Surface crack detection by liquid penetrant testing

Liquid penetrant testing (PT).

Liquid penetrant testing, sometimes referred to as dye penetrant inspection, is one of the most commonly used surface crack detection methods.

This type of testing is limited to the detection of surface-breaking discontinuities, or
discontinuities that are open to the surface where the penetrant has been applied.

It can’t detect discontinuities that are sealed within the body of the weld, such as internal porosity or fusion defects.

It’s not usually suitable for testing rough or porous materials because interpretation of the test results can be hindered by false indications.

When compared to unassisted visual inspection, this type of inspection is more likely to detect smaller and finer surface-breaking discontinuities, such as hairline cracks and micro surface porosity.

This type of inspection may be suitable for both ferrous and nonferrous materials.

Liquid penetrant testing is done with either “visible dye” or fluorescent dye.

With fluorescent penetrant inspection, a highly fluorescent liquid is applied to the surface of the inspection area(s). A developer is then applied to draw the penetrant to the surface, and then a black light is used to inspect the weld.

The high contrast between the fluorescent material and the object allows the inspector to detect traces of penetrant that indicate surface defects.

With a visible penetrant inspection, the process is the same; however, instead of fluorescent dye, a highly visible, colored dye is used with a white developer, which makes any contrast visible under regular light.

To perform a liquid penetrant test, follow these steps.

1.) Clean the area of the weld where the inspection will take place. Allow time for the area to dry completely.

2.) Apply the dye penetrant by spraying, brushing, or dipping the weld into it.

3.) Let the dye sit and fully absorb into the surface. This can take anywhere from one to three hours, depending on the level of detail on the weld.

4.) Carefully remove any excess penetrant using a solvent or a water wash.

5.) Apply your developer.

Once you have completed these steps, you will be able to detect any penetrant bleed out from within discontinuities.