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14 Cards in this Set

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Conditions for Creep to start:
- Temperature higher than 0.4 Melting point.
- Stress (Less than Yield Stress)
- Time
3 Creep Mechanisms
- Viscous Creep
- Dislocation Creep
- Diffusion Creep
Viscous Creep
- Occurs in Polymers and Glass
- Uncoiling and sliding of large molecules
Dislocation Creep
- Occurs in Metals
- Some dislocations mobilise above Tc (0.4Tm), becuase diffusion of atoms allows them to break away from obstructions
- Predominates at high stress, low temperatures
Diffusion Creep
- Occurs in Metals and Ceramics
- Diffusion of atoms in grains result in grain elongation in direction of load
The Creep Process
- Initial elastic strain (not creep)
- Primary Creep
- Secondary Creep
- Tertiary Creep
- Rupture
Primary Creep
Initial plastic flow occurs.
As dislocations break away from obstructions in metals, or molecules start to slide in Polymers, which results in a relatively high strain rate occurs.
Secondary Creep
Hardening processes start acting.
Strain hardening in metals, or molecular entanglement in Polymers, which limit slip and give steady state creep.
Slope of graph in this section gives creep rate.
Tertiary Creep
Diffusion of atoms leads to voids at grain boundaries orthoganal to the loading direction. Cracks/necking may occur. Both Reduce components cross sectional area, and thus increase stress and hence creep rate.
Stress Relaxation
The micro-mechanisms that result in creep may cause a reduction in stress in a dimensionally constained system.
Avoiding Creep - Metals
- Select material with high Tm
- Use strengthening mechanisms stable at high temperatures, including Solid Solution and dispersion hardening to limit dislocation creep.

- Grain Boundary precipitates, which inhibit grain boundary sliding

- Directional solidification, produce components with no transverse grain boundaries to accumulate voids.
Avoiding Creep - Ceramics
- Select Materials with high Tm
- Use high purity materials, which reduce defects which allow diffusion
- Large grain size, increases diffusion distances, slowing deformation rate
Creep resistance in Polymers is affected by:
- Glass transition temperature, above Tg, chains are mobile relative to each other, allowing deformation
- Crystallinity, inhibits molecular movement, reducing creep rate.
- Molecular Mass, large molecules form more entanglements
- Cross linking, limits viscous flow in amorphous polymers above Tg, ie rubber
- Fibre Re-enforcement, effectiveness depends on fibre length and volume fraction
Creep Modulus (Polymers)
The Elastic Modulus (stress/strain) at a specific temperature and time.