STRUCTURAL INSPECTION

Once all engine parts have been visually inspected, key components must be structurally inspected to verify their integrity. If a structural inspection reveals a faulty part, the part is immediately removed from the overhaul process and replaced with a new part. Some of the more common struc-tural inspections that are used include magnetic particle, liquid penetrant, eddy current, ultrasonic, and radiography inspection.

MAGNETIC PARTICLE INSPECTION

The nondestructive inspection method most often used for parts made of iron or iron alloys is magnetic particle inspection. Magnetic particle inspection is useful for detecting cracks, splits, seams, and voids that form when a metal ruptures. It is also useful for detecting cold shuts and inclusions of foreign mat-ter that occured when the metal was cast or rolled.

With magnetic particle inspection, a part is magne-tized and an oxide containing magnetic particles is poured or sprayed over the part's surface. If you recall from your general studies, when a material containing large amounts of iron is subjected to a strong magnetic field, the magnetic domains within the material align themselves and the part becomes magnetized. When this happens, the part develops a north and south pole and lines of flux flow in a con-tinuous stream from the north pole to the south pole. If a break occurs within the part, another set of magnetic poles appears, one on either side of the break. Therefore, when conducting magnetic parti-cle inspection, the poles produced by a fault attract the magnetic particles in the oxide, thereby giving you an indication of the break.

In order to detect a crack with magnetic particle inspection, the part must be magnetized so the lines of flux are perpendicular to the fault. This is because a flaw that is parallel to the lines of flux causes a

2-46 Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul

Figure 2-41. When a part is magnetized in a coil, or solenoid, the lines of flux pass through the material longitudinally. As the flux lines pass through the part, faults that run across the part or at an angle are detected.

Figure 2-42. When current passes through a part, lines of flux encircle the part, making it circularly magnetized. This circular magnetization allows for the detection of faults extending lengthwise and at an angle along the part.

minimal disruption in the magnetic field. On the other hand, a defect that is perpendicular to the field creates a large disruption and is relatively easy to detect. To ensure that the flux lines are nearly per-pendicular to a flaw, a part should be magnetized both longitudinally and circularly.

In longitudinal magnetization, the magnetizing cur-rent flows either through a coil that encircles the part being tested, or through a coil around a soft iron yoke. In either method, the magnetic field is ori-ented along the material so that magnetic fields form on either side of faults located across a material.

[Figure 2-41]

With circular magnetization, current flows through the part being inspected, creating lines of magnetic flux that encircle the part. When this occurs, flaws or faults located along the material are magnetized and, therefore, attract magnetic particles. Current is sent through the part by placing it between the heads of the magnetizing equipment. However, if the part is tubular, it is slipped over a conductive rod that is then placed between the heads of a mag-netizing machine. [Figure 2-42]

Large flat objects are circularly magnetized by using test probes that are held firmly against the surface

with current passed through them. The magnetic field is oriented perpendicular to current that flows between the probes. [Figure 2-43]

The medium used to indicate the presence of a fault by magnetic particle inspection is ferromagnetic. In other words, the material is finely divided, has a high permeability, and a low retentivity.

Furthermore, for operator safety it is nontoxic.

There is no one medium that is best for all applica-tions. However, in general, a testing medium con-sists of extremely fine iron oxides that are dyed gray, black, red, or treated with a dye that causes them to fluoresce when illuminated with an ultra-violet lamp.

The iron oxides may be used dry, or they can be mixed with kerosene or some other light oil and sprayed over a surface. Dry particles require no spe-cial preparation, making them well suited for field applications where portable equipment is used. Dry particles are typically applied with hand shakers, spray bulbs, or powder guns.

Wet particles are flowed over a part as a bath. The wet method is typically used with stationary equip-ment that continuously agitates the bath to keep the particles in suspension. Particles are either mixed

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-47

Figure 2-43. Current flowing through a flat object from high-current probes magnetizes the part circularly and detects cracks or faults that are in line with the probes.

in the vehicle with proportions recommended by the manufacturer or they come pre-mixed. The par-ticle concentration of the bath requires close moni-toring with adjustments made as necessary each time the system is used. Measuring particle concen-tration is accomplished by collecting a sample of the agitated bath in a centrifuge tube. A volume of

parti-cles settles to the bottom of the tube allowing mea-surement and comparison to be made against the manufacturer's standardization guide.

