Liquid Penetrant Testing (DP Test)

What is Penetrant Testing or DPT?

Liquid penetrant testing (LPT) or also known as Dye penetrant test (DPT) is one of the most popular Nondestructive Testing (NDT) methods. It is cheap, handy, and requires minimum training when compared to other NDT methods. Like other nondestructive test methods, DPI allows inspectors to test materials without cutting, boring, or otherwise disturbing the surface. Dye penetrant inspection helps to reveal surface cracks or other defects that may not be visible to the naked eye. One of the biggest limitations to dye penetrant inspection testing is that it can only be used on non-porous surfaces. Porous materials like concrete or stone simply soak up the dye, making it easy to miss problem areas. This test method may also be ineffective on objects with a roughly textured surface. Dye penetrant tests generally work best on relatively smooth surfaces, where the dye will penetrate only into cracks and other defects.

This NDT method uses a colored dye, which can easily find its way into clean and dry discontinuities. After sufficient penetrant dwell time has been allowed for the test medium to enter discontinuities, the excess surface penetrant is removed. A blotting material (developer) is then applied to draw the highly contrasting liquid out onto the specimen’s
surface where it forms visible indications.

liquid penetrant examination has been called many names:

Other names of PT

  1. Penetrant testing (PT),
  2. Liquid penetrant testing (LP), and
  3. Dye penetrant testing (DP). 

The ASNT considers the name as liquid penetrant testing (LPT)ASME B & PVC) and the NBIC (National Board Inspection Code) call it liquid penetrant examination (LPE) but the term is same as PT in both ASNT and ASME.

Dye penetrant solvent removable method

The dye penetrant solvent removable method is most popular because it is low cost and very versatile. It typically comes in three aerosol cans –

  1. cleaner,
  2. penetrant, and
  3. developer.

Most nonporous materials (steel, stainless steel, cast iron, aluminum, brass, bronze, titanium, rubber, plastics, and glass) can be examined using PT. Porous materials (concrete, wood, paper, cloth, and some types of fiberglass if the fibers are exposed to the surface) should not be examined using PT. 


  • High sensitivity to small surface discontinuities
  • Easy inspection of parts with complex shapes
  • Quick and inexpensive inspection of large areas and large volumes of parts/materials
  • Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive can all be inspected)
  • A visual representation of the flaw is indicated directly on the part surface
  • Aerosol spray cans make the process portable, convenient, and inexpensive
  • Indications can reveal the relative size, shape, and depth of the flaw
  • It is easy and requires a minimal amount of training


  • Detects flaws only open to the surface
  • Materials with porous surfaces cannot be examined using this process
  • Only clean, smooth surfaces can be inspected. (Rust, dirt, paint, oil, and grease must be removed.)
  • Metal smearing from power wire brushing shot blasting, or grit blasting must be removed prior to liquid penetrant examination
  • Examiner must have direct access to the surface being examined
  • Surface finish and roughness can affect examination sensitivity. (It may be necessary to grind surfaces before PT.)
  • Multiple process steps must be performed and controlled
  • Post cleaning of parts and material is required, especially if welding is to be performed
  • Proper handling and disposal of chemicals is required
  • Fumes can be hazardous and flammable without proper ventilation

For PT to be used on ASME Code construction or NBIC repair or alterations, a written procedure must be followed. This must comply with ASME Boiler and Pressure Vessel Code, Section V, Article 6, and address all essential and nonessential variables. Many liquid penetrant examinations are done for informational purposes only and do not follow a written procedure. For instance, a written PT procedure does not need to be followed if a welder grinding out only a weld crack for repair and using PT to ensure removal of the entire crack. However, if the PT is being done to comply with the Code, the written procedure needs to be followed by qualified NDT personnel.

Penetrant testing steps:

Basic Steps:

The penetrant testing process consists of six basic steps regardless of the type of penetrant and removal method:

  1. Preclean and dry the test object.
  2. Apply liquid penetrant to surfaces of interest and allow sufficient dwell time.
  3. Carefully remove excess penetrant from the surface without removing penetrant from discontinuities.
  4. Apply developer to the prepared surface and allow adequate development time for test indication formation.
  5. Visually examine surfaces for test indications; interpret and evaluate indications.
  6. Postclean the specimen if process residues will be detrimental to subsequent operations—for instance, painting—or the part’s intended function.
  1. Pre-clean part. 

The importance of precleaning cannot be overstated because adverse surface conditions, including paint, oils, and surface soils, or corrosion products, chemically active residues, and moisture, can interfere with the following: wetting of the test surface by the liquid penetrant, entry of the liquid penetrant into discontinuities, and the subsequent bleed out of liquid penetrant to form discontinuity indications.

penetrant testing

2. Apply penetrant. 
Penetrant is applied by spraying penetrant from the aerosol can or applying it with a brush. A dwell (soak) time needs to be observed to allow for the penetrant to permeate into cracks and voids. This is typically 5 to 20 minutes but should never be long enough for the penetrant to dry. The penetrant manufacturer’s recommendations and written procedure should be followed.

penetrant testing

3. Remove penetrant. 
All penetrant should be removed with clean, dry, lint-free rags until thoroughly clean. The part or material should be rubbed vigorously until the penetrant is not visible on the dry rags. Next, cleaner/ remover should be sprayed on another clean, dry, lint-free rag and used to vigorously rub the part again until there is no penetrant visible on the rag.

4. Apply developer.
A thin, light coating of the developer should be sprayed on the part being examined. A dwell time needs to be observed to allow time for the dye to exit the flaws and create an indication (flaw) in the developer. The dwell time for the developer is typically 10 to 60 minutes. The developer manufacturer’s recommendations and written procedure should be followed closely.

penetrant testing

5. Evaluate indications.
It is critical to examine the part within the time frame designated in the written procedure.  Length of an indication can grow over time as penetrant bleeds out, causing an acceptable indication to be a rejectable defect.  Length of indication is measured for evaluation, not length of the flaw.  Here, the two linear indications are rejectable defects.  The round indication is nonrelevant. 

penetrant testing

6. Post-clean part.
The part needs to be cleaned to remove all developers after it has been evaluated.

penetrant testing

ASME Section V also requires the dye penetrant solvent removable method to be evaluated with a minimum light intensity of 100-foot candles on the part surface. The other unit is Lux (1 Foot candle= 10.764 Lux) Proper quantity of light must be verified using a light meter. Below picture of Light meter showing 109.9 footcandles (ftc) of light.

penetrant testing lux meter

In the ASME B & PV Codes of Construction, magnetic particle examination or liquid penetrant examination is called out many times to detect the possibility of surface defects. If material is nonmagnetic the only choice is PT.

Example of ASME Code required examinations include:

  • castings for surface defects
  • plates for laminations in corner joints when one plate’s edge is exposed and not fused into the weld joint
  • head spin hole plug welds
  • weld metal build-up on plates
  • removal of defects before welding repair

Once the fabricated parts such as boilers, vessels, piping and other components are in service, PT can be a very valuable tool.

Related Article:

ASNT NDT Level III Penetrant testing mock examination.

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