Fundamentals of ultrasound
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Fundamentals of ultrasound

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  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies
  • Acoustics: waves of lower frequencies

Fundamentals of ultrasound Fundamentals of ultrasound Presentation Transcript

  • Fundamentals of Ultrasonics
  • Ultrasonics
    • Definition : the science and exploitation of elastic waves in solids, liquids, and gases, which have a frequency above 20KHz.
    • Frequency range : 20KHz-10MHz
    • Applications :
    • Non-destructive detection (NDE)
    • Medical diagnosis
    • Material characterization
    • Range finding
    • ……
  • Elastic wave
    • Definition : An elastic wave carries changes in stress and velocity. Elastic wave is created by a balance between the forces of inertia and of elastic deformation.
    • Particle motion : elastic wave induced material motion
    • Wavespeed : the propagation speed of the elastic wave
    • Particle velocity is much smaller than wavespeed
  • Wave Function
    • Equation of progressive wave :
    • Amplitude : A
    • Wavelength : 
    • Frequency/Time period : f=1/T
    • Velocity U : U= f  =  /T
    • Energy:
    • Intensity:
  • Waveform & Wave front Waveform : the sequence in time of the motions in a wave
  • Propagation and Polarization Vector Propagation vector : the direction of wave propagation Polarization vector : the direction of particle motion
  • Wave Propagation
    • Body wave : wave propagating inside an object
      • Longitudinal (pressure) wave: deformation is parallel to propagation direction
      • Transverse (shear) wave: deformation is perpendicular to propagation direction, v T =0.5v L, generated in solid only
    • Surface wave : wave propagating near to and influenced by the surface of an object
      • Rayleigh wave: The amplitude of the waves decays rapidly with the depth of propagation of the wave in the medium. The particle motion is elliptical. v R =0.5v T
      • Plate Lamb wave: for thin plate with thickness less than three times the wavelength
  • Parameters of Ultrasonic Waves
    • Velocity : the velocity of the ultrasonic wave of any kind can be determined from elastic moduli, density, and poisson’s ratio of the material
      • Longitudial wave:
        • is density and  is the Poisson’s Ratio
      • Transverse wave:
      • Surface wave:
  • Attenuation
    • Definition : the rate of decrease of energy when an ultrasonic wave is propagating in a medium. Material attenuation depends on heat treatments , grain size , viscous friction , crystal structure , porosity , elastic hysterisis , hardness , Young’s modulus , etc.
    • Attenuation coefficient : A=A 0 e -  x
  • Types of Attenuation
    • Scattering : scattering in an inhomogeneous medium is due to the change in acoustic impedance by the presence of grain boundaries inclusions or pores, grain size, etc.
    • Absorption : heating of materials, dislocation damping, magnetic hysterisis.
    • Dispersion : frequency dependence of propagation speed
    • Transmission loss : surface roughness & coupling medium.
  • Diffraction
    • Definition : spreading of energy into high and low energy bands due to the superposition of plane wave front.
    • Near Field :
    • Far Field :
    • Beam spreading angle :
  • Acoustic Impedance
    • Definition: the resistance offered to the propagation of the ultrasonic wave in a material, Z=  U. Depend on material properties only.
  • Reflection-Normal Incident
    • Reflection coefficient:
    • Transmission coefficient:
  • Reflection-Oblique Incident
    • Snell’s Law:
    • Reflection coefficient:
    • Transmission coefficient:
  • Total Refraction Angle
  • Mode Conversion
    • When a longitudinal wave is incident at the boundary of A & B, two reflected beams are obtained.
    • Selective excite different type of ultrasonic wave
  • Surface Skimmed Bulk Wave
    • The refracted wave travels along the surface of both media and at the sub-surface of media B
  • Resonance Quality factor
  • Typical Ultrasound Inspection System
    • Transducer : convert electric signal to ultrasound signal
    • Sensor : convert ultrasound signal to electric signal
  • Types of Transducers
    • Piezoelectric
    • Laser
    • Mechanical (Galton Whistle Method)
    • Electrostatic
    • Electrodynamic
    • Magnetostrictive
    • Electromagnetic
  • What is Piezoelectricity?
    • Piezoelectricity means “pressure electricity”, which is used to describe the coupling between a material’s mechanical and electrical behaviors.
      • Piezoelectric Effect
        • when a piezoelectric material is squeezed or stretched, electric charge is generated on its surface.
      • Inverse Piezoelectric Effect
        • Conversely, when subjected to a electric voltage input, a piezoelectric material mechanically deforms.
  • Quartz Crystals
    • Highly anisotropic
