Par 5-94 Page 5-48
b. Application of Ultrasonic Inspection. Application to bonded structures must be examined in detail because of the many inspection methods and structural configurations. The advantages and limitations of each inspection method should be considered, and reference standards (representative of the structure to be inspected) should be ultrasonically inspected to verify proposed techniques.
c. Internal Configuration. Complete information on the internal configuration of the bonded test part must be obtained by the inspector. Drawings should be reviewed, and when necessary, radiographs of the test part should be taken. Knowledge of details such as the location and boundaries of doublers, ribs, etc., is required for valid interpretation of ultrasonic inspection results. The boundaries of internal details should be marked on the test part using a grease pen or other easily removable marking.
d. Reference Standards. Standards can be a duplicate of the test part except for the controlled areas of unbond. As an option, simple test specimens, which represent the varied areas of the test part and contain controlled areas of unbond, can be used. Reference standards must meet the following requirements:
(1) The reference standard must be similar to the test part regarding material, geometry, and thickness. This includes containing closure members, core splices, stepped skins, and internal ribs similar to the test part if bonded areas over or surrounding these details are to be inspected.
(2) The reference standard must contain bonds of good quality except for controlled areas of unbond fabricated as explained below.
(3) The reference standard must be bonded using the adhesive and cure cycle prescribed for the test part.
e. Types of Defects. Defects can be separated into five general types to represent the various areas of bonded sandwich and laminate structures as follows:
(1) Type I. Unbonds or voids in an outer skin-to-adhesive interface.
(2) Type II. Unbonds or voids at the adhesive-to-core interface.
(3) Type III. Voids between layers of a laminate.
(4) Type IV. Voids in foam adhesive or unbonds between the adhesive and a closure member at core-to-closure member joints.
(5) Type V. Water in the core.
f. Fabrication of NDI Reference Standards. Every ultrasonic test unit should have sample materials that contain unbonds equal to the sizes of the minimum rejectable unbonds for the test parts. Information on minimum rejectable unbond sizes for test parts should be obtained from the OEM’s manuals, FAA requirements, or the cognizant FAA ACO engineer. One or more
Par 5-94 Page 5-49 of the following techniques can be used in fabricating reference defects; however, since bonding materials vary, some of the methods may not work with certain materials.
(1) Standards for Types I, II, III, and IV unbonds can be prepared by placing discs of
0.006 inch thick (maximum) Teflon sheets over the adhesive in the areas selected for unbonds.
For Type II unbonds, the Teflon is placed between the core and adhesive. The components of the standard are assembled and the assembly is then cured.
(2) Types I, II, and III standards can also be produced by cutting flat-bottomed holes of a diameter equal to the diameter of the unbonds to be produced. The holes are cut from the back sides of bonded specimens, and the depths are controlled to produce air gaps at the applicable interfaces. When using this method, patch plates can be bonded to the rear of the reference standard to cover and seal each hole.
(3) Type II standards can be produced by locally undercutting (before assembly) the surface of the core to the desired size unbond. The depth of the undercut should be sufficient to prevent adhesive flow causing bonds between the undercut core and the skin.
(4) Type IV standards can be produced by removing the adhesive in selected areas prior to assembly.
(5) Type V standards can be produced by drilling small holes in the back of the standard and injecting varying amounts of water into the cells with a hypodermic needle. The small holes can then be sealed with a small amount of water-resistant adhesive.
g. Inspection Coverage. Examples of several different configurations of bonded structure along with suggested inspection coverages with standard ultrasonic test instruments are shown in Figure 5-21. In many cases, access limitations will not permit application of the suggested inspections in all of the areas shown. The inspection coverages and suggested methods contained in Figure 5-21 and Table 5-7 are for reference only. Details of the inspection coverage and inspections for a particular assembly should be obtained from the OEM’s manuals, or other FAA-approved requirements.
h. Inspection Methods. Table 5-8 lists the various inspection methods for bonded structures along with advantages and disadvantages of each inspection method.
FIGURE 5-21. EXAMPLES OF BONDED STRUCTURE CONFIGURATIONS AND
SUGGESTED INSPECTION COVERAGE CORE WITH "Z" CLOSURE MEMBER CORE AND SPLICE JOINT CORE (A) (B) (C) CORE WITH "Z" CLOSURE CORE AND CLOSURE CORE AND INTERNAL RIBS WITH
MEMBER AND LAND IN SKIN MEMBER FLIM ADHESIVE BOND TO TOP OF CORE (D) (E)
TEST PER DETAIL CORE AND INTERNAL RIBS
CORE AND WHEN GAP IS CORE, CLOSURE WITH FOAM ADHESIVE BOND CLOSURE 1 INCH OR WIDER MEMBER, AND TO TOP OF CORE
MEMBER LANDS IN SKINS (G)
CORE AND CLOSE OUT CORE AND CLOSURE CORE, CLOSURE MEMBER
MEMBER WITH SHEAR AND DOUBLER PLATES (K) "Z" OR HAT LAMINATED "T," DOUBLER, TRIPLER, COMPOSITE COMPOSITE
LAMINATES WEB, OR CHANNEL OR COMPOSITES SPLICE JOINT SPLICE JOINT (N) (P) (Q) (R) (S) NOTES:
1. THE NUMBERED LINES SURROUNDING EACH VIEW INDICATE THE SCAN PLANES. THE
NUMBER ON EACH LINE IS USED TO DETERMINE THE ACCEPTABLE INSPECTION METHODS BY REFERRING TO TABLES.
