Text-only reference. Published from the recorded official FAA General Chapter 10 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
10-29 CRT H. F. oscillator Pulse amplifier Transducer Tuning capacitor Horizontal time-base generator Contacts Material Motor system. application of acoustic emission testing to aircraft has required a new level of sophistication in testing technique and data interpretation.
Magnetic Particle Inspection
Magnetic particle inspection is a method of detecting invisible cracks and other defects in ferromagnetic materials, such as iron and steel. It is not applicable to nonmagnetic materials. In rapidly rotating, reciprocating, vibrating, and other highly- stressed aircraft parts, small defects often develop to the point that they cause complete failure of the part. Magnetic particle inspection has proven extremely reliable for the rapid detection of such defects located on or near the surface. With this method of inspection, the location of the defect is indicated and the approximate size and shape are outlined.
The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected. The ferromagnetic particles (indicating medium) may be held in suspension in a liquid that is flushed over the part; the part may be immersed in the suspension liquid; or the particles, in dry powder form, may be dusted over the surface of the part. The wet process is more commonly used in the inspection of aircraft parts. If a discontinuity is present, the magnetic lines of force are disturbed and opposite poles exist on either side of the discontinuity. The magnetized particles thus form a pattern in the magnetic field between the opposite poles. This pattern, known as an “indication,” assumes the approximate shape of the surface projection of the discontinuity. A discontinuity may be defined as an interruption in the normal physical structure or configuration of a part, such as a crack, forging lap, seam, inclusion, porosity, and the like. A discontinuity may or may not affect the usefulness of a part.
Development of Indications When a discontinuity in a magnetized material is open to the surface and a magnetic substance (indicating medium) is available on the surface, the flux leakage at the discontinuity tends to form the indicating medium into a path of higher permeability. (Permeability is a term used to refer to the ease that a magnetic flux can be established in a given magnetic circuit.) Because of the magnetism in the part and the adherence of the magnetic particles to each other, the indication remains on the surface of the part in the form of an approximate outline of the discontinuity that is immediately below it. The same action takes place when the discontinuity is not open to the surface, but since the amount of flux leakage is less, fewer particles are held in place and a fainter and less sharply defined indication is obtained.
If the discontinuity is very far below the surface, there may be no flux leakage and no indication on the surface. The flux leakage at a transverse discontinuity is shown in Figure 10-26 . The flux leakage at a longitudinal discontinuity is shown in Figure 10-27. Types of Discontinuities Disclosed The following types of discontinuities are normally detected by the magnetic particle test: cracks, laps, seams, cold shuts, inclusions, splits, tears, pipes, and voids. All of these may affect the reliability of parts in service. Cracks, splits, bursts, tears, seams, voids, and pipes are formed by an actual parting or rupture of the solid metal.
Cold shuts and laps are folds that have been formed in the metal, interrupting its continuity. Inclusions are foreign material formed by impurities in the metal during the metal processing stages. They may consist, for example, of bits of furnace lining picked up during the melting of the basic metal or of other foreign constituents. Inclusions interrupt the continuity of the metal, because they 10-30 prevent the joining or welding of adjacent faces of the metal. Preparation of Parts for Testing Grease, oil, and dirt must be cleaned from all parts before they are tested. Cleaning is very important since any grease or other foreign material present can produce nonrelevant indications due to magnetic particles adhering to the foreign material as the suspension drains from the part.
Grease or foreign material in sufficient amount over a discontinuity may also prevent the formation of a pattern at the discontinuity. It is not advisable to depend upon the magnetic particle suspension to clean the part. Cleaning by suspension is not thorough and any foreign materials so removed from the part contaminates the suspension, thereby reducing its effectiveness. In the dry procedure, thorough cleaning is absolutely necessary. Grease or other foreign material holds the magnetic powder, resulting in nonrelevant indications and making it impossible to distribute the indicating medium evenly over the part’s surface. All small openings and oil holes leading to internal passages or cavities must be plugged with paraffin or other suitable nonabrasive material.
Coatings of cadmium, copper, tin, and zinc do not interfere with the satisfactory performance of magnetic particle inspection, unless the coatings are unusually heavy or the discontinuities to be detected are unusually small. Chromium and nickel plating generally do not interfere with indications of cracks open to the surface of the base metal, but prevent indications of fine discontinuities, such as inclusions. Because it is more strongly magnetic, nickel plating is more effective than chromium plating in preventing the formation of indications. Effect of Flux Direction To locate a defect in a part, it is essential that the magnetic lines of force pass approximately perpendicular to the defect.
