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1 BASIC CONCEPTS AND INITIAL

STATEMENTS

1.1 The science of strength of materials
All real bodies under the action of external forces change their
shapes and sizes, that is deformed, and at a certain value of applied
forces can be destroyed. Deformation is the result of changes in
the mean distance between particles (molecules, atoms) of the
substance of the body. It may be due to the action of forces on the
body, changing its temperature or influence fields of other nature,
i.e. the so-called external factors. Applying external factors to the
body is called load, and their removal – off-load.
The ability of the body to restore their size and shape after
offloading is called elasticity and the deformation which
completely disappears immediately after offloading - elastic.
The deformation that remains in the body after offloading is
called residual or plastic, and the body's ability to acquire a plastic
deformation - plasticity.
For certain values of external factors the deformation can reach
values at which the existence of the body as a whole becomes
impossible and it is destroyed or it obtains significant residual
strains. The ability of the body to perceive the external factors
without being destroyed or receiving a residual strain is called
strength.
Sometimes the elastic deformation of the body obtains such
value that it being strong is unable to perform its functions.
Therefore, in the design, along with the strength of their bodies it
should be provided with the necessary rigidity. The ability of the
body to resist a deformation is called rigidity.
The deformation of the body under the influence of a certain
load can be negligible as long as the value of the load is less than
the so-called critical value. If this critical value is exceeded, then
there is a qualitative change in the nature of deformation - loss of
stability. Hereby, deformations of the body are soaring. A striking

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