d) Dcam Part 66 - Module 6

Full text

(1)DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. g in. n i ra. f T o MODULE 6: MATERIALS ANDg HARDWARE y 66 CATEGORY n r i (DCAMa PART B1.1) r t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011.

(2) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. WARNING. g in. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. n i ra. This document is intended for the purposes of training only. The information contained herein is as accurate as possible at the time of issue, and is subjected to ongoing amendments where necessary according to any regulatory journals and documents. Where the information contained in this document is in variation with other official journals and/or documents, the latter must be taken as the overriding document. The contents herein shall not be reproduced in any form without the expressed permission of ETD.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011.

(3) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE TABLE OF CONTENTS. 6 AIRCRAFT MATERIALS .................................................................................................................................................................. 1 6.1 FERROUS METALS.................................................................................................................................................................. 5 6.2 NON-FERROUS METALS....................................................................................................................................................... 21 6.3 COMPOSITE STRUCTURES.................................................................................................................................................. 21. g in. 6.4 TYPES OF CORROSION ........................................................................................................................................................ 55. n i ra. 6.5 FASTENERS ........................................................................................................................................................................... 90 6.6 PIPES AND UNIONS..............................................................................................................................................................184. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 6.7 SPRINGS................................................................................................................................................................................201 6.8 BEARINGS .............................................................................................................................................................................208 6.9 TRANSMISSIONS ..................................................................................................................................................................212 6.10 CONTROL CABLES .............................................................................................................................................................233 6.11 ELECTRICAL CABLES ........................................................................................................................................................258. For Training Purposes Only. Issue 1 Revision 0 Jan 2011.

(4) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE TABLE OF CONTENTS. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M THIS PAGE INTENTIONALLY LEFT BLANK. For Training Purposes Only. Issue 1 Revision 0 Jan 2011.

(5) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. 6 AIRCRAFT MATERIALS Knowledge and understanding of the uses, strengths, limitation and other characteristics of structural metals is vital to properly construct and maintain any equipment especially airframes. In aircraft maintenance and repair, even slight deviation of from design specification of interior materials result in the loss of both lives and equipment. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals.. g in. Strength, weight, and reliability are three factors which determine the requirements to be met by any material used in airframe construction and repair. The material must possess the strength required by the dimensions, weight and use. There are five basic stresses which metals may be required to withstand. These are: 1. 2. 3. 4. 5.. n i ra. Tension Compression Shear Bending Torsion. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Tension. Tension. Deformation. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 1.

(6) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. Tensile Strength When a piece of sheet metal is pulled from each end, the resultant force is called tension. The ability to withstand tension is called tensile strength, and is measured in pounds per square inch. Yield Strength. g in. The ability of a metal to resist deformation is called yield strength. Example:. n i ra. P P. f T o g y n r i r a t e e e i n r i p g o n r E P S A M A simplified view of a material. The same material this time with an applied force. It breaks once the force exceeds ultimate strength of the material. When a tensile load is applied to a material, the material resists any deformation until its yield point is reached. However, once the yield point is reached, the metal stretches, and its molecular structure changes enough to increase the metal’s strength and therefore, resist further deformation. This continues until the ultimate load is reached, at which time, the material breaks.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 2.

(7) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. Tension. g in. n i ra. Tension. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Shear Strength. Shear strength describes a metal’s ability to resist opposing forces. A rivet that holds two or more sheets of metal together, resisting the force of the sheet trying to slide apart, is an example of shear load. When the rivets installed in a joint have more strength than the metal surrounding them, the joint is said to be loaded in shear. Example:. P. Two simplified view of materials. For Training Purposes Only. P. The two materials, joined with other materials, can withstand certain amount of force without deformation. P. P. The joint breaks once the force exceed ultimate strength of the material for the joint. Issue 1 Revision 0 Jan 2011. Page 3.

(8) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE. Compression. Tension. Tension. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Bearing Strength. Bearing strength is the ability of a joint to withstand any form of crushing or excessive compressive distortion. Material under a compression load usually fails by buckling or bending. The force at which something buckles while being compressed varies with an objects length, cross sectional area and shape. Example:. P. A simplified view of materials. For Training Purposes Only. P. The same material this time with an applied force. P. P. The material buckles once the force exceed the ultimate strength. Issue 1 Revision 0 Jan 2011. Page 4.

(9) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). 6.1 FERROUS METALS PROPERTIES OF METALS The various properties of metals can be assessed, by accurate laboratory tests on sample pieces. The terminology, associated with these properties, is outlined in the following paragraphs.. g in. 1. BRITTLENESS The tendency of the metal to shatter, without significant deformation. It will shatter under a sudden, low stress but will resist a slowly-applied, higher load.. n i ra. 2. CONDUCTIVITY The ability of a metal to conduct heat, (thermal conductivity) and electricity. Silver and copper are excellent thermal and electrical conductors.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 3. DUCTILITY The property of being able to be permanently extended by a tensile force. It is measured during a tensile, or stretching, test, when the amount of stretch (elongation), for a given applied load, provides an indication of a metal’s ductility. 4. ELASTICITY The ability of a metal to return to its original shape and size after the removal of any distorting force. The ‘Elastic Limit’ is the greatest force that can be applied without permanent distortion. 5. HARDNESS The ability of a metal to resist wear and penetration. It is measured by pressing a hardened steel ball or diamond point into the metal’s surface. The diameter or depth of the resulting indentation provides an indication of the metal’s hardness. 6. MALLEABILITY The ease with which the metal can be forged, rolled and extruded without fracture. Stresses, induced into the metal, by the forming processes, have to be subsequently relieved by heat-treatment. Hot metal is more malleable than cool metal. 7. PLASTICITY The ability to retain a deformation after the load producing it has been removed. Plasticity is, in fact, the opposite of elasticity. 8. TENACITY The property of a metal to resist deformation when subjected to a tensile load. It is proportional to the maximum stress required to cause the metal to fracture.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 5.

