MultiaxialFatigue-LCF Flashcards
Mention the 3 main criterias for the Multiaxial Fatigue - LCF
- Strain based static Yield criteria
- Energy model
- Critical Plane Approach
which are the models for the Strain based static yield criteria?
VON MISES - theory of the octahedral shear strain theory
which are the models for the Energy Model?
- Morrow
- Garud
- Ellyin
which are the models for the Critical Plane Approach?
- Brown Miller
- Fatemi socie
- Smith Watson Tapper Model
- FatemiSocie+SWTM
Main characteristics of VON MISES model
Strain tensor in function of time
vaule of each strain i & j values
-evaluate in Tension-compression uniaxial loading & Torsional loading
equivalent strain approach is unsuitable for the case of non-proportional loading. The equivalent STRAIN AMPLITUDE is the same for both porportional and non proportional loading
VON MISES model PROS
- simple
- non additional parameters
VON MISES model CONS
Equivalent Strain cannot account for experimentally observed differences in the ratio of tension-torsion fatigue strength for many materials.
- Unsuitable for non-proportional loading path
- Does not address the effect of a superimposed mean STRESS / STRAIN
- Equivalent Strain approaches do not explain the observed nucleation and propagation of fatigue cracks and specific planes
Main characteristics of MORROW model
- Plastic strain is related to the movement of dislocations and the cycle stress is related to the resistance to ther motion.
- Fatigue resistance for a metal maybe characterized in terms of its capacity to absorb and dissipate plastic strain energy.
- On one side we have the STRAIN related of the movement of dislocations and on the other the resistance motion of dislocations
Main characteristics of GARUD model (derive it)
- if the material is NOT Masing we cannot use MORROW due to the fact that is based on masing material concept.
- We need to fit experimental data with a straight line
- GARUD takes MORROW’s uniaxial hypothesis loop energy concepts to multiaxial fatigue.
*P & q = parameters added because are NOT masing material
Main characteristics of GARUD model (derive it)
- if the material is NOT Masing we cannot use MORROW due to the fact that is based on masing material concept.
- We need to fit experimental data with a straight line
- GARUD takes MORROW’s uniaxial hypothesis loop energy concepts to multiaxial fatigue.
*P & q = parameters added because are NOT masing material
Mention some GARUD CONS
- not address the effect of a superimposed mean STRESS/STRAIN
- Strain energy is scalar quantity (crack nucleates and propagate along specific planes)
- In HCF plastic work per cycle is very small and is difficult to accurately measure or calculate.
- Reducing STRAIN AMPLITUDE lead to Increas Nf and hysteresis loop become norrow so the area decrease
Main characteristics of ELLYIN model
- If we are close to a HCF cycle
- Wt = Total energy per cycle [elastic+plastic]
- Plastic part define = as GARUD model (DELTAWp)
- Elastic part define = just the positive part of elastic strain energy
Mention some ELLYIN PROS
- no additional parameters
- GAROUD aproximaiton
- with positive STRAIN energy provides away to introduce mean stress effects into energy based criterion
- Avoids the problems associated with calculating small plastic strains
Mention some ELLYIN PROS
- no additional parameters
- GAROUD aproximaiton
- with positive STRAIN energy provides away to introduce mean stress effects into energy based criterion
- Avoids the problems associated with calculating small plastic strains
Main characteristics of CRITICAL PLANE MODELS
NUCLEATION = TFatigue life represented by crack-nucleation or the formation of a non-propagation crack that is a solid line.
SHEAR = Failure mode that is dominated by shear crack growth
TENSION = Shear crack nucleation is followed by crack growth on planes of maximum principal strain/stress
Main characteristics of BROWN-MILLER
- Close to Matake Criterion
- Cycle shear & normal strain on the plane of maximum shear.
- Insted of have STRESS we have STRAIN
- New parameter = S
- Radius of minimum circumstrcibed circle -> to shear strain
- Change THETHA and PHI to look fot max shear strain amplitude (plane with max shear strain amp in the critical plane)
Mention some BROWN-MILLER PROS
- suitable for the case of non-prop. loading path.
- mean stress effects is introduced using MORROW’s
- observe cracks nucleate & Propagate along specific planes.
- ration tension-torsion fatigue strength is material dependent (through S)
Mention some BROWN-MILLER PROS
- suitable for the case of non-prop. loading path.
- mean stress effects is introduced using MORROW’s
- observe cracks nucleate & Propagate along specific planes.
- ration tension-torsion fatigue strength is material dependent (through S)
Critical plane Model FATEMI SOCIE
Shear failure mode (region A)
- depending on material stress state environment strain amplitude fatigue life.
- usually will be dominate by CRACK growth along either shear planes or tensile planes.
- models can be able to predict fatigue life and dominant failure plane.
Critical plane Model SMITH WATSON TAPPER
Tensile failure mode (region B)
- Alternative model to crack growth predominantly fails for materials on planes of maximum tensile strain or stress.
- These materials crack nucleates in shear but early life is controlled by crack growth on planes perperndicular to the maximum princpial stress and strain
-SWT parameter for multiaxial loading is based on the maximum normal strain range (DELTASTRAINMAX) and maximum stress on the maximum normal strain range plane (SIGMA N,max)
Mention some SMITH WATSON TAPPER PROS
-Suitable for the case of non-proportional loading.
-Mean stress is introduced in the equivalent
-It addresses the observation that cracks nucleate and propagate along specific planes.
-Suitable for both tensile and shear dominant failure modes
Nf = min [NfsocieNfswt]
Mention some SMITH WATSON TAPPER PROS
-Suitable for the case of non-proportional loading.
-Mean stress is introduced in the equivalent
-It addresses the observation that cracks nucleate and propagate along specific planes.
-Suitable for both tensile and shear dominant failure modes
Nf = min [NfsocieNfswt]
Mention some SMITH WATSON TAPPER CONS
-One additional material parameter is introduced into the analysis (K parameter)
