Free Body Diagrams – Advanced Utilities to Ease the Process

Detailed part analysis and optimization involves the creation of sub-models, or breakout models, extracted from global or internal loads models.  The creation of these sub-models can be difficult and time consuming using traditional methods.  Utilizing Free Body Diagram (FBD) concepts, one can simplify and streamline the understanding, creation and setup of breakout models.  FBDs allow the user to understand load paths and to create free body loads of interest for detailed models as boundary conditions (BCs) within a sub-modeling scheme.

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FBDs also allow for equivalent force and moment results to be calculated on cross-sections for each loadstep.  From these results, Shear-Moment Plots (VMT) can be created for determining maximum equivalent loads.  These results can also be used to create Potato Plots for performing loadstep screening in advance of the detailed part analysis and optimization.

Altair HyperMesh Free Body Diagram Tools

The Free Body Diagram (FBD) tools facilitate the extraction and post-processing of grid point force (GPF) and displacement data from OptiStruct and Nastran .op2 files, Abaqus .odb files, and Ansys .rst files.  FBD is typically utilized for breakout and/or sub-modeling, where balanced loadsteps are extracted from a coarse grid model for eventual optimization and/or analysis on a fine grid sub-model of the component of interest.  FBD is also used to extract equivalent cross-sectional forces and moments – typically at the centroid of a cross-section – for use in traditional strength calculations.  These use-cases have implementations within HyperMesh called FBD Forces, FBD Displacements, and Resultant Force and Moment, respectively.

Free Body Force Extraction

The HyperMesh FBD Forces tool operates on GPF data contained within OptiStruct and Nastran .op2 files, Abaqus .odb files, and Ansys .rst files.  The tool requires that the components/elements of interest be organized into a single entity set.  In order to facilitate the creation and visualization of entity sets, a tool called the Set Browser is available.  The FBD Forces tool supports GPF data for all element types (including rigid, 1D, 2D, and 3D), SPC and MPC constraints, and all applied forces, moments, pressures and body forces (including gravity and temperature loads).

The steps to create a “free body” with the FBD Forces utility are as follows:

1.    Load a User Profile supported by the FBD Forces tool (RADIOSS (Bulk), OptiStruct, Nastran, Abaqus or Ansys).

2.    Open the FBD Forces tool using the Post->Free Body->Forces pull-down menu.

3.    Select the result file (.op2, .odb or .rst) containing the GPF data.

4.    Select the loadstep(s) for which free bodies are to be extracted.

5.    Select the element set that contains the elements defining the free body.

a.    Use the Set Browser to create element sets, or define them using the FBD Forces tool on-the-fly.

6.    Optionally, select a coordinate system into which the grid point force and moment result vectors are transformed and output; the default is the global system.

7.    Optionally, select a summation node.  This is useful for verifying free body behavior through zero-sum values for all force and moment components about any node.  It is also useful for calculating the result of applied or reaction forces about any node.  The default is (0,0,0).

8.    Optionally, select from several output options including the FBD type (all loads, applied load only, reaction loads only), the zero-tolerance value (the cut-off point below which a result quantity is considered zero), and options for displaying and reviewing the nodal force and moment results for the free body (vector representations via load collectors, tabular output in a summary table window, and a comma separated value (.csv) file that can be opened with MS Excel).

9.    Click Accept.

a.    If the option to create load collectors containing the result vectors is specified, several load collectors are created for each selected loadstep.  These load collectors can be visualized via the FBD Results Manager tool.

Free Body Vector Visualization with FBD Results Manager

One benefit of the HyperMesh FBD utilities is that they can create and store free body

results for multiple loadsteps with a single extraction run.  Since free bodies are being

generated and stored for multiple loadsteps, there is a need for a tool to simplify managing and reviewing the data.  The FBD Results Manager tool manages the load collectors created by the FBD Forces, FBD Displacements and Resultant Force and Moment tools, and allows the user to customize the vector display within the HyperMesh model window for any selected loadstep or cross-section.  The functionality also allows for graphical inspection of the free body within HyperMesh, as opposed to sorting through the tabular output to find the quantities of interest.

The steps to visualize the results of any FBD Forces, FBD Displacements or Resultant Force and Moment extraction are as follows:

1.    Select the element set containing the free body of interest.

2.    Select the result type of interest.

3.    Select the loadstep from the list of loadsteps available for that element set and result type.

4.    For FBD Displacements and Resultant Force and Moment extractions, select the node set of interest.

5.    For FBD Forces and Resultant Force and Moment extractions, select the vector components and/or resultant forces and moments to display.

6.    Click Accept.

Equivalent Cross-Section Force and Moment Extraction

The Resultant Force and Moment tool within HyperMesh allows for the calculation of

equivalent cross-section forces and moments, either automatically at the nodally averaged centroid of the cross-section, or at any user-defined node.  Equivalent forces and moments are typically used in traditional strength calculations.  Extracting equivalent forces and moments along the length of a beam at all cross-sections obtains the data necessary to develop VMT (V = Shear; M = Moment; and T = Torque) plots.  The Resultant Force and Moment tool has the capability to output a HyperGraph .fbd file that contains the equivalent forces and moments for all defined cross-sections for all selected loadsteps. With the HyperGraph Shear Moment Plot utility (Utilities->Free Body Diagrams->Shear Moment Plot), plots for selected cross-sections and loadsteps can automatically be generated from the .fbd file, and searched for the maximum equivalent loads.  Additionally, a Potato Plot utility is available for performing loadstep screening.

The steps necessary to extract equivalent cross-section forces and moments for any cross-section and any number of loadsteps is as follows:

1.    Load a User Profile supported by the FBD Forces tool (RADIOSS (Bulk), OptiStruct, Nastran, Abaqus or Ansys).

2.    Open the Resultant Force and Moment tool using the Post->Free Body->Resultant Force and Moment pull-down menu.

3.    Select the result file (.op2, .odb or .rst) containing the GPF data.

4.    Select the loadstep(s) for which equivalent cross-section forces and moments are to be extracted.

5.    Select the cross-section(s) of interest.

a.    Cross-sections consist of an element set, a node set, an optional coordinate system, and an optional summation node.  Cross-sections are created and managed by the FBD Cross-section Manager (Post->Free Body->Cross-Section pull-down menu).

6.    Optionally, select a coordinate system to use to output the nodal coordinates (x,y,z) in the summary table, .csv file, and .fbd file output; the default is the global system.

7.    Optionally select from several output options including the zero-tolerance value, and options for displaying and reviewing the force and moment resultant for the cross-section (vector representations via load collectors, tabular output in a summary table window, a comma separated value (.csv) file that can be opened with MS Excel, and an FBD file (.fbd) that can be opened with HyperGraph).

8.    Click Accept.

a.    If the option to create load collectors containing the result vectors is specified, several load collectors are created for each selected loadstep and cross-section.  These load collectors can be visualized via the FBD Results Manager tool.

The HyperWorks Free Body Diagram tools allow for fast creation and manipulation of FBD result data, as well as for easy setup of detailed part analysis and optimization.

► Learn about the HyperWorks product used in this article

Paul Eder

Senior Director - Geometry/Meshing/SWAT team at Altair
Paul Eder

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