Liquid Penetrant Inspection (LPI)

Basic Principles:

Liquid penetrant inspection (LPI), also called fluorescent penetrant inspection (FPI) or penetrant testing (PT), is a widely used, low-cost inspection method used to locate surface-breaking defects in nonporous materials. Penetrant inspection is used to detect casting, forging and welding surface defects such as hairline cracks, porosity, leaks in new products and fatigue cracks on in-service components.

Liquid penetrant inspection is based on the principal of capillary action, where specially formulated liquid penetrates into clean and dry surface discontinuities. Penetrant may be applied to the test component by dipping, spraying or brushing. After an adequate penetration time has been allowed, the excess penetrant is removed and a developer applied. The developer helps to draw out any penetrant that was entrapped in a flaw so that the flaw becomes readily visible to the inspector. Inspection is usually performed under ultraviolet light but can be performed under white light, depending on the type of dye used and sensitivity required.


  • Rapid, cost-efficient method for large surface areas and large volumes.
  • Facilitated inspection of parts with complex geometry.
  • Indications are produced directly on the surface of the part, providing a visual image of the discontinuity.


  • Can only detect surface-breaking defects.
  • Proper surface preparation and cleaning is critical.
  • Requires a relatively smooth and nonporous surface.
  • Post-inspection cleaning is necessary to remove residual inspection products.
  • Requires multiple operations under controlled conditions.
  • Proper chemical handling precautions are necessary (toxicity, fire, waste).

Liquid Penetrant Inspection for these sectors

aviation safety inspection


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vehicle safety inspection


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industrial ndt


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