Par 3-40 Page 3-32
d. Polishing and Finishing. Scratches and repair marks, within certain limitations, can be removed from acrylic plastic. No sanding that could adversely affect the plastic’s optical properties and distort the pilot’s vision should be done on any portion of a windshield.
(1) If there are scratches or repair marks in an area that can be sanded, they may be removed by first sanding the area. Use 320- or 400-grit abrasive paper that is wrapped around a felt or rubber pad.
(2) Use circular rubbing motions, light pressure, and a mild liquid soap solution as a lubricant. After the sanding is complete, rinse the surface thoroughly with running water. Then, using a 500-grit paper, continue to sand lightly. Keep moving to higher grit paper and sand and rinse until all of the sanding or repair marks have been removed.
(3) After using the finest abrasive paper, use rubbing compound and buff in a circular motion to remove all traces of the sanding.
e. Cleaning. Acrylic windshields and windows may be cleaned by washing them with mild soap and running water. Rub the surface with your bare hands in a stream of water. Follow with the same procedure but with soap and water. After the soap and dirt have been flushed away, dry the surface with a soft, clean cloth or tissue and polish it with a windshield cleaner especially approved for use on aircraft transparent plastics. These cleaners may be purchased through aircraft supply houses.
f. Waxing. A thin coating of wax will fill any minute scratches that may be present and will cause rain to form droplets that are easily blown away by the wind.
3-41. PROTECTION. Acrylic windshields are often called “lifetime” windshields, to distinguish them from those made of the much shorter-lived acetate material. However, even acrylic must be protected from the ravages of the elements.
a. When an aircraft is parked in direct sunlight, the windshield will absorb heat and will actually become hotter than either the inside of the aircraft or the outside air. The sun will cause the inside of a closed aircraft to become extremely hot, and this heat is also absorbed by the plastic windshield.
b. To protect against this damage, it is wise to keep the aircraft in a hangar. If this is not possible, some type of shade should be provided to keep the sun from coming in direct contact with the windshield. Some aircraft owners use a close-fitting, opaque, reflective cover over the windshield. In many cases, this has done more harm than good. This cover may absorb moisture from the air and give off harmful vapors, and if it touches the surface of the plastic, it can cause crazing or minute cracks to form in the windshield. Another hazard in using such a cover is that sand can blow up under the cover and scratch the plastic.
3-42. WINDSHIELD INSTALLATION. Aircraft windshields may be purchased either from the original aircraft manufacturer or from any of several FAA-PMA sources. These windshields are formed to the exact shape required, but are slightly larger than necessary so they may be trimmed to the exact size.
Par 3-42 Page 3-33
a. After removing the damaged windshield, clean all of the sealer from the grooves and cut the new windshield to fit. New windshields are covered with either protective paper or film to prevent damage during handling or installation. Carefully peel back just enough of this covering to make the installation. The windshield must fit in its channels with about 1/8- to 1/4-inch clearance to allow for expansion and contraction. If any holes are drilled in the plastic for screws, they should be about 1/8 inch oversize.
b. Place the sealing tape around the edges of the windshield and install the windshield in its frame. Screws that go through the windshield should be tightened down snug and then backed out a full turn, so the plastic can shift as it expands and contracts.
c. Do not remove the protective paper or film until the windshield is installed and all of the securing screws are in place.
3-43. –3-47. [RESERVED.]
Par 4-1 Page 4-1 CHAPTER 4. METAL STRUCTURE, WELDING, AND BRAZING SECTION 1. IDENTIFICATION OF METALS
4-1. GENERAL. Proper identification of the aircraft structural material is the first step in ensuring that the continuing airworthiness of the aircraft will not be degraded by making an improper repair using the wrong materials.
a. Ferrous (iron) alloy materials are generally classified according to carbon content.
(See Table 4-1.)
TABLE 4-1. FERROUS (IRON) ALLOY MATERIALS
MATERIALS CARBON CONTENT Wrought iron Trace to 0.08% Low carbon steel 0.08% to 0.30% Medium carbon steel 0.30% to 0.60% High carbon steel 0.60% to 2.2% Cast iron 2.3% to 4.5%
b. The strength and ductility, or toughness of steel, is controlled by the kind and quantity of alloys used and also by cold-working or heat-treating processes used in manufacturing. In general, any process that increases the strength of a material will also decrease its ductility.
c. Normalizing is heating steel to approximately 150 °F to 225 °F above the steel’s critical temperature range, followed by cooling to below that range in still air at ordinary temperature. Normalizing may be classified as a form of annealing. This process also removes stresses due to machining, forging, bending, and welding. After the metal has been held at this temperature for a sufficient time to be heated uniformly throughout, remove the metal from the furnace and cool in still air. Avoid prolonging the soaking of the metal at high temperatures, as this practice will cause the grain structure to enlarge. The length of time required for the soaking temperature depends on the mass of the metal being treated. The soaking time is roughly 1/4 hour per inch of the diameter of thickness (refer to Military Technical Order (TO) 1-1A-9).
