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LECTURER 4

Fundamental Mechanical Properties

Fatigue

Creep

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Fatigue

Fatigue is caused by repeated application of stress tothe metal. It is the failure of a material by fracture whensubjected to a cyclic stress.

Fatigue is distinguished by three main features.

i) Loss of strength

ii) Loss of ductility

iii) Increased uncertainty in strength andservice life

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Fatigue

Fatigue is an important form of behaviour in all materials includingmetals, plastics, rubber and concrete.

All rotating machine parts are subjected to alternating stresses.

Example: aircraft wings are subjected to repeated loads, oil and gaspipes are often subjected to static loads but the dynamic effect of

temperature variation will cause fatigue.There are many other situations where fatigue failure will be veryharmful.

Because of the difficulty of recognizing fatigue conditions, fatiguefailure comprises a large percentage of the failures occurring inengineering.

To avoid stress concentrations, rough surfaces and tensile residualstresses, fatigue specimens must be carefully prepared.

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Fatigue

The S-N Curve

A very useful way to visual the failure for a specific material is with

the S-N curve.

The S-N means stress verse cycles to failure, which when plotted

using the stress amplitude on the vertical axis and the number ofcycle to failure on the horizontal axis.

An important characteristic to this plot as seen is the fatigue limit.

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Fatigue

The point at which the curve flatters out is termed as fatigue limitand is well below the normal yield stress.

The significance of the fatigue limit is that if the material is loadedbelow this stress, then it will not fail, regardless of the number oftimes it is loaded.

Materials such as aluminium, copper and magnesium do not show afatigue limit; therefore they will fail at any stress and number ofcycles.

Other important terms arefatigue strengthandfatigue life.

The fatigue strength can be defined as the stress that producesfailure in a given number of cycles usually 107.

The fatigue life can be defined as the number of cycles required fora material to fail at a certain stress.

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Factors affecting fatigue properties

Surface finish:

Scratches dents identification marks can act as stress raisers and so

reduce the fatigue properties.

Electro-plating produces tensile residual stresses and have adeterimental effect on the fatigue properties.

Temperature:

As a consequence of oxidation or corrosion of the metal surface

increasing, increase in temperature can lead to a reduction in fatigueproperties.

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Factors affecting fatigue properties

Residual stresses:

Residual stresses are produced by fabrication and finishing

processes.

Residual stresses on the surface of the material will improve

the fatigue properties.Heat treatment:

Hardening and heat treatments reduce the surfacecompressive stresses; as a result the fatigue properties of thematerials are getting affected.

Stress concentrations:

These are caused by sudden changes in cross section holes orsharp corners can more easily lead to fatigue failure. Even asmall hole lowers fatigue-limit by 30%.

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Stress Cycles

There are different arrangements of fatigue loading.

The simplest type of load is the alternating stress where the stress

amplitude is equal to the maximum stress and the mean or average stress

is zero. The bending stress in a shaft varies in this way.

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Fatigue Failure

Fatigue fracture results from the presence of fatigue cracks, usuallyinitiated by cyclic stresses, at surface imperfections such as machinemarking and slip steps.

The initial stress concentration associated with these cracks are toolow to cause brittle fracture they may be sufficient to cause slow

growth of the cracks into the interior.Eventually the cracks may become sufficiently deep so that thestress concentration exceeds the fracture strength and suddenfailure occurs.

The extent of the crack propagation process depends upon thebrittleness of the material under test.

In brittle materials the crack grows to a critical size from which itpropagates right through the structures in a fast manner, whereaswith ductile materials the crack keeps growing until the remainingarea cannot support the load and an almost ductile fracture suddenlyoccurs.

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Fatigue Failure

Failure can be recognized by the appearance of fracture.

For a typical fracture ,Two distinct zones can be distinguishedasmooth zone near the fatigue crack itself which, has beensmoothened by the continual rubbing together of the crackedsurfaces, and a rough crystalline-looking zone which is the finalfracture.

Occasionally fatigue cracks show rough concentric rings whichcorrespond to successive positions of the crack.

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Creep

The creep is defined as the property of a material by virtue of whichit deforms continuously under a steady load.

Creep is the slow plastic deformation of materials under theapplication of a constant load even for stressed below the yieldstrength of the material.

Usually creep occurs at high temperatures.

Creep is an important property for designing I.C. engines, jetengines, boilers and turbines. Iron, nickel, copper and their alloysexhibited this property at elevated temperature.

But zin, tin, lead and their alloys shows creep at room temperature.

In metals creep is a plastic deformation caused by slip occurringalong crystallographic directions in the individual crystals togetherwith some deformation of the grain boundary materials.

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Creep

The creep curve usually consists of three \ stages of creep.

Primary Stage:

In this stage the creep rate decreases with time, the effect of workhardening is more than that of recovery processes. The primary stage is ofgreat interest to the designer since it forms an early part of the total

extension reached in a given time and may affect clearness providedbetween components of a machine.

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Creep

Secondary Stage:

In this stage, the creep rate is a minimum and is constant withtime. The work hardening and recovery processes are exactlybalanced. It is the important property of the curve which is used

to estimate the service life of the alloy.Tertiary Stage:

In this stage, the creep rate increases with time until fractureoccurs. Tertiary creep can occur due to necking of the specimenand other processes that ultimately result in failure.

The Creep Limit is the stress at which a material can be formed

by a definite magnitude during a given time at a giventemperature. The calculation of creep limit includes thetemperature, the deformation and the time in which thisdeformation appears.

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Factors affecting Creep

Heat Treatment

Creep resistance of steel is affected by heat treatment.

At temperatures of 300C or higher maximum creep resistance is usuallyproduced. But the quacking and drawing decreases the creep resistance.

Grain size The major factor in creep is grain size.

Normally large grained materials exhibit better creep resistance than finegrained one based on the temperature.

At temperatures below the lowest temperature of recrystallisation, a finegrained structure possesses the greater resistance whereas at temperatureabove this point a large grained structure possesses the greater resistance

and we must select it for high temperature applications.

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Mechanism of Creep

Some mechanisms that play vital roles during the creep process are:

Dislocation climb

Vacancy Diffusion

Grain boundary sliding

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Mechanism of Creep

At high temperature, the appreciate atomic movement causes thedislocation to climb up or down.

By a simple climb of edge dislocation the diffusion rate of vacanciesmay produce a motion in response to the applied stress.

Thus edge dislocations are piled up by the obstacles in the glideplane and the rate of creep is governed by the rate of escape ofdislocation.

Another mechanism of creep is called diffusion of vacancies.

In this mechanism, the diffusion of vacancies controls the creep ratebut does not involve the climb of edge dislocations.

It depends on the migration of vacancies from one side of a grain toanother. In response to the applied stress, the vacancies move fromsurfaces of the specimen transverse to the stress axis

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Mechanismof Creep

The third mechanism of creep is sliding of grain boundaries.

It means sliding of neighboring grains with respect to the boundarythat separates them.

Grain boundaries become soft at low temperature as compared to

individual grains.Grain boundaries play a major role in the creep of polycrystals at

high temperatures as they side past each other or create vacancies.

At high temperature, ductile metals begin to lose their ability to strainharden and become viscous to facilitate the sliding of grainboundaries.

As the temperature increases the grain boundaries facilitate thedeformation process by sliding, whereas at low temperature, theyincrease the yield strength by stopping the dislocations.