Since the bath is continuously recycled, it often becomes contaminated and discolored. When this happens, you must drain and clean the equipment, then refill with a fresh bath.

Different types of magnetizing procedures must be used for different applications. The two methods you must be familiar with are the residual magnet-ism method and the continuous magnetmagnet-ism method.

When a part is magnetized and the magnetizing force is removed before the testing medium is applied, the part is tested by the residual method.

This procedure relies on a part's residual or perma-nent magnetism. The residual procedure is only used with steels that have been heat-treated for stressed applications. Continuous magnetization requires a part to be subjected to a magnetizing force when the testing medium is applied. The continu-ous process of magnetization is most often used to locate invisible defects since it provides a greater sensitivity in locating subsurface discontinuities than does residual magnetism. When performing an engine overhaul, most overhaul manuals specify the type of magnetization to be used on various parts.

[Figure 2-44]

PART METHOD OF

MAGNETIZATION

D.C.

AMPERES

CRITICAL AREAS POSSIBLE

DEFECTS CRANKSHAFT CIRCULAR AND

LONGITUDINAL

2500 JOURNALS, FILLETS, OIL HOLES,THRUST FLANGES,

PROP FLANGE

FATIGUE CRACKS, HEAT CRACKS

CONNECTING ROD CIRCULAR AND LONGITUDINAL

1800 ALL AREAS FATIGUE CRACKS

CAMSHAFT CIRCULAR AND

LONGITUDINAL

1500 LOBES, JOURNALS HEAT CRACKS

PISTON PIN CIRCULAR AND LONGITUDINAL

1000 SHEAR PLANES, ENDS,

CENTER FATIGUE CRACKS

ROCKER ARMS CIRCULAR AND LONGITUDINAL

800 PAD, SOCKET UNDER SIDE ARMS AND BOSS

FATIGUE CRACKS

1500 TEETH, SPLINES FATIGUE CRACKS

SHAFTS CIRCULAR AND

LONGITUDINAL

LONGITUDINAL 500 THREADS UNDER HEAD FATIGUE CRACKS

Figure 2-44. A typical magnetic particle inspection schedule for aircraft engine parts provides details on method of magnetization, current required, critical areas, and a list of possible defects. In this case, the fluorescent method is preferred and a wet continuous procedure is required.

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Magnetization of a part after it is inspected is often detrimental to its operation in an aircraft. Therefore, before a part is returned to service, it must be thor-oughly demagnetized. In order to demagnetize a part, the magnetic domains must be disorganized.

To accomplish this, the part is subjected to a tizing force opposite that of the force used to magne-tize it. For example, if the magnetizing force was AC, the domains alternate in polarity, and if the part is slowly removed from the field while current is still flowing, the reversing action progressively becomes weaker. Thus, the domains are left with random orientation and the part is demagnetized.

AC current does not penetrate a surface very deeply.

For this reason, complete demagnetization of some parts require DC demagnetization. To accomplish this, a part is placed in a coil and subjected to more current than initially used to magnetize the part.

Current is flowed through the coil and then the direction of current flow is reversed while decreas-ing the amount. The direction of current flow con-tinues to be reversed in direction and decreased until the lowest value of current is reached.

The presence of any residual magnetism is checked with a magnet strength indicator. Parts must be demagnetized to within the limits specified in the appropriate overhaul manual before they are returned to service. Although in many cases over-haul manual instructions specify the magnetic par-ticle method of inspection for a certain component, the best advice is to consult the overhaul manual for warnings or restrictions before proceeding. For example, magnetizing the plunger assembly of a hydraulic valve lifter interferes with the steel check valve's ability to seat properly and it is difficult to demagnetize the assembly sufficiently to prevent further problems.