(10) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). 9. TOUGHNESS The ability of a metal to resist suddenly applied loads. A metal’s toughness is tested by impact with a swinging pendulum of known mass. 10. STRENGTH There are several different measurements of the strength of a metal, as may be seen from the following sub-paragraphs. g in. 10.1 TENSILE STRENGTH The ability to resist tension forces applied to the metal. n i ra. 10.2 YIELD STRENGTH The ability to resist deformation. After the metal yields, it is said to have passed its yield point.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 10.3 SHEAR STRENGTH The ability to resist side-cutting loads - such as those, imposed on the shank of a rivet, when the materials it is joining attempt to move apart in a direction normal to the longitudinal axis of the rivet. 10.4 BEARING STRENGTH The ability of a metal to withstand a crushing force.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 6.

(11) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). ALLOYING INGREDIENTS The main alloying agents of steel are: Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel and allows heat treatment of steel to occur. Hardness and tensile strength increases as carbon content increases up to about 0.85% carbon. Low carbon steel contains 0.1 to 0.3 % carbon. Low carbon steels are used for the manufacture of safety wire and secondary structures. Medium carbon steel contains 0.3 and 0.5 % carbon. These steels are employed where a machining processes are required or where surface hardness is desireable. High carbon steels contain 0.5-- 1.05% carbon. These steels are used where extreme hardness is required, typical applications include springs, files and cutting tools.. g in. n i ra. . Sulphur decreases ductility and weldability with increasing content. Sulphur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability.. . Manganese contributes to strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel.. . Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in increasing as--rolled strength and hardness. In low--carbon steels, silicon is generally detrimental to surface quality.. . Phosphorous increases strength and hardness and corrosion resistance but decreases ductility. . Nickel increases the hardenability and impact strength of steels.. . Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength.. . Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low--alloy steels at elevated temperatures.. . Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels.. . Titanium is used to improve toughness. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 7.

(12) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Alloying elements. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 8.

(13) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). MATERIAL DESIGNATIONS Designations given to most low alloy steels are based upon an AISI (American Iron and Steel Institute) system that refers to the chemical composition of the alloy.. g in. The first two digits refer to the specific primary alloying elements, the last two digits (or the last three in a five-digit number) refer to the percentage of carbon contained in the alloy.  10XX -- refers to plain carbon steels (contain only carbon and manganese)  41XX -- refers to chromium and molybdenum alloy steels  43XX -- refers to nickel, chromium and molybdenum alloy steels  52100 -- refers to a chromium alloy with 1% carbon  93XX -- refers to a nickel, chromium and molybdenum alloy steel (with a different ratio between these elements than is contained in the 43XX alloys). For example, 4340 refers to a nickel-chromium-molybdenum alloy containing .40% carbon.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 9Ni - 4Co.30C is a specific trade name assigned to a nickel-cobalt alloy with .30% carbon. The 9 and 4 refer to the nominal percentages of nickel and cobalt in the alloy. The normally-used low alloy steels and their applicable strength ranges are shown. Use of these alloys is limited to the strength ranges shown. The European designations are slightly different. For further information refer to the ’Metallic Material List’ in the Structural Repair Manual (SRM) of the specific aircraft manufacturer.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 9.

(14) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Material designations. Metalworking Processes. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 10.

(15) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). After metal alloys are produced, they must be formed into useful shapes. Wrought objects are those formed by physically working the metal into shape, whereas cast items are formed by pouring molten metal into moulds. When it comes to mechanically working metal into a desired shape, there are three methods commonly used: 1. Hot-working 2. Cold-working 3. Extruding. g in. n i ra. Hot – Working. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Hot-working is the process of forming metal at an elevated temperature when it is in its annealed or soft condition. Almost all steel is hot-worked from the ingot into a form which is either hot or cold worked to a finished shape. The ingot is then placed in a soaking pit to slow the cooling process until the molten interior gradually solidifies.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 11.

(16) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). After soaking, then it is worked into its desired shape through rolling and forging. Rolling consists of forming hot metal ingots with rollers to form sheets, bars and beams. Forging is a process where in a piece of metal is worked at temperature above its critical range. Forging is typically used to form shape through either pressing or hammering.. g in. Pressing. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Pressing is used to form large and heavy parts. Since a press is slow acting, its force is uniformly transmitted to the centre of the material being pressed. This affects the interior grain structure resulting in the best possible structure throughout.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 12.

(17) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Drop Forging. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Drop forging is a hammering process whereby a hot ingot is placed between a pair of formed dies in a machine called a drop hammer and a weight of several tons is dropped on the form upper die. This results in the hot metal being forced to take the form of the dies. Because the process is very rapid, the grain structure of the metal is altered, resulting in significant increases in the strength of the finished part.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 13.

(18) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Hammering. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Hammering is a type of forging that is usually used on small parts because it requires a metal worker to physically hammer a piece of metal into its finished shape. The advantage of hammering is that the operator has control over both the amount of pressure applied and the finishing temperature. Forging is usually referred to as smith forging and is used extensively where only a small number of parts are needed. In addition to the forming operation, hammering hardens the metal.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 14.

(19) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Cold – Working Cold-working is performed well below a metal’s critical temperature and ranges from the manual bending of sheet metal for skin repairs to drawing seamless tubing and wire. There are several cold-working processes; the two that are most common are cold-rolling and cold-drawing.. g in. Cold-Rolling. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Cold -rolling usually refers to the rolling of metal at room temperature.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 15.

(20) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Cold – Drawing. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Cold – drawing is used in making seamless tubing, wire and other forms of stock.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 16.

(21) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Extrusion. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Extrusion is the process of forcing metal through a die which imparts a required cross- section to the metal. Metals such as lead, tin and aluminium may be extruded cold, however most metals are heated. The advantage of the extrusion process is its flexibility. Example: Because of its workability, aluminium can be economically extruded to more shapes and larger sizes than is practicable with other metals. Many structural parts such as channels, angles, T-sections and Z-section are formed by the extrusion process.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 17.

(22) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Ferrous Metal (Iron) Any alloy containing iron as its chief constituent is called ferrous metal. The most common ferrous metal in aircraft structure is steel, an alloy of iron with a controlled amount of carbon added.. g in. Iron is a chemical element which is fairly soft malleable and ductile in its pure form. It is silvery white in color and is quite heavy. Iron combines readily with oxygen to form iron oxide, which is more commonly known as rust. Iron poured from a furnace into moulds is known as cast iron and normally contains more than two percent carbon and some silicon.. n i ra. Cast iron has few aircraft applications because of its low strength to weight ratio. However, it is used in engines for items such as piston rings where its porosity and wear characteristic allow it to hold a lubricant film.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 18.