4-2. IDENTIFICATION OF STEEL STOCK. The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) use a numerical index system to identify the composition of various steels. The numbers assigned in the combined listing of standard steels issued by these groups represent the type of steel and make it possible to readily identify the principal elements in the material.
a. The basic numbers for the four digit series of the carbon and alloy steel may be found in Table 4-2. The first digit of the four number designation indicates the type to which the steel belongs. Thus, “1” indicates a carbon steel, “2” a nickel steel, “3” a nickel chromium steel, etc. In the case of simple alloy steels, the second digit indicates the approximate percentage of the predominant alloying element. The last two digits usually indicate the mean of the range
Par 4-2 Page 4-2 of carbon content. Thus, the designation “1020” indicates a plain carbon steel lacking a principal alloying element and containing an average of 0.20 percent (0.18 to 0.23) carbon. The designation “2330” indicates a nickel steel of approximately 3 percent (3.25 to 3.75) nickel and an average of 0.30 percent (0.28 to 0.33) carbon content. The designation “4130” indicates a chromium molybdenum steel of approximately 1 percent (0.80 to 1.10) chromium, 0.20 percent
(0.15 to 0.25) molybdenum, and 0.30 percent (0.28 to 0.33) carbon.
TABLE 4-2. NUMERICAL SYSTEM FOR STEEL IDENTIFICATION
TYPES OF STEELS NUMERALS AND DIGITS Plain carbon steel 10XX Carbon steel with additional sulfur for easy machining 11XX Carbon steel with about 1.75% manganese 13XX .25% molybdenum 40XX 1% chromium, .25% molybdenum 41XX 2% nickel, 1% chromium, .25% molybdenum 43XX
1.7% nickel, .2% molybdenum 46XX
3.5% nickel, .25% molybdenum 48XX 1% chromium steels 51XX 1% chromium, 1.00% carbon 51XXX
1.5% chromium steels 52XX
1.5% chromium, 1.00% carbon 52XXX 1% chromium steel with .15% vanadium 61XX .5% chromium, .5% nickel, .20% molybdenum 86XX .5% chromium, .5% nickel, .25% molybdenum 87XX 2% silicon steels, .85% manganese 92XX
3.25% nickel, 1.20% chromium, .12% molybdenum 93XX
b. There are numerous steels with higher percentages of alloying elements that do not fit into this numbering system. These include a large group of stainless and heat resisting alloys in which chromium is an essential alloying element. Some of these alloys are identified by three digit AISI numbers and many others by designations assigned by the steel company that produces them. The few examples in Table 4-3 will serve to illustrate the kinds of designations used and the general alloy content of these steels.
Par 4-2 Page 4-3 TABLE 4-3. EXAMPLES OF STAINLESS AND HEAT-RESISTANT STEELS
NOMINAL COMPOSITION (PERCENT) ALLOY
DESIGNATION CARBON CHROMIUM NICKEL OTHER GENERAL CLASS OF STEEL 302 0.15 18 9 Austenitic 310 0.25 25 20 Austenitic 321 0.08 18 11 Titanium Austenitic 347 0.08 18 11 Columbium or Tantalum Austenitic 410 0.15 12.5 Martensitic, Magnetic 430 0.12 17 Ferritic, Magnetic 446 0.20 25 Nitrogen Ferritic, Magnetic PH15-7 Mo 0.09 15 7 Molybdenum, Aluminum Precipitation Hardening 17-4 PH 0.07 16.5 4 Copper, Columbium, or Tantalum Precipitation Hardening
c. “1025” welded tubing as per SAE Specification AMST5066B and “1025” seamless tubing are interchangeable.
4-3. INTERCHANGEABILITY OF STEEL TUBING.
a. “4130” welded tubing conforming to SAE Specification AMST6736B and “4130” seamless tubing are interchangeable.
b. NE-8630 welded tubing conforming to SAE Specification AMST6736B and NE-8630 seamless tubing are interchangeable.
