Learning Forum

[karim.el-ghamrawy]

Hello,

I am modelling a structure with tubular K, Y, and T joints for an offshore application. As per the standard for these structures, if I would like to evaluate the fatigue on these joints, the stresses to evaluate the fatigue have to account for the geometry of the welds at the joints to avoid modelling the weld itself and therefore a more efficient simulation. There are formulations to compute the stresses at the joints (also called notch stress or hot spot stress) and they are based on the dimensions of the tubes. They way this is done is by extrapolating either principal or normal stresses near the joint. the following schematic shows what I mean:

My question is, is there a way where this is done in ansys automatically somehow to avoid coding? 

Another question is, is there a way to manually edit the principal stress output from ansys at the desired locations?

Any help would be much appreciated

Thanks in advance

[danielshaw]

You should use the SeamWeld solver in Ansys nCode DesignLife for this type of analysis.  In lieu of using DesignLife, you can use stress linearization in Mechanical to obtain a “hot spot” stress.  You could linearize along the length of the weldment (as shown in the image), but it is probably more accurate to linearize through the weldment thickness.

Why do you want to edit principal stress output?  What output do you want?

[karim.el-ghamrawy]

Hi,

Thank you very much for your reply. There are two issues here, I do not have the license for the seamweld in ncode (as far as I know it is a separate license). The other issue is I am not modelling the weld itself, so as per the guidelines for  fatigue analysis for offshore structures, in order to have accurate results at the joints, I need to account for the hot spot stress by extrapolating the stresses around the joint. As I understand, in the fatigue analysis, I need a scalar value at each node to compute the life of the structure (combination method). Again as per the DNV rp c203 guideline, the principal stress is an acceptable measure. 

So if we imagine there is a way to compute the hotspot stress at the desired locations in mechanical; then I would like to modify the stress tensor with the new values at these locations to be able to compute the updated principal stresses which takes into account the notch effect. I am not sure if this makes sense.

Please let me know what you think.

 

Many thanks,

[danielshaw]

Are you attempting to use the Mechanical Fatigue Tool (FT) for this evaluation?  If so, it does not have the capability to consider a “hot spot” stress.  You can use stress linearization to obtain the hot spot stress, but you would need to manually calculate the fatigue damage.

It might be possible to use the DNSOL/DESOL MAPDL commands to modify stored stress results, but I would not recommend doing it.  Those commands are difficult to accurately use with standalone MAPDL.  I do not know what issues might arise when using them with Mechanical.  It would be easier (and safer) to manually calculate the fatigue damage.