Compression vs Shear Wave UT: Differences That Matter
Compression vs shear waves in UT: particle motion, velocity and wavelength in steel, mode conversion, critical angles, and which mode a procedure uses.
Reviewed by Anoop Rayavarapu, ASNT NDT Level III (UT, RT, MT, PT, VT). Updated 18 October 2026.
Ultrasonic testing uses two main wave modes in solid metal. In a compression (longitudinal) wave the particles move back and forth along the direction the wave travels. In a shear (transverse) wave they move at right angles to it. The difference sets the speed, the wavelength, where each wave can travel and which flaws each one sees well, and so which probe a procedure calls for.
Compression wave vs shear wave at a glance
| Property | Compression (longitudinal) | Shear (transverse) |
|---|---|---|
| Particle motion | Parallel to the direction of travel | Perpendicular to the direction of travel |
| Velocity in steel | About 5,900 m/s | About 3,230 m/s, a little over half |
| Wavelength in steel at 5 MHz | About 1.2 mm | About 0.65 mm |
| Travels through liquids and gases | Yes (so it crosses couplant and water paths) | No; liquids and gases do not support shear stress |
| Usual probe | Straight-beam (normal) and dual-element probes; immersion | Angle-beam probe on a wedge, by mode conversion |
| Typical work | Thickness and corrosion measurement, laminations, forgings, castings | Weld examination: lack of fusion, cracks and other planar flaws across the beam |
The wavelengths follow from velocity divided by frequency. Halving the wavelength at the same frequency is the resolution gain that makes shear waves the usual choice for weld volumes.
Mode conversion and the critical angles
When a sound beam meets an interface at an angle, part of it changes mode. An angle-beam probe uses this on purpose: a longitudinal wave in the wedge refracts into the steel as both a longitudinal and a shear wave, at angles set by Snell's law and the two materials' velocities. Past the first critical angle the refracted longitudinal wave no longer enters the steel and only the shear wave remains; past the second critical angle the shear wave is replaced by a surface wave travelling along the part. Weld probes are designed to work between the two, which is why they are specified by their refracted shear angle in steel.
Mode conversion also happens inside the part, at back walls, weld roots and flaw faces, so an experienced technician learns to recognise converted-mode echoes rather than report them as indications.
Which mode a procedure uses
Compression waves are used where the beam should enter normal to the surface: wall-thickness and corrosion surveys, lamination scans of plate, and forgings and castings examined from flat faces. Shear waves are used where the flaw is expected to lie at an angle to the surface, above all on welds, where the beam is aimed to meet the fusion faces as near to perpendicular as possible. Laminations lie parallel to the surface and are found with a straight-beam compression scan, not with shear waves; that scan is done on the base metal before angle-beam weld examination. For the angle, frequency and coverage choices on welds, see shear wave angle beam inspection.
Where each mode struggles
Compression waves have a near-surface dead zone and miss planar flaws that lie at an angle to the beam. Shear waves are strongly affected by coarse-grained structures: in austenitic stainless and dissimilar-metal welds the columnar grains scatter and skew the shear beam, so procedures switch to low-frequency dual-element longitudinal probes or to time-of-flight diffraction there.
Questions about compression and shear waves
Why don't shear waves travel through water?
A shear wave needs a medium that resists shear, and liquids and gases do not. That is why a shear wave stops at the couplant layer, why immersion testing launches a longitudinal wave in water and relies on mode conversion at the part surface, and why a shear wave cannot cross a gas-filled crack.
Which wave mode is used for ultrasonic thickness gauging?
Compression (longitudinal) waves, sent normal to the surface with a straight-beam or dual-element probe. Thickness is the measured round-trip time multiplied by the longitudinal velocity and halved, so the velocity setting must match the material.
Why are angle-beam probes rated by their shear angle in steel?
Because the wedge is designed so that only the refracted shear wave enters the steel, and that wave's angle in steel is what sets beam path, skip distance and coverage. The real angle drifts with wedge wear and differs in materials with other velocities, so it is checked on a calibration block before use.
What is mode conversion in ultrasonic testing?
The change of part of a sound wave from one mode to another, for example longitudinal to shear, when it meets an interface at an angle. Angle-beam probes depend on it, and it also creates extra echoes from back walls and flaw faces that have to be recognised during evaluation.
Plan beam paths with the free angle beam and skip distance calculator, read ASTM E164 weld UT for the governing practice, and practise both modes in the ultrasonic testing simulator.