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CrackTip Command

The CrackTip coverts all elements at the crack tip to be quarter point elements. Quarter point elements are 6 or 8-node quadratic where the midside nodes near the crack tip are 1/4 of the way from the tip node to the next edge node rather than half way as in conventional quadratic elements. Quarter point elements at the crack tip are created by creating a mesh with 8-node quadrilaterals or 6-node triangles and then adding the command:

CrackTip (keyid)

   or

CrackTip (x),(y)

where

Notes

  1. In theory, quarter point elements represent the singular stress field of a crack tip better than conventional elements. They were first proposed by Barsoum (1974)
  2. Quarter point element are available in NairnFEAMPM for research use, but they have drawbacks, and therefore are not recommended. Experience with crack-closure calculations show they converge more slowly to the correct result than conventional mid-side nodes and prior literature has errors in the crack-closure equations (see Nairn, 2007c). Furthermore, NairnFEAMPM plots strain from FEA results by differentiation of the displacement field. Since quarter-point elements have singular strains at the crack tip, the strain in those element will be undefined. These elements can still plot stresses.