Relaxation of shear and normal stress components in step-strain experiments

The relaxation of torque and axial thrust following a rapidly imposed step in shear strain ( 0 < γ ≤ 3.33) was measured for several undiluted polymers and polymer solutions using the cone-and-plate geometry in a Rheometrics mechanical spectrometer. Instrumental response times were favorable for calculating both the shear stress σ(γ, t) and the first normal stress difference N₁(γ,t) from torque and thrust data on two of the solutions. Those results confirm the relationship N₁(γ,t) = γσ(γ, t). Apparent departures from this relationship appear if σ(γ,t) and N₁(σ, t are calculated from torque and thrust data when the instrumental response criteria are not met.