Assignment 1 - Shell Meshing 

Shell Meshing

Aim: To create the shell (2-D) meshing for the given models by eliminating geometrical errors and considering the given element quality criteria using Hypermesh.

Objective:

1. To clean the geometry.

2. To extract mid surface and clean mid surface geometry.

3. To create 2D mesh and assign proper thickness, by creating a generic property.

Introduction on Finite Element Analysis:

The finite element method is a numerical technique for obtaining an approximate solution to a wide variety of engineering problems. FEA (Finite Element Analysis) is done by three phases.

1. Pre-processing

2. Solution

3. post-Processing

This project comes under Pre-processing. The goals of Pre-processing are to develop an appropriate finite element mesh, assign suitable material ID and boundary conditions. ANSA And Hyper-mesh are the example tools and in this project Hypermesh is used. Hypermesh interface is shown in the fig 0.2.

Fig.0.2: Hypermesh Interface.

Types of the elements

1. One-dimensional elements: These elements are suitable for one-dimensional problems such as bar, truss and beams.

Example- Linear elements have 2 nodes etc.

2. Two-dimensional elements: These elements are suitable for Two-dimensional problems such as plane stress and plane strain in sheet metal parts up to thickness of 5mm.

Example- Tria Elements, Quad elements,

2. Three-dimensional elements: These elements are suitable for three-dimensional analysis. uses for solid parts and sheets of thickness more than 5mm.

Example- Tetra elements and hexa elements.

Above all types of elements are done under user defined Quality criteria. for example refer table1.1.

Element Quality Criteria:

S.No	Quality Criteria	Value
1	Target/Average length	5
2	Minimum Length	3.5
3	Maximum Length	7

Table1.1: Element Quality Criteria

Procedure:

The meshing is done by following steps

1. Import the files in to Hypermesh

2. CAD Geometry clean up and topology

3. Mid-surface extraction

4. Defeaturing

5. Shell meshing

6. Creating properties and Thickness assigning.

1. Import .hm in to Hypermesh : file>open>model, browse the file and open. in the figure 1.1 shows 3 given models.

Fig.1.1: Imported file.

From the imported file we can see components are uniform thickness.

2. CAD Geometry clean up and topology: In this step the given model undergoes geometry check. Where all the defects such as collapsed faces, cracks, free edges, T-edges are clear. In the fig.2.1 show description on surface.

Fig.2.1: Description on surface.

Tools can use in topology

Tools>Edge Edit Panel

A. Equivalence (multiple edges at a time) Search surfaces for pairs of free edges and combine into shared edges

B. Toggle (edge pair at a time) Select an edge; equivalences with other free edges found within a user specified tolerance

C. Replace (1 edge pair at a time) Select 2 edges to equivalence together. Control which edge to retain and which to move.

D. Geom>Surfaces>Ruled or Geom> Quick edit > filler surface or 2d> spline> surfaces only (to create missing surfaces).

For model 1 we have missing surfaces T- junction edges, and free edges. In the other two models we don't have any geometry error shown in the figure 2.1.1. In the wire frame we can see all the geometry error.

.

Fig.2.1.1: Model 1 with geometry error

errors are cleared one by one using different tools explained in the following steps.

To clear all free edges toggle tool is used (Tools>Edge Edit Panel>toggle or F11 from keyboard)

Toggle (edge pair at a time) Select an edge; equivalences with other free edges found within a user specified tolerance.

In the fig.2.1.2 shows before and after toggle

Fig.2.1.2: Before and after toggle

Replace points: Move points from their current locations to the locations of other points, effectively combining them. Use the moved selector to select the point you wish to move (or use window selection to select multiple points). Use the retain selector to specify the point that you wish the moved point to combine with. In the fig.2.1.3 shows before and after replace points

Fig.2.1.3: Before and after Replace Points

Filler surface: Use the line(s) selector to select a closed-loop line (a hole) in the model to create a surface that fills the hole. In the fig.2.1.4 shows before and after Filler surface

Fig.2.1.4: Before and after Filler surface

All T-edges in the models are due to presence of duplicate surfaces so this T edge can be cleared by deleting the duplicate surfaces. In the fig.2.1.5 shows before and after

Fig.2.1.5: Before and after deleting

Intersecting surfaces are cleared by following steps trim and delete.

a. Use the Trim with Surfs/Plane subpanel to trim or split surfaces with another surface or a plane. This function determines the intersection of the selected surfaces and a plane or a surface, and then trims the original surfaces at this intersection.

b. When surfaces are trimmed, they are broken into smaller pieces (surfaces). After the surfaces are trimmed, you can delete any unwanted surfaces.

In the fig.2.1.5 shows before and after Trim, Delete.

Fig.2.1.6: Before and after Trim, Delete.

3. Mid-surface extraction: The given model is a floor panel and before going to the Mid surface extraction check for thickness of entities and measure (Geom>distance). We found thickness is same for throughout components of thickness in table below

Model	Name	Thickness (units)
1	Part 1	0.9
2	Part 2	2.0
3	Part 3	1.8

Automid: for extraction of mid surface Auto midsurface (Geo>midsurface>Automidsurface) and select the surface, click extract so that it will create midsurface. We can change the type selection by Extraction option.

Steps

a. Select the surfaces or solids from which you wish to extract midsurfaces.

closed solid: Select a single surface of the solid geometry and click extract. The enclosed volume the selected surface belongs to is automatically determined, and the midsurface is extracted from it.

incomplete: solid Enable additional options for selecting and orienting the surfaces defining the body to extract.

Extract the midsurface by clicking on extract. Here skin offset and offset are selected shown in the fig.3.1 for all 3 models' mid surface.

Fig.3.1: Mid-Surface.

