Fundamentals of Ultrasonics

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Fundamentals of Ultrasonics
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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

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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

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Wave Function

• Equation of progressive wave

• Amplitude A
• Wavelength l
• Frequency/Time period f1/T
• Velocity U Ufll/T
• Energy
• Intensity

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Waveform Wave front
Waveform the sequence in time of the motions in
a wave
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Propagation and Polarization Vector
Propagation vector the direction of wave
propagation Polarization vector the direction of
particle motion
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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, vT0.5vL,
  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.
  vR0.5vT
• Plate Lamb wave for thin plate with thickness
  less than three times the wavelength

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Parameters of Ultrasonic Waves

• Velocity the velocity of the ultrasonic wave of
  any kind can be determined from elastic moduli,
  density, and poissons ratio of the material
• Longitudial wave
• is density and m is the Poissons Ratio
• Transverse wave
• Surface wave

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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, Youngs
  modulus, etc.
• Attenuation coefficient AA0e-ax

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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.

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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

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Acoustic Impedance

• Definition the resistance offered to the
  propagation of the ultrasonic wave in a material,
  ZrU. Depend on material properties only.

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Reflection-Normal Incident

• Reflection coefficient
• Transmission coefficient

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Reflection-Oblique Incident

• Snells Law
• Reflection coefficient
• Transmission coefficient

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Total Refraction Angle
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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

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Surface Skimmed Bulk Wave

• The refracted wave travels along the surface of
  both media and at the sub-surface of media B

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Resonance
Quality factor
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Typical Ultrasound Inspection System

• Transducer convert electric signal to ultrasound
  signal
• Sensor convert ultrasound signal to electric
  signal

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Types of Transducers

• Piezoelectric
• Laser
• Mechanical (Galton Whistle Method)
• Electrostatic
• Electrodynamic
• Magnetostrictive
• Electromagnetic

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What is Piezoelectricity?

• Piezoelectricity means pressure electricity,
  which is used to describe the coupling between a
  materials 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.

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Quartz Crystals

• Highly anisotropic
• X-cut vibration in the direction perpendicular
  to the cutting direction
• Y-cut vibration in the transverse direction

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Piezoelectric Materials

• Piezoelectric Ceramics (man-made materials)
• Barium Titanate (BaTiO3)
• Lead Titanate Zirconate (PbZrTiO3) 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
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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
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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.

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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.
• kxy, 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
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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

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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

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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.
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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
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Comparison of different PZ materials for
Actuation and Sensing
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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

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Different Types of PZ Transducer
Normal beam transducer
Dual element transducer
Angle beam transducer
Focus beam transducer
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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
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Beam Profile vs. Distance
Beam profile vs. distance
Intensity vs. distance
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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

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Comparison of Ultrasound Generation
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Ultrasonic Parameter Selection

• Frequency
• Penetration decreases with frequency
• 1-10MHz NDE work on metals
• lt1MHz 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