Element Quality Check

In order to prevent analyses from being carried out on badly discretized models, an element quality check is incorporated into the pre-processing phase.

The element quality check is controlled by the CHECKEL parameter (see PARAM, CHECKEL), the ELEMQUAL Bulk Data Entry, and the SYSSETTING(ELMERRCMF=YES) command.

Three types of element quality checks are performed:
  1. Validity check of maximum allowable limits - a bound based on mathematical limitations. Violation will cause singular or ill-conditioned element matrices. Examples are: a re-entrant angle (equal to or greater than 180 degrees) in quadrilateral element/surface, 0 collapse in tetra.
  2. Quality check of error limits - whether an element is in the acceptable range.
  3. Quality check of warning limits - whether an element is in the recommended range. Violation may cause poor result quality, but will not stop the solution process.

With the CHECKEL parameter, type 2 and 3 checks may be controlled and the ELEMQUAL Bulk Data Entry may be used to set the error and warning limits. However, the "validity" check, type 1, is always performed - even when the value of PARAM, CHECKEL is NO.

Element quality checks are automatically turned on even if PARAM,CHECKEL,NO is present. Along with regular element quality errors, the elements which failed due to negative Jacobian and Collapsed grids are also printed to the HM.elcheck.cmf file in new categories called “jacobian” and “collapsed," respectively. Element Jacobians cannot be negative and elements that violate this rule are printed in the "jacobian" section of the HM.elcheck.cmf file.

A check for collapsed element nodes is performed for all elements. All other property checks are performed after this collapsed node check, and will be skipped if collapsed nodes are found. If the element quality check is activated, the checks for each element are performed according to the following precedence:
  1. Validity check
  2. Quality check for error limits
  3. Quality check for warning limits. Violation of any check will skip the subsequent check(s)

The warp angle check on CQUAD4 elements is relaxed for topography optimization. This relaxation prevents premature error termination of the optimization due to element quality concerns. However, be aware that the resulting mesh from a topography optimization may fail the CQUAD4 warp angle check when reanalyzed.

The checks performed and the default bound values for each element type are outlined in the following topics.