Constant Stress/Creep

In creep testing a constant extensional stress σ↓0 is applied at time t=0 and the subsequent Hencky strain is followed as function of time. The extensional creep compliance is defined as J↑Ε(t)=∈(t)/σ↓0 A large creep complience signifies a soft material, while a small creep compliance signifies a hard material. For viscoelastic liquids, the creep compliance ultimately increases linearly in time with a slope that is the inverse of the steady extensional viscosity.

Extensional stress as a function of time for poly(methyl methacrylate)-based copolymer (poly(MEA-co-UPYMA) 8%).		Creep compliance as a function of time for poly(methyl methacrylate)-based copolymer at 60C. The red line is the prediction from small amplitude oscillation.