Creep Flashcards
What is the definition of creep in metals?
The time-dependent plastic deformation of metals and alloys caused by a constant load applied even for long periods at temperatures generally higher than room-temperature.
How can creep testing be done?
By measuring the strain evolution of a specimen subjected to a constant load (usually the force is kept constant) along time.
Explain what is happening at the three different stages of creep.
- Primary stage: Metal starts to creep after application of a constant force/stress.
At the end voids start to form because of diffusion at GB. - Secondary stage: The resulting strengthening from initial workhardening lowers the creep rate to a minimum value, which remains constant. Because of balanced deformation-related strengthening (increase in the dislocation density) and thermally related softening (recovery, decrease in the dislocation density). At the end the voids gradually coalesce so that microcracks start to appear
- Tertiary stage: Increased creep rate due to internal damage and necking. Failure.
Why does the creep rate remain at a constant minimum value in the secondary stage?
Deformation related strenghtening will increase the dislocation density, while at the same time thermally related softening will decrease the dislocation density. We obtain a balancce between these two.
What are the six different mechanisms by which creep can happen?
- Tangential stress exceeding the ideal shear stress of the lattice
- Glide of dislocations
- Twinning
- Plasticity at high temperature, due to glide and climb of dislocations
- Diffusion within the crystal lattice (Nabarro-Herring creep)
- Diffusion at grain boundary (Coble creep)
What are Ashby-maps?
They predict what is the main creep machanisms under a known temperature and material
Where and why do wedge cracks usually happen?
At triple joints of GBs. Due to sliding of GBs.
