Problem with Mesh Sensitivity Analysis of a Masonry wall

[Ravi K Mudragada]

A Micro model of Masonry wall was modelled in Abaqus and surfaced based cohesive behavior used between the masonry brick units. Drucker-Prager material model was used for the compressive behavior of brick units.
Wall was subjected to a pressure load on the top and a nonlinear static analysis(load controlled) was carried out.
I have considered three different mesh sizes and sensitivity analysis was carried out. But, result(displacement history) was decreasing with decreasing mesh size.
Very odd.
Can anyone explain what could be the reason for getting stiffer solution for the finer mess,..?

 

[FEA way]

Can you attach some pictures of this model ? How did the stresses change ?

 

[Ravi K Mudragada]

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[Ravi K Mudragada]

 

[bkal]

Your top surface looks quite flat (in a deformed shape), but are you comparing displacements of a particular node, or an average displacement? Could you apply a displacement load so that your loaded surface remains flat and then compare total applied forces?

Out of curiosity, what is the ultimate purpose of this analysis? How did you account for mortar joints having different mechanical properties to masonry units?

 

[Ravi K Mudragada]

Displacement are measured at particular node not average displacement. In fact, I would like to try that also.
I have not modelled mortar joints explicitly instead I have used surface based cohesive behavior between the masonry units and tried to incorporate the joint interaction.

 

[bkal]

It would be interesting to compare plots of principal stresses (max), some sort of damage parameters / plastic strains, and also applied force vs displacement curve. It is not clear from the above if any non-linearity has taken place in your model and if so what is the extent of it. Loading the model in a displacement controlled manner with a rigid frictionless contact would probably give you more insight into what is going on.

Regarding masonry joints, mortar has different mechanical properties compared to masonry units so would induce additional stresses. I am not sure if it is important for what you are doing as I do not know what is the ultimate goal of your analysis.