Frame Load Cases

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There are a number of FEA load cases which should be applied to the frame in order to verify the design and ensure the Baja vehicle's roll cage is structurally sound. These load cases come in two major forms, those using gravitational/acceleration loads and those using discrete force loads. For the many impact scenarios that are analyzed, either a gravity load can be applied or discrete reaction forces can be solved for and then utilized. For analyzing the stresses in very specific members or at the impact locations, it is usually preferred to apply force loads and turn on the Inertial Relief flag in the NX solution properties. When looking at the overall rollcage structure and deformation, and when the deceleration due to the driver and drivetrain masses are significant, it is best to use a gravitational load.

Setting up the Model[edit]

Please refer to NX FEA for general details on setting up the frame model in NX for simulation. Once that is done, there are several specific operations which should be carried on the the FEM. Most important is adding concentrated masses for the driver and drivetrain. The locations of their respective centers of mass can be taken from SolidWorks, and are also recorded in the Car Weight 2016 Spreadsheet on Drive. Points should first be created in either the Part or Idealized Part models, then a 0D CONM2 mesh can be applied to each point. When editing the Mesh Associated Data, it is only necessary to include a mass. All other entries may be left empty. Again, the masses can be taken either from SolidWorks or the Car Weight spreadsheet. Next, the concentrated masses must be connected to the frame. There are two main ways to do this, either by using a 1D RBE2 or RBE3 link:

  • RBE2
    • Select several master and several slave nodes
    • Each node's displacement and rotation are set equal, creating infinitely stiff links between the master and slave nodes
  • RBE3
    • Select one master and several slave nodes
    • Displacement and Rotation of master node is set as the weighted averaged of all of the slave nodes

The drivetrain concentrated mass should be connected to the frame using a RBE2 link, with the CONM2 mesh as the master node, and the approximate locations of the drivetrain tabs as the slave nodes. This approximates the stiffness added to the lower plane of the rear bracing by the drivetrain braces. Connecting the driver concentrated mass is somewhat more complicated, as neither links well approximate the seat and seatbelt attachments. The model developed in the 2015-16 design uses both a RBE2 and RBE3. Like with the drivetrain, the concentrated mass is set as the master node, with the slaves selected as listed:

  • RBE2 slave nodes (4 total)
    • Selected at the approximate location where the shoulder- and waist-belts attach to the frame (ignoring the submarine belt)
  • RBE3 slave nodes (4 total)
    • Selected at the approximate locations of the seat-back tabs

Suspension Forces[edit]

Front Gravitational[edit]

This load cases is meant to simulate the vehicle going off a jump and landing on just the front wheels. A drop acceleration of 8G is used, as calculated by the Suspension Project Team.

  • Apply a 8G gravitational load downwards
  • Fix the frame in XYZ translation at the front shock mounting points
    • Also constrain those points in rotation about the axis directed through them

Rear Gravitational[edit]

This load cases is meant to simulate the vehicle going off a jump and landing on just the rearwheels. A drop acceleration of 8G is used, as calculated by the Suspension Project Team.

  • Apply a 8G gravitational load downwards
  • Fix the frame in XYZ translation at the rearshock mounting points
    • Also constrain those points in rotation about the axis directed through them

Rear Discrete Force[edit]

This analysis is meant to better model the forces through the rear suspension in the case of landing a jump on the rear wheels. The axial forces through each of the suspension components (H-Arms, links, shocks) are solved from a series of free body diagrams for the upright and lower control arm, and are then applied to the frame at their respective mounting locations. Since the masses of the drivetrain and driver take relatively long to react to these forces, they should both be fixed in all six degrees of freedom.

Frontal and Rollover Impact[edit]

Frontal Impact[edit]

This analysis models the vehicle crashing into a rigid obstacle, such as a tree or wall, at a speed of 35 MPH. An impulse time of .2 seconds is assumed, and assuming constant deceleration gives a value of 3080 in/s^2. This is very nearly 8G, so it is approximated as such.

  • Apply a 8G Gravitational load pointed forwards along the forwards axis of the vehicle
  • Fix the frame at the front of the vehicle where the SIMs meet the LFSs (Points G)
    • Fixtures should be in XYZ translation, and in rotation in the axis which runs through the two fixture points

Rollover Impact[edit]

This impact scenario is a worst case where the vehicle rolls over forwards and sideways, causing impact at the front of one RHO on the bend at point C. The load is assumed to be a quarter of the other impact loads (somewhat arbitrarily), and is directed from the FEM model's center of mass through the constraint point.