(23) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Steel. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. To make steel, pig iron is re-melted in a special furnace. Pure oxygen is then forced through the molten metal where it combines with carbon and burns. The molten steel is then poured into moulds where it solidifies into ingots. The ingots are placed in a soaking pit where they are heated to a uniform temperature of about 2,200º F/ 1204.4º C. They are then taken from the soaking pit and passed through steel rollers to form plate or sheet plate. Much of the steel used in aircraft construction is made in electric furnaces, which allow better control of alloying agents then gas-fired furnaces. An electric furnace is loaded with scrap steel, limestone and flux. The intense heat from the arcs melts the steel and the impurities mix with flux. Once the impurities are removed, controlled quantities of alloying agents are added, and the liquid metal in poured into moulds.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 19.

(24) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Stainless Steel. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Stainless steel is the classification of corrosion-resistant steels that contain large amounts of chromium and nickel. Their strength and resistance to corrosion make them well suited for high-temperature application such as firewalls and exhaust system components. The principal alloy stainless steel is chromium. The corrosion resistant steel most often used in aircraft construction is known as 18-8 steel because of its content of 18 percent chromium and 8 percent nickel. Stainless steel may be rolled, drawn, bent or formed to any shape.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 20.

(25) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Molybdenum One of the most widely used alloying elements for aircraft structural steel is molybdenum. It reduces the grain size of steel and increases both its impact strength and elastic limit. Molybdenum steels are extremely wear resistant and possess a great deal of fatigue strength and it’s used in high-strength structural members and engine cylinder barrels.. g in. Chrome-molybdenum (chrome-moly) steel is the most commonly used in aircraft. Its Society of Automotive Engineers (SAE) designation of 4130 denotes an alloy of approximately 1 per cent molybdenum and 0.30 percent carbon. It machines readily, is easily welded by either gas or electric arc, and responds well to heat treatment.. n i ra. Heat- treated SAE 4130 steel has an ultimate tensile strength about four times that of SAE 1025 steel, making for landing gear structure and engine mounts.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 21.

(26) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Heat Treatment of Steel Iron is an allotropic metal, meaning it can exist in more than one type of lattice structure, depending on temperature. Pure molten iron begins to solidify at 2,800 º F. Its structure at this point is known as the Delta form. If cooled to 2,554 º F, the atoms rearrange themselves into a Gamma form. Iron in this form is nonmagnetic. When nonmagnetic gamma iron in this form is cooled to 1,666 º F, another change occurs and the iron is transformed into a nonmagnetic form of Alpha structure.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Ferrite. g in. n i ra. There are two basic forms of steel when it comes to heat treatment. They are ferrite and austenite.. Austenite. Ferrite is an alpha solid solution of iron containing some carbon and exists at temperature below the lower critical temperature. Above this lower critical temperature, the steel begins to turn into austenite, which consists of gamma iron containing carbon. As the temperature increases the transformation of ferrite into austenite until the upper critical temperature is reached.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 22.

(27) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Below the alloy’s lower critical temperature, the carbon which exists in the steel in the form of iron carbides is scattered throughout the iron matrix as a physical mixture. When the steel is heated to its upper critical temperature, this carbon dissolves into matrix as a physical mixture.. g in. Heat Treatment. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Heat treatment is a series of operations involving the heating and cooling of metal in the solid state. Its purpose is to make the metal more useful, serviceable, and safe for a definite purpose. By heat treating a metal can be made harder, stronger and more resistant to impact. Heat treating can also make a metal softer and more ductile. All heat-treating processes are similar in that they involve the heating and cooling of metals. They differ however in the temperatures to which the metal is heated and the rate at which it is cooled. A pure metal cannot be hardened by heat treatment because there is little change in its structure when heated.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 23.

(28) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Heat –Treating Equipment. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Successful heat treating requires close control over all factors affecting the heating and cooling of metals. The furnace must be of the proper size and type and must be so controlled that temperature are kept within prescribed for each operation. Even the atmosphere within the furnace affects the condition of the part being heat–treated. The quenching equipment and the quenching medium must be selected to fit the metal and the heat–treating operation. There are many different types and sizes of furnaces used in heat treatment. Furnaces are designed to operate in certain specific temperature ranges and attempted use in other rangers frequently results in work of inferior quality. Furnaces heated by electricity the heating elements are generally in the form of wire or ribbon. Such furnaces commonly operate up to a maximum temperature of about 2000 º F.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 24.

(29) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Heating The object in heating is to transform parasite (a mechanical mixture of iron carbide that exists in a finely mixed condition) to austenite as the steel is hated through the critical range. Steel begins to appear dull red at about 1000 º F and as the temperature increases the colour changes gradually through various shades of red to orange, to yellow and finally to white.. g in. Soaking. n i ra. The temperature of the furnace must be held constant during the soaking period, since it is during this period that rearrangement of the internal structure of the steel takes place. The length of the soaking period depends upon the type of steel and the size of the part. As a general rule, a soaking period of 30 minutes to 1 hour is sufficient for the average heat-treating operation.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 25.

(30) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Cooling Various rates of cooling are used to produce the desired results, still air is a slow cooling medium, but is much faster than furnace cooling. Liquids are the fastest cooling media and therefore used in hardening steels. There are three commonly used quenching liquids brine, water and oil. Brine is the most severe medium, water is next and oil is the least severe. Generally an oil quench is used for alloy steels and brine or water for carbon steels.. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Portable Quench Tank. Quenching Media. Quenching solutions act only through their ability to cool the steel. Most requirements for quenching media are met satisfactorily by water. The rate of cooling is relatively rapid during quenching in brine, somewhat less rapid in water and slow in oil. Brine usually is made of a 5 to 10 percent solution of salt (sodium chloride) in water. In addition to its greater cooling speed, brine has the ability to “throw” the scale from steel during quenching.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 26.

