Ratcheting VS Incremental Plastic Strain

[Paulettea]

Dear All,

I am trying to understand the difference between two situations. Please help me understand this.

In ASME BPVC VIII-2, the definition of ratcheting is as follows:

18. Ratcheting: A progressive incremental inelastic deformation or strain that can occur in a component subjected to variations of mechanical stress, thermal stress, or both (thermal stress ratcheting is partly or wholly caused by thermal stress). Ratcheting is produced by a sustained load acting over the full cross section of a component, in combination with a strain controlled cyclic load or temperature distribution that is alternately applied and removed. Ratcheting causes cyclic straining of the material, which can result in failure by fatigue and at the same time produces cyclic incremental growth of a structure, which could ultimately lead to collapse.

In the definition provided by the code a sustained load is superimposed by a cyclic displacement controlled load . What happens if there is a monotonic displacement control load superimposed by a cyclic load control load? An example is an imposed displacement on a nozzle in the initial assembly then the vessel goes through cyclic pressurizing and depressurizing.
My guess is that there will be a progressive inelastic stress and may lead to some stress relaxation after each cycle.

Warm Regards,

 

[TGS4]

You'll need to research the work by Bree. He answered this type of question.

I have a figure that shows this. I'll try to find it on Monday.

 

[TGS4]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1519761021/tips/Bree3_dlxq4w.bmp[/url]

 

[Paulettea]

The image is not a good quality. I think the P1 is cyclic plasticity and S1 is elastic shakedown.

TGS4, I have a more basic question regarding ratcheting. My question is why ratcheting is important at all?

Suppose a case in which there is a primary stress and no secondary stress. If it is subject to a cyclic load in such a way that it undergoes some inelastic strain then upon unloading there will be some plastic strain remaining in that location. Then the cycle is repeated and an incremental plastic strain will occur. This is a case of fatigue loading in which there is a mean stress more than zero. Apparently since there is no secondary stress there is no ratcheting at all (as per the definition in the code). However, there will be incremental plastic strain.

An incremental plastic strain which is caused by a secondary stress acting on a primary stress: Ratcheting--As per the code must be prevented and must not occur at all.
An incremental plastic strain which is caused by a primary stress: Not Ratcheting--As per the code need not be prevented and has to be dealt with in fatigue design.

Why is there this much obsession regarding ratcheting and not about incremental plastic straining caused by primary stresses.

 

[TGS4]

In cyclic primary membrane stress and constant secondary stress, ratcheting cannot occur. Go search for Bree diagrams via google - you will see many different situations (hopefully some have better quality). Region P1 is cyclic plasticity without ratcheting, while S1 is shakedown after the first half-cycle.

Your example will not ratchet because the primary stress limits prohibit stresses that high.

Why is there this obsession with Ratcheting? Because when it happens it can cause catastrophic failure. Look at all of the different Bree diagrams - there are certainly situations where cyclic secondary stresses can occur to cause ratcheting. More importantly, though, is the assurance that when you have either purely elastic action or achieve shakedown in the first half-cycle, then the assumption of no cyclic plasticity built-in to the fatigue curves is assured. When you have cyclic plasticity, then you need to adjust the pseudo-elastic fatigue stress range - hence the K_e factor.

 

[victorpbr]

I recommend this reading about Bree,

http://www.rickbradford.co.uk/BreeDiagram.pdf

It's a derivation of how the graph is made. Its good to get a better understanding of it.

 

[Paulettea]

TGS4, I have a couple of questions. Can you help me understand?
1-Are the stresses referred in table 3-F.9 of ASME BPVC VIII-2 true stress or engineering stress? (unfortunately I did not find the units for the stresses I assume it is ksi)
2-What is the meaning of maximum possible effect of mean stress in 5.5.1.4?

5.5.1.4 Stresses and strains produced by any load or thermal condition that does not vary during the cycle need not be considered in a fatigue analysis if the fatigue curves utilized in the evaluation are adjusted for mean stresses and strains. The design fatigue curves referenced in 5.5.3 and 5.5.4 are based on smooth bar test specimens and are adjusted for the maximum possible effect of mean stress and strain; therefore, an adjustment for mean stress effects is not required. The fatigue curves referenced in 5.5.5 are based on welded test specimens and include explicit adjustments for thickness and mean stress effects.

From what I see in this paragraph the effect of mean stress cannot be neglected at all. If you have mean stress you must either have access to some S-N curve data which includes the effect of mean stress or one must consider some theories like Goodman to include the effect of mean stress.
If I want to have a judgment about what code says about maximum possible mean stress then I say the maximum stress is very close to UTS. Like what I have shown here.

 

[TGS4]

Do you have access to ASME PTB-1? The mean stress correction is discussed there. Suffice it to say that the mean stress level is premised on the stress levels covered in the other failure modes.

Otherwise, elastic fatigue is based on the stresses being pseudo-elastic. There is no plasticity, except with the Ke factor. Unless you use elastic-plastic fatigue, in which case the curves are well described (if not extremely difficult to implement).

 

[Paulettea]

Thank you TGS4. I have access to PTB-1 like many other valuable books. Unfortunately, I have not gone through these books except for a couple of times and that was due to you referring to them. I took a look at PTB-1 Annex A where I could see an explanation of what was the basis for the criteria in the code. It needs much deeper study. Thank you again TGS4.

Do I have to perform ratcheting analysis if my fatigue analysis is elastic-plastic?

 

[TGS4]

Do I have to perform ratcheting analysis if my fatigue analysis is elastic-plastic?

Strictly-speaking, yes. The EP fatigue analysis won't detect ratcheting, but will detect cyclic plasticity.

 

[Paulettea]

4.1.1.4 A screening criterion shall be applied to all vessel parts designed in accordance with this Division to determine if a fatigue analysis is required. The fatigue screening criterion shall be performed in accordance with 5.5.2. If the results of this screening indicate that a fatigue analysis is required, then the analysis shall be performed in accordance with 5.5.2. If the allowable stress at the design temperature is governed by time-dependent properties, then a fatigue screening analysis based on experience with comparable equipment shall be satisfied (see 5.5.2.2).

Protection against ratcheting shall be considered for all operating loads listed in the User’s Design Specification and shall be performed even if the fatigue screening criteria are satisfied (see 5.5.2).

TGS4, do I have to perform ratcheting analysis if I design the equipment as per Part 4?

 

[TGS4]

No. The rules are considered to adequately consider ratcheting. However, if a fatigue analysis is required, then a ratcheting assessment is needed in the course of that fatigue analysis (if the fatigue analysis is the elastic version).

 

[Paulettea]

TGS4, my understanding is that the reason behind considering ratcheting as a mode of failure along with fatigue is that the S-N curve data in 3-F are based on strain controlled tests. I mean when developing these data there is no chance of incremental plastic strain. The worst case in a strain controlled cyclic loading will be cyclic plasticity.

If I want to use an S-N curve (not the ones in 3-F) in which the data are developed based on stress controlled tests, do I have to bother considering ratcheting?

 

[TGS4]

In that situation, I would recommend to use your own engineering judgement.