Learning Forum

[Kathi1988]

Hello everyone,I am trying to model the cracking behaviour of unreinforced concrete or screed. I am using the Menetrey-Willam-Model and according to the underlying crack-band model, I need to define a tension softening behavior via the yield function Omega_t (curve of tensile relative stress over plastic strain kappa) without losing the objectivity of the model. I can either use exponential softening or linear softening. In the case of exponential softening the parameters fracture energy G_ft and residual relative stress Omega_tr have to be defined. In linear softening, the parameters are again the residual relative stress Omega_t and the plastic strain limit kappa_tr.nNow my question concerns the mesh dependency: In the Menetrey-Willam documentation in the Ansys Help there is a parameter effective element length to overcome mesh-depenedent softening behavior. If I use exponential softening, the effective element length influences the model obviously through the yield function (equation 4-47 in the documentation). But in linear softening I have to define the yield function directly through the parameter of the plastic strain limit kappa_tr. nSo the effective element length doesn't influence the material model in case of linear softening, does it? So the linear softening behavior is fully mesh-dependent, isn't it?nOr is there another way, in which the effective element length influences the material behavior in the case of linear softening?nThank you in adavance for your help!nKind regardsnKatharinan

[John Doyle]

Hi Katharina: nIn general, softening plasticity models are always mesh dependent unless the formulation somehow takes into account the mesh size. The Menetrey-Willam exponential softening option does account for the element size as you have pointed out. The objective here is for the plastic deformation, during softening, to dissipate the same amount of energy, independent of the mesh size. nThe linear softening option doesn't do anything to address mesh dependency, so it will always be mesh dependent. The magnitude of this dependency might not be significant enough to affect the results, but the effect should be investigated. nIt is generally recommended that you calibrate the model and results to an experiment that closely matches the real service conditions.nRegards,nJoh

[Kathi1988]

Hi Katharina: In general, softening plasticity models are always mesh dependent unless the formulation somehow takes into account the mesh size. The Menetrey-Willam exponential softening option does account for the element size as you have pointed out.  The objective here is for the plastic deformation, during softening, to dissipate the same amount of energy, independent of the mesh size.   The linear softening option doesn't do anything to address mesh dependency, so it will always be mesh dependent. The magnitude of this dependency might not be significant enough to affect the results, but the effect should be investigated. It is generally recommended that you calibrate the model and results to an experiment that closely matches the real service conditions.Regards,John/forum/discussion/comment/102334#Comment_102334

Hi John,nthank you for your reply, it was very helpful! The magnitude of the dependency is higher for unreinforced concrete, I believe. And if experimantal data is neccessary for calibrating the model, I cannot really use it for predicting large-scale behaviour...?nI have another question concerning the Meterey-Willam Model, maybe you (or someone else?) can help with that as well: In the documentation I found the constriction 'excluding plane stress' concerning the use of element type PLANE182/183 and 'excluding layered structural solid' for SOLID185/186. Do you know, why plane stress and layered solids are excluded for the Menetrey-Willam model? The documentation doesn't give any explanation as far as I can tell.nThank you in advance!nKatharinan