4-4. IDENTIFICATION OF ALUMINUM.
a. To provide a visual means for identifying the various grades of aluminum and aluminum alloys, such metals are usually marked with symbols such as a government specification number, the temper or condition furnished, or the commercial code marking.
b. Plate and sheet are usually marked with specification numbers or code markings in rows approximately 5 inches apart. Tubes, bars, rods, and extruded shapes are marked with specification numbers or code markings at intervals of 3 to 5 feet along the length of each piece.
The commercial code marking consists of a number which identifies the particular composition of the alloy. In addition, letter suffixes (see Table 4-4) designate the basic temper designations and subdivisions of aluminum alloys.
Par 4-4 Page 4-4 TABLE 4-4. BASIC TEMPER DESIGNATIONS AND SUBDIVISIONS FROM
ALUMINUM ALLOYS
NON HEAT-TREATABLE ALLOYS HEAT-TREATABLE ALLOYS Temper Designation Definition Temper Designation Definition -0 Annealed recrystallized (wrought products only) applies to softest temper of wrought products.
-0 Annealed recrystallized (wrought products only) applies to softest temper of wrought products.
-H1 Strain-hardened only. Applies to products which are strain-hardened to obtain the desired strength without supplementary thermal treatment.
-T1 Cooled from an elevated temperature shaping process (such as extrusion or casting) and naturally aged to a substantially stable condition.
-H12 Strain-hardened one-quarter-hard temper.
-T2 Annealed (castings only).
-H14 Strain-hardened half-hard temper. -T3 Solution heat-treated and cold-worked by the flattening or straightening operation.
-H16 Strain-hardened three-quarters-hard temper.
-T36 Solution heat-treated and cold-worked by reduction of 6 percent.
-H18 Strain-hardened full-hard temper. -T4 Solution heat-treated.
-H2 Strain-hardened and then partially annealed. Applies to products which are strain-hardened more than the desired final amount and then reduced in strength to the desired level by partial annealing.
-T42 Solution heat-treated by the user regardless of prior temper (applicable only to 2014 and 2024 alloys).
-H22 Strain-hardened and partially annealed to one-quarter-hard temper.
-T5 Artificially aged only (castings only).
-H24 Strain-hardened and partially annealed to half-hard temper.
-T6 Solution heat-treated and artificially aged.
-H26 Strain-hardened and partially annealed to three-quarters-hard temper.
-T62 Solution heat-treated and aged by user regardless of prior temper (applicable only to 2014 and 2024 alloys).
-H28 Strain-hardened and partially annealed to full-hard temper.
-T351 -T451 -T3510 -T3511 -T4510 -T4511 Solution heat-treated and stress relieved by stretching to produce a permanent set of 1 to 3 percent, depending on the product.
Par 4-4 Page 4-5
NON HEAT-TREATABLE ALLOYS HEAT-TREATABLE ALLOYS Temper Designation Definition Temper Designation Definition -H3 Strain-hardened and then stabilized.
Applies to products which are strain-hardened and then stabilized by a low temperature heating to slightly lower their strength and increase ductility.
-T651 -T851 -T6510 -T8510 -T6511 -T8511 Solution heat-treated, stress relieved by stretching to produce a permanent set of 1 to 3 percent, and artificially aged.
-H32 Strain-hardened and then stabilized. Final temper is one-quarter hard. -T652 Solution heat-treated, compressed to produce a permanent set, and then artificially aged.
-H34 Strain-hardened and then stabilized. Final temper is one-half hard. -T8 Solution heat-treated, cold-worked, and then artificially aged.
-H36 Strain-hardened and then stabilized. Final temper is three-quarters hard. -T/4 Solution heat-treated, cold-worked by the flattening or straightening operation, and then artificially aged.
-H38 Strain-hardened and then stabilized. Final temper is full-hard. -T86 Solution heat-treated, cold-worked by reduction of 6 percent, and then artificially aged.
-H112 As fabricated; with specified mechanical property limits. -T9 Solution heat-treated, artificially aged and then cold-worked.
-F For wrought alloys; as fabricated.
No mechanical properties limits.
For cast alloys; as cast.
-T10 Cooled from an elevated temperature shaping process artificially aged and then cold-worked.
-F For wrought alloys; as fabricated.
No mechanical properties limits.
For cast alloys; as cast.
4-5.–4-15. [RESERVED.]
