Modelling of an automotive suspension assembly using ANSA

Problem Statement -

For the given suspension assembly, check for the geometrical errors and mesh with the given element Quality criteria. After meshing the component the thickness has to be assigned for mid surfaces. Use tetra elements to model non-uniform thickness components. Also, use connections to connect the components.

Theory -

To complete this challenge, we'll be using a solid/volume as well as shell mesh.

To reduce the simulation time without the risk of decreased accuracy we, mesh uniform thickness components using a 2D shell mesh that consists of triangular(trias) and quadrangle(quads) elements. Such components are then assigned a thickness which can then be visualized as a solid.

A solid mesh can be created in two ways namely, 'Structured' and 'Unstructured'. Both ways require different procedures. An unstructured mesh is constructed using 'Pyramid' and 'Tetrahedron' elements and a structured mesh is constructed using 'Tetrahedron', 'Pyramid', 'Hexahedron', and 'Wedge' elements.

Both ways require an initial surface mesh, which is then extruded through the thickness of the part. The manner in which this extrusion occurs differentiates the structured and unstructured way of meshing.

An unstructured mesh consists of 'Tria' and 'Ortho tria' elements on the surface and using automated algorithms that have growth ratios tailored for each application, these tria elements are converted to tetra elements to fill the complete volume of the geometry.

A structured mesh consists of 'Quad' and 'Tria' elements on the surface. This mesh on the top surface is then mapped to the bottom surface by dividing the thickness in a structured way to fill the complete volume of the geometry.

We'll be using an unstructured mesh to complete this challenge.

The given geometry is the 'Rear Left Suspension Assembly' The assembly comprises of the following sub-assemblies/parts -

Tyre Wheel Wheel hub & Upright Assy.

Lower Control Arm Steering Assy Chassis Mounting Bracket

Brake Disc & Calliper Assy. Sway Bar Assy. McPherson Strut

Each part/assembly has to be meshed using shell/solid mesh and then connected to each other to form the complete subsystem. A part of pre-processing after the finite element model is created is the assignment of connections between the individual parts. For the given model we'll be creating the following connections -

RBE2 (Two Node/Many(Cluster) Node) - Defines a rigid body element of form 2 (RBE2) between selected grid points, 3D-points, etc. This element is a rigid body with independent degrees of freedom specified at a single grid point and with dependent degrees of freedom at an arbitrary number of grid points. The RBE2 is represented by a set of thick cyan lines connected to the grid point with independent degrees of freedom (displayed as a small red cross). When the grid point with dependent degrees of freedom is only one, the RBE2 is represented by a yellow line connected to the grid point with independent degrees of freedom (again, displayed as a small red cross).

Procedure -

1) Import CAD Geometry - The part file should be of a format supported by ANSA(STP., IGES., STEP., etc.).

2) Refine the geometry to target length(4) for better visualization of the part - Find the function under the mesh module and change the length of perimeters and macros to the specified target length.

3) Run the 'TOPO' function by selecting the complete geometry - This function automatically creates topological connections between selected adjacent faces. These connections are bifurcated as single, double, and triple connections as per the number of surfaces that are connected.

4) Run Geometry Check for the part and once cleared, reorient the part - Geometrical errors occur in the part because it is modeled in another software, and importing it leads to some translational errors. The following errors occurred in the part -

Triple Cons - These connections are formed where three surfaces meet and this is an error for such a part since as per its manufacturing process, such an occurrence is not possible. Hence these false connections need to be released, cleaned, and converted into double and single cons before moving forward. Triple Cons are shown as 'blue lines'.

Needle Faces - These are very tiny cross-section surfaces formed as translation errors when a part is imported from different software.

Unchecked Faces -

Overlapping Faces -

Cracks between faces -

5.) Each part is assigned different property IDs to better understand the connections.

6.) Mid-Surface Extraction - The auto mid surface function turns the solid description of a thin part into a thin shell description by isolating the outer or inner skin of the solid description. The function only requires you to select the faces after which it automatically comprehends its thickness and creates a new face accordingly. ANSA provides also a function named offset which can be used to create the mid surface of planar surfaces by manually selecting each surface and offsetting it by a distance that is half of the total thickness.

The part that will receive a shell mesh is the control arm but it has variation in thickness of all 4 faces and hence each face will be manually offset to create the mid surface. Surfaces are rejoined using functions like 'Extend' to get the final part.

7.) Setting Mesh and Quality parameters - These are set as per part's complexity, load case, and level of accuracy required. Tightened tolerances will lead to longer simulation times due to finer mesh which will result in increased nodes and thus more calculations. Although in our case, the quality criteria and mesh parameters are already given for both - shell and solid mesh as per part thickness.