(31) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). STEEL APPLICATIONS General The base material iron is a chemical element which, in its pure form, is a very soft, malleable and ductile metal which is easy to form and shape. It readily combines with oxygen to form iron oxide (rust), and so is alloyed, primarily with carbon, but also with other elements. When molten iron is alloyed with more than 2% Carbon and poured into a mould, cast iron is formed. Cast iron has limited uses in the aviation industry due to low strength to weight ratio and brittleness.. g in. n i ra. Iron is extracted from iron ore by mixing it with coke and limestone and heating it in a furnace. The process extracts the oxygen from the ore, and allows the iron to sink to the bottom of the furnace. The limestone reacts with any impurities in the molten iron and floats to the surface to form a slag.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. To make steel, the pure iron is remelted in a special furnace where carbon is introduced along with other alloying elements to achieve the desired characteristics. Description. Steel is an excellent engineering material with many applications. For aircraft use, however, it does have some significant problems. The main restrictions are its high density (approximately 3 times the density of aluminium) and its susceptability to corrosion. The corrosion of steel can be reduced by the addition of certain alloying elements, but this can have significant effects on properties and costs. Between 9 and 16% (Airbus A320: 9% , Boeing B777: 11%) of an aircraft’s structure is alloy steel and stainless steel. The high strength and high modulus of elasticity are the primary advantages of the high-strength steels. This is useful for designs with space limitations such as with some landing gear components. Alloy selection considerations include service temperature, strength, stiffness fatigue properties and fabricability.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 27.

(32) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Steel Application. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 28.

(33) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). TESTING OF MATERIALS The mechanical properties of a material must be known before that material can be incorporated into any design. Mechanical property data is compiled from extensive material testing. Various tests are used to determine the actual values of material properties under different loading applications and test conditions.. g in. Tensile Testing Tensile testing is the most widely-used mechanical test. It involves applying a steadily increasing load to a test specimen, causing it to stretch until it eventually fractures. Accurate measurements are taken of the load and extension, and the results are used to determine the strength of the material. To ensure uniformity of test results, the test specimens used must conform to standard dimensions and finish as laid down by the appropriate Standards Authority (BSI, DIN, ISO etc).. n i ra. The cross-section of the specimen may be round or rectangular, but the relationship between the cross-sectional area and a specified "gauge length", of each specimen, is constant. The gauge length, is that portion of the parallel part of the specimen, which is to be used for measuring the subsequent extension during and/or after the test.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Tensile Strength Tensile strength in a material is obtained by measuring the maximum load, which the test piece is able to sustain, and dividing that figure by the original cross-sectional area (c.s.a.) of the specimen. The value derived from this simple calculation is called STRESS.. Stress . Load (N) Original c.s.a. (mm 2 ). Note: The units of Stress may be quoted in the old British Imperial (and American) units of lbf/in2, tonf/in2 (also psi and tsi), or the European and SI units such as kN/m2, MN/m2 and kPa or MPa.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 29.

(34) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Example 1 A steel rod, with a diameter of 5 mm, is loaded in tension with a force of 400 N. Calculate the tensile stress.. Stress . Load Area. 400 400   20  37 N / mm 2 2 2 r   25. Exercise 1 Calculate the tensile stress in a steel rod, with a cross-section of 10 mm x 4 mm, when it is subjected to a load of 100 N.. g in. n i ra. Example 2 A structural member, with a cross-sectional area of 05m2, is subjected to a load of 10 MN. Calculate the stress in the member in; (a) MN/m2 and (b) N/mm2 (a). (b). f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Stress . Load Area. 10  20MN / m 2 05. 1N/mm  1 MN/m 2. 2. So Stress  20 N/mm. 2. As the load in the tensile test is increased from zero to a maximum value, the material extends in length. The amount of extension, produced by a given load, allows the amount of induced STRAIN to be calculated. Strain is calculated by measuring the extension and dividing by the original length of the material. Note: Both measurements must be in the same units, though, since Strain is a ratio of two lengths, it has no units.. Strain . Extension Original Length. Exercise 2 Calculate the cross-sectional area of a tie rod which, when subjected to a load of 2,100N, has a stress of 60 N/mm2.. Note: When calculating stress in large structural members, it may be more convenient to measure load in Mega-Newtons (MN, or N6) and the area in square metres (m2). When using such units, the numerical value is identical to that if the calculation had been made using Newtons and mm2. i.e. A Stress of 1 N/mm2 = l MN/m2. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 30.

(35) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Example 3 An aluminium test piece is marked with a 20 mm gauge length. It is subjected to tensile load until its length becomes 2115 mm. Calculate the induced strain.. Extension  21 15 - 20  1  15 mm Strain . g in. Extension 1  15   0  0575 (no units) Original Length 20. n i ra. Exercise 3 A tie rod 1.5m long under a tensile load of 500 N extends by 12 mm. Calculate the strain.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 31.

(36) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Load/Extension Diagrams If a gradually increasing tensile load is applied to a test piece while the load and extension are continuously measured, the results can be used to produce a Load/Extension diagram or graph (refer to Fig. 1). Obviously a number of different forms of graph may be obtained, depending on the material type and condition, but the example shows a Load/Extension diagram which typifies many metallic materials when stressed in tension. Load/Extension Diagram. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Fig 1. The graph can be considered as comprising two major regions. Between points 0 and A, the material is in the Elastic region (or phase), i.e. when the load is removed the material will return to its original size and shape. In this region, the extension is directly proportional to the applied load. This relationship is known as ‘Hooke's Law’, which states:. Within the elastic region, elastic strain is directly proportional to the stress causing it.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 32.

(37) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Point A is the Elastic Limit. Between this point and point B, the material continues to extend until the maximum load is reached (at point B). In this region the material is in the plastic phase. When the load is removed, the material does not return to its original size and shape, but will retain some extension. After point B, the cross-sectional area reduces and the material begins to ‘neck’. The material continues to extend under reduced load until it eventually fractures at point C.. g in. Aircraft structural designers’ interest in materials does not extend greatly beyond the elastic phase of materials. Production engineers, however, are greatly interested in material properties beyond this phase, since the forming capabilities of materials are dependent on their properties in the plastic phase.. n i ra. An examination of a graph, obtained from the results of a tensile test on mild steel (refer to Fig. 2), shows that considerable plastic extension occurs without any increase in load shortly after the elastic limit is reached. The onset of increasing extension, without a corresponding increase in load, at point `B', is known as the ‘yield point’ and, if this level of stress is reached, the metal is said to have ‘yielded’. This is a characteristic of mild steel and a few other, relatively ductile, materials.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 33.

