UT Method Fundamentals and Principles
Core physics candidates must own
Ultrasonic testing rests on elastic wave propagation: particle displacement transfers energy through a medium, and the mode (longitudinal, shear, surface/Rayleigh, Lamb) determines what can be detected and how. Know the relationships among velocity, frequency, and wavelength (λ = c/f), and how acoustic impedance (Z = ρc) governs reflection at an interface. The reflection/transmission coefficients at normal incidence, and Snell's law for refracted beams, are frequent calculation targets.
Near-field length and beam spread are central. The near field N ≈ D²/(4λ) for a round piston transducer; beyond it the beam diverges, approximated by sin θ = 1.22λ/D (half-angle to first null). Beam spread widens with lower frequency and smaller elements, which trades resolution for penetration.
Traps that show up in questions
- Confusing near-field length with skip distance or with the 6 dB beam width.
- Forgetting that shear velocity is roughly half longitudinal velocity in the same material, so wavelength and thus sensitivity change with mode.
- Attenuation: absorption plus scattering; scattering rises with frequency and grain size, so coarse-grain material favors lower frequency.
- Mode conversion at non-normal incidence: the refracted angle applies to the mode you are using, and a second mode usually exists.
- Acoustic impedance mismatch drives reflectivity-air backing gives near-total reflection, which is why couplant matters.
Rule of thumb
For a 5 MHz, 12.7 mm (0.5 in) element in steel (c ≈ 5900 m/s longitudinal), λ ≈ 1.18 mm, so N ≈ D²/(4λ) ≈ 161/4.72 ≈ 34 mm. Anything inside ~34 mm is in the near field where amplitude-versus-depth is non-monotonic-a classic exam pitfall when interpreting a first-cycle signal.