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Thus changes to the frame rail geometry or the position of cross-members can be quickly meshed and exported to ANSYS. Likewise, beam elements that represent bolt connections follow components as they are moved to new positions. Subassemblies, such as cross-members, can also be suppressed so that they are not included in the mesh. This makes it very easy to turn on and off different cross-member designs and develop meshes of each configuration. After the mesh is created, it is read into ANSYS, and a custom macro is run that renumbers the nodes where forces and constraints will eventually be applied in ADAMS. This ensures that the same node numbers are always used. Nodes for constraint mode calculation are then selected and the modal neutral file for ADAMS is created. The file is then ready for input to the ADAMS model.
As described earlier, the ADAMS simulation loads are transferred to ANSYS and the stresses acting on a finite element model of the frame are calculated. This can be the same finite element model that was used to develop the modal neutral file for ADAMS or it can be a different model with a finer mesh. For a different finite element model to work, its mesh must have the same node numbers at the connection points as the original model used for ADAMS. This is easily achieved by executing the node renumbering macro as used earlier. Consequently, a model with a refined mesh can be used for stress calculations, while a model with a coarse mesh can be used to calculate the modal properties for ADAMS. A sub modelling approach was also developed so that cross-member stresses could be computed more accurately. First a finite element analysis is performed on the same coarse finite element model as used for ADAMS.
Custom macros in ANSYS are then used to read the results and extract the forces and torques from beam elements that represent the bolt connections between the frame rails and the cross-members. These are then applied to a detailed finite element model of the cross member alone. The bolt forces are applied using ANSYS RBE3 elements that are configured to distribute the bolt loads to nodes that would be under the bolt head of the two mating components. Again, this is achieved with custom macros and is completed in a matter of minutes. Mesh controls in Pro/MESH are used to ensure that an annular ring of nodes and elements exist to represent the bolt hole and the area under the bolt head. This technique can be applied to finite element models with shell or solid elements.
VALIDATION OF FRAME FLEXIBILITY
In order to establish the accuracy of the flexible frame model, a modal test was conducted on a bare truck frame. An ANSYS finite element model of the same frame was then built using the procedures described above and the eigenvalues and eigenvectors were computed for comparison. As can be seen in Table 1, the finite element model agrees very well with the test results. A modal neutral file was then produced for ADAMS and the modes were recomputed using the ADAMS/Linear option. The ADAMS modal frequencies also agree well with the test data and confirm that the flexible frame provides an accurate representation of the real structure. Note, only modes up to 56 Hz were extracted fromthe modal test data. Higher modes exist, but the test did not contain enough measurement points to clearly define the mode shapes. The frame model was then incorporated into the full vehicle MSS model using the procedures described above. The modes for the vehicle were then computed and compared to modal test data for a linehaul truck with a similar configuration. See Table 2. Again the agreement is good and indicates that the dynamics of a typical linehaul truck are well represented by the model.
CONCLUSION
The aim of the project outlined in this paper was to develop a process by which design changes to a truck frame could be quickly evaluated such that concurrent design and analysis would be possible. As described above, this goal has been achieved by combining current computer aided design and engineering codes with custom software routines. The process takes advantage of the strengths of each code to create a high fidelity environment where the impact of subtle design changes to the truck frame can be measured against vehicle performance and durability requirements. Design alternatives can be quickly evaluated and fed back to the designer while it is still possible to make changes. Although this process is being used successfully, there are many areas where further enhancements can be made and will be the focus of future development. These include: