Stress Relaxation

Elastic materials keep their stress after deformation. Viscous fluids (such as water or glycerine) by contrast relax the stress immediately upon cessation of deformation. Polymers and soft materials are examples of viscoelastic materials where the stress decays gradually aften deformation. The VADER 1000 offers unique measurements of stress relaxation after large extensional deformation. This may be used to probe the molecular relaxation times in the non-linear regime.

Figure 3: Stress divided by stretch rate during steady deformation and subsequent stress relaxation. Red: Polystyrene, Blue: Low density polyethylene. The gray data corresponds to a complete extension experiment for sample.		Figure 4: Measurements from Figure 3 shifted to start at the instant of flow cessation. A power law relaxation is revealed over two decades of time.