For the inspection to be thorough, the part must be inspected in two directions. That is to say, magnetization must be established circumferentially as well as along the longitudinal axis. Reliable observation of discontinuities can only be accomplished if the defect intersects the magnetic lines of flux at a favorable angle. This usually ranges from 45 to 90 degrees in most cases. As with fluorescent penetrant testing, all lighting conditions must be monitored.
Magnetic particle testing can also be accomplished with the use of dry oxide powders. For most aircraft, these powders lack sensitivity and are not a good choice for the steel alloys used in aviation. They should not be employed unless specifically called out in the test procedure.
Magnetic particle testing is economical. The equipment comes in all price ranges. The fixed "bench type" units can be pricy, but they are designed for high volume production. Field equipment is less costly. Some handheld portable mag yokes can be had for a few hundred dollars. All in all, magnetic particle inspection will remain a viable, cost-efficient method for some time to come.
3. Radiographic testing
Radiography, or X-ray inspection, has proven itself to be highly beneficial in the evaluation of contemporary and aging aircraft. X-ray is commonly employed to detect cracks, corrosion, assess internal damage, and detect foreign objects in airframe structures and powerplants.
For field applications, radiography is comparatively more expensive than its sister methods. The costs affiliated with equipment and film make it a tad pricey not to mention the hassles associated with regulatory requirements and the recruitment of qualified personnel. The cost efficiency depends upon its proper application. Radiography is costly if cheaper, more suitable methods of inspection are not fully explored. However, when it is the right tool for the job, it can save a lot of heartburn and money.
Before inspection, the aircraft and surrounding area must be evacuated to avoid exposing maintenance personnel to the effects of ionizing radiation. The importance of safety practices cannot be overstated, the fact remains that the risk to the general public and technical personnel remains negligible provided all warning signs and established barriers are heeded.
The X-ray tube converts electrical energy into X-rays. The power of the tube is rated in kilovolts or (KV), the higher the KV, the more powerful the tube. Most aircraft tubes run approximately 150 KV. This is a relatively modest energy level.
The real heart of the inspection is the exposed film or "radiograph." X-ray film is composed of a sheet of clear cellulose or triacetate that is treated on both sides with an emulsion of gelatin and silver halide compounds. These compounds undergo a chemical change when exposed to X-rays, gamma rays, or light. When the exposed film is treated in a chemical solution (developer), a further chemical reaction takes place. The silver halide compounds form tiny crystals of black metallic silver. It is this silver, suspended in gelatin on both sides of the cellulose base, that forms the radiographic image.
A radiograph is about "density" (light and dark) as well as image. The film resembles a photographic negative. Thinner sections of material will appear darker than thicker ones. Thicker cross sections absorb radiation more readily. This diminishes the X-ray's ability to expose the film. For example, a badly corroded piece of aircraft aluminum loses mass due to oxidation. On a radiograph, the most severely pitted or exfoliated areas will appear darker than the rest of the specimen. Where the material has become thinner, more radiation comes in contact with the film.
4. Eddy current testing
The concept of eddy current testing is far older than one would suspect, but it wasn't until the 1940s that it became crudely functional. By the 1960s, the technique had proliferated into a wide variety of industrial applications. Besides being highly adaptable, eddy current evaluation introduced new possibilities in testing methodology that could not be accomplished by other NDI disciplines.
Eddy currents are electromagnetically induced currents; therefore, the method is limited to materials that are good conductors. A coil is mounted into a probe that is connected to the test unit. The coil's magnetic field fluctuates into the test material at high frequency. This generates the circular flow of eddy currents, which produce a fluctuating magnetic field of their own. This field is in direct opposition to the field of the test coil and creates impedance. Any factor affecting the flow of eddy currents causes a change in the impedance of the coil. This registers on the equipment's display screen, meter, or CRT.
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