T.O. 33B-1-1
5-1
CHAPTER 5
SECTION I
GENERAL ULTRASONIC PRINCIPLES
5
ULTRASONIC TESTING.
This method uses ultrasound to detect internal discontinuities. Ultrasound can be used on most all materials and locate
very small defects. It can be used to measure the overall thickness of a material and the specific depth of a defect. The
part needs little or no preparation. However, knowledge of the internal geometry of a part is very critical to
interpretation of any defect signal.
5.1
GENERAL ULTRASONIC PRINCIPLES.
NOTE
Whenever possible, definitions are taken from ASTM E1316, Standard
Terminology
for
Nondestructive
Examinations.
Section
1,
Ultrasonic
Examinations.
5.1.1
Characteristics of Ultrasonic Energy.
5.1.1.1
Definition—Ultrasonic.
The term ultrasonic pertains to sound waves having a frequency greater than 20,000 Hz.
5.1.1.2
Characteristics of Sound.
The transmission of ultrasound is characterized by periodic vibrations of a particle or small volume element of matter.
As a particle is displaced from its rest position by any applied stress, it moves to a maximum distance away from its rest
position (this is called a maximum displacement). The particle then reverses direction and moves past its rest position
to a maximum position in the negative direction (a second displacement). The particle then moves back to its rest
position that completes one cycle. This process continues until the source of vibration is removed.
a. The term "period" indicates the amount of time it takes to complete one cycle.
b. The term "velocity" indicates the distance traveled per unit time (second).
c. The term "frequency" indicates the number of complete cycles that occur in one second. The term
"Hertz" indicates cycles per second.
d. The term "wavelength" is the distance a wave travels while going through one cycle. Wavelength is
defined by the formula:
l(lambda) = v/f
Where:
l = wavelength (normally inches or centimeters)
v = velocity (inches or centimeters per second)
f = frequency (Hertz)
5.1.2
Generation and Receiving of Ultrasonic Vibrations.
Ultrasonic vibrations are generated by applying high-frequency electrical pulses to a transducer element (piezoelectric
element) contained within a search unit. The transducer element transforms the electrical energy into mechanical
energy (ultrasonic energy). The transducer element can also receive ultrasonic energy and transform it into electrical
