Aviation has changed quite a bit since Orville and Wilber Wright made their first flight at Kitty Hawk, NC, aircraft like life was much simpler. A visual inspection was all that was needed to locate any possible damage. The life extension programs we have today were never dreamed of. Because of this simpler construction, this first generation of aircraft did not require much if any nondestructive testing.
Over the years with changes in aircraft design and specifically the materials used for construction, nondestructive testing has become an integral part of aircraft manufacture and operation. By applying different methods of nondestructive testing during the manufacturing process, defective parts can be identified and removed before they are put into service. Using nondestructive testing while the aircraft is in service ensures continued structural integrity of the airframe, engine, and components.
When the Wright brothers designed and flew the Wright Flyer, inspecting the structure probably was never considered. Even the term “nondestructive testing” (NDT) was not readily used until 1914.
At this time the common methods of NDT were liquid penetrant and magnetic particle. Variations of both of these methods had been used for many years. The penetrant inspection method was first developed by the railroad industry. This inspection technique was referred to as “Oil and Whiting”. Heavy oil was commonly available in the railroad workshops; this oil was diluted with kerosene and applied to the surface of parts. After a period of time the surface was carefully cleaned and coated with a chalk powder in alcohol. Once the alcohol evaporated a coating of chalk was left on the surface of the part. The part was then struck with a hammer causing the residual oil in a surface defect to seep out and leave a stain in the chalk coating. Can you imagine the damage one would cause by hammering on an aluminum part to do penetrant inspections like this? In the 1920s more effective penetrating oils with highly visible dyes were developed by Magnaflux and the Switzer Brothers in Cleveland, OH.
It wasn’t until 1942 that the penetrant method we know today was introduced by Magnaflux, the Zyglo system of penetrant inspection where fluorescent dyes were added. This is when the five different levels of penetrant sensitivity were developed. Today, we use levels 2 and 3 sensitivity for general structure and level 4 ultrahigh sensitivity penetrant for critical structure and rotating engine parts. And all penetrant materials are evaluated to verify they meet current industry requirements. Aerospace Material Specification (AMS) 2644 is now the primary specification used in the United States. Once a material has been accepted by this specification it is added to the Qualified Products List (QPL). Only materials on this list may be used for the inspection of aircraft structure and materials.
Magnetic particle was being used in machine shops, a fine ferromagnetic powder was applied to magnetized parts, and this powder collected over flaws and formed a visible indication. In the early 1930s magnetic particle started to replace the liquid penetrant method in the rail industry. While steel is not widely used in aviation, there are a number of parts made from high strength steel requiring inspection and magnetic particle is still the preferred method.
The eddy current and ultrasound inspection methods came to being when aircraft started to be manufactured from aluminum. Eddy current inspection techniques were first developed during the 1940s and used to inspect metal aircraft during World War II. After the war the advances made in construction of aircraft for military applications were being applied to civilian aircraft. The new metal structures were lighter and stronger than the old materials. With the use of these new materials came new problems like metal fatigue, cracking, and corrosion.
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