mitesh patel ppt

8/3/2019 Mitesh Patel Ppt

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PREPARED BY

NAME: PATEL MITESH P.

ROLL.NO: 916

SUBJECT: ELECTOMETALLURGY &CORROSION


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In materials science, fatigue is the progressiveand localized structural damage that occurs

when a material is subjected to cyclic loading.

The nominal maximum stress values are less

than the ultimate tensile stress limit, and maybe below the yield stress limit of the material.


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The premature failure of a material from theexposure to the combined action of corrosiveenvironment and cyclic ( or Alternating) stress

is called Corrosion Fatigue.

The number of cycles for failure decreases asthe stress is increased. Below a certain stressthe metal will last indefinitely. This level istermed as 'Endurance Limitof the material.


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FATIGUE FAILURE:THEY SHOW LARGER SMOOTHAREA AND SMALLER AREAWHICH HAS ROUGHENED ANDSOMEWHAT CRYSTALLINEAPPEARANCE

CORROSION FATIGUE:THEY SHOW LARGER AREACOVERED WITH CORROSIONPRODUCTS AND SMALLER AREARESULTING FROM FINAL BRITTLEFRACTURE.


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Ships Offshore platforms Drilling rigs Navigation tuners

Aircrafts Submarine pipelines Communication equipments Heat exchanger tubes Pump shafts

Alternate dry and wet zones Pump shafts Rotors Steam turbine blades Boiler and steel equipment.

Nuclear parts


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ASTM defines fatigue life, Nf , as the numberof stress cycles of a specified character that aspecimen sustains before failure of a specifiednature occurs.

One method to predict fatigue life of materialsis the Uniform Material Law (UML).

UML was developed for fatigue life predictionof aluminum and titanium alloys by the end of20th century and extended to high-strengthsteels.


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Continue.....

The fatigue life of

steel and otherferrous materialsusually becomesindependent ofstress at low stress

levels. As shown infigure , this is calledfatigue limit.

Nonferrous metalssuch as aluminumand magnesium donot possess a fatiguelimit.


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Cyclic stress state Geometry

Surface quality

Material Type

Residual stresses

Size and distribution of internal defects

Direction of loading

Grain size Environment

Temperature


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Stress Ratio The most commonly used

stress ratio is R, the ratio of theminimum stress to themaximum stress

R= ( S min. / S max. ).

If the stresses are fully reversed,then R = -1.

If the stresses are partially reversed,R = a negative number less than 1.

If the stress is cycled between amaximum stress and no load, R =zero.

If the stress is cycled between two

tensile stresses, R = a positivenumber less than 1.


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Corosionfatigue

Mechanical

MetallurgicalEnvironmental


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Evan has given electrochemical mechanismmetal according to which failure takes place in twostages.

1.In first stage the pits of various depth areformed on the surface of metal as a result ofcorrosion.

2.In second stage there is appearance anddevelopment of cracks.


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The main distinguishing feature ofcorrosion fatigue cracks is the presence ofseveral cracks near the fracture. In a normal

fatigue failure, usually one crack is present. The corrosion fatigue cracks run parallel to

each other and are aligned perpendicular tothe direction of principal stress.

Corrosion fatigue cracks in carbon steelsoften propagate generally from the base ofcorrosion pits.


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Generally, the fatiguecorrosion cracks incertain steels aretransgranular in nature .

The striations formed incorrosion fatigue failureare less pronounced than

in normal fatigue failure,as shown by electronmicroscopy.


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In this figure, curve A shows the fatigue behavior of a materialtested in air. A fatigue threshold or limit is seen in Acorresponding to the horizontal part of the curve.

Curves B and C represent the fatigue behavior of the same

material in two corrosive environments.

In curve B, the fatigue failure at high stress levels is retardedand the fatigue limit is eliminated.

In curve C, the whole curve is shifted to the left indicating ageneral lowering in the fatigue strength, accelerated initiationat higher stresses and elimination of a fatigue limit.


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Effect of Corrosion on S-N Diagram


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Discontinuities in metal, such as near the surface.

Inclusions or second-phase particles.

Scratches on a metal surface.

Sites of pitting or intergranular corrosion.

Twin boundaries.


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Crack Propagation Studies inCorrosin Fatigue In normal fatigue testing of smooth specimens,

about 90% of the life is spent in crack nucleationand only the remaining 10 % in crack propagation.

However , in corrosion fatigue , crack nucleation is

facilitated by corrosion and typically about 10 % oflife is sufficient for this stage. The rest, 90 % of lifeis spent in crack propagation.

Fatigue crack grows in a stable fashion below thecritical stress intensity factor for fracture (fracturetoughness), the process is called sub-critical crackgrowth.


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Continue..

In this log-log plot,the crackpropagation velocity is plottedagainst the applied stress intensityrange.

Three stages may be visualized inthis plot. Near the threshold,crack

propagation velocity increasesconsiderably with increasing stress

intensity range where the curve is nearly linear

and follows the Paris' law where crack propagation velocity

increases rapidly with stress

intensity range leading to fractureat the fracture toughness value.


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Corrosion fatigue can be reduced by increasing tensile strength by

alloy addition corrosion inhibitors cathodic protection

All of above reduce pitting.

Also surface treatment like

plating

cladding nitriding shot-peening


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There is a great demand for high static strengthmaterials and a desire for higher structuralperformance from these materials.

an understanding of corrosion fatigue and offatigue-environment interaction is extremelyimportant for selection of materials resistant tocorrosion fatigue.

This phenomenon limits the use, particularly ofsteels, in marine environment, chemical andpetrochemical industries.


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Corrosion Engineering By Mars G. Fontana.

Principle of Corrosion Engineering andcorrosion control By Zaki Ahmad.

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