The liquid penetrant method of nondestructive testing has been used since the 1940’s. But even after being used by generations of NDT professionals, there are still some common areas of confusion or misunderstanding.
Here we set the record straight on 10 misperceptions we’ve seen in the field in recent years.
1. The highest sensitivity penetrant is the best penetrant for my application
The best penetrant for an application is the one that finds the right indications with the least amount of money and time. Sometimes this means not using the highest sensitivity penetrant.
While it is true that a higher sensitivity penetrant will produce indications for very small discontinuities, a higher sensitivity penetrant will probably not give you the best inspection results if you only need to find medium discontinuities since you will see far more indications than are relevant to the inspection.
To start selecting a penetrant, review any governing specifications and work procedures for required sensitivity levels.
Take into consideration the surface finish and configuration of the part.
A high sensitivity level fluorescent penetrant is appropriate for smooth, highly machined surfaces. However, a high sensitivity level fluorescent penetrant may leave excessive fluorescent background on a rough cast part, making inspection difficult.
A lower sensitivity fluorescent penetrant is a better choice for rough surfaces.
2. A penetrant indication is a discontinuity
A penetrant indication is the visual results or response of the penetrant test which must be interpreted to determine its relevance.
Penetrant indications must be evaluated by a qualified inspector to determine if they are nonrelevant or relevant.
Nonrelevant indications may be present on parts because of inherent surface roughness or seams. Fingerprints or fibers may also cause nonrelevant indications.
Relevant indications are the result of a discontinuity, or interruption in the physical structure of an object, and are evaluated according to acceptance criteria. After evaluation, the part is accepted as is, reworked or discarded.
3. Water washable penetrants are water based
Some water washable penetrants are water based. However, this is not always the case. A penetrant can be water washable and not contain water.
Water-washable penetrants contain surfactants which allow the penetrant to be easily removed from the part surface with water rinsing, regardless of if they are water-based or oil-based.
4. Penetrants are only used on nonferrous metals
Penetrants can be used to inspect ferrous and nonferrous metals.
Penetrant inspection will find discontinuities open to the surface on ferrous and nonferrous metals.
Penetrant testing should not be done on porous surfaces, as the pores will act as discontinuities to trap penetrant and prevent accurate inspection.
5. Penetrant will be able to penetrate a discontinuity that contains water
Penetrant cannot seep into a discontinuity if it is already filled with water or other liquid.
Likewise, penetrant will not displace or penetrate through paint, particulate, oil or grease.
This is one of the reasons why an important prerequisite for a valid penetrant inspection is to start with properly cleaned and dried parts.
6. Tanks and an inspection booth are required for penetrant inspection
Penetrant inspection is easy to adapt to different environments and job sites.
Penetrant inspection systems with stationary tanks and booths are commonly seen in production environments. However, both fluorescent and visible dye penetrants are available in aerosol cans and kits for convenience and portability.
7. Penetrant is all that is needed to perform a penetrant inspection
At a minimum, penetrant and developer are required to perform a water washable penetrant inspection.
Additional products such as cleaner/removers and emulsifiers are required for solvent removable and post emulsifiable penetrant inspections.
8. Special lighting is required for penetrant inspection
Fluorescent penetrants do require inspection in a darkened area with specification compliant UV lighting. The UV lights may be mounted or hand-held for flexibility and portability.
Visible dye penetrants only require adequate white light, typically 100 foot candles minimum, for inspection.
9. Penetrant inspection should be the final check in a manufacturing process
Penetrant inspection is useful immediately after any manufacturing process which is known to cause discontinuities. This allows parts to be reworked or discarded earlier in the manufacturing process, which saves time and cost.
Penetrant inspection may sometimes be performed more than once during the manufacture of a part.
The placement of each penetrant inspection process should be optimized to locate manufacturing-induced discontinuities and reduce the amount of scrap or rework done later in the manufacturing process.
10. Penetrant inspection can take place at any point in the manufacturing process
As discussed, it is important to perform penetrant inspection after manufacturing operations likely to cause discontinuities open to the surface in parts.
However, care must be taken to perform penetrant inspection prior to mechanical operations that will smear the metal surface. Machining operations such as shot blasting, peening or grinding may close surface discontinuities, which can prevent subsequent penetrant inspections from finding these discontinuities.
Penetrant inspection should take place before machining operations like shot blasting, peening or grinding unless chemical etching can be used between these operations and the penetrant testing to reliably expose the discontinuities.