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
E  x  tg ,

x
i.e. the value of modulus of elasticity equals the slope angle
tangent of the straight section diagram to the x-axis.
If the sample (after the load that corresponding to the liquid
limit) is unloaded, this process is illustrated by the line МО
1
nearly parallel to the section of elastic ОА Relative longitudinal
.
deformation of a sample consists of two parts: elastic deformation
 , which disappears after unload, and residual (plastic)  , that
pr pl
remains after unloading the sample. If the sample is loaded once
again, the process of repeated loading will be depicted by a line
that almost coincides with the line O M , of unloading. Both lines
1
(unloading and loading) form a hysteresis loop (it is conditionally
replaced by a straight line). Loop area equals mechanical energy
losses for one cycle, they are rather small. These losses are caused
by so-called internal (molecular) friction. When performing a large
number of cycles (unloading - loading), such as vibrations,
mechanical power losses are significant.
With further loading (after point M) the curve continues as if
there was not intermediate unloading. Therefore, in the sample
material after a preliminary deformation there is the increase of
border proportionality  , border of elasticity  (compare
pt pr
section ОВ and О 1М) and liquid limit  (with the disappearance
t
of fluidity area), but there is the decrease of the value of permanent
deformation. This phenomenon is called defamation.
If the defamation sample is not re-loaded immediately, but after
a while (about 100 hours), it will increase not only the size  ;
pt pr
and  , strength but also the border  . This change in the
t st
material properties of the defamation sample is called natural
aging.
Material characteristics of elasticity and strength
 , , and  are called mechanical characteristics.

pt pr t st
When testing the tensile of plasticity and the characteristics of
the material:
- relative residual elongation   % :

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