Par 5-15 Page 5-195 Inspection Mirrors. An inspection mirror is used to view an area that is not in the normal line of sight. The mirror should be of the appropriate size to easily view the component, with the reflecting surface free of dirt, cracks, worn coating, etc ., and a swivel joint tight enough to maintain its setting.
Simple Magnifiers. A single converging lens, the simplest form of a microscope, is often referred to as a simple magnifier. Magnification of a single lens is determined by the equation M = 10/f. In this equation, “M” is the magnification, “f” is the focal length of the lens in inches, and “10” is a constant that represents the average minimum distance at which objects can be distinctly seen by the unaided eye. Using the equation, a lens with a focal length of 5 inches has a magnification of 2, or is said to be a two-power lens.
BORESCOPES. These instruments are long, tubular, precision optical instruments with built-in illumination, designed to allow remote visual inspection of internal surfaces or otherwise inaccessible areas. The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the distant, or distal tip of the borescope. Rigid and flexible borescopes are available in different designs for a variety of standard applications and manufacturers also provide custom designs for specialized applications. Figure 5-1 shows three typical designs of borescopes.
Borescopes Uses. Borescopes are used in aircraft and engine maintenance programs to reduce or eliminate the need for costly tear-downs. Aircraft turbine engines have access ports that are specifically designed for borescopes. Borescopes are al so used extensively in a variety of aviation maintenance programs to determine the airworthiness of difficu lt-to-reach components.
Borescopes typically are used to inspect interiors of hydraulic cylinders and valves for pitting, scoring, porosity, and tool marks; inspect for crack ed cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans; verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult to reach areas; and assess Foreign Object Damage (FOD) in aircraft, airframe, and powerplants. Borescopes may also be used to locate and retrieve foreign objects in engines and airframes.
Optical Designs. Typical designs for the optical connection between the borescope viewing end and the distal tip are:
A rigid tube with a series of relay lenses;
A flexible or rigid tube with a bundle of optical fibers; and
A flexible or rigid tube with wiring that carries the image signal from a Charge Couple Device (CCD) imaging sensor at the distal tip.
These designs can have either fixed or adjustable focusing of the objective lens at the distal tip. The distal tip may also have prisms and mirrors that define the direction and field of view. A fiber optic light guide with white light is generally used in the illumination system, but ultraviolet light can also be used to inspect surfaces treated with liquid fluorescent penetrant or to inspect for contaminants that fluoresce. Some borescopes with long working lengths use light-emitting diodes at the distal tip for illumination.
VISUAL INSPECTION PROCEDURES.
Corrosion can be an extremely critical defect.
Therefore, NDI personnel should be familiar with the appearance of common types of corrosion and have training and
Eyepiece lens Relay Lens Objective lens Incandescent Lamp
(a) A rigid borescope with a lamp at the distal end.
Eyepiece lens Focusing ring Control handles tor tour-way tip articulation Interchangeable Image guide tips Objective lens Light guide Protective sheath Light source Light guide exit Projection lamp -
(b) A flexible fiberscope with a light source.
Field of View Orbital scan Control Focus control Shaft rotation for orbital scan Working length Light guide connector (inside handle) Light guide To light source (integral or separate)
(c) A rigid borescope with a light guide bundle in the shaft.
FIGURE 5-1. Typical borescope designs.
experience on corrosion detection on aircraft structure and engine materials. (Reference: AC 43-4A, Corrosion Control for Aircraft, for additional information on corrosion.
Preliminary Inspection. Perform a preliminary inspection of the overall general area for cleanliness, presence of foreign objects, deformed or missing fasteners, security of parts, 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.
Corrosion Treatment. Treat any corrosion found during preliminary inspection after completing a visual inspection of any selected part or area.
Par 5-18 Page 5-196
Par 5-18 Page 5-197 NOTE: Eddy current, radiography, or ultrasonic inspection can determine the loss of metal to corrosion.
Lighting. Provide adequate lighting to illuminate the selected part or area.
Personal Comfort. Personal comfort (temperature, wind, rain, etc.) of the inspector can be a factor in visual inspection reliability.
Noise. Noise levels while conducting a visual inspection are important. Excessive noise reduces concentration, creates tensi on, and prevents effective communication. All these factors will increase the likelihood of errors.
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 (prima ry access) and doing the visual inspection (secondary access). Poor access can affect the inspector’s interpretation of discontinuities, decision making, motivation, and attitude.
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.
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.
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.
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; delaminations of composites; and damaged protective finishes.
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.
Par 5-18 Page 5-198 (and 5-14)
F IGURE 5-2. Using a flashlight to inspect for cracks.
