10.1 UT Physics

How Ultrasound Interrogates Materials

Ultrasonic Testing (UT) uses high-frequency mechanical vibrations, typically 1 to 10 MHz for most industrial work and occasionally 0.5 to 25 MHz, that travel through a part as elastic (mechanical) waves. When the beam meets a discontinuity or a boundary, part of its energy reflects or scatters back to a receiver. The transit time locates the reflector along the beam, and the echo amplitude gives a relative measure of its size. Because sound must be mechanically coupled to travel, UT physics, wave mode, velocity, impedance, and attenuation govern nearly every Basic-exam UT question.

Wave Modes

Sound propagates in solids as several distinct modes, distinguished by how particles vibrate relative to the direction of travel.

• Longitudinal (compressional) waves vibrate particles parallel to propagation. They exist in solids, liquids, and gases and have the highest velocity. Straight-beam UT uses longitudinal waves for thickness gauging and lamination detection.
• Shear (transverse) waves vibrate particles perpendicular to propagation. They exist only in solids because a liquid cannot sustain shear, and they travel at roughly half the longitudinal velocity. Angle-beam weld inspection relies on refracted shear waves.
• Surface (Rayleigh) waves travel along a free surface to a depth near one wavelength and follow gentle contours, making them useful for surface-crack detection.
• Lamb (plate) waves are whole-plate resonant modes in material a few wavelengths thick; they are used for thin sheet and tubing over long distances.

Velocity, Frequency, and Wavelength

Velocity (v) is a material property set by elastic modulus and density and is essentially independent of frequency. Wavelength is tied to both by:

λ = v / f

A 5 MHz longitudinal beam in steel (v ≈ 5,900 m/s = 5.9 mm/µs) has λ = 5,900,000 / 5,000,000 ≈ 1.18 mm. Because the smallest reliably detectable reflector is roughly half a wavelength, higher frequency shortens the wavelength and improves resolution and small-flaw sensitivity, while lower frequency penetrates deeper and tolerates coarse grain.

Acoustic Impedance and Reflection

Acoustic impedance (Z) is the product of density and velocity, Z = ρ × v. At any interface the fraction of energy reflected depends on the impedance mismatch:

R = ((Z2 − Z1) / (Z2 + Z1))²

A steel-to-air boundary has an enormous mismatch, so nearly 100% of the sound reflects. That is exactly why a couplant (water, gel, glycerin, oil) is needed to displace air and let energy enter the part, and why a back-wall or a lamination gives a strong echo. Large impedance differences make good reflectors; small differences (steel to a tight metallurgical bond) reflect weakly.

Attenuation

As the beam travels, its energy weakens through attenuation, the sum of two effects. Absorption converts sound to heat and rises with frequency. Scattering redirects energy off grain boundaries, inclusions, and porosity; it grows sharply when grain size approaches the wavelength. Coarse-grained cast austenitic stainless steel scatters heavily, which is the standard exam reason conventional UT struggles there, the fix is a lower frequency to lengthen the wavelength relative to the grains.

Near Field, Far Field, and Beam Divergence

Close to the transducer, interference creates the near field (Fresnel zone), where amplitude fluctuates and sizing is unreliable. Its length is approximately N = D² / (4λ) (D = element diameter). Beyond N lies the far field (Fraunhofer zone), where the beam spreads and amplitude falls off predictably. In the far field the beam diverges; the half-angle of spread follows sin θ ≈ 1.22 λ / D, so larger elements and higher frequency give a tighter, more directional beam.

Refraction, Snell's Law, and Critical Angles

When a beam crosses an interface at an angle, it refracts by Snell's law:

sin θ1 / V1 = sin θ2 / V2

Mode conversion at the interface produces both longitudinal and shear waves in the part. As the incident angle grows, the refracted longitudinal wave bends toward 90°. The first critical angle is where the longitudinal wave just reaches 90° and disappears, leaving only a refracted shear wave, this is the region angle-beam wedges use to make clean 45°, 60°, or 70° shear beams for welds. The second critical angle is where the shear wave itself reaches 90°, generating a surface (Rayleigh) wave.

Worked Snell example. To create a 45° refracted shear wave in steel from an acrylic wedge: sin θ1 / 2,730 = sin 45° / 3,230, so sin θ1 = 2,730 × 0.7071 / 3,230 = 0.598, giving an incident wedge angle of about 36.7°. Because velocity changes at the interface, the wedge angle and the beam angle in the steel are never the same, and every angle-beam procedure verifies the actual refracted angle on a calibration block. (ASNT updated its published UT topic coverage for exams after July 2026, but these physics fundamentals are unchanged.)

Common Exam Traps

• Shear waves cannot travel in liquids, only in solids, so immersion coupling always uses a longitudinal water path before mode conversion refracts sound into the part.
• Velocity is a material constant; changing frequency changes the wavelength and resolution, not the speed of sound.
• Higher frequency gives better resolution but less penetration, while lower frequency penetrates coarse grain and thick sections.
• The first critical angle leaves only a refracted shear wave; the second critical angle creates a surface (Rayleigh) wave. Memorize which angle produces which mode.