    • X-cut: vibration in the direction perpendicular to the cutting direction
    • Y-cut: vibration in the transverse direction
  • Piezoelectric Materials
    • Piezoelectric Ceramics (man-made materials)
      • Barium Titanate (BaTiO 3 )
      • Lead Titanate Zirconate (PbZrTiO 3 ) = PZT, most widely used
      • The composition, shape, and dimensions of a piezoelectric ceramic element can be tailored to meet the requirements of a specific purpose.
    Photo courtesy of MSI, MA
  • Piezoelectric Materials
    • Piezoelectric Polymers
      • PVDF (Polyvinylidene flouride) film
    • Piezoelectric Composites
      • A combination of piezoelectric ceramics and polymers to attain properties which can be not be achieved in a single phase
    Image courtesy of MSI, MA
  • Piezoelectric Properties
    • Anisotropic
    • Notation: direction X, Y, or Z is represented by the subscript 1, 2, or 3, respectively, and shear about one of these axes is represented by the subscript 4, 5, or 6, respectively.
  • Piezoelectric Properties
    • The electromechanical coupling coefficient, k , is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or vice versa.
      • k xy , The first subscript (x) to k denotes the direction along which the electrodes are applied; the second subscript (y) denotes the direction along which the mechanical energy is developed. This holds true for other piezoelectric constants discussed later.
      • Typical k values varies from 0.3 to 0.75 for piezoelectric ceramics.
    or
  • Piezoelectric Properties
    • The piezoelectric charge constant, d, relates the mechanical strain produced by an applied electric field,
      • Because the strain induced in a piezoelectric material by an applied electric field is the product of the value for the electric field and the value for d, d is an important indicator of a material's suitability for strain-dependent (actuator) applications.
      • The unit is Meters/Volt, or Coulombs/Newton
  • Piezoelectric Properties
    • The piezoelectric constants relating the electric field produced by a mechanical stress are termed the piezoelectric voltage constant, g,
      • Because the strength of the induced electric field in response to an applied stress is the product of the applied stress and g, g is important for assessing a material's suitability for sensor applications.
      • The unit of g is volt meters per Newton
  • SMART Layer for Structural Health Monitoring
    • Smart layer is a think dielectric film with built-in piezoelectric sensor networks for monitoring of the integrity of composite and metal structures developed by Prof. F.K. Chang and commercialized by the Acellent Technology , Inc. The embedded sensor network are comprised of distributed piezoelectric actuators and sensors.
    Image courtesy of FK Chang, Stanford Univ.
  • Piezoelectric Wafer-active Sensor
    • Read paper:
      • “ Embedded Non-destructive Evaluation for Structural Health Monitoring, Damage Detection, and Failure Prevention” by V. Giurgiutiu, The Shock and Vibration Digest 2005; 37; 83
    • Embedded piezoelectric wafer-active sensors (PWAS) is capable of performing in-situ nondestructive evaluation (NDE) of structural components such as crack detection.
    Image courtesy of V. Giurgiutiu, USC
  • Comparison of different PZ materials for Actuation and Sensing
  • Thickness Selection of a PZ transducer
    • Transducer is designed to vibrate around a fundamental frequency
    • Thickness of a transducer element is equal to one half of a wavelength
  • Different Types of PZ Transducer Normal beam transducer Dual element transducer Angle beam transducer Focus beam transducer
  • Characterization of Ultrasonic Beam
    • Beam profile or beam path
    • Near field: planar wave front
    • Far field: spherical wave front, intensity varies as the square of the distance
    • Determination of beam spread angle
    • Transducer beam profiling
    Near field planar wave front
  • Beam Profile vs. Distance Beam profile vs. distance Intensity vs. distance
  • Laser Generated Ultrasound (cont’)
    • Thermal elastic region : ultrasound is generated by rapid expansion of the material
    • Ablation region : ultrasound is generated by plasma formed by surface vaporization
  • Comparison of Ultrasound Generation
  • Ultrasonic Parameter Selection
    • Frequency :
      • Penetration decreases with frequency
        • 1-10MHz: NDE work on metals
        • <1MHz: inspecting wood, concrete, and large grain metals
      • Sensitivity increases with frequency
      • Resolution increases with frequency and bandwidth but decrease with pulse length
      • Bream spread decrease with frequency
    • Transducer size:
      • active area controls the power and beam divergence
      • Large units provide more penetration
      • Increasing transducer size results in a loss of sensitivity
    • Bandwidth
      • A narrow bandwidth provides good penetration and sensitivity but poor resolution