5-19.5-24. [RESERVED.]
Par 5-25 Page 5-199
SECTION 3. EDDY CURRENT INSPECTION
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.
Eddy Current Instruments. A wide variety of eddy current test instruments are av ailable. The eddy current test instrument performs three basic functions:
generating, receiving, and di splaying. 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, a nd electrical or electronic relays.
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.)
Flaws in or thickness changes of the testpiece 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.
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.
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.
FIELD APPLICATION OF EDDY CURRENT INSPECTION. Eddy current techniques are particularly well-suited for detection of serviceinduced cracks in the field.
Par 5-25 Page 5-200
FIGURE 5-5. Typical instrument displays.
Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion. Both types of cracks initiate at th e surface of a part. If this surface is accessible, a hi gh-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 st rip surface coatings, inspect by other methods, and then refinish.
SURFACE INSPECTION. 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.
Equipment Requirements. The following are typical eddy current equipment re quirements for surface crack inspections.
Instruments must meet the liftoff and sensitivity requirements of the applicable NDI procedures. The frequency requirement is generally 100 Hz to 200 kHz.
Many types of probes are available such as:
flat-surface; spring-loaded; pencil; shielded pencil; right-angle pencil; or fastener hole probes.
Par 5-28 Page 5-201 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.
SUBSURFACE INSPECTION.
Eddy current inspection techniques are used to inspect for subsurface cracks such as those shown in figure 5-7.
The following are typical eddy current equipment requirements for subsurface crack inspections.
Use a variable frequency instrument with frequency capability from 100 Hz to 500 MHz.
The probe used would be a low-freque ncy; spot, ring, or sliding probe.
Use a reference standard appropriate for the inspection being performed.
CORROSION INSPECTION.
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. The following are typi cal eddy current equipment requirements for corrosion inspection.
Use a variable frequency instrument with frequency capability from 100 Hz to 40 kHz.
Use a shielded probe with coil diameter between 0.15 and 0.5 inch and designed to operate at the lower frequencies.
A reference standard made from the same alloy, heat treatment, and thickness as the test structure will be required.
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-tonoise 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 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.
Fastener hole crack, fastener in place Skin Chord > Skin Angle Upper member crack Crack Second or deeper member crack Skin Gap Chord Splice Plate Skin FIGURE 5-7. Typical subsurface cracks.
Chord Skin Corrosion Skin - Bonded - Stringer Web - doubler Skin and chord web Body skin lap splice Skin and bonded doubler FIGURE 5-8. Typical structural corrosion.
Par 5-28 Page 5-202
Par 5-40 Page 5-203
5-32.5-39. [RESERVED.] SECTION 4. MAGNETIC PARTICLE INSPECTION
GENERAL. Magnetic particle inspection is a method for detecting cracks, laps, seams, voids, pits, subsurface holes, and other surface, or slightly subsurface, discontinuities in ferro-magnetic materials. Magnetic particle inspection can be used only on ferro-magnetic 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 inspecti on, in-process inspection;
and quality control, maintenance, and overhaul.
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.
FIGURE 5-10. Crack detection by magnetic particle inspection.
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 cr itical 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 pa rts before performing the inspection.
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:
Sharp radii, less than 0.10 inch radius, in fillets;
Thread roots, keyways, and drilled holes; and
Abrupt changes in geometry or in magnetic properties within the part.
Par 5-40 Page 5-204 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.
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.
During field repair operations, disassembly is often not necessary, except when th e 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.
Magnetic rubber examination material is useful for in-field service examinations of fastener holes in areas where the accessibility is lim ited or restricted, where particle suspensions may cause unwanted contamination, when a permanent record is desired, and when the examination area cannot be observed visually.
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.
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, AD’s, SSID’s, or manufacturer’s SB’s. 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 engineeringapproved 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 5-205 TABLE 5-2. Listing of commonly accepted standards and specifications for magnetic particle inspection.
NUMBER TITLE
ASTM STANDARDS
ASTM A275/A275 M-96
ASTM A456/A456 M Rev. A.
ASTM D96
ASTM E125-63 (1993)
ASTM E1316-95C
Standard Test Method for Magnetic Particle Examination of Steel Forgings.
1995 Standard Specification for Magnetic Particle Examination of Large Crankshaft Forgings. 1995 Standard Test Methods for Water and Sediment in Crude Oils by Centrifuge Method (Field Procedure). 1988 Standard Reference Photographs for Magnetic Particle Indications on Ferrous Castings. (Revised 1993) 1963 Standard Terminology for Nondestructive Examination. 1995 (Replaces ASTM E269).
