3
Technical Notes
e. Applying Ultrasound
Ultrasonic nondestructive testing introduces high-frequency sound waves
into a test object to obtain information about the object without altering or
damaging it in any way. Two basic quantities are measured in ultrasonic
testing; they are time of flight, or the amount of time for the sound to travel
through the sample, and the amplitude of the received signal. Based on
velocity and round trip time of flight through the material, the material's
thickness can be calculated as follows:
Eqn. 4
T = Material Thickness
c = Material Sound Velocity
t = Time of Flight
Measurements of the relative change in signal amplitude can be used in
sizing flaws or measuring the attenuation of a material. The relative change
in signal amplitude is commonly measured in decibels. Decibel values are
the logarithmic value of the ratio of two signal amplitudes. This can be
calculated using the following equation. Some useful relationships are also
displayed in the table below;
Eqn. 5
dB = Decibels
A
1
= Amplitude of signal 1
A
2
= Amplitude of signal 2
A
1
Ratio dB
A
2
100%
1.4142 3
70.71%
100%
2 6
50%
100%
4 12
25%
100%
10 20
10%
100%
100 40
1%
f. Sensitivity and Resolution
- Sensitivity is the ability of an ultrasonic system to detect
reflectors (or defects) at a given depth in a test material.
The greater the signal that is received from a given reflector,
the more sensitive the transducer system.
- Axial resolution is the ability of an ultrasonic system to
produce simultaneous and distinct indications from reflectors
Iocated at nearly the same position with respect to the sound
beam.
- Near-surface resolution is the ability of the ultrasonic system
to detect reflectors located close to the surface of the test
piece.
2. Advanced Definitions and Formulas
a. Transducer Waveform and Spectrum
Transducer waveform and spectrum analysis is done according to test
conditions and definitions of ASTM E1065. Typical units are MHz for
frequency analysis, microseconds for waveform analysis, and dB down from
peak amplitude. Figure 4 illustrates waveform duration at the –14 dB level or
20% amplitude of peak. The –40 dB waveform duration corresponds to 1%
amplitude of peak.
Fig. 4
Figure 5 illustrates peak frequency, upper and lower –6 dB frequencies, and
MHz bandwidth measurements.
Fig. 5
The relationship between MHz bandwidth and waveform duration is shown
in Figure 6. The scatter is wider at –40 dB because the 1% trailing end of
the waveform contains very little energy and has very little effect on the
analysis of bandwidth. Because of the scatter, it is most appropriate to
specify waveforms in the time domain (microseconds) and spectra in the
frequency domain.
Fig. 6
-14 dB
Amplitude
Time (Microseconds)
WAVEFORM
DURATION
-6 dB
Amplitude
Frequency (MHz)
BANDWIDTH
PEAK
LOWER UPPER
Waveform Duration
-6 dB Bandwidth (MHz)
-40dB
-14dB
(Microseconds)
.01 .1 1 10 100
.1 1 10 100