Because it is more strongly magnetic, nickel plating is more effective than chromium plating in preventing the formation of indications.
FIGURE 10-2. Flux leakage at longitudinal discontinuity.
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 will contaminate the suspension, thereby reducing its effectiveness.
In the dry procedure, thorough cleaning is absolutely necessary. Grease or other foreign ing in nonrelevant indications and making it impossible to distribute the indicating medium evenly over the part's surface.
Effect of Flux Direction In order 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 Longitudinal magnetization Attraction of particles at defects Circular magnetization сили томи terial.
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 will not interfere with indications of cracks open to the surface of the base metal but will prevent indications of fine discontinuities, such as inclusions.
Attraction of particles at defects B FIGURE 10-3. Effect of flux direction on strength of indication.
FIGURE 10-4. Circular magnetization of crankshaft.
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 10-3.
Circular magnetization is the inducing of a magnetic field, consisting of concentric circles of force about and within the part, by passing electric current through the part. This type of magnetization will locate defects running approximately parallel to the axis of the part.
Circular magnetization of a part of solid cross section is illustrated in figure 10-4. Each head of the magnetizing unit is electrically connected to a pushbutton control, so that when contact is made the magnetizing current passes from one head to the other through the part.
Figure 10-5 illustrates circular magnetization of a hollow part by passing the magnetizing current FIGURE 10-5. Circular magnetization of piston pin with conductor bar.
through a conductor bar located on the axis of the part.
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. (See figure 10-6.) 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 FIGURE 10-6. Longitudinal magnetization of crankshaft (solenoid method).
FICURE 10-7. Longitudinal magnetization of steel propeller blade (cable method).
obtained by a solenoid may be accomplished by wrapping a flexible electrical conductor around the part, as shown in figure 10-7. Although this method is not as convenient, it has the advantage that the coils conform more closely to the shape of the part, thus producing somewhat more uniform magnetization.
The flexible coil method is also useful for large or irregularly shaped parts for which 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.
Excessively high flux densities, however, may form nonrelevant indications; for example, patterns of the grain flow in the material. These indications will 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 of which the part is 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.
Inasmuch as the continuous procedure will reveal 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 which 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, in general, are more valuable in distinguishing between types of discontinuities than in determining their severity. However, careful observation of the character of the magnetic particle pattern should 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 suriace.
These discontinuities include fatigue cracks, heat- 472
Main gear outer cylinder Fatigue crack Torsion link lugs FIGURE 10-8. Fatigue crack in a landing gear.
treat cracks, shrink cracks in welds and castings, and grinding cracks.
Fatigue cracks give sharp, clear patterns, generally uniform and unbroken throughout their length and with good buildup. They are often jagged in appearance, as compared with the straight indications of a seam, and may also change direction slightly in localized areas. Figure 10-8 illustrates a fatigue crack.
Fatigue cracks are found in parts that have been in service but are never found in new parts. They are usually in highly stressed areas of the part or where a stress concentration exists for some reason. It is important to recognize that even a small fatigue crack indicates positively that failure of the involved parts is in progress.
Heat-treat cracks have a smooth outline but are usually less clear and have less buildup than fatigue cracks. On thin sections, such as cylinder barrel walls, heat-treat cracks may give very heavy patterns (figure 10-9). These heat-treat cracks have a characteristic form, consisting of short jagged lines grouped together.
Shrink cracks give a sharp, clear pattern and the line is usually very jagged. Since the walls of shrink cracks are close together, their indications generally build up to less extent than do indications of fatigue cracks.
Grinding cracks are fine and sharp but seldom have a buildup because of their limited depth.
Grinding cracks vary from single line indications to a heavy network of lines. Grinding cracks are generally related to the direction of grinding. For example, the crack usually begins and continues at right angles to the motion of a grinding wheel, giving a rather symmetrical pattern. Indications of grinding cracks can frequently be identified by means of this relation.
Indications of seams are usually straight, sharp, and fine. They are often intermittent and sometimes have very little buildup.
