Explicit Dynamics in Ansys (Tutorial 2)

In this tutorial, we will learn how to do a basic explicit dynamics analysis in Ansys Mechanical. I will do my level best to share as much theoretical information about dynamic analysis as I can. We will be simulating a simple ball penetrating a thin glass sheet. Feel free to leave a comment if you have any doubts or corrections.

Pre-requisites:
1. A basic understanding of Ansys (preferably should be able to perform static structural analysis)
2. Understanding of core mechanical subjects like FEM and SOM.
3. Should have Ansys Mechanical (student version will do).

  1. Step 1: Understanding Dyamics

    An easy way to understand it is that Dynamic analysis in FEA is a type of analysis where things change over time.

    Dynamic analysis is done when there is a non-linearity in the system. There are 3 types of non-linearities:

    1. Material Non-Linearity: The material data is non-linear.
    2. Geometric Non-Linearity: The bodies move in space (this can occur if non-linear contacts or joints are defined)
    3. Boundary Condition Non-Linearity: The boundary conditions change over time (like if a force variable with time is applied)


    There are 2 general kinds of dynamic analysis:

    1. Implicit Dynamics: It uses several iterations to achieve a solution equilibrium at each step. Usually preferred for analysis with small non-linearities, like bending of a beam, dynamic loading on a suspension, etc.
    2. Explicit Dynamics: It conducts the analysis by calculating nodal acceleration. The can be used for problems with large non-linearities, like crash tests, chip formation analysis.


    Most dynamic problems can be done using both types of models. While explicit analysis does yield better results at higher non-linearity, it also consumes a lot more processing power.


    Ansys Workbench has an explicit dynamics model. While there is no direct implicit dynamics model in ansys, there are several that use implicit dynamics like:

    1. Transient Structural,
    2. Rigid Dynamics,
    3. Motion, etc.


    There are 2 types of bodies used in dynamic analysis:

    1. Rigid: These bodies cannot be deformed.
    2. Flexible: These bodies can be deformed.
  2. Step 2: Contacts and Joints

    A contact is basically used to define how two bodies interact with one another at the points or surfaces at which they come into contact with one another.


    There are generally 5 types of contacts:

    1. Bonded: These contacts can be used in static and dynamic analysis. The two surfaces in bonded contacts cannot penetrate one another, cannot separate and cannot slide over one another.
    2. No separation: These contacts can be used in static and dynamic analysis. The two surfaces in contacts cannot penetrate one another, cannot separate but they can slide over one another freely with no friction between them (coefficient of friction = 0).
    3. Frictionless: These contacts can only be used in dynamic analysis. The two surfaces in contacts cannot penetrate one another, but they can separate and they can slide over one another freely with no friction between them (coefficient of friction = 0).
    4. Frictional: These contacts can only be used in dynamic analysis. The two surfaces in contacts cannot penetrate one another, but they can separate and they can slide over one another with a user defined coefficient of friction between the two surfaces.
    5. Rough: These contacts can only be used in dynamic analysis. The two surfaces in contact cannot penetrate one another and cannot slide over one another (coefficient of friction tends to infinity) but they can be separated.


    In Ansys, if you don't define the contacts then Ansys will assume bonded contacts by default.


    Joints are used to define how a body moves in an analysis. You can either define a joint relative to another body, or relative to ground. It is better to use body to ground contacts, as in most analyses they are easier to define.


    The types of joints found in Ansys are:


    By default, Ansys does not create any joints as they aren't necessary for most analyses, however, they make your work a lot easier.

  3. Step 3: Let's start the analysis

    Simulating a small ball penetrating a thin glass sheet is one of the simplest analyses that you can do in explicit, so we shall perform that to get a beginner level understanding.


    Why explicit?

    This is a very important question you must ask yourself before you start an explicit dynamic analysis, as you may save yourself a lot of time and computational power. In this case, we are using explicit because we want to simulate the ball going through the plate, which introduces a high level of non-linearity to the problem.


    Let us begin:

    Go to Ansys Workbench and drag and drop the explicit dynamics model into the project schematic, like so:



  4. Step 4: Geometry

    Right click on geometry and open the design modeler. You can also make the geometry in space claim or discovery, but design modeler is much simpler and ideal for basic geometries such as the one we will use. You can also design the assembly in another CAD software like Fusion 360 or SolidWorks, then import it to Ansys, preferably as a .stp, .stl or .iges file.

    You are free to create whatever geometry you like, however, if you wish to replicate my project exactly, then here's the geometrical specifications:


    The glass plate is 2000x2000x10 mm. The ball is at the centre of the plate, 200 mm away from it. The ball has a radius of 30 mm.


  5. Step 5: Connections and Bodies

    After you are done with your geometry, close design modeler and double click on model (inside the explicit dynamics box).


    In the tree outline, click on geometry and click on ball. Change the stiffness behaviour of the ball from flexible to rigid, like so:


    We want the plate to be flexible as we want the plate to break. Click on the square plate and click on material->assignment. Search for glass and select common glass.




    Now we can define the connections, which basically means we will define the contact and joint.


    Contact:

    Double click on connections and delete anything below it, as we want to define everything manually.

    Now, double click on connections->insert->manual contact region. Choose the front face of the square plate as the contact body. Select the surface of the sphere as the target body. If you switch these two, it will give an error. Now, below definition, in type, put rough. We define rough because we don't want the ball to slide against the surface. This will also save computational time.


    Joint:

    Double click on connections->insert->joint. In scope, select the ball and click apply. Under definition, change type from fixed to translational. This step is not necessary, but will make your work easier; click on reference geometry and adjust the x-axis in the direction the ball will travel. In case you don't do this step, just check all the boundary conditions before solving to make sure the displacement vector is facing in the right direction. Then, drag and drop the translational joint on explicit dynamics. We will define the translation when we get to boundary conditions.


  6. Step 6: Mesh

    Let's get the mesh done before we move on to our boundary conditions. we need to mesh.


    To make things simple, we will auto-generate the mesh.


    Make sure element preference is explicit (so that an explicit mathematical model is assigned to the nodes and elements).


    Make sure the element order is linear. Linear order means h-refinement, i.e. the nodes are present at the corners of each element. We do this to reduce the number of nodes, which reduces computational power required and to prevent problems like mesh entanglement.




  7. Step 7: Boundary Conditions

    Under explicit dynamics, click on joint-displacement. Select type as velocity and enter velocity as 500 mm/s.


    Right click on Explicit Dynamics->Insert->fixed support. Choose the 4 corner faces of the glass plate and click ok.


    Go to analysis settings and create a time period table as follows:


    Enter a very high number for maximum energy error (like 1e10). This is to prevent an energy error during solving.


  8. Step 8: Solution

    Click on solution->insert->total deformation. You can calculate von-mises stress too, if you like.

    Solve and be patient, as it may take some time to solve.


    Result:

    Deformation:

    Stress:

    Notice how there is no stress contour on the ball (because the ball is rigid)



  9. Step 9: Final Note

    I'll drop all the files on my GitHub soon incase you wanna check it out for yourself.


    If you want to get better at explicit dynamics, I would advise you try this simulation by yourself and then modify it. Play around with the mesh, try different materials, try different angles of attack, etc.


    Thank you!!

Comments