Longitudinal Wave

A longitudinal (compression) wave is the ultrasonic mode in which particle motion is parallel to the direction of propagation; the fastest wave mode, used for thickness measurement and 0° flaw detection.

Definition

Longitudinal waves compress and rarefy the medium along the propagation axis. They can propagate in solids, liquids, and gases, and travel faster than shear waves. In carbon steel, longitudinal velocity is ~5,920 m/s.

The principle

A wave whose particle motion is along the direction of travel — compression and rarefaction. It is the fastest mode in a given material and the only one that propagates in liquids and gases, since they cannot sustain shear.

How it shows up in practice

The straight-beam workhorse: thickness measurement, lamination detection and corrosion mapping all use it. Its higher velocity means that for a given frequency the wavelength is longer than shear, so resolution is coarser.

Where the codes address it

ASME Section V Article 5 for thickness measurement; Article 4 for straight-beam weld examination.

Where practitioners get caught

Using a longitudinal velocity to size a shear-wave path or vice versa. The velocities differ by roughly a factor of two in steel, so the resulting depth is wrong by about the same factor.

Where Longitudinal Wave fits in an inspection programme

A term is only useful when it connects to a decision. Longitudinal Wave appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.

Related terms

Shear Wave — A shear wave is an ultrasonic mode in which particle motion is perpendicular to the direction of propagation; in steel, shear waves travel at ~3,250 m/s and are produced by mode conversion at angled wedges for weld inspection.
Normal-Beam Probe — A normal-beam probe (0°) launches a longitudinal wave perpendicular to the test surface, used primarily for thickness measurement and for detecting laminar defects parallel to the surface.
Mode Conversion — Mode conversion is the partial conversion of an ultrasonic wave from one mode (longitudinal, shear, surface) to another at a refracting or reflecting interface, governed by Snell's law and the angle of incidence.

More physics terms

Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Surface Wave · Lamb Wave · Attenuation · Geometric Unsharpness · Radiographic Density

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.