Capillary Action

Capillary action is the physical phenomenon by which a liquid is drawn into a narrow space against gravity by surface tension and adhesion forces, and is the fundamental mechanism of liquid penetrant testing.

Definition

Capillary action enables low-viscosity penetrants to enter tight surface-breaking cracks during dwell time, then to be drawn back out into the developer coating. The narrower the defect, the stronger the capillary effect and the longer the dwell time required.

Related terms

  • Dwell Time — Dwell time is the duration that penetrant is allowed to remain on the test surface before excess removal, allowing capillary action to draw the dye into surface-breaking discontinuities.
  • Penetrant Testing (PT) — Penetrant Testing (PT) is a surface NDT method that uses a colored or fluorescent liquid drawn into surface-breaking defects by capillary action; after excess penetrant is removed and a developer applied, indications bleed out and become visible.
  • Developer — A developer is a powdered or suspended absorbent material applied after penetrant removal that draws trapped penetrant out of surface defects by reverse capillary action, producing visible indications.

More physics terms

Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion

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.