Term
| what is fatigue and what are some variables of interest |
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Definition
-damage caused by oscillating stress below the fracture stress.
-accounts for ~90% of all mechanical failures
-it involves the generation of defects by plastic flow and the movement of dislocations
-variables if interest include: stress, strain, temperature, frequency, amplitude, environment, surface finish, coating |
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Term
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Definition
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Term
| in graphs of G,R v. crack length, what region is dominated by fatigue |
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Definition
| between the two intersections of R and G |
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Term
what is high cycle fatigue?
approximately how many cycles would this include? |
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Definition
| low amplitude stresse induce elastic strains which results in long life (>10,000 cycles) |
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Term
| what is low cycle fatigue? approximately how many cycles would there be? |
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Definition
| considerable plastic deformation during cyclic loading result in an endurance limit below 10,000 cycles and behavior dominated by plastic deformation |
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Term
| What is thermo-mechanical fatigue |
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Definition
| varying both stress and temperature to give strain cycles in or out of phase with the temperature cycle |
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Term
| what are the stages of fatigue in ductile materials? |
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Definition
1.intiial micro-structural changes leading to the nucleation of permanent damage
2.nucleation of the first micro-cracks
3.growth and coalescence of these flaws to produce a dominant crack
4. stable propagation of the dominant crack
5. failure
1-3 intiation, 4-5 propagation |
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Term
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Definition
this is a strategy to predict the total life and retire the component at a fixed proportion of this to include a considerable margin for error (before the crack forms)
This c an be wasteful as much useful life remains unused
Most of material life is spent forming crack |
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Term
| Damage-tolerant/fail-safe |
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Definition
this approach recognizes that all structures contain defects and that these grow at a stable and predictable rate. Involves periodic inspection of the structure and repairs or replaces when cracks are found
Used when failure would not result in component failure
a greater proportion of the useful life is used
ex. if amax is konwn, the interval between inspections is determined by teh time predicted for this crack to achieve critical size (t1)
spends most of its life propagating crack |
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Term
| is safe-life or fail-safe more efficient? |
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Definition
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Term
| what is leak before break? |
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Definition
a failsafe approach used for pressure vessels and pipes
the thickness are arranged so that a crack does not propagate catastrophically (crack remains below critical size)
a therefore leak can be detected and repaired without rupture
variables: thickness, KIC, stress to manipulate a |
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Term
| what is total life approach and what relationship does it show? |
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Definition
relates the oscillating stress v. lnN
Usually is asymptotic to a fatigue limit and often an endurance limit is set |
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Term
| conditions for Basquins Law |
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Definition
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Term
| conditions for Coffin Manson Law |
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Definition
short life
low cycle fatigue
total strain amplitude has an added plastic component
ef' is the plastic strain at failure |
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Term
| what is the significance of Soderberg and Goodman relations |
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Definition
they assume that mean stress decreases fatigue life
Soderberg is the orginal and most conservative method
Goodman relation is good for brittle materials and conservative for metals
both are linear |
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Term
| what information does a Goodman diagram provide? |
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Definition
the effect of mean stress and R value
-straight lines show constant values of R
-curved lines are known as "life lines" and correspond to constant values of number of cycles |
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Term
| What is miners law? What are the shortcomings of this law? |
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Definition
provides consideration of changing levels of stress or strain over the life of a material
the life of a component can be assessed by expressing the number of cycles at each amplitude as a proportion of total life and summing the fractions. When this fraction equals 1, the fatigue life is exhausted
Shortcomings: 1-no account can be taken of the impact of prior damage on the later exposure at a different stress
2-low stress regions could harden the material |
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Term
what effects can occur during fatigue tests with strain held constant?
sketch graphs to illustrate these effects.
Why do these occur? |
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Definition
cyclic hardening: stress increases with time (UTS/YS>1.4). Initally dislocation density is low, then they increase hardening material
cyclic softening: stress decreases with time (UTS/YS<1.2) initially density dislocaiton is high and stress puts them in stable networks
[image]
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Term
| What is the Bauschinger Effect? |
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Definition
| the change in yield stress during cyclic deformation of a material due to 'memory' effects. This is reversible and can occur by dislocaiton pile ups at precipitates |
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Term
| what is the relationship between stacking fault energy and cross slip? |
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Definition
| High SFE>>easy cross slip>>rapid formation of stable cell structure |
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Term
| Discuss cyclic softening of precipitation hardened alloys |
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Definition
dislocations have the strength to cut precipitations and once they are below a particular size they dissolve in the material
slip is concentrated in narrow slip bands and it leaves regions of unstrengthened matrix vulnerable to high plastic deformation and early crack formation
often leads to precipitation free channels |
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Term
| discuss slip bands and crack initiation |
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Definition
- for the nucleation of cracks, there must be some local yielding. this comes from regions of high stress concentration (ie persistent slip bands)
- PSBs result from specific arrangements of dislocations as parallel walls with relatively low dislocaiton density between them
- the distribution of strain is not even in the PSB ldeading to the creation of extrusions and intrusions
[image]
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Term
where do cracks usually initiate in fatigue situations
describe the appearance of them |
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Definition
the surface of a material
sometimes internally at sites of defects or due to internal porosity. once they propagate to the surface, however, air is admitted, stress intensity increases, and growth rate increases leading to characteristic surface |
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Term
Paris Law
what is the shape of the graph and what to the different regions represent |
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Definition
- crack initiation folowing slip planes, there is usually some treshold value before start @ 45*
- crack propagates at 90* to tensile axis and striations are formed
- final rupture
[image]
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Term
| oxidation/corrosion and crack nucleation in fatigue situations |
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Definition
- no fatigue limit: material is immune from corrosion or oxidation
- brittle oxide layers crack and raise the stress at the tip sufficiently for crack to propagate into the substrate thus allowing further oxidation
- coatings with different mechanical properties to the substrate can accelerate cracking by promoting rapid initiation
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Term
| How can one design against failure in fatigue situations? |
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Definition
- reduce surface damage (scratches etc.) as they act as stress concentrators
- corrosion protection
- work harden
- less thermal barrier coatings
- heat at high pressure/stress to reduce pores
- crack interface bridging
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Term
| What law is Miner's law based on? |
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Definition
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