# How Element Quality is Calculated

The quality of elements in a mesh can be gauged in many ways, and the methods used often depend not only on the element type, but also on the individual solver used.

When possible, the most common or standard methods are used, but there is no truly standardized set of element quality checks. When a solver does not support a specific check within HyperMesh, HyperMesh uses its own method to perform the check.

## HyperMesh

When possible, HyperMesh checks strive to maintain compatibility with popular solvers.

### 2D and 3D Element Checks

- Aspect Ratio
- Ratio of the longest edge of an element to either its shortest edge or the shortest distance from a corner node to the opposing edge ("minimal normalized height"). HyperMesh uses the same method used for the Length (min) check.
- Chordal Deviation
- Largest distance between the centers of element edges and the associated surface.
- Interior Angles
- Maximum and minimum interior angles are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Deviation of an element from its ideal or "perfect" shape, such as a triangle’s deviation from equilateral.
- Length (min)
- Minimum element lengths are calculated using one of two methods.
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.
- The shortest distance from a corner node to its opposing edge (or face, in the case of tetra elements); referred to as "minimal normalized height".

Note: This setting affects the calculation of the Aspect Ratio check. - Minimum Length / Size
- Minimum element size is calculated using:
- Shortest edge
- Length of the shortest edge of each element is used.
- Minimal normalized height
- Is a more accurate, but more complex height.
- Minimal height
- The same as minimal normalized height, but without a scaling factor.

- Skew
- Skew of triangular elements is calculated by finding the minimum angle
between the vector from each node to the opposing mid-side, and the
vector between the two adjacent mid-sides at each node of the
element.
The minimum angle found is subtracted from ninety degrees and
reported as the element’s skew.Note: Skew for quads is part of the HyperMesh-Alt quality check.
- Taper
- Taper ratio for the quadrilateral element is defined by first finding the area of the triangle formed at each corner grid point. These areas are then compared to one half of the area of the quadrilateral.
- Warpage
- Amount by which an element, or in the case of solid elements, an element face, deviates from being planar. Since three points define a plane, this check only applies to quads. The quad is divided into two trias along its diagonal, and the angle between the trias’ normals is measured.

### 3D Element Only Checks

- Minimum Length / Size
- Two methods are used to calculate the minimum element size.
- Shortest edge
- Length of the shortest edge of each element is used.
- Minimal normalized height
- More accurate, but more complex.

- Tetra Collapse
- The height of the tetra element is measured from each of the four nodes to its opposite face, and then divided by the square root of the face’s area. The minimum of the four resulting values (one per node) is then normalized by dividing it by 1.24. As the tetra collapses, the value approaches 0.0, while a perfect tetra has a value of 1.0. Non-tetrahedral elements are given values of 1 so that HyperMesh will not mistake them for bad tetra elements.
- Vol. Aspect Ratio
- Tetrahedral elements are evaluated by finding the longest edge length and dividing it by the shortest height (measured from a node to its opposing face). Other 3D elements, such as hex elements, are evaluated based on the ratio of their longest edge to their shortest edge.
- Volume Skew
- Only applicable to tetrahedral elements; all others are assigned values of zero. Volume Skew is defined as 1-shape factor, so a skew of 0 is perfect and a skew of 1 is the worst possible value.

## HyperMesh-Alt

HyperMesh includes some alternate methods of calculating certain element types, which only apply to quads or rectangular faces of solids, and only include alternate checks for Aspect Ratio, Skew, Taper and Warpage.

- Aspect Ratio
- ratio1 = V1/H1
- Skew
- First, HyperMesh constructs lines connecting the midpoints of each edge of the quad, dotted in the picture below. Next, HyperMesh constructs a third line, green in the picture below, perpendicular to one of the initial lines, then finds the angle between this third line and the remaining initial line – with which is it most likely not perpendicular, unless the quad is a perfect rectangle.
- Taper
- First, the quad’s nodes are projected to plane defined by the
orthonormal vectors U-V found as follows:
- Z = X × Y
- V = Z × X
- U = X

- Warpage
- Only applies to quads or rectangular faces of solids. Warpage = 100 * h / max { Li }, where h is the minimum distance between the diagonals.

## OptiStruct

For the most part, OptiStruct uses the same checks as HyperMesh. However, OptiStruct uses its own method of calculating Aspect Ratio, and it does not support 3D element checks.

