Meshing on Backdoor of a Car using HYPERMESH

• Aim

To obtain the discretized backdoor of a car for the virtual analysis.

• Objective

To prepare the backdoor of a car for the virtual analysis by discretizing it into a finite number of elements.

PROCEDURE

The backdoor model was comprised of 3 parts including the inner extract, the upper outer extract, and the lower outer extract. Each part was kept as different components. To obtain the completely discretized or meshed hood model, each part had to be meshed separately. The inner extract was so uneven in shape, and thus, it was taken firstly for meshing. The three different parts can be seen below.

Figure 1: Inner extract.

Figure 2: Upper outer extract.

Figure 3: Lower outer extract.

• Geometry Clean-up

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. This might include missing faces, distorted faces, misconnection of edges, or the presence of free-edges. The part is made up of many faces. Each face has to be connected with the neighboring faces. This connection is represented by the green line, in Hypermesh, and if this connection is missing between any two neighboring faces, it is represented by a red-line which represents a free edge of that particular face. The line can also be suppressed if no division between faces is required. Fig. 4 shows the presence of these three kinds of lines.

Figure 4: Different kinds of edges.

To make a line, free from sharing the edges, or connect the edges, or suppress the edges, there is an option called the ‘toggle edge’ in the Geom→quick edit. It can also be used from the Geom→edge edit→toggle.

In 2d meshing, the elements are deployed on a surface of no thickness or midsurface. Solid parts have no role in 2d meshing. Therefore, before cleaning the geometry, it is necessary to check if any solids present in the model. If present, it should be deleted.

In this model, no solids were present. It was not even having many errors. The only error found was in the lower outer extract. This error was related to the presence of free-edge, as shown in fig. 5.

Figure 5: Edges were not connected in the lower outer extract.

To connect those free-edges, points were added on both of the edges, and using the ‘replace point’ option, in the quick edit tool, edges were connected.

Figure 6: Connected edges of the lower outer extract and the part is free from errors.

• Extracting Mid-surface

After cleaning the geometry, the next stage of the discretization procedure is to obtain the mid-surface or the middle surface of the complete part. Mid-surface is the surface of the sheet metal part which has no thickness. A property has to be assigned separately to that surface, which defines its thickness. The reason behind extracting the mid-surface is to reduce the computational time. If the part is meshed with 3-d elements, it would take more computational time to solve the problem compare to 2-d elements. For the sheet metal and plastic components parts with thickness less than 5-6 mm, generally, 2-d elements are used.

Figure 7: Mid-surface of half portion of backdoor’s inner extract.

Figure 8: Mid-surface of half-portion of backdoor’s upper outer extract.

Figure 9: Mid-surface of half-portion of backdoor’s lower outer extract.

In fig. 7, 8, and 9, the mid-surface of only half-portion was obtained. It was because of the symmetry of the part. Symmetric parts reduce the efforts of meshing the part. If the parts are symmetric, only the half portion has to be meshed, and the elements on the other side can be mirrored. It can be performed by using the ‘reflect’ tool, in the Tool module.

In fig. 7, some face was not symmetric, and therefore, they were obtained completely (and not half) and the elements were not reflected.

• Preparation for meshing

While meshing a part, the flow of elements has to be smooth. It provides a smooth flow of energy. To maintain the smooth flow of energy some other criteria are like no trias should be facing each other, trias shouldn’t be present at the edge, etc. Rotating quads are also not required. Tria elements are generally kept very limited. In this work, trias are limited to 15%.

To mesh the complete geometry with the smooth flow of elements, the geometry is split into a number of smaller parts bearing in mind that it will not disturb any feature of the geometry. Thus, meshing and observation of the smaller parts become easier.

• 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 changes in the shape of any part, the triangle and square elements change 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. Quality criteria required are set and it is provided to the CAE team. Depending on the criteria, the part is meshed. The criteria provided for this part is as shown below table:

Table 1: Quality criteria for elements.

Target/Average length	5 mm
Aspect	5
Skewness	45
Warpage	15
Taper	0.5
Minimum length	3 mm
Maximum length	7 mm
Minimum Quad Angle	45
Maximum Quad Angle	135
Minimum Tria Angle	30
Maximum Tria Angle	110
Tria Percentage	15%

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 10: 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 is meshed on-by-one. The figures below show all the meshed parts of the hood model.

Figure 11: Discretized backdoor assembly including inner extract, the outer upper extract, and the lower outer extracts.

Figure 12: Meshed inner extract.

Figure 13: Close view of meshed backdoor inner extract.

Figure 14: Backdoor’s meshed upper outer extract.

Figure 15: Close view of backdoor’s meshed upper outer extract.

Figure 16: Backdoor’s meshed lower outer extract.

• 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 as shown in fig. 17.

Figure 17: Window to check failed elements.

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, 4 elements have failed for warpage, 6 for taper, and 131 for minimum length. All these failed elements are due to space constraints. However, there are no elements failing for aspect and skewness.

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.