It is, therefore, necessary to induce magnetic flux in more than one direction, since defects are likely to exist at any angle to the major axis of the part. This requires two separate magnetizing operations, referred to as circular magnetization and longitudinal magnetization. The effect of flux direction is illustrated in Figure 10-28. Circular magnetization is the induction of a magnetic field consisting of concentric circles of force about and within the part. This is achieved by passing electric current through the part, locating defects running approximately parallel to the axis of the part. Figure 10-29 illustrates circular magnetization of a crankshaft. In longitudinal magnetization, the magnetic field is produced in a direction parallel to the long axis of the part. This is accomplished by placing the part in a solenoid excited by electric current. The metal part then becomes the core of an electromagnet and is magnetized by induction from the magnetic field created in the solenoid.
In longitudinal magnetization of long parts, the solenoid must be moved along the part in order to magnetize it. [Figure 10-30] This is necessary to ensure adequate field strength throughout the entire length of the part. Solenoids produce effective magnetization for approximately 12 inches from each end of the coil, thus accommodating parts or sections approximately 30 inches in length. Longitudinal magnetization equivalent to that obtained by a solenoid may be accomplished by wrapping a flexible electrical conductor around the part. Although this method is not as convenient, it has an advantage in that the coils conform more closely to the shape of the part, producing a somewhat more uniform magnetization. The flexible coil method is also useful for large or irregularly-shaped parts when standard solenoids are not available.
Effect of Flux Density The effectiveness of the magnetic particle inspection also depends on the flux density or field strength at the surface of the part when the indicating medium is applied. As the flux density in the part is increased, the sensitivity of the test increases, because of the greater flux leakages at discontinuities and the resulting improved formation of magnetic particle patterns. 10-31 Longitudinal magnetization Attraction of particles at defects Attraction of particles at defects Circular magnetization A B Excessively high flux densities may form nonrelevant indications, such as patterns of the grain flow in the material.
These indications interfere with the detection of patterns resulting from significant discontinuities. It is therefore necessary to use a field strength high enough to reveal all possible harmful discontinuities, but not strong enough to produce confusing nonrelevant indications. Magnetizing Methods When a part is magnetized, the field strength in the part increases to a maximum for the particular magnetizing force and remains at this maximum as long as the magnetizing force is maintained. When the magnetizing force is removed, the field strength decreases to a lower residual value depending on the magnetic properties of the material and the shape of the part. These magnetic characteristics determine whether the continuous or residual method is used in magnetizing the part.
In the continuous inspection method, the part is magnetized and the indicating medium applied while the magnetizing force is maintained. The available flux density in the part is thus at a maximum. The maximum value of flux depends directly upon the magnetizing force and the permeability of the material that the part is made of. The continuous method may be used in practically all circular and longitudinal magnetization procedures. The continuous procedure provides greater sensitivity than the residual procedure, particularly in locating subsurface discontinuities. The highly critical nature of aircraft parts and assemblies and the necessity for subsurface inspection in many applications have resulted in the continuous method being more widely used. Since the continuous procedure reveals more nonsignificant discontinuities than the residual procedure, careful and intelligent interpretation and evaluation of discontinuities revealed by this procedure are necessary.
The residual inspection procedure involves magnetization of the part and application of the indicating medium after the magnetizing force has been removed. This procedure relies on the residual or permanent magnetism in the part and is more practical than the continuous procedure when magnetization is accomplished by flexible coils wrapped around the part. In general, the residual procedure is used only with steels that have been heat-treated for stressed applications. Identification of Indications The correct evaluation of the character of indications is extremely important but is sometimes difficult to make from observation of the indications alone. The principal distinguishing features of indications are shape, buildup, width, and sharpness of outline. These characteristics are more valuable in distinguishing between types of discontinuities than in determining their severity. Careful observation of the character of the magnetic particle pattern 10-32 method).
must always be included in the complete evaluation of the significance of an indicated discontinuity. The most readily distinguished indications are those produced by cracks open to the surface. These discontinuities include fatigue cracks, heat-treat cracks, shrink cracks in welds and castings, and grinding cracks. An example of a fatigue crack is shown in Figure 10-31. Magnaglo Inspection Magnaglo inspection is similar to the preceding method, but differs in that a fluorescent particle solution is used and the inspection is made under black light. [Figure 10-32] Efficiency of inspection is increased by the neon-like glow of defects allowing smaller flaw indications to be seen. This is an excellent method for use on gears, threaded parts, and aircraft engine components. The reddish- brown liquid spray or bath that is used consists of Magnaglo paste mixed with a light oil at the ratio of 0.10 to 0.25 ounce of paste per gallon of oil. After inspection, the part must be demagnetized and rinsed with a cleaning solvent.
