Autodesk Nastran In-CAD 2019 X64 Free Download
Lucille Minasian
This is the latest in a series of review articles looking at FEA software products. Each overview is formatted as a walkthrough using a simple structural example. The full capabilities of each product cannot be covered in a few pages, but the hope is to provide a feel for the basic workflow required for each product.
A few years ago, Autodesk acquired NEi Software, a small company that developed an independent finite element analysis (FEA) solver that closely emulated the traditional Nastran input and output structure. Since then, Autodesk has embedded much of the solver technology into its Autodesk Inventor environment as Autodesk Nastran In-CAD. The full solver capability also continues to be available as a standalone product, Autodesk Nastran.
For this review I have created a simple bracket model. The file is available for download at autodesknastran. Versions using Parasolid, Autodesk Inventor and SOLIDWORKS formats are available. The Autodesk Nastran input files and result files are also available for download.
Fig. 1 shows the Autodesk Inventor user interface (UI), with the bracket geometry shown. The layout is traditional, with a tree view, graphics area and a main menu running horizontally across the top. Autodesk refers to this as the ribbon. Selecting a tab on the ribbon opens a set of commands and drop-down menus. Fig. 1 shows the Tools options selected.
I opened the Parasolid version of the geometry and was interested to see how the geometry conversion was handled. The source file holds the default input units, which in this case picks up meter units. This can be changed to inches easily. However, it is always a good idea to check units and dimensions during an import. The Inspect tab has an easy-to-use measure tool, which is always a good check. The vertical bracket edge is correctly reported as 2.75 in.
Users can check model units by using Document Settings under the Tools tab. The units are reported as inches and pound mass. I was a bit skeptical as to whether strict mass units were being used. I was carrying out a dynamic analysis, so it is important to be clear on this point.
I selected alloy steel from the provided material list. It is also straightforward to set up a new material library or define a new material. The file created is clearly indicated and can be copied or made centrally accessible.
Tabs appear that cover the usual FEA actions of pre-processing, analysis control and post-processing. The new tree view is in two sections: the Analysis tree that shows the options applied to the current part for this analysis and the model tree that shows all the options that can be set. The options defined in the model tree are available for use in the Analysis but are not activated until dragged across or created directly from within the Analysis tree.
Fig. 2 shows a warning that a mesh is required. Right-clicking on Mesh Model or using the ribbon icon generates the mesh. The result is shown in Fig. 3. Right-clicking on the Model Mesh or using the ribbon icon accesses the mesh settings shown in Fig. 4.
The default or global mesh size is 0.1911 in. This is a very useful parameter, as it indicates the starting point for adding local refinement. Guessing a value can result in a huge number of elements or a very coarse mesh. Reducing the global size is one refinement option, but it is a very expensive solution. The number of elements with the default size is 8,332.
Halving the element size to 0.095 gives 67,844 elements, halving again to 0.0475 gives 474,797 elements. As usual the objective is to put fine mesh control where it matters. Autodesk Nastran has local mesh controls as shown in Fig. 5.
Any number of local controls can be used to target vertex, line or surface geometry. As an exercise, I assumed that a fine mesh is needed around the bolt holes on the horizontal faces to investigate stresses. Fig. 6 shows the result of increasing the mesh refinement locally on the edges of both of these holes.
The final Fig. 6(d) is a result of reducing the Element Growth Rate to 1.2 from a default of 1.6. The effect is to keep a fine mesh bias over a larger distance away from the bolt hole. The number of elements using this method is 33,113.
The bracket is assumed to be connected to a stiff backing structure at the outer pair of bolt holes on the YZ plane flange. This represents a case where the inner bolt pair has been missed in assembly. The bolts act as a standoff, rather than clamping the structure to the backing. The constraint setup is shown in Fig. 7.
The constrained degrees of freedom (DOF) can be set manually, or simple constraint options such as symmetry can be invoked. Cylindrical constraints such as bolt hole rotation can be set up, but the required coordinate system must be defined in the Autodesk Inventor Model first and then referenced here.
The task for this overview is to carry out a Normal Modes analysis and a subsequent Modal Transient Dynamic analysis on the bracket. The default analysis is linear static. Right mouse-clicking on the Analysis object in the Analysis tree, and selecting edit allows any analysis to be set up, as shown in Fig. 8.
The output file is useful for debugging and understanding the calculation flow. Experienced analysts will welcome this additional insight, and it is a great tool for newcomers to start to investigate the process more deeply.
The first two modes from the analysis results are shown in Fig. 11. The plots show the first bending mode and the first torsional mode. I have modified the default settings to give a banded contour view, switched element edges off and deselected maximum and minimum markers. It is straightforward to move between the frequencies, retaining this view. Animation also has good controls. I increased the number of frames to 24 and used the oscillate option. I found it easier to move between modes with animation by using the control panel available from the Analysis tree view, rather than the ribbon controls.
These plots are important for carrying out a modal survey. In this case, the first two modes are similar at relatively low frequencies, but the frequencies climb rapidly with more complex modes. A typical response spectrum cutoff may be at 2000 Hz, and the first two modes are the only two of concern. Alternatively, if a payload is attached to the bracket, approximate hand calculations can estimate the drop in frequency and whether higher modes may become important. The modal effective mass graph shows mode 1 contributing in a vertical (Z) sense, but mode 2 does not. This allows excellent predictions for base motion excitation and approximate indications for external excitation.
I edited the Normal Modes analysis and converted it to a Modal Transient analysis. This spawned new objects in the tree view. There are other possibilities, such as creating a duplicate analysis and then editing. The motivation is to inherit the Normal Modes analysis settings.
Using the Loads object in the tree view, loading is applied in the usual way as a static loading, with an option to create the time dependency on the fly via a table. Alternatively, a table can be separately defined, with the ability to paste external data.
Time step size and number of time steps are input via the Dynamics Setup object created in the tree view. If you know the step size and duration, which is the typical case, the number of steps is calculated automatically.
The first analysis used the standard Nastran file setup from Autodesk In-CAD. The second analysis used a modified Nastran file to look at the contribution from mode 4 only. The file modification was carried out from within the Autodesk Nastran File tab using the direct editing tools available and the local help documentation.
Tony Abbey is a consultant analyst with his own company, FETraining. He also works as training manager for NAFEMS, responsible for developing and implementing training classes, including e-learning classes. Send e-mail about this article to [email protected].
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