12. Stress relaxation after a step deformation

The relaxation process

When a plastic body is suddenly deformed then a stress will be created inside the body. The deformation is kept constant. First the stress will be relatively high, but it will continuously reduce with time. The speed of the stress reduction is fast in the beginning and slows down later, as shown in the figure below.

The reduction of stress with time is called stress relaxation. It occurs both in the glass phase and in the rubber phase.

In the glass phase the stress relaxation is caused by rotation of chain segments (Kuhn segments). The fast decay in the beginning is due to the high stress that reduces the segmental rotation time.

In the rubber phase the stress relaxation is caused by reptation of the deformed polymer molecules into new positions. The fast decay in the beginning is again due to the high stress that reduces the reptation time.

Summary

  • Stress relaxation after a sudden deformation occurs both in the glass phase and in the rubber phase.
  • In the glass phase the relaxation is caused by rotation of the chain segments.
  • In the rubber phase the relaxation is caused by reptation of the polymer molecules into new positions.
  • The decay of stress is fast in the beginning and slows down later. This is caused by the influence of the stress on the segmental rotation time and the reptation time.

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