NONLINEAR VISCOELASTICITY DETERMINED FROM INTERNAL-FRICTION

Internal friction (IF) peaks measured as function of temperature or fr equency are associated with nonlinear processes only if a systematic s tudy determines the influence of the amplitude of the applied stress o n the relaxation process. Nonlinear viscoelasticity, however, can be m ore easily determined just by noting incompatibilities between experim ental IF data and the postulates linear viscoelasticity. The aim of th is work is to demonstrate that the logarithmic derivative of the inter nal friction with respect to the frequency is a useful than one then n onlinear mechanisms are involved in the relaxation process. Nothing ca n be said about the linearity, however, if the derivative is constrain ed to the interval [-1,1]; only measurements at different stress ampli tudes should determine nonlinear effects. Not only the theoretical fun damentals are detailed but also several examples are shown as an appli cation of the procedure developed to find nonlinear effects. Finally, the linearity of the Snoek and Zener relaxations is analyzed through t he calculation of the logarithmic derivative from IF measurements made as a function of both frequency and temperature.