point loads/ point constraints

[amoncur]

i made a toothpick and have two points towards the ends i'm

using as constraints.



i made another point right in the middle of the toothpick i'm using for my point load.



i specified material, constraints, and load (the load is unidirectional in the opposite direction of constraint)



but i get an error saying the highlighted point loads can cause singular stress concentrations



and when i run it it terminates with a fatal error.



any ideas how to fix it?

 

[Israr]

Point loads and constraints cause singularities, yes Pro/Mechanica is correct. What you should do is; instead of making your toothpick as solid, create 3 points in pro/engineer (if you'r using integrated Pro/Mechanica) and then go to Mechanica, then Structure, then Model, then Idalizations, and then Beams.

If you need a tutorial on this, I will send you one.



Israr

 

[Kantikumar]

Hi,

Singularities occur when infinitesimally small areas are stressed. Physically, such small areas do not exist in the world. Mathematically, Pro/MECHANICA STRUCTURE is trying to map elements exactly as the model is defined. This can result in abnormally high stresses or displacements.



There are two types of singularities that occur in Pro/MECHANICA STRUCTURE: those due to constraint conditions and those due to geometry.



Singularities cause edges in a model to require a high p-level for convergence thus resulting in a longer solution time. Additionally, the result of singularities can cause anything from abnormally high results to models which will not converge. Typically, peculiar fringe plots are an indication that singularities in the model may exist.



Singular conditions will vary depending upon the entity being loaded or constrained and the type of element being used. You must know combinations of loads or constraints and different types of elements result in infinite stresses or displacements for 3D models.



Along with loads and constraints, reentrant corners or sharply varying geometry can cause singularities. It may be necessary to alter an internal angle of an element or smooth out discontinuous geometry to achieve convergence. If this is not possible, excluding elements is an option. The following examples will illustrate where singularities occur and how changing the model definition can help to achieve expected results.



Well I can conclude that .......it becomes obvious how changing the model definition can affect model results. Singularities can occur in structural models either due to constraint conditions or in model geometry. Below are further tips to avoiding singularities:



Try to create constraints on as large an area that is physically reasonable



Consider Poisson's Ratio effects



In models that tend to have sharp corners or sharp cutouts, use excluded elements in the analysis



If possible, alter geometry to smooth sharp corners or discontinuities. Use fillets if possible this will also reduce high stresses in the model.



I hope this anyhow help you..if you need more information..please let me know..



Regards,

Kantikumar Lakhankar

 

[Kantikumar]

In last reply I forgot to write this..One of the following techniques is recommended for this situation: Attach point laod directly to point on tooth peak and then constrain MX, MY, MZ at the point. And then run beam elements along the edges of the solid that connect to the attachment point. This way the tooth peak is actually attached to the point load and it should have a relatively small cross-section so that it has a negligible effect on the overall stiffness of the model.