Unit 9 - Mixed Mode Regimes

Question 1

What are the characterisitics of “CREEP - Fatigue” interaction?

Answer 1

• This is creep accelerated by fatigue
• Cyclic stress or strain amplitude is small compared to mean stress
• Temperature is high and applied cyclic frequency is low
• Fracture surfaces exhibit a tendancy towards intergranular fracture

Question 2

What are the characterisitics of “FATIGUE - Creep” interaction?

Answer 2

• This is fatigue accelerated by creep
• Cyclic stress or strain amplitude is large copared to mean stress
• Temperature is low and applied frequency is high
• Fracture surfaces are manifested by fatigue striations and regions of transgranular fracture

Question 3

How does fatigue crack nucleation occur?

Answer 3

• Cyclic slip
• Grain boundary cavitation
• Grain boundary sliding and associated development of wedge cracks
• Nucleation growth of voids at inclusions/precipitates
• Oxidation and corrosion

Question 4

What can cause an embrittling effect on the fatigue life of materials?

Answer 4

• Oxygen in the air
• Elevated temperatures
• Frequency of cyclic loading

Question 5

What effects can high temperature or low frequency have during Low Cycle Fatigue (LCF) testing?

Answer 5

Positive:
* Slip dispersal
* Strain ageing
* Crack-tip blunting
* Microstructural coarsening

Negative:
* Creep damage
* Environmental effects

Question 6

What effects can low temperature or high frequency have during Low Cycle Fatigue (LCF) testing?

Answer 6

Positive:
* Elimination of creep and environmental effects

Negative:
* Increased planarity of slip

Question 7

What effects can tensile holds during Low Cycle Fatigue (LCF) testing have?

Answer 7

Positive:
* Development of a compressive mean stress

Negative:
* Creep damage in the form of grain boundary cavities

Question 8

What effects can compressive holds during Low Cycle Fatigue (LCF) testing have?

Answer 8

Positive:
* Sintering of grain boundary cavities

Negative:
* Development of tensile mean stress

Question 9

What effects can fatigue followed by creep loading have during Low Cycle Fatigue (LCF) testing?

Answer 9

Positive:
* Cyclic work hardening (depending on the material)

Negative:
* Cyclic work softening (depending on the material)

Question 10

What effects can creep followed by fatigue loading have during Low Cycle Fatigue (LCF) testing?

Answer 10

Positive:
* Precipitation of strengthening phases

Negative:
* Little effect when grain boundary damage does not occur

Question 11

What factor can determine whether isothermal or thermo mechanical fatigue occur?

Answer 11

Constraint due to:
* Thermal gradients
* Material anisotropy
* Geometry effects

Question 12

Which form of fatigue is more detrimental out of isothermal and thermo mechanical?

Answer 12

Thermo mechanical fatigue (TMF) is far more damaging than typical isothermal fatigue. This is even the case at the maximum operating temperature.

Question 13

What is the phase angle for a TMF test?

Answer 13

The shift which occurs between the mechanical strain and the temperatrue applied to the material. If the strain and temp increase/decrease in proportion with eachother, the cycle is in-phase (IP). If one always increases as the other decreases, the cycle is out-of-phase (OOP), more accurately, 180 degrees out of phase.