Examination method
Liquid penetrant testing (PT)
A coloured or fluorescent liquid is drawn into surface-breaking flaws by capillary action, then bled back out onto a developer so tight cracks become visible indications.
How it works
The surface is cleaned thoroughly, then penetrant is applied and left for a dwell period. Capillary action draws the liquid into any opening that reaches the surface, and the narrower the opening the stronger the capillary pull, which is why the method finds cracks far too tight to see directly.
Excess penetrant is then removed from the surface without washing it back out of the flaw, which is the most delicate step in the whole process. Over-cleaning strips the trapped liquid and erases the indication; under-cleaning leaves background that masks it. Water-washable, solvent-removable, and post-emulsifiable systems each handle this trade-off differently.
A developer, usually a fine white powder in suspension or dry form, is then applied. It acts as a blotter, reversing the capillary flow and drawing penetrant back out of the flaw while spreading it sideways. That spreading magnifies a hairline crack into a visible line, in contrasting colour under white light or fluorescing under ultraviolet light.
Strengths
- Works on any non-porous material regardless of whether it is magnetic, so it covers stainless, aluminium, and titanium
- Equipment is inexpensive and portable, with aerosol kits usable anywhere
- Indications are visual and directly proportional in length to the flaw, so interpretation is straightforward
- Covers complex shapes and large areas at once without scanning point by point
Limitations
- Detects only discontinuities that are open to the surface; nothing subsurface is found
- Surfaces must be genuinely clean, and porous or very rough material gives unusable background
- Chemicals require ventilation, compatibility checks, and disposal control, particularly on stainless where halogen content matters
- Openings blocked by grinding smear, paint, or shot peening can hide flaws that are physically present
Key variables
- Surface preparation
- Determines whether penetrant can reach the flaw at all; smeared, painted, or oily surfaces defeat the method entirely.
- Penetrant dwell time
- Must be long enough for capillary action to fill tight flaws, and is extended for very fine cracks and cold surfaces.
- Excess removal method
- The balance point of the whole technique: enough removal to clear background, not so much that the flaw is emptied.
- Developer type and thickness
- A thin even coat blots and magnifies; too heavy a layer buries fine indications under white powder.
- Viewing conditions
- Colour contrast systems need good white light, while fluorescent systems need a darkened area and adequate ultraviolet intensity.
What an inspector watches for
- Confirm the pre-clean removed grinding smear, since a smeared surface can close a crack mouth and hide it completely
- Check that dwell and developing times were observed, because rushing either step produces false confidence rather than false indications
- Distinguish relevant indications from bleed-out at threads, corners, and rough surfaces before calling a rejection
- Verify penetrant chemistry compatibility on stainless and nickel alloys, where residual halogens can cause later damage