Nonlinear Shear Behavior of Poly(vinyl acetate) Material
Tóm tắt
Because of the strong environmental sensitivity of poly(vinyl acetate), PVAc, especially with respect to moisture, and the fact that shear deformation is essentially equivoluminal up to moderate strain levels, little has been reported in the literature on the nonlinear mechanical creep behavior of this polymeric material loaded in shear. This paper presents the results of torsional tests which establish the shear response through the linear zone and well into the nonlinear region. Test specimens were thin-walled cylinders giving an approximately uniform deformation field. Because of carefully chosen wall thickness to length ratio, it is considered that these measurements represent some of the most accurate nonlinear shear results to date in the strain range above 1%. Measurements of stress, strain and creep compliance were made at temperatures near the glass transition temperature and somewhat below it. Isochronal shear stress-strain dependence into the nonlinear range was used to establish limits of viscoelastic linearity during creep. As temperature is increased toward the glass transition, the limit shows a greater dependence on stress than on strain. The stored distortional strain energy at the limit of linearity was not a constant but varied with temperature and load. Thus, these results appear not to support the concept of stored energy as a material property defining the threshold for nonlinear viscoelastic behavior. Strain during the short-time load-up period gives evidence that PVAc is also subject to nonlinear elasticity in the glassy response region.
Tài liệu tham khảo
Bertilsson, H. and Jansson, J., 'The limits of linear viscoelasticity in poly(methyl methacrylate) and poly(ethyl methacrylate)', Journal of Applied Polymer Science 19, 1975, 1971-1978.
Brueller, O.S., 'On the nonlinear characterization of the long term behavior of polymeric materials', Polymer Engineering and Science 27, 1987, 144-148.
Brueller, O.S. and Schmidt, H.H., 'On the linear viscoelastic limit of polymers - Exemplified on poly(methyl methacrylate)', Polymer Engineering and Science 19, 1979, 883-887.
Emri, I. and Pavsek, V., 'On the influence of moisture on the mechanical behavior of polymers', in Proceedings of VII International Congress on Experimental Mechanics, Vol. II, Society for Experimental Mechanics, Bethel, CT, 1992, 1429-1437.
Kenner, V.H., Knauss, W.G. and Chai, H., 'A simple creep torsiometer and its use in the thermorheological characterization of a structural adhesive', Experimental Mechanics 22, 1982, 75-80.
Knauss, W.G. and Emri, I., 'Volume change and the nonlinearly thermo-viscoelastic constitution of polymers', Polymer Engineering and Science 27, 1987, 86-100.
Knauss, W.G. and Kenner, V.H., 'On the hygrothermomechanical characterization of polyvinyl acetate', Journal of Applied Physics 51, 1980, 5131-5134.
Lu, H.B. and Knauss, W.G., 'Nonlinear polymer response under biaxial stress states', in Abstract Proceedings of VIII International Congress on Experimental and Applied Mechanics, Society for Experimental Mechanics, Bethel, CT, 1996, 182-183.
Schapery, R.A., 'On the characterization of nonlinear viscoelastic materials', Polymer Engineering and Science 9, 1969, 295-310.
Sedath, R.H., 'Analysis of the nonisothermal and nonlinear viscoelastic properties of amorphous polymers using a thermodynamic equation of state', Ph. D. Thesis, Purdue University, West Lafayette, IN, 1987, 149-158.
Volmir, A.S., Stability of Deformable Systems, Publishing Office of Physico-Mathematical Literature, Moscow, 1967, 557-561 [in Russian].
Williams, M.L. and Arenz, R.J., 'The engineering analysis of linear photoviscoelastic materials', Experimental Mechanics 4, 1964, 249-262.