- Aspect Ratio
- Ratio between the minimum and maximum side lengths.
- Chordal Deviation
- Chordal deviation of an element is calculated as the largest distance between the centers of element edges and the associated surface. 2nd order elements return the same chordal deviation as 1st order, when the corner nodes are used due to the expensive nature of the calculations.
- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Deviation of an element from its ideal or "perfect" shape, such as a triangle’s deviation from equilateral. The Jacobian value ranges from 0.0 to 1.0, where 1.0 represents a perfectly shaped element. The determinant of the Jacobian relates the local stretching of the parametric space which is required to fit it onto the global coordinate space.
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.
- The shortest distance from a corner node to its opposing edge (or face, in the case of tetra elements); referred to as "minimal normalized height".

- Skew
- Skew of triangular elements is calculated by finding the minimum angle between the vector from each node to the opposing mid-side, and the vector between the two adjacent mid-sides at each node of the element. The minimum angle found is subtracted from ninety degrees and reported as its skew.
- Warpage
- Amount by which an element, or in the case of solid elements, an element face, deviates from being planar. Since three points define a plane, this check only applies to quads. The quad is divided into two trias along its diagonal, and the angle between the trias’ normals is measured.

## Abaqus

Abaqus-specific checks used to calculate element quality for 2D and 3D elements.

### 2D and 3D Element Checks

These checks apply to both types of elements, but when applied to 3D elements they are generally applied to each face of the element. The value of the worst face is reported as the 3D element’s overall quality value.

- Aspect Ratio
- Ratio of the longest edge of an element to its shortest edge.
- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Deviation of an element from its ideal or "perfect" shape, such as a triangle’s deviation from equilateral. The Jacobian value ranges from 0.0 to 1.0, where 1.0 represents a perfectly shaped element. The determinant of the Jacobian relates the local stretching of the parametric space which is required to fit it onto the global coordinate space.
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.
- The shortest distance from a corner node to its opposing edge (or face, in the case of tetra elements); referred to as "minimal normalized height".

- Skew (tria only)
- Defined by shape factor. Abaqus determines triangular element shape factor by dividing the element’s area by the area of an ideally shaped element. The ideally shaped element is defined as an equilateral triangle with the same circumradius—the radius of a circle that passes through the three vertices of the triangle—as the element.

### 3D Element Only Checks

- Volume Skew
- Only applicable to tetrahedral elements; all others are assigned values of zero.

## ANSYS

ANSYS-specific checks used to calculate element quality for 2D and 3D elements.

### 2D and 3D Element Checks

These checks apply to both types of elements, but when applied to 3D elements they are generally applied to each face of the element. The value of the worst face is reported as the 3D element’s overall quality value.

- Aspect Ratio (tria)
- For tria elements, a line is drawn from one node to the midpoint of the opposite edge. Next, another line is drawn between the midpoints of the remaining two sides. These lines are typically not perpendicular to each other or to any of the element edges, but provide four points (three midpoints plus the vertex).
- Aspect Ratio (quad)
- If the element is not flat, it’s projected to a plane which is based on the average of the element’s corner normals. All subsequent calculations are based on this projected element rather than the original (curved) element.
- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Deviation of an element from its ideal or "perfect" shape, such as a triangle’s deviation from equilateral. The Jacobian value ranges from 0.0 to 1.0, where 1.0 represents a perfectly shaped element. The determinant of the Jacobian relates the local stretching of the parametric space which is required to fit it onto the global coordinate space.
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.

- Angle Deviation (Skew)
- Only applicable to quadrilateral elements, and relies upon the angles between adjacent legs at each corner node (that is, the interior angles at each corner). Each angle is compared to a base of 90 degrees, and the one with the largest deviation from 90 is reported as the angle deviation. Triangular elements are given a value of zero.
- Warping Factor
- Only applicable to quadrilateral elements as well as the quadrilateral faces of 3D bricks, wedges, and pyramids.

### 3D Element Only Checks

ANSYS does not use any exclusively 3D checks within HyperMesh, but HyperMesh does use its own when ANSYS is set as the solver. For details on 3D checks, refer to HyperMesh.

## I-deas

I-deas-specific checks used to calculate element quality for 2D and 3D elements.

Additional element checks not listed here are not part of the solver’s normal set of checks, and therefore use HyperMesh check methods.

### 2D and 3D Element Checks

- Stretch (Aspect Ratio)
- Stretch is evaluated differently depending on whether the element is
triangular or quadrilateral:
- For trias, the radius of the largest circle that fits within the element is divided by the longest edge, then multiplied by the square root of 12.
- For quads, the minimum edge length is divided by the maximum diagonal length. The result is multiplied by the square root of 2.