Hairlines are very fine seams in which the faces of the seam have been forced very close together during fabrication. Hairline indications are very fine and sharp, with very little buildup. Discontinuities of this type are normally considered detrimental only in highly stressed parts.
Inclusions are nonmetallic materials, such as slag materials and chemical compounds, that have been trapped in the solidifying ingot. They are usually elongated and strung out as the ingot is worked in subsequent processing operations.
FIGURE 10-9. Heat-treat cracks on cylinder barrel wall.
Inclusions appear in parts in varying sizes and shapes, from stringers easily visible to the eye to particles only visible under magnification. In a finished part they may occur as either surface or subsurface discontinuities.
Indications of subsurface inclusions are usually broad and fuzzy. They are seldom continuous or of even width and density throughout their length.
Larger inclusions, particularly those near or open to the surface, appear more clearly defined. Close examination, however, will generally reveal their lack of definition and the fact that the indication consists of several parallel lines rather than a single line. These characteristics will usually distinguish a heavy inclusion from a crack.
When cavities are located considerably below the surface of a part, the magnetic particle test is not a reliable method of detecting them. If any indication is obtained, it is likely to be an indistinct and inexact outline of the cavity, with the magnetic substance tending to distribute over the whole area rather than to outline clearly the boundary of the discontinuity. Defects of this type are detected more easily by radiographic pro- Laps may be identified by their form and location. They tend to occur at the ends or flash line of a forging. The indications are usually heavy and irregular. Islands and short branch indications usually break a lap indication of any length, and the scale included in the lap invariably gives fuzzy or small fernlike patterns stemming from the main indication.
When an ingot solidifies, the distribution of the various elements or compounds, generally, is not uniform throughout the mass of the ingot. Marked segregations of some constituents may thus occur.
As the ingot is forged and then rolled, these segregations are elongated and reduced in cross section. Upon subsequent processing, they may appear as very thin parallel lines or bands, known as banding.
Segregation in the form of banding is sometimes revealed by magnetic particle inspection, particularly when high field strengths are used.
Banding is not normally considered significant.
The most serious forms of segregation probably occur in castings. Here the basic condition of the metal remains unaltered in the finished part, and any segregations occur as they were originally formed. They may vary in size and will normally be irregular in shape. They may occur on or below Magnaglo Inspection Magnaglo inspection is similar to the preceding method, except that a fluorescent particle solution is used and the inspection is made under black fight. Efficiency of inspection is increased by the neon-like glow of defects, and smaller flaw indications are more readily 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 .10 to .25 ounce of paste per gallon of oil.
After inspection, the part must be demagnetized and rinsed with a cleaning solvent.
MAGNETIZING EQUIPMENT Fixed Unit (Nonportable) A fixed general-purpose unit is shown in figure
10-10. This unit provides direct current for wet continuous or residual magnetization procedures.
Either circular or longitudinal magnetization may be used, and it may be powered with rectified
a.c. as well as d.c.
The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position. Its contact plate is mounted on a shaft surrounded by a pressure spring, in order 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 movable head slides horizontally in longitudinal guides and is motor driven. It 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 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.
Ammeter Pressure Contact Solenoid spring plate Nozzle Contact Movable head plate
• Fixed head Movable head switch Push button Pump switch Rheostat Short-circuiting- Circulating switch pump Solenoid* switch FIGURE 10-10. Fixed general-purpose magnetizing unit.
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, which 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 the wooden grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.
General-Purpose Portable Unit It is often necessary to perform the magnetic particle inspection at locations where fixed generalpurpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. This has occurred, particularly on landing gears and engine mounts suspected of having developed cracks in service. Equipment suitable for this purpose, supplying both alternating-current and directcurrent magnetization, is available. A typical example is shown in figure 10-11.
This unit is only a source of magnetizing and demagnetizing current and does not provide a means for supporting the work or applying the susperision. It operates on 200-volt, 60-cycle, alternating current and contains a rectifier for producing direct current when required.
