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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
  • Transcript

    • 1. Fundamentals of Ultrasonics
    • 2. 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
      • ……
    • 3. 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
    • 4. Wave Function
      • Equation of progressive wave :
      • Amplitude : A
      • Wavelength : 
      • Frequency/Time period : f=1/T
      • Velocity U : U= f  =  /T
      • Energy:
      • Intensity:
    • 5. Waveform & Wave front Waveform : the sequence in time of the motions in a wave
    • 6. Propagation and Polarization Vector Propagation vector : the direction of wave propagation Polarization vector : the direction of particle motion
    • 7. 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
    • 8. 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:
    • 9. 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
    • 10. 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.
    • 11. 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 :
    • 12. Acoustic Impedance
      • Definition: the resistance offered to the propagation of the ultrasonic wave in a material, Z=  U. Depend on material properties only.
    • 13. Reflection-Normal Incident
      • Reflection coefficient:
      • Transmission coefficient:
    • 14. Reflection-Oblique Incident
      • Snell’s Law:
      • Reflection coefficient:
      • Transmission coefficient:
    • 15. Total Refraction Angle
    • 16. 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
    • 17. Surface Skimmed Bulk Wave
      • The refracted wave travels along the surface of both media and at the sub-surface of media B
    • 18. Resonance Quality factor
    • 19. Typical Ultrasound Inspection System
      • Transducer : convert electric signal to ultrasound signal
      • Sensor : convert ultrasound signal to electric signal
    • 20. Types of Transducers
      • Piezoelectric
      • Laser
      • Mechanical (Galton Whistle Method)
      • Electrostatic
      • Electrodynamic
      • Magnetostrictive
      • Electromagnetic
    • 21. 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.
    • 22. Quartz Crystals
      • Highly anisotropic
      • X-cut: vibration in the direction perpendicular to the cutting direction
      • Y-cut: vibration in the transverse direction
    • 23. 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
    • 24. 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
    • 25. 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.
    • 26. 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
    • 27. 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
    • 28. 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
    • 29. 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.
    • 30. 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
    • 31. Comparison of different PZ materials for Actuation and Sensing
    • 32. 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
    • 33. Different Types of PZ Transducer Normal beam transducer Dual element transducer Angle beam transducer Focus beam transducer
    • 34. 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
    • 35. Beam Profile vs. Distance Beam profile vs. distance Intensity vs. distance
    • 36. 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
    • 37. Comparison of Ultrasound Generation
    • 38. 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