Introduction

Liquid penetrant testing (PT) is one of the most widely used techniques in the field of non-destructive testing (NDT). This method allows the detection of surface discontinuities in materials such as metals, ceramics, and plastics. The purpose of this article is to highlight the importance of liquid penetrant testing in non-destructive testing (NDT), explaining its operation, types, regulations, and most common applications to ensure the quality and safety of the materials inspected.

How does liquid penetrant testing work?

Its fundamental principle lies in the phenomenon of capillarity, which is the ability of a liquid to penetrate and fill surface discontinuities in a material. This effect occurs due to the adhesion forces between the liquid and the walls of the discontinuity. These overcome the surface tension of the liquid, allowing it to infiltrate into defects open to the surface, such as cracks, pores, or fissures.

Penetrant liquids are specially formulated to take advantage of this principle, allowing the liquid to move through minute discontinuities by capillary action. This process is complemented by other physical principles, such as cohesion, viscosity, adhesion and surface tension, which work together to maximize defect detection. In essence, these liquids can rise, fall or spread between nearby walls, facilitating an inspection of welds, equipment and materials, ensuring accurate identification of surface irregularities.

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Types of penetrant liquids

There are three main types of liquid penetrant. The choice of penetrant type depends on the material to be inspected, the environmental conditions and the level of sensitivity required.

Visible liquid penetrants: They use pigments, usually red, to highlight surface discontinuities. The inspection is performed under white light and a white developer is applied, which establishes a high contrast between the red pigment and the white background.
Fluorescent penetrant liquids: In this technique, the pigment is fluorescent, and evaluation is performed under ultraviolet light, which distinguishes fluorescent pigments effectively even in low light conditions.
Dual penetrant liquids: These liquids combine the characteristics of the two previous types. They can be evaluated both under white light with a visible penetrant and under ultraviolet light with fluorescent penetrants.

Removal methods

They are classified into four methods according to the way in which the excess penetrating liquids are removed.

Method A water washable: The penetrant liquid is removed directly with water, ideal for quick and practical inspections.
Lipophilic Post Emulsifiable Method B: Requires an oil-based emulsifier prior to rinsing with water, used in high sensitivity applications.
Method C Solvent Washable: Excess penetrant liquid is removed with a specific solvent, suitable for areas without access to water.
Hydrophilic Post Emulsifiable Method D: Requires a water-based emulsifier prior to rinsing, offering high accuracy in flaw detection.

Sensitivity

Liquid penetrant sensitivity is the ability to detect surface discontinuities, such as cracks, pores, or fissures. It is classified in different levels, from low to very high sensitivity, depending on the minimum size of the imperfections that can be identified. In any case, a higher sensitivity level allows detection of smaller defects, but may increase the possibility of false indications due to contaminants or minor surface irregularities. The choice of the appropriate level depends on the specific application, the material to be evaluated and the quality requirements of the process.

Sensitivity level ½: Ultra low sensitivity
Sensitivity level 1: Low sensitivity
Sensitivity level 2: Medium sensitivity
Sensitivity level 3: High sensitivity
Sensitivity level 4: Ultra high sensitivity

The procedure consists of the following steps

Surface preparation: The surface is cleaned to remove contaminants such as grease, rust or paint, which could obstruct the penetrant.
Penetrant application: A highly fluid liquid is applied to the surface of the material. This liquid enters the discontinuities thanks to capillarity.
Penetration time: The liquid is allowed to act for a specific time, known as Dwell Time, to ensure that it fully infiltrates the discontinuities.
Excess liquid removal: The liquid is removed from the surface without removing the liquid that has penetrated the discontinuities.
Developer application: A developer is used to draw the penetrating liquid from the discontinuities to the surface, making the defect visible.
Visual inspection: The inspector evaluates the discontinuities that appear as visible indications.
Final cleaning: After inspection, the part is cleaned to remove developer or penetrant residue and make it ready for further use or processing.

Types of liquid penetrant inspection

Manual: Applied directly by the inspector and inspected visually.

The following video shows the steps to follow in the manual inspection process with visible penetrant dyes. Source: CWB Group.

Liquid penetrant inspection.

Automatic: Uses mechanized or robotic systems equipped with cameras and sensors to perform faster and more accurate discontinuity detection. These systems are especially useful in mass production processes or when inspecting components with complex geometries. In addition, they can be integrated with software for automatic analysis and documentation of results, thus reducing the possibility of human error. This system is more efficient because it distributes the liquid uniformly over the surface, has better control of the penetration time, cleans without affecting the penetrant in the defects and ensures a uniform distribution of the liquid.

