Week 3

Meshing a car Hood Using Ansa

Aim:

To Check the Quality for the given car hood using various tools in Ansa software before manufacturing the product.

Abstract:

The hood or bonnet is the hinged cover over the engine of motor vehicles that allows access to the engine compartment, or trunk on rear-engine and some mid-engine vehicles for maintenance and repair. The car hood is manufactured such that it is strong and less in weight so the it protects the engine components and the passengers in the vehicle in case of an accident. Therefore to maintain the quality of the hood design the component must satisfy the basic quality criteria as required. This part is done in a CAE software before proceeding for manufacturing process. In this challenge Ansa software is being used for the design topo cleanup and meshing the given component.

Methodology:

The following method is being done for the challenge

• The imported component first undergoes a procedure called topo cleanup. The imported geometry will be having geometrical errors which will be fixed in this process so that the geometry is prepared for the next process.
• A mid surface should be created form the geometry and to be extracted so the given 3D geometry is converted to 2D component. Make sure the mid surface has no geometrical errors for further process. The original and the extracted mid surface are represented as parent geometry and child geometry respectively.
• A Mesh is to be created for the particular mid surface. Meshing is the process where the geometry is covered with a mesh or net structure which contains nodes and elements. In this process the software shows the errors representing respective colours for the different reasons of failure. Th e user should identify the errors and rectify them using the tools for the particular operation. The mesh should flow uniformly through the given geometry which represents the good quality of the component. After meshing the component should be given the thickness as that of the parent geometry.

Introduction:

The bonnet system is an access panel to the engine compartment to enable maintenance of power train, drive belts, battery, fluid levels and lamp units. It is fundamentally a reinforced skin panel with many safety and quality requirements.

Procedure:

Geometrical Cleanup:

The imported geometry contains many errors which are to me rectified before preceding for the process.

Outer panel Inner panel

The imported geometry in the Ansa software consists of cons and hot points which are responsible for maintaining the structure of the geometry. Hot points are responsible for connecting the cons and maintaining the measured distance of the cons. Cons are of three types

1. Single Cons : Single cons are represented in red colour which connects the surface in single dimension.
2. Double Cons : Double cons are represented in yellow colour which connects two surface as a two dimensional surface.
3. Triple Cons: Triple cons are represented in light blue colour which connects the surfaces in three dimensional surface.

The software is usually set to ENT mode which shows the component’s positive and negative phases. The positive and negative phase is represented in grey and yellow colours respectively.

The surfaces of the components should be attached to each other so that there should be no cracks in the component. To attach the surfaces a function called ‘topo’ is performed by selecting the total component into one.

To check if there are any kind of errors a geometry check is to be done so that the user can easily identify the errors.

It is a tool where the errors can be identified and can be fixed automatically or manually by the user.

After rectifying the errors the component can be taken for further process.

Mid Surfacing:

Mid surface is a process where a 2D surface is created in between the given geometrical surface. The given geometrical surface is represented as parent geometry where as the mid surface as child geometry. To create child geometry first the thickness of the geometry is to be measured so that the child surface is created in between the parent geometry.

Measurement of the outer panel Measurement of the inner panel

To create a mid surface a tool known as offset face is used to select the positive or negitive phase of the component.The software shows the direction of creating the mid surfaceas an arrow on the selected surface. After selecting a perticular phase the user has to mention the location to be created in between the parent geometry.

The Software asks for a conformation for deleting the original parent geometry. However it is the users choice to delete or not to delete the surface. Finally the child geometry is created in between the parent geometry and the mid surface is to be extracted from the given component.

Mid surface Parent geometry

The mid surface is to be checked again perticularly for the geometrical errors and are to be rectified if need. There must be not fillet areas present in a mid surface so that the child geometry will fail to met the quality criteria.

Both the mid surfaces ie., outer panel and inner pannel should have their own properties to identify and mentioning the thickness. So a PSHELL card should be created in the property manager and the thickness is to be mentioned. The software alotes different colours for each PSHELL card crated.

Property Manager Creating new PSHELL card

PSHELL Card

Quality Criteria:

Quality Criteria are the specific elements or functions that will be selected, tested and measured in order to confirm that the quality objectives have been met. The mid surface created should met the given quality criteria as shown in the table. Each quality criteria is mentioned with unique colours so that while meshing the failed area can be easily identified and can be fixed.

