  • 4.
    Grishenkov, Dmitry
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Brismar, Torkel B.
    Karolinska University Hospital.
    Paradossi, Gaio
    Università di Roma Tor Vergata.
    On the acoustic properties of polymer-shell ultrasonic contrast agents.2008Conference paper (Other academic)
    Abstract [en]

    Polymer-shelled microbubbles have become the focus of intense research for their enhanced shelf life and demonstrated chemical versatility.  These are properties highly sought after in the ultrasonic contrast agents (UCAs) of the next generation, which will be engineered not only to enhance the contrast of ultrasound-based images, but also to function as carriers of drugs to be delivered locally.  Here, the results of an experimental investigation of three potentially new UCAs are presented.  These microbubbles are stabilized by thick poly (vinyl alcohol) shells.  These UCAs differ from each other in their dimensions and shell thickness (order of 0.5 microns).  Fundamental to their use as drug carrier is the knowledge of the pressure threshold at which the shell of these UCAs fractures.  Therefore, the dependence of this quantity on temperature, number of cycles of the incident pulse, nominal central frequency and pulse repetition frequency of the emitting transducer is examined.  The effect of using blood instead of deionized water is also considered.  The main conclusion to be drawn from these results is that their thick shell notwithstanding, these microbubbles begin to fracture at values of MI which can be acceptable in clinical applications.  This claim is supported also by images acquired by means of commercially available imaging systems.  Finally, these values of the pressure threshold are correlated with estimates of the shells’ visco-elastic constants obtained by fitting Church’s model to the frequency-dependent attenuation coefficient and phase velocity.  Open questions arising from this comparison are briefly discussed.

  • 5. Mendelsohn, D. A.
    et al.
    Vedachalam, S.
    Mokashi, P. S.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    A perturbation approach to the nonlinear vibration of a damaged beam2008In: AIP Conf. Proc., 2008, p. 1395-1402Conference paper (Refereed)
    Abstract [en]

    The vibration characteristics of a cohesively cracked Euler-Bernoulli beam are investigated using the modified line-spring method. The crack plane is statically loaded in bending into the nonlinear region and small amplitude vibrations are superposed about this state. The deflection solution form is then separated into the zeroth order linear and first order nonlinear components. Several vibration characteristics such as deflection, slope and modal amplitude are studied as a function of the applied static moment.

  • 6.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    A note on the sensitivity of SV wave scattering to surface-breaking cracks2005In: Ultrasonics, ISSN 0041-624X, E-ISSN 1874-9968, Vol. 43, no 7, p. 508-511Article in journal (Refereed)
    Abstract [en]

    The results presented in this communication show that the conventional configuration adopting a shear vertical wave at 45 degrees of incidence to detect cracks breaking the surface opposite the one on which the inspecting transducer is placed is not optimal when the crack depth is of the order of one wavelength of the inspecting wave or smaller. Angles of incidence proximal to the critical angle of longitudinal wave should be used instead.

  • 7.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Adhesion and nonlinear scattering by rough surfaces in contact: Beyond the phenomenology of the Preisach-Mayergoyz framework2004In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 116, no 4, p. 1938-1947Article in journal (Refereed)
    Abstract [en]

    Phenomenological models reproducing the elasticity and acoustic properties of geomaterials and materials with damage have been successfully developed. These models yield macroscopic stress-strain constitutive equations featuring hysteresis with end-point memory, and predict the efficient generation of higher harmonics accompanying the propagation of monochromatic waves. The assumption common to these models is that the material's microstructure is characterized by nonlinear compliant components of an unspecified nature which can exist in two states: "open" or "closed." The density of the compliant units is defined on a mathematical continuum (the Preisach-Mayergoyz space) whose elements identify the dynamic behavior of the components. In this work, adhesion is shown to introduce hysteresis with end-point memory in the macroscopic behavior of an interface between two rough surfaces in contact, and, upon scattering, to generate higher harmonics bearing a striking similarity to those observed in wave propagation phenomena in media with distributed damage and in geomaterials. It appears, therefore, that two rough surfaces interacting via adhesion forces offer a meaningful example of macroscopic interface or bond with dynamics resembling that of the fictitious elements of the Preisach-Mayergoyz space, and acoustic nonlinear properties similar to those of rocks and damaged materials.

  • 8.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Spring boundary model for a partially closed crack2008In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 46, no 2, p. 182-188Article in journal (Refereed)
    Abstract [en]

    The spring boundary conditions have been widely employed to describe the acoustic properties of rough surfaces in partial contact. Central to this model is the role of the interfacial stiffness. Recently, a singular stiffness model has been proposed to remove inconsistencies which arise when these boundary conditions are used to describe the acoustic response of partially closed cracks with a position-independent and finite stiffness. Here, an alternative solution to this problem is discussed which is based on the micromechanics of rough surfaces in contact. For cracks that are partially closed, it prescribes a finite crack stiffness that varies along the crack faces and, rather than diverging, becomes null within a small but finite neighborhood of the crack tip. The conditions under which the local stiffness of a non-planar crack can be evaluated are also reviewed.

