On the use of stress growth data to determine strain-dependent material functions for factorable K-BKZ equations

The procedure of using experimental stress growth data to determine strain-dependent, or damping, functions for factorable K-BKZ constitutive equations is examined. Damping functions obtained using stress growth data from simple shear and planar extensional flows in conjunction with integrated forms of factored K-BKZ models are compared with the damping function obtained from shear step-strain experiments. Results for the low-density polyethylene (LDPE) known as IUPAC X are presented that demonstrate the difficulties encountered with this method, which can be quite serious for planar extensional flow. These results appear to be consistent with previous studies on several polymer melts where uniaxial extension stress growth data were used to determine damping functions. This body of evidence raises important questions about the use of this procedure and the strain-dependent and, apparently, test-dependent functions that are obtained. An explanation for these results is proposed which suggests that these observations may be due to an intrinsic shortcoming of K-BKZ theories.