Mechanical properties

In order to characterise the deformation of coated fabrics un­der load, the most important factors are the mechanical be­haviour of the materials under the effects of loading, time, and temperature. Because coated fabrics are plastic compos­ites constructed for a purpose, their mechanical behaviour cannot easily be compared with the mechanical behaviour of traditional materials. Fabric threads and coating have dif­fering stiffness as individual elements and are subject to dif­fering stresses causing differing deformations. Altogether, they form a statically indeterminate system.

can be evaded and deformations can be accepted.[30] The de­formation of flexible elements helps to relax peaks of stress through the flexible material. A larger deformability (ductili­ty) of the material allows the expectation of smaller deforma­tions in the system under certain circumstances.

This contrast to traditional materials shows most clearly in the elastic behaviour of synthetic plastics. Conventional con­struction materials, with few exceptions, show an almost elastic relationship between stress and strain. Because of the chemical composition and physical structure of polymers, deformations arising under loading, depending on temper­

ature range, are partly of an elastic (reversible) and partly of a viscous-plastic (irreversible) nature. This has the result that important mechanical properties not only depend on the type of loading and the temperature, but above all on the time, the duration of the loading and the speed of loading.1 Behaviours under mechanical loading and dependant on time, like creep and relaxation, are considerably more typical for plastic fabrics than for other materials. As a consequence, any description of the material properties and their values must also include the time scale.

The fact that the deformation depends on the structuring of the individual elements regarding their alignment in the en­tirety also has the greatest significance for the description of fabric behaviour. This dependence on alignment results in different deformations under load along, across and diago­nally to the fibre orientation.

The interaction of all these properties, and additional effects like the temperature dependence or the behaviour of tears extending, fall outside the scope of conventional material properties. They are, however, typical for the material group of fabric membranes and demand an entire range of geo­metrical and structural analyses on one and two-axis fabric strips in short and long-term tests.

The most important material properties for the description of the physical, geometrical and material characteristics of flex­ible structural elements under mechanical loading are elas­ticity behaviour, strength and stiffness, the tendency of tears to extend, the resistance to kinking and creep and relaxation. Questions of stability, oscillation and structural safety of light­weight structures cannot be discussed here.

PVC-coated polyester fabrics have quantitatively the largest representation in textile construction. This section is there­fore primarily devoted to the load, time and temperature
dependant mechanical properties of flexible materials in the group of polyester/PVC membranes, which are most relevant to practical applications in construction.