Abstract
A micromechanical investigation of the transverse creep behavior of unidirectionally reinforced glass fibre composites with an unsaturated polyester matrix is presented. More specifically the influence of initial strains induced by polymerization shrinkage of the unsaturated polyester is studied. A previously established nonlinear viscoelastic model of the deformational behavior of the unsaturated polyester matrix is used in the numerical calculations. The matrix model includes effects of physical ageing of the resin and, in its general 3D-formulation, is able to describe time-dependent lateral contraction (Zhang, Ernst and Brouwer, 1997. Mech. Mater. 26, 141–166, 167–195). It has been shown that this uniaxially characterized 3D model can be used to describe three-dimensional loading situations successfully. In another paper (Zhang, Ernst and Brouwer, 1998. Mech. Mater. 27 (1998) 13–36), the theory has been further applied to model the transverse creep of the composites. New fibre packing geometries have thereto been presented. These packing geometries overcome the shortcomings of the traditional rectangular and hexagonal geometries, as in some sense they take into account the inhomogeneous fibre distribution in the transverse plane. In this paper, the initial strains induced by curing shrinkage of the unsaturated polyester during the manufacturing process of the composite are introduced. The model calculations show that the stresses caused by the initial strains gradually relax in the composite even without external loading. Complete relaxation of these stresses however takes a very long time and may never be realized in practice. It is further shown that, although the global creep behavior is relatively insensitive to the presence of initial strains, the local stress distribution is completely distorted. This result suggests that polymerization shrinkage significantly influences the onset of crack initiation in composites.
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