The field of NDT (nondestructive testing) and inspection is varied. There are various methods that are available for use in aircraft maintenance. The effectiveness of any particular method of NDT inspection depends upon the skill, experience and training of the persons applying the process. Each process is limited in its usefulness as an inspection tool through its adaptability to the particular component to be inspected. It is often necessary to consult the aircraft or product manufacturer for specific instructions regarding NDT inspection of their product.
INSPECTION BY MAGNIFYING GLASS AFTER WELDING. Careful examination of all joints with a medium-power magnifying glass (at least 10-power), after first removing all scale, is considered an acceptable method of inspection for repaired structures. The practice of filling steel tubular structures with hot linseed or petroleum base oils, under pressure, in order to coat the inside surface and inhibit corrosion, assists in the detection of weld cracks, as the hot oil will seep through cracks invisible to the eye. This practice, though not justifiable in all cases, is suggested where a very large portion of the structure has been rewelded by aerospace engineers.
Magnetic particle inspection
Magnetic particle inspection can be used only on magnetic materials; i.e., iron and steel. Most stainless or high chromium nickel and manganese alloy steels, being nonmagnetic, cannot be inspected by this method. The method consists essentially of detection of discontinuities (cracks, voids, defects, pits, subsurface holes, etc.) by means of accumulation of magnetic particles on the discontinuities when the part has been magnetized aerospace tech consultant. The magnetic particles are applied either dry as a powder or suspended in light oil. For complete magnetic inspection, both circular and longitudinal, magnetization should be deployed. Improper operation of Magnaflux equipment, because of faulty equipment or by untrained persons, can jeopardize the airworthiness of parts being inspected. Minute electrical arc burns caused during inspection, can result in eventual failure of the part.
Circular magnetization is produced by transmitting an electric current directly through the article being tested, or through a central conductor placed through the part, in which case, defects parallel to the flow of current may be detected. As an example, circular magnetization of a round steel bar would be produced by placing the ends of the steel bar between the heads of the magnetic inspection machine and passing a current through the bars. Magnetic particles applied either during or after passage of the current, or after passage of the current in magnetically retentive steels, would disclose discontinuities parallel to the axis of the bar.Longitudinal magnetization is induced in a part by placing the part in a strong magnetic field, such as the center of a coil. Thus, longitudinal magnetization of a round steel bar would be produced by placing the ends of the bar between the heads of a magnetic-inspection machine and placing the D.C. solenoid around the bar. After application of the magnetic particles, either during or subsequent to magnetization, discontinuities perpendicular to the axis of the bar would be disclosed.
Red, black, and sometimes grey particles are used in the wet or dry methods. In the case of wet inspection, a fluorescent magnetic particle may also be used. Articles inspected are illuminated by so-called black light, and the magnetic particles glow by florescence causing any defects or indications to be easily visible. The wet inspection procedure provides better control and standardiza tion of the concentration of magnetic particles, easier application to complex shapes, and indications that are easier to interpret. This is due to the difficulty of obtaining efficient distribution of the dry powder during magnetization. The dry procedure is particularly suitable for detecting subsurface defects, such as, when inspecting heavy welds, forgings, castings, etc. The wet continuous process is recommended by aerospace engineers for most aircraft work.
The presence of accumulations of magnetic particles in magnetic inspection does not necessarily mean that a defect exists. Changes in section of the part, particularly where the change in section is very sharp, and also holes drilled through a part, will frequently cause indications. Surface defects are most easily detected, however, since a crack will cause a sharp line of magnetic particles to appear. Subsurface defects are less easily detected, only a general collection of magnetic particles will be observed.
After magnetic inspection carefully demagnetize and clean the parts. Examine for possible evidence of electrical arc burns that may have occurred during inspection. All metal particles must be removed and the serviceable bits coated with a suitable preservative.
In house NDT
Portable type magnetic particle inspection equipment has been developed for use in the detection of surface or slight surface discontinuities in ferromagnetic aircraft materials and parts. This type of equipment usually gives better results when the wet pension type of indicator, such as that conforming to Specification MIL-I-6868, is used with the powder supplied by the equipment manufacturer.
X-ray or radiographic inspection may may be used on either magnetic or nonmagnetic materials for detecting subsurface voids such as open cracks, blowholes, etc. When a photographic film or plate is used to record the X-ray (in a similar manner to exposing a photographic film), the process is known as radiography. When the X-rays are projected through the part onto a fluorescent screen, the process is known as fluoroscopy. The technique used for radiography should be capable of indicating the presence of defects having a dimension parallel to the X-ray beam of 3 percent of the thickness of the part being radiographed for magnesium alloys, and 2 percent for all other metals and alloys. Inspection using a fluoroscopic screen is much less sensitive. Consequently, the radiographic method is usually used for inspection and the fluoroscopic method is used for culling.
Radiographic inspection is extensively used by aerospace engineers in the aircraft industry for the inspection of all types of castings including sand castings, permanent-mould castings, die castings, etc. X-ray is particularly useful for this application, since it is capable of disclosing defects which exist below the surface, and also since the open types of defects which may occur in castings (shrinks, blowholes, dross inclusions, etc.) are readily disclosed by proper use of X-rays.
In the inspection of forged or wrought metals, on the other hand, X-ray inspection is not used so extensively. This is due to the fact that the process of forging or working may cause defects which originally existed in the metal to become tightwalled cracks. Such defects are somewhat difficult to disclose by X-rays. If doubt exists as to the suitability of the X-ray examination, abandon in house NDT and consult a laboratory familiar with the X-ray examination of aircraft parts.