The default failure criteria for ANSYS is Von Mises however

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 (Aluminium 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? 5. Observe the two stress distribution plots below, what is the difference between the two plots?

Solution

1) For brittle materials, maximum principal stress theory is used instead of von mises stress theory. If the option of maximum principal stress theory is missing, one can note the highest value of stress occuring in the material and then relate it to the yield strength to confirm for fracture.

2) Torsion loading of the material can cause very high stresses along the sharp edges of a material becuase torsion stress is directly proportional to the distance from the neutral axis of the material. More the distance greater will be the stress along the outr edges.

3) Stress caused in a material only depends on the loading, geometry and boundary condition and not on the material properties. Hence both will have the same stress field

4) The difference is that the mesh on plot 1 is of low refinement and the refinement of plot 2 is higher.

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.

Element Types

Geometry Topology

Mapped meshing can be used on geometry with the following shapes:

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