Materials Science > Mechanical Properties > Creep
Creep in Materials Science:
Creep is a time-dependent and permanent deformation of materials when subjected to a constant load or stress. Unlike elastic deformation, which occurs rapidly when a load is applied and removed, creep occurs slowly and progressively over an extended period, especially under high temperatures relative to the material’s melting point.
Creep Mechanisms:
Creep can be explained by multiple mechanisms that become dominant depending on the stress and temperature:
Dislocation Creep: Predominant at relatively high stresses, this occurs via the movement of dislocations through a crystalline lattice. The dislocations can bypass obstacles at high temperatures through processes such as climb, enabled by the diffusion of atoms.
Diffusion Creep: Predominant at lower stresses and high temperatures, this can occur via:
- Nabarro-Herring Creep: Atomic diffusion through the crystal lattice, leading to a change in shape.
- Coble Creep: Diffusion of atoms along grain boundaries.
Grain Boundary Sliding: At high temperatures, grains can slide past each other, which contributes to creep deformation.
Stages of Creep:
Creep typically progresses through three stages:
- Primary (Transient) Creep: This initial stage features a decreasing creep rate. The material undergoes strain hardening, which temporarily impedes further deformation.
- Secondary (Steady-State) Creep: During this stage, the creep rate becomes relatively constant. It represents a balance between strain hardening and recovery processes.
- Tertiary Creep: This final stage is characterized by an accelerating creep rate leading to material failure. It often involves necking, internal cracking, and other forms of localized deformation.
Mathematical Description of Creep:
The creep strain, \(\epsilon_{cr}(t)\), as a function of time \(t\), is often modeled empirically. One common representation for steady-state creep is the Norton-Bailey law, given by:
\[
\dot{\epsilon}_{cr} = A \sigma^n e^{-\frac{Q}{RT}}
\]
where:
- \(\dot{\epsilon}_{cr}\) is the creep rate,
- \(A\) is a material constant,
- \(\sigma\) is the applied stress,
- \(n\) is the stress exponent,
- \(Q\) is the activation energy for the creep process,
- \(R\) is the universal gas constant,
- \(T\) is the absolute temperature.
These parameters must be determined experimentally for each material and condition.
Importance in Engineering:
Understanding creep is critical in engineering applications where materials are exposed to high temperatures for long periods. This includes components in jet engines, nuclear reactors, and even everyday electronics. Materials engineers must choose materials and design components to minimize creep and ensure the longevity and safety of the structure.
Overall, creep is a fundamental aspect of materials science that combines principles of thermal dynamics, solid mechanics, and materials characterization to ensure the reliability and durability of engineered structures under long-term stress.