Nonlinear thermoviscoelastic behavior of polymers

• Main Author: Losi, Giancarlo Umberto Maria
• Format: Others
• Language: en
• Published: 1990
• Online Access: https://thesis.library.caltech.edu/4381/1/Losi_gu_1990.pdf; Losi, Giancarlo Umberto Maria (1990) Nonlinear thermoviscoelastic behavior of polymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2dk7-z575. https://resolver.caltech.edu/CaltechETD:etd-11032003-103818 <https://resolver.caltech.edu/CaltechETD:etd-11032003-103818>

The rheological behavior of polymers in the neighborhood of the glass transition has been investigated in the framework of the free volume theory of nonlinear viscoelastic behavior. Free volume theory as normally applied above the glass transition was modified to account for the effect of the residual volume of vacancies below the glass transition; this modification was accomplished by modelling the changes in the state of the polymer as the sum of viscoelastic changes and a random disturbance deriving from the thermal collisions between molecules. The changes in mechanical properties going across the glass transition follow from the freezing-in of relaxation mechanisms and of free volume. The pressure dependence of the glass transition was found to be in qualitative agreement with measurements on PVAc, while the ratio of the glassy and rubbery heat capacities was found to coincide with the ratio of the equilibrium bulk compliances in the glassy and rubbery domains. The predictions of the model for the problem of transient and residual thermal stresses were compared with those of two simpler models. The second part of the thesis studies the consequences of the nonlinear viscoelastic behavior on the decohesion zone in front of a crack propagating through an adhesive layer. The softening of the material response in the cohesive zone is taken to be effected by free volume induced change in relaxation times of the cohesive material and by void growth; the latter is assumed to depend on a critical value of strain at the beginning of the cohesive zone. The stress intensity factor for steady crack propagation is obtained by imposing the finiteness of strains at the crack tip. For the case where the properties of the adherends are the same as the linearized properties of the adhesive, the predictions show three regimes of crack propagation: a low speed regime where the adherends behave elastically with the rubbery properties, an intermediate range where their response becomes increasingly stiffer, and a high speed regime characterized by glassy behavior of the adherends and control of the crack growth process exclusively by the nonlinearly viscoelastic behavior of the failing material.