Stress Intensity determination through FEA 2

[phodgson]

Hey,

I am performing a pressure vessel analysis using ASME boiler code Section VIII Division 2 and Algo's FEMPRO (latest edition). This requires examining the stress intensity. The stress linearization software provides an average value of the stress intensity Pm + Pb, but the code requires finding the greatest intensity through a section; not an average.

However, the Tresca value is define as a measure of the maximum shear stress. Stress intensity is defined as 2*maximum shear. Therefore, is the Tresca*2 stress result output a good approxiation of the stress intensity at a node?

Thanks,

Pat Hodgson
Mechanical Engineer, E.I.T

 

[rb1957]

this clearly isn't the fracture mechanics term "stress internsity" ... is it ?

 

[corus]

Although I've answered this elsewhere regarding the definition of stress intensity, I think your interpretation of the code and the results from the FEA is wrong.

In general the stresses through a wall will be composed of primary membrane, primary bending, and a peak stress component. The primary stresses may of course be classed as secondary depending on whether or not they are strain dependent. ASME will give examples on the type of stress classification. In linearising the stress through the thickness the FEA code will remove the peak stress component (required for fatigue) to leave the primary plus bending stress at the surface which will be compared against some factor of the design stress (related to yield). The primary membrane stress can be considered as the average stress through the thiockness. Primary plus bending is the stress at the surface and is not an average as such.

The Von Tresca stress will be the stress intensity you compare against design limits. I've posted links showing definitions of this for you elsewhere. You can check the calculation of the Von Tresca stress intensity by looking at the principal stresses calculated and taking the maximum of the absolute value of the principal stress difference. This definition of stress intensity will be in the code somewhere too.

corus