Creep Test Flashcards
Creep
Time-dependent and permanent deformation of materials when subjected to a constant load or stress over a long period of time
Creeping temperature depends on the melting point
Creep Behavior
- instantaneous deformation: mainly elastic
- primary/transient creep: slope of strain vs. time decreases with time
- secondary/steady-state creep: rate of straining is constant
- tertiary creep: rapidly accelerating strain rate up to failure - formation of internal cracks, voids, grain boundary separation, necking
Displacement-limited applications
precise dimensions or small clearances must be maintained such as in turbine rotors in jet engines
Rupture-limited applications
precise dimensions are not essential but fracture must be avoided such as in high-pressure steam tubes and pipes
Stress-relaxation-limited appliations
initial tension relaxes with time such as in suspended cables and tightened bolts
Creep variables
material properties, exposure time, exposure temperature, applied load (stress)
Stress and temperature effects on creep
Strain is independent of time when T
Creep Test methods
Tensile Creep Testing, Compressive Creep Testing, Flexural Creep Testing
Minimum/Steady State Creep Rate
Slope of secondary portion of the creep curve which is considered for long-life applications
Rupture Lifetime/Time to Rupture
Time required till rupture at a constant load considered for many relatively short-life creep situations
Creep Modulus
Ratio of initial applied stress to creep strain at a specific time
Creep Strength/Limit
Highest stress that a material can withstand for a specified length of time without exceeding the specified deformation at a give temperature
Larson-Miller Parameter
Φ=T(C+log tr)
T=temp in Kelvin
C=20 (constant)
tr=time in hours
