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Linear stress state that believed to be equal in strength to that
one is called equivalent. Denote:  – the principal stresses of
eqv
equivalent stress state (fig. 3.13).

Figure 3.13

The condition of strength in difficult stress state generally is
written as
  f ( ,  , )    (3.32)

eqv 1 2 3
Today they have not managed to formulate a universal criterion
of equal strength which is able to take into account the totality of
the reasons that affect the strength. Therefore, several theories of
strength are used that complement each other.
The first theory, or the theory of the most normal stresses, is the
hypothesis that the strength of the material is provided, if the most
normal stress does not exceed the allowable established for linear
stress state.
According to this theory condition of strength, for example, for
a material with different resistance to tensile and compression, is:


       , if       0 ;
eqv 1 t 1 2 3

 Й      , if       0 ; (3.33)
eqv 3 c 3 2 1

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