FEA Questions The default failure criteria for ANSYS is Von

FEA Questions

The default failure criteria for ANSYS is Von Mises, however you have been asked to complete a FEA on a brittle material, what failure criteria would be more appropriate and why? How would you apply this criterion if there is no option for it within the program? A colleague of yours has come to you with a question about their mesh, they have a general mesh on most of the object with refinement around the sharp edges. They find that as they increase the refinement level the stress keeps increasing and is now beyond the failure stress of the material. What sort of loading could cause this to happen and why? Consider two objects made of different isotropic materials (Alumininum and Steel) but with the same geometry, loads and boundary conditions. Which of the materials would have a higher stress and why? When meshing you have two options; free and mapped. Explain the advantages and disadvantages of each of the methods? Give an example of when you would use each method and justify why? Observe the two stress distribution plots below, what is the difference between the two plots?

Solution

1) For a brittle material, the principal stress theory would be better applicable over von mises theory because brittle materials have almost no plastic deformation and von mises theory is almost exclusively used for ductile materials only. If the FEA package has no option for principal stress theory, you can apply it by carryinf out the FEA analysis on the part and then finding it the maximum principal stress on the part and then compare it to the yield strength to determine for fracture.

2) Torsional loading can have this effect, generally on parts with sharp edges high stress concentration factors actually increase the nominal stress value. Hence increasing the refinement causes the edges to have a better result of the stress field.

3) Stress in a body is independent of the material properties and only depends on the load applied , geometry and boundary conditions.

4) A free mesh has no restrictions in terms of element shapes, and has no specified pattern applied to it.

Compared to a free mesh, a mapped mesh is restricted in terms of the element shape it contains and the pattern of the mesh. A mapped area mesh contains either only quadrilateral or only triangular elements, while a mapped volume mesh contains only hexahedron elements. In addition, a mapped mesh typically has a regular pattern, with obvious rows of elements. If you want this type of mesh, you must build the geometry as a series of fairly regular volumes and/or areas that can accept a mapped mesh.

5) the difference between the two plots is in the level of mesh refinement. Plot 1 has a lower mesh refinement and is visible by the fact that the stress distribution is not so vivid whereas in Plot 2 because of a higher mesh refinement stress distribution is much more detailed.