Meshing of Hood in Hypermesh

Objective:

The objective of this project is to create a 2D mesh of the given 'Hood' component. The given 'Hood' consists of a total of five different components. All the components of the hood need to have mid-surfaces extracted separately & then meshed according to the quality criteria given below. The target element length to be used is 5mm.

In this project, we will see the process of creating a 2D mesh for thin-walled components & components with complex shapes & features. For this purpose, we will be using Hypermesh software.

Procedure:

Let's first understand why we need a 2D mesh of a 3D component. For thin-walled components, i.e, where one dimension is very small compared to the other two dimensions, it is preferred to create a 2D shell mesh rather than a 3D mesh. By doing so, we save a considerable amount of time in processing & still get similarly accurate results as from 3D mesh. This is achieved by extracting the mid-surface of the component & assigning thickness to it, to capture the original thickness. And then the mesh is created for that mid-surface.

Now, let's see the process to generate a 2D mesh. The given hood component consists of the following five individual components.

First, the given model is imported as geometry into the Hyperworks Optistruct workspace. After importing the model, it is important to properly understand the geometry of the model and clean up any errors like missing surfaces, duplicate surfaces, etc. In these components, there were no such errors.

The next step is to extract the mid-surfaces for all the five components separately and assign thickness, property, and material to them. Mid-surfaces are extracted by going to mid-surface>auto midsurface>surfs>closed solid, under the Geometry (Geom) tool panel. Then a material named Steel is created by right click>Create>Material in the model tree tab. Similarly, a property named midsurface is created by right click>Create>Property. Under property configuration thickness (T) is assigned a value of 2mm. Card image & Material is set to PSHELL & Steel, respectively. Then, this newly created property is assigned to all the extracted midsurfaces, and material also gets applied automatically.

After extracting the mid-surfaces, it again needs to be carefully examined. Then cleanup is done for errors like missing surfaces, duplicate surfaces, distorted surfaces, unexpected free edges, etc. For the given components software itself extracted mid-surfaces accurately, apart for the inner body. As the geometry of the inner body component is very complex at certain places, there were some irregular/distorted surfaces as shown below. To correct this error, these irregular surfaces were deleted and then recreated by using either the surfs>spline/filler tool or surfs>Ruled tool under the Geometry (Geom) tool panel.

Also, there are several instances where we have more green lines (shared edge) than it is required to capture the feature. Such extra edges are toggled as they will create problems while meshing by driving the nodes on them. An example is shown in the image below.

After cleaning the geometry, the next step is to create the mesh. But before meshing such large and complex shaped components, it is better to plan before.
We can see that both the inner body and the outer body are symmetrical along the x-axis direction. So, it is better to just work on one half and then mirror the first half to generate a complete mesh. This way, geometry cleanup is also required for one half only. Similarly, the two hinge reinforcements are also a mirror image of each other. Therefore, in the case of the inner body & outer body, both geometry cleanup and meshing is done for only the right half while the other half was masked. Similarly, in the case of the two hinge reinforcements geometry cleanup and meshing is done only for one and then mirrored for the other one. Also, it is better to divide large and/or complex surfaces into small sections by splitting those surfaces. And then meshing for each section one by one, instead of meshing for the whole surface. This way it becomes very easy to control the quality, type, and flow of mesh elements. Adding a washer split around the circular/curved holes also helps to capture the geometry during meshing.

Images below show the midsurface of the inner body and outer body and the way I split the surface into sections.

It can be noticed that the right half of both are cleaned up and surfaces split into sections of simpler shapes which are easy to mesh.

The next part is meshing the midsurfaces. To create the mesh automesh tool under the 2D tools panel is used. Element size is set to 5mm and mesh type is configured to mixed.

Here, we are assuming that this mesh will be used for crash analysis. From that perspective, it is important to ensure that there are only quad elements. To do so is very simple if the surface section we are meshing can have an equal number of nodes on the opposite sides. There are tools in the automesh tool's interactive panel, using which we can force the mesh to have an equal number of nodes on opposite sides & therefore have only quad elements. But if the surface is converging, then sometimes having only quads will increase the mesh density and the elements might fail for quality criteria. In such cases, it is unavoidable and better to have some tria elements, as shown in the image below.

                                                             
And sometimes the shape of the surface itself is such that having only quad elements will introduce some rotational quad elements. For crash analysis, quad elements are not at all suitable. In such cases also, it is better to have some tria elements. And while dealing with the tria elements, it is important to ensure that there are no opposite trias and no back to back trias touching each other. Another thing to ensure is that the flow of mesh elements is consistent throughout as much as possible.

And most importantly all the elements must pass for the quality criteria. Quality criteria can be checked by going to Tool>check elems or 2D>qualityindex. There are a number of tools available under the 2D & Tool panel which are used to control the mesh quality. It also needs to be checked that there are no duplicate elements. Next, the mesh is checked for free edges to ensure there are no connectivity issues between the elements. Next, elements normal check is done, to ensure all the elements normal are in one direction.

Conclusion:

The 2D mesh is generated for the given hood components successfully, with zero element failure based on the quality criteria & with just 0.66% of tria elements.

After clearing all the checks, the mesh generated for the right half of the inner body & outer body is mirrored for the other half using the reflect tool. Then the elements on the centerline of both the halves are connected using the equivalence tool. Similarly, mesh generated for one hinge_reinforcement is duplicated and reflected for the second one.

Finally, the free edge check is done to ensure that there are no connectivity issues between the elements. Elements normal check also ensured that all elements normal were aligned properly.

Note: .hm file of the mesh is attached.