021 01-00-00 System Design Loads Stresses Maintenance Amend0

8/9/2019 021 01-00-00 System Design Loads Stresses Maintenance Amend0

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TEXTBOOK

SYSTEM DESIGN, LOADS, STRESSES, MAINTENANCE

020 00 00 00 AIRCRAFT GENERAL KNOWLEDGE

021 00 00 00 AIRCRAFT GENERAL KNOWLEDGE AIRFRAME AND SYSTEMS,

ELECTRICS, POWERPLANT, EMERGENCY EQUIPMENT

021 01 00 00 SYSTEM DESIGN, LOADS, STRESSES, MAINTENANCE


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System Design, Loads, Stresses, Maintenance

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Table of Contents:

021-01-00-00 System Design, Loads, Stresses, Maintenance______________ 3 Loads and combination loadings applied to an aircrafts structure______________ 3Fatigue ___________________________________________________________ 6Corrosion ________________________________________________________ 11


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021-01-00-00 System Design, Loads, Stresses, Maintenance

Loads and combination loadings applied to an aircrafts structure

An aircraft structure like any physical object is subjected to the five basic

stresses: tension, compression, torsion, bending and shear. Tension and

compression are the basic stresses and the other three are a combination of

these two.

A stress is a force within an object that tries to prevent an outside force from

changing its shape. A strain is a deformation or a physical change caused by

stress. A material that is strained within its elastic limit will return to its original

shape after the stress is removed. A material that is strained beyond its elastic

limit will stay permanently deformed.

Tension tries to pull an object apart. A chain of a hoist supporting a weight is

under tension or has a tensile stress within it.


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Bending is also made up of tension and compression.

The wing of an airplane is under a bending stress. When this aircraft is on the

ground the top skin is under tensile stress and the bottom skin is under a

compressive stress. In flight these forces are reversed. The top skin is under

compressive stress and the bottom skin under a tensile stress

A shear stress tries to slide an object apart.

This clevis bolt is subject to shear stress. The force on the cable puts a tensile

stress in the clevis bolt towards the right while the fixed fitting puts a tensile

stress into the bolt toward the left. These two tensile stresses act alongside each

other rather than opposite each other and the result is a force that tries to shear

the bolt or to slide it apart.

Hoop Stress is stress in a pipe wall acting circumferentially in a plane

perpendicular to the longitudinal axis of the pipe.

The amount of Hoop Stress depends on the pressure of the fluid in the pipe, the

outside diameter and the normal wall thickness of the pipe or pressure vessel.

On an airplane hydraulic pipe, pressure accumulators and aircraft tires are

subjected to Hoop Stress.


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Fatigue

During the normal working life of an aircraft its structure is constantly subject to

varying stresses, which occur due to:

flight manoeuvres

atmospheric turbulence

ground loads

cabin pressurisation and de-pressurisation

thermal effects

vibrations

In areas where a structure is subject to high repetitive or cyclic tensile stresses a

minute crack may eventually develop and if left unchecked will continue to

enlarge. It will steadily propagate into the cross-section and when the remaining

material is no longer able to support the applied stresses a sudden fracture will

occur as a result of fatigue failure. In practice the time taken for this to occur is

directly related to the magnitude of the applied cyclic stresses.


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It follows that the higher the cyclic stress the fewer the number of reversals

required to cause fatigue failure. Areas of high stress concentration must

therefore be avoided, particularly in the main load bearing structures, such as,

spars, longerons and stressed skins, where any such failure may prove

catastrophic.

Stress loads of any kind which is repetitive or cyclic can eventually lead to a

weakening or deterioration of metal or other material. This is called "fatigue

strain".

In this condition the actual single stress load is well within the elastic limit of the

material. It is the repetitive loading and unloading of the material that can

eventually lead to a failure.

Structures which are known to have to withstand fatigue stress such as landing

gear, wings or certain structural attachment fittings have a life limit based on the

number of takeoffs, landings or flying hours

Bending

Bending (or beam stress) is a combination of compression and tension.


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In a bar for example the upper half is subject to tensile stress, whilst the lower

half is subject to compressive stress. At the centre of the bar the two stresses

oppose each other and cause maximum shear stress in this region.

When a structural member is subject to a force or load it will tend to distort. For

example if a member is subject to tension it will tend to stretch, and this is

commonly known as strain or elongation.

Basic Structural Members

A structure normally consists of a series of individual elements or members,

which when loaded in any direction exhibit different properties. These elements

are known as beams, struts and ties.