  • 9.
    Pecorari, Claudio
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Grishenkov, Dmitry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Ultrasound.
    Characterization of ultrasound-induced fracture of polymer-shelled ultrasonic contrast agents by correlation analysis2008Conference paper (Other academic)
  • 10.
    Pecorari, Claudio
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Grishenkov, Dmitry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Characterization of ultrasound-induced fracture of polymer-shelled ultrasonic contrast agents by correlation analysis2007In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 122, no 4, p. 2425-2430Article in journal (Refereed)
    Abstract [en]

    Beyond a characteristic value of the negative peak pressure, ultrasound fracture the shell of ultrasonic contrast agents (UCAs). Existing criteria for ascertaining this threshold value exploit the dependence of the amplitude of the UCA acoustic response on the incident pressure. However, under the common experimental conditions used in this work, these criteria appear to be unreliable when they are applied to UCAs that are stabilized by a thick polymeric shell. An alternative criterion for determining the onset of shell fracture is introduced here, which uses variations of the shape of the acoustic time-domain response of an UCA suspension. Experimental evidence is presented that links the changes of the cross-correlation coefficient between consecutive time-domain signals to the fracture of the shells, and consequent release of air microbubbles. In principle, this criterion may be used to characterize similar properties of other types of particles that cannot undergo inertial cavitation.

  • 11.
    Pecorari, Claudio
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Poznic, Milan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Nonlinear acoustic scattering by a partially closed surface-breaking crack2005In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 117, no 2, p. 592-600Article in journal (Refereed)
    Abstract [en]

    A theoretical model describing the nonlinear scattering of acoustic waves by surface-breaking cracks with faces in partial contact is presented. The nonlinear properties of the crack are accounted for by suitable boundary conditions that are derived from micromechanical models of the dynamics of elastic rough surfaces in contact. Both linear and nonlinear responses of the crack are shown to be largest for a shear vertical wave incident on the surface containing the crack at an angle just above the critical angle for longitudinal waves. These findings question the fitness for the purpose of a conventional inspection method, which utilizes shear vertical waves at 45degrees of incidence to search for surface-breaking cracks in many engineering components. For angles of incidence proximal to the critical angle of longitudinal waves, the efficiency of the second harmonic's generation appears to be the highest. Thanks to the increased sensitivity to surface-breaking cracks, this configuration seems to offer a solution to the localization problem, a task that has eluded nonlinear techniques operating under other circumstances. Finally, this model suggests a simple interpretation of the highly localized nonlinear response of delaminations in composite materials.

  • 12.
    Pecorari, Claudio
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Poznic, Milan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    On the linear and nonlinear acoustic properties of dry and water-confining elasto-plastic interfaces2006In: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences, ISSN 1364-5021, E-ISSN 1471-2946, Vol. 462, no 2067, p. 769-788Article in journal (Refereed)
    Abstract [en]

    An experimental investigation into the linear reflection and generation of the second harmonic component following the incidence of an ultrasonic wave onto a dry or water-confining interface between elasto-plastic steel-steel surfaces in contact is presented. The results on dry interfaces show that, although the theoretical models currently used to estimate the stiffness of such interfaces constitute a valid framework, the statistics of asperities in contact requires further development to account for the effects of the elasto-plastic deformation. Similarly, the results with water-confining interfaces indicate that current models not accounting for the liquid-mediated forces between the solid surfaces cannot explain the results presented here. In fact, the experimental evidence is interpreted as suggesting that structural repulsive forces may be responsible for the observed phenomena. Finally. the level of second harmonic generation for these interfaces offers support by the development of inspection techniques which exploit such wave phenomena for the detection of partially closed and dry, or nearly open and water-trapping stress-corrosion surface-breaking cracks.

  • 13.
    Pecorari, Claudio
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Rokhlin, Stanislav I.
    Elasto-plastic micromechanical model for determination of dynamic stiffness and real contact area from ultrasonic measurements2007In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 262, no 7-8, p. 905-913Article in journal (Refereed)
    Abstract [en]

    The interaction of ultrasonic waves with rough surfaces in contact is sensitive to the micromechanics of the contacts and to the topography of the surfaces. A continuous effort to develop a link between ultrasonic measurements and interfacial properties has evolved to the extent that elasto-plasticity occurring during loading can now be taken into account. Here, a state-of-the-art model is critically revisited in light of recent results on the micromechanics of elasto-plastic asperities in contact during unloading. A new probability density function of the asperity height is derived, which takes into account plastic deformation. This function is used to evaluate the pressure during unloading as a function of the relative approach between the mean planes of the surfaces. It is also shown that this relation can be cast in terms of the probability density function prior deformation, provided that a suitable transformation variable is utilized. These results are used to obtain a new expression for the dynamic stiffness of the elasto-plastic interface which determines the response of the latter to an incident wave. Finally, some results illustrating the behavior of the static and dynamic interfacial stiffness during a complete loading-unloading cycle are presented.

