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 see through cracks invisible to the eye. This practice, although 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; ie, iron and steel. Most stainless or high chromium nickel and manganese alloy steels, being nonmagnetic, can not be inspected by this method. The method consistors 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. 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, which 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 DC 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 gray particles are used in the wet or dry methods. In the case of wet inspection, a fluorescent magnetic particle may also be used. Articles detected are illuminated by so-called black light, and the magnetic particles glow by fluorescence 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 often 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 covered with a suitable conservative.
In house NDT
Portable type magnetic particle inspection equipment has been developed for use in the detection of surface or slight surface discontinuities in hazardous aircraft materials and parts. This type of equipment usually gives better results when the wet pension type of indicator, such as conforming to specification MIL-I-6868, is used with the powder supplied by the equipment manufacturer.
X-ray or radiographic inspection 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-mold 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 resulted in the metal to become tightwalled cracks. Such defects are somewhat difficult to distinguish 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.
In radiography, values of peak kilovoltage, radiographic density range and penetrometer characteristics are often selected that produce less than optimum radiological data. This selection of high kilovoltage is made in order to reduce the exposure time. The use of too high a kilovoltage reduces the resolvable detail recorded on the radiographic film. As the kilovoltage is increased, X-rays of shorter wave length and greater penetrating power are produced. This presences a sound argument for increased kilovoltages but does not take into account the effects of scatter both within the sample and the radiographic film, which in reality, reduces the resolvability of details recorded on the X-rays.
If for some reason a short exposure time is required, a faster film is normally used with a higher kilovoltage; however, this has the effect of increasing the granularity and reducing the resolution on the radiographic material. The use of low voltages results in improved radiographic signal-to-noise ratio and improved resolution.
Dye penetrant inspection
FLUORESCENT PENETRANT. In this method of inspection, the article, which may be of metal, plastic material, etc., is first carefully cleaned to permit the fluorescent material to penetrate cracks and defects. It should be noted that cleaning of aluminum may necessitate stripping of any anodising, since the anodized film, if formed after the defect, could prevent penetration of the fluorescent material and an anodized film tend to hold penetrants which may obscure defect indications.
After the article is cleaned, it is either sprayed, painted, or immersed in a bath of fluorescent penetrant. The penetrant is a light oil which has the property of fluorescing or emitting visible light when excited by invisible radiation in the near ultra-violet range (so-called black light). It is important that the penetrant be given sufficient time to penetrate cracks and defects, and for fatigue cracks a minimum of 30 minutes is stipulated by MIL-I-6866. Heat may also be applied to facilitate entry of the penetrant. After the penetrant has had sufficient time to enter any defects, the excess on the surface of the article is washed off water spray. This washing should be checked by inspection with black light, which which penetrant left on the surface may be detected.
After washing, a developer is used to bring out the indication. This developer may in a liquid form or may be a light powder which absorbs the penetrant as it oozes from cracks and defects in the part. The development also be aided by application of heat. After the indications have been developed, the part is inspected under black light. Any crevices into which the fluorescent material has etrated will show as luminous areas. Indications which appear are usually checked by close inspection with a magnifying glass, by etching with a suitable acid or call solution, or it may be necessary to cross-secti the part, a procedure that, of course, stroys its usefulness. Usually a skilled operator can determine whether an indication actually shows a defect or whether it is a false indication.
Also, the internal extent of the defect sometimes be estimated with fair accuracy. This process of inspection, like all others, has its limits. If the fluorescent material for any reason is not able to penetrate into a defect, such a defect can not detected. DYE PENETRANTS. Several dye penetrant type inspection kits are now available for in house NDT by aerospace engineers which will disclose the presence of surface cracks or defects and subsurface flaws which extend to the surface of the part being inspected. These penetrant type inspection methods are considered acceptable, provided the part being inspected has been thoroughly cleaned, all areas are readily accessible for viewing, and the manufacturer's recommendations as to method of application are closely followed.
Cleaning. An inspection is initiated by first cleaning the surface to be inspected of dirt. loose scale, oil, and grease. Precleaning may typically be accomplished by vapor degreasing or with volatile cleaners. Use a volatile cleaner as it will evaporate from the defects before applying the penetrant dye. Sand blasting is. not desirable as a cleaning method, since surface indications may be obscured. It is not necessary to remove anodic films from parts to be inspected, since the dye readily penetrates such films. Special procedures for removing the excess dye should be followed.
Application of Penetrant. The penetrant is applied by brushing, spraying, or by dipping and allowed to stand for a minimum of 2 minutes. Dwell time may be extended up to 15 minutes, depending upon the temperature of part and fineness of the defect or when the surface being inspected is ground. Parts being monitored should be dry and heated to at least 70 ° F., but not over 130 ° Very small indications require increased penetration periods.
Removal of Dye Penetrant. Surplus penetrant is usually removed by application of a special cleaner or remover, or by washing with plain water and the part allowed to dry. Water rinse may also be used in conjunction with the remover, subject to the Manufacturer's recommendations.
Application of Developer. A light and even coat of developer is applied by spraying, brushing, or dipping. When dipping, avoid excess accumulation. Penetrant which has penetrated into cracks or other openings in the surface of the material will be drawn out by the developer resulting in a bright red indication. Some idea of the size of the defect may be obtained after experience by watching the size and rate of growth of the indication.
Ultrasonic flaw detection.
Ultrasonic flaw detection equipment has made it possible to locate defects in all types of materials without damaging the material being inspected. Very small cracks, checks, and voids, too small to be seen by X-ray, are located by means of ultrasonic inspection. An ultrasonic test instrument requires access to only one surface of the material to be inspected, and can be used with either straight line or angle beam testing techniques.
The instrument electronically generates ultrasonic vibrations and sends them in a pulsed beam through the part to be tested. Any discontinuity within the part, or the opposite end, will reflect the vibration back to the instrument, which measures the elapsed time between the initial pulse and the return of all reflections and indications such time lapse on a cathode ray indicator or paper recorder. Ultrasonic inspection requires a skilled operator who is familiar with the equipment being used as well as the inspection method to be used for the many different parts being tested.