Par 4-16 Page 4-6 SECTION 2. TESTING OF METALS
4-16. HARDNESS TESTING. If the material type is known, a hardness test is a simple way to verify that the part has been properly heat-treated. Hardness testers such as Rockwell, Brinell, and Vickers can be useful to check metals for loss of strength due to exposure to fire or abusive heating. Also, understrength bolts can be found and removed from the replacement part inventory by checking the hardness of the bolt across the hex flats. Although hardness tests are generally considered nondestructive, hardness testing does leave a small pit in the surface;
therefore, hardness tests should not be used on sealing surfaces, fatigue critical parts, load bearing areas, etc., that will be returned to service. These hardness tests provide a convenient means for determining, within reasonable limits, the tensile strength of steel. It has several limitations in that it is not suitable for very soft or very hard steels. Hardness testing of aluminum alloys should be limited to distinguishing between annealed and heat-treated material of the same aluminum alloy. In hardness testing, the thickness and the edge distance of the specimen being tested are two factors that must be considered to avoid distortion of the metal. Several readings should be taken and the results averaged. In general, the higher the tensile strength, the greater its hardness. Common methods of hardness testing are outlined in the following paragraphs. These tests are suitable for determining the tensile properties resulting from the heat treatment of steel.
Care should be taken to have case-hardened, corroded, pitted, decarburized, or otherwise nonuniform surfaces removed to a sufficient depth. Exercise caution not to cold-work, and consequently harden, the steel during removal of the surface.
4-17. ROCKWELL HARDNESS TEST. The Rockwell hardness test is the most common method for determining hardness of ferrous and many nonferrous metals. (See Table 4-5.) It differs from Brinell hardness testing in that the hardness is determined by the depth of indentation made by a constant load impressing on an indenter. In this test, a standard minor load is applied to set a hardened steel ball or a diamond cone in the surface of the metal, followed by the application of a standard major load. The hardness is measured by depth of penetration.
Rockwell superficial hardness tests are made using light minor and major loads and a more sensitive system for measuring depth of indentation. It is useful for thin sections, very small parts, etc. Calibration of Rockwell hardness testers is done in accordance with ASTM International (ASTM) E-18 specifications.
Par 4-17 Page 4-7 TABLE 4-5. HARDNESS VALUES FOR ALUMINUM ALLOYS (refer to AMS2772H) Material Commercial Designation Hardness Temper Brinell Number 500 kg. Load 10 mm. Ball 1100 0 23 H18 44 3003 0 28 H16 47 2014 0 45 T6 135 2017 0 45 T6 105 2024 0 47 T4 120 2025 T6 110 6151 T6 100 5052 0 47 H36 73 6061 0 30 T4 65 T6 95 7075 T6 135 7079 T6 135 195 T6 75 220 T4 75 C355 T6 80 A356 T6 70
4-18. BRINELL HARDNESS TEST. In this test, a standard constant load (usually 500 kg to 3,000 kg) is applied to a smooth flat metal surface by a hardened steel-ball type indenter 10 mm in diameter. The 500-kg load is usually used for testing nonferrous metals such as copper and aluminum alloys, whereas the 3,000-kg load is most often used for testing harder metals such as steels and cast irons. The numerical value of Brinell Hardness (HB) is equal to the load divided by the surface area of the resulting spherical impression.
Wh ere:
P=load in kg D=diameter of the ball in mm d=diameter of the indentation in mm
Par 4-18 Page 4-8
a. General Precautions. To avoid misapplication of Brinell hardness testing, the fundamentals and limitations of the test procedure must be clearly understood. To avoid inaccuracies, the following rules should be followed:
(1) Do not make indentations on a curved surface having a radius of less than 1 inch.
(2) Do make the indentations with the correct spacing. Indentations should not be made too close to the edge of the work piece being tested.
(3) Apply the load steadily to avoid overloading caused by inertia of the weights.
(4) Apply the load so the direction of loading and the test surface are perpendicular to each other within 2 degrees.
(5) The thickness of the work piece being tested should be such that no bulge or mark showing the effect of the load appears on the side of the work piece opposite the indentation.
(6) The indentation diameter should be clearly outlined.
b. Limitations. The Brinell hardness test has three principal limitations:
(1) The work piece must be capable of accommodating the relatively large indentations.
(2) Due to the relatively large indentations, the work piece should not be used after testing.
(3) The limit of hardness, 15 HB with the 500-kg load to 627 HB with the 3,000-kg load, is generally considered the practical range.
c. Calibration. A Brinell hardness tester should be calibrated to meet ASTM standard E10 specifications.
4-19. VICKERS HARDNESS TEST. In this test, a small pyramidal diamond is pressed into the metal being tested. The Vickers Hardness number (HV) is the ratio of the load applied to the surface area of the indention. This is done with the following formula:
a.
