Viscoelasticity in thermoforming

When polymeric liquids undergo large‐amplitude shearing oscillations, the shear stress responds as a Fourier series, the higher harmonics of which are caused by fluid nonlinearity. Previous work on large‐amplitude oscillatory shear flow has produced analytical solutions for the first few harmonics of a Fourier series for the shear stress response (none beyond the fifth) or for the normal stress difference responses (none beyond the fourth) [JNNFM, 166, 1081 (2011)], but this growing subdiscipline of macromolecular physics has yet to produce an exact solution. Here, we derive what we believe to be the first exact analytical solution for the response of the extra stress tensor in large‐amplitude oscillatory shear flow. Our solution, unique and in closed form, includes both the normal stress differences and the shear stress for both startup and alternance. We solve the corotational Maxwell model as a pair of nonlinear‐coupled ordinary differential equations, simultaneously. We choose the corotational Maxwell model because this two‐parameter model (η0 and λ) is the simplest constitutive model relevant to large‐amplitude oscillatory shear flow, and because it has previously been found to be accurate for molten plastics (when multiple relaxation times are used). By relevant we mean that the model predicts higher harmonics. We find good agreement between the first few harmonics of our exact solution, and of our previous approximate expressions (obtained using the Goddard integral transform). Our exact solution agrees closely with the measured behavior for molten plastics, not only at alternance, but also in startup.

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“…Eqs. (48) and (50) also demonstrate why the interpretation of torque in oscillatory paralleldisk flow is more complicated than in cone-plate flow.…”

mentioning

confidence: 96%

“…For a detailed treatment of the corotational Maxwell model see Chapters 7 and 8 of [44], and for its evaluation in large-amplitude oscillatory shear, see [15]. The corotational Maxwell model can also be used to explore complex flows arising in plastics processing, both analytically [44,45,46,47,48,49,50], and numerically [51,52].…”

mentioning

confidence: 99%