(38) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). UTS. Point B Yield Point. g in. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. n i ra. Load/Extension Diagram for Mild Steel Fig. 2. If, after passing the yield point, the load is further increased, it may be seen that mild steel is capable of withstanding this increase until the Ultimate Tensile Stress (UTS) is reached. Severe necking then occurs and the material will fracture at a reduced load. The unexpected ability of mild steel to accept more load after yielding is due to strain-hardening of the material. Work-hardening of many materials is often carried out to increase their strength.. As previously stated, various forms of load/extension curves may be constructed for other materials (refer to Fig 3), and their slopes will depend on whether the materials are brittle, elastic or plastic.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 34.

(39) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Point of Fracture. g in. Plastic Region. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Small Elongation. Zero Elongation. Large Elongation. (a). (b). (c). Load/Extension Graphs for Brittle, Elastic and Plastic Materials Fig. 3. (a) represents a brittle material (glass). (b) represents a material with some elasticity and limited plasticity (high-carbon steel. (c) represents a material with some elasticity and good plasticity (e.g. soft aluminium).. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 35.

(40) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Ductility After fracture of a specimen, following tensile testing, an indication of material ductility is arrived at, by establishing the amount of plastic deformation which occurred. The two indicators of ductility are:  Elongation. g in.  Reduction in area (at the neck). n i ra. Elongation is the more reliable, because it is easier to measure the extension of the gauge length than the reduction in area. The standard measure of ductility is to establish the percentage elongation after fracture.. Final Extension  100 Original Gauge Length. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Percentage elongation. Example 4 In a tensile test, on a specimen with 150.5 mm gauge length, the length over the gauge marks at fracture were 176.1 mm. What was the percentage elongation?. Elongation . Final Extension 176.1 - 150.5  100   100  17.009%  17% Gauge Length 150  5. Proof Stress Many materials do not exhibit a yield point, so a substitute value must be employed. The value chosen is the ‘Proof Stress’, which is defined as: The tensile stress, which is just sufficient to produce a non-proportional elongation, equal to a specified percentage of the original gauge length.. Usually a value of 0.1% or 0.2% is used for Proof Stress, and the Proof Stress is then referred to as the 0.1% Proof Stress or the 0.2% Proof Stress respectively. The Proof Stress may be acquired from the relevant Load/Extension graph (refer to Fig 4) as follows:. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 36.

(41) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). If the 0.2% Proof Stress is required, then 0.2% of the gauge length is marked on the extension axis. A line, parallel to the straight-line portion of the graph, is drawn until it intersects the non-linear portion of the curve. The corresponding load is then read from the graph. Proof Stress is calculated by dividing this load by the original cross-sectional area. 0.1% Proof Stress will produce permanent set equivalent to one thousandth of the specimen's original length.. g in. 0.2% Proof Stress will produce permanent set equivalent to one five hundredth of the original length.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Acquiring the Proof Stress from a Load/Extension Graph Fig. 4. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 37.

(42) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Stiffness Within the elastic range of a material, if the Strain is compared to the Stress causing that extension, it will provide a measure of stiffness/rigidity or flexibility.. Stress is a measure of stiffness Strain. ie .. g in. This value, which is of great importance to designers, is known as ‘the Modulus of Elasticity, or Young’s Modulus’, and is signified by use of the symbol E. Thus E = Stress divided by Strain and, since Strain has no units, the unit for `E' is the same as Stress. i.e. lbf/in2, tonf/in2 (also psi and tsi), or the European and SI units such as kN/m2, MN/m2 and kPa or MPa.. n i ra. The actual numerical value is usually large, as it is a measure of the stress required to theoretically double the length of a specimen (if it did not break first).. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. A typical value of E for steel would be 30 x 106 psi. or 210,000 MN/m2. Relative stiffness values for some common materials (using Rubber as a datum), are:  Wood. 2000 x.  Aluminium. 10,000 x.  Steel. 30,000 x.  Diamond. 171,000 x. Tensile Testing of Plastics This is conducted in the same way as for metals, but the test piece is usually made from sheet material. Although the basic load/extension curve for some plastics is somewhat similar to metal curves, changes in test temperature or the rate of loading can have a major effect on the actual results. Even though the material under test may be in the elastic range, the specimen may take some time to return to its original size after the load is removed.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 38.

(43) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Compression Test Machines for compression testing are often the same as those used for tensile testing, but the test specimen is in the form of a short cylinder. The Load/Deflection graph in the elastic phase for ductile materials is similar to that in the tensile test. The value of `E' is the same in compression as it is in tension. Compression testing is seldom used as an acceptance test for metallic or plastic materials (except for cast iron).. g in. Compression testing is generally restricted to building materials and research into the properties of new materials.. n i ra. Hardness Testing The hardness of materials is found by measuring their resistance to indentation. Various methods are used, but the most common are those of the Brinell, Vickers and Rockwell Hardness Tests.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 1. Brinell Test. In the Brinell Hardness Test (refer to Fig. 5), a hardened steel ball is forced into the surface of a prepared specimen, using a calibrated force, for a specified time. The diameter of the resulting indentation is then measured accurately, using a graduated microscope and, thus, the area of the indentation is calculated. The hardness number is determined by reference to a Brinell Hardness Number (BHN) chart.. Diameter (Area) of resulting Indentation. The Brinell Hardness Test Fig. 5. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 39.

(44) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). 2. Vickers Test The Vickers Hardness Test is similar to the Brinell test but uses a square-based diamond pyramid indenter (refer to Fig. 6). The diagonals, of the indentation, are accurately measured, by a special microscope, and the Hardness Value (HV) is again determined by reference to a chart.. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M The Vickers Hardness Test Fig. 6. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 40.

(45) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). 3. Rockwell Test The Rockwell Hardness Test (refer to Fig. 7) also uses indentation as its basis, but two types of indenter are used. A conical diamond indenter is employed for hard materials and a steel ball is used for soft materials. The hardness number, when using the steel ball, is referred to as Rockwell B (e.g. RB 80) and the diamond hardness number is known as Rockwell C (e.g. RC 65).. g in. Note: Whereas Brinell and Vickers hardness values are based upon the area of indentation, the Rockwell values are based upon the depth of the indentation.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M The Rockwell Hardness Test Fig. 7. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 41.