LIQUID PENETRANT INSPECTION

Liquid penetrant inspection is a method of nonde-structive inspection suitable for locating cracks, porosity, or other types of faults open to the surface.

Penetrant inspection is usable on ferrous and nonfer-rous metals, as well as nonporous plastic material. The primary limitation of dye penetrant inspection is that a defect must be open to the surface.

Dye penetrant inspection is based on the principle of capillary attraction. The area being inspected is covered with a penetrating liquid that has a very low viscosity and low surface tension. This penetrant is allowed to remain on the surface long enough to allow the capillary action to draw the penetrant into any fault that extends to the surface. After sufficient

Figure 2-45. (A) When performing a liquid penetrant inspection, the penetrant is spread over the surface of the material being examined, and allowed sufficient time for capillary action to take place. (B) The excess penetrant is then washed from the surface, leaving any cracks and surface flaws filled. (C) An absorbent developer is sprayed over the surface where it blots out any penetrant.

The crack then shows up as a bright line against the white developer.

time, the excess penetrant is washed off and the sur-face is covered with a developer. The developer, by the process of reverse capillary action, blots the pen-etrant out of cracks or other faults, forming a visible line in the developer. If an indication is fuzzy instead of sharp and clear, the probable cause is that the part was not thoroughly washed before the developer was applied. [Figure 2-45]

There are two types of dyes used in liquid penetrant inspection: fluorescent and colored. An ultraviolet light is used with a fluorescent penetrant and any flaw shows up as a green line. With the colored dye method, faults show up as red lines against the white developer.

When using liquid penetrant it is important that the surface be free of grease, dirt, and oil. Only when the surface is perfectly clean can the penetrant be ensured of getting into cracks or faults. The best method of cleaning a surface is with a volatile petro-leum-based solvent, which effectively removes all traces of oil and grease. However, some materials are damaged by these solvents, so care must be taken to ensure the proper cleaner is used. If vapor degreas-ing is not practical, the part is cleaned by scrubbing with a solvent or a strong detergent solution. Parts to be inspected with liquid penetrant should not be cleaned by abrasive blasting, scraping, or heavy brushing. These methods tend to close any discontinuities on the surface and hide defects that could

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-49

otherwise be detected. After the part is clean, rinse and dry it thoroughly.

Once clean, penetrant is typically applied to a sur-face by immersing the part in the liquid or by swab-bing or brushing a penetrant solution onto a part's surface. However, some manufacturer's do offer dye penetrant in spray cans to allow application in small areas for localized inspection. Whichever system is used, the area inspected is completely covered with the penetrating liquid which is then allowed to remain on the surface for the manufacturer's recom-mended length of time.

The amount of time required for a penetrant to cure is called its dwell time and is determined by the size and shape of the discontinuities for which you are looking. For example, small, thin cracks require a longer dwell time than large and more open cracks. Dwell time is decreased if a part is heated; however, if the part gets too hot the pene-trant evaporates.

Once the appropriate dwell time passes, liquid pen-etrants are removed using either water, an emulsifying agent, or a solvent. Water-soluble penetrants are the easiest to remove. Typically, this type of penetrant is flushed away with water that is sprayed at a pressure of 30 to 40 psi. The spray nozzle is held at a 45 degree angle to the surface to avoid washing the penetrant out of cracks or faults.

Post-emulsifying penetrants are not water soluble.

They must be treated with an emulsifying agent before they can be washed from a part's surface. This allows you to control the amount of penetrant that is removed prior to cleaning. By varying the emulsifier dwell time, surface penetrant can be emulsified while the penetrant absorbed into cracks or other defects is left untouched. As a result, the surface penetrant is rinsed off but the absorbed penetrant remains to expose the defect.

Some penetrants are neither water soluble nor emul-sifiable, but instead are solvent-removeable.

When using this type of penetrant, excess penetrant is removed with an absorbent towel, and the part's surface is then wiped with clean towels dampened with solvent. The solvent should not be sprayed onto the surface nor should the part be immersed in solvent, since this will wash the penetrant out of faults or dilute it enough to prevent proper indication in the developer.

