"The Big Five"
By Joseph Stump
Nondestructive inspection (NDI) in the field of aviation came into its own in the 1940s. While still in its infancy, it represented a major innovation in aircraft maintenance and inspection technology. Nondestructive inspection implies a method that will detect "defective" parts and assemblies while avoiding any damage to serviceable items during the course of the inspection process.
NDI has grown exponentially over the past 25 years in both application and innovation. It has done more to improve safety than just about any other existing technology.
This article will provide a general overview, "The Big Five," of NDI. These are the core disciplines that represent the bulwark of the aircraft maintenance and manufacturing industries.
1. Fluorescent penetrant testing
This is a nondestructive testing method best suited for finding discontinuities open to the surface in solid materials that are essentially nonporous. Penetrants are low viscosity liquids specifically engineered to seep into the tightest flaws and hairline fractures by way of capillary action. The penetrant contains a fluorescent compound that illuminates when exposed to ultraviolet light. This gives fluorescent penetrants a considerable edge in sensitivity over visible dyes. Visible dye penetrants are generally unsuitable for use on flight-related hardware and simply do not possess the sensitivity required to maintain a good margin of safety. Most current aircraft NDI manuals and procedures forbid their use.
To conduct a fluorescent penetrant inspection (FPI), the item to be inspected must be clean and free from paints, dirt, and oil as these inhibit the penetrant's ability to enter open flaws. The penetrant is then applied and allowed to "dwell" for a specified time established in the applicable procedure. After the penetrant is removed, a developer is applied to visually enhance any indications that may appear.
Before the inspection can be performed, the NDI technician must use a radiometer to monitor ambient white light conditions as well as black light intensity. If ambient white light levels are too great, the fluorescent radiance of the penetrant becomes compromised. The same is true if the energy output from the black light falls below required levels; the ultraviolet light will fail to illuminate the fluorescent dye effectively. The effects of excessive white light can be overcome by tenting the inspection area.
Penetrant inspection ranks as one of the most cost-effective test methods. The initial outlay for equipment and materials is probably the most modest of all NDI techniques.
The major liability of this process rests in its inherent inability to locate defects below the surface as well as its incompatibility with some materials and surface finishes.
2. Magnetic particle testing
Since its start in the 1930s, magnetic particle testing has been used extensively to test such fracture critical items as crankshafts, connecting rods, engine mounts, and landing gear components for surface and subsurface discontinuities. This testing process is ideally suited for examination of ferromagnetic materials (iron, nickel, and cobalt alloys), but not for nonferromagnetic materials (alloys of aluminum, magnesium, copper, titanium, and austenitic stainless steels).
The basic concepts behind magnetic particle testing are pretty straightforward. The part under test is magnetized with low voltage high amperage equipment to a specified value. A mixture of base oil and fluorescent iron oxide particles is then applied over the entire surface of the test article. If a flaw is present, such as a stress-related fracture, a magnetic leakage field forms around the defect. The leakage field establishes its own pair of North and South Poles. It is here the magnetic lines of flux are the strongest. The fluorescent iron oxide particles migrate to the defect by way of the leakage field. They congregate at the site where the break in the alloys crystalline structure occurs. Visual examination by black light readily detects the indication.