Siginificant Digits for Div2 Part5 Ratcheting Assessment

[fraccionadora]

Dear All,

When performing a elastic-plastic ratcheting assessment in accordance with the Div2 Part5, I am checking the maximum plastic strain magnitude at each pressure cycle on integration point to check that if plastic strain progress.

The question is that the software is providing seven digits and plastic strain magnitude slightly increases numerically. Please check the plastic strain magnitude at each pressure cycle within the table and the plot from the link below.

When we look at the magnitudes, it does not make sense to conclude that the component fails from ratcheting. However, i need a basis just like three significant figures is sufficient.
Thaks for your help.

https://files.engineering.com/getfi...4486-45e0-98ef-c33dad761798&file=Untitled.png

 

[TGS4]

Two thoughts:

1) Use your engineering judgement.

2) Ratcheting is a failure mode for displacement, not plastic strain magnitude. Monitor the displacement of critical dimensions.

 

[fraccionadora]

Thanks for the reply TGS4.

I am also checking the displacement as well. Please refer to the attached figure.
The component is 45 deg branch connection. Within plot, nodal displacements at the crotch section along the circumferential direction is plotted. Plot shows no significant progression. However, as an example, according to the nodal displacements shown at the bottom of the figure, it is seen that when four digits is considered, displacement slightly increase.

What i am trying to do is, to determine a criteria with backing on the code.
Looked for Div.2 Section 1.4, however, this paragraph was about conversion factors and did not make sense either.

Since the displacement is directly dependent on the dimensions, maybe the criteria should be dependent on header diameter. In this example, header is 42" and displacement progression at each pressure cycle is no more than 0.0001 mm. If we divide this displacement to header dia. it equals approximately 1e-7. Maybe something like that,
If (Displacement progression / Header dia.) <= (1e-6), pass. It is not easy to say.
I need some criteria in order not to get lost and also prevent other designers to make different judgements. Thanks for your help again.

 

[TGS4]

There is no criteria in the Code (nor will there be) in this matter. Use your engineering judgement.

 

[cbPVme]

What are your units? millimeters? Differences in the realm of 1e-4 is pretty darn small and well below any reasonable manufacturing tolerance for a pressure vessel. If you are still concerned, you can run more increments to see if things change.

You are also using some pretty coarse tet elements between the area of interest and the fixtures. This is probably generating some additional error since you are measuring global displacements.