Par 5-18 Page 5-12 corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, use visual aids such as a mirror or borescope.
b. Corrosion Treatment. Treat any corrosion found during preliminary inspection after completing a visual inspection of any selected part or area.
NOTE: Eddy current, radiography, or ultrasonic inspection can determine the loss of metal to corrosion.
c. Lighting. Provide adequate lighting to illuminate the selected part or area.
d. Personal Comfort. Personal comfort (temperature, wind, rain, etc.) of the inspector can be a factor in visual inspection reliability.
e. Noise. Noise levels while conducting a visual inspection are important. Excessive noise reduces concentration, creates tension, and prevents effective communication. All these factors will increase the likelihood of errors.
f. Inspection Area Access. Ease of access to the inspection area has been found to be of major importance in obtaining reliable visual inspection results. Access consists of the act of getting into an inspection position (primary access) and doing the visual inspection (secondary access). Poor access can affect the inspector’s interpretation of discontinuities, decision making, motivation, and attitude.
g. Precleaning. Clean the areas or surface of the parts to be inspected. Remove any contaminates that might hinder the discovery of existing surface indications. Do not remove the protective finish from the part or area prior to inspection. Removal of the finish may be required at a later time if other NDI techniques are required to verify any visual indications of flaws that are found.
h. Inspection. Carefully inspect the area for discontinuities, using optical aids as required.
An inspector normally should have available suitable measuring devices, a flashlight, and a mirror.
(1) Surface Cracks. When searching for surface cracks with a flashlight, direct the light beam at a 5 to 45 degree angle to the inspection surface, towards the face. (See Figure 5-2.)
Do not direct the light beam at such an angle that the reflected light beam shines directly into the eyes. Keep the eyes above the reflected light beam during the inspection. Determine the extent of any cracks found by directing the light beam at right angles to the crack and tracing its length.
Use a 10-power magnifying glass to confirm the existence of a suspected crack. If this is not adequate, use other NDI techniques, such as penetrant, magnetic particle, or eddy current to verify cracks.
Par 5-18 Page 5-13 FIGURE 5-2. USING A FLASHLIGHT TO INSPECT FOR CRACKS (
2) Other Surface Discontinuities. Inspect for other surface discontinuities, such as:
discoloration from overheating; buckled, bulging, or dented skin; cracked, chafed, split, or dented tubing; chafed electrical wiring; delamination of composites; and damaged protective finishes.
i. Recordkeeping. Document all discrepancies by written report, photograph, and/or video recording for appropriate evaluation. The full value of visual inspection can be realized only if records are kept of the discrepancies found on parts inspected. The size and shape of the discontinuity and its location on the part should be recorded along with other pertinent information, such as rework performed or disposition. The inclusion on a report of some visible record of the discontinuity makes the report more complete.
5-19.–5-24. [RESERVED.]
Par 5-25 Page 5-14 SECTION 3. EDDY CURRENT INSPECTION
5-25. EDDY CURRENT INSPECTION. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine alloy and heat-treat condition.
a. Eddy Current Instruments. A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating current to the test coil.
The receiving section processes the signal from the test coil to the required form and amplitude for display. Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays.
b. Principles of Operations. Eddy currents are induced in a test article when an alternating current is applied to a test coil (probe). The alternating current in the coil induces an alternating magnetic field in the article which causes eddy currents to flow in the article. (See Figure 5-3.)
(1) Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. (See Figure 5-4.) Instruments display the impedance changes either by impedance plane plots or by needle deflection.
(2) Figure 5-5 shows typical impedance plane display and meter display instrument responses for aluminum surface cracks, subsurface cracks, and thickness.
