Inspection of Aircraft Bonded Structures (Part 2) - Inspection Concepts and Techniques | Aircraft Systems

Inspection of Aircraft Bonded Structures (Part 2) - Inspection Concepts and Techniques

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 1] 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.

Figure 1. Magnaglo inspection

Magnetizing Equipment

Fixed (Nonportable) General Purpose Unit A fixed, general purpose unit provides direct current (DC) for wet, continuous, or residual magnetization procedures. [Figure 2]

Figure 2. Fixed general-purpose magnetizing unit

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 3]


Figure 3. Portable magnetic particle inspection equipment

The magnetizing current is supplied through the flexible cables with prods or contact clamps, as shown in Figure 4. The cable terminals may be fitted with prods or with contact clamps. Circular magnetization may be developed by using either the prods or clamps.


Figure 4. Magnetic particle inspection accessories

Longitudinal magnetization is developed by wrapping the 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 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 5]


Figure 5. Radiograph

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 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 post 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. Voids 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 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.


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