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The calculations on the stability of the critical stress is
devastating as liquid limit or boundary strength calculations for
strength. Therefore, we introduce the concept of allowable
stresses for resistance  :
ст
 ст   кр k ,
ст
where k – factor of stability.
ст
Stability condition requires that the tension that arises when
compressed, will not exceed the allowable stress for resistance:
P
  max   ст  .
F

However, calculating the allowable stress for resistance is
complicated by the fact that the critical stress depends not only on
the material properties, but also the flexibility of the rod.
We find the relationship between the allowable stress on stability
and allowable stress for compressive strength:
 ст    кр n н   ,
  k  н (5.22)

ст

where    n – allowable stress; n – factor of safety,

н н н
для пластичних матеріалів
  , n  for plastic material; ;
 н ,n н    Т Т
for fragile materials.
  м , n  для крихких матеріалів .
м

The required dependence
 ст     , (5.23)
where  – the main factor reducing the allowable stress in the
calculation of the stability.
The coefficient  for each material can be determined at any
value of flexibility and represented as a table or graph depending
 on  .
Thus, given the dependence (5.23) stability condition takes the
form

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