The first strength hypothesis
The first strength hypothesis is also called the maximum normal stress hypothesis because it accepts the maximum normal stress as the criterion. It can be formulated like this: the limit material state in the combined stress state occurs when the maximum normal stress reaches the value of the limit stress in the uniaxial stress, i.e.
where The material strength in the combined stress state is ensured if the maximum normal stress is lesser than the normal allowable working stress accepted for the uniaxial stress. We see this hypothesis takes into account the maximum principal stress influence alone ignoring the influence of two other principal stresses on the material strength.
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(6.1)
is the limit maximum normal stress; 
is the limit stress in the uniaxial stress.