  • Apply a 2G gravitational load upwards diagonally as described
  • Fix the frame in all six degrees of freedom at the midpoint of the bend at point C on only one side of the vehicle (say, the left hand side)

Driver Safety[edit]

Shoulder Belt Mounting Members[edit]

This analysis scenario is meant to model the forces on the specifically on the shoulder restraint mounting tube in a sudden frontal impact. It is derived from the same 8G load used for the full frame frontal impact, and assumes that 62.8% percent of the driver's weight (disregarding the legs and feet) is supported by the member. This number comes from a chart listing the mass percents of various body parts, and can be found in the Frame Project Team folder on Drive. Assuming the heaviest driver weighs 200lbf, the force on the USMs can be solved for using F=ma.

  • Apply a distributed load totaling 1000 lbf at the the mounting locations of the shoulder restraint, acting forwards
  • Constrain the frame in XYZ translation at the top and bottom of the RRHs (Points A and B)

USMs[edit]

This analysis is meant to verify that the USMs can support the driver's weight in a sudden vertical impact with the ground. Accordingly, it uses the same 8G drop used by the other drop analyses and assumes that 72.8% of the driver's weight (disregarding the mass of the lower legs and feet) is supported by the USMs. This number comes from a chart listing the mass percents of various body parts, and can be found in the Frame Project Team folder on Drive. Assuming the heaviest driver weighs 200lbf, the force on the USMs can be solved for using F=ma.

  • Apply an evenly distributed load of 1160lbf total along the USMs, acting downwards
  • Fix the frame in XYZ translation at the intersections of the LFSs and RRHs/FBMs (Points A and F)

Front Cockpit LC[edit]

The front cockpit LC was added to the frame to help support the driver's weight during ingress and egress. This analysis assumes that the total weight of the heaviest driver supported by the aforementioned member. In reality, that is unlikely as the the driver will be hoisting themselves with the aid of their hands, so the load should allow for some amount of impulse on the member.

  • Apply a distributed load of 200lbf total at two nodes, roughly shoulder-width apart and acting downwards
  • Fix the frame in XYZ translation at the intersections of the LFSs and RRHs/FBMs (Points F and A)

Torsional Analysis[edit]

The torsional analysis is done primarily to include on the Design Spec Sheet, and is fairly simple in nature.

  • Apply a 100 lbf load on each of the front shock mounting points
    • One force should act upwards, and the other downwards. This creates a torque on the front of the frame
  • Constrain the rear shock mounting points in XYZ translation
    • They should also be constrained in rotation in the axis directed through both of them

The stiffness of the frame (in ft-lb/deg) can be computed by T/theta, where theta is the rotational displacement of either shock mounting point (they should be roughly the same) and T=FB. F is then the force applied on each mounting point and B is the width (in feet) between the two mounting points.

Drivetrain Forces[edit]

The forces acting on the frame due to the drivetrain are fairly complicated in nature, and can be found with derivations in the Drivetrain Project Team folder on Drive. In order to run an analysis of them on the the frame, it is necessary to add the drivetrain braces to the FEM model. This can be done by following the standard wireframe import procedure, but instead of creating a new part they should be imported into the existing NX frame wireframe model. Once meshed, the braces still need to be attached to the frame. This can be accomplished by creating RBE2 links between the mounting points on the braces and respective mounting points on the frame. Now, the forces can be applied directly to the braces.

Improvements to be made[edit]

There are always improvements which can be made to frame load cases and how the model is set up. Regarding the latter, the model for connecting the driver to the frame leaves much to be desired. One reason which makes developing a single model for driver attachment very difficult is that with gravitational loads, the tubes which support the driver are entirely dependent on the direction of the load. A drop would place the majority of the driver's deceleration force on the USMs, whereas a frontal impact would place most of it on the shoulder- and waist-belt mounting locations. As for load cases, the gravitational loads for vertical drops do a poor job of approximating the forces through the suspension elements. They are useful for analyzing the stresses due to the deceleration of the driver and drivetrain, but the forces through the shocks are not well approximated, and the rest of the suspension arms and links are completely disregarded. The rear suspension force analysis developed in the 2015-16 design year did a good job of solving that problem for the rear, but the front suspension was left untouched by the analysis. Also, the force analysis was performed on the 2014-15 rear suspension which attached the shock directly to the upright, whereas it is mounted on the H-Arm for the 2015-16 design. Assuming that that lower shock mounting is kept for subsequent years, it would be beneficial to update the model accordingly. A force analysis should also be formed on the front suspension so that more objective footbox design may be performed. The 8G load historically used for drop load cases has long been questioned and there's no real data to justify using it over a smaller (say 4G or 5G) load. Work should be done to test drop accelerations and either verify the accuracy of this load case or develop a smaller one.