Ultrasonic Testing: Three Methods, Five Display Modes
Through-transmission, pulse-echo and tandem testing do not measure the same thing, and the echo can be shown five ways. Sorting out both lines makes a test report readable.
1. Three methods
Through-transmission puts a transmitter on one side of the part and a receiver on the other, judging internal quality from how much sound is attenuated on the way through. It is sensitive to defects but gives no depth.
Pulse-echo uses one probe for both transmission and reception, reading defect position and size from echo arrival time and amplitude. It is the workhorse of industrial testing, and the method the rest of this article follows.
Tandem testing places two probes on the same side at a fixed spacing, so the beam reflects off the backwall and sweeps the weld section. It is reserved for thick-wall welds.
2. Why pulse-echo can locate a defect
When sound crosses from one medium into another, the greater the difference in acoustic impedance across the boundary, the stronger the reflection. A defect is effectively a patch of material with different acoustic properties embedded in the steel, so it reflects.
Arrival time corresponds to distance from the surface; amplitude corresponds to size and to how different the defect is. Combine the two with probe position and the defect location can be reconstructed.
3. Five display modes
A-scan: a waveform, depth across and echo height up. The most basic display, and the standard one for industrial testing.
M-scan: one brightness-modulated signal unfolded against time, used to watch moving objects; in medicine, the heart and vessels.
B-scan: several brightness signals placed side by side into a two-dimensional section, for static internal structure. Medical B-ultrasound works this way.
C-scan: both axes are probe position on the part, and echo amplitude is shown as brightness, giving a plan view of the defect with no depth.
F-scan: a C-scan with depth added, giving a three-dimensional impression. Rarely used in engineering.
4. How to choose
Most industrial inspection judges on the A-scan, with B or C imaging added where a record of defect distribution helps. Imaging is worth paying for only when showing that distribution is the point.
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