Meshing of Door Inner Panel using HYPERMESH

DISCRETIZATION OF SIDE DOOR INNER PANEL

Aim

To obtain the discretized inner panel of the side door of a car for the virtual analysis.

Objective

To prepare the inner panel of the side door of a car for the virtual analysis by discretizing it into a finite number of elements.

• PROCEDURE

The .hm file of the inner panel model when imported in Hypermesh, it was observed that the panel included many ribs and attachment components, as can be seen in fig. 1 and 2. The uneven shape and many small geometrical features can be seen in fig. 2.

Figure 1: Outer side of the inner panel.

Figure 2: Inner side of the inner panel.

Such small features increase the complexity in obtaining the midsurface. There were some features with varying thickness and some other features with no thickness. While performing pre-processing, and if that too with only 2d elements, these are the concerns that have to be paid attention to.

Geometry Clean-up & Mid-surface Extraction

The primary stage of meshing any part is to clean the geometry. If the part created in any modeling software and then opened in any other CAE software, because of a change in the format of the file some errors occur in the geometry. The file that was imported had so many geometrical errors, however, it was possible to obtain the mid-surface without cleaning the geometry. Therefore, without wasting time on cleaning the geometry, the work was directly skipped to the extraction of mid-surface.

It was tried to obtain the mid-surface using an automatic option of obtaining the mid-surface, however, after doing that it was found that many surfaces were not obtained as perfect as was required. Thus, instead of deleting all the automatically obtained surfaces, only the surfaces which were not obtained correctly were deleted. Manually, those deleted mid-surfaces were then obtained. In some regions, new surfaces were also created to make the mid-surface the exact representation of the imported inner panel geometry. Fig. 3 shows the mid-surface of the side-door inner panel.

Figure 3: Mid-surface of the side-door inner panel.

Preparation for meshing

While meshing any component, the one thing one has to be kept in mind is that the flow of elements has to be smooth. It provides a smooth flow of energy. To maintain the smooth flow of energy there are some criteria are like, no trias should be facing directly each other, trias shouldn’t be present at the edge, rotating quads are also not required if the preparation is for the durability analysis, etc. Tria elements are generally kept very limited. In this work, trias are limited to 10%.

Setting Quality Criteria

Meshing with 2d elements is basically performed using two types of elements viz. quads and trias (or squares and triangles). The accuracy of the results depends on the quality of elements. An ideal element is a perfect square or perfect triangle. But due to the ups and downs in the shape of the component, the shape of the triangle and square changes slightly. These changes can be in their lengths or angles. To mesh the model with only ideal elements is very difficult. Thus, a criterion is always set for the quality of elements based on some parameters viz. the aspect ratio, the warpage, the minimum, and maximum length, etc. of an element. The criteria provided for this part is as shown below table:

Table 1: Quality criteria for elements.

Target/Average length	4 units
Aspect	5
Skewness	45
Warpage	15
Taper	0.5
Minimum length	2 units
Maximum length	6 units
Minimum Quad Angle	45
Maximum Quad Angle	135
Minimum Tria Angle	20
Maximum Tria Angle	120
Tria Percentage	10%

The quality of the elements required is set by clicking the Preferences→Criteria file settings, in the menu bar. A dialog box would open as shown in fig.7 in which a required quality criterion can be set.

Figure 4: Dialog box that opens after clicking the Preferences → Criteria file settings in the menu bar to set quality parameters of elements.

Discretization or meshing

Once the quality of elements is set, the next stage is the meshing of the model. To mesh the model, there is an option called ‘automesh’ in the 2D section (shortcut is F12). Using this tool, the faces of the model are meshed on-by-one. Figure 5 shows the meshed inner panel component.

Figure 5: Meshed side door inner panel.

Assigning the Thickness Property

Many features of the geometry had uniform thickness, however, some of them were having varying thicknesses. For the features with uniform thickness, a property card was created, providing it with the value of the thickness, and assigned it to the respective surfaces. And for the features with varying thicknesses, automatic midmesh thickness was provided. In the menu bar, Mesh → Assign → Midmesh Thickness, when clicked, a dialog box opens as shown in fig. 6.

Figure 6: Dialog box for automatically assign the thickness.

In this dialog box, the surfaces of parent geometry and the created elements are provided as input. Clicking the ‘Calculate Thickness’ button automatically calculates the thickness and also creates separate property cards. Fig. 7 shows the rib with varying thickness, whose thickness was provided using the automatic thickness method.

Figure 7: A rib with varying thickness.

Quality Check

After completing the meshing part, the last stage of the process is to check if all the elements are within the set criteria or not. To do that, there is an option called ‘elems check’ in the Tool section. When clicked, a window of a set of quality parameters opens. Failed elements can be found by clicking on any of the parameters. Once the failed elements are found it has to be corrected manually. It can be done by using the different tools available in 2D→qualityindex→cleanup tools, as shown in fig. 10. In this work, 6 elements have failed for warpage, 2 for skew, 8 for taper, and 49 for minimum length. All those elements are failing because of space constraints. However, there are no elements failing for aspect.

• CONCLUSION

Meshing a model is the primary and important stage of virtual testing. Thus, while meshing the geometry with quad/tria elements, attention should be given to maintain the mesh flow. Because, if it has gone wrong, the result would be inaccurate and thus would be the waste of time and money.