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properties in different directions are "averaged" and it can be
considered isotropic. Sometimes in practice we have to deal with
materials with directional anisotropy (fiberglass, paper, wood) or
structural anisotropy (concrete, plywood, composites). Such
materials should be considered as continuous and homogeneous,
but anisotropic.
4) The material is perfectly elastic. According to this
hypothesis, the material has the ability to restore completely the
original shape and size after offload and between deformation and
load there is a directly proportional relationship.

1.2.2 Model of the form of a deformable body

The geometrical shape of the elements of a structure is often
quite complex, and consideration of all its features leads to
complex models. Therefore, most of the elements of engineering
structures that can be calculated are limited to the design schemes
of a beam, a shell, a plate or an array.
1) Beam - a body, which two dimensions are of the same order,
and the length of the third is much larger (fig. 1.2 a). The
geometrical axis of a beam is the line that connects the centers of
gravity of cross sections. The beam with the rectilinear axis is
called rod.
2) Shell - a body limited by two curved surfaces, the distance
between them is small in comparison with other dimensions (fig.
1.2 b).
3) Plate - a body limited by two planes, the distance between them
(thickness) was compared to other dimensions (fig. 1.2 c).
4) Array – a body, in which all dimensions are of the same order
(fig. 1.2 d).

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