Why do K-Values not converge when refining the mesh with quadratic tetrahedrons?

- November 21, 2019 at 8:33 am
  
  charlesk
  Subscriber
  
  Hello everybody,
  
  I’m doing fracture analysis on steel structures mostly using the premeshed-crack option in ANSYS workbench (I used the arbitrary crack before and had the same problems). Due to the fact that I have to do many iterations to calculate the crack propagation, modelling every iteration with crack tip elements does not seem feasible. Using the “semi-elliptical crack” is not an option because my crack will propagate close to the border of the steel part later.
  
  My main problem is this: when refining my mesh around the crack the SIF (K-Values) keep getting better until a certain element size. From then on, the results get worse and worse again. This does not count for the non-fracture-tool-results (stress, strain, deformation, ...)
  
  I’m modelling my crack by extruding and then deleting a very thin (0,001 mm) semi-elliptical surface from my body. Afterwards I use the pinch command to ignore the small face within the crack. I’m meshing my whole body using quadratic tetrahedrons.
  
  Right now I’m working with ANSYS 19 R2, I have encountered the same problem in other versions aswell.
  
  I added some pictures showing the geometry around the crack and the mesh after deformation. I can upload more information if needed.
  
  Thank you!

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