Mechanics of aircraft materials 1 cdio

Mechanics of Aircraft Materials

Department of Aerospace Engineering – Faculty of Transportation Engineering

Properties of Aircraft MetalsFerrous Aircraft Metals

Heat Treatment of Ferrous Metals

Dr Ly Hung Anh

Assoc Prof Dr Ly Hung Anh

• Aircraft structures carry loads on the aircraft • Structural failure can be catastrophic

• To prevent structural failure, aircraft structures have to fulfill a number of requirements, e.g :

 accurate outer shape and dimension  aerodynamics  rigid and strong

 reasonable production cost  reasonable maintenance cost  long design life

Background

There are various alternative to fulfill the requirements e.g : 1 Structure is assembled from

many simple components

limited number of complex components 2 Half products used : plate

extrusion casting 3 Materials : metals

4 Manufacturing process : cutting, forming, or chemical milling 5 Joining method : rivet, adhesive bonded, welding

6 Durability measures : corrosion, fatigue, inspection

Background

Density is defined as mass per unit volume (g/cm3)Density of some metallic materials

• Strength is the ability of a material to resist deformation,

• Strength is also the ability of a material to resist stress without breaking.

The type of load or stress on the material affects the strength it exhibits.

1 Properties of Metals

• Elastic deformation: return to original position after load is released • Elastic deformation is proportional to the load (Hooke)  E (Young

modulus) is the proportional constant

• Young modulus of some metallic materials :

• E is obtained from tensile test

1 Properties of Metals

Ductility (Plastic behavior)

• Plastic deformation: deformation that remains after load is released • Ductility is the property of a metal which permits it to be

permanently drawn, bent, or twisted into various shapes without breaking.

• Plastic deformation has to be avoided in structures

• High stress concentration areas can cause plastic deformation • Plastic behavior can be obtained from tensile test

• Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads.

• Chrome molybdenum steel is also easily formed into desired shapes • Ductility is similar to malleability.

1 Properties of Metals

• Hardness refers to the ability of a material to resist abrasion, penetration, cutting action, or permanent distortion.

• Hardness may be increased by cold working the metal and, in the case of steel and certain aluminum alloys, by heat treatment.

1 Properties of Metals

• Capacity of a metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effects.

• This property is necessary in sheet metal that is worked into curved shapes, s such as cowlings, fairings, or wingtips.

• Copper is an example of a malleable metal.

1 Properties of Metals

• Ability of a material to withstand tearing or shearing and be stretched or otherwise deformed without breaking.

• Toughness is a desirable property in aircraft

1 Properties of Metals

• The property of a metal which allows little bending or deformation without shattering.

• A brittle metal is apt to break or crack without change of shape.

• Brittleness is not a very desirable property.

• Cast iron, cast aluminum, and very hard steel are examples of brittle metals.

1 Properties of Metals

• The ability of a metal to become liquid by the application of heat.

• Metals are fused in welding • Steels fuse around 2,600 °F

• Aluminum alloys fuse at approximately 1,100 °F.

1 Properties of Metals

• The property which enables a metal to carry heat or electricity.

• The heat conductivity of a metal is especially important in welding because it governs the amount of heat that will be required for proper fusion.

• Conductivity of the metal determines the type of jig to be used to control expansion and contraction.

• In aircraft, electrical conductivity must also be considered in conjunction with bonding, to eliminate radio interference.

1 Properties of Metals

Thermal Expansion

• Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling.

• Heat applied to a metal will cause it to expand or become larger.

• Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material.

1 Properties of Metals

Four requirements for selecting substitute metals:

1 Maintaining the original strength of the structure 2 Maintaining contour or aerodynamic smoothness

3 Maintaining original weight, if possible, or keeping added weight to a minimum

4 Maintaining the original corrosion resistant properties of the metal

1 Properties of Metals

Common chemical symbols and metallurgical symbols

2 Ferrous Aircraft MetalsIron

carbon steel = iron + carbon (1%)

alloy steels = carbon steel + other elements

A base metal (such as iron) to which small quantities of other metals have been added is called an alloy

2 Ferrous Aircraft Metals

The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) use a numerical index to identify the

chemical compositions of the structural steels.

 a four-numeral series is used to designate the plain carbon and alloy steels;  a five numerals are used to designate certain types of alloy steels.

