PT Penetrant Testing
What is PT or Dye Penetrant Testing?
Penetrant testing (PT), also known as dye penetrant testing or liquid penetrant inspection, is a non-destructive testing method used to detect surface-breaking discontinuities in materials, such as cracks, pores, or seams.
It involves applying a liquid dye to the surface of a clean, dry part, allowing the dye to seep into flaws through capillary action. After the required dwell time, excess dye is removed, and a developer is applied to draw out the dye from defects, making them visible under appropriate lighting.
This method is widely used in industries like aerospace, automotive, and manufacturing to ensure material integrity without damaging the component.
Basic PT Process
The following steps outline the basic process involved with dye penetrant testing
Pre-Clean
Cleaning of the part is performed utilizing a cleaner approved in the written procedure
The cleaner needs to be compatible with the dye penetrant that will be used
Cleaning removes contaminants that will reduce or eliminate the performance of the dye penetrant
Allow for the proper off-gassing of the cleaner before applying the dye penetrant
PT Application
Apply a generous amount of dye penetrant to the inspection area of interest
Allow for the proper amount of dwell time, which allows for the penetrant to enter and fill any discontinuities that are open to the surface
The PT procedure and governing codes will determine the required amount of dwell time
Excess penetrant removal
Remove the excess penetrant after the proper dwell time has been accomplished
The proper technique must be employed to remove the excess penetrant while not removing the removal of any penetrant contained in discontinuities
Developer
Apply the developer to the area of interest being tested, apply evenly and allow to dry
Developer has an absorbent quality that draws the penetrant out from discontinuities
The white color of the developer provides a background high in contrast with the penetrant allowing for improved visibility of indications and visual detection
Post-Clean
Properly clean the inspection area the same as performed in the pre-clean step
Remove all penetrant and developer from the inspected area leaving the inspected area in the same condition as it was before the inspection began
PT : Pros
PT can be used on many surfaces and material types, with simple or complex shapes
PT can detect small surface discontinuities, and has a high level of sensitivity amongst NDE methods
PT testing equipment is generally simple and involves unsophisticated equipment
PT magnifies the apparent size of discontinuities making them easier to visually detect
PT sensitivity can be adjusted through selection of penetrants, removal methods, and type of developer
PT : Cons
PT only reveals discontinuities open to the surface
PT relies on its ability to flow into discontinuities through capillary action, temperature and surface roughness can hinder the process
Surfaces to be PT inspected must be very clean and free of contaminants that would interfere
Mechanical surface treatments such as shot peening, machining, honing, buffing, grinding, abrasive blasting or sanding can smear the surface layer of material which can reduce or close the surface opening of the discontinuities
Dye penetrants may be impractical on porous materials, and may cause permanent staining
Dye Penetrant Types
The penetrant testing method relies on the visibility of the dye on the surfaces of parts. The amount of dye penetrant that enters a discontinuity is relatively small, so the penetrant needs to be highly visible.
Dye penetrants types are classified as one of the three types below:
Fluorescent Penetrant
Visible Penetrant
Dual Mode Penetrant (Visible & Fluorescent)
PT Removal Method Classifications
Dye penetrant is also classified by the method used to remove it from the surface of the part, after the prescribed dwell time. Penetrants are designed to minimize displacement from discontinuities during the removal process while still being reasonably manageable to clean from large surface areas in the inspection area.
Dye penetrants removal methods are classified as one of the three types below
Water Washable Penetrant
Solvent Removable Penetrant
Postemulsifiable Penetrant
Lipophilic :
“Lipo”-”phillic” : having an affinity for oil
an emulsifier that can combine with oil based penetrants that can be removed by water
Hydrophilic :
“Hydro”-”philic” : having an affinity for water
emulsifying agents are dissolved in water, and used in an immersion dip with air or mechanical agitation, or as a forced water spray rinse.
PT Developer Types
Developer provides absorbent action, which functions to draw the penetrant out from discontinuities, and provides visual contrast with the penetrant to aid in visual detection of relevant indications.
Dye penetrants developer types are classified as one of the three types below
Dry Powder
Water Soluble
Water suspendible
Nonaqueous
PT Sensitivity
When applied to PT dye penetrant testing sensitivity refers to the ability to detect discontinuities. Higher sensitivity can detect smaller discontinuities than a less sensitive process.
PT sensitivity is influenced by the following factors
Cleaning method
Penetrant type : fluorescent or visible penetrant
Viscosity of the penetrant
Lighting - visible or black light
The procedure by which the test is processed will also affect the sensitivity
Qualification / Certification
Qualification and Certification Codes and Standards
ASNT SNT-TC-1A Personnel Qualification and Certification in Nondestructive Testing
ASNT CP-189 Standard for Qualification and Certification of Nondestructive Testing Personnel
ISO 9712 Non-destructive testing — Qualification and certification of NDT personnel
BS EN 473 Non-destructive testing. Qualification and certification of NDT personnel. General principles
ASME ANDE-1 Nondestructive Examination and Quality Control Central Qualification and Certification Program