NLFE Bodies

NLFE stands for Non Linear Finite Elements. The NLFE implementation in MotionView/MotionSolve is based on Absolute Nodal Coordinate Formulation or ANCF. In this approach, only absolute coordinates and global slopes are used to define the element nodal coordinates without the need for using infinitesimal or finite rotations. In complex multi-body simulations, flexible bodies are needed to improve model fidelity. In cases where the deformations and rotations are expected to be large and/or exceed linear assumptions, NLFE becomes a necessity.

  1. If the Bodies panel is not currently displayed, select the desired body by clicking on it in the Project Browser or in the modeling window.
    The Bodies panel is automatically displayed.
  2. Choose a type from the Properties tab.
    If Beam is chosen:
    1. Select a cross-section type from the drop down menu.
    2. Enter the cross-section dimensions for the start and end.
    3. Define the material, Rayleigh damping, and number of segments to be used for the cross-section.
      Rayleigh damping represents damping that is proportional to the mass matrix of the body.
    Note: To view material properties, click the Manage Materials button. See the Materials topic for additional information.
    If Cable is chosen:
    1. Enter a value for the radius of the cross-section and the number of fibers.
    2. Define the material, Rayleigh damping, and number of segments to be used for the cross-section.
      Rayleigh damping represents damping that is proportional to the mass matrix of the body.
    Note: The cross-section is positioned such that the point profile defined in the Connectivity tab lies at the centroid of the cross-section.
  3. Click the Connectivity tab.
  4. Define a No Load profile.
    • Append, insert, and delete selected points by clicking the Point collectors and making your selection in the modeling window (or the Model Tree by double-clicking Point).
    • Click the Import Points button and browse for a .csv file from which point coordinates can be extracted.
    Note: The profile is generally defined as a no load configuration of the beam. This means that the beam modeled in this profile does not have any load on it at the beginning of the simulation. Defining a no load profile is mandatory.
  5. Define a loaded configuration.
    The loaded profile data is different than the no load profile and this difference introduces the pre-load in the body at the beginning of the simulation. Defining a loaded configuration is optional. If a loaded configuration is not specified, the body is in an unloaded state at the beginning of the simulation.
    1. Select Loaded from the View drop-down menu.
    2. Import a .csv file.
    An additional set of constraints is introduced by default at an NLFE grid when a marker (an explicit marker or implicit by means of joints/forces) is associated with the NLFE body at that grid position. This constraint rigidifies the grid. The constraints are of the form <CONN0 id = “conn_id” gid = “grid_id” conn = “TTTTTT” />. The statement constrains the gradient vector. The first three keywords ‘T’ in the conn attribute represent the constraint on the gradients along X, Y, and Z direction, while the remaining three ‘T’ keywords represent the constraint in the rotational (shear) direction. The form of these constraints can be changed using an environment variable HW_NLFE_CONN_TYPE = ssssss, where s can be either ‘T’ or ‘F’
  6. Click the Orientation tab.
    Note: The Orientation tab only becomes available if Beam is chosen in the Properties tab.
  7. Orient the elements at the start and end of the NLFE beam.
    The X-Axis of these two markers, by default, will be pointing along the length of the beam profile and hence is grayed out. The X axis of the Start Orientation marker is aligned with the second point defined in the beam profile, while that of the End Orientation marker is aligned along the vector formed by the last but one point and the last point in the beam profile.
  8. Click the Mass Properties tab and review the computed mass and mass-moment-of-inertia properties of the body along with its CG location with regard to global.
  9. Define initial conditions.
    1. Click the Initial Conditions tab on the panel.
    2. Activate the desired check boxes and enter values for translational and rotational velocity.
    3. Select a marker for VM and WM.
      Note: The translational initial conditions for a body are assumed to be with respect to the global reference frame in absence of a unique VM (linear velocity reference marker), and the rotational initial conditionals are applied about the center of mass maker in absence of a unique WM (angular velocity reference marker).