MotivationThe goal of the project was to develop a representative model for Class 8 truck tractor-semitrailer vehicles. Tractors and trailers are available in many different sizes based on model options, so they vary widely in their dimensional, inertial and impact characteristics. The tractor model developed by the FHWA was based on a sleeper cab version of the vehicle that had relatively long wheelbase, which is outside the NCHRP Report 350 recommended range for the representative vehicle models. The full-scale crash tests available for comparison were all conducted with shorter wheelbase tractors, and trailers of various lengths. Those basic dimensional differences make it difficult to compare simulations and experiments.
It would be beneficial to researchers to be able to account for those variations without having to build a new vehicle model from ground up for each new vehicle configuration. A set of model modification programs was developed that allows for tailoring of the wheelbase of the tractor and length of the trailer FE models to desired dimensions. This gives researchers more options for simulations, and helps to better validate FE models against existing experiments.
These programs are reasonably general and can be used for other FEM model modifications. These options have not been pursued as the scope of this project task was to develop tractor wheelbase modification procedures that would set FE model overall dimensions close to overall dimensions of vehicles in the available physical tests. The model generation is executed using scripts, which perform sequence of model basic modifications through stand-alone programs. Each modification step results in a syntactically valid LS-DYNA input file, which allows for easier procedure development and debugging. The emphasis of the project was not on computational efficiency of the modification process, but on accuracy and flexibility. In any case, the modification is executed in several minutes, which is negligible compared to new model development.
Tractor Model Geometry Modification Process
Several basic operations were developed for LS-DYNA model modifications:
- Elimination of connections of a (list of) part(s) with other parts in the structure,
- Removal of a (list of) part(s),
- Cutting of a segment from a (list of) part(s),
- Scaling of the geometry of a (list of) part(s),
- Rotation of the geometry a (list of) part(s),
- Translation of a (list of) part(s),
- Merging of a (list of) part(s),
- Joining of parts
All of the above operations can be applied to specified part region, only. We can now combine the basic operations to yield the desired geometry through a sequence of steps. This sequence is obviously not unique. The following figures illustrate the basic concept and graphically show the modification steps. The procedure shown here is the modification of the original tractor FE model from a sleeper cab style with an original wheelbase of 203 in. to a day-cab style tractor with a wheelbase of 129 in. The sequence is shown in the order how they were applied to the model based on the development of the above programs.
To accomplish a rather substantial wheelbase reduction of 1.6m, it was decided to cut the region from few inches behind the cab door all the way to and including the stand-on plate connecting the two main rails. The first step was to disconnect from the model and remove parts that were entirely in the cut region and determined to be unnecessary for the final model.
The next step was to disconnect and cut the part of the sleeper cab, which included the sides and the floor of the cab.
A new geometry to close and complete the day cab was generated, meshed and attached to the model in the next step. The nodes of the edge of the new part were merged with edge nodes of the existing sides, roof, and the floor of the cabin.
At this stage the wheelbase of the tractor is still unchanged. The next step is to disconnect and intermediate two sets of fairings.
Each segment of fairings is independently supported so that it is simpler to completely remove a set than to cut each one partially and adjust the support brackets. Next, a segment of the main rails were cut for the difference of the current to desired wheelbase.
We next cut the rear set of fairings so that the distance between the edges of the front and rear fairings matches the cut distance of the main rails.
As it can be seen from the above figure, this difference in cut plane locations between rails and fairings was to accommodate other parts such as fuel tanks and the front rail cross member. Note that the cabin's cut plane is even more to the front on the tractor.
In the next modification step, the drive-axle parts were scaled with respect to their front and rear connections.
Another possible approach was to cut segments from the axles, and can be applied as needed. Upon inspection of the model, the edges of the front and rear fairings do not match. Therefore, the rear fairing and its support rail were scaled in the lateral and vertical directions with respect to their rear console connections to the main rail, in order to match the location of the front edges of the respective parts.
We are now able translate the front part of the tractor to the rear for the length of the cut segment.
At this time, the model is not yet merged. The cabin mounts are not connected to the cabin, as well.
The location of the meeting plane of two model segments in the main rails is indicated by a slight difference in the element length. The final steps are to merge the nodes of the corresponding parts at the cut edges, add the exhaust stack and connect it to the main rail and the rear cabin wall, and to connect the cabin mounts with the cabin floor. The final, day cab model is shown below. The last several figures show the fit of the fairings that had to be scaled to match their front counterparts in order to be able to reconnect to each other.
The connection location of the two sides is again indicated by a slight change of the element size, and this variation does not impart any negative effects on the model's performance.
The reduced wheelbase model version was used for the simulation of the tests problem. Different wheelbases and cab designs can be accommodated by variation of parameters and modification steps. The same procedure was followed for adjusting the length of the trailer.