What is XFEM

[Fea_pather]

Hello community,

I am trying to use XFEM for my Fracture simulations in ABAQUS. I find it difficult to grasp at first glance. Can anyone please explain XFEM in simpler words for me?

Can I use XFEM in place of VCCT or the Cohesive approach? Under what cases can I use XFEM?

 

[SWComposites]

Extended finite element method - Wikipedia

en.wikipedia.org

Lots of other info from a simple google search.

 

[FEA way]

In practice, you can use XFEM in Abaqus to model bulk crack propagation with crack location independent of the mesh. It can be used together with CZM or VCCT (delamination in conjunction with bulk crack propagation). Crack path and location don't have to be predefined.

For more details, check the Abaqus documentation.

 

[Vishakh_Rajendran]

Put in a very simple way, XFEM is the art of an element saying to the Solver - 'LOOK, I AM NOT LIKE OTHERS, HANDLE ME WITH CARE'.

In an ABAQUS model, try plotting Displacement ('U') contours in the model. The contour lines will be smooth and continous. In an undamaged material, displacement is continuous across the body, meaning that any point's displacement is smoothly connected to the adjacent points. But across a crack, this continuity is broken. When a crack forms in a solid, the material on either side of the crack is no longer connected, and the displacement field across the crack faces will be discontinuous.

So, a crack means = discontinuity is broken.

Until XFEM came about, people had no way to represent this displacement discontinuity within an element. So, they put this displacement discontinuity along the element edges (this is why VCCT crack method is through a contact definition).

With XFEM, they found a way to put this discontinutiy within an element's interior. They decided to modify the shape function by multiplying it with some enrichment terms. Shape functions define how field variables (like displacement or temperature) vary within an element. By modifying the shape function, we are telling the software how to do the displacement discontinuity and thereby bringing in the effect of the crack.

These enrichments are two - one heaveside enrichment (for those elements cut by the crack) and a crack tip enrichment (for that element which contains the crack tip).

An example of a shape function affected by the heaveside enrichment term is attached. The new shape function has a jump in the shape function at X = 1 (where the crack is at). This is how the effect of the crack is brought into the meshing. So, now, we have a way to tell the position of the crack to the software.

We give the initial crack using *INITIAL CONDITIONS, TYPE = ENRICHMENT. Heaveside enrichment only introduces the effects of the crack on the element (thereby allowing FEA to have a crack even in the element interior).

Best Regards,

Vishakh Rajendran.