Allowable Fillet Weld Stress Limits

[DSB123]

Hi Members,
Can anyone help? I have a situation where carbon steel is welded to stainless steel using fillet welds. There is a differential expansion between the two materials which results in shear stresses within the attachment weld. The problem is what is the allowable stress limits which should be used to check on the weld sizes? Although the Codes give values for allowable shear for fillet welds these are primarily for a situation where the load is of a Primary nature and not of a secondary nature as in my case. Can we use the allowable shear stress for the weld (or weakest base material) multiplied by 3 (ASME allows 3Sm for secondary stresses)? Please give your thoughts

Thanks in advance.

 

[prex]

If that thermal stress is the only one present in the weld, then I think you can do what you are planning (just because the allowable stress in fillet welds is 0.5S_m for Div.2), but don't forget that the 3Sm limit applies to a range of stresses: for a thermal stress the whole temperature variation from coldest condition to the hottest one must be condidered. prex

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