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Introduction to Ultrasonic Principles
What is Ultrasound? Ultrasound is a mechanical wave with a
frequency exceeding the upper limit of human hearing, which is 20 000Hz (20kHz)
Industrial Ultrasound: 1MHz to 12MHz
Spectrum of Sound
Atomic Structures
Understanding wave propagation
Understanding wave propagation
Particle oscillation and wave propagation along parallel directions
Understanding wave propagation
Particle oscillation and propagation along perpendicular directions
Understanding wave propagation
»The wavelength (mm) is given by :Where;
V : sound velocity (m/s)
f : probe frequency (Hz)
Longitudinal (in the steel)
Transverse (in the steel)
Coupling (in the water)
No Ultrasonic Wave
traveling trough the air
Understanding wave propagation
f
V
Behavior at an interface
Behavior at an Interface
Ultrasound Production
Ultrasound Production
Piezoelectric Effect
Piezoelectric Effect
Short voltage pulse (<1µs) generates an oscillation at the crystal resonant frequency
What’s a UT Probe
Ultrasonic probes
Ultrasonic pulses
»Piezoelectric element transforms electrical energy into sound waves and vice-verca : electric voltage mechanical displacements
»Piezoelectric element produces damped oscillation ; ultrasonic pulse and in reception the electric RF signal
»Sound waves are emitted into the material and are eventually reflected and received back by the probe, thus generating echoes
Frequency vs Crystal Thickness
Frequency role The frequency also affects the QUALITY of
the ultrasound image– The HIGHERHIGHER the frequency, the BETTERBETTER the
resolution– The LOWERLOWER the frequency, the LESSLESS the
resolution
12MHz transducer has very good resolution, but cannot penetrate very deep in the body
3MHz transducer can penetrate deep into the body, but the resolution is not as good as high frequencies.
Types of Resolution
Types of Resolution
Inspection modes
Direct Contact
Through transmission
Immersion Testing
Inspection Modes
6320 m/s in Aluminium 3130 m/s in Aluminium
Longitudinal Wave Mode (LW)– Straight beam probe
»Detection of flaws parallel to the surface: Process defects, inclusions, porosity.
»Comparison of the backwall (BW) echo, and intermediate echoes.
Shear Wave Mode (SW)
»Straight beam probe at angle with the surface change mode fromLW -> SW
»Detection of subsurface defects: Process defects, cracks, inclusions.
»Comparison of Backwall Echo, and intermediate echoes
Coupling Mode »Straight beam probe
Longitudinal Wave»Detection of water
between the probe and the surface of the bar :
»Gate verify the presence of the Frontwall Echo
»If not, no water»No Ultrasonic wave
travel in the air
Limitations of UT systems
Limitations of UT systemsDead zones at the beginning and at the end of the bar
Limitations of UT systemsDead zones inside the bar