Non-Linear Scan — Definition & NDT Use
A scanning pattern in ultrasonic testing that follows material geometry rather than straight parallel paths. Used on cylindrical pipe welds, circumferential scans follow the pipe circumference. Non-linear scans may follow weld contours for complex components. The key is maintaining consistent technique and beam angles throughout the scan. Data from non-linear scans must be properly referenced and interpreted.
From the inspector's bench, Non-Linear Scan is run as a defined sequence: equipment verification on a known reference, scan setup against the procedure, scanning the part, and writing the indications into the report. On piping, the inspection is usually a circumferential band of UT thickness readings, a girth-weld RT or PAUT shot, and a follow-up MT/PT on the toes — each method picking up a different failure mode at the same weld. Procedure writing, inspector qualification, and the reference block establish the chain that lets a remote engineer trust an indication called a kilometre away from the office.
Non-Linear Scan is selected when the failure mode the engineer cares about — surface crack, internal void, wall loss, lack of fusion — lines up with what the technique is physically capable of detecting. On welded fabrication it is most often paired with VT and one volumetric method (RT or UT) so surface and internal defects are both addressed. On in-service piping the inspection is scheduled against an API 570 corrosion monitoring plan rather than a one-off; trend data is what matters.
AWS D1.1
Structural Welding Code — Steel; defines visual and NDE acceptance for static and dynamically loaded welds.
ASME Section IX
Welding, brazing, and fusing qualifications referenced by every U.S. pressure-equipment code.
ASME Section V Article 4
Ultrasonic examination methods for welds and components.
ASTM E114 / E164 / E2375
ASTM straight-beam, contact, and wrought-product UT practices.
The most expensive mistake with Non-Linear Scan is treating it as a yes/no test rather than a characterisation — an indication called without a sizing strategy forces a repair where a fitness-for-service review might have left the part in service.
What does "Non-Linear Scan" mean in NDT?
A scanning pattern in ultrasonic testing that follows material geometry rather than straight parallel paths. Used on cylindrical pipe welds, circumferential scans follow the pipe circumference
Which standards govern the use of Non-Linear Scan?
Non-Linear Scan is most often referenced under ASME Section V together with the relevant ASTM practice or the matching ISO standard for the method; the contract or purchase order will name the controlling document and edition for any specific job.
What other NDT concepts should I read alongside Non-Linear Scan?
The most directly related entries in this glossary are "parallel scan", "scan pattern", "circumferential scan"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.