2. WHERE SURFACES ARE SYMMETRICAL, THE COVERAGE ILLUSTRATED SHALL BE CONSIDERED TYPICAL FOR BOTH SIDES.
3. SHADED AREAS REPRESENT FOAM ADHESIVE. I
4. WHEN THE SAME METHOD(S) ARE SPECIFIED IN MORE THAN ONE SCAN PLANE, CALI- BRATION SHALL BE VERIFIED FOR EACH PLANE.
Par 5-94 Page 5-50
Par 5-94 Page 5-51 TABLE 5-7. ACCEPTABLE ULTRASONIC INSPECTION METHODS ASSOCIATED
WITH THE EXAMPLE BONDED STRUCTURE CONFIGURATIONS SHOWN IN FIGURE 5-21
NUMBER ACCEPTABLE METHODS 1 Either
(a) Pulse-echo straight or angle beam, on each side;
or
(b) Through-transmission.
2 Pulse-echo, straight beam, on each skin.
3 Refer to 1 for methods. If all these methods fail to have sufficient penetration power to detect reference defects in the reference standard, then the ringing method, applied from both sides, should be used. Otherwise, the ringing method is unacceptable.
4 Either
(a) Ringing, on each skin of the doubler;
or
(b) Through-transmission;
or
(c) Damping.
5 Ringing.
6 Through-transmission. Dotted Line represents beam direction.
7 Either
(a) Through-transmission;
or
(b) Damping.
NOTE:
A variety of ultrasonic testing methods and instruments are available for adaptation to specific inspection problems. Other bond inspection instruments can be used if detailed procedures are developed and proven on the applicable reference standards for each configuration of interest.
Some representative instruments, which can be used for the inspection of bonded structures are the Sondicator, Harmonic Bond Tester, Acoustic Flaw Detector, Audible Bond Tester, Fokker Bond Tester, 210 Bond Tester, and Bondascope 2100.
Par 5-94 Page 5-52 TABLE 5-8. ULTRASONIC INSPECTION METHODS FOR BONDED STRUCTURES
INSPECTION METHOD
THROUGH
TRANSMISSION
PULSE-ECHO RINGING DAMPING Advantages Applicable to structures with either thick or thin facing sheets.
Applicable to structures with multiple layers bonded over honeycomb core.
Detects unbonds on either side Detects broken, crushed, and corroded core.
In some cases water in core can be detected.
Applicable to structures with either thick or thin facing sheets.
Determines which side is unbonded.
Detects small unbonds.
Detects broken, crushed, and corroded core.
In some cases water in core can be detected.
Applicable to complex shapes.
Detects small near surface unbonds (larger than diameter of search unit).
Applicable to structures with either thick or thin facing sheets.
Applicable to multiple-layered (more than two layers) structures.
Detects unbonds on either side.
Detects small unbonds (larger than diameter of receiving search unit).
Disadvantages Access to both sides of part is required.
Does not determine layer position of unbonds.
Alignment of search units is critical.
Couplant is required.
Inspection rate is slow.
Inspection from both sides required, does not detect far side unbonds.
Applicable only to skin-to-honeycomb core structures.
Reduced effectiveness on structures with multiple skins over honeycomb core.
Couplant required.
Applicable only to near surface unbonds, works best on unbonds between top sheet and adhesive, may miss other unbonds.
Reduced effectiveness on thick skins.
Couplant required.
Applicable only to doublers and laminate-type structures.
Access to both sides required.
Does not determine layer positions of unbonds.
Couplant required.
5-95. BOND TESTING INSTRUMENTS. Standard ultrasonic inspection instruments can be used for bond testing as previously noted; however, a wide variety of bond testing instruments are available for adaptation to specific bonded structure inspection problems.
a. General Principle. Two basic operating principles are used by a variety of bond testers for single-sided bond inspection.
Par 5-95 Page 5-53
(1) Ultrasonic Resonance. Soundwaves from a resonant transducer are transmitted into and received from a structure. A disbond in the structure will alter the sound wave characteristics, which in turn affect the transducer impedance.
