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.
Definition
When an ultrasonic wave strikes an interface at non-normal incidence, part of the energy refracts and reflects in modes different from the incident wave. For an angled wedge on steel, the incident longitudinal wave produces both refracted longitudinal and refracted shear waves; choosing the wedge angle selects which mode dominates.
The principle
When a wave strikes an interface at an angle, part of its energy converts to another wave mode — longitudinal to shear and back — each travelling at its own velocity and refracting at its own angle. It is why angle-beam probes work at all, and why unexpected signals appear where geometry is complex.
How it shows up in practice
Wedge angles are chosen to exploit it: beyond the first critical angle only shear survives in the part, which is what gives a clean angle-beam examination. Beyond the second critical angle, surface waves dominate.
Where the codes address it
Underpins the angle-beam techniques of ASME Section V Article 4 and ISO 17640.
Where practitioners get caught
Calling a mode-converted signal from a corner or a counterbore a flaw. Geometry produces reproducible, predictable indications, and knowing the component profile is what separates them from real reflectors.
Where Mode Conversion fits in an inspection programme
A term is only useful when it connects to a decision. Mode Conversion 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.
- 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.
- Angle-Beam Probe — An angle-beam probe couples ultrasonic energy through a plastic wedge to produce a refracted shear or longitudinal wave at a fixed angle (typically 45°, 60°, or 70°) in the test piece, used primarily for weld inspection.
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.