contact errors 2

[saja11]

Hi all
I am modelling neuropathic foot bones. I have segmented 19 bones via MIMICS software. For meshing I used MATIC and Hypermesh. Now I am trying to simulate the neutral standing position of foot with Abaqus/Implicit (Quasi Static).I considered bones to be elastic and giving material (Young's Modulus=7300, poisson's ratio=0.3, density=4.49e-10) to each bone.I fixed the upper surface of tibia and giving concentrated force of 350N from below the ground.

I want to have a surface to surface contact between bones (finite sliding, frictionless) and between foot and ground (hard contact,finite sliding,penalty 0.6) but due to convergence error job get aborted.I tried changing increment in step and through solver control but still getting increment errors.In warnings I am getting some unconnected regions but as its a neuropathic foot it have some bones eroded. I am stuck in this for several days and can't figure out problem in my project. Kindly can someone guide me what I am doing wrong.
Below are some pics of model and warnings.

 

[saja11]

I want to kinematic couple my calcaneal bone with a point reference point but getting errors...giving achilles concentrated force on that reference point

Overconstraint checks: node 2 instance m1-1 is used more than once as a coupling node in the *kinematic coupling keyword. Remove multiple usage of this node as a coupling node.

Overconstraint checks: node 2 instance m2-1 is used more than once as a coupling node in the *kinematic coupling keyword. Remove multiple usage of this node as a coupling node.

Overconstraint checks: node 2 instance m3-1 is used more than once as a coupling node in the *kinematic coupling keyword. Remove multiple usage of this node as a coupling node.

Overconstraint checks: node 2 instance m4-1 is used more than once as a coupling node in the *kinematic coupling keyword. Remove multiple usage of this node as a coupling node.

Overconstraint checks: node 2 instance m5-1 is used more than once as a coupling node in the *kinematic coupling keyword. Remove multiple usage of this node as a coupling node.

1 nodes are missing degree of freedoms. The MPC/Equation/kinematic coupling constraints can not be formed. The nodes have been identified in node set ErrNodeMissingDofConstrDef.

 

[FEA way]

Apparently some nodes in your model’s mesh have more than one kinematic coupling constraint assigned. Check where the nodes indicated in error messages are located and make sure that only one coupling is assigned to each of them.

 

[saja11]

This seems a layman question but What does first value in Von Mises box represents?
Like to report max stress on a bone I avaerage values on bone nodes near the red point(max stress pt.. Max stress on foot bones is not more than 10/12 MPa but first value in S, Mises box is like 3.61e+1 and similar

If I want to comapare diff models by first value of Simulation Result Box how can I report it??

 

[FEA way]

I’m not sure what you mean. Can you show a picture with these values ? If you are talking about the lowest value (the one on the bottom) displayed in von Mises contour plot color legend. This is simply the minimum von Mises stress calculated for the model. Usually of no concern. But you can go to contour plor settings and change Limits so that different models can be compared using the same color scale.

3.61e+1 is in scientific format. This equals to 3.61*10^1 = 36.1

 

[saja11]

Well If I want to see peak von mises stress in calcaneus bone only..Isn't it that I have to average sme nodes stress values near red contor area as peak stress at a single point is not considered in real scenarios...

 

[saja11]

And one more query plz... PLA, TPU, PEEK, Ca-hydroxyapatite, UHMWPE can be considered good biomaterials with low young modulus in load bearing prosthesis?

 

[FEA way]

Yes, such single element stress concentrations are often artificial and should be treated carefully. If you turn off averaging, switch to quilt plot and measure stresses in this element and its neighbors, you may notice large differences. You can hide this elements using Display Group tools and then contour plot scale will automatically adjust.

From what I know such materials exhibit high biocompatibility. They are also pretty good at bearing loads. Of course not as good as titanium but in case of foot prostheses their low weight is great advantage.

 

[FEA way]

That's rather unusual. Carefully check all units. If they are correct then maybe the analysis experiences instabilities that occur when parts in contact are stiff. One idea is that there are some rigid body motions that don't appear when material is flexible enough to deform in such way that contact between the parts is closed and they are not experiencing rigid body motions anymore.

 

[saja11]

Well job is completing now...But one thing i realized in animate scale factor that Plantar fascia truss elements are not connecting properly to calcaneus..I did same as you suggested..Made a reference point at end of truss element and couple that RP to some nodes of calcaneus..There is no warning of unconnected regions and job is completing but truss elements are not connecting properly

 

[FEA way]

Is it unconnected all the time or separates at some point of the analysis ? Did you try with connectors instead of truss element ? They are meant to be used in such cases and you can choose from various types as well as include damage criteria and connector stop option.

 

[saja11]

They disconnect in middle of analysis...I have used axial connectors and couple them to phalanges and calcaneus..But foot bones move to a side while remaining in contact..Is this due to BCs or I am doing something wrong in connectors?

 

[FEA way]

Keep in mind that axial connectors, as well as truss elements, normally can't take lateral loads. You would have to apply pretension to them first (or specify other directions in case of connectors).