(2) Mechanical Impedance. Low frequency, pulsed ultrasonic energy is generated into a structure. Through ultrasonic mechanical vibration of the structure, the impedance or stiffness of the structure is measured, analyzed, and displayed by the instrumentation.
b. Operation. In general, operation of the adhesive bond test instruments noted is similar.
The test probe is moved over the surface in smooth overlapping strokes. The direction of the stroke with regard to the surface is generally immaterial; however, when using the Sondicator models, the direction of the stroke becomes critical when the test probe is operated near a surface edge. Edge effects on vibration paths give a test reading that may be misinterpreted. To avoid edge effects, the test probe should be moved so that the inspection path follows the surface edge, giving a constant edge for the test probe to inspect. Edge effects are more pronounced in thicker material. To interpret meter readings correctly, the operator should determine whether there are variations in the thickness of the material.
c. Probe Sending Signal. With the exception of the Sondicator models, the test probes of the testers emit a sending signal that radiates in a full circle. The sending signal of the Sondicator probe travels from one transducer tip to the other. For this reason, the test probe should be held so that the transducer tips are at right angles to the inspection path. When inspecting honeycomb panels with a Sondicator model, the transducer tips should be moved consistently in the direction of the ribbon of the honeycomb or at right angles to the ribbon so that a constant subsurface is presented.
5-96. THICKNESS MEASUREMENTS. Ultrasonic inspection methods can be used for measurement of material thickness in aircraft parts and structures.
a. Applications. Ultrasonic thickness measurements are used for many applications, such as: checking part thickness when access to the back side is not available; checking large panels in interior areas where a conventional micrometer cannot reach; and in maintenance inspections for checking thickness loss due to wear and/or corrosion.
b. Pulse-Echo Method. The most commonly used ultrasonic thickness measurement method. The ultrasonic instrument measures time between the initial front and back surface signals or subsequent multiple back reflection signals. Since the velocity for a given material is a constant, the time between these signals is directly proportional to the thickness. Calibration procedures are used to obtain direct readout of test part thickness.
c. Thickness Measurement Instrument Types. Pulse-echo instruments designed exclusively for thickness measurements are generally used in lieu of conventional pulse-echo instruments; however, some conventional pulse-echo instruments also have direct thickness measurement capabilities. Conventional pulse-echo instruments without direct thickness measuring capabilities can also be used for measuring thickness by using special procedures.
Par 5-96 Page 5-54
d. Thickness Measurement Ranges. Dependent upon the instrument used and the material under test, material thickness from 0.005 inches to 20 inches (or more) can be measured with pulse-echo instruments designed specifically for thickness measuring.
5-97. LEAK TESTING. The flow of a pressurized gas through a leak produces sound of both sonic and ultrasonic frequencies. If the gas leak is large, the sonic frequency sound it produces can probably be detected with the ear or with such instruments as stethoscopes or microphones;
however, the ear and these instruments have limited ability to detect and locate small leaks.
Ultrasonic leak detectors are frequently used to detect leaks that cannot be detected by the above methods, because they are very sensitive to ultrasonic energy and, under most conditions, background noise at other frequencies does not affect them.
a. Standard Method. A standard method of testing for leaks using ultrasonics is provided in ASTM E1002. The method covers procedures for calibration, location, and estimated measurements of leakage by the ultrasonic technique (sometimes called ultrasonic translation).
b. Detection Distance. Ultrasonic energy in the relatively low-frequency range of 30-50 KHz travels easily through air; therefore, an ultrasonic leak detector can detect leakage with the probe located away from the leak. The maximum detection distance depends on the leakage rate.
c. Typical Applications. Some typical applications for the ultrasonic leak detector on aircraft are fuel system pressurization tests, air ducts and air conditioning systems, emergency evacuation slides, tire pressure retention, electrical discharge, oxygen lines and valves, internal leaks in hydraulic valves and actuators, fuel cell testing, identifying cavitation in hydraulic pumps, arcing in wave guides, cabin and cockpit window and door seals, and cabin pressurization testing.
5-98.–5-104. [RESERVED.]
Par 5-105 Page 5-55 SECTION 8. TAP TESTING
5-105. GENERAL. Tap testing is widely used for a quick evaluation of any accessible aircraft surface to detect the presence of delamination or debonding.
a. The tap testing procedure consists of lightly tapping the surface of the part with a coin, light special hammer with a maximum of 2 ounces (see Figure 5-22), or any other suitable object. The acoustic response is compared with that of a known good area.
b. A “flat” or “dead” response is considered unacceptable. The acoustic response of a good part can vary dramatically with changes in geometry, in which case a standard of some sort is required. The entire area of interest must be tapped. The surface should be dry and free of oil, grease, and dirt. Tap testing is limited to finding relatively shallow defects in skins with a thickness less than .080 inch. In a honeycomb structure, for example, the far side bondline cannot be evaluated, requiring two-side access for a complete inspection. This method is portable, but no records are produced. The accuracy of this test depends on the inspector’s subjective interpretation of the test response; therefore, only qualified personnel should perform this test.
