Definition : the science and exploitation of elastic waves in solids, liquids, and gases, which have a frequency above 20KHz.
Frequency range : 20KHz-10MHz
Non-destructive detection (NDE)
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
Equation of progressive wave :
Amplitude : A
Frequency/Time period : f=1/T
Velocity U : U= f = /T
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
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
is density and is the Poisson’s Ratio
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.
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 :
Definition: the resistance offered to the propagation of the ultrasonic wave in a material, Z= U. Depend on material properties only.
Total Refraction Angle
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
Mechanical (Galton Whistle Method)
What is Piezoelectricity?
Piezoelectricity means “pressure electricity”, which is used to describe the coupling between a material’s mechanical and electrical behaviors.
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.
X-cut: vibration in the direction perpendicular to the cutting direction
Y-cut: vibration in the transverse direction
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
PVDF (Polyvinylidene flouride) film
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
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.
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.
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
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
“ 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
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
active area controls the power and beam divergence
Large units provide more penetration
Increasing transducer size results in a loss of sensitivity
A narrow bandwidth provides good penetration and sensitivity but poor resolution