Quality Criteria

Meshing:

Once the geometry is in an appropriate state, a mesh will be created to approximate the geometry. Either a beam mesh (1-D), shell mesh (2-D) or a solid mesh (3-D) will be created. This meshing step is crucial to the finite element analysis as the quality of the mesh directly reflects on the quality of the results generated. At the same time the number of elements (number of nodes) affects the computation time. That is the reason why in certain cases a 2D and 1D mesh is preferred over 3D mesh. Each node in a mesh is considered as Degree of Freedom (DOF) by the software. Each degree of freedom consists of 6 axes [3 translations (Ux , Uy , Uz) and 3 rotations (θx , θy , θz)] and applies all the quality criteria for the component. The total DOFs for a given mesh model is equal to the number of nodes multiplied by the number of DOF per node. As the mesh lines increase the error margins reducies.

Before meshing outer panel After meshing outer panel

Mesh pattern of outer panel

Before meshing inner panel After meshing inner panel

Mesh flow of inner panel

The mesh flow must me uniform according to the shape of the component. The mesh on the mid surface are the 2D elements.

1. 2D Elements:

2D Elements are 3 or 4 node elements with only 2 DOF, Y and Z translation, and are normally created in the YZ plane. They are used for Plane Stress or Plane Strain analyses.

2D elements

The majority of the mesh consists of quadrilateral (Square) elements and the triangular elements are restricted in less percentage. The triangular elements again are of two types

• Linear-strain triangle (LST)
• constant-strain triangle (CST)

A CST element is a three noded linear triangular element having 2 nodes per side while an LST element is six noded quadratic triangular element having 3 nodes per side.

A CST element has straight sides where as the LST element may or may not have curved sides. A curved geometry can be meshed with curved side LST elements whereas a rectangular (straight sided) domain may be discretised with straight sided but higher order LST elements.

2. 1^st or and 2^nd order elements:

A Linear element or First order element will have nodes only at the corners. This is something like the Edge Centered Cubic Structure. However, a Second order element or Quadratic element will have mid side nodes in addition to nodes at the corner.

3. Tools used for proper meshing:

In many cases, the imported geometry is not ready for meshing. Quite often the geometry needs tobe cleanup first due to

• “broken” surfaces
• surfaces which are not stitched together
• redundant (multiple) surfaces
• surfaces which are too small to be meshed in a reasonable way later on

To overcome these problems particular tools are used for good flow of the mesh. They are:

1. Reconstruction:

When a mesh is having too many errors and triangular elements, they can be fixed using the reconstruct tool by selecting the area to be reconstructed for meshing the region. Must of the mesh will be solved automatically by this tool. However few region must be solved manually by the user where the reconstruct my not give appropriate results.

1. Split:

Split is a tool which is used for creating another line of mesh by splitting triangular elements or uneven quad surfaces.

Splitting the mesh

1. Swap:

Swap is a tool which is used for swapping a triangular side to another side so that the triangle can be used for a smooth mesh flow or for splitting the triangle

Before Swaping After swaping

1. Smoothing:

Smooth is a tool used for spreading the uneven surfaces to even and uniformmesh flow. It may also be used for solving the errors formed in the mesh.

Smoothening the mesh flow

1. Pasting:

Pasting is a tool used for stiching to lines of mesh to a single line of mesh by selecting the nodes in parallel. This tool is mostly used when there are two triangles opposite to each other.

Paste tool window

Shell to Solid conversion:

After the correction of the mesh flow the mid surface model is in shell mode. The shell mode should be converted to solid mode so that the model can be given a thickness as that of parent geometry

Inner Hood Panel shell to solid

Outer hood panel shell to solid

Result:

The Quality checking using the mesh process is done without any errors and less percentage of triangle elements.

CAD model FE model

CAD model FE model

Learning Outcome :

• Learned to check the geometry and extracting the mid surface.
• Learned to mesh the mid surface .
• Learned to solve the errors using the tools.
• Learned how the mesh is used to improve the quality.

Conclusion:

Present work draws conclusion as given below:

• The outer hood panel has total 49266 elements, the quad elements are 99.6% and triangular elements are 0.40%
• The inner hood panel has 38792 elements, the quad elements are 97.18% and triangular elements are 2.82%.

https://drive.google.com/file/d/1VBSZ91hW7tKdTkXfi20sHPMEkkHYjNEB/view?usp=sharing