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Penetrant Testing (PT):

 

PT reveals surface flaws by the “bleed-out” of a penetrating medium against a contrasting background. This is done by applying penetrant to the pre-cleaned surface and flaw of the item being inspected. The penetrant is applied to the surface and allowed to remain on the surface for a prescribed time (dwell time); the penetrant liquid will be drawn into any surface opening by capillary action. Following removal of excess penetrant an application of a developer reverses the capillary action and draws penetrant from the flaw. The resultant indications reveal the presence of the flaw so that it can be visually inspected and evaluated.

 

Within the categories of penetrant inspection are a family of tests, these relate to the type of penetrant used. There are also two ways in which penetrant materials are classified, typically the type of indication produced and the method of removal. Penetrant testing results are displayed in two ways, visible and fluorescent. The visible penetrant type produces a bold red line or strain indication against a white developer background when viewed under good white light conditions. The fluorescent penetrant produces a green, fluorescent indication when observed under an ultraviolet (black) light. Because the human eye can more readily perceive a fluorescent indication than a visible indication, the use of fluorescent penetrant inspection will result in a more sensitive test.

 

The second way in which penetrants are classified is the method by which the excess penetrant is removed from the test surface. The penetrants are either water washable, solvent removable or post-emulsifiable. Water washable penetrants contain an emulsifier that allows the penetrant to be rinsed off using low-pressure water spray; sometimes a water-dampened towel is used. Solvent removal penetrants require a solvent to remove the excess penetrant from the test surface. Post-emulsifiable penetrants are removed by adding an emulsifier after the penetrant dwell time. By combining the characteristics of these two classifications, six different types of penetrants can be used.

 

Types of penetrant

 

(1) Visible/water – washable

(2) Visible/solvent – removable

(3) Visible/post – emulsifiable

(4) Fluorescent/water – washable

(5) Fluorescent/solvent – removable

(6) Fluorescent/post – emulsifiable

 

With any of these types, application and use is considered the same, except for postemulsifiable penetrant that requires an emulsifier application step. With any of the methods, there are several steps to follow. It is important that each of the steps is performed carefully in its correct sequence; otherwise, the test results may not be reliable.

 

Liquid penetrant testing is used by all industries where there is concern about the surface condition of a material or welded joint. It is used extensively on non-magnetic materials, i.e. stainless steel and aluminium that cannot be evaluated using magnetic particle testing.

 

Limitations

 

• Access required for surface preparation and cleaning
• Surface condition must be satisfactory
• Non-relevant indications from irregular surfaces
• Will only detect surface flaws
• Flaw must be clean and not contaminated

 

Advantages

 

• Economical
• Aid to VT
• Portable
• Can inspect a wide range of materials and components

 

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