The Allowable Stresses Limits as per ASME VIII Div.2

The Allowable Stresses Limits should be determined and will be compared with actual stress occured on structure. This value is taken from ASME BPV Sect II part D.

And the limits can be taken from ASME VIII div 2 as follow:

Primary Membrane and Bending (combined) Stress Intensity = 1.5 f_allow

Local Membrane and Bending (combined) Stress Intensity = 3 f_allow

Stress intensity is defined as twice the maximum shear stress (which is equivalent to the difference between the largest principal stress and the smallest principal stress as a given point).

Primary membrane stress refers to a membrane stress occurring across thickness of any section or is average stress across full width.

Primary Membrane and Bending (combined) Stress Intensity refers to primary membrane stress plus bending stress excluding secondary and peak stresses.

Local Membrane and Bending (combined) Stress Intensity refers to primary membrane stress plus bending stress including secondary stress but not peak stresses.

Secondary Stress refers to normal stress or shear stress developed by the constraint adjacent parts or by self-constraint of a structure.

Peak Stress refers to stress which does not cause any noticeable distortion and is objectionable only as a possible source of fatigue crack or brittle fracture.