Anelastic and Viscoelastic Behaviour

Definition

Anelasticity is a time-dependent elastic deformation where the material returns to its original shape after the load is removed, but it does not do so instantaneously. Viscoelasticity is a property of materials that exhibit both viscous (fluid-like) and elastic (solid-like) characteristics when undergoing deformation, where the stress-strain relationship depends on time.

Main Content

1. Anelastic Behaviour

It is a form of time-dependent elastic deformation; the material is elastic, but there is a "lag" in the response.
This delay is primarily caused by local atomic processes, such as the rearrangement of atoms or the movement of small defects within the crystal lattice.

2. Viscoelastic Behaviour

This behavior is common in polymers and some metals at high temperatures, where molecular chains slide past each other.
It involves energy dissipation; when the load is removed, the material shows a permanent strain or a very slow, non-linear recovery process.

3. Stress-Strain Time Dependency

Unlike purely elastic materials (which follow Hooke’s Law instantly), anelastic and viscoelastic materials show a curve where strain increases over time under constant stress (Creep).
When the load is released, the strain recovery is not immediate but follows an exponential decay pattern.

Stress/Strain Response:
Stress: |-------| (Load applied)
Strain:  \      / (Instant elastic)
          \____/  (Time-dependent creep/recovery)

Working / Process

1. Application of Load (Loading Phase)

When a force is applied, the material undergoes immediate elastic deformation due to atomic bond stretching.
Following this, time-dependent deformation begins as internal structures reorganize or polymer chains uncoil.

2. Sustained Loading (Steady State)

If the load remains constant, the material may exhibit "creep," where strain continues to increase at a decreasing rate.
In viscoelastic materials, this represents the viscous flow component where energy is dissipated as heat.

3. Removal of Load (Unloading/Recovery)

Upon removing the stress, the material exhibits immediate elastic recovery.
The remaining delayed deformation gradually disappears over time as the internal configuration returns to the equilibrium state.

Advantages / Applications

Damping Systems: Viscoelastic materials are used in vibration isolators and shock absorbers because they dissipate kinetic energy as heat, protecting structures.
Polymer Processing: Understanding these behaviors is critical for injection molding and extrusion, ensuring plastic components maintain dimensional stability.
Aerospace Components: Predicting anelasticity is vital for turbine blades and airframes that experience cyclic loading and need to avoid permanent structural sag.

Summary

Anelasticity and viscoelasticity describe materials that respond to stress over time rather than instantaneously. While anelastic materials eventually recover their full shape, viscoelastic materials often retain some permanent deformation due to viscous flow. These behaviors are essential for designing systems that require vibration dampening and long-term structural reliability.

Important terms to remember: Creep, Hysteresis, Viscosity, Elastic Recovery, Energy Dissipation.