SAE-AMS
SPECIFICATIONS
AMS 2300G
MAM 2300A
AMS 2303C
MAM 2303A
AMS 2641 AMS 3040B AMS 3041B
AMS 3042B AMS 3043A
AMS 3044C AMS 3045B AMS 3046B
Premium Aircraft-Quality Steel Cleanliness Magnetic Particle Inspection Procedure. 1991 (Revised 1995) Premium Aircraft Quality Steel Cleanliness Magnetic Particle Inspection Procedure Metric (SI) Measurement. 1992 Aircraft Quality Steel Cleanliness Martensitic Corrosion Resistant Steels Magnetic Particle Inspection Procedure. 1993 Aircraft Quality Steel Cleanliness Martensitic Corrosion Resistant Steels Magnetic Particle Inspection Procedure Metric (SI) Measurement. 1993 Vehicle, Magnetic Particle Inspection Petroleum Base. 1988 Magnetic Particles, Nonfluorescent, Dry Method. 1995 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Ready-To-Use.
1988 Magnetic Particles, Nonfluorescent, Wet Method, Dry Powder. 1988 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Aerosol Packaged.
1988 Magnetic Particles, Fluorescent, Wet Method, Dry Powder. 1989 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle Ready-to-Use. 1989 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Aerosol Packaged.
1989 U.S. GOVERNMENT
SPECIFICATIONS
DOD-F-87935 Mil-Std-271F Mil-Std-410E MIL-HDBK-728/1 MIL-HDBK-728/4A
Fluid, Magnetic Particle Inspection, Suspension. 1993 Requirements for Nondestructive Testing Methods. 1993 Nondestructive Testing Personnel Qualifications and Certifications. 1991 Nondestructive Testing. 1985 Magnetic Particle Testing. 1993
OTHER PUBLICATIONS
SNT-TC-1A
ATA No. 105 ASM Handbook, Volume 17
American Society for Nondestructive Testing. Recommended Practice . 1992 (Personnel Qualification and Certification in Nondestructive Testing and Recommended Training Courses) Note: Updated every 4 years - 1996 edition due in early 1997.
Air Transport Association of America. Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods, (Revision 4 1993) Nondestructive Evaluation and Quality Control. 1989
Par 5-44 Page 5-206 PREPARATION OF SURFACE.
Remove protective coatings according to the manufacturer’s instructions if necessary. Unless otherwise specified, magnetic particle examination should not be performed with coatings in place that could prevent the detection of surface defects in the ferro-magnetic substrate. Such coatings include paint or chromeplate thicker than 0.003 inch, or ferro-magnetic coatings such as electropl ated nickel thicker than
0.001 inch.
Parts should be free of grease, oil, rust, scale, or other substances which will interfere with the examination process. If required, clean by vapor degrease, solvent, or abrasive means per the manufacturer’s instructions.
Use abrasive cleaning only as necessary to completely remove scale or rust. Excessive blasting of parts can affect examination results.
Exercise extreme care to prevent any cleaning material or magnetic particles from becoming entrapped where they cannot be removed. This may require extracting components such as bushings, bearings, or inserts from assemblies before cleaning and magnetic particle examination.
A water-break-free surface is required for parts to be examined by water suspension methods. If the suspension completely we ts the surface, this requirement is met.
Magnetic particle examination of assembled bearings is not recommended because the bearings are difficult to demagnetize. If a bearing cannot be removed, it should be protected from the magnetic particle examination materials and locally magnetized with a magnetic yoke to limit the magnetic field across the bearing.
METHODS OF EXAMINATION. Magnetic particle examination generally consists of: the application of magnetic particles;
magnetization; determination of field strength;
special examination techniques; and demagnetization and post-examination cleaning. Each of these steps will be described in the following paragraphs.
APPLICATION OF MAGNETIC PARTICLES. The magnetic particles used can be nonfluorescent or fluorescent (dependent on the examination required) and are applied suspended in a suitable substance. Fluorescent particles are preferred due to their higher sensitivity.
Wet Continuous Method. Unless otherwise specified, use only the wet continuous method. In the wet continuous method, the particle suspension is liberally applied to wet all surfaces of the part. The magnetizing current is applied at the instant the suspension is diverted from the part. Apply three shots of magnetizing current, each 1/2 second long.
Wet suspensions of fluorescent particles, either in water or oil, should be used for most overhaul and in-service examinations except where the material, size, or shape of the part prohibits its use.
Water, with a suitable rust inhibitor and wetting agent, may be used as a liquid vehicle, provided that magnetic examination equipment is designed for use or is satisfactorily converted for use with water.
Dry Continuous Method. This method is not recommended for use on aerospace components because of its lower sensitivity level.