Solid mesh properties -

Quality Criteria for Tria Elements (Tyre and Rim):

S.No	Quality Criteria	Value
1	Target/Average length	6.5
2	Minimum Length	5
3	Maximum Length	10
4	Skewness	45

Quality criteria for Tria (For other components):

S.No	Quality Criteria	Value
1	Target/Average length	4
2	Minimum Length	2
3	Maximum Length	6
4	Skewness	45

Common Quality Criteria for Tetra elements are:

S.No	Quality Criteria	Value
1	Tet-collapse	0.2

Shell mesh properties -

Quality criteria for Mixed elements (For Midsurface only):

S.No	Quality Criteria	Value
1	Target/Average length	4
2	Minimum Length	2
3	Maximum Length	6
4	Aspect	3
5	Warpage	15
6	Skewness	45
7	Jacobian	0.7
8	Minimum Quad Angle	45
9	Maximum Quad Angle	135
10	Minimum Tria Angle	30
11	Maximum Tria Angle	120

8) Meshing -

Firstly we'll shell mesh the control arm and we generate a mesh for the given area using one of the automatic algorithms that suit our purpose under the mesh generation tab. In this case, I have used the 'Best' function. This function automatically meshes Macro Areas using all alternative meshing algorithms and keeps only the mesh of the highest quality according to quality check.

Although the algorithm generates the best mesh possible, there are still some best practices to be taken into account before moving onto the next area. Since most areas are not perfect rectangles, the mesh is a mix of tria and quad elements. Since trias do not deform as well as quads, they are avoided in structural analysis near sensitive regions such as fillets and bends. Also opposing trias and rotating quads can form which need to be removed. In some cases, quality issues might remain unresolved even though the 'Best' function is used. In all such cases, elements can be iterated to achieve the desired quality using the functions shown in the image below,

Some functions majorly used in this model are -

Reconstruct - It examines a given mesh and performs re-meshing to correct problematic areas and optimize the overall quality of the mesh according to the mesh parameters.

Smooth - This function automatically applies Smoothing attributes so as to 'relax' the mesh on already meshed Macro Areas or on FE-model shell mesh.

Split - This function splits quadrilateral and triangular shell elements into smaller ones. Element splitting is performed between selected nodes and/or inserted nodes along selected element edges. The function can be applied to individual or groups of shell elements in order to improve the mesh orientation, reduce the number of triangles, or refine locally or globally the mesh of a part.

Swap - The Swap function rotates the shell edge picked by the user, inside the two shells which have that edge as common.

Next, we create the volume mesh for the rest of the parts.

We firstly select an area to start from. A good practice would be to move from the middle of the part towards the outer edges. This leaves some flexibility for rearrangements while in the other case, the area becomes constrained by the surrounding mesh. Once selected, using one of the automated algorithms such as 'Spot Mesh' or 'Best' create a mesh on all the surfaces. Reconstruct where elements fail to meet the user-defined quality criteria.

Once the surface mesh is generated we move to the volume mesh module. Here firstly we define each of the volumes, by manually selecting them and assigning a PID to them.

When each volume is defined we'll mesh them using one of the automated unstructured mesh generation algorithms that each have different growth ratios as per the application requirement.

Lastly, we need to mesh the spring. Since the spring has a cross-section that cannot be captured with a 2D plane and meshing it using 3D elements will increase computation time, we mesh it using 1D line elements. To achieve this we firstly create a curve such that it passes through the center of the complete spring.

Once the curve is ready, we move to the NASTRAN deck and create a CBAR element using this curve. Provide the target element length as 4 and define the cross-section of the line element as per the spring's dimensions to get the line element definition of the given spring.

9.) Connections - Since we're dealing with solid-solid or shell-solid connections, ANSA can only deploy RBE2 connections. Under the NASTRAN module, we select RBE2 many nodes or two nodes as per the required number of nodes to be constrained between two parts. They can also be used in conjunction with each other for applications such as to create a bolt connection. Take for example the connection between control arm and chassis mounting bracket -

Firstly we create two cluster node RBE2 connections using the nodes around the mounting hole. Once this has been created, a two-node RBE2 can be used to connect the master nodes of the two cluster RBE2 connections which can represent the stem of a bolt. Since the Control arm has a rotation about the X-axis as per model configuration, we'll remove the constraint by editing the degrees of freedom for the two-node RBE2. This can be done by removing '4' from '123456' where '123' are translational degrees about x,y, and z respectively and '456' are rotational degrees of freedom about x, y, and z respectively in the

In this manner, all parts need to be connected keeping in mind their type of connection and relative motion between the respective parts.

10.) Lastly, since the rear suspension is symmetric, we used the transform function to copy and get a symmetric mirror image of the right rear suspension about the working plane.

Output -

Learning Outcome -

1) Geometry cleanup and performing topology.

2) Generating mid surface

3) Assigning mesh parameters, quality parameters, and properties.

4) Mesh Generation using different algorithms and fine-tuning the mesh using various functions.

5) Converting shell mesh to solid

6) 2D surface meshing with Tria and Ortho Tria elements

7) Types of 3D mesh - Structured and Unstructured

8) Unstructured mesh algorithms

9) Different types of connections in an automotive part.

10) ANSA's forms of representation for each type of connection.

Conclusion -

This project enabled me to understand the complete concept of a very important process in the life cycle of a part's computer-aided design - Pre-Processing. The accuracy and precision of virtual simulations with real-world testing depend highly on the part's pre-processing stage. A clean geometry, good mesh flow, and the right connections can help mimic the real-world scenarios that a part undergoes and save a lot of time, money, and effort that can be invested in enhancements and innovations.

Google Drive link -

https://drive.google.com/file/d/17GWYYe16SDPbh9Xla6GkB0pSpAY94YJ2/view?usp=sharing