4) Defeaturing: Update the Criteria length as per given element Quality criteria which is Target/Average length 5. Removing all unnecessary edges, fillet radiuses etc. also maintain distance between two edges or nodes not less than Minimum Length of 3.5. also remove holes of dia less than 5 units

tools to Defeaturing

A. Toggle edge

Left-click a free edge (red) to make it shared (green), or a shared edge to make it suppressed (blue).

Right-click a suppressed edge to make it shared, or a shared edge to make it free.

In either case, the edge must meet the criteria for the type of edge you wish to change it to. For example, toggling a free edge to a shared edge requires the presence of a similar edge within the distance you specify in the tolerance field.

B. Replace (1 edge pair at a time) Select 2 edges to equivalence together. Control which edge to retain and which to move.

C. Geom> Quick edit > filler surface (to create missing surfaces).

In the fig.4.1 shows the all 3 models Defeature

In 1st and 2nd model holes are removed

Fig.4.1: Defeature

5. Shell Meshing: Before meshing Assign given element Quality criteria in to hypermesh by Preferences>criteria file setting shown in Fig.5.1

here to know more on each criteria

Fig.5.1: Quality Criteria

Start meshing Process by Automesh (2D>Automesh )

steps to mesh

a. Select surfaces for initial mesh and for remesh select elements.

b. select size and bias abd select mesh size which is Target/Average length 5. Use the Size and Bias subpanel to perform size and bias meshing, a flexible and powerful meshing method. A minimum of inputs are required for the element criteria settings. The algorithm options set preferences on how to handle certain situations when encountered in the geometry. When using existing finite elements as the basis for mesh generation, feature recognition settings allow the mesher to break up the areas defined by the selected elements into logical groupings with mesh controls set for each group boundary.

c. Select the type of mesh as mixed (Quad, Tria, mixed, R tria etc) and clock on alin and size in order to get proper mesh near features.

clock on mesh or middle mouse button

Modify mesh flow by changenging surfaces to elements and select the elements. Make sure that while doing mesh all the element Quality criteria must be satisfied. Avid maximum opposite traits and. In fig.5.2 show the mesh flow for model 1.

Fig.5.2: Mesh Flow

Model 2 & 3 are symmetric in both X and Y axes.

a. So both the models are divided into 4 parts..

b. Select one quarter part for meshing.

In fig.5.3 show the mesh flow for model 1 & 2

Fig.5.3: Mesh Flow

c. reflect the mesh along x then y by selecting base point.

In fig.5.4 show the mesh flow after reflect for model 1 & 2

Fig.5.4: Mesh Flow

Find Free Edges: Use the Edges panel to find the free edges in a group of shell elements, find "T" connections in a group of shell elements. Free edges are defined as a pair of nodes identifying an element edge that are not shared with another element. Free edges are normally found around the outer perimeter of a part or around openings within the part.

Free edges found in your model are displayed as red plot elements in their own component called ^edges. Free edges within a field of elements typically indicate a discontinuity within the mesh. Such discontinuities may be intentional, such as crack modeling for durability simulation, or unintentional, due to geometry discontinuities, meshing methods or component boundaries. In cases where the mesh has a discontinuity, there will typically be more than one node at approximately the same location in space. Such nodes are called "coincident". You can use equivalence to remove the coincident nodes, based on a search tolerance which you specify.

Steps

a. Go to tools>Edge

b. Select the elements whose associated edges you wish to find.

c. Specify a spherical search radius around each node in the selected component or group of elements. If multiple nodes are found within this search tolerance they are considered coincident, and can be resolved with the equivalence function. tolerance is approximately60 to 70 % of thickness

d. Click on preview equivalence. and equivalence to join nodes.

e. Click on find edges to see the free edges in Red color for mesh show in the fig.5.5

Fig.5.5: Find Free Edges

Remove all quality failing elements by smoothing Mesh, by changing distance, angle b/w the nodes of failing elements we can see 0 elements are failed

6. Creating properties and Thickness assigning: Not only property we can create components, material, Cross sections etc by right click on Tab area and select requirement shown in the fig 6.1.

Fig.6.1: Create

Before creating properties we need to create components for elements as per the different thickness. right click on the Tab area and select component to create components. If we have varying thickness (taper).Then divide that region with respect to mesh flow and assign thickness value from maximum to minimum with equal intervals.

Create property as per the components thickness. To create right click on the Tab area and select property to create properties.

a. select property, we can see the properties in the tab area.

b. click on the property and rename, change the thickness, select material, card image and etc. for each component we need to create property shown in the fig6.2

Fig.6.2. Components and Properties

Each property card has the following properties. as per the solver we have to change the properties.

Slover keyword

Id

color

card image

thickness T etc shown in the fig.6.4

Fig.6.4: properties card

Assign properties to each component by right click on comp> assign> select property. In the graphics area we can see the model after applying properties shown in the fig. 6.5 with 2D detailed elements representation

Fig.6.5: After Applying Properties & 2D detailed elements representation

Check for normals: use the Normals panel to display and reverse the normals of elements or surfaces. The orientation of element normals can also be adjusted. The normal of an element is determined by following the order of nodes of the element using the right-hand rule.

Steps

a. Use the Elements subpanel to display and reverse the normals of elements. Select elements indirectly by components or directly by elements, or elect to have all of the displayed 2D elements selected automatically. Click display to review the current normal orientation of the selected elements. Optionally, click reverse to flip the normals to the opposite side (reverse the normals).

b. Adjust normals based on the orientation method you specified

c. Flip all of the normals of the selected elements shown in the fig. 6.6.2

Fig.6.6.2: Normals

Result:

Shell (2-D) mesh is done Successfully for 3 models by eliminating geometrical errors and considering the given element quality criteria also assigning thickness using Hypermesh.

In the fig.6.7 Shows the final FE model

Fig.6.7: Result