NDT Method Overview: What is Liquid Penetrant Testing?

While many of our readers are well-versed in NDT, some might be just learning about our industry. This series will serve as a primer to the major NDT methods covered in ASNT’s publications, educational programming, and events. The following is an excerpt from the second edition of Principles & Applications of Liquid Penetrant Testing, published by ASNT in 2020.

This book was developed to be used by technicians for classroom training, self-instruction, and as a reference. It is available from the ASNT Store both in print and now as an ebook.

HISTORY OF PT

The liquid penetrant process is one of the oldest nondestructive testing (NDT) methods. It originated in the 1890s, where it was used in railroad maintenance shops. It was noticed that parts such as axles, crank pins, and couplings that were coated with oil would exude the oil from cracks or other surface openings after first being wiped clean.

This led to the “oil and whiting” test. Parts were immersed in oil for a period of time and, after removal, the surface oil was wiped off with kerosene-moistened cloth. The part was then coated with powdered chalk or a mixture of powdered chalk and alcohol (whiting). Any oil trapped in cracks or other surface discontinuities crept out and stained the chalk coating leaving a visible indication.

Most railroad maintenance shops introduced innovations, such as the use of dirty or used oil to improve the visibility of the indication. Another innovation included the reduction of the oil viscosity by heating or diluting it with kerosene and then hammering or vibrating the part to “work” the oil out of the surface opening.

The applications for the oil and whiting test on steel and ferrous materials were greatly reduced in the 1930s when the magnetic particle test came into general use. However, industries using aluminum, magnesium, and other nonmagnetic metals, especially aircraft manufacturers, needed a more reliable and sophisticated tool than discolored machine oil and chalk.

In the late 1930s, fluorescent dye materials were added to highly penetrating oil, creating a fluorescent penetrant material. The fluorescent dye materials process was patented in 1941. In the mid-1940s, colored dyes, primarily red, were introduced to the dye materials. Since then, a large number of penetrant systems or families have evolved. These include developments in the various types and concentrations of dye materials, types of penetrating oils and additives, materials and methods of removing the excess surface penetrant, and various materials and forms of developing agents.

BASIC PENETRANT PROCESS

Modern PT materials and procedures have been significantly improved over the oil and whiting days; however, the basic fundamentals have not changed. This introduction provides a simplified description of three basic penetrant steps. Figure 1 illustrates the three basic steps associated with the PT process.

1. In the first step, a penetrating liquid containing dyes is applied to the surface of the object to be inspected. The part surface must be clean and dry before applying the penetrant. Cleaning is not part of the penetrant process but is essential for reliable results. Contaminants, soils, or moisture on the part surface or inside the discontinuity can hinder or prevent the penetrant from entering the discontinuity opening. This reduces the effectiveness of the inspection.
2. After the penetrant is applied, it is allowed to remain on the part surface for a period of time referred to as the “penetrant dwell,” which allows the penetrant to enter and fill any surface openings or discontinuities. After a suitable dwell time, the penetrant is removed from the part surface as shown in step 2. This operation is critical. Care must be taken to minimize the removal of penetrant trapped in discontinuities; excess removal would reduce the size and visibility of any indications formed.
3. The third step is the application of a material called the “developer.” Developer assists in extracting penetrant entrapped in discontinuities and in forming and enhancing indications on the part surface.

BASIC PT PROCESS REQUIREMENTS

The following principles affect the reliability and capability of the penetrant process:

• The penetrant must enter the discontinuity to form an indication. Any foreign materials (such as contaminants, soils, moisture, or smeared metal) that restrict the entry of penetrant will reduce the effectiveness of the inspection.
• The penetrant must remain on the part surface for a sufficient time to allow it to enter and fill the discontinuity. This period of time varies widely depending on a number of factors, such as the size and type of discontinuity and the type of penetrant.
• Overemulsification or overwashing of the penetrant from the part surface will remove penetrant trapped within discontinuities, reducing the size and visibility of indications.

NDT PERSONNEL REQUIREMENTS

The apparent simplicity of the penetrant process is deceptive. Very small processing variations during penetrant application, removal of excess surface penetrant, application of developer, or examination can produce large differences in inspection results. Therefore, the process is highly dependent on the operator’s knowledge, skill, and experience. The reliability and confidence in the inspection are totally dependent upon the operator and inspector.

The operator who is responsible for processing the parts must be trained, have some experience in processing requirements, and must be aware of the detrimental effects of processing variations.

Inspectors who examine and evaluate indications must also be trained and have experience in the processing requirements and must be capable of making decisions based on the interpretation of penetrant indications.

