The skin thickness based on ultimate axial stress is
The skin thickness based on ultimate shear stress is
The largest of the two will be the minumum skin thickness that would be used based on the material ultimate strength values.
Notice that no factor of safety is used in this example. Normally there is a factor of safety associated with every design. For example, if we were to use a factor of safety of 2 for normal stress and 3 for shear, we would use 68000/2 = 34000 psi for max allowable normal stress and 41000/3 = 13666.67 psi for max allowable shear stress. Under these conditions the minimum allowable skin thickness would be twice as large as the one found earlier. i.e., tmin = .007782"
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