FIGURE 5-22. SAMPLE OF SPECIAL TAP HAMMER
5-106.–5-111. [RESERVED.]
Par 5-112 Page 5-56 SECTION 9. ACOUSTIC-EMISSION
5-112. GENERAL. Acoustic-Emission is an NDI technique that involves the placing of acoustical-emission sensors at various locations on the aircraft structure and then applying a load or stress. The level of stress applied need not reach general yielding, nor does the stress generally need to be of a specific type. Bending stress can be applied to beamed structures, torsional stress can be applied to rotary shafts, thermal stresses can be applied with heat lamps or blankets, and pressure-induced stress can be applied to pressure-containment systems such as the aircraft fuselage. The materials emit sound and stress waves that take the form of ultrasonic pulses that can be picked up by sensors. Cracks and areas of corrosion in the stressed airframe structure emit sound waves (different frequencies for different size defects) which are registered by the sensors.
These acoustic-emission bursts can be used both to locate flaws and to evaluate their rate of growth as a function of applied stress. Acoustic-emission testing has an advantage over other NDI methods in that it can detect and locate all of the activated flaws in a structure in one test.
Acoustic-emission testing does not now provide the capability to size flaws, but it can greatly reduce the area required to be scanned by other NDI methods.
5-113. APPLICATIONS.
a. A wide variety of structures and materials, such as: wood, plastic, fiberglass, and metals can be inspected by the acoustic-emission technique by applying stress on the test material. The emission-producing mechanism in each type of material may differ, but characteristic acousticemissions are produced and can be correlated to the integrity of the material. Acoustic-emission technology has been applied quite successfully in monitoring proof tests of pressure vessels and tests of fiber-reinforced plastic structures of all kinds. There are now ASTM standards and ASME codes applying to its use in testing gas cylinders and both metal and fiber-reinforced plastic vessels, tanks, and piping.
b. For a welded structure such as a pressure vessel, acoustic-emission testing works well with relatively simple instrumentation. However, slight movement of bolted or riveted joints can also generate acoustic signals. Thus a complex structure may have many acoustic sources besides flaws in its components. These unwanted emission sources greatly complicate acoustic-emission tests of complex structures. The difficulties are not prohibitive, but they put a premium on the intelligent use of signal processing and interpretation. Therefore, because of the complexity of aircraft structures, application of acoustic-emission testing to aircraft has required a new level of sophistication, both in testing techniques and data interpretation. Research and testing programs are currently in progress to determine the feasibility of acoustic-emission testing on several different types of aircraft.
5-114.–5-119. [RESERVED.]
Par 5-120 Page 5-57 SECTION 10. THERMOGRAPHY
5-120. GENERAL. Thermography is an NDI technique that uses radiant electromagnetic thermal energy to detect flaws. The presence of a flaw is indicated by an abnormal temperature variant when the item is subjected to normal heating and cooling conditions inherent to the in-service life, and/or when artificially heated or cooled. The greater the material’s resistance to heat flow, the more readily the flow can be identified due to temperature differences caused by the flaw.
5-121.–5-126. [RESERVED.]
Par 5-127 Page 5-58 SECTION 11. HOLOGRAPHY
5-127. GENERAL. Holography is an NDI technique that uses visible light waves coupled with photographic equipment to create a 3D image. The process uses two laser beams, one called a reference beam and the other called an object beam. The two laser beams are directed to an object, between beam applications the component is stressed. The beams are then compared and recorded on film, or other electronic recording medium, creating a double image. Indications of applied stresses or defects are shown as virtual images with a system of fringe lines overlaying the part. Holography is most commonly used for rapid assessment of surface flaws in composite structures.
5-128.–5-133. [RESERVED.]
Par 5-134 Page 5-59 SECTION 12. SHEAROGRAPHY
5-134. GENERAL. Shearography was developed for strain measurements. The process now provides a full-field video strain gauge, in real time, over large areas. It is an enhanced form of holography, which requires the part to be under stress. A laser is used for illumination of the part while under stress. The output takes the form of an image processed video display. This technique has been used effectively in locating defects, such as disbonds and delaminations, through multiple bondlines. It is capable of showing the size and shape of subsurface anomalies when the test part is properly stressed. Shearography has been developed into a useful tool for NDI. It can be used easily in a hangar environment, while meeting all laser safety concerns.
Other applications include the testing of honeycomb structures, such as flaps and control surfaces. Shearography offers a great increase in the speed of inspection by allowing on-aircraft inspections of structures without their removal, as well as inspections of large areas in just seconds.
5-135.–5-140. [RESERVED.]