Nodal Stress Calculation - Average over Adjacent Elements of the Same Material Only

w1961

Hi, I'm using version 23. Two issues got my attention: Calculation of the nodal stress using average over all adjacent elements and uniqueness of the element stress.
Currently, Mecway V23 calculates the nodal stress by taking the average over all adjacent elements. It is not difficult to show that this approach can lead to significant error if the elements have very different material properties. This will become apparent if we include the free space into the mesh and set the elastic modulus to approach zero. Then the stress calculated at the interface nodes would become 50% too low. By default, at the interface nodes, we should take average only over the elements of the same material properties and only report the higher stress.
Turning to the 2nd issue, I found that the stress for the same element can sometime have different values when Mecway displays the nodal stress, which is calculated from the average over adjacent elements. As far as I understand, the solution for displacements is unique and the element stress calculated from the displacement (defined at the nodal points) must also be unique. Is this correct?

JohnM

This issue is present in many FEA codes. Try to disconnect the nodes and use bonded contact.

w1961

Thank you for the response.

Victor

I'm not sure using the higher stress at material boundaries is the right way. While it might be practical and conservative, it wouldn't be obvious that it's doing that and there's ambiguity about how to order tensors.

I'm not sure what you mean in your second point about the different values on the same element. Element stress should be unique (single valued) at each of the element's nodes.