(46) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). No precise relationship exists between the various hardness numbers, but approximate relationships have been compiled. Some comparative values between Brinell Vickers and Rockwell are shown in Table 1. Table 1 COMPARATIVE HARDNESS VALUES MATERIAL Aluminium alloy Mild steel Cutting tools Note:. BHN 100 130 650. HV 100 130 697. ROCKWELL B 57 B 73 C 60. g in. n i ra. There is a good correlation between hardness and U.T.S. on some materials (e.g. steels). f T o g y n r i r a t e e e i n r i p g o n r E P S A M. 4. Hardness Testing on Aircraft It is not normal to use Brinell, Rockwell or Vickers testing methods on aircraft in the hangar. There are, however, portable Hardness Testers, which may be used to test for material hardness on items such as aircraft wheels, after an over-heat condition, because the over-heat condition may cause the wheel material to become soft or partially annealed.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 42.

(47) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Impact Testing The impact test (refer to Fig. 8) is designed to determine the toughness of a material and the two most commonly used methods are those using the ‘Charpy’ and ‘Izod’ impact-testing machines.. g in. Both tests use notched-bar test pieces of standard dimensions, which are struck by a fast-moving, weighted pendulum. The energy, which is absorbed by the test piece on impact, will give a measure of toughness. A brittle material will break easily and will absorb little energy, so the swing of the pendulum (which is recorded against a calibrated scale) will not be reduced significantly. A tough material will, however, absorb considerably more energy and thus greatly reduce the recorded pendulum swing.. n i ra. Most materials show a drop in toughness with a reduction in temperature, though some materials (certain steels in particular) show a rapid drop in toughness as the temperature is progressively reduced. This temperature range is called the Transition Zone, and components, which are designed for use at low temperature, should be operated above the material’s Transition Temperature.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Nickel is one of the most effective alloying elements for lowering the Transition Temperature of steels. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 43.

(48) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Test Piece. Impact Test Fig. 8. .. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 44.

(49) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Other Forms of Material Testing Although some of the more important forms of material testing have already been discussed, there are several other forms of material testing to be considered, not least important of which are those associated with Creep and Fatigue Testing.. g in. a. Creep Creep can be defined as the continuing deformation, with the passage of time, of materials subjected to prolonged stress. This deformation is plastic and occurs even though the acting stress may be well below the yield stress of the material.. n i ra. At temperatures below 0.4T (where T is the melting point of the material in Kelvin), the creep rate is very low, but, at higher temperatures, it becomes more rapid. For this reason, creep is commonly regarded as being a high-temperature phenomenon, associated with super-heated steam plant and gas turbine technology. However, some of the soft, low-melting point materials will creep significantly at, or a little above, ambient temperatures and some aircraft materials may creep when subjected to over-heat conditions.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. b. Creep in Metals When a metallic material is suitably stressed, it undergoes immediate elastic deformation. This is then followed by plastic strain, which occurs in three stages (refer to Fig. 9):  Primary Creep - begins at a relatively rapid rate, but then decreases with time as strain-hardening sets in.  Secondary Creep - the rate of strain is fairly uniform and at its lowest value..  Tertiary Creep - the rate of strain increases rapidly, finally leading to rupture. This final stage coincides with gross necking of the component, prior to failure. The rate of creep is at a maximum in this phase.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 45.

(50) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Three Stages of Creep Fig. 9. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 46.

(51) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). c.. Effect of Stress and Temperature on Creep. Both stress and temperature have an effect on creep. At low temperature or very low stress, primary creep may occur, but this falls to a negligible value in the secondary stage, due to strain-hardening of the material. At higher stress and/or temperature, however, the rate of secondary creep will increase and lead to tertiary creep and inevitable failure.. g in. It is clear, from the foregoing, that short-time tensile tests do not give reliable information for the design of structures, which must carry static loads over long periods of time, at elevated temperatures. Strength data, determined from long- time creep tests (up to 10,000 hours), are therefore essential.. n i ra. Although actual design data are based on the long-time tests, short-time creep tests are sometimes used as acceptance tests.. d. The Effect of Grain Size on Creep Since the creep mechanism is partly due to microscopic flow along the grain boundaries, creep resistance is improved by increased grain size, due to the reduced grain boundary region per unit volume. It is mainly for this reason that some modern, high-performance turbine blades are being made from directionally solidified (and, alternatively, improved single-crystal) castings.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. e. Creep in Plastics Plastics are also affected by creep and show similar, though not identical, behaviour to that described for metals. Since most plastics possess lower thermal properties than metals, the choice of plastic for important applications, particularly at elevated temperature, must take creep considerations into account. f. Fatigue An in-depth survey, in recent years, revealed that over 80 percent of failures of engineering components were caused by fatigue. Consequences of modern engineering have led to increases in operating stresses, temperatures and speeds. This is particularly so in aerospace and, in many instances, has made the fatigue characteristics of materials more significant than their ordinary, static strength properties. Engineers became aware that alternating stresses, of quite small amplitude, could cause failure in components, which were capable of safely carrying much greater, steady loads. This phenomenon of small, alternating loads causing failure was likened to a progressive weakening of the material, over a period of time and hence the attribution of the term ‘fatigue’. Very few constructional members are immune from it, and especially those operating in a dynamic environment. Experience in the aircraft industry has shown that the stress cycles, to which aircraft are subjected, may be very complex, with occasional high peaks, due to gust loading of aircraft wings. For satisfactory correlation with in-service behaviour, full-size or large-scale mock-ups must be tested in conditions as close as possible to those existing in service.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 47.

(52) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g. Fatigue Testing An experiment, conducted in 1861, found that a wrought iron girder, which could safely sustain a mass of 12 tons, broke when a mass of only 3 tons was raised and lowered on the girder some 3x106 times.. g in. It was also found that there was some mass, below 3 tons, which could be raised and lowered on to the beam, a colossal number (infinite) of times, without causing any problem.. n i ra. Some years later, a German engineer (Wohler), did work in this direction and eventually developed a useful fatigue-testing machine which bears his name and continues to be used in industry. The machine uses a test piece, which is rotated in a chuck and a force is applied at the free end, at right angles to the axis of rotation (refer to Fig. 10). The rotation thus produces a reversal of stress for every revolution of the test piece.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Various other types of fatigue testing are also used e.g. cyclic-torsional, tension-compression etc. Exhaustive fatigue testing, with various materials, has resulted in a better understanding of the fatigue phenomenon and its implications from an engineering viewpoint.. Test Piece made to vibrate or oscillate against load (Stress Cycles).. Test Piece. Load. Simple Fatigue Testing Fig 10. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 48.