FIGURE 5-3. GENERATING AN EDDY CURRENT
FIGURE 5-4. DETECTING AN EDDY CURRENT Alternating current
- Probe coil Primary magnetic field Conductor - Eddy current Secondary magnetic field FIGURE 5-5. TYPICAL INSTRUMENT DISPLAYS Aluminum surface crack Aluminum subsurface crack Thickness Impedance response Crack response Crack plane thickness a, b, c, \ a instrument response Liftoff Balance point Liftoff Balance point Liftoff Balance point Balance Crack Balance Crack point response point response Meter Balance display point instrument
5-26. EDDY CURRENT COILS AND PROBES. A wide variety of eddy current coils and probes is available. Coils and probes are not always interchangeable between various types of instruments and, for best results, should be matched to a specific instrument and frequency range. Special probe holders can be fabricated to facilitate eddy current inspection of contoured or shaped parts including part edges.
Par 5-25 Page 5-15
Par 5-27 Page 5-16
5-27. FIELD APPLICATION OF EDDY CURRENT INSPECTION. Eddy current techniques are particularly well-suited for detection of service-induced cracks in the field.
Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion.
Both types of cracks initiate at the surface of a part. If this surface is accessible, a high-frequency eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity. If the surface is less accessible, such as in a subsurface layer of structure, low-frequency eddy current inspection can usually be performed. Eddy current inspection can usually be performed without removing surface coatings such as primer, paint, and anodic films.
Eddy current inspection has the greatest application for inspecting small localized areas where possible crack initiation is suspected rather than for scanning broad areas for randomly-oriented cracks. However, in some instances it is more economical to scan relatively large areas with eddy current rather than strip surface coatings, inspect by other methods, and then refinish.
5-28. SURFACE INSPECTION.
a. Inspection Technique. Eddy current inspection techniques are used to inspect for surface cracks such as those shown in Figure 5-6.
FIGURE 5-6. TYPICAL SURFACE CRACKS
b. E quipment Requirements. The following are typical eddy current equipment requirements for surface crack inspections.
(1) Instruments must meet the liftoff and sensitivity requirements of the applicable NDI procedures. The frequency requirement is generally 100 hertz (Hz) to 200 kilohertz (kHz).
Par 5-28 Page 5-17
(2) Many types of probes are available such as: flat-surface; spring-loaded; pencil;
shielded pencil; right-angle pencil; or fastener hole probes.
(3) A reference standard is required for the calibration of Eddy Current test equipment.
A reference standard is made from the same material as that which is to be tested. A reference standard contains known flaws or cracks and could include items such as: a flat surface notch, a fastener head, a fastener hole, or a countersink hole.
5-29. SUBSURFACE INSPECTION.
a. Eddy current inspection techniques are used to inspect for subsurface cracks such as those shown in Figure 5-7.
b. The following are typical eddy current equipment requirements for subsurface crack inspections.
(1) Use a variable frequency instrument with frequency capability from 100 Hz to 500 megahertz (MHz).
(2) The probe used would be a low frequency; spot, ring, or sliding probe.
(3) Use a reference standard appropriate for the inspection being performed.
FIGURE 5-7. TYPICAL SUBSURFACE CRACKS
Par 5-30 Page 5-18
5-30. CORROSION INSPECTION.
a. Eddy current inspection is used to detect the loss of metal as a result of corrosion. An estimation of material loss due to corrosion can be made by comparison with thickness standards. Figure 5-8 shows typical structural corrosion that may be detected by the use of eddy current inspection. Remove all surface corrosion before performing the eddy current corrosion inspection.
b. The following are typical eddy current equipment requirements for corrosion inspection.
(1) Use a variable frequency instrument with frequency capability from 100 Hz to 40 kHz.
(2) Use a shielded probe with coil diameter between 0.15 and 0.5 inch and designed to operate at the lower frequencies.
(3) A reference standard made from the same alloy, heat treatment, and thickness as the test structure will be required.