The indenter is made of diamond, and is in the form of a square-based pyramid having an angle of 136 degrees between faces. The facets are highly polished, free from surface imperfections, and the point is sharp. The loads applied vary from 1 to 120 kg; the standard loads are 5, 10, 20, 30, 50, 100, and 120 kg. For most hardness testing, 50 kg is maximum.
b. A Vickers hardness tester should be calibrated to meet ASTM Standard E10 specifications, acceptable for use over a loading range.
Par 4-20 Page 4-9
4-20. MICROHARDNESS TESTING. This is an indentation hardness test made with loads not exceeding 1 kg (1,000 g). Such hardness tests have been made with a load as light as 1 g, although the majority of microhardness tests are made with loads of 100 g to 500 g. In general, the term is related to the size of the indentation rather than to the load applied.
a. Fields of Application. Microhardness testing is capable of providing information regarding the hardness characteristics of materials which cannot be obtained by hardness tests such as the Brinell or Rockwell, and are as follows:
(1) Measuring the hardness of precision work pieces that are too small to be measured by the more common hardness testing methods.
(2) Measuring the hardness of product forms such as foil or wire that are too thin or too small in diameter to be measured by the more conventional methods.
(3) Monitoring of carburizing or nitriding operations, which is sometimes accomplished by hardness surveys taken on cross-sections of test pieces that accompanied the work pieces through production operations.
(4) Measuring the hardness of individual microconstituents.
(5) Measuring the hardness close to edges, thus detecting undesirable surface conditions such as grinding burn and decarburization.
(6) Measuring the hardness of surface layers such as plating or bonded layers.
b. Indenters. Microhardness testing can be performed with either the Knoop or the Vickers indenter. The Knoop indenter is used mostly in the United States; the Vickers indenter is more widely used in Europe.
(1) Knoop indentation testing is performed with a diamond, ground to pyramidal form, that produces a diamond-shaped indentation with an approximate ratio between long and short diagonals of 7 to 1. The indentation depth is about one-thirtieth of its length. Due to the shape of the indenter, indentations of accurately measurable length are obtained with light loads.
(2) The Knoop Hardness number (HK) is the ratio of the load applied to the indenter to the unrecovered projected area of indentation. The formula for this follows:
Wh ere:
P=the applied load in kg A=the unrecovered projected area in square mm l=the measured length of the long diagonal in mm C=0.07028 (a constant of the indenter, relating projected area of the indentation to the square of the length of the long diagonal)
Par 4-21 Page 4-10
4-21. INDENTATIONS. The Vickers indenter penetrates about twice as far into the work piece as does the Knoop indenter. The diagonal of the Vickers indentation is about one-third of the total length of the Knoop indentation. The Vickers indenter is less sensitive to minute differences in surface conditions than is the Knoop indenter. However, the Vickers indentation, because of the shorter diagonal, is more sensitive to errors in measuring than is the Knoop indentation. (See Figure 4-1.)
FIGURE 4-1. COMPARISON OF INDENTATION MADE BY KNOOP AND VICKERS
INDENTERS IN THE SAME WORK METAL AND AT THE SAME LOADS 4 -22. MAGNETIC TESTING. Magnetic testing consists of determining whether the specimen is attracted by a magnet. Usually, a metal attracted by a magnet is iron, steel, or an iron-base alloy containing nickel, cobalt, or chromium. However, there are exceptions to this rule since some nickel and cobalt alloys may be either magnetic or nonmagnetic. Never use this test as a final basis for identification. The strongly attracted metals could be pure iron, pure nickel, cobalt, or iron-nickel-cobalt alloys. The lightly attracted metals could be cold-worked stainless steel or Monel. The nonmagnetic metals could be aluminum, magnesium, silver, or copper-base alloy, or an annealed 300-type stainless steel.
4-23. ALUMINUM TESTING. Hardness tests are useful for testing aluminum alloy chiefly as a means of distinguishing between annealed, cold-worked, heat-treated, and heat-treated and aged material. It is of little value in indicating the strength or quality of heat-treatment. Typical hardness values for aluminum alloys are shown in Table 4-5.
a. Clad aluminum alloys have surface layers of pure aluminum or corrosion-resistant aluminum alloy bonded to the core material to inhibit corrosion. Presence of such a coating may be determined under a magnifying glass by examination of the edge surface which will show three distinct layers. In aluminum alloys, the properties of any specific alloy can be altered by work hardening (often called strain hardening), heat treatment, or by a combination of these processes.