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.]
Par 6-1 Page 6-1 CHAPTER 6. CORROSION, INSPECTION, AND PROTECTION SECTION 1. GENERAL
6-1. GENERAL. The purpose of this chapter is to provide information that will help maintenance personnel prevent, control, identify, and treat various types of corrosion. (Refer to Advisory Circular (AC) 43-4, Corrosion Control for Aircraft, for a more in-depth study on the detection and treatment of corrosion.)
a. Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action and converts it back to a metallic compound.
b. Four conditions must exist before electrochemical corrosion can occur.
(See Figure 6-1.) They are:
(1) A metal subject to corrosion (Anode);
(2) A dissimilar conductive material (Cathode), which has less tendency to corrode;
(3) Presence of a continuous, conductive liquid path (Electrolyte); and
(4) Electrical contact between the anode and the cathode (usually in the form of metal-to-metal contact such as rivets, bolts, and corrosion).
FIGURE 6-1. SIMPLIFIED CORROSION CELL SHOWING CONDITIONS WHICH
MUST EXIST FOR ELECTROCHEMICAL CORROSION
Par 6-1 Page 6-2
c. Elimination of any one of these conditions will stop electrochemical corrosion.
(See Figure 6-2.)
FIGURE 6-2. ELIMINATION OF CORROSION BY APPLICATION OF AN ORGANIC
FILM TO METAL SURFACE NOTE: P aint can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, painted surfaces should be inspected often for irregularities such as blisters, flakes, chips, and lumps.
6-2. FACTORS INFLUENCING CORROSION.
a. Some factors which influence metal corrosion and the rate of corrosion are:
(1) Type of metal;
(2) Heat treatment and grain direction;
(3) Presence of a dissimilar, less corrodible metal;
(4) Anodic and cathodic surface areas (in galvanic corrosion);
(5) Temperature;
(6) Presence of electrolytes (hard water, saltwater, battery fluids, etc.);
(7) Availability of oxygen;
(8) Presence of biological organisms;
Par 6-2 Page 6-3
(9) Mechanical stress on the corroding metal;
(10) Time of exposure to a corrosive environment; and
(11) Lead/graphite pencil marks on aircraft surface metals.
b. Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys. Most alloys are made up entirely of small crystalline regions, called grains. Corrosion can occur on surfaces of those regions which are less resistant and also at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most active metals (those which lose electrons easily), such as magnesium and aluminum, corrode easily. The most noble metals (those which do not lose electrons easily), such as gold and silver, do not corrode easily.
c. Corrosion is quickened by high-temperature environments that accelerate chemical reactions and increase the concentration of water vapor in the air.
d. Electrolytes (electrically conducting solutions) form on surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion.
e. When some of the electrolyte on a metal surface is partially confined (such as between faying surfaces or in a deep crevice), the metal around this area corrodes more rapidly. This type of corrosion is called an oxygen concentration cell. Corrosion occurs more rapidly because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic to other metal surfaces on the same part that are immersed in electrolyte or exposed to air.
f. Slime, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area usually remains damp, increasing the possibility of corrosion.
g. Manufacturing processes such as machining, forming, welding, or heat treatment can leave residual stress in aircraft parts and can cause cracking in a corrosive environment.
6-3. COMMON CORROSIVE AGENTS. Substances that cause corrosion are called corrosive agents. The most common corrosive agents are acids, alkalis, and salts. The atmosphere and water, the two most common media for these agents, may also act as corrosive agents.
a. Any acid will severely corrode most of the alloys used in airframes. The most destructive are sulfuric acid (battery acid), halogen acids (hydrochloric, hydrofluoric, and hydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals.
b. Alkalis, as a group, are not as corrosive as acids. Aluminum and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a
Par 6-3 Page 6-4 corrosion inhibitor. Substances particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust).
c. The major atmospheric corrosive agents are oxygen and airborne moisture.
Corrosion often results from the direct action of atmospheric oxygen and moisture on metal, and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys. The atmosphere may also contain other corrosive gases and contaminants, particularly industrial and marine salt spray.
d. The corrosiveness of water depends on the type and quantity of dissolved mineral and organic impurities and dissolved gases (particularly oxygen) in the water. One characteristic of water that makes it corrosive is its conductivity. Physical factors, such as water temperature and velocity, also have a direct bearing on its corrosiveness.
6-4. MICRO-ORGANISMS.
a. Bacteria may be either aerobic or anaerobic. Aerobic bacteria require oxygen to live.
They accelerate corrosion by oxidizing sulfur to produce sulfuric acid. Bacteria living adjacent to metals may promote corrosion by depleting the oxygen supply or by releasing metabolic products. Anaerobic bacteria, on the other hand, can survive only when free oxygen is not present. The metabolism of these bacteria requires them to obtain part of their sustenance by oxidizing inorganic compounds, such as iron, sulfur, hydrogen, and carbon monoxide. The resultant chemical reactions cause corrosion.
b. Fungi are the growths of micro-organisms that feed on organic materials. While low humidity does not kill microbes, it slows their growth and may prevent corrosion damage. Ideal growth conditions for most micro-organisms are temperatures between 68 and 104 °F (20 and 40 °C) and relative humidity between 85 and 100 percent.
c. Damage resulting from microbial growth can occur when any of three basic mechanisms, or a combination of these, is brought into play. First, fungi have a tendency to hold moisture, which contributes to other forms of corrosion. Second, because fungi are living organisms, they need food to survive. This food is obtained from the material on which the fungi are growing. Third, these micro-organisms secrete corrosive fluids that attack many materials, including some that are not fungi nutrient.
d. Microbial growth must be removed completely to avoid corrosion. Microbial growth should be removed by hand with a firm, nonmetallic bristle brush and water. Removal of microbial growth is easier if the growth is kept wet with water. Microbial growth may also be removed with steam at 100 pounds per square inch (psi). Protective clothing must be used when using steam for removing microbial growth.
6-5.–6-10. [RESERVED.]