Local peak stresses in solid models
Local peak stresses in solid models
(OP)
Hi,
I am regularly doing analyses with SOLID45 elements and plastic material properties with TB,KINH defined.
In many models I find very local peak stresses and strains. These are higher than allowable according to classic design codes (yield/1.5, tensile/2.35, yield*0.6). These codes do not allow for plasticity in the first place.
While I am sure that the stresses I find are local and no problem to the structural integrity of the model, I am having difficulty to relate the stresses to codes. Statements like "The red elements are OK because the element right next to it is green" are very slippery and not satisfactory.
Does anyone know a way to evaluate local stresses around yield stress? Is there a code which is more suitable for FEM analyses? I am thinking of a statement like "peak stresses only occur on 5% of the load bearing surface".
Thanks,
I am regularly doing analyses with SOLID45 elements and plastic material properties with TB,KINH defined.
In many models I find very local peak stresses and strains. These are higher than allowable according to classic design codes (yield/1.5, tensile/2.35, yield*0.6). These codes do not allow for plasticity in the first place.
While I am sure that the stresses I find are local and no problem to the structural integrity of the model, I am having difficulty to relate the stresses to codes. Statements like "The red elements are OK because the element right next to it is green" are very slippery and not satisfactory.
Does anyone know a way to evaluate local stresses around yield stress? Is there a code which is more suitable for FEM analyses? I am thinking of a statement like "peak stresses only occur on 5% of the load bearing surface".
Thanks,





RE: Local peak stresses in solid models
RE: Local peak stresses in solid models
I do not think there are any codes of the type you are looking for. Either use the codes for design or use the analysis. The proof for the analysis has to be provided by
testing or comes from the experience of the analyst.
Gurmeet
RE: Local peak stresses in solid models
In fact they come to the same thing - no plasticity allowed.
The field I am in, which is somewhat conservative, I do not only need to convince my self, I also have to convince colleagues, third parties, warranty surveyors and insurance companies. It would therefore be nice to be able to say "occurring value is less than allowable value, therefore it's OK". Just waiving away high local stresses as not important will not fly in my world, however much I would like to.
RE: Local peak stresses in solid models
The allowable stress (based on the lesser of yield/1.5 or tensile/2.35) is to provide a suitable margin against plastic collapse and "burst" of a general pressure vessel. The localized stresses, however, have different, shall we say, allowables. For example, the local membrane stress may be allowed to reach 1.5 times the basic allowable, while the local membrane-plus-bending stress may be allowed to reach 3 times the basic allowable stress (all of this assuming that you have performed an elastic analysis). If, OTOH, you are performing an elastic-plastic analysis, the rules again are completely different.
Suffice it to say, this is not something where you can just "run" an FEA on a problem, say the local stresses at-a-point are below some value, and be done...
RE: Local peak stresses in solid models
And, your model doesn't have any numerical discontinuities either, because the stresses at those locations are identically infinite, so...
RE: Local peak stresses in solid models
There is a WRC bulletin 429 that recommends that it is inappropriate to consider calculated stresses in sharp transition regions, as for one thing it is considered very unlikely that ratcheting could occur there.
One method to estimate a stresse at a junction which is more in keeping with the intent of the pressure vessel codes is to obtain stresses at say 2t, 1t, 0.4t and use a quadratic extrapolation to estimate a stress at the junction.
RE: Local peak stresses in solid models
The element solution PLES is the same as the nodal solution PLNS, and I have tried mesh refinement but there is no significant change.
The load is not pressure but axial load and bending, so the application of pressure vessel code is doubtfull. If PD5500 states that 3xbasic allowable is allowed for local stress, this will be 2x yield with elastic material properties, correct? How do you define local stress? When does local stress become global stress?
Thanks for your help.
RE: Local peak stresses in solid models
corus
RE: Local peak stresses in solid models
What section in PD5500 or ASME VIII indicates the use of local stresses?
Regards
RE: Local peak stresses in solid models
You still have not yet told us what is the (legally mandated) Code of Construction for this "thick"-walled pipe with holes that you are designing.
(As a further aside - D/t>8-10 can be considered to be thin-wall - Reference Timoshenko's Theory of Thin-Wall Structures).