Materials Engineering - Creep

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Conditions for Creep to start:

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- Temperature higher than 0.4 Melting point.
- Stress (Less than Yield Stress)
- Time

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3 Creep Mechanisms

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- Viscous Creep
- Dislocation Creep
- Diffusion Creep

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Viscous Creep

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- Occurs in Polymers and Glass
- Uncoiling and sliding of large molecules

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Dislocation Creep

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- 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

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Diffusion Creep

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- Occurs in Metals and Ceramics
- Diffusion of atoms in grains result in grain elongation in direction of load

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The Creep Process

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- Initial elastic strain (not creep)
- Primary Creep
- Secondary Creep
- Tertiary Creep
- Rupture

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Primary Creep

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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.

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Secondary Creep

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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.

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Tertiary Creep

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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.

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Stress Relaxation

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The micro-mechanisms that result in creep may cause a reduction in stress in a dimensionally constained system.

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Avoiding Creep - Metals

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- 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.

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Avoiding Creep - Ceramics

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- Select Materials with high Tm
- Use high purity materials, which reduce defects which allow diffusion
- Large grain size, increases diffusion distances, slowing deformation rate

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Creep resistance in Polymers is affected by:

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- 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

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Creep Modulus (Polymers)

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The Elastic Modulus (stress/strain) at a specific temperature and time.