The following video demonstrates automated application with penetrant dyes. Source: MiracleanUltrasonics.

Automated clean and penetrant application.

Importance of liquid penetrant testing

Liquid penetrant testing plays an important role in the inspection of welds and other industrial components. Their importance lies in several aspects:

Early detection of defects: They allow the identification of surface discontinuities such as cracks, pores, and fissures that could compromise structural integrity.
Cost-effectiveness: Compared to other NDT methods, they are relatively inexpensive and easy to implement.
Versatility: They can be applied to a wide range of materials, including metals, plastics, and ceramics.
Complementary to other techniques: They are often used in conjunction with other non-destructive testing (NDT) methods, such as ultrasound or radiography, to provide a more complete evaluation.
Regulatory compliance: Many industries require liquid penetrant testing to ensure the quality and safety of their products.

Liquid penetrant detectable defects

This method is effective to identify discontinuities open to the surface in non-porous materials, such as:

Cracks: generated by stress, corrosion, fatigue or thermal processes.
Porosities: Defects caused by gas inclusion during welding or casting processes.
Surface laminations: Defects in laminated materials.
Cracks: Resulting from mechanical stresses or stress corrosion.
Welding failures: As lack of fusion or inclusions.

Standards governing dye penetrant testing

Several international bodies have established standards to ensure the correct application and evaluation of liquid penetrant testing. Some of the most relevant are:

ASTM E165: Specifies procedures for liquid penetrant testing of metallic materials.
ISO 3452: Defines general requirements for liquid penetrant testing.
ASME Section V: Provides guidelines for the application of liquid penetrant for weld and critical component inspections.
EN 571: European standard regulating liquid penetrant testing of non-metallic materials.

Personal level certifications

In liquid penetrant testing, there are numerous certifications that accredit the competence of professionals in this technique, the most recognized being those granted by ASNT (The American Society for Nondestructive Testing) and ISO (International Organization for Standardization). These certifications are granted to the technicians who perform the tests, after meeting specific training, experience and examination requirements. The accredited levels are as follows:

ASNT (SNT-TC-1A or CP-189).
- Level I: Technician qualified to perform tests under the supervision of a Level II or III certified professional.
- Level II: Professional with advanced skills to perform and interpret results, in addition to supervising Level I technicians.
- Level III: Technical expert with the ability to develop procedures, train personnel and lead NDT programs.
ISO 9712 (International Organization for Standardization).
- Certification: Level I, II and III, with characteristics very similar to those of ASNT.

Applications of liquid penetrant testing

They are widely used in various industries due to their efficiency and versatility:

Aerospace industry: inspection of welds and critical components such as wings, landing gear and airframes.
Welding fabrication and repair: Detection of surface defects in the different passes of welded joints to ensure quality and safety.
Automotive industry: Evaluation of parts such as axles, gears, and engine blocks.
Power plants and equipment: Inspection of welds, piping, valves and pressure vessels, wind turbine blades, among others.
Shipbuilding: Inspection of metal structures in ship hulls and offshore platforms.
Petrochemical industry: Integrity control of storage tanks and hydrocarbon transportation pipelines.

Advantages

High sensitivity to detect surface discontinuities.
Easily implemented, low-cost non-destructive testing technique.
Compatible with a wide variety of materials, including ferrous/non-ferrous, conductive/non-conductive and magnetic/non-magnetic.
Can be used to inspect large areas quickly.
Findings can be seen visually on the surface of materials and can show the dimensions of the defect.

Limitations

Only detects surface discontinuities (or discontinuities “open” to the surface).
Porous or rough materials cannot be inspected with penetrant dyes.
Requires thorough cleaning before and after testing.
Direct access to the material is required
Limited for high temperatures
There are several steps in the inspection process, each of which could affect the quality of the findings.
Presence of chemicals, so inspectors must follow established protocols for handling and disposal.

Recent advances in liquid penetrant testing

Recent advances in liquid penetrant testing have significantly improved both the sensitivity and efficiency of the process. Innovations such as new penetrant and developer formulations have improved visibility and reduced drying times, increased resistance to high temperatures, addressing the limitations of traditional methods that were prone to inconsistencies and subjectivity. In addition, the integration of automated systems has revolutionized the testing process.

Conclusion

Liquid penetrant testing is a significant tool in the non-destructive testing arsenal. Their ability to detect surface defects quickly, accurately and economically makes them an indispensable method for quality and safety assurance in a wide range of industries. Understanding how they work, the types available, the regulations governing them and their applications allows you to maximize their effectiveness and contribute to meeting the highest quality standards.