Beams

Beams are members, which under load, are subject to bending. They can either

be supported at both ends (simply supported), or supported at one end only

(cantilever.)

When a solid beam is under load one side is in tension whilst the other side is incompression.


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The centre of the beam is however neither in compression nor tension, so

material in this area is unnecessary, and is normally removed, giving an overall

weight saving. Beams used in aircraft constructions are normally of H section

with tensile and compressive loadings being borne by flanges held together by a

lightly loaded web.

Beams under load are also subject to bending moments.

LOAD LOAD

In the simply supported beam the maximum bending moments occur beneath the

load, whereas in the cantilever beam they occur at the support. Failure is

therefore most likely to occur in these regions.

The fuselage of an aircraft is an example of a simply supported beam, whilst its

wings are examples of cantilever beams. Aircraft wings are therefore normally

tapered in section towards the wing tip where bending moments are least.

LOAD

MAXIMUMBENDING MOMENT


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Struts

Struts are designed to withstand mainly compressive loads, and unless they are

extremely short will tend to bend under load before failure occurs.

Long struts therefore behave like cantilever beams when under load, with one

side in compression and the other side in tension. Struts of this type are

consequently normally manufactured as hollow tubes. An example of a strut is

the external bracing employed on some high winged aircraft, which help support

the wings when the aircraft is on the ground.

Ties

Ties are members which are designed to withstand mainly tensile loads, and are

normally constructed from a solid rod, or even a wire of relatively small diameter.

An example of a tie is the external wing bracing employed on some high winged

aircraft to prevent the wings from lifting in flight.

Material inTension

Load

Material inCompression


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Corrosion

Many aircraft component failures have been

caused by corrosion.

It is important to recognize corrosion and to

understand how to prevent it, since corrosion,

if undetected, may cause catastrophic

failures, as was almost the case with the aircraft shown in the photograph.

Due to poor inspection practices, the corrosion on this aircraft was not detected

until the fuselage strength was reduced to almost zero. When the cabin was

pressurized, the stress caused the fuselage to split. This occurred at 17,000 feetand caused the aircraft to be scrapped. Several passengers and crew were killed

or injured.

Corrosion can occur very quickly in high humidity environments. During the

summer months it is not uncommon for unprotected parts to corrode overnight.

These parts must then be cleaned, repaired and protected against further

corrosive attacks. This is expensive! It would be far better if we could stop the

corrosion before it began.

Galvanic or Dissimilar metal corrosion will occur any time two dissimilar metals

are connected so that a galvanic current flows through them. This electric current

occurs because all materials are in possession of what is called an "electric

potential".

Since two dissimilar metals have their own potential, there will be a potential

difference (voltage) between the two, resulting in a current flow.

In other words, an ANODE (positive electrode) and a CATHODE (negative

electrode) will be formed.


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In the drawing, the two dissimilar metals are the steel of the rivet forming a

cathode and 2024 aluminium alloy sheet metal forming the anode. The current

will cause electrons and-or ions to leave the surface of one material and to be

deposited on the surface of the other. This slow loss of material will weaken the

surface, which will eventually fail.

To avoid galvanic corrosion we must insulate dissimilar metals (anode-cathode)

from each other and ensure moisture (a current conductor) does not remain

trapped on the material.

Crevice corrosion is severe localized corrosion along adjoining surfaces.

It is caused by the penetration of oxygen and moisture into small cracks andcrevices surrounding a joint, as can be seen in the enlarged photograph.

It may be prevented by sealing all cracks and crevices with an appropriate

sealing compound.


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Stress corrosion occurs when metal is subjected to stress (a result of mechanical

loading), while in a corrosive environment. The stress may be external (fasteners,

interference fits) or internal (poor quenching technique, holes).

In either case, corrosion starts between the grains of metal and will result in

surface and internal cracks. The simplest way to avoid stress corrosion is to coat

the material to protect it from the corrosive environment, and to ensure that all

internal stresses are minimized by using the appropriate heat treatment (and shot

peening).

Stress corrosion is often found around rivets in a stressed skin of an aircraft,

around fit bushings and tapered pipe fittings.

Filiform corrosion is a form of surface corrosion

which tends to propagate longitudinally. In the

advanced stages it appears as if worms have

burrowed just under the material's surface coating.

It usually occurs once the original paint coating

was damaged. This corrosion may be repaired by

sanding, priming and repainting.

Intergranular corrosion occurs in aluminium alloys when the grains of aluminium

are allowed to become too large. Once a certain grain size is reached, the slight

differences in grain composition will cause a galvanic reaction to occur. This

event causes the grains to corrode into various sulfates and salts.

