mitesh patel ppt
TRANSCRIPT
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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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