(53) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). S-N Curves One of the most useful end-products, from fatigue testing, is an S-N curve, which shows, graphically, the relationship between the amount of stress (S), applied to a material, and the number of stress cycles (N), which can be tolerated before failure of the material.. g in. Using a typical S-N curve, for a steel material (refer to Fig. 11), it can be seen that, if the stress is reduced, the steel will endure a greater number of stress cycles. The graph also shows that a point is eventually reached where the curve becomes virtually horizontal, thus indicating that the material will endure an infinite number of cycles at a particular stress level.. n i ra. This limiting stress is called the ‘Fatigue Limit’ and, for steels, the fatigue limit is generally in the region of 40% to 60% of the value of the static, ultimate tensile strength (U.T.S.). f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 49.

(54) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Stress (S). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Fatigue Limit. 40 – 60 % UTS. Number of Cycles (N). A S-N Curve for a Steel Material Fig. 11. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 50.

(55) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Many non-ferrous metals, however, show a different characteristic from steel (refer to Fig. 12). In this instance there is no fatigue limit as such and it can be seen that these materials will fail if subjected to an appropriate number of stress reversals, even at very small stresses. When materials have no fatigue limit an endurance limit together with a corresponding number of cycles is quoted instead. It follows that components made from such materials must be designed with a specific life in mind and removed from service at the appropriate time. The service fatigue lives of complete airframes or airframe members are typical examples of this philosophy.. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M An S-N Curve for an Aluminium Alloy Fig. 12. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 51.

(56) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Non-metallic materials are also liable to failure by fatigue. As is the case with metals, the number of stress cycles, required to produce a fatigue failure, will increase as the maximum stress in the loading cycle decreases. There is, however, generally no fatigue limit for these materials and some form of endurance limit must be applied. The importance of fatigue strength can be illustrated by the fact that, in a high- cycle fatigue mode, a mere 10% improvement in fatigue strength can result in a 100-times life improvement.. g in. n i ra. Causes of Fatigue Failure As the fatigue characteristics of most materials are now known (or can be ascertained), it would seem reasonable to suppose that fatigue failure, due to lack of suitable allowances in design, should not occur.. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Nevertheless, fatigue cracking occurs frequently, and even the most sophisticated engineering product does not possess immunity from this mode of failure. Such failures are often due to unforeseen factors in design, environmental or operating conditions, material, and manufacturing processes. Two essential requirements for fatigue development in a material are: . An applied stress fluctuation of sufficient magnitude (with or without an applied steady stress).. . A sufficient number of cycles of that fluctuating stress.. The stress fluctuations may be separated by considerable time intervals, as experienced in aircraft cabin pressurisation, during each take-off (e.g. daily), or they may have a relatively short time interval, such as encountered during the aerodynamic buffeting/vibration of a wing panel. The former example would be considered to be low-cycle fatigue and the latter to be high-cycle fatigue. In practice, the level of the fluctuating stress, and the number of cycles to cause cracking of a given material, are affected by many other variables, such as stress concentration points (stress raisers), residual internal stresses, corrosion, surface finish, material imperfections etc.. Vibration Vibration has already been quoted as being a cause of high-cycle fatigue and, because most dynamic structures are subjected to vibration, this is undoubtedly the most common origin of fatigue. All objects have their own natural frequency at which they will freely vibrate (the resonant frequency). Large, heavy, flexible components vibrate at a low frequency, while small, light, stiff components vibrate at a high frequency.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 52.

(57) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Resonant frequencies are undesirable (and in some cases could be disastrous), so it is important to ensure that, over their normal operating ranges, critical components are not vibrated at their natural frequencies and so avoid creating resonance. The resonant frequency, of a component, is governed by its mass and stiffness and, on certain critical parts, it is often necessary to do full-scale fatigue tests to confirm adequate fatigue life before putting the product into service.. g in. Fatigue Metallurgy Under the action of fatigue stresses, minute, local, plastic deformation on an atomic scale, takes place along slip planes within the material grains. If the fatigue stresses are continued, then micro cracks are formed within the grains, in the area of the highest local stress, (usually at or near the surface of the material). The micro cracks join together and propagate across the grain boundaries but not along them. A fatigue fracture generally develops in three stages (refer to Fig. 13):   . f T o g y n r i r a t e e e i n r i p g o n r E P S A M. n i ra. Nucleation Propagation (crack growth) Ultimate (rapid) fracture.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 53.

(58) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Nucleation. Propagation (crack growth). Ultimate (rapid) fracture. The Three Stages of Fracture Fig. 13. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 54.

(59) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). The resultant fractured surface often has a characteristic appearance of:  An area, on which a series of curved, parallel, relatively smooth ridges are present and are centred around the starting point of the crack. These ridges are sometimes called conchoidal lines or beach marks or arrest lines.  A rougher, typically crystalline section, which is the final rapid fracture when the cross-section is no longer capable of carrying its normal, steady load.. g in. The arrest lines are, normally, formed when the loading is changed, or the loading is intermittent. However, in addition to these characteristic and informative marks, there are similar, but much finer lines (called ‘striations’), which literally show the position of the crack front after each cycle. These striations are obviously of great importance to metallurgists and failure investigators when attempting to estimate the crack initiation and/or propagation life. The striations are often so fine and indistinct that electron beam microscopes are required to count them.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. In normal circumstances, a great deal of energy is required to `weaken' the material sufficiently to initiate a fatigue crack, and it is not surprising, therefore, to find that the nucleation phase takes a relatively long time. However, once the initial crack is formed, the extremely high stress concentration (present at the crack front) is sufficient to cause the crack to propagate relatively quickly, and gaining in speed as the crack front not only increases in size, but also reduces the component cross-sectional area. A point is eventually reached (known as the 'critical crack length') at which the remaining cross-section is sufficiently reduced to cause a gross overloading situation, and a sudden fracture finally occurs. It is not unusual for the crack initiation phase to take 90% of the time to failure, with the propagation phase only taking the remaining 10%. This is one of the major reasons for operators of equipment being relatively unsuccessful in detecting fatigue cracks in components before a failure occurs. Fatigue Promoters. As fatigue cracks initiate at locations of highest stress and lowest local strength, the nucleation site will be:  dictated largely by geometry and the general stress distribution  located at or near the surface or.  centred on surface defects/imperfections, such as scratches, pits, inclusions, dislocations and the like. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 55.

