October 20, 2012 | In: Liquid Penetrant Inspection

LPI Glossary of Terms

Following are some common terms used in LPI method of Non Destructive Testing/Inspections


Background Fluorescence:  Fluorescent residues observed over the general surface of he part during Fluorescent Penetrant Inspection (FPI)

Bath:  A term used colloquially to designate the liquid penetrant inspection materials into which parts are immersed during inspection process.

Black Light:  Ultraviolet light energy between the range of 320-400 nanometers (that is near or just below the visible range).  Also known as UV-A, this energy range reacts strongly on certain dyes causing them to emit visible light at about 550 nm,  i.e. fluoresce.

Black Light Filter:  A filter that transmits black light while suppressing the transmission of visible light.

Black Light Intensity:  Intensity is the measure of black light strength in microwatts per centimeter squared (µW/cm²) at any given point on the inspection surface.

Bleed-out:  The action by which the penetrant exudes from the discontinuities onto the surface of the material.

Blotting:  The action of an absorbing medium soaking up the entrapped penetrant from surface discontinuities.  This in-turn, forms penetrant indications with increased contrast and sensitivity.

Capillary Action:  The tendency of certain liquids to penetrate, or migrate when exposed to small openings such as cracks or fissures.

Carrier Fluid:  A fluid in which liquid penetrant inspection materials are dissolved or suspended.  A good example would be the the non-aqueous solution that holds a dye penetrant.

Clean:  A part to be inspected that is substantially free of solid or liquid contamination on the surface as well as in the voids of the flaws.

Color-Contrast Dye:  When viewed under visible (white) light, this dye is designed to impart sufficient color intensity, enabling a good contrast of indications from the inspection surface.

Color-Contrast Penetrant:  A penetrant – usually non-fluorescent – that incorporates the color-contrast dye, which is sufficiently intense to give good visibility of indications under white light.

Comparative Test Block:  An intentionally cracked metal block that has two separate but adjacent areas for the application of different penetrants so that a direct comparison can be obtained.

Contrast:  The striking difference of the colors used in liquid penetrant when compared under white light.

Dark Adaptation:  The adjustment of the eyes when one passes from a bright to a dark area, usually a measure of time.

Defect:   a) A discontinuity that interferers with the usefulness of an article.   b)  A fault in any material or part that is detrimental to its serviceability.

Degreasing Fluid:  A cleaning agent employed to dissolve or remove oil and grease from the surface of components before the penetrant is applied.

Developer:  A finely divided material – usually white in colour – that is applied to the surface of the component. The developer accentuates the bleed-out process and intensifies the detectability of flaw indications.

Developer Time:  The length of time the developer is allowed to remain on the surface of the part under inspection to permit reverse capillary action.

Discontinuity:  An interruption in the normal physical structure or configuration of an item such as cracks,  forging laps, inclusions, sudden changes in thickness or hardness etc.   A discontinuity may or may not affect the usefulness of the material.

Drain Time:  The period of time allowed for excess liquid to gradually flow off a part after immersion in a bath.

Dwell Time:  The total period of time that the penetrant is in contact with the surface of the part.  Drain time is considered to be a portion of the dwell time.


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