The magnetizing current is supplied through the flexible cables. The cable terminals may be fitted with prods, as shown in the illustration, or with contact clamps. Circular magnetization may be developed by using either the prods or clamps.
FIGURE 10-11. General-purpose portable unit.
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 length of time for which it is applied is regulated by an automatic cutoff similar to that used in the standard general-purpose unit.
This portable unit also serves as a demagnetizer and supplies high-amperage, low-voltage, alternating current for this purpose. For demagnetization, the alternating current is passed through the part and gradually reduced by means of a current When it is desired to use cable with the standard general-purpose unit as a source of power, a contact block is useful. This consists of a wooden block fitted at each end with copper plates spaced to receive the terminals of the cable.
When the contact block is placed between the heads of a standard unit, the regular controls and timing switches of the unit can be used to regulate the magnetizing current. This provides a convenient way of connecting the cable to the source of power and eliminates the necessity for bolted connections.
When magnetizing current is passed through a steel propeller blade for circular magnetization, it SES is possible to burn the blade tip if necessary precautions are not taken. This possibility can be avoided by the use of a hinged clamp attached to the movable head of the inspection unit.
The clamp is lined with copper braid, which provides good electrical contact by conforming to the curvature of the propeller blade faces. This fixture avoids electrical contact at the thin edge of the blade tip and eliminates high current intensities which may cause burning at the point.
The butt end of the blade is supported by a plug mounted on the other head.
Indicating Mediums The various types of indicating mediums available for magnetic particle inspection may be divided into two general types: wet process materials and dry process materials. The basic requirement for any indicating medium is that it produce acceptable indications of discontinuities 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 standard generalpurpose unit as well as the portable unit. The part or assembly being tested may be suspended above the standard unit and the suspension hosed onto the area; excessive suspension drains into the tank. The dry procedure may also be used.
Prods should be held firmly against the surface being tested. There is a tendency for a highamperage current to cause burning at contact areas, but with proper care, such burning will usually be slight. For applications where prod magnetization is acceptable, slight burning is normally not objectionable.
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 will be required to attract the material to flux leakage caused by discontinuities.
Low retentivity ensures that the mobility of the magnetic particles will not be hindered; that is, by the particles themselves becoming magnetized and attracting one another.
The magnetic substance for the wet process is usually supplied in paste form. The red paste improves visibility on dark surfaces. Although the 476
exact amount of magnetic substance to be added may vary somewhat, a concentration of 2 ounces of paste per gallon of liquid vehicle has been found generally acceptable. The paste must not be dumped into the suspension liquid in the tank, since the unit agitator and pump cannot be depended on to do the mixing.
The proper procedure for preparing a suspension is to place the correct amount of paste in a container and add small quantities of the suspension liquid, working each addition with a flat paddle until the paste has been diluted to a uniform watery mixture which can then be poured into the tank.
It is important that new magnetic substance always be used in preparing suspensions. When the suspension becomes discolored or otherwise contaminated to the extent that the formation of magnetic particle patterns is interfered with, the unit should be drained, cleaned, and refilled with clean suspension.
DEMAGNETIZATION 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 will 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. Previously, this operation was considered necessary to remove completely the existing field in a part before it was magnetized in a different direction.
Demagnetization may be accomplished in a number of different ways. Possibly the most 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 simplest procedure for developing a reversing and gradually decreasing magnetizing force in a part involves the use of a solenoid coil energized by alternating current. As the part is moved away from the alternating field of the solenoid, the magnetism in the part gradually decreases.
A demagnetizer as near the size of the work as practicable should be used; and, for maximum effectiveness, small parts should be held as close to the inner wall of the coil as possible.
Parts that do not readily lose their magnetism should be 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 should be kept directly opposite the opening until it is 1 or 2 feet from the demagnetizer.
The demagnetizing current should never be cut off until the part is 1 or 2 feet from the opening;
otherwise, the part will usually be re-magnetized.
Another procedure used with portable units is to pass alternating current through the part being demagnetized and gradually reduce the current to zero.
DYE-PENETRANT INSPECTION Penetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.