Note: The inverse of stretch displays on-screen in HyperMesh as the aspect. - Chordal Deviation
- Largest distance between the centers of element edges and the associated surface. Second order elements return the same chordal deviation as first order, when the corner nodes are used due to the expensive nature of the calculations.
- Jacobian
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.

- Skew
- Deviation of an element’s corners from 90 degrees (for quads) or 60 degrees (for trias).
- Taper
- Taper ratio for the quadrilateral element is defined by first finding the area of the triangle formed at each corner grid point.
- Warpage
- The amount by which an element, or in the case of solid elements, an element face, deviates from being planar. Since three points define a plane, this check only applies to quads. The quad is divided into two trias along its diagonal, and the angle between the trias’ normals is measured.

### 3D Element Only Checks

- Stretch (volume aspect ratio)
- Stretch is evaluated differently depending on whether the element is a
tetrahedron, Wedge, Brick, or Pyramid.
- Tetras
- The radius of the largest sphere that fits within the element is divided by the longest edge. This value is then multiplied by the square root of 24.
- Wedges
- Each face is evaluated for its 2D stretch, and the worst value is reported. This means that the value reported for vol AR should always be the same as that reported for aspect.
- Bricks
- The minimum edge length is divided by the maximum diagonal length. The result is multiplied by the square root of 3.
- Pyramids
- No check is defined, so HyperMesh performs its standard check in which each face is evaluated as a 2D object and the worst result reported.

## Medina

Medina-specific checks used to calculate element quality for 2D and 3D elements.

Additional element checks not listed here are not part of the solver’s normal set of checks, and therefore use HyperMesh check methods.

### 2D and 3D Element Checks

- Aspect Ratio (Edge Ratio)
- Edge Ratio is calculated as the ratio between an element’s shortest edge and its longest edge; For the sake of consistency, HyperMesh inverts this result, effectively making it the ratio of longest to shortest, and reports the result as the element’s aspect ratio.
- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.

- Maximum Angle
- Largest angle between adjacent edges of the element is reported.
- Minimum Angle
- Smallest angle between adjacent edges of the element is reported.
- Skew
- Element’s interior corner angles are compared to 90 degrees (for quads) or 60 degrees (for trias). The absolute values of these deviations are summed and reported.
- Taper
- Quadrilateral elements are split into two triangles.
The area of the smaller of the two triangles is compared to the
total area of the quadrilateral. In Figure 33, $$taper=\frac{{A}_{b}}{{A}_{quad}}$$.Note: To improve consistency with other taper checks, HyperMesh displays a value of 0.5 minus this value so that 0 implies no taper. However, this is not completely consistent with other taper checks, because in this case taper ranges from 0 (no taper) to 0.5 (full taper), whereas HyperMesh’s own taper check reports a 1.0 for full taper.
- Warpage
- Elements with more than three nodes are split into triangles. The largest angle between the normals of triangle pairs is reported as the warpage.

### 3D Element Only Checks

Medina does not use any 3D specific checks. HyperMesh uses its own checks instead.

## Moldflow

Moldflow-specific checks used to calculate element quality for 2D and 3D elements.

Additional element checks not listed here are not part of the solver’s normal set of checks, and therefore use HyperMesh check methods.

### 2D and 3D Element Checks

- Aspect Ratio
- Only applied to triangles, with quadrilaterals given a value
of:$$\frac{2.0}{\sqrt{3}}$$

### 3D Element Only Checks

- Vol. Aspect Ratio
- Finds the perpendicular height h of each node, and then dividing the
longest edge length L by the shortest height h and multiplying by the
square root of 1.5:$$\frac{\sqrt{1.5}\times L}{h}$$This results in an equilateral tetrahedron having a volume aspect ratio of 1.5. Non-tetrahedral elements are assigned a value of 1.0.

## Nastran

Nastran-specific checks used to calculate element quality for 2D and 3D elements.

### 2D and 3D Element Checks

- Aspect Ratio
- Ratio of the longest edge of an element to its shortest edge.

- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Skew
- HyperMesh creates lines between the midpoints of opposite sides of the element, then measures the angles between these lines. The angle with the greatest deviation from the ideal value is used to determine skew.
- Taper
- HyperMesh finds the taper of quadrilateral elements by treating each node as the corner of a triangle, using one of the quad’s diagonals as the triangle’s third leg. The areas of each of these four "virtual" triangles are compared to one half of the total area of the quadrilateral element to produce a ratio; the largest of these ratios is then compared to the tolerance value. A value of 1.0 is a perfect quadrilateral, and higher numbers denote greater taper.
- Warpage
- First, HyperMesh constructs a plane based on the mean
of the quad’s four points. This means that the corner points of a warped
quad are alternately H units above and below the constructed plane. This
value is then used along with the length of the element’s diagonals in
the following equation:$$WC=2H/(D1+D2)$$Where WC is the Warping Coefficient, H is the "height" or distance of the nodes from the constructed plane, and D1 and D2 are the lengths of the diagonals. Thus, a perfect quad has a WC of zero.