Since this corrosion takes place internally, (within the material) it may only be

detected in the early stages by ultrasonic or eddy current inspection.

In the latter stages this corrosion may be detected visually by the presence of

small blisters on the surface of the metal. In advanced cases, black discolorationwill also become apparent.


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Corrosion is an electro-chemical process which converts a metal into an oxide,

hydroxide or sulfate. In its simplest form, the corrosion occurs through oxidation,

also referred to as dry" corrosion, which occurs in the following way:

Here we see a piece of unprotected base metal. This base metal, for instance,

aluminium or steel, is exposed to a gas containing oxygen (e.g. AIR).

A chemical reaction takes place between the metal atoms and the oxygen.

Initially, corrosion will cause a discoloration.

In the latter stages, a metal oxide such as aluminium-oxide or, in the case of

steel, iron oxide (rust), will form in a layer.

Aluminium oxide is non-porous, so once the surface is completely sealed by the

oxide coating, corrosion will stop.

Iron oxide is porous and once iron or steel begins to corrode the process will

continue until the material is totally "eaten away".

To stop the oxidation process, the material may be coated with any non-porous

material such as paint, oil or grease.

How to prevent corrosion? First of all, ensure that all (aircraft) parts are kept

clean and dry. Do not allow any part to touch other parts, your skin, the floor or

any chemicals.

The human skin contains fatty acid that is made up of all sorts of elements (e.g

hydrogen, oxygen, nitrogen and carbon) that induce forms of corrosion. It is

therefore advised to wear specified gloves when handling aircraft parts.


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Lastly, we must remember that once we see corrosion, the damage has already

occurred. It makes far more sense to take the precautions that were just

mentioned and not have to deal with corrosion in the first place.

Controlling the appearance and spread of corrosion is of primary importance to

all aircraft operators.

Corrosion weakens primary structural members, which must then be replaced or

reinforced in order to sustain flight loads. Such replacements or reinforcements

are costly, time consuming, and result in unscheduled delays.

Most metals used in for example an aircraft or an aluminium statue exist in nature

as chemical compounds such as oxides or chlorides. Aluminium is never found in

nature in its pure state, but must be refined from an ore such as alumina (Al 2O3).

When pure aluminium is exposed to the elements, it combines or chemically

reacts with oxygen and eventually changes back into alumina. Corrosion then, is

simply a process whereby metals return to their natural state.

Corrosion is a natural phenomenon that attacks metal by chemical or

electrochemical action and converts the metal into a metallic compound, such as

an oxide, hydroxide, or sulfate.Substances that cause corrosion are called corrosive agents.

Water or vapor containing salt combined with oxygen in the atmosphere to

produce the most prominent corrosive agents.

Additional corrosive agents include acids, alkalis and salts.

The appearance of corrosion varies with various metals.

For example, on aluminium alloys and magnesium it appears as surface pitting or

etching, often combined with a grey or white powdery deposits.However on copper and copper alloys corrosion forms a greenish film and on

steel a reddish rust.


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When corrosion deposits are removed, the metal's surface may appear etched

and pitted, depending upon the length and severity of the attack.

If deep enough, these pits may become sites for crack development.

Some types of corrosion can travel beneath surface coatings and can spread

until the part fails.

The photograph shows the terrible result of poor inspection practices. The

corrosion on this aircraft was not detected until the fuselage strength was

reduced to almost zero. When the cabin was pressurized the stress caused the

fuselage to split causing the terrible damage shown here. This occurred at 17,000

feet and caused the aircraft to be scrapped. Several passengers and crew were

either killed or injured.

Corrosion is not always easily detected and can sometimes only come "out in the

open" after a full disassembly of a component.

The photograph shows a heavily corroded shaft of an aircraft engine's gearbox

shaft.


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Corrosion is usually classified as chemical or electrochemical; however, both

types involve two simultaneous changes.

The metal that is attacked or oxidized suffers an anodic change, and the

corrosive agent is reduced and suffers a cathodic change.

Pure CHEMICAL CORROSION results from direct exposure of a bare surface to

caustic liquid or gaseous agents.

The most common agents causing direct chemical corrosion include:

- SPILLED BATTERY ACID OR FUMES FROM BATTERIES,

- RESIDUAL FLUX DEPOSITS RESULTING FROM INADEQUATELY

CLEANED, WELDED, BRAZED OR SOLDERED JOINTS, and lastly

- ENTRAPPED CAUSTIC CLEANING SOLUTIONS.

