Finite Element Analysis

What is Finite Element Analysis (FEA)?

Finite element analysis is a mathematical process that can simulate physical, real-world conditions inside of a digital environment. FEA is a very useful tool in design, as it allows engineers to asses the validity of their model without having to conduct a large number of real-world tests or experiments. Take the design of an armchair for example. If an engineer would like to know the feasibility of their current design, without using any digital simulations, they would have to conduct lengthy physical testing of the chair to determine the eventual modes of failure. This information would be used to improve the design, and the lengthy testing process would begin again. This would continue until the engineer was satisfied with the testing results of their model. However when using FEA, the physical testing conditions can be defined in the software, and the program will simulate 1,000,000 cycles of loading. The results of this time-efficient simulation (modes of failure, cycles before failure, etc.) can then be used to improve the design. Once the design has been changed, the updated parameters can simply be input into the software, and the simulation can be run again. This results in a significantly accelerated design cycle.

FEA Workflow

For the Armchair Life Cycle Testing project, FEA simulations were carried out using the ANSYS Discovery AIM 2020 R2 software. This software can be used to run many different types of simulations, such as: structural, fluid flow, thermal, and electromagnetic. The necessary steps that must be completed in order to set up and run a structural simulation, as well as properly interpret the simulation results, are described in detail below. The pictures shown throughout correspond to an example structural simulation run on an armchair geometry. There are three main sections to the simulation workflow: "Geometry", "Physics", and "Results".

Physics

Material Assignments

In order to run a simulation, every single body in the imported geometry must be given a material assignment. This means that during the simulation calculations, each body will be treated as its designated material with its specific corresponding properties (density, Young's modulus, yield strength, etc.). Material assignments can be set using the following steps:

1. 1. 1. Under the "Physics" tab, select the "Material Assignments" option
      2. Click the "Body selection" tool on the top right side of the workspace
      3. Select a body and press the "Replace with selected entities" button that will have become blue
      4. If any other bodies in the geometry are the same material as the previously selected body, select those bodies and press the "Add selected entities" button
      5. Once all bodies of a common material have been added, click the "Material" drop-down menu and select the desired material (if the desired material is not a pre-defined material, select the "Create New" option and specify known properties)
      6. Return to the "Physics" tab. For any other sets of bodies that share a common material, select the "Add" option besides "Material Assignments" and repeat the above steps

Structural Conditions

The next step in setting up the simulation is the most important. The structural conditions section is where you will define all of the conditions that the geometry will face during the simulations. This tool can be used to define structural conditions such as forces, supports, temperatures, pressures, and many more. Structural conditions can be defined using the following steps:

1. 1. 1. Select the "Add" dropdown menu beside the "Structural Conditions" option and select the desired condition 
      2. Fully define the parameters of the condition based on the goal of the simulation. For example, when defining a force, the faces to which the force will be applied, the magnitude of the force, and the direction of the force must all be specified
      3. Repeat the above steps until all conditions desired to be in the simulation have been added

Note: A support can either be defined as the faces of the geometry that would be resting on the "ground" in reality, or by adding a body to the geometry that serves as the ground (flat slab of concrete for example), and defining that body as a support.

Advanced Setting: Interface Conditions

Interface conditions present in the simulation define how different bodies/faces that are in contact interact. For simpler geometries, this step is often not needed in order to obtain realistic simulation results, as the pre-defined conditions generated by the software are typically adequate. However for more complicated geometries, defining proper interface conditions important. There are five different types of interface conditions in Discovery Aim 2020:

• • • Bonded: By default, all contacts in the geometry are modelled as bonded. This means that the bodies in contact are treated as a single body and not allowed to slide, rotate, or separate from one another
    • Frictional: This behavior treats bodies in contact as distinct bodies, allowing them to slide and rotate against one another with a user-specified coefficient of friction
    • Frictionless: This behavior treats bodies in contact as distinct bodies, allowing them to slide and rotate against one another without the presence of friction
    • No Separation: This behavior treats bodies in contact as distinct bodies, allowing them to slide against one another, but does not allow for separation
    • Rough: This behavior is best used when one or both of the bodies in contact are significantly rough (ex. rough concrete surface)

Actually defining the interface conditions in the simulation can be done using the following steps:

1. 1. 1. In the "Physics" tab, select the "Interface Conditions" option
      2. A list of automatically detected bodies in contact will be displayed, ensure that the software did not miss any contacts
      3. For any contact for which bonded behavior is not suitable, click on the contact title
      4. In the "Contact behavior" dropdown menu, select "Create New"
      5. Select the behavior from the displayed list that is most suitable for the given contact
      6. Repeat the above steps for all necessary contacts

Solving Physics

Once all of the necessary simulation parameters have been defined, the physics task will display a yellow "Out-of-date" notification. This signals to the user that no more changes need to be made to this section before the simulation can be run. At this point, the user may click the "Solve Physics" button to begin running the simulation calculations. Depending on the complexity of the model, the solution process can take anywhere between a couple of minutes to a couple of hours to complete. Once the solution is complete, the physics task will display a green "Up-to-date" notification.

References

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