Creep Flashcards
Define ‘creep’.
Creep is the permanent elongation of a component under static load maintained for a period of time.
Define ‘stress relaxation’.
The decrease in stress in a component under a constant strain over a period of time.
Why conditions are typically present for creep to occur?
- Under load/time/temperature.
- Typically only significant at elevated temperatures (40% of T_m for metals, above T_g for plastics).
What mechanisms allow creep occur at elevated temperatures for the following:
A) Metals
B) Plastics
A) Grain boundary sliding
B) Slip of polymer chains and alignment parallel to applied stress.
What behaviours occur at the following stages of creep:
A) Stage I - Primary, Transient
B) Stage II - Secondary, Steady State
C) Stage III - Tertiary, Transient
A) Increasing dislocations, work hardening, reducing creep rate.
B) Balanced (work hardening vs thermal softening), design region.
C) Necking, cracking, increasing rate.
What is reverse creep?
Reverse creep is the phenomenon of creep strain reversal on unloading. However, creep in metals is typically unrecoverable.
List the primary assumption of creep strain rate functions.
Assuming that for a given creep time or creep strain, the creep rate is a function of current stress and temperature only.
- Time-hardening
- Strain-hardening
Define viscoelasticity. How is it modelled? What are the three key models?
Viscoelasticity is the typical behaviour of non-metallic materials (e.g. thermoplastics). They are modelled using combined spring (elastic) and dashpot (viscous) models.
- Series “Maxwell” model.
- Parallel “Voigt/Kelvin” model.
- Standard Linear Solid model.
Why is the Standard Linear Solid (SLS) model useful?
- Provides the simplest model providing good representation of observed creep and recovery in polymers.
- Instantaneous and time dependent creep.
- Instantaneous (recovery) and time dependant (relaxation) when load removed.
