Damage And Strength Behavior Analysis Of Some Polymers

Polymers are typical viscoelastic materials, and their mechanical properties depend ontemperature, loading time, loading speed and strain etc, among which the loading time andthe temperature are the main influence factors. In practice, long-time creep behavior underconstant stress is needed in most engineering structures. On the foundation of time-stressequivalent theory, long-time creep features can be evaluated in short time by enlarging thestress level. In this paper, PMMA specimens are used in creep experiment under differentconstant stresses. A nonlinear viscoelastic constitutive equation is established throughanalysis of the test results, and the corresponding long-time creep can be predicted fromthose results. Polymers usually show nonlinear or local deformation under high stress or long-timeload. The deformation localization will result in damage in the material, and the craze is atypical pattern in the damaged region of polymers. According to the existing research, therupture of molecules induces the craze, and the extending of the craze will result in macro-crack. So the growth process of craze plays a very important role in the rupture of polymers.In this paper, the crazing process of PMMA is experimentally investigated under differentconstant stresses. The damage evolvement rule is derived from the test, and the stress in thedamaged area is defined that is very useful for the understanding of polymer' strengthbehavior and their life prediction. Finally, the nonlinear viscoelastic constitutive equation and the damage evolvementrule are combined to construct a relationship of stress-strain in the damaged area, which iseffective for PMMA with damage. The effect of damage evolvement on mechanical properties of polymers has beendescribed in this paper, and the stress in the damaged region is defined. Compared withtraditional craze-stress hypothesis, the defined stress and the proposed stress-strainrelationship in damaged area are more applicable.