Sec 8 Div 2 - primary + secondary equivalent stress range from FEA

[McT178]

Is it a legitimate practice to obtain delta Sn from a course meshed FEA model. I am looking at a report where delta Sp is taken from a model where a very fine mesh is used around a discontinuity, and then the model is run with a course mesh to obtain delta Sn. I can see how this would remove some effect of the stress intensity, but this does not seem to be the same as a linearized stress.

 

[TGS4]

Do the results of the "coarse" model match the "fine" model? If not, then one could argue that the "coarse" model is not sufficiently refined as to be mesh-converged, and therefore should be thrown out.

Regardless, the M+B stresses are to be linearized, like you said, and not "peak" values from a coarse mesh taken. Perhaps the analyst does not know how to linearize?

 

[McT178]

The "course" results are much lower than results produced with the fine mesh. If I understand the code correctly, this will affect the fatigue penalty factor - perhaps giving an artificially low value.

 

[McT178]

sorry...coarse not course

 

[TGS4]

are you talking about the stress a ta point, or the linearized stresses? (I understand that the report may not have done linearization, but you might be able to eye-ball it).

Sounds to me, though, like the coarse mesh model has not converged. Punt it back to them, pointing out Article 5.1.2.3:

Recommendations on a stress analysis method, modeling of a component, and validation of analysis results are not provided. While these aspects of the design process are important and shall be considered in the analysis, a detailed treatment of the subject is not provided because of the variability in approaches and design processes. However, an accurate stress analysis including validation of all results shall be provided as part of the design.

Get them to prove to you that the results are right. The burden of proof is not to you to say that the results are wrong.

Good catch.

 

[McT178]

In both cases a maximum point stress is found at the area of interest. A note states that "a coarse mesh was used therefore peak stresses due to notches are not included." I think this implies that PL+PB+Q can be obtained with a coarse mesh, while PL+PB+Q+F is obtained with a fine mesh.

 

[TGS4]

If that's the case, then I would call bull$h!t. That is definitely NOT permitted. Punt it back to them.

 

[McT178]

Thanks. I am new to Div 8 Sec 2, but this just seemed wrong.