Question about Ratcheting (ASME Sec. VIII Div.2)

[victor6397]

Hello mates, I have some doubts regarding ratcheting, and I hope you can help me a bit.

As I have read, and as can be seen in the Bree diagram, for ratcheting to take place a source of sustained primary stresses and a source of cyclic secondary loads is necessary. In that case, why is it mandatory to evaluate this failure mode always according to ASME Sec. VIII Div. 2? Based on the above, if we only have internal pressure in a cylindrical shell, regardless of whether it is cyclic or not, ratcheting would not be possible, would it?

Also, one of the criteria to verify this failure mode from the ASME Sec. VIII Div. 2 limit load method is the following:
"There is an elastic core in the primary-load-bearing boundary of the component".

Is this location the same as the location where the primary membrane stress (Pm), also called "structural element", occurs?

If so, the effect of a cyclic bending moment in a nozzle on a pressurised cylindrical shell, for example, could not lead to ratcheting on the latter, as the stresses would be considered secondary, right?
The only case I can understand is that of a cyclic temperature gradient. Could you give me another example?

I'm aware that I'm probably misunderstanding some concepts, and that the questions can be a bit silly, so please don't be too hard on me, I'm trying to learn

 

[IdanPV]

Hi,

I rocommend to start with reading the below articles, it can give you better understanding:

ASME Section VIII, Division 2 Elastic Analysis Discussion: Collapse vs Ratcheting

So, you think you know what load case combinations to use for your ASME Section VIII, Division 2, Part 5 analysis to satisfy Protection Against? Common sense says to follow to Code rules in Table 5.3 – but do you fully understand what that means? And, what loads should you use to satisfy Protection

becht.com

My notes on ratcheting failure and local failure criteria

Codes or standards for pressurized components requires design and analysis engineers to asses the component design for ratcheting and local failure modes. It’s very important to understand the physical phenomenon of the failure modes to correctly apply it during design and analysis.

www.linkedin.com

 

[victor6397]

Hi,

I rocommend to start with reading the below articles, it can give you better understanding:

ASME Section VIII, Division 2 Elastic Analysis Discussion: Collapse vs Ratcheting

So, you think you know what load case combinations to use for your ASME Section VIII, Division 2, Part 5 analysis to satisfy Protection Against? Common sense says to follow to Code rules in Table 5.3 – but do you fully understand what that means? And, what loads should you use to satisfy Protection

becht.com

My notes on ratcheting failure and local failure criteria

Codes or standards for pressurized components requires design and analysis engineers to asses the component design for ratcheting and local failure modes. It’s very important to understand the physical phenomenon of the failure modes to correctly apply it during design and analysis.

www.linkedin.com

Thanks for the articles, very interesting, but my question is more related to the areas where this failure mode is possible.

I know that ratcheting is a global failure mode, and that cyclic plastic strains at local discontinuities are considered to reach shakedown.However, the concept of ‘primary-load-bearing boundary’ is a bit confusing for me, and I don't understand how a secondary stress produced for example by loads on a nozzle, affect this ‘primary-load-bearing boundary’.

 

[TGS4]

You’re mixing the elastic analysis method with the elastic-plastic methodology. The two are not interchangeable. Focus on one or the other only.

 

[victor6397]

You’re mixing the elastic analysis method with the elastic-plastic methodology. The two are not interchangeable. Focus on one or the other only.

Thank you so much for your response.

Right, maybe I'm mixing both methods, because unlike other failure modes, some post-processing tasks are necessary even with the elastic plastic method, and therefore I was trying to understand the basics of this failure mode.

From what I can understand, ASME Sec. VIII Div.2 evaluates global ratcheting, i.e. if there is change in geometry, there is still a wide margin with respect to plastic collapse.

However, I don't understand why the ratcheting analysis is necessary to validate the subsequent fatigue analysis.

I understand that the fatigue assessment zones (mainly local discontinuities) the plastic deformation produced will stabilize after a certain number of cycles.

But isn't the number of cycles needed for shakedown important? How do fatigue curves take into account whether few or hundreds of cycles are needed for stabilization?

 

[TGS4]

From what I can understand, ASME Sec. VIII Div.2 evaluates global ratcheting, i.e. if there is change in geometry, there is still a wide margin with respect to plastic collapse.

You’re mixing failure modes now. Don’t do that. Plastic collapse and ratcheting are completely different.

However, I don't understand why the ratcheting analysis is necessary to validate the subsequent fatigue analysis.

The fatigue curves have the built-in assumption that you don’t ratchet.

But isn't the number of cycles needed for shakedown important? How do fatigue curves take into account whether few or hundreds of cycles are needed for stabilization?

Not really. And if you take hundreds of cycles to demonstrate that you don’t ratchet, can you really say that you don’t ratchet? You may exhaust your material’s ductility.

 

[victor6397]

You’re mixing failure modes now. Don’t do that. Plastic collapse and ratcheting are completely different.

Thanks again for your help.

My comparison between plastic collapse and ratcheting is based on the paper "Global Ratcheting by elastic plastic FEA according to ASME Section VIII Rules" by Kalnins where you can read, among other things:

According to equation 1 (hoop membrane stress for a cylindrical shell), the incremental change of the shape of increased radius and decreased thickness has reduced the resistance to collapse and moved the shell closer to eventual collapse by 2/3 of the limit pressure.

Hence my confusion about mixing failure modes and methodologies.

If you know of any other resources that can give me a better understanding, I would really appreciate it . I saw the example shown in PTB-3, but unfortunately there wasn't plastic strain and the first criteria of the elastic plastic methodology of the code was fulfilled, so I couldn't solve my doubts that comes mainly from the following criteria.