FIGURE 5-8. TYPICAL STRUCTURAL CORROSION 5-
31. ESTABLISHING EDDY CURRENT INSPECTION PROCEDURES. When establishing eddy current inspection procedures, where no written procedures are available, the following factors must be considered: type of material to be inspected; accessibility of the inspection area; material or part geometry, the signal-to-noise ratio, test system; lift-off effects, location and size of flaws to be detected; scanning pattern; scanning speed; and reference standards. All of these factors are interrelated. Therefore, a change in one of the factors may
Par 5-31 Page 5-19 require changes in other factors to maintain the same level of sensitivity and reliability of the eddy current inspection procedure. Written procedures should elaborate on these factors and place them in their proper order.
5-32.–5-39. [RESERVED.]
Par 5-40 Page 5-20 SECTION 4. MAGNETIC PARTICLE INSPECTION
5-40. GENERAL. Magnetic particle inspection is a method for detecting cracks, laps, seams, voids, pits, subsurface holes, and other surface, or slightly subsurface, discontinuities in ferromagnetic materials. Magnetic particle inspection can be used only on ferromagnetic materials (iron and steel). It can be performed on raw material, billets, finished and semifinished materials, welds, and in-service assembled or disassembled parts. Magnetic particles are applied over a surface either dry, as a powder, or wet, as particles in a liquid carrier such as oil or water.
Common uses for magnetic particle inspection are final inspection, receiving inspection, in-process inspection; and quality control, maintenance, and overhaul.
5-41. PRINCIPLES OF OPERATION. Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an applied field to pass through the metal rather than through the air. A defect at or near the metal’s surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface. (See Figure 5-9.) The field strength is increased in the area of the defect and opposite magnetic poles form on either side of the defect.
Fine magnetic particles applied to the part are attracted to these regions and form a pattern around the defect. The pattern of particles provides a visual indication of a defect. (See Figure 5-10.)
FIGURE 5-9. MAGNETIC FIELD DISRUPTED F IGURE 5-10. CRACK DETECTION BY MAGNETIC PARTICLE INSPECTION
Par 5-41 Page 5-21
a. To locate a defect, it is necessary to control the direction of magnetization, and flux lines must be perpendicular to the longitudinal axes of expected defects. Examination of critical areas for defects may require complete disassembly. Two methods of magnetization, circular and longitudinal, are used to magnetize the part and induce perpendicular flux paths. Parts of complex configuration may require local magnetization to ensure proper magnetic field direction and adequate removal of surface coatings, sealants, and other similar compounds. Possible adverse influence of the applied or residual magnetic fields on delicate parts such as instruments, bearings, and mechanisms may require removal of these parts before performing the inspection.
b. Certain characteristics inherent in the magnetic particle method may introduce errors in examination results. Nonrelevant errors are caused by magnetic field distortions due to intentional design features, such as:
(1) Sharp radii, less than 0.10-inch radius, in fillets;
(2) Thread roots, keyways, and drilled holes; and
(3) Abrupt changes in geometry or in magnetic properties within the part.
c. Operators must understand nonrelevant error indications and recognize them during examination. Proper analysis of indications in these regions will require considerable skill and experience, and supplemental methods may be required before a final evaluation can be made.
Special techniques for examination of these areas are given in subsequent paragraphs.
5-42. APPLICATIONS. Use magnetic particle inspection on any well-cleaned surface that is accessible for close visual examination. Typical parts deserving magnetic particle examination are: steel fasteners and pins; critical structural elements; linkages; landing gear components;
splice and attach fittings; and actuating mechanisms.
a. During field repair operations, disassembly is often not necessary, except when the parts have critical areas or delicate installed components. However, for overhaul operations, a more thorough and critical examination may be obtained with stationary equipment in a shop environment with completely disassembled, and thoroughly cleaned and stripped parts.
b. Magnetic rubber examination material is useful for in-field service examinations of fastener holes in areas where the accessibility is limited or restricted, where particle suspensions may cause unwanted contamination, when a permanent record is desired, and when the examination area cannot be observed visually.
