• Crack grows incrementally typ. 1 to 6 a ~ increase in crack length per loading cycle • Failed rotating shaft --crack grew even though K max < K c --crack grows faster as • increases • crack gets longer • loading freq. crack origin Fatigue Mechanism Fatigue Mechanism m K dN da
27
Embed
Crack grows incrementally typ. 1 to 6 increase in crack length per loading cycle Failed rotating shaft --crack grew even though K max < K c --crack grows.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Improving Fatigue LifeImproving Fatigue Life1. Impose a compressive surface stress (to suppress surface cracks from growing)
N = Cycles to failure
moderate tensile mLarger tensile m
S = stress amplitude
near zero or compressive mIncreasing
m
--Method 1: shot peening
put surface
into compression
shot--Method 2: carburizing
C-rich gas
2. Remove stress concentrators. Adapted from
Fig. 8.25, Callister 7e.
bad
bad
better
better
Adapted fromFig. 8.24, Callister 7e.
• Corrosion: -- the destructive electrochemical attack of a material. -- Al Capone's ship, Sapona, off the coast of Bimini.
• Cost: -- 4 to 5% of the Gross National Product (GNP)* -- this amounts to just over $400 billion/yr**
* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.**Economic Report of the President (1998).
Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
THE COST OF CORROSIONTHE COST OF CORROSION
What is CORROSION?What is CORROSION?
Corrosion is a natural event It represents a return of metals to their more natural state as minerals (oxides)
Metal “Wants” to be DirtMetal “Wants” to be Dirt
ENERGY
METALORE
Basics of CorrosionBasics of Corrosion
Corrosion is essentially the oxidation of metal
Need:1. An Anode (where oxidation is taking place)2. A Cathode (where reduction is taking place)3. Conductive electrolyte4. Electrical contact between the Anode and Cathode
Source: Moore, J.J. Chemical Metallurgy
ElectrochemistryElectrochemistry
Corrosion is an electrochemical reactionCorrosion is an electrochemical reaction– ½ reaction at the anode½ reaction at the anode:: M MM Mn+n+ + ne- + ne-
– Possible ½ reactions at the cathode:Possible ½ reactions at the cathode: 2H2H++ + 2e + 2e-- H H22
Acid Solutions:Acid Solutions: H H22O + eO + e-- ½ H ½ H22 + OH + OH--
½ O½ O22 + 2H + 2H++ + 2e + 2e-- 2OH 2OH--
Important thing to note is the flow of electronsImportant thing to note is the flow of electrons
Thermodynamic Driving ForceThermodynamic Driving Force
Like all chemical reactions – ThermodynamicsLike all chemical reactions – Thermodynamics What is the driving force for the reaction? What is the driving force for the reaction?
(otherwise stated as what is the electrochemical (otherwise stated as what is the electrochemical potential for the reaction)potential for the reaction)– Dissimilar metalsDissimilar metals– Different cold work statesDifferent cold work states– Different grain sizesDifferent grain sizes– Difference in local chemistryDifference in local chemistry– Difference in the availability of species for a reaction Difference in the availability of species for a reaction
Derivation of Nernst EquationDerivation of Nernst Equation
eFeFe 22
tsreac
productsoo
a
aRTGQRTGG
tan
ln)ln(
)(22 2 gHeH
For:
)(2 22 gHFeHFe
2
2
22
2
][
][ln
)()(
)()(ln
H
FeG
HaFea
HfFeaGG oo
1
1
Derivation of Nernst Equation…Derivation of Nernst Equation… Introduce: The total electropotential isIntroduce: The total electropotential is
G = -nFEG = -nFEWhere:Where:
F = Faraday’s constant (total charge on Avogadro’s number of electrons)F = Faraday’s constant (total charge on Avogadro’s number of electrons)n = the number of electrons transferredn = the number of electrons transferredE = The electrode potentialE = The electrode potential
= Area Inside Crevice (Anodic)= Area Outside Crevice (Cathodic)
Aa << Ac
Aa
Ac
i
+oi H Ac
Effect of Oxidizer Concentration (e.g., Oxygen) on the Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive MetalElectrochemical Behavior of an Active - Passive Metal
Log i
M M+
[Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]
Increasing OxidantConcentration
Effect of Effect of TemperatureTemperature and Dissolved O and Dissolved O22
Potential and Current Fields in Electrolyte in Potential and Current Fields in Electrolyte in the Vicinity of a Localized Corrosion Sitethe Vicinity of a Localized Corrosion Site