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The applicability and service time of polymer details working in various media are limited by the degradation of their physical and mechanical properties. From the structural point of view the weakest places strongly affected by ageing are the surface and boundary regions of the crystallites.
If a material is subjected to high-strain deformation, it deforms permanently (plastic deformation) and ultimately fails. For sufficiently low stresses and strains, the polymeric material behaves as a linear elastic solid. The point where the behaviour starts to be non-linear is called the proportional limit. The local maximum in the stress-strain curve is called the yield point and indicates the permanent deformation.
The corresponding stress and elongation are called yield strength and elongation at yield. Beyond the yield point the material stretches out considerably and this region is called the plastic region. Further elongation leads to strain hardening and the ultimate rupture of the material. At the rupture point the corresponding stress and strain are called the ultimate strength and the elongation at break.
The stress-strain behaviour of a polymeric material depends on various parameters such as molecular characteristics, microstructure, strain-rate and temperature. High-strain behaviour and failure for different kinds of polymers is described in this paper.
Furthermore, the physical ageing of glassy polymers, due to the heat effect on the mechanical properties, is also described. Composite materials, as polymers, are being tested due to its favourable characteristics which can be used in wind turbine blade technology to enable the development of longer blades that are lighter, more structurally and aerodynamically efficient, and impart reduced loads to the system, in the near future.
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