# CQUAD4 Element Checks and Default Bound Values

## Element Check

The following qualities are checked for CQUAD4 elements:

- Vertex Angle
- For each vertex, the angle is measured between its two adjacent edges. The minimum and maximum values of the four nodes are reported for the element.
- Aspect Ratio
- The aspect ratio of a CQUAD4 element is the length of its longest side, divided by the length of its shortest side.
- Skew Angle
- The skew angle in a CQUAD4 element is calculated by finding the minimum angle between two lines joining opposite mid-sides of the element. Ninety degrees minus the minimum angle found is reported.
- Warp Angle
- The warpage of a CQUAD4 element is calculated by splitting the quad into two trias and finding the angle between the two planes which the trias form. The quad is then split again, this time using the opposite corners and forming the second set of trias. The angle between the two planes, which the trias form, is then found. The maximum angle found between the planes is the warp angle of the element.

## Default Bounds

Information | Default values for warning message | Default values for error message | Default values for validity check | |||
---|---|---|---|---|---|---|

Lower Limit | Upper Limit | Lower Limit | Upper Limit | Lower Limit | Upper Limit | |

Aspect Ratio | - | 100.0 | - | 1000.0 | - | 1.0E5 |

Skew Angle | - | 60.0 | - | 75.0 | - | 90.0 |

Warp Angle^{1} |
- | 30.0 | - | 60.0 | - | 180.0 |

Vertex Angle | 15.0 | 165.0 | 3.0 | 177.0 | 0.0 | 180.0 |

The values used for warning and error checks may be adjusted by the ELEMQUAL Bulk Data Entry, but validity checks are hard-coded.

^{1}Warp angle limits are relaxed to 140/160/179 in topography optimization problems.