Ultrasonic Testing (UT) - Level II Study Guide
Core concepts
Ultrasonic testing uses high-frequency sound, typically 1 to 10 MHz, generated by a piezoelectric transducer. Know wave modes (longitudinal, shear, surface, and Lamb), how velocity depends on material and mode, acoustic impedance and reflection at interfaces, couplant function, near field length N = D squared times f divided by 4v, beam spread with sine of the half angle equal to 1.22 lambda over D, and refraction by Snell's law. Also study calibration blocks such as IIW, V1, and V2, distance amplitude correction, time corrected gain, and sizing by the 6 dB and 20 dB drop methods.
Traps to expect
- Flaws inside the near field cannot be reliably sized because amplitude behavior is erratic there.
- Higher frequency improves resolution and shrinks wavelength but increases attenuation.
- Increasing the incident angle increases the refracted shear angle; mode conversion occurs beyond the critical angles.
- Couplant affects transmission efficiency, not the velocity through the part.
- The 6 dB drop method suits reflectors larger than the beam; small reflectors need other sizing approaches.
- Poor coupling produces lost back wall echoes that mimic defects.
Rule of thumb
In steel, compression velocity is about 5,900 m/s and shear about 3,230 m/s. For a 45-degree angle beam, the sound path equals 1.414 times the depth, since path = depth divided by cosine of 45 degrees.
Worked example: a 45-degree shear wave shows an echo at 2 in depth. The sound path is 2 times 1.414, or 2.83 in.