Liquid Penetrant Testing – The History and Principles

Liquid Penetrant Testing – the history and principles of the method

The Liquid Penetrant method should be considered by all NDT & CWI personnel seeking to maintain their certification status and enhance their “toolbox” for performing effective and efficient inspections. But before discussing the basic principals of the method, it’s equally important to know the history of how and why the method was developed.

Liquid penetrant testing is a versatile nondestructive test method used for detecting surface discontinuities in a wide variety of solid, nonporous materials. The effectiveness of the test is determined by the training, skill and dedication of the penetrant technician, the cleaning and preparation of the test object and the materials and procedure used to perform the test. Liquid penetrant testing is used primarily for detecting surface discontinuities.

The oil and whiting method used in the railroad industry in the early 1900s was the first recognized use of the principles of penetrants to detect cracks. The oil and whiting method used an oil solvent for cleaning followed by the application of a whiting or chalk coating, which absorbed oil from the cracks revealing their locations. Soon a dye was added to the liquid. By the 1940s, fluorescent or visible dye was added to the oil used to penetrate test objects.

Experience showed that temperature and soak time were important. This started the practice of written instruction to provide standard, uniform results. The use of written procedures has evolved, giving the ability for design engineers and manufacturers to get the same high standards results from any properly trained and certified liquid penetrant testing technician.

Liquid penetrant testing can detect surface discontinuities. It is a relatively inexpensive test method considering personnel training time, materials used and the ease with which surface discontinuities can be located. When performed properly with proper process controls and qualified controls and qualified technician, it will provide reliable indications. Liquid penetrant testing is used to locate surface discontinuities on the final finished surface of the test objects but can also be used for in-service checks to fatigue damage or production problems, for example the root pass of some welds and fatigue cracks. Fatigue induced cracks originate or propagate to the surface, making liquid penetrant tests useful in their detection.

Many materials fail during use. The cause of a high percentage of these failures is a linear discontinuity left in the test object during the manufacturing process or developed during use of the object. This gas convinced design engineers, manufacturers, users, safety and regulatory agencies to use reliable, inexpensive surface testing to minimize the undesirable consequences and expenses of object failure. The goal of any inspection process is to achieve as high a probability of detection as possible to mitigate risk of part failure.



When a dye is added to a liquid with a certain combination of properties of cohesion, adhesion, surface tension and viscosity, the liquid is called penetrant, penetrant dye or in some specifications just dye. When this penetrant is placed on a clean, dry surface it will wet the surface properly and migrate by capillary action. This capillary action causes the liquid to enter discontinuities that are open to the surface and flow up or down into the opening.

Some specifications and company procedures require different penetrant dwell times for different types of material or discontinuities. These times will be specifically required for the particular test object or procedure referenced. Typical penetrant dwell time allows the capillary action to cause the liquid to enter discontinuities.

After excess surface penetrant is removed, only the penetrant in the discontinuities remain. Reverse capillary action causes the penetrant to migrate or bleed back out to form a penetrant indication.

The use of a contrasting developer aids the reverse capillary action bleedout and makes the indications easier to see. A liquid penetrant testing indication is the penetrant bleedout spot against the contrasting developer background. After a developer dwell time, which is usually also a minimum of 10 minutes, any liquid penetrant indications are evaluated to the specification acceptance criteria. Developer dwell times vary with specification, company procedures or techniques; types of materials; type of discontinuity to be detected; and type of developer used. These dwell times typically range between 5 minutes and 4 hours.

Because of the ease of application, relative portability and the comparatively low cost Liquid Penetrant is a valuable and highly used inspection method in many instances.


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