Liquid Penetrant Testing (PT)
PT is a widely applied NDT method that is utilized to detect surface breaking flaws in ferrous & non-ferrous materials. PT is based on capillary action which is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. Because PT is based on capillary action pre-cleaning is of the up most importance when using PT as an examination method.
PT can be used to examine small parts, large parts, parts with complex geometry, and welds
After penetrant is applied it has to stay on the part for specified dwell time usually ranging from 5 to 60 minutes depending on the specification. The dwell time allows the penetrant time to enter the discontinuity. After the dwell time the excess penetrant is carefully removed and a developer is applied.
The developer is also given a specified dwell time usually ranging from 5-60 minutes depending on the specification. The developer causes reverse capillary action also referred to as bleed out. Bleed out draws the penetrant out from discontinuity. The developer also provides a contrasting background when utilizing the visible penetrant testing method. making discontinuities to be easily seen by the technician.
Advantages of PT
Works on complicated geometric shapes
Few materials Limitations
Disadvantages of PT
Works on porous surface materials only
No depth sizing
Surfacing breaking flaws only