Differences from Linear Flexible Bodies
There are a number of ways in which using the nonlinear option on the Adams flexible body differs from the modal-based linear flexible body:
■Nonlinearity: The linear flexible body is well suited when the body in question is not expected to undergo large deformation (often this means roughly more than 10% of the characteristic dimension of the part) nor have material properties which are expected to change by an important amount throughout the course of the simulation. The nonlinear flexible body is able to accurately represent large deformations and material nonlinearity.
■Source File: The linear flexible body is based on a modal neutral file (MNF) which is a specific output of a linear modes analysis from a finite element analysis solver. For the nonlinear option, the Adams flexible body is based on a run-ready data deck suitable for MSC Nastran SOL400, the implicit nonlinear analysis mode within MSC Nastran. And, if working through an Adams GUI like Adams View or Adams Car, only a valid mesh definition is required via bulk data file (BDF). That BDF is imported into the GUI and then at analysis time the GUI generates the run-ready data deck to be used in the analysis which will have the .dat extension so one can distinguish it from the originating file (.bdf) in the Adams working directory. The processes to generate MNF's requires that those users specify the model units. The same is not true for BDF's; so, in cases where the units are not present in the BDF the Adams user must declare them upon import and should therefore confirm the BDF units with its author. Subject to some limitations, the BDF used to create an MNF can be used as the source file for the nonlinear flexible body. See "
Appendix I: Using a BDF from which an MNF was Generated" for more details.
Note: | The BOLT element is not supported by Adams-NLFE Solver. |
■Licensing: The nonlinear option on the flexible body is licensed separately from the linear flexible body. So, a different license feature is used.
■Modeling Capabilities: The nonlinear flexible body does not support load bearing/applying objects located on solid element nodes. So, only markers which are used, for example, for output requests can placed on such nodes. Markers used for constraint or force definitions cannot be placed at solid element nodes. No such limitation exists for shell-elements. The use of master nodes connected to the load bearing surfaces of the structure through rigid-body elements (RBE) is recommended as the means to connect nonlinear solid-element nonlinear flexible bodies to the rest of the Adams model.
The nonlinear flexible body does not yet support contact with rigid bodies or flexible bodies. And, unlike the linear flexible body, it does support self-contact.
The concept of the modal force (MFORCE) from linear flexible bodies is replaced with BDF-resident load cases (TLOAD and DLOAD cards in the MSC Nastran language) for the nonlinear flexible body. These are means to employ distributed loads on the flexible body which would otherwise be inconvenient to create in Adams. For more details see "
BDF-Resident Load Cases" within the "
Modifying Nonlinear Flexible Bodies in Adams View" section.
The nonlinear flexible body is supported only by the default Adams Solver choice (C++) and not the FORTRAN choice.
■Analysis Capabilities: Models containing nonlinear flexible bodies are not supported by Adams Linear and do not support Adams Controls usage except for external system library (ESL) import mode. Models containing nonlinear flexible bodies will also set the integrator order (kmax) to 1 because it is recommended for the most robust simulation of nonlinear flexible bodies.
■Damping: For the linear (MNF-based) flexible body damping is done on a modal basis since those flexible bodies are essentially a superposition of individual modes. Since the nonlinear flexible body is not such a simplification from FEA but, rather, more like an FE model directly embedded in the Adams model, the damping definition is fundamentally different and applied directly to the body itself, primarily as structural and Rayleigh damping. See the "
Damping" section of this guide for details.
■Post-Processing Capabilities: Because they are not based on linear modes the nonlinear flexible bodies do not contain result sets or other tools to determine the participation of individual modes on the overall deformation of the part. While the nonlinear flexible body does support stress and strain recovery via Adams Durability, it provides only elemental VonMises stress and strain results as opposed to the broader set of stress and strain channels available from the linear flexible body. However, the nonlinear flexible body results come directly from MSC Nastran technology so the VonMises stress and strain results are direct as opposed to being based on modal stress/strain recovery. Furthermore, Adams exports complete MSC Nastran result files (.op2, .f06, and so on.) for the nonlinear flexible bodies in the model. So, those files can be used for fully detailed post-processing of the flexible body results within a finite element post-processor like Patran or SimXpert.
■Hardware Requirements: Nonlinear finite element analysis is typically much more computationally intensive than multi-body dynamics simulation. So, when using nonlinear flexible bodies within Adams models one should be aware that more memory and CPU time will likely be consumed than otherwise. The solve times for Adams analyses containing nonlinear flexible bodies will typically be somewhat longer than those for a standalone MSC Nastran implicit nonlinear analysis on the same body because the Adams analysis is coupled MBD-FEA solution. See "
Simulating Adams Nonlinear Flexible Bodies" for information on how to manage memory and CPU usage. Also, frequently more hard disk space will be required than Adams analysts are accustomed to. Specifically the primary results file for the nonlinear flexible body, the .op2 file, can be expected to be roughly 10-times larger than the results files to which Adams users are accustomed. Its size is dependent on the size of the mesh (number of nodes, elements, degrees of freedom), the number of output steps in the Adams analysis and the presence of stress or strain data in the output (these each increase file size). Finally, to increase simulation speed typically Adams analysts think about running their analyses in environments with more memory (RAM), greater CPU speed or multiple CPU's (for use with Adams Solver SMP multi-threading). However, for models with nonlinear flexible bodies the analyses usually involve a dramatically larger amount of file I/O activity compared with other Adams models As a result, often the simulation speed is reduced more by running the analyses on faster disk drives than on faster CPU's and larger memory environments. See "
Hardware Recommendations" in the "
Simulating Adams Nonlinear Flexible Bodies" section for more details on improving simulation time through faster drives and, for remote solving, improving the network speed between the machines solving the nonlinear flexible bodies and the rest of the Adams model.