About Flexible Bodies in Adams Car
Adams Car integrates and fully supports the Adams Flex, Adams ViewFlex, and Adams MaxFlex features. These tools give you several methods to enhance your multi-body vehicle model by allowing part elasticity to contribute to the system dynamics. The following sections further explain flexible bodies:
The following sections further explain flexible bodies:
About Integrating Flexible Bodies
Integrating flexible bodies into templates or subsystems lets you capture inertial and compliance effects, study deformations of your flexible components, and predict loads with greater accuracy, therefore achieving more realistic results. Once the flexible body is created, Adams Car displays its geometric representation in the main window.
The flexible body characteristics are defined in one of three ways:
■A finite element modeling (FEM) output file, called modal neutral file (MNF). This file can be produced by several FEM programs, or by Adams ViewFlex.
■A Nastran MASTER database file.
■A Nastran Bulk Data File (BDF). This method uses Adams MaxFlex to run a nonlinear finite element analysis during the time domain solution.
When you integrate a flexible body into a template, you must supply the following:
■A MNF, MASTER, or BDF file. This file should have been previously created and stored in a shared or private database.
■Location and orientation information for the part that you will create. Adams Car uses this information to model the flexible body at the desired design position.
■Inertia coupling and the damping ratio.
You can now integrate flexible bodies into your subsystems or assemblies. The process of swapping rigid bodies with flexible bodies is fast, easy, and convenient, and it eliminates the need for multiple templates. With rigid-to-flexible swapping, flexibility becomes a property of the general part. When you integrate a flexible body into a subsystem, you must:
■Supply a file. This means that the MNF, MASTER, or BDF file should have been previously created and stored in a shared or private database.
■Swap the rigid body for a flexible body: from the Adjust menu, point to General Part, select Rigid to Flex, and then press F1 for help. Select the FEM file type, and supply the path to the file.
■Specify the connectivity. On the Swap dialog, select the Connections tab to get a list of markers that will be moved from the rigid body to the flexible body. You can specify the node(s) with which you want each marker to associate.
To successfully integrate a flexible body into an Adams Car template or subsystem and run simulations, consider these precautions:
■Use flexible bodies if component flexibility affects the dynamic behavior of your model or if you are interested in accurate deformations of the flexible body under various load conditions.
■Flexible bodies defined with MNF and MASTER files use a linear combination of modal deformation shapes. Therefore geometric nonlinearity (for example, deformation greater than roughly 10% of the characteristic length) cannot be simulated with this method. Adams MaxFlex is the recommended method for this use case.
■Consider the computational load that a flexible body representation demands, . For MNF and MASTER representations, the number of nodes and elements does not affect computation time. The number of active modes, and the frequency of those modes is what drives computation time. Adams MaxFlex requires significantly more simulation time than the Adams Flex modal method, because it performs a nonlinear finite element solution at every step of the dynamics analysis.
■Verify your flexible body and check the mass and inertia properties.
■For flexible bodies represented with Adams Flex, check the natural frequencies associated with the active mode shapes. Compare to FEA results to ensure a valid representation in Adams.
About Flexible Body Damping Ratio
Dynamic system simulations are greatly complicated when the time integration must traverse a signal with very high frequency components. To achieve the desired accuracy, Adams Solver must integrate the signal with a time step short enough to capture the frequency content. Flexible bodies can contribute large amounts of high frequency content and can, therefore, be difficult to simulate.
Carefully applying modal damping can help you successfully simulate a model containing flexible bodies.You can specify a single scalar damping value applied to all the modes, control the damping using a DMPSUB user-written subroutine, or accept the default nonzero damping that Adams Flex applies to all the modes.
If you do not specify the damping, Adams Flex applies the following defaults:
■1% of critical damping for all modes with frequency lower than 100 Hz.
■10% of critical damping for modes with frequency between 100 and 1000 Hz.
■100% critical damping for modes with a frequency higher than 1000 Hz.
During simulations, Adams Car displays in the Message Window the type of damping that you selected for each flexible body in the model.
We suggest you start with the default damping ratio. If simulation time is prohibitive, or your frequencies of interest are significantly different than the table above, or the undamped vibration of your flexible body at high frequencies is important, modify the damping accordingly.