 The first two digits indicate the type of steel

 The second digit generally gives the approximate amount of the major

(copper 0.35%); (nickel 0.25%); (chromium 0.20%); (molybdenum 0.06%).

Steel and Steel Alloys

2 Ferrous Aircraft Metals

SAE numerical index

Steel and Steel Alloys

2 Ferrous Aircraft Metals

• Sheet metal is made in a number of sizes and thicknesses Specifications designate thicknesses in thousandths of an inch.

• Bars and rods are supplied in a variety of shapes, such as round, square, rectangular, hexagonal, and octagonal.

• Tubing can be obtained in round, oval, rectangular, or streamlined shapes The size of tubing is generally specified by outside diameter and wall thickness.

The sheet metal is usually formed cold in such machines as presses, bendingbrakes, draw benches, or rolls Forgings are shaped or formed by pressing orhammering heated metal in dies Castings are produced by pouring molten metalinto molds The casting is finished by machining.

Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubing, extrusions, forgings, and castings.

Steel and Steel Alloys

2 Ferrous Aircraft MetalsSteel and Steel Alloys

To identify various ferrous metals: Spark testing

The piece of iron or steel is held against a revolving grinding stone and the metal is identified by the sparks thrown off

Each ferrous metal has its own peculiar spark characteristics The spark streams vary from a few tiny shafts to a shower of sparks several feet in length.

• Wrought iron produces long shafts that are straw colored as they leave the stone and white at the end

• Cast iron sparks are red as they leave the stone and turn to a straw color

• Low carbon steels give off long, straight shafts having a few white sprigs As the carbon content of the steel increases, the number of sprigs along each shaft

increases and the stream becomes whiter in color

• Nickel steel causes the spark stream to contain small white blocks of light within the main burst.

2 Ferrous Aircraft MetalsSteel and Steel Alloys

To identify various ferrous metals: Spark testing

Wrought iron sparks flow out instraight lines The tails of the sparkswiden out near the end, similar to a leaf.

Mild steel sparks are similar to wroughtiron's, except they will have tiny forksand their lengths will vary more Thesparks will be white in color.

Carbon tool steel has a bushy sparkpattern (lots of forking) that starts at thegrinding wheel The sparks are not asbright as the medium-carbon steel ones.

Gray Cast Iron has straw-yellow sparkstream, length about 25 in Sprigs aresmall, repeat along length of eachstreamer.

White Cast Iron has straw-yellow sparkstream, length about 20 in Sprigs aresmall, repeat along length of eachstreamer.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Low carbon steel: Carbon containing (0.10% - 0.30%).

SAE numbers (1010 - 1030).

Steels of this grade are used for making such items as safety wire, certain nuts,cable bushings, or threaded rod ends This steel in sheet form is used for secondarystructural parts and clamps, and in tubular form for moderately stressed structuralparts.

Medium carbon steel: Carbon containing (0.30% - 0.50%).

This steel is especially adaptable for machining or forging, and where surface hardness is desirable

Certain rod ends and light forgings are made from SAE 1035 steel.

High carbon steel: Carbon containing (0.50% - 1.05%).

The addition of other elements in varying quantities adds to the hardness of thissteel In the fully heat-treated condition it is very hard, will withstand high shearand wear, and will have little deformation It has limited use in aircraft.

SAE 1095 in sheet form is used for making flat springs and in wire form formaking coil springs.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Nickel steels: carbon steel + nickel (3% – 3.75%).

Nickel increases the hardness, tensile strength, and elastic limit of steel withoutappreciably decreasing the ductility It also intensifies the hardening effect of heattreatment.

SAE 2330 steel is used extensively for aircraft parts: bolts, terminals, keys,clevises, and pins.

Chromium steel: high hardness, strength, and corrosion resistant properties, and

is particularly adaptable for heat-treated forgings which require greater toughnessand strength than may be obtained in plain carbon steel It can be used for sucharticles as the balls and rollers of antifriction bearings.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Chrome-nickel or stainless steels (the corrosion resistant metals): The principal

alloy of stainless steel is chromium The corrosion resistant steel most often usedin aircraft construction is known as 18-8 steel because of its content of 18 percentchromium and 8 percent nickel One of the distinctive features of 18-8 steel is thatits strength may be increased by cold working.

•Stainless steel may be rolled, drawn, bent, or formed to any shape.

•Stainless steel are more difficult to weld because they expand about 50% morethan mild steel and conduct heat only about 40% as rapidly.

