Roof crash analysis according to FMVSS 216 standards

Objective:

• Arrange the position and orientation of impactor as required and simulate a roof crush situation with suitable interfaces.
• Boundary conditions are applied to the model with a load condition of imposed displacement. Control cards are checked accordingly while simulating for 200ms using constant nodal time step. The frequencies of time history and animation files are set as 0.0001s and 0.005s respectively.
• Required output requests were given to the model. Force vs displacement plot is created and the model is checked to meet FMVSS target load of 47000N.
• Energy plots are plotted.

Case setup:

• The reduced roof crush model and impactor are located in the hypermesh with a radioss as its solver.

• Point A in the impactor model as shown below will be the base for all rotations given to the impactor.

• The impactor is rotated by 180 deg about Z-axis, 5 deg about axis AB, 25 deg about axis AC with A as the base point. Impactor position and orientation are according to the FMVSS 216 standards.
• The whole impactor is translated by a distance of 3500mm in X-axis, 584.822mm in Y-axis and 1343.06mm in Z-axis.
• Existing interfaces and output blocks are deleted from the roof crush model to avoid the confusion.
• Model is found to have free parts i.e. free components which are missing for connectivity. These components are connected using connectors with springs elements.

• Multi usage (Type 7) contact is applied to the roof crush model with self impact having all components of roof crush model as both slave and masters. Recommended values are used to define the interface.

• Another multi-usage (Type 7) contact is applied while selecting the impactor as master and roof crush model as a slave. Recommended values are used to define the interface.

• Model is checked for initial interactions and penetrations.
• An output block for time history files to record the data from the interfaces is created.
• Master node rigid body in the right side of the roof crush model is arrested in all DOF's.

• Individual skews are created and a boundary condition is applied to the nodes at the free edges of the suspension shock tower of the roof crush model and motion is locked in Z- direction.

• The free end of the spring in the impactor is fixed using boundary condition definition for the stability of the impactor.

• A moving skew to define the movement of the impactor in its normal direction is defined.
• A boundary condition is created to guide the master node of the impactor rigid body so that it is free to translate normal to the face of the impactor, but is fixed in all other DOFs.

• Imposed displacement of the following function is applied to the impactor and an abrupt change in the acceleration of the impactor is avoided.

• Following control cards are checked for their definition.

• The constant nodal time step is used and the time step is monitored.

• Run time is set to 200ms. Time history is printed for every 0.0001s and animation file is created every 0.005s.

• Following output requests are created in the animation files in the solver browser (under engine keywords).

• Parallel arithmetic is turned ON.

• Model is made to run for its simulation.

Results and conclusions:

Energy and mass errors

The simulation of roof crush ran for 200ms run time with normal termination. The total number of cycles is 200001 and total elapsed time is 12998.01 sec. Energy error started at 0% and ends with -11.7% while following a decreasing trend. Energy error is almost constant and zero, till 105500 cycle (i.e. 105.5 ms) and then decreases up to -11.7%. This is due to the initial gap between the impactor and the roof. Mass error is almost constantly increasing and is below 2% with a maximum value of 0.04448. There was no deletion of elements.

The time step plot is very well controlled to be constant and change in time step can be observed at the time when impactor comes into contact with roof i.e. at 105.5ms.

Force vs displacement

Using a cross plot, force vs displacement curve is obtained.

Maximum resistance force from the roof to impactor is found to be 7.033KN.

According to FMVSS 216 specified strength to weight ratio (i.e. SWR) requires a resistance force of 2.5 times the GVW (Gross vehicle weight) to protect the passengers from oof crush scenarios (like Rollover). Current weight of the model is 179.4 Kg. Resistance force required is 2.5*179.4*9.81 = 4399.785 N. Since maximum resistance force from the above plot is greater than the required force, Target load has been met.

Considering SWR to be 3 times the GVW, Required force is 5279.742 N. With this rating, target load has been met while considering current weight as GVW.

But for a full-scale model having a weight of 1216.55 Kg (From the full-scale reference model), target load condition i.e 3*1216.55*9.81 = 35.8KN, target load is way below to meet the standards.

Target load of 47,000 N (3*GVW) is not met and has a huge difference. The maximum force from the roof can be different if the model is simulated while considering all components and their weights and above FMVSS standard (target load = 47,00 N) can be met.

Animation:

Displacement

From the animation, it can be observed, elemental displacement from the roof impact is induced towards the A-pillar elements and can be a better case to protect the passengers.

Specific energy

Maximum specific energy is found to be 1.394 KJ.

Equivalent plastic strain

Hourglass energy

From values of hourglass energy, a very minimum amount of energy is associated with the hourglass effect.

Von mises stress

From animation, it can be said the stresses from roof crush are well distributed into the roof components. The maximum value of 0.247 GPa is observed at node 140357.

Nodal mass

Nodal mass is almost kept constant and added mass is very less as observed in below plot.

Energy vs time plots

Hourglass energy

Very less amount of hourglass energy is associated with the crash simulation and this can be seen to be below 5% which is acceptable.

Energies

Kinetic energy is almost constant until the impact and this is due to the movement of impactor under imposed displacement. It can be observed till impact no change in roof model elements and kinetic energy is constant and this can be considered as no abrupt accelerations are induced due to imposed displacement.

As the roof is crushed by the impactor, an increase in internal energy can be seen.