Penetrant inspection will detect such defects as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat-treat cracks, seams, forging laps, and bursts. Penetrant inspection will also indicate a lack of bond between joined metals.
The main disadvantage of penetrant inspection is that the detect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material which is magnetic, the use of magnetic particle inspection is generally recommended.
Penetrant inspection depends for its success upon a penetrating liquid entering the surface opening and remaining in that opening, making it clearly visible to the operator. It calls for visual examination of the part after it has been processed, but the visibility of the defect is increased so that it can be detected. Visibility of the penetrating material is increased by the addition of dye which may be either one or two types- visible or The visible penetrant kit consists of dye penetrant, dye remover-emulsifier and developer.
The fluorescent penetrant inspection kit contains a black light assembly as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position Briefly, the steps to be taken when performing a penetrant inspection are:
1. Thorough cleaning of the metal surface.
2. Appiying penetrant.
3. Removing penetrant with remover-emulsifier or cleaner.
4. Drying the part.
5. Applying the developer.
6. Inspecting and interpreting results.
Interpretation of Results inspection depends upon the thoroughness with which the part was prepared. Several basic principles applying to penetrant inspection are:
1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter.
2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant will be removed from within the defect, as well as from the surface.
3. Clean cracks are usually easy to detect.
Surface openings that are uncontaminated, regardless of how fine, are seldom difficult to detect with the penetrant inspection.
4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores.
5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method, if the equipment is available.
6. Visible penetrant-type developer, when applied to the surface of a part, will dry to a smooth, even, white coating. As this developer dries, bright red indications will appear where there are surface defeets. If no red indications appear, there are no surtace
7. When conducting the fluorescent penetranttype inspection, the defects will show up (under black light) as a brilliant yellow-green color and the sound areas will appear deep
8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes to which the part has been subjected.
The size of the indication, or accumulation of penetrant, will show the extent of the defect, and the brilliance will be a measure of its depth. Deep cracks will hold more penetrant and, therefore, will be broader and more brillant. Very fine openings can hold only small amounts of penetrants and, therefore, will appear as fine lines.
Figure 10-12 shows some of the types of defects that can be located using dry penetrant.
False Indications With the penetrant inspection there are no false indications in the sense that such things occur in the magnetic particle inspection. There are, however, two conditions which may create accumulations of penetrant that are sometimes confused with true surface cracks and discon- The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the penetrant dwell time, the unremoved penetrant will be visible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part should be completely repro- 478
Pits of porosity Tight crack or Crack or partially welded lap similar opening FIGURE 10-12. Types of defects.
cessed. Degreasing is recommended for removal of all traces of the penetrant.
False indications may also be created where parts press-fit to each other. If a wheel is press-fit onto a shaft, penetrant will show an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together.
Indications of this type are easy to identify since they are so regular in form and shape.
RADIOGRAPHY X- and gamma radiations, because of their unique ability to penetrate material and disclose discontinuities, 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, in a portable unit, provides a fast and reliable means for checking the integrity of airframe structures and engines.
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, which usually require removal, disassembly, and stripping of paint from the suspected part before it can be inspected. Due to the nature of X-ray, extensive training is required to become a qualified radiographer, and only qualified radiographers are allowed to operate the X-ray units.
Three major steps in the X-ray process discussed in subsequent paragraphs are: (1) Exposure to radiation, including preparation, (2) processing of film, and (3) 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:
(a) Material thickness and density.
(b) Shape and size of the object.
(c) Type of defect to be detected.
(d) Characteristics of X-ray machine used.
(e) The exposure distance.
(f) The exposure angle.
(g) Film characteristics.
(h) Types of intensifying screen, if used.
Knowledge of the X-ray unit's capabilities should 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 should 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 will provide specific data as a guide for future radiographs.
Film Processing 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.
The film consists of a radiation-sensitive silver salt suspended in gelatin to form an emulsion. The developer solution converts radition-affected elements in the emulsion to black metallic silver.
These black metallic particles form the image. The longer the film remains in the developer, the more metallic silver is formed, causing the image to become progressively darker. Excessive time in the developer solution results in overdevelopment.
