How to correctly consider Von Mises (maximum distortion energy theory) in an AutoPIPE combination for ASME B31.4 2006?
Von Mises stress is used to determine if a material XYZ will yield.
Combined stress are based on Code Tresca stress. User can specify Von Mises stress using "Total stress" option in Result > Result Options > Model. However, this option is disabled for B31.4 (2006 Edition). Calculations are based on which category is selected for a combination dialog i.e. Von Mises, or Eq Tensile.)
Question:
How does AutoPIPE calculate the Von Mises stresses?
Answer:
in AutoPIPE, Von Mises is known as the Octahedral Shear Stress. See AutoPIPE help for information on how and other similar values are calculated:
s1 and s2 - principal plane stresses
s3 - principal out-of-plane stress, for a pipe cross section is zero.
sa - longitudinal stress
sh - hoop stress
t - torsional shear stress
Total stress is evaluated every 15deg to find the maximum Von Mises or Max shear stress (as defined under Tools/model options/results) since The Principal Max and Min stresses do not always occur at the same location around the pipe circumference. Please refer to a 'Strength of Materials" reference book in evaluating principal vs Von Mises stresses.
Allow me to try to clarify that.
The Principal Max may be at one location on the pipe and the Principal Min location may be at another location, but the total compares the Max and Min at the same location (with the highest comparative results - widest range). The single Max location may not have a comparable Min and may therefore not represent the widest range (and visa versa).
Note:
1. Selecting Von Mises stress category automatically adds grayed out (LONG) AND (HOOP) which cannot be removed unless switching to a different stress category which will remove (LONG) and (HOOP).
2. In order to obtain consistent results for Equivalent Stress, a pressure extension analysis should be performed (enable the "pressure extension analysis" option in the ’Static Load Cases’ dialog). This type of static analysis insures that the "F/A" pressure term, which can be substantial in an axially restrained system, is included in the stress calculation.
3. The location on the pipe cross section where maximum shear stress or octahedral shear stress occurs is reported as a clockwise angle relative to the out-of-plane axis of the cross-section.
4. Tools>Model Options>Results>Total Stresses (Oct/Max). 'O' represents the maximum octahedral shear (Von Mises) stress, and M represents Max Shear Stress (where Max Shear Stress = Tresca stress/2).
5. Calculated Tresca stress in the Tresca category is different from the Code Tresca stress in the Functional or Func+Env stress categories in that it includes the maximum of s1, s2 and s1-s2 stresses. Code Tresca stress include only the s1-s2 stress. s1 and s2 are the principal stresses.
6. General Stress report has no allowable stress since it is not a piping code with explicitly defined allowable stress criteria.
7. For B31.4-2009 and later code years and B31.8-2012 and later code years only:
To be able to see all the combinations, use Piping Restraint Options (Insert > Xtra Data > Piping Restraint Options) "Show Both" to enable AutoPIPE to show all types of equations. Also, using Equivalent will allow user to see the equations as were available in earlier editions like B31.4 (2006).
ASME B31.4 Piping Code Calculation Issues
Bentley AutoPIPE