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LECTURER 4

Fundamental Mechanical PropertiesFatigue Creep

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Fatigue

Fatigue is caused by repeated application of stress to the metal. It is the failure of a material by fracture when subjected to a cyclic stress. Fatigue is distinguished by three main features.

i) Loss of strengthii) Loss of ductilityiii) Increased uncertainty in strength and

service life

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Fatigue

Fatigue is an important form of behaviour in all materials including metals, plastics, rubber and concrete. All rotating machine parts are subjected to alternating stresses.Example: aircraft wings are subjected to repeated loads, oil and gas pipes 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 very harmful. Because of the difficulty of recognizing fatigue conditions, fatigue failure comprises a large percentage of the failures occurring in engineering. To avoid stress concentrations, rough surfaces and tensile residual stresses, 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 of cycle to failure on the horizontal axis.

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

6 10 14 16

22 18

26 30 3438

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Fatigue

The point at which the curve flatters out is termed as fatigue limit and is well below the normal yield stress. The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Materials such as aluminium, copper and magnesium do not show a fatigue limit; therefore they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The fatigue strength can be defined as the stress that produces failure in a given number of cycles usually 107. The fatigue life can be defined as the number of cycles required for a 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 a deterimental 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 fatigue properties.

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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 surface

compressive stresses; as a result the fatigue properties of the materials are getting affected.

Stress concentrations: These are caused by sudden changes in cross section

holes or sharp corners can more easily lead to fatigue failure. Even a small 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, usually initiated by cyclic stresses, at surface imperfections such as machine marking and slip steps.The initial stress concentration associated with these cracks are too low 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 the stress concentration exceeds the fracture strength and sudden failure occurs. The extent of the crack propagation process depends upon the brittleness of the material under test. In brittle materials the crack grows to a critical size from which it propagates right through the structures in a fast manner, whereas with ductile materials the crack keeps growing until the remaining area cannot support the load and an almost ductile fracture suddenly occurs.

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Fatigue Failure

Failure can be recognized by the appearance of fracture. For a typical fracture ,Two distinct zones can be distinguished – a smooth zone near the fatigue crack itself which, has been smoothened by the continual rubbing together of the cracked surfaces, and a rough crystalline-looking zone which is the final fracture. Occasionally fatigue cracks show rough concentric rings which correspond to successive positions of the crack.

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Design for Fatigue

To secure satisfactory fatigue life Modification of the design to avoid stress concentration eliminating

sharp recesses and severe stress raisers. Precise control of the surface finish by avoiding damage to surface

by rough machining, punching, stamping, shearing etc. Control of corrosion and erosion or chemical attack in service and to

prevent of surface decarburization during processing of heat treatment.

Surface treatment of the metal.

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Creep

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

• Creep is the slow plastic deformation of materials under the application of a constant load even for stressed below the yield strength of the material.

• Usually creep occurs at high temperatures. • Creep is an important property for designing I.C. engines, jet

engines, boilers and turbines. Iron, nickel, copper and their alloys exhibited 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 occurring

along crystallographic directions in the individual crystals together with 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 work

hardening is more than that of recovery processes. The primary stage is of great interest to the designer since it forms an early part of the total extension reached in a given time and may affect clearness provided between components of a machine.

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Creep

Secondary Stage: In this stage, the creep rate is a minimum and is constant with time. The work hardening and recovery processes are exactly balanced. 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

fracture occurs. Tertiary creep can occur due to necking of the specimen and 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 given temperature. The calculation of creep limit includes the temperature, the deformation and the time in which this deformation appears.

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Types of Creep

Creep are classified based on temperature Logarithmic Creep Recovery Creep Diffusion Creep

At low temperature the creep rate decreases with time and the logarithmic creep curve is obtained. At high temperature, the influence of work hardening is weakened and there is a possibility of mechanical recovery. As a result, the creep rate does not decrease and the recovery creep curve is obtained. At very high temperature, the creep is primarily influenced by diffusion and load applied has little effect. This creep is termed as diffusion creep or plastic creep.

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Factors affecting Creep

Heat Treatment• Creep resistance of steel is affected by heat treatment. • At temperatures of 300°C or higher maximum creep resistance is usually

produced. 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 fine

grained one based on the temperature. • At temperatures below the lowest temperature of recrystallisation, a fine

grained structure possesses the greater resistance whereas at temperature above this point a large grained structure possesses the greater resistance and we must select it for high temperature applications.

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Factors affecting Creep

Strain Hardening Strain hardening of steel increases its creep resistance. Particularly below the equicohesive temperature at which the fracture

changes from intra crystalline to inner-crystalline strain hardening increases the creep resistance and hence there is no measurable creep. So the second stage of creep curve is almost horizontal.

At temperature above the equicohesive temperature yield rate exceeds the strain hardening rate and creep will proceed even under low stresses.

Alloying additions At temperatures, below the lowest temperatures of recrystallation the creep

resistance of steel may be improved by the finite forming elements like nickel, cobalt and manganese or by the carbide forming elements like chromium molybdenum, tungsten and vanadium.

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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 the dislocation to climb up or down. By a simple climb of edge dislocation the diffusion rate of vacancies may produce a motion in response to the applied stress. Thus edge dislocations are piled up by the obstacles in the glide plane and the rate of creep is governed by the rate of escape of dislocation.Another mechanism of creep is called diffusion of vacancies. In this mechanism, the diffusion of vacancies controls the creep rate but does not involve the climb of edge dislocations. It depends on the migration of vacancies from one side of a grain to another. In response to the applied stress, the vacancies move from surfaces of the specimen transverse to the stress axis

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Mechanism of Creep

The third mechanism of creep is sliding of grain boundaries. It means sliding of neighboring grains with respect to the boundary that 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 strain – harden and become viscous to facilitate the sliding of grain boundaries. As the temperature increases the grain boundaries facilitate the deformation process by sliding, whereas at low temperature, they increase the yield strength by stopping the dislocations.