Once the excess penetrant is removed, a developer is applied. There are three kinds of developers used to draw penetrants from faults. While all three

types do the same job, the method of application differs. One thing to keep in mind is that penetrant begins to bleed out of any fault as soon as the sur-face penetrant is removed. Because of this, cover-ing the surface to be inspected with developer as soon as possible helps to pinpoint the location of any fault.

A dry developer consists of a loose powder material such as talcum that adheres to the part and acts as a blotter to draw the penetrant out of any surface faults. When using a dry developer, the part is typi-cally placed in a bin of loose developer. For larger components, dry powder is applied with a soft brush, or blown over the surface with a powder gun.

After the powder remains on the surface for the rec-ommended time, the excess is removed with low-pressure air flow.

The penetrant used with a dry developer is often treated with a fluorescent or colored dye. These parts are typically examined under black light so faults appear as a green indication as the light causes the dye to fluoresce, or glow. Colored dye penetrants are usually red and any faults appear as red marks, clearly visible on the surface.

A wet developer is similar to a dry developer in that it is applied as soon as the surface penetrant is rinsed off the part. However, a wet developer typi-cally consists of a white powder mixed with water that is either flowed over the surface or a part is immersed in it. The part is then air-dried and inspected in the same way as a part on which dry developer was used. Wet developers are typically used with penetrants that are treated with either flu-orescent or colored dyes.

The most commonly used developer for field main-tenance is the nonaqueous type. Nonaqueous devel-oper consists of a white, chalk-like powder suspended in a solvent that is normally applied from a pressure spray can, or sprayed onto a surface with a paint gun. The part being inspected must be thoroughly dry before a thin, moist coat of devel-oper is applied. The develdevel-oper dries rapidly and pulls out any penetrant that exists within a fault.

The penetrant stains the developer and is easily seen with a black light when a fluorescent pene-trant is used. If a white light is desired, use a col-ored dye.

EDDY CURRENT INSPECTION

Eddy current inspection is a testing method that requires little or no part preparation and can detect surface and subsurface flaws in most metals.

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Furthermore, it can differentiate among metals and alloys, as well as a metal's heat treat condition. Eddy current inspection is based on the principle of current acceptance. In other words, it determines the ease with which a material accepts induced current. As AC is induced into a material being tested, the AC is mea-sured to determine the material's characteristics.

Eddy currents are electrical currents that flow through electrically conductive material under the influence of an induced electromagnetic field. The ease with which a material accepts the induced eddy currents is determined by four properties: its conductivity, permeability, mass, and by the pres-ence of any voids or faults.

ULTRASONIC INSPECTION

Ultrasonic testing equipment is based on an elec-tronic oscillator that produces AC of the proper fre-quency, which is amplified to the proper strength and sent to a transducer that is touching the mater-ial being tested. The transducer causes the test mate-rial to vibrate at the oscillator's frequency. When the vibrations reach the other side of the material and bounce back, they create an electrical impulse at the transducer that is seen on the CRT display.

RADIOGRAPHIC INSPECTION

One of the most important methods of nondestruc-tive inspection available is radiographic inspection.

Radiographic inspection allows a photographic view inside a structure. In other words, this method uses certain sections of the electromagnetic spec-trum to photograph an object's interior.

X-ray and gamma ray radiation are forms of high energy, short wavelength electromagnetic waves.

The amount of energy these rays contain is related inversely to their wavelength. In other words, the shorter the wavelength, the greater the energy. They have no electrical charge or mass, travel in straight lines at the speed of light, and are able to penetrate matter. The depth of penetration is dependent upon the ray's energy.

There are certain characteristics that make x-rays and gamma rays especially useful in nondestructive inspection. For example, both types of rays are absorbed by the matter through which they pass.

There are certain characteristics that make x-rays and gamma rays especially useful in nondestructive inspection. For example, both types of rays are absorbed by the matter through which they pass.