(60) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). a. Design Apart from general stressing, the geometry of a component has a considerable influence on its susceptibility to fatigue. A good designer will therefore minimise stress concentrations by:  avoiding rapid changes in section and. g in.  using generous blend radii or chamfers to eliminate sharp corners. b. Manufacture While the designer may specify adequate blend radii, the actual product may still be prone to fatigue failure if the manufacturing stage fails to achieve this sometimes-seemingly unimportant drawing requirement.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. Several other manufacturing-related causes of premature fatigue failure exist, the most common of which are:  Inherent material faults: e.g. cold shuts, pipe, porosity, slag inclusions etc.  Processing faults: e.g. bending, forging, grinding, shrinking, welding, etc..  Production faults: e.g. incorrect heat-treatment, inadequate surface protection, poor drilling procedures, undue force used during assembly, etc  In-service damage: e.g. dents, impact marks, scratches, scores, tooling marks etc.. c. Environment One of the most potent environmental promoters of fatigue occurs when the component is operating in a corrosive medium. Steel (normally), has a welldefined fatigue limit on the S-N curve but, if a fatigue test is conducted in a corrosive environment, not only does the general fatigue strength drop appreciably, but the curve also resembles the aluminium alloy curve (e.g. the fatigue failure stress continues to fall as the number of cycles increases). Other environmental effects such as fretting and corrosion pitting, erosion or elevated temperatures will also adversely affect fatigue strength.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 56.

(61) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Fatigue Preventers If a component is prone to fatigue failure in service, then several methods of improvement are available, in the form of:  Quality.. Correct and eliminate any failure-related manufacturing or processing shortcomings..  Material.. Select a material with a significantly better fatigue strength, or corrosion-resistance or corrosion-protection if relevant.. g in.  Geometry.. a) Increase the size (c.s.a.) to reduce the general stress level or modify the local geometry to reduce the change in section (large radius).. n i ra. b) Modify the geometry to change the vibration frequency or introduce a damping feature, to reduce the vibration amplitudes. c) Improve the surface finish or put a compressive stress in the skin (e.g. shot peen or cold expand).. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. a. Cold Expansion (Broaching) Most fatigue failures occur whilst a material is subject to a tensile, alternating stress. If the most fatigue-prone areas, such as spar fastener holes, have a compression stress applied (refer to Fig. 14), they are significantly more resistant to fatigue failure. The fastener hole is initially checked for defects (using, usually, an Eddy Current NDT procedure) and the surface finish is further improved by reaming (and checked once again). A tapered mandrel is then pulled through the hole, resulting in a localised area of residual (compressive) stress which will provide a neutral or, at least, a significantly reduced level of fatigue in the area around the fastener hole. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 57.

(62) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M Area around hole pre-stressed in compression. Tapered Mandrel pulled through fastener hole. Cold Expansion of Fastener Hole Fig.14. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 58.

(63) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). Do's and Dont's – Preventing Fatigue Failures DO  Be careful not to damage the surface finish of a component by mishandling.  Use the right tools for assembling press-fit components etc.  Maintain drawing sizes and tolerances.  Keep the correct procedures (e.g. don't overheat when welding).  Avoid contact or near contact of components that might cause fretting when touching. DON'T  Leave off protective coverings - plastic end caps etc.  Score the surface.  Leave sharp corners or ragged holes.  Force parts unnecessarily to make them fit.  Work metal unless it is in the correct heat-treated state.. g in. f T o g y n r i r a t e e e i n r i p g o n r E P S A M. n i ra. STRUCTURAL HEALTH MONITORING (SHM) Obviously it is extremely important, that the level of fatigue, imposed on an aircraft structure (and associated components), be monitored and recorded so that the respective fatigue lives are not exceeded. Several methods have been developed to assist in the vital tasks involved with SHM. a. Fatigue Meters Fatigue meters are used to check overall stress levels on aircraft and to monitor the fatigue history of the aircraft. Fatigue meters also allow a check to be made on the moment in time when the aircraft exceeds the design limits imposed on it.. b. Strain Gauges Strain gauges may be used to monitor stress levels on specific aircraft structures. Strain gauges are thin-foil, electrical, resistor elements, bonded to the aircraft structure. Their resistance varies proportional to the applied stress loading.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 59.

(64) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2). c. Fatigue Fuses Fatigue fuses are metallic fuses, which are bonded to the structure and which fail at different fatigue stresses. The electrical current, flowing through the fuse, will vary and thus, provide an indication of the stress level.. g in. d. Intelligent Skins Development Modern developments in aircraft structures will allow the structures to be designed and built with a variety of sensors and systems embedded into the structure and skin. This would mainly be restricted to structures manufactured from composite materials. One major benefit of this is to allow the structure to monitor it's own loads and fatigue life.. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M i. Smart Structures The generic heading ‘Smart Structures’ actually covers three areas of development:. . Smart Structures. These are structures, which have sensors, actuators, signal-processing and adaptive control systems built in. . Smart Skins. These have radar and communications antennae embedded in, or beneath, the structural skin. . Intelligent Skins. Skin embedded with fibre optic sensors. Smart Structures perceived benefits include:. . Self-diagnostic in the monitoring of structural integrity. . Reduced life cycle costs. . Reduced inspection costs. . Potential weight saving/performance improvements derived from increased knowledge of composite material characteristics. . From a military point of view – an improvement in ‘Stealth’ characteristics.. A fully monitored and self-diagnostic system could: . Assess structural integrity.. . Pinpoint structural damage.. For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 60.

(65) DCAM PART 66 CAT B1.1 MODULE 6 MATERIALS AND HARDWARE AIRCRAFT MATERIALS - FERROUS (DCAM 6.1 L1 & L2) . Process flight history.. Composite laminates, containing embedded fibre optic sensors can be used for SHM, including fatigue monitoring and flight envelope exceedance monitoring and their advantages include:  Cover a greater area of structure   Not prone to electrical interference Less vulnerable to damage when embedded in the plies Increased knowledge of structural loads aids designers. g in. n i ra. f T o g y n r i r a t e e e i n r i p g o n r E P S A M For Training Purposes Only. Issue 1 Revision 0 Jan 2011. Page 61.

No results found