Nondestructive testing personnel must see:penetration testing of the ten misunderstandings

Liquid penetration methods for non-destructive testing have been in use since the 1940s. But even after generations of use by NDT professionals, there are still some common confusions and misconceptions in this field.

We have directly documented in this article the top 10 misconceptions found in this field in recent years.

1. The most sensitive penetrant is the best penetrant for the application

The best penetrant for the application is the one that will find the problem with the least amount of money and time, not necessarily the one with the highest sensitivity. While it is true that a higher sensitivity penetrant can find very small discontinuities, if you only need to find moderate discontinuities, a higher sensitivity penetrant may not provide you with the best probing results because you will see more discontinuities that are beyond the scope of the probe. Before selecting a penetrant, review any guidance specifications and work procedures for the relevant sensitivity level. Consider the surface finish and shape of the workpiece. High sensitivity fluorescent penetrants are suitable for smooth, highly machined surfaces. However, high sensitivity fluorescent penetrants may leave an excessive fluorescent background on rough castings, making flaw detection difficult. Lower sensitivity fluorescent penetrants are better suited for rough surfaces.

2. Penetration display must be discontinuous

Penetration displays are visual results or responses to penetration testing and must be analyzed to determine if they are associated with discontinuities. Penetration displays must be evaluated by a qualified flaw detectorist to determine if they are associated with discontinuities. Irrelevant displays may be present on the workpiece due to inherent surface roughness or seams. Fingerprints or fibers may also cause extraneous displays. Irrelevant displays are the result of discontinuities or interruptions in the physical structure of the object and are evaluated according to the acceptance criteria. After evaluation, the workpiece is accepted as is, returned to the factory, or discarded.

3. Water-washable penetrants are water-based penetrants

Some washable penetrants are water-based penetrants. However, this is not always the case. Water-washable penetrants may not contain water. Water-washable penetrants contain surfactants that can be easily removed from the surface of the workpiece by rinsing with water, whether water-based or oil-based.

4. Penetrant for non-ferrous metals only

Penetrant can be used to detect both ferrous and non-ferrous metals. Penetrant flaws can detect discontinuities in openings in the surface of ferrous and non-ferrous metals. Penetrant probing cannot be performed on porous surfaces because the pores trap the penetrant like discontinuities and affect the detection of defects.

5. Penetrant can penetrate discontinuities containing water.

If the discontinuity is filled with water or other liquids, the penetrant will not penetrate. Similarly, penetrants cannot penetrate through paint, magnetic powder, oil or grease displacement. This is one of the reasons why effective penetration testing starts with a properly cleaned and dried part.

6. Penetration testing requires tanks and inspection chambers

Penetration testing can be easily adapted to different environments and construction sites. On the factory floor, it is common to find penetration testing systems with fixed tanks and inspection chambers. However, for convenience and portability, fluorescent penetrants and color penetrants are available in aerosol cans and kits.

7. Only penetrant is required to perform penetration testing.

Water-washed penetrant flaws require at least penetrant and visualizer. Solvent-removal penetrant flaws and post-emulsification penetrant flaws require additional products such as cleaners/removers and emulsifiers.

8. Penetrant flaws require special illumination

Fluorescent penetrant inspection requires a darkened area irradiated by a code-compliant UV lamp. For flexibility and portability, UV lamps are available in mounted and hand-held versions. Only sufficient white light is required for probing with colored penetrants, and a minimum of 100 footcandles is usually required for inspection.

9. Penetrant flaw detection should be the final inspection process in the manufacturing process

Penetrant inspection is useful after any manufacturing process that is known to cause discontinuities, allowing the part to be reworked or discarded early in the manufacturing process, saving time and cost Penetrant probing may sometimes be performed more than once when manufacturing a part. Penetration testing points are optimized at points in the manufacturing process where discontinuities may occur to reduce the amount of scrap and rework later in the manufacturing process.

10. Penetration testing can be performed at any time during the manufacturing process.

As discussed, it is important to perform penetration testing after machining operations that may result in open discontinuities on the surface of the part. However, penetration testing must be performed carefully prior to machining or contamination of the metal surface may occur. Machining operations such as shot blasting, sandblasting or grinding may bridge surface discontinuities and prevent subsequent penetration testing from finding these discontinuities. Penetration testing must be performed prior to machining operations such as sandblasting, shot peening, or grinding unless chemical etching is used between these operations and penetration testing to reliably expose the discontinuities.