ASNT provides guidelines for training, qualifying, and certifying NDT personnel in Recommended Practice No. SNT-TC-1A: Personnel Qualification and Certification in Nondestructive Testing (2020).

ADVANTAGES AND CAPABILITIES OF PT

• PT has a number of advantages and capabilities compared to other inspection methods.
• PT is capable of examining the entire external surface of an object in one operation, even though the object may have a complex shape. Most other inspection methods require multiple applications for different surfaces or scanning a small area with a probe or transducer. With PT, small parts are usually dipped or immersed in the liquid penetrant. Parts that are too large to be immersed can be coated with penetrant by spraying, brushing, flowing, or pouring.
• Liquid penetrant can detect very small discontinuities. The penetrant “bleed-out” of an indication also magnifies the apparent size of discontinuities, making them easier to see. In addition, the indications are formed over the discontinuity on the surface of the part. This provides information on location, orientation, and approximate length, simplifying interpretation and evaluation. A typical fluorescent penetrant indication of a crack on a bolt is shown in Figure 2.

• PT can be used on a wide variety of materials including ferrous and nonferrous metals, fused ceramics and cermets, glass, and many types of plastics. There are some limitations on nonmetallics.
• The sensitivity of PT can be varied by selection of materials. A range of sensitivities are possible and, when desired, can be adjusted to suppress indications from small, inconsequential discontinuities while indicating large discontinuities of concern. Figure 3 shows indications from two different sensitivity penetrants on a cracked chromium reference panel.
• PT is relatively inexpensive and does not require high technology equipment.
• The sensitivity level of the inspection can be adjusted by the careful selection of different liquid penetrant materials and processes.
• PT is often the first NDT method for industrial products because it requires less capital expenditure for implementation than other methods. PT is also used as an efficient in-process inspection to prevent wasteful processing of product that would be rejected at the final stage.

DISADVANTAGES AND LIMITATIONS OF PT

There are a number of disadvantages and limitations to the PT process. Some of these are:

• PT cannot reveal discontinuities that are not open to the part surface.
• The part surface and the interior of any discontinuity must be clean and free of contaminants, soils, and moisture. Organic and inorganic materials on the part surface at the discontinuity, discontinuity edges, or inside the discontinuity can restrict or prevent the penetrant from entering and filling the discontinuity; this decreases the effectiveness of the inspection.
• PT cannot be used on porous materials or materials with interconnecting subsurface networks. Penetrant can be absorbed or enter the pores and migrate through the interconnecting network, producing an overall fluorescence or color background that would mask any discontinuity indications.
• Penetrants usually consist of petroleum-based vehicles that can soften or attack some nonmetallic materials, such as certain rubbers and plastics. Penetrants may also permanently stain or discolor porous or coated surfaces.
• Most of the materials used in penetrants, emulsifiers, cleaners, and developers have very good wetting and detergent properties. They can also dissolve and remove the natural oils from skin if they come in contact with skin surfaces for any length of time. This causes drying and cracking that is a source for secondary infections.
• Most liquid penetrant materials are petroleum-based and should not be used on parts or assemblies where the oil residue would have a negative impact on further use. For example, the use of petroleum-based liquid penetrants on parts used in liquid oxygen systems is extremely dangerous and extra processes must be used to ensure the penetrant residue is totally removed.
• Mechanical operations such as shot peening, honing, abrasive blasting, or grinding will smear or peen the surface of the metals. These operations close or reduce the surface openings of existing discontinuities. PT will not reliably detect discontinuities when performed after these types of mechanical operations without further processing, such as chemical etching.

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This blog post is excerpted from the second edition of Principles & Applications of Liquid Penetrant Testing, published by ASNT in 2020. Originally written by Bernie Boisert, it was updated in 2020 by Charles W. Eick. The book is available from the ASNT store: asnt.org/Store/ProductDetail?productKey=b3eeb8e0-34c2-4bf5-91f2-d59e43d718de

This book was developed to be used by technicians for classroom training, self-instruction, and as a reference. It is available from ASNT both in print and now as an ebook.

To learn more about liquid penetrant testing:

• Nondestructive Testing Handbook, Fourth Edition: Volume 1, Liquid Penetrant Testing
• ASNT Questions & Answers Book: Liquid Penetrant Testing Method
• ASNT Level II Study Guide: Liquid Penetrant Testing, Third Edition
• ASNT Level III Study Guide: Liquid Penetrant Testing Method, Second Edition
• Liquid Penetrant Testing Classroom Training Book, Second Edition
• Relevant Discontinuities: Magnetic Partical and Liquid Penetrant Testing
• For webinars and refresher courses, go to ASNT Learn