Magnetizing Equipment Fixed (Nonportable) General Purpose Unit A fixed, general purpose unit provides direct current (DC) for wet, continuous, or residual magnetization procedures. [Figure 10-33] Circular or longitudinal magnetization may be used, and it may be powered with rectified AC, as well as DC. The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position with its contact plate mounted on a shaft surrounded by a pressure spring so that the plate may be moved longitudinally. The plate is maintained in the extended position by the spring until pressure transmitted through the work from the movable head forces it back.
The motor-driven movable head slides horizontally in longitudinal guides and is controlled by a switch. The spring allows sufficient overrun of the motor-driven head to avoid jamming it and also provides pressure on the ends of the work to ensure good electrical contact. A plunger-operated switch in the fixed head cuts out the forward motion circuit of the movable head motor when the spring has been properly compressed. In some units, the movable head is hand operated, and the contact plate is sometimes arranged for operation by an air ram. Both contact plates are fitted with various fixtures for supporting the work.
The magnetizing circuit is closed by depressing a pushbutton on the front of the unit. It is set to open automatically, usually after about one-half second. The strength of the magnetizing current may be set manually to the desired value by means of the rheostat or increased to the capacity of the unit by the rheostat short circuiting switch. The current utilized is indicated on the ammeter. Longitudinal magnetization is produced by the solenoid that moves in the same guide rail as the movable head and is connected in the electrical circuit by means of a switch. The suspension liquid is contained in a sump tank and is agitated and circulated by a pump. The suspension is applied to the work through a nozzle. The suspension drains from the work through a nonmetallic grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.
Portable General Purpose Unit It is often necessary to perform the magnetic particle inspection at locations where fixed general purpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. It is particularly useful for inspecting landing gear and engine mounts suspected of having developed cracks in service. Portable units supply both AC and DC magnetization. This unit is a source of magnetizing and demagnetizing current but does not provide a means for supporting the work or applying the suspension. It operates on 200 volt, 60 cycle AC and contains a rectifier for producing DC when required.
[Figure 10-34] The magnetizing current is supplied through the flexible cables with prods or contact clamps, as shown in Figure 10-35. The cable terminals may be fitted with prods or with contact clamps. Circular magnetization may be developed by using either the prods or clamps. Longitudinal magnetization is developed by wrapping the 10-33 shock absorber. cable around the part. The strength of the magnetizing current is controlled by an eight-point tap switch, and the duration that it is applied is regulated by an automatic cutoff similar to that used in the fixed general purpose unit. This portable unit also serves as a demagnetizer and supplies high amperage, low-voltage AC for this purpose.
For demagnetization, the AC is passed through the part and gradually reduced by means of a current reducer. In testing large structures with flat surfaces where current must be passed through the part, it is sometimes impossible to use contact clamps. In such cases, contact prods are used. Prods can be used with the fixed general purpose unit, as well as the portable unit. The part or assembly being tested may be held or secured above the standard unit and the suspension hosed onto the area, while excess suspension drains into the tank. The dry procedure may also be used. Prods are held firmly against the surface being tested. There is a tendency for a high-amperage current to cause burning at contact areas, but with proper care, such burning is usually slight. For applications where prod magnetization is acceptable, slight burning is normally acceptable.
Indicating Mediums The various types of indicating mediums available for magnetic particle inspection may be divided into two general material types: wet and dry. The basic requirement for any indicating medium is that it produce acceptable indications of discontinuities in parts. The contrast provided by a particular indicating medium on the background or part surface is particularly important. The colors most extensively used are black and red for the wet procedure and black, red, and gray for the dry procedure. For acceptable operation, the indicating medium must be of high permeability and low retentivity. High permeability ensures that a minimum of magnetic energy is required to attract the material to flux leakage caused by discontinuities.
