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Creep: Materials Science Stresses Yield Strength

Creep is the slow deformation of materials under persistent mechanical stress over time. It occurs when materials are subjected to stresses below their yield strength, especially at high temperatures near their melting points. Creep deformation happens in three stages - primary creep where the strain rate decreases over time, secondary creep where the strain rate is constant, and tertiary creep where damage accumulates and the strain rate rapidly increases until failure. The rate of creep depends on the material properties, exposure time and temperature, and applied load. Creep can cause failure in components over time and is an important consideration in designing structures operating under heat and stress.

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494 views3 pages

Creep: Materials Science Stresses Yield Strength

Creep is the slow deformation of materials under persistent mechanical stress over time. It occurs when materials are subjected to stresses below their yield strength, especially at high temperatures near their melting points. Creep deformation happens in three stages - primary creep where the strain rate decreases over time, secondary creep where the strain rate is constant, and tertiary creep where damage accumulates and the strain rate rapidly increases until failure. The rate of creep depends on the material properties, exposure time and temperature, and applied load. Creep can cause failure in components over time and is an important consideration in designing structures operating under heat and stress.

Uploaded by

Saad khan
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Creep

In materials science, creep (sometimes called cold flow) is the tendency


of a solid material to move slowly or deform permanently under the
influence of persistent mechanical stresses. It can occur as a result of
long-term exposure to high levels of stress that are still below the yield
strength of the material. Creep is more severe in materials that are
subjected to heat for long periods and generally increases as they near
their melting point.
The rate of deformation is a function of the material's properties,
exposure time, exposure temperature and the applied structural load.
Depending on the magnitude of the applied stress and its duration, the
deformation may become so large that a component can no longer
perform its function — for example creep of a turbine blade could
cause the blade to contact the casing, resulting in the failure of the
blade. Creep is usually of concern to engineers and metallurgists when
evaluating components that operate under high stresses or high
temperatures. Creep is a deformation mechanism that may or may not
constitute a failure mode. For example, moderate creep in concrete is
sometimes welcomed because it relieves tensile stresses that might
otherwise lead to cracking.
Unlike brittle fracture, creep deformation does not occur suddenly
upon the application of stress. Instead, strain accumulates as a result of
long-term stress. Therefore, creep is a "time-dependent" deformation.
It works on the principle of Hooke's law (stress is directly proportional
to strain).

Stages of Creep
Strain as a function of time due to constant stress over an extended
period for a Class M material.
Creep behavior can be split into three main stages.
Primary Creep
In primary, or transient, creep, the strain rate is a function of time. In
Class M materials, which include most pure materials, strain rate
decreases over time. This can be due to increasing dislocation density,
or it can be due to evolving grain size. In class A materials, which have
large amounts of solid solution hardening, strain rate increases over
time due to a thinning of solute drag atoms as dislocations move.
Secondary Creep
In the secondary, or steady-state, creep, dislocation structure and grain
size have reached equilibrium, and therefore strain rate is constant.
Equations that yield a strain rate refer to the steady-state strain rate.
Stress dependence of this rate depends on the creep mechanism.
Tertiary Creep
In tertiary creep, the strain rate exponentially increases with stress. This
can be due to necking phenomena, internal cracks, or voids, which all
decrease the cross-sectional area and increase the true stress on the
region, further accelerating deformation and leading to fracture.
Temperature Dependent
The temperature range in which creep deformation may occur differs in
various materials. Creep deformation generally occurs when a material
is stressed at a temperature near its melting point. While tungsten
requires a temperature in the thousands of degrees before creep
deformation can occur, lead may creep at room temperature, and ice
will creep at temperatures below 0 °C (32 °F).] Plastics and low-melting-
temperature metals, including many solders, can begin to creep at
room temperature. Glacier flow is an example of creep processes in
ice.The effects of creep deformation generally become noticeable at
approximately 35% of the melting point for metals and at 45% of
melting point for ceramics.

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