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
RF pulser Amplifier Transducer Rate Flaw generator Specimen Timing circuit Cathode ray oscilloscope FIGURE 10-13. Block diagram of basic pulse-echo system.
transmitting and receiving, the picture on the oscilloscope appears to be stationary.
A few microseconds after the sweep is initiated, the rate generator electrically excites the pulser and the pulser in turn emits an electrical pulse.
The transducer converts this pulse into a short train of ultrasonic sound waves. If the interfaces of the transducer and the specimen are properly orientated, the ultrasound will be reflected back to the transducer when it reaches the internal flaw and the opposite surface of the specimen. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen is measured by the timing circuits. The reflected pulse received by the transducer is amplified, then transmitted to the oscilloscope, where the pulse received from the flaw is displayed on the CRT screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is in relation to the front and back surfaces of the specimen. (See figure
10-14.)
The Reflectoscope is a pulse-echo type instrument. The Reflectoscope can be used for the detection of defects such as cracks, folds, inclusions, delaminations, partial welds, voids, shrinks, porosity, flaking, and other subsurface defects.
The principle of operation is pictured in figure 10-15, where electrical pulses are transformed by the crystal into ultrasonic vibrations which are transmitted into the material. The portion of the electrical pulse delivered to the cathode-ray tube causes an initial pulse indication, as shown in figure 10-15, view A. The back reflection has formed in view B, the vibrations having traveled to the bottom of the part and reflected back to the searching unit, which transforms them back into electrical pulses. The screen's vertical indication of their return is known as the "first back-reflection indication." If a defect is present (figure 10-15, view C), a portion of the vibrations traveling through the material is reflected from the defect, causing an additional indication on the screen. The horizontal-sweep travel indicates the time elapsed since the vibrations left the crystal.
This type of operation, referred to as straightbeam testing, is suitable for the detection of flaws whose planes are parallel to the plane of the part.
By means of angle-beam testing, also referred to as shear-wave testing, the usefulness of the Reflectoscope includes the following:
1. Flaws whose planes lie at an angle to the plane of the part.
2. Discontinuities in areas that cannot be reached with the standard straight-beam technique.
3. Some internal defects in plate and sheet stock.
4. Some types of internal defects in tubing, pipe and bar stock, such as inclusions and small cracks near the surface.
5. Cracks in parent metal resulting from welding.
6. Some defects in welds.
Transducer Flaw
- Flaw Speciman Cathode ray tube FICURE 10-14. Oscilloscope display in relationship to flaw location.
Coaxial cable Quartz crystal - Material.
material face Echo from Echo from material face and back face material face, defect, Echo from - Defect and back face FIGURE 10-15. Reflectoscope operation-straight-beam testing.
Angle-beam testing differs from straight-beam testing only in the manner in which the ultrasonic waves pass through the material being tested. As shown in figure 10-16, the beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam or a portion thereof reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight-beam testing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle-beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.
Resonance System This system differs from the pulse method in that the frequency of transmission is, or can be, continuously varied. The resonance method is principally used for thickness measurements when the two sides of the material being tested are smooth and parallel. The point at which the frequency matches the resonance point of the Coaxial cable
- Quartz crystal Material Defect FICURE 10-16. Reflectoscope operation-angle-beam testing.
material being tested is the thickness-determining factor. It is necessary that the frequency of the ultrasonic waves, corresponding to a particular dial setting, be accurately known. Checks should be made with standard test blocks to guard against possible drift of frequency.
If the frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen (fundamental frequency), then the reflected wave will arrive back at the transducer in the same phase as the original transmission so that strengthening of the signal, or a resonance, will occur. If the frequency is increased so that three times the wavelength equals four times the thickness, then the reflected signal will return completely out of phase with the transmitted signal and cancellation will occur. Further increase of the frequency, so that the wavelength is equal to the thickness again, gives a reflected signal in phase with the transmitted signal and resonance occurs once more.
Transducer incident wave Reflected wave 1 Reflecting
• surface Wavelength T = 2 F = F, (Fundamental frequency) T = WF = 2F, (2nd Harmonic) Material under test I = 1½W F = 3F, (3rd Harmonic) T = 2W F = 4F, (4th Harmonic) FICURE 10-17. Conditions of ultrasonic resonance in a metal plate.
Pulse H.F.
amplifier Oscillator
I CRT I Motor - Tuning capacitor Transducer Horizontal Material time-base Contacts generator FIGURE 10-18. Block diagram of resonance thickness measuring system.
By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. (See figure 10-17.)
In some instruments, the oscillator circuit contains a motor-driven capacitor which changes the frequency of the oscillator. (See figure 10-18.)
In other instruments, the frequency is changed by electronic means.
The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis thus represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to present these vertically. Calibrated transparent scales are then placed in front of the tube, and the thickness can be read directly. The instruments normally operate between 0.25 mc.
and 10 mc. in four or five bands.
