Ratcheting VS Incremental Plastic Strain
Ratcheting VS Incremental Plastic Strain
(OP)
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:
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,
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:
Quote (ASME BPVC VIII-2 (2015) Paragraph 5.12)
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,





RE: Ratcheting VS Incremental Plastic Strain
I have a figure that shows this. I'll try to find it on Monday.
RE: Ratcheting VS Incremental Plastic Strain
RE: Ratcheting VS Incremental Plastic Strain
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.
RE: Ratcheting VS Incremental Plastic Strain
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.
RE: Ratcheting VS Incremental Plastic Strain
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.
RE: Ratcheting VS Incremental Plastic Strain
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?
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.
RE: Ratcheting VS Incremental Plastic Strain
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).
RE: Ratcheting VS Incremental Plastic Strain
Do I have to perform ratcheting analysis if my fatigue analysis is elastic-plastic?
RE: Ratcheting VS Incremental Plastic Strain
RE: Ratcheting VS Incremental Plastic Strain
TGS4, do I have to perform ratcheting analysis if I design the equipment as per Part 4?
RE: Ratcheting VS Incremental Plastic Strain
RE: Ratcheting VS Incremental Plastic Strain
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?
RE: Ratcheting VS Incremental Plastic Strain