The Development of Dye Penetrants in Aviation

By Joseph E. Stump

The application of dye penetrants for finding minute fractures in aircraft structural materials started in the U.S. aircraft industry in 1942. As with most developments that had a significant impact on aviation, this is a story of evolution, not revolution.

The growth of contemporary dye penetrants can be traced back to the mid- 1930s when advancements in the magnetic particle inspection method accentuated the need for a similar method for nonferrous materials. The term "penetrant" relates to the principal property of the material itself, namely, the liquid's ability to penetrate or seep into minute openings such as hairline fractures. Fortunately, the same properties that cause the liquid to penetrate in the first place are also responsible for its re-emergence from the defect after the surface has been cleaned.

Oil and whiting

In the train industry, the "oil and whiting" method was used to find cracks in steam locomotives and heavy parts such as couplers. The method can be traced back to the 1890s, and it was the first use of a penetrating substance designed to locate flaws.

The penetrating oil used in this technique was a heavy 600W available in most railroad shops. The oil was thinned with kerosene to lower the viscosity; this also gave it a rich dark brown hue. The parts to be inspected were cleaned in a tank of boiling caustic soda, dried, and dipped into the oil mixture. The parts were allowed to dwell in the oil for up to 24 hours. The excess surface oil was removed until the assembly was clean and dry. "Whiting" was then applied to the surface of the part. This was a mixture of powdered chalk and denatured alcohol. The alcohol would evaporate, leaving an even coating of chalk over the part surface. As part of the procedure, the item under test was often struck with a heavy hammer to "squeeze" the oil out of the cracks where it would be absorbed by the chalk, leaving a dark brown stain at the point of fracture.

The oil and whiting method left much to be desired. It lacked the sensitivity to locate fine, shallow fatigue cracks at a point where salvage of the part is nearly always possible. In addition, the oil and whiting procedure lacked standardization. The method varied widely from shop to shop according to the individual organization's own recipe; the results varied widely as well. The oil and whiting method, while lacking in sensitivity, did hold the essential answer to the problem of finding a way to reliably test nonferrous alloys. It was the first penetrant/developer combination used in industry.

The development of fluorescent

penetrant

In 1938 Taber de Forest, a researcher for Magnaflux, experimented extensively with color-contrast penetrants. The results were mixed. While making considerable headway, the product was not yet ready for the marketplace. At about the same time, Robert C. Switzer, of Switzer Brothers, began his experiments independent of Magnaflux. His work focused on the use of color-contrasting penetrants as a means of flaw detection. Switzer realized a superior contrast could be achieved with the use of fluorescent dyes and the application of ultraviolet light to induce the penetrants to fluoresce. Switzer was granted U.S. Patent 2,259,400 in October 1941. This covered both of his developments in color-contrasting and fluorescent penetrants. The Patent Office later required a separate patent number to differentiate between the two developments. Patent # 2,259,400 became the basic patent that covered fluorescent dye penetrants.

Switzer offered Magnaflux an exclusive license under his patent in 1942. While Magnaflux realized the enormous potential of Switzer's development, more work was needed to produce a viable, cost-effective test system. Greer Ellis and Taber de Forest led the Magnaflux research team. After months of tedious research, the first water-washable fluorescent dye penetrant system emerged. This was marketed under the trade name "Zyglo".

In July 1942 this new inspection system was offered commercially, and it was presented to the Air Arms Division of the Army and Navy for evaluation. Being responsible for the wartime production of aircraft, Air Arms quickly moved to approve the method for the location of surface flaws in nonmagnetic materials.

Early aviation applications

During the war years, the use of fluorescent penetrants expanded quickly. It was employed on a wide variety of aircraft parts, such as:

Lack of bond in bearings

Stainless steel supercharger impeller wheels

Stellite tools

Hard-faced exhaust valves

Sintered tungsten-carbide tools

Propellers

Piston engine cylinder heads

Piston engine crankcases

Aluminum and magnesium castings

During the following decade, penetrants continued to improve and expand into a wide range of industrial applications. Special-use penetrants were developed to solve specific problems. Equipment for both general use and specialized testing were developed and marketed. It was during this era the dye penetrant inspection method proved its cost effectiveness, viability, versatility, and limitations.

Color-contrasting penetrants

Magnaflux had explored color-contrasting penetrant systems in the 1930s. R.C. Switzer thoroughly investigated this approach before his decision to focus on the more promising fluorescent penetrants. By the mid-1940s, further developments in color-contrasting penetrants were taken up by Rebecca Sparling and other private researchers. This method, despite improvements, failed to achieve widespread general use. As the understanding of penetrants continued to advance, color-contrasting dye penetrants, though less sensitive than their fluorescent counterparts, have since found acceptance in many industrial applications. It is important to note that color-contrasting penetrants are not really suitable for aviation use. Colored dye penetrants are commonly found in hangars all across the United States, however, they should only be used for diagnostic purposes. Flight hardware should never be returned to service via a color-contrasting method unless the manufacturer okays it in a documented procedure. Current aircraft NDT manuals almost universally forbid the use of colored dye penetrants due to their decreased sensitivity.

The greatest advantage of the color-contrast method lies in its portability. It is easily applied by aerosol spray or brushed on straight from the container. There is no need for electrical outlets, black lights, or any of the other specialized equipment often associated with fluorescent penetrants. Contemporary color-contrast penetrant kits are composed of a three-part system. They consist of a contrasting dye penetrant--usually red--a cleaner/remover, and a developer to accentuate any indications. All three items are readily available in aerosol form.

The state of the art

After more than 60 years of use, fluorescent penetrant inspection has proven to be a crucial quality control method for the aviation industry. Penetrant testing is currently one of the three most widely used NDT methods: the other two are radiography and magnetic particle inspection.

As a result of many years of development and service, much knowledge has been accumulated about how to obtain the best possible results from penetrant systems under a wide range of conditions. Research has developed a wide variety of special-application penetrants engineered to meet specific needs. This can range from highly porous materials to compatibility with volatile elements such as liquid oxygen. Penetrants will continue to be one the most widely used NDT methods available to the aviation industry. For cost effectiveness, reliability, and simplicity of use, penetrant inspection in all its many forms will remain an indispensable inspection tool for the foreseeable future.

Note: this article is based on research by C.E. Betz.

VENDOR LISTING

Magnaflux

Phone: 847-657-5300

Web: www.magnaflux.com

Staveley NDT

Phone: 509-736-2751

Web: www.staveleyndt.com

Yxlon International

Phone: 330-798-4800

Web: www.yxlon.com

GE Inspection Technologies