Low retentivity ensures that the mobility of the magnetic particles is not hindered by the particles themselves becoming magnetized and attracting one another. Demagnetizing The permanent magnetism remaining after inspection must be removed by a demagnetization operation if the part is to be returned to service. Parts of operating mechanisms must be demagnetized to prevent magnetized parts from attracting filings, grindings, or chips inadvertently left in the system or steel particles resulting from operational wear. An accumulation of such particles on a magnetized part may cause scoring of bearings or other working parts. Parts of the airframe must be demagnetized so they do not affect instruments.
Demagnetization between successive magnetizing operations 10-34 is not normally required unless experience indicates that omission of this operation results in decreased effectiveness for a particular application. Demagnetization may be accomplished in a number of different ways. A convenient procedure for aircraft parts involves subjecting the part to a magnetizing force that is continually reversing in direction and, at the same time, gradually decreasing in strength. As the decreasing magnetizing force is applied first in one direction and then the other, the magnetization of the part also decreases.
Standard Demagnetizing Practice The basic procedure for developing a reversing and gradually decreasing magnetizing force in a part involves the use of a solenoid coil energized by AC. As the part is moved away from the alternating field of the solenoid, the magnetism in the part gradually decreases. A demagnetizer whose size approximates that of the work is used. For maximum effectiveness, small parts are held as close to the inner wall of the coil as possible. Parts that do not readily lose their magnetism are passed slowly in and out of the demagnetizer several times and, at the same time, tumbled or rotated in various directions. Allowing a part to remain in the demagnetizer with the current on accomplishes very little practical demagnetization.
The effective operation in the demagnetizing procedure is that of slowly moving the part out of the coil and away from the magnetizing field strength. As the part is withdrawn, it is kept directly opposite the opening until it is 1 or 2 feet from the demagnetizer. The demagnetizing current is not cut off until the part is 1 or 2 feet from the opening as the part may be remagnetized if current is removed too soon. Another procedure used with portable units is to pass AC through the part being demagnetized, while gradually reducing the current to zero.
Radiographic
Because of their unique ability to penetrate material and disclose discontinuities, X and gamma radiations have been applied to the radiographic (x-ray) inspection of metal fabrications and nonmetallic products. The penetrating radiation is projected through the part to be inspected and produces an invisible or latent image in the film. When processed, the film becomes a radiograph or shadow picture of the object. This inspection medium and portable unit provides a fast and reliable means for checking the integrity of airframe structures and engines. [Figure 10-36] Radiographic Inspection Radiographic inspection techniques are used to locate defects or flaws in airframe structures or engines with little or no disassembly. This is in marked contrast to other types of nondestructive testing that usually require removal, disassembly, and stripping of paint from the suspected part before it can be inspected. Due to the radiation risks associated with x-ray, extensive training is required to become a qualified radiographer. Only qualified radiographers are allowed to operate the x-ray units.
Three major steps in the x-ray process discussed in subsequent paragraphs are: exposure to radiation, including preparation; processing of film; and interpretation of the radiograph. Preparation and Exposure The factors of radiographic exposure are so interdependent that it is necessary to consider all factors for any particular radiographic exposure. These factors include, but are not limited to, the following: • Material thickness and density • Shape and size of the object • Type of defect to be detected • Characteristics of x-ray machine used • The exposure distance • The exposure angle • Film characteristics • Types of intensifying screen, if used Knowledge of the x-ray unit’s capabilities form a background for the other exposure factors. In addition to the unit rating in kilovoltage, the size, portability, ease of manipulation, and exposure particulars of the available equipment must be thoroughly understood. Previous experience on similar objects is also very helpful in the determination of the overall exposure techniques. A log or record of previous exposures provides specific data as a guide for future radiographs. After 10-35 Prod set Contact clamp exposure to x-rays, the latent image on the film is made permanently visible by processing it successively through a developer chemical solution, an acid bath, and a fixing bath, followed by a clear water wash.
Radiographic Interpretation From the standpoint of quality assurance, radiographic interpretation is the most important phase of radiography. It is during this phase that an error in judgment can produce disastrous consequences. The efforts of the whole radiographic process are centered in this phase, where the part or structure is either accepted or rejected. Conditions of unsoundness or other defects that are overlooked, not understood, or improperly interpreted can destroy the purpose and efforts of radiography and can jeopardize the structural integrity of an entire aircraft. A particular danger is the false sense of security imparted by the acceptance of a part or structure based on improper interpretation.