### 3D Element Only Checks

- Vol. Aspect Ratio
- HyperMesh evaluates Tetrahedral elements by finding the longest edge length and dividing it by the shortest height, measured from a node to its opposing face. Other 3D elements, such as hex elements, are evaluated based on the ratio of their longest edge to their shortest edge.
- Warpage
- HyperMesh evaluates warpage on solid element faces by dividing the quad face into two trias along its diagonal, and measuring the cosine of the angle between the trias’ normals. This value will be 1.0 for a face where all nodes lie on the same plane.

## Patran

Patran-specific checks used to calculate element quality for 2D and 3D elements.

### 2D and 3D Element Checks

- Aspect Ratio (triangle)
- The length of a side is divided by the height of the triangle from that side to its opposite node, then multiplied by ½ of the square root of 3. In a perfect equilateral triangle, this formula produces a value of 1. The process is performed for each of the three sides, and the largest value of the three is reported as the aspect ratio.
- Aspect Ratio (quads)
- If the element is not flat, it is projected to a plane which is based on the average of the element’s corner normals. All subsequent calculations are based on this projected element rather than the original (curved) element.
- Interior Angles
- Maximum and minimum values are evaluated independently for triangles and quadrilaterals.
- Jacobian
- Length (min)
- Minimum element lengths are calculated using one of two methods:
- The shortest edge of the element. This method is used for non-tetrahedral 3D elements.

- Skew (triangle)
- Patran evaluates triangular skew by constructing a line from one of the triangle’s nodes to the midpoint of its opposite side, and another line connecting the midpoints of the remaining two sides.
- Skew (Quad)
- The skew test begins by bisecting the four element edges. This creates an origin at the vector average of the four corners, with the x-axis extending from the origin to the bisector on edge 2. Next, finding the cross-product of the x-axis and the vector that stretches from the origin to the midpoint of edge 3 defines the z-axis. With the x and z axes defined, their cross-product defines the y-axis. Finally, subtracting the angle α (located between the y axis and the line bisecting edges 1 and 3) from 90 degrees reveals the element skew.
- Taper
- Patran calculates taper by first averaging the corner nodes to find the
element center, and creating lines between this center and the corner
nodes to split the element into four triangles.$$taper=\frac{4{\alpha}_{smallest}}{\alpha 1+\alpha 2+\alpha 3+\alpha 4}$$
- Warpage
- The warpage test bisects the element edges, creating a point at the
vector average of the element corners. This point serves as the base
node for a plane, with the plane’s x-axis extending from the base node
to the bisector on edge 2 of the element. The plane normal (z-axis) is
in the direction of the cross-product of this x-axis and the vector from
the origin to the bisector of edge 3. Each corner of the quad is then
the same distance, h, from the plane. Next, Patran measures the length
of each half-edge, and calculates the arcsine of the ratio of h to the
shortest half-edge length (L):$$\Theta ={{\displaystyle \mathrm{sin}}}^{-1}\frac{h}{L}$$

### 3D Element Only Checks

- Vol. Aspect Ratio (Tetrahedron)
- Patran finds the aspect ratio of Tetra elements by finding the ratio between a vertex height and ½ the area of the opposing face. This process is repeated for each vertex, and the largest ratio found. Next, Patran multiplies the largest ratio found by 0.805927, the corresponding ratio of an equilateral tetrahedron. The result is reported as the element’s aspect ratio, with a value of 1 representing a perfect equilateral tetrahedron.
- Vol. Aspect Ratio (pyramid)
- Ratio of the element’s longest edge length to its shortest edge length.
- Vol. Aspect Ratio (wedge)
- This test begins by averaging the triangular faces of the element to create a triangular mid-surface. Next, it finds the aspect ratio of the mid-surface, as for a tria element. Then it compares the average height (h1) of the wedge element to the mid-surface’s maximum edge length (h2). If the wedge height h1 exceeds the edge length h2, the wedge’s aspect ratio equals the mid-surface aspect ratio multiplied by h2, then divided by the average distance between the triangular faces (h3).
- Vol. Aspect Ratio (hexahedron)
- Each face of the hex element is treated as a warped quadrilateral, and its center point found. The volume aspect ratio is simply the ratio of the largest distance h between the center points of any two opposing faces, to the smallest such distance.