Electrochemical corrosion is similar to the electrolytic reaction that takes place in

a dry cell battery.

To understand how this happens, recall that the

number of electrons matches the number of protons in

an atom, the atom is said to be electrically balanced.

However, if there are more or fewer electrons than

protons, the atom is said to be charged and is calledan ion. If there are more electrons (negative charge)

than protons (positive charge), it is called a negative

ion, but if there are more protons than electrons, it is a

positive ion.

An ion is unstable, always seeking to lose or gain electrons so it can change back

into a balanced, or neutral, atom.

A metal that easily gives up its electrons is known as an anodic metal and will

CORRODE EASILY. This principle is reflected in the so called NOBILITY of ametal. The NOBLER the metal the less it will be willing to give up its electrons

and therefore will be less prone to corrosion. Metals that do not give up their

electrons easily are called cathodic metals

Many metals become ionized due to galvanic action when brought into contact

with dilute acids, salts or alkalis, such as those found in industrially contaminated

air.


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For example, if an aluminium structure (aircraft!) is in contact with moisture

having a trace of hydrochloric acid, a chemical reaction takes place between the

acid and the aluminium to form aluminium chloride and hydrogen. The hydrogen

is released as a gas, and the aluminium chloride, which is a salt, forms as a white

powder on the surface of the metal. This powder is the visible evidence of

corrosion. Corrosion is an electrochemical reaction in which one metal is

changed into a chemical salt.

When two dissimilar metals are in contact with each other in the presence of

some electrolyte, such as hydrochloric acid or plain water, the less active metal

acts as the cathode (negative electrode) and attracts electrons from the anode

(positive electrode, the metal). As the electrons are pulled away from the anode

the metal corrodes. This process can be visualized if you consider it to be similar

to the action of a battery.

Perhaps the easiest way to visualize what is actually taking place is to consider

the action of a battery.

If two metals, for instance, Aluminium and Copper are immersed in an electrolyte

of acid, in this particular example a weak solution of hydrolchloric acid, saline or

alkaline solution, a battery is formed and produces a flow of electrons betweenthe two metals. This process continues as long as there are active materials in

the metal and electrolyte and the cathode and anode are connected by a

conductive path.


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How can this process be explained in terms of the nobility of the metals?

In the electrochemical series, aluminium is considerably more active than copper,

which is the nobler material of the two. When electrons flow from the aluminium,

through the conductor to the copper, positive aluminium ions are left.

Two of these ions attract six negative chlorine atoms from the acid and form two

molecules of aluminium chloride (AlCl 3) on the surface of the aluminium. This

eats away some of the base metal. The six positive hydrogen ions remaining in

the acid are attracted to the copper by the electrons which came from the

aluminium. These electrons neutralize the hydrogen molecules (3H 2) and leave

the surface as free hydrogen gas.

One of the basic characteristics of metals is their electrode potential. In other

words, when two dissimilar metals are placed in an electrolyte, an electrical

potential exists.

This potential forces electrons in the more negative material, the anode, to flow to

the less negative material, the cathode, when a conductive path is provided.

Corrosion occurs when electrons leave an element.

If all of the aluminium in the construction of aircraft were pure aluminium,

corrosion would not be a problem. However, aluminium must be alloyed withother metals to increase its strength. The most common alloying agent is copper.

In a structural piece of alloyed aluminium the microscopic grains of copper and

aluminium serve as the cathode and anode of a galvanic cell. Aluminium is more

negative than copper and acts as the anode in the electrochemical action. There

is no flow of electrons between the two alloying agents within the metal until an

external path is furnished by the electrolyte, which can be a surface film of

moisture containing some pollutants as acids, salts, or other industrial

contaminants.

If the entire area is covered with a strong electrolyte, corrosion can develop

uniformly over an extensive area.

This type of corrosion is called direct chemical attack. The illustration shows how

an electrolyte can "cross a bridge" between the steel material of a fastener (rivet)

and the aluminium sheet metal it is attached to.

The illustration shows the importance of the use of a sealant that isolates both

materials and thus protecting them against galvanic corrosion.


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In summary we have seen that there are four requirements for the formation of

corrosion:

- The presence of a metal that will corrode (anode),

- The presence of a dissimilar conductive material (cathode) which has less

tendency to corrode,

- The presence of a conductive liquid (electrolyte) and

- An electrical contact between the anode and cathode, usually metal-to-

metal contact, or a fastener.

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021 01-00-00 System Design Loads Stresses Maintenance Amend0