As a first impression, radiographic interpretation may seem simple, but a closer analysis of the problem soon dispels this impression. The subject of interpretation is so varied and complex that it cannot be covered adequately in this type of document. Instead, this chapter gives only a brief review of basic requirements for radiographic interpretation, including some descriptions of common defects. Experience has shown that, whenever possible, it is important to conduct radiographic interpretation close to the radiographic operation. When viewing radiographs, it is helpful to have access to the material being tested. The radiograph can thus be compared directly with the material being tested, and indications due to such things as surface condition or thickness variations can be immediately determined. The following paragraphs present several factors that must be considered when analyzing a radiograph.
There are three basic categories of flaws: voids, inclusions, and dimensional irregularities. The last category, dimensional irregularities, is not pertinent to these discussions, because its prime factor is one of degree and radiography is not exact. V oids and inclusions may appear on the radiograph in a variety of forms ranging from a two-dimensional plane to a three-dimensional sphere. A crack, tear, or cold shut most nearly resembles a two-dimensional plane, whereas a cavity looks like a three-dimensional sphere. Other types of flaws, such as shrink, oxide inclusions, porosity, and so forth, fall somewhere between these two extremes of form.
It is important to analyze the geometry of a flaw, especially for items such as the sharpness of terminal points. For example, in a crack-like flaw, the terminal points appear much sharper in a sphere-like flaw, such as a gas cavity. Also, material strength may be adversely affected by flaw shape. A flaw having sharp points could establish a source of localized stress concentration. Spherical flaws affect material strength to a far lesser degree than do sharp-pointed flaws. Specifications and reference standards usually stipulate that sharp-pointed flaws, such as cracks, cold shuts, and so forth, are cause for rejection.
Material strength is also affected by flaw size. A metallic component of a given area is designed to carry a certain load plus a safety factor. Reducing this area by including a large flaw weakens the part and reduces the safety factor. Some flaws are often permitted in components due to these safety factors. In this case, the interpreter must determine the degree 10-36 Dark area Light area After processing Film Void Radiation source Dark area Specimen of tolerance or imperfection specified by the design engineer. Both flaw size and flaw shape are considered carefully, since small flaws with sharp points can be just as bad as large flaws with no sharp points.
Another important consideration in flaw analysis is flaw location. Metallic components are subjected to numerous and varied forces during their effective service life. Generally, the distribution of these forces is not equal in the component or part, and certain critical areas may be rather highly stressed. The interpreter must pay special attention to these areas. Another aspect of flaw location is that certain types of discontinuities close to one another may potentially serve as a source of stress concentrations creating a situation that must be closely scrutinized. An inclusion is a type of flaw that contains entrapped material. Such flaws may be of greater or lesser density than the item being radiographed. The foregoing discussions on flaw shape, size, and location apply equally to inclusions and to voids. In addition, a flaw containing foreign material could become a source of corrosion.
Radiation Hazards Radiation from x-ray units and radioisotope sources is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided. Personnel must keep outside the primary x-ray beam at all times. Radiation produces change in all matter that it passes through. This is also true of living tissue. When radiation strikes the molecules of the body, the effect may be no more than to dislodge a few electrons, but an excess of these changes could cause irreparable harm. When a complex organism is exposed to radiation, the degree of damage, if any, depends on the body cells that have been changed.
Vital organs in the center of the body that are penetrated by radiation are likely to be harmed the most. The skin usually absorbs most of the radiation and reacts earliest to radiation. If the whole body is exposed to a very large dose of radiation, death could result. In general, the type and severity of the pathological effects of radiation depend on the amount of radiation received at one time and the percentage of the total body exposed. Smaller doses of radiation could cause blood and intestinal disorders in a short period of time. The more delayed effects are leukemia and other cancers.
Skin damage and loss of hair are also possible results of exposure to radiation.
Inspection of Composites
Composite structures are inspected for delamination (separation of the various plies), debonding of the skin from the core, and evidence of moisture and corrosion. Previously discussed methods including ultrasonic, acoustic emission, and radiographic inspections may be used as recommended by the aircraft manufacturer. The simplest method used in testing composite structures is the tap test. Newer methods, such as thermography, have been developed to inspect composite structures. Tap Testing Tap testing, also referred to as the ring test or coin test, is widely used as a quick evaluation of any accessible surface