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Modelling of Mechanical Behavior of Materials - A > MultiaxialFatigue-LCF > Flashcards

MultiaxialFatigue-LCF Flashcards

1
Q

Mention the 3 main criterias for the Multiaxial Fatigue - LCF

A
  • Strain based static Yield criteria
  • Energy model
  • Critical Plane Approach
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2
Q

which are the models for the Strain based static yield criteria?

A

VON MISES - theory of the octahedral shear strain theory

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3
Q

which are the models for the Energy Model?

A
  • Morrow
  • Garud
  • Ellyin
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4
Q

which are the models for the Critical Plane Approach?

A
  • Brown Miller
  • Fatemi socie
  • Smith Watson Tapper Model
  • FatemiSocie+SWTM
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5
Q

Main characteristics of VON MISES model

A

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

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6
Q

VON MISES model PROS

A
  • simple

- non additional parameters

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7
Q

VON MISES model CONS

A

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
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8
Q

Main characteristics of MORROW model

A
  • 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
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9
Q

Main characteristics of GARUD model (derive it)

A
  • 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

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10
Q

Main characteristics of GARUD model (derive it)

A
  • 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

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11
Q

Mention some GARUD CONS

A
  • 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
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12
Q

Main characteristics of ELLYIN model

A
  • 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
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13
Q

Mention some ELLYIN PROS

A
  • 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
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14
Q

Mention some ELLYIN PROS

A
  • 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
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15
Q

Main characteristics of CRITICAL PLANE MODELS

A

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

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16
Q

Main characteristics of BROWN-MILLER

A
  • 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)
17
Q

Mention some BROWN-MILLER PROS

A
  • 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)
18
Q

Mention some BROWN-MILLER PROS

A
  • 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)
19
Q

Critical plane Model FATEMI SOCIE

A

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.
20
Q

Critical plane Model SMITH WATSON TAPPER

A

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)

21
Q

Mention some SMITH WATSON TAPPER PROS

A

-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]

22
Q

Mention some SMITH WATSON TAPPER PROS

A

-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]

23
Q

Mention some SMITH WATSON TAPPER CONS

A

-One additional material parameter is introduced into the analysis (K parameter)

Modelling of Mechanical Behavior of Materials - A (10 decks)

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