The resonant thickness instrument can be used to test the thickness of such metals as steel, cast iron, brass, nickel, copper, silver, lead, aluminum, and magnesium. In addition, areas of corrosion or wear on tanks, tubing, airplane wing skins, and other structures or products can be located and evaluated.
Direct-reading, dial-operated units are available that measure thickness between .025 inch and 3 inches with an accuracy of better than I1 percent.
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.
EDDY CURRENT TESTING Electromagnetic analysis is a term which describes the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current.
Eddy currents are composed of free electrons, which are made to "drift" through metal, under the influence of an induced electromagnetic field.
Eddy current is used in aircraft maintenance to inspect jet engine-turbine shaft and veins, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat or frame damage. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions.
Basic Principles When an alternating current is passed through a coil it develops a magnetic field around the coil which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current.
If this coil is placed so that the magnetic field passes through an electrically conducting specimen, eddy currents will be induced into the specimen.
The eddy currents create their own field which varies the original field's opposition to the flow of original current. Thus the specimen's susceptibility to eddy currents determine the current flow through the coil (see figure 10-19).
Oscillator Amplifier The magnitude and phase of this counter field is dependent primarily upon the resistivity and permeability of the specimen under consideration, and it is this fact that enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a wheatstone bridge.
The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process--whereby electric fields are made to explore a test piece for various conditionsinvolves the transmission of energy through the specimen much like the transmission of X-rays, heat, or ultrasound. In the transmission of X-rays, heat, or ultrasound, the energy flows in beams having a recognizable direction and intensity and obeys the laws of absorption, reflection, diffraction, and diffusion. Receiver elements can be placed into the beams and a direct measurement of energy flow is possible. However, in electromagnetic tests.
the energy distributes itself in a vaguely known manner and undergoes a transformation in the process, from magnetic to electric energy, and subsequently, back to magnetic energy. Since the induced currents flow in closed circuits, it is neither convenient nor generally possible, to intercept them at the specimen boundaries.
Meter Probe Sample part FIGURE 10-19. Eddy current inspection circuit.
FIGURE 10-20. Examples of good welds.
VISUAL INSPECTION Nondestructive testing by visual means is the oldest method of inspection. Defects which would escape the naked eye can be magnified so they will be visible. Telescopes, borescopes, and magnifying glasses aid in performing visual inspection.
A discussion of visual inspection in this chapter will be confined to judging the quality of completed welds by visual means. Although the appearance of the completed weld is not a positive indication of quality, it gives a good clue to the care used in making it.
A properly designed joint weld is stronger than the base metal which it joins. The characteristics of a properly welded joint are discussed in the following paragraphs.
A good weld is uniform in width; the ripples are even and well feathered into the base metal, which shows no burn due to overheating. (See figure
10-20.) The weld has good penetration and is free of gas pockets, porosity, or inclusions. The edges of the bead illustrated in figure 10-20 (B) are not in a straight line, yet the weld is good, since penetration is excellent.
Penetration is the depth of fusion in a weld.
Thorough fusion is the most important characteristic which contributes to a sound weld. Penetration is affected by the thickness of the material to be joined, the size of the filler rod, and how it is added. In a butt weld the penetration should be 100 percent of the thickness of the base metal. On a fillet weld the penetration requirements are 25 to 50 percent of the thickness of the base metal. The width and depth of bead for a butt weld and fillet weld are shown in figure 10-21.
To assist further in determining the quality of a welded joint, several examples of incorrect welds are discussed in the following paragraphs.
The weld shown in figure 10-22 (A) was made too rapidly. The long and pointed appearance of the ripples was caused by an excessive amount of heat or an oxidizing flame. If the weld were Reinforcement Bead width ¼ to ½ T Throat 3 to 5 T 2 to 3T Leg 1 1/3 to 1 1/2 T
• Approx. ½ T 25 to 50% T 100% Penetration (A) (B) FIGURE 10-21. (A) Butt weld and (B) fillet weld, showing width and depth of bead.
D FIGURE 10-22. Examples of poor welds.
cross-sectioned, it probably would disclose gas pockets, porosity, and slag inclusions.
Figure 10-22 (B) illustrates a weld that has improper penetration and cold laps caused by insufficient heat. It appears rough and irregular and its edges are not feathered into the base metal.
The puddle has a tendency to boil during the welding operation if an excessive amount of acetylene is used. This often leaves slight bumps along the center and craters at the finish of the weld. Cross-checks will be apparent if the body of the weld is sound. If the weld were cross-sectioned, pockets and porosity would be visible. Such a condition is shown in figure 10-22 (C).
A bad weld with irregular edges and considerable variation in the depth of penetration is shown in D of figure 10-22. It often has the appearance of a cold weld.