5-43. ELECTRICAL MAGNETIZING EQUIPMENT. Stationary equipment in the range of 100 to 6000 amperes is normal for use within the aerospace industry for overhaul operations.
Mobile equipment with similar amperage outputs is available for field examination of heavy structures, such as landing gear cylinders and axles. Small parts and local areas of large components can be adequately checked with the use of small, inexpensive permanent magnets or electromagnetic yokes. In procuring magnetizing equipment, the maximum rated output should be greater than the required examination amperage. Actual current flow through a complex part may be reduced as much as 20 percent by the resistance load of the rated output.
Par 5-44 Page 5-22
5-44. MATERIALS USED IN MAGNETIC PARTICLE INSPECTION. The particles used in magnetic particle inspection are finely divided ferromagnetic materials that have been treated with color or fluorescent dyes to improve visibility against the various surface backgrounds of the parts under inspection. Magnetic particles, particle-suspension vehicles, and cleaners are required for conducting magnetic particle inspection. Requirements for magnetic particle inspection materials, other than cleaners, are contained in the aerospace industry standard, ASTM-E1444, Inspection, Magnetic Particle (as revised). A certification statement which will certify that the material meets applicable specification requirements will generally be received when a magnetic particle inspection material is purchased. Magnetic particle inspection materials for use on a specific part or component will generally be specified by the aircraft or component manufacturer or the FAA in documents such as; maintenance or overhaul manuals, Ads, SSIDs, or manufacturer’s SBs. However, if the magnetic particle inspection materials are not specified for the specific part or component to be inspected, it is recommended that personnel use materials meeting the aircraft or component manufacturers’ specifications or materials meeting the requirements of ASTM E1444. Other FAA engineering-approved materials may also be used.
Table 5-2 provides a partial listing of commonly accepted standards and specifications for magnetic particle inspection.
Par 5-44 Page 5-23 TABLE 5-2. LISTING OF COMMONLY ACCEPTED STANDARDS AND
SPECIFICATIONS FOR MAGNETIC PARTICLE INSPECTION
NUMBER TITLE
ASTM STANDARDS ASTM A275/A275M Standard Practice for Magnetic Particle Examination of Steel Forgings. 1995.
ASTM A456/A456M Rev. A. Standard Specification for Magnetic Particle Examination of Large Crankshaft Forgings. 1995.
ASTM D96 Standard Test Methods for Water and Sediment in Crude Oils by Centrifuge Method (Field Procedure). 1988.
ASTM E125 Standard Reference Photographs for Magnetic Particle Indications on Ferrous Castings. (Revised 1993). 1963.
ASTM E1316 Standard Terminology for Nondestructive Examination.
1995 (Replaces ASTM E269).
SAE-AMS SPECIFICATIONS AMS 2300 Steel Cleanliness, Premium Aircraft-Quality Magnetic Particle Inspection Procedure. 1991 (Revised 1995.)
MAM 2300 Steel Cleanliness, Premium Aircraft Quality Magnetic Particle Inspection Procedurez, Metric (SI) Measurement.
1992.
AMS 2303 Steel Cleanliness, Aircraft Quality Martensitic Corrosion-Resistant Steels Magnetic Particle Inspection Procedure. 1993.
MAM 2303 Steel Cleanliness, Aircraft Quality Martensitic Corrosion-Resistant Steels, Magnetic Particle Inspection Procedure, Metric (SI) Measurement. 1993.
AMS 2641 Vehicle, Magnetic Particle Inspection Petroleum Base.
1988.
AMS 3040 Magnetic Particles, Nonfluorescent Dry Method. 1995.
AMS 3041 Magnetic Particles, Nonfluorescent Wet Method, Oil Vehicle, Ready-To-Use. 1988.
AMS 3042B Magnetic Particles, Nonfluorescent Wet Method, Dry Powder. 1988.
AMS 3043 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Aerosol Packaged. 1988.
AMS 3044 Magnetic Particles, Fluorescent Wet Method, Dry Powder.
1989.