Liquid penetrant test, also known as dye penetrant inspection (DPI) or liquid penetrant inspection (LPI), is a non-destructive testing method that is used for detection of different types of cracks in concrete such as fatigue cracks, quench cracks, grinding cracks, overload cracks, impact fractures, and porosity cracks.

The liquid penetrant test is affordable, flexible, and easy to use, and considered when visual inspection is not enough to detect cracks in concrete, especially in the case of Liquid retaining structure. This test method is able to observe cracks, but it neither specifies its depth nor monitors the crack development.

It works on the principle of Capillary action i.e., the ability of a liquid to flow into narrow cracks without help, even in opposition to the forces such as gravity, to detect discontinuities of concrete surface.

The liquid penetrant testing involves application of a liquid to concrete surface where cracks are appearing. Then excess liquid from the cracks is removed. After that, a developer is applied to pull out the trapped penetrant liquid from the cracks. Finally, the cracks in concrete are detected by the visual inspection process.

Principle of Liquid Penetrant Test
Fig. 1: Principle of Liquid Penetrant Test

Purpose of the Test

Liquid penetrant testing is used to detect cracks in structural concrete elements. It is specifically employed when visual inspection is not adequate for cracks detection.

Procedures

The liquid penetrant test can be conducted based on the procedures which are provided by various international codes such as ASTM E 1417, Standard Practice for Liquid Penetrant Testing, and EN 1371-1, Founding – Liquid penetrant inspection – Part 1: Sand, gravity die and low pressure die castings. The following steps can be considered to carry out liquid penetrant test:

1. Cleaning of Concrete Surface

Concrete surface must be cleaned from oil, grease, water, or other contaminants that possibly prevent penetrant from entering cracks.

2. Application of Liquid Penetrant

After cleaning and drying of concrete surface, the liquid penetrant is applied to the concrete surface. The penetrant is then allowed time to soak into any cracks for generally 10 to 30 minutes.

3. Excess Penetrant Removal

The excess liquid penetrant is then removed from the concrete surface. Method for removal of penetrant is based on the type of penetrant used. Water-washable, lipophilic post-emulsifiable, solvent-removable, or hydrophilic post-emulsifiable are the common materials for removal of extra penetrants.

During the removal of penetrant using solvent and lint-free cloth, the solvent should not be directly sprayed on the surface with defects, as it can remove the liquid penetrant from the cracks.

4. Application of Developer(Powder)

After removal of excess penetrants from the cracks in concrete, a thin layer of developer is applied on the surface. This developer draws the penetrants out from the defects or cracks and forms a visible indication for propagation detection.

Any color stains indicate the positions and types of defects on the surface. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

5. Inspection of Cracks in Concrete

Visible light or ultra-violet lights of adequate intensity are used for inspection of cracks in concrete. Inspection of defects is done after 10 minutes of developer(powder) application.

Any delay after application of developer for inspection of cracks, may bleed out the liquid penetrants from cracks and correct interpretation may be hindered.

6. Post Cleaning of Concrete Surface

The concrete surface is cleaned after inspection and recording of defects if any found.

Procedure for Liquid Penetrant Test
Fig. 2: Procedure for Liquid Penetrant Test

Advantages

  • It is easy and requires a minimal amount of training.
  • Complicated geometric shapes can be tested easily.
  • High sensitivity to small surface discontinuities.
  • Cost-effective
  • Few material limitations such as—works on non-metallic, metallic, non-magnetic, magnetic, non-conductive and conductive materials
  • Liquid penetrant testing materials are individually very cost-effective
  • Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
  • Rapid inspection of large areas and volumes.

Disadvantages

  • Requires precleaning and extensive surface contamination may appear as defects.
  • Surface finish and roughness can affect inspection sensitivity.
  • Sensitive to surface-breaking defects only
  • Only materials with a relatively nonporous surface can be inspected.
  • Post-cleaning is necessary
  • User dependent
  • Environmental concerns—may require disposing of chemicals and expensive handling
  • It is impossible to monitor the advancement of defects or determine the severity of its depth.
  • Multiple process operations must be performed and controlled.
  • The inspector must have direct access to the surface being inspected.

Applications

Liquid penetrant test method can be used to examine various materials such as:

  1. Concrete
  2. Metals (aluminum, copper, steel, titanium, etc.)
  3. Glass
  4. Many ceramic materials
  5. Rubber Plastics