•Stainless steel can be used for almost any part of an aircraft Some of itscommon applications are in the fabrication of exhaust collectors, stacks andmanifolds, structural and machined parts, springs, castings, tie rods, andcontrol cables.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Chrome-vanadium steels: (18% vanadium + 1% chromium).

When heat treated, they have strength, toughness, and resistance to wear andfatigue A special grade of this steel in sheet form can be cold formed into intricateshapes It can be folded and flattened without signs of breaking or failure.

•SAE 6150 is used for making springs,

•SAE 6195 (chrome-vanadium with high carbon content), is used for ball androller bearings.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Chrome-molybdenum steel: (Small molybdenum% + chromium) Molybdenum

is a strong alloying element It raises the ultimate strength of steel withoutaffecting ductility or workability.

Molybdenum steels are tough and wear resistant, and they harden throughoutwhen heat treated They are especially adaptable for welding and, for this reason,are used principally for welded structural parts and assemblies This type steel haspractically replaced carbon steel in the fabrication of fuselage tubing, enginemounts, landing gears, and other structural parts For example, a heat-treated SAEX4130 tube is approximately four times as strong as an SAE 1025 tube of thesame weight and size.

A series of chrome-molybdenum steel most used in aircraft construction:

Carbon (0.25% 0.55%) + molybdenum (0.15 % 0.25 %) + chromium (0.50% -1.10%).

These steels, when suitably heat treated, are deep hardening, easily machined,readily welded by either gas or electric methods, and are especially adapted tohigh temperature service.

2 Ferrous Aircraft Metals

Types, Characteristics, and Uses of Alloyed Steels

Inconel: (a 50% nickel-chromium-iron alloy) closely resembling stainless steel.

Aircraft exhaust systems use both alloys interchangeably Because the two alloyslook very much alike, a distinguishing test is often necessary One method ofidentification is to use an electrochemical technique to identify the nickel (Ni)content of the alloy.

The tensile strength of Inconel is 100,000 psi annealed, and 125,000 psi when hardrolled It is highly resistant to salt water and is able to withstand temperatures ashigh as 1,600 °F Inconel welds readily and has working qualities quite similar tothose of corrosion resistant steels.

3 Heat Treatment of Ferrous Metals

• heating to a temperature above its upper critical point

• holding it at that temperature for a time sufficient to permit certain internal changes to occur

• cooling to atmospheric temperature under predetermined,

3 Heat Treatment of Ferrous Metals

The Fe-C phase diagram

3 Heat Treatment of Ferrous Metals

The Fe-C phase diagram

α-ferrite - solid solution of C in BCC Fe

• Stable form of iron at room temperature.• The maximum solubility of C is 0.022 wt%• Transforms to FCC γ-austenite at 912 °C

γ-austenite - solid solution of C in FCC Fe

• The maximum solubility of C is 2.14 wt %.• Transforms to BCC δ-ferrite at 1395 °C• Is not stable below the eutectic temperature(727 ° C) unless cooled rapidly

δ-ferrite solid solution of C in BCC Fe

• The same structure as α-ferrite

• Stable only at high T, above 1394 °C• Melts at 1538 °C

Fe3C (iron carbide or cementite)

This intermetallic compound is

metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α-Fe and C (graphite) at 650 - 700 °C

3 Heat Treatment of Ferrous Metals

The Fe-C phase diagram

BCC: Body-Centered Cubic structure

FCC: Face-Centered Cubic structure

3 Heat Treatment of Ferrous Metals

• heating the steel to a temperature just above the upper critical point

• soaking or holding for the required length of time

• cooling it rapidly by plunging the hot steel into oil, water, or brine.

 The maximum hardness depends almost entirely on the carbon content of the steel.

 The carbon content increases, the ability of the steel to be hardened increases

 Hardening treatment:

 Hardening increases the hardness and strength of the steel but makes it less ductile.

3 Heat Treatment of Ferrous Metals

• the time limit for the temperature drop to 1,000°F increases, • above the 1 second limit for carbon steels,

• use a slower quenching medium steel must be tempered just before it becomes cold and should,

• be removed from the quenching bath at a temperature of approximately 200°F.

Carbon Steel Hardening

 Steel must be cooled to below 1,000 °F in less than 1 second.

 After the 1,000 °F temperature is reached, the rapid cooling must continue if the final structure is to be all martensite.

 Alloy hardening: