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