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• The frame is represented by a single modal neutralfile. As a result, it is very easy to reuse the frame in other MSS models. The files can be stored in common directory for archiving and future use.
• In the MSS model the frame is represented as a single flexible body and not a large number of rigid bodies. bodies. This makes it much easier to manipulate the frame in the model.
• Every flexible body mode adds only one degree-of freedom to the simulation. Previous methods added many more degrees of freedom since they used a large number of rigid bodies and each of these added six degrees-of-freedom.
• The linear, flexible characteristics of frame model are more accurate since they are based on a full finite element model and not a collection of rigid bodies and force elements. This makes it much easier to tune the model to agree with modal test results.
• Damping is added on a modal basis. Thus, damping results from modal testing can be easily added for increased accuracy.
• Modal participation during a simulation can be tracked based on the strain energy contribution. Modes that do not contribute significantly can be deactivated for improved computational efficiency.
• Visualization of simulation results isimproved since the FEA mesh exists in the MSS environment and it can be used for viewing frame deformations during animation.
• The transfer of MSS loads back to the original FEA model for stress analysis is improved since loads are associated with nodes in the finite element mesh. Although, this method has many advantages, it can still be time consuming to implement. For example, not all joint and force elements are supported for direct connection to the frame mesh when it is the MSS environment. These must first be connected to massless rigid bodies that are then locked to nodes of the mesh using fixed joints which remove the degrees-of-freedom added by the massless rigid bodies. Since a truck frame can have 36 or more connection points in a MSS model, it can be very time consuming to connect a flexible frame. Also, if an existing flexible frame needs to be replaced with a new one to evaluate a design change, more modeling effort is required and the potential to introduce modeling errors is great.
To overcome this difficulty custom programs were developed to integrate a flexible frame in a vehicle model. The process begins with a full vehicle model with a rigid frame. A copy of the model is made and then a series of macro programs are executed that perform the following tasks on the copy:
• Read the flexible body modal neutral file and position the flexible body in the vehicle model.
• Create and connect massless rigid bodies at each node where forces and constraints are applied to the flexible body.
• Modify all connections to the existing rigid body of the frame so that they connect to the appropriate massless rigid bodies.
• Delete the rigid body that previously represented the frame. In a typical analysis, a simulation will be run with the rigid frame as a baseline, and then after importing the flexible frame, it is repeated. The results of both are studied to understand the influence of the frame’s flexibility. Then another copy of the model with the rigid frame is made and it is converted to have a different flexible body (a different modal neutral file is read into the modelling database). This new flexible body will have some design change that needs to be evaluated such as a new crossmember .The same simulations are repeated and all the results are compared.
With this process the burden of adding a flexible frame to a model is greatly reduced. For example, it takes less than one minute to convert a model with a rigid frame to have a flexible frame. As a result, this procedure works very well for concurrent design and analysis. FINITE ELEMENT MESH MODELLING – In order for these methods to work effectively, the frame’s finite element model must be easily created and modified to reflect changes required by designers. The approach adopted here begins with a Pro/Engineer solid model assembly of the frame rails and cross-members similar to the one shown in Figure 1.The solid models for each component are created specifically for finite element analysis meshing, and as a result, are simplified versions of the actual designs. The finite element mesh is created with an add-on module for Pro/ENGINEER called Pro/MESH. It contains features to simplifyfinite element meshing such as automatically determining midplane locations for shell elements, applying global and local mesh controls, and defining element properties. When requested, a mesh of shell, beam, and mass elements is created that reflects the current CAD Creating the mesh while inside the Pro/ENGINEER environment has many advantages. For example, changes to the solid models are automatically reflected in the mesh.