  • 14.
    Poznic, Milan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Discriminating surface-breaking from subsurface cracks in pressurized water pipes by means of a parametric modulation technique2008In: NDT & E international, ISSN 0963-8695, E-ISSN 1879-1174, Vol. 41, no 3, p. 208-216Article in journal (Refereed)
    Abstract [en]

    The discrimination of surface-breaking cracks from cracks that are entirely embedded within the material is paramount to the assessment of the integrity of pressurized water pipes. This work addresses this issue investigating the use of a parametric modulation technique towards this end. Tests are conducted on 1D surface-breaking cracks that are either dry or contain water. The response of these defects is investigated as a function of crack depth and closure, as well as of amplitude and frequency of the modulation. The ensuing results suggest that fluid-filled and dry cracks can be discriminated from each other. The origin of the different response of these defects is discussed in terms of both crack's stiffness and modulation-induced dynamics.

  • 15.
    Poznic, Milan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Nonlinear scattering by a partially closed surface breaking crack2005In: Review of Progress in Quantitative Nondestructive Evaluation, Vols 24A and 24B / [ed] Thompson, DO; Chimenti, DE, MELVILLE: AMER INST PHYSICS , 2005, Vol. 760, p. 43-50Conference paper (Refereed)
    Abstract [en]

    Nonlinear scattering from a partially closed, two dimensional, surface-breaking crack in a homogenous linear elastic half-space is considered. The boundary conditions along the crack are described in terms of the mechanics of two distributions of nonlinear springs. Both the linear and the nonlinear acoustic response of the crack shows to be highest when the surface breaking crack is insonified by a shear vertical wave at the longitudinal critical angle. The increased efficiency of generation of higher harmonics under the above circumstances suggests a means of localizing the nonlinear crack. Finally, the model provides a simple interpretation of the highly localized nonlinear response of delaminations observed in thin composites.

  • 16.
    Poznic, Milan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    So… is this a surface-breaking crack?2006In: Journal of Mechanics of Materials and Structures, ISSN 1559-3959, Vol. 1, no 4, p. 743-762Article in journal (Refereed)
    Abstract [en]

    An inspection technique used to assess the structural integrity of critical components in a nuclear power plant must be able to discern surface-breaking cracks from subsurface cracks. This work proposes an ultrasonic method to provide that information and presents a theoretical investigation into it. The main assumption of the model is that water carried by pressurized pipes infiltrates and fills a surface-breaking crack, while a subsurface crack is dry. The model simulates an inspection in which the modulation technique is employed and the surface hosting the crack is not accessible. A ratio, R, constructed with signals recorded in backscattering configuration during a modulation cycle, is examined and shown to provide a clear criterion allowing subsurface cracks to be distinguished from surface-breaking cracks when a shear vertical wave at 45 degree incidence is employed as a probe.

  • 17. Ueda, Sei
    et al.
    Biwa, Shiro
    Watanabe, Kazumi
    Heuer, Rudolf
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    On the stiffness of spring model for closed crack2006In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 44, no 13-14, p. 874-888Article in journal (Refereed)
    Abstract [en]

    Implications of the spring model used for a closed crack are discussed in terms of the resulting asymptotic crack-tip singularity as well as the field solutions for specific examples of an anti-plane crack. The spring stiffness distribution on the crack surfaces is assumed to be represented by the sum of a constant term and a singular term that is inversely proportional to the distance from the crack tip. Exact expressions for the eigenfunction around the crack tip are obtained and the stress singularity is discussed in some details. The stress singularity at the crack tip is found to be governed by the parameter of the singular stiffness term but not by the constant stiffness term. When the singular stiffness vanishes, the stress singularity reduces to the well-known inverse square-root singularity of an open (traction-free) crack, irrespective of the constant stiffness. In order to delineate the influence of the stiffness distribution on the stress intensity factor (SIF), the crack opening displacement (COD) and the stress distribution on the crack plane, analytical descriptions as well as numerical solutions are also demonstrated for an anti-plane crack under static and dynamic shear loading by the aid of the singular integral equation technique.

  • 18. Watanabe, Kazumi
    et al.
    Ueda, Set
    Biwa, Shiro
    Pecorari, Claudio
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Heuer, Rudolf
    Closed interface crack with singular spring stiffness model2007In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 45, no 2-8, p. 210-226Article in journal (Refereed)
    Abstract [en]

    The singularity of the stress field at the tip of a partially closed interface crack is discussed based on a non-uniform spring stiffness model that includes an inverse function of the radial distance from the crack edge. The stress singularity is found to be governed by the coefficient of the inverse function, henceforth referred to as "stiffness intensity" due to its resemblance to the well-known stress intensity factor. Two distinct non-oscillatory singular stress fields, corresponding to Modes I and 11 deformations, respectively, are found to coexist. An oscillatory singularity may also appear, but only when the two stiffness intensities are close to each other. This means that the oscillatory singularity exists when the normal and tangential interactions between the upper and lower crack faces are close.

1 - 18 of 18
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