An acid rinse bath, sometimes referred to as a stop bath, instantly neutralizes the action of the developer and stops further development. Due to the soft emulsion and the nonabsorbent quality of the base of most negative materials, only a very weak acid bath is required.
The purpose of the fixing bath is to arrest the image at the desired state of development. When a radiation-sensitive material is removed from the developing solution, the emulsion still contains a considerable amount of silver salts which have not been affected by the developing agents. These salts are still sensitive, and if they are allowed to remain in the emulsion, ordinary light will ultimately darken them and obscure the image. Obviously, if this occurs, the The fixing bath prevents this discoloration by dissolving the salts of silver from the developed free-silver image. Therefore, to make an image permanent, it is necessary to fix the radiationsensitive material by removing all of the unaffected suver salt from the emulsion.
After fixing, a thorough water rinse is necessary to remove the fixing agent which, if allowed to remain, will slowly combine with the silver image to produce brownish-yellow stains of silver sulfide, causing the image to fade.
NOTE: All processing is conducted under a subdued light of a color to which the film is not readily sensitive.
Radiographic Interpretation standpoint of quality 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; the part or structure is either accepted or rejected.
Conditions of unsoundness or other defects which 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 will give only a brief review of basic requirements for radiographic interpretation, including some descriptions of common defects.
Experience has shown that, whenever possible, radiographic interpretation should be conducted close to the radiographic operation. It is helpful, when viewing radiographs, 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 which 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 that exacting. 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 will most nearly resemble a two-dimensional plane, whereas a cavity will look like a three-dimensional sphere.
Other types of flaws, such as shrink, oxide inclusions, porosity, etc., will fall somewhere between these two extremes of form.
It is important to analyze the geometry of a flaw, especially for such things as the sharpness of terminal points. For example, in a crack-like flaw the terminal points will appear much sharper than they will for 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 sharppointed flaws. Specifications and reference standards usually stipulate that sharp-pointed flaws, such as cracks, cold shuts, etc., are cause for Material strength is also affected by flaw size.
A metallic component of a given area is designed to 480
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 because of 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 should be 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; therefore, this type of situation should be closely scrutinized.
An inclusion is a type of flaw which contains entrapped material. Such flaws may be either 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 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 changes in all matter through which it passes. This is also true of living tissue. When the 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 which of its body cells have been changed.
The more vital organs are in the center of the body; therefore, the more penetrating radiation is likely to be the most harmful in these areas. The skin usually absorbs most of the radiation and, therefore, reacts earliest to radiation.
If the whole body is exposed to a very large dose of radiation, it could result in death. 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. The smaller doses of radiation may cause blood and intestinal disorders in a short period of time. The more delayed effects are leukeria and cancer. Skin damage and loss of hair are also possible results of exposure to radiation.
ULTRASONIC TESTING Ultrasonic detection equipment has made it possible to locate defects in all types of materials without damaging the material being inspected Minute cracks, checks, and voids, too small to be seen by X-ray, are located by 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.
Two basic methods are used for ultrasonic inspection. The first of these methods is immersion testing. In this method of inspection, the part under examination and the search unit are totally immersed in a liquid couplant, which may be water or any other suitable fluid.
The second method is called contact testing, which is readily adapted to field use, and is the method discussed in this chapter. In this method the part under examination and the search unit are coupled with a viscous material, liquid or a paste, which wets both the face of the search unit and the material under examination.
There are two basic ultrasonic systems: (1) Pulsed and (2) resonance. The pulsed system may be either echo or through-transmission; the echo is the most versatile of the two pulse systems.
Pulse-Echo Flaws are detected by measuring the amplitude of signals reflected and the time required for these signals to travel between specific surfaces and the discontinuity. (See figure 10-13.)
The time base, which is triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen of the CRT (Cathode ray tube). The spot sweeps from left to right across the face of the scope 50 to 5,000 times per second, or higher if required for high-speed automated scanning. Due to the speed of the cycle of