Frontal crash simulation of a Dodge Neon BIW using FMVSS 208.

Objective: To conduct a simulation of a Frontal Crash for Dodge Neon BIW based on the guidelines below: 

-Check the unit system of the model

- Create an appropriate interface, friction 0.2, and recommended properties. 

- Make sure no intersections and penetrations arise in the model

- Correct rigid bodies

- Impart an initial velocity of 35kmph

- Run the model checker

- Compart the weight of the model with full-scale model provided to us and reach a target weight of 700 kg while moving the C.G to the expected range

- Timestep: 0.5 to 1 microsecond

- Run 80 ms

Frontal-Crash Tests:

A frontal crash is the most common type of crash resulting in fatalities. Major strides have been made in frontal protection, thanks in large part to the crash test program that the National Highway Traffic Safety Administration (NHTSA) began in the late 1970s and the crashworthiness evaluations

IIHS conducts three different frontal crash tests: a moderate overlap test (formerly known as the frontal offset test), a driver-side small overlap test, and a passenger-side small overlap test.

FMVSS 208 Frontal Crash Simulation:
The Full Frontal Fixed Barrier Crash test (or Rigid Barrier test) represents a vehicle-to-vehicle full-frontal engagement crash with each vehicle moving at the same impact velocity. The test is intended to represent most real-world crashes (both vehicle-to-vehicle and vehicle-to-fixed object) with significant frontal engagement in a perpendicular impact direction. In the rigid barrier test, the vehicle changes velocity very quickly upon hitting the barrier. The crash produces a high deceleration crash pulse of short time duration – frequently referred to as a “stiff” pulse. The rigid barrier test is used in the New Car Assessment Programs (NCAP) of the U.S., Japan, and Australia.

Moderate overlap frontal test:

In the moderate overlap frontal test, a vehicle travels at 40 mph toward a barrier with a deformable face made of aluminum honeycomb. The barrier face is just over 2 feet tall. A Hybrid III dummy representing an average-size man is positioned in the driver seat. Forty percent of the total width of the vehicle strikes the barrier on the driver's side.

The forces in the test are similar to those that would result from a frontal offset crash between two vehicles of the same weight, each going just under 40 mph.

Driver side small overlap frontal test:

The driver-side test is designed to replicate what happens when the front left corner of a vehicle collides with another vehicle or an object like a tree or utility pole. This crash test is a challenge for some safety belt and airbag designs because occupants move both forward and toward the side of the vehicle.

In the driver-side small overlap frontal test, a vehicle travels at 40 mph toward a 5-foot-tall rigid barrier. A Hybrid III dummy representing an average-size man is positioned in the driver seat. Twenty-five percent of the total width of the vehicle strikes the barrier on the driver side.

Passenger-side small overlap frontal test:

The passenger-side test is the same as the driver-side test except the vehicle overlaps the barrier on the right side. In addition, instead of just one Hybrid III dummy, there are two — one in the driver seat and one in the passenger seat.

Most modern cars have safety cages encapsulating the occupant compartment and built to withstand head-on collisions and moderate-overlap frontal crashes with little deformation. Crush zones in the passenger vehicle help manage crash energy to reduce forces on the occupant compartment. These crush zones are mainly located in the middle 50% of the front end. When a crash involves these structures, the occupant compartment is protected from intrusion, and front airbags and safety belts can effectively restrain and protect occupants.

Study of Dodge Neon full-scale model:

For this study, FMVSS 208 Rigid barrier test scenario was setup for simulation. 

The above file has been provided as a reference for setting up the model and it assists in going for a model quality check of the BIW structure provided to us.

From the above full-scale model, we need only the mass of the vehicle. Using a mass balancing tool in hypercrash, the weight of the vehicle comes out to be 1216 kg.

Partial BIW structure:

The partial BIW structure given to us contains 74 components. Each component is described by a unique property and there are only 68 material cards used to define this BIW structure. 

Around 390 rigid bodies constitute the components. However, many of these rigid bodies are being defined for very few nodes. This is to represent the actual behavior of a full-scale model. 6 contact interfaces are provided in the current model and none of them are required for the simulation. It is also noted that around 89 additional masses are present in the model with most of these being of 0.001 kg. These additional masses neither help nor harm the simulation in any way. They can be deleted from the model.

There are 55 Joints defined in the system using Spring property. During the final moel check, the use of these joints will be evaluated. An interface exists in the system of Type 21 (Drucker-Pager interface). A Rock and Concrete material card is assigned to the components which are a part of the above-mentioned contact. These cards overlapped with the bumper-shell component. 

Model setup:

Based on the provided guidelines, the units system used for this simulation is [kg], [mm], [ms]

1) Setting up interfaces:

A type 7 self-impact contact is setup for contact between the rigid-wall and the BIW structure.

- It is a general purpose contact used to model most of the contact conditions

- A gap is an area around the node defined such that, while in contact, if the node from the slave penetrates the gap area; the counter forces are activated.

- It works on the logic of Impact penalty mehtod wherein the gap area and the behavior in the gap area is treated like a spring. These springs generate resisitive forces as counter-forces to the induced penetration from the slave nodes to the master

- The type 7 contact provides a cylindrical gap around the edges to take them into consideration while making the simulation more accurate.

- Certain parameters are used to define the type 7 contact:

Igap : The size of the gap is decided by the choice of this parameter

Gap_min : A default parameter to decide the minimum gap to activate the interface conditions

Inacti: Action to take in case of initial penetrations

Istf : Used to model the sitffness at the interface

Iform: Friction formulation

Stmin: Minimum stiffness to be used at the interface

Idel: Model the behavior of slave and master segments in case of element failures that are attached to the segments

The reason for defining the minimum stiffness and minimum gap is because of their effect on the timestep. 

2) Setting up rigid-wall:

A rigid wall is setup perpendicular to the global x-axis and in front of BIW by a small gap

This rigid wall was created with a friction value of 0.1

3) Defining additional masses for targe weight of 700 kg and balancing C.O.G requirements:

The target weight of the model is 700 kg and the optimum C.O.G location is between the driver and passenger seats. 

Mass is added to the nodes outlined in red. A mass of 512 is added to those nodes. This brings out the C.G to the required position.

4) Defining initial velocity:

An initial velocity of 15.646 mm/ms is defined. 

5) Defining cross-sections to measure sectional forces:

The output of the following cross-sections has been requested:

-Sectional forces in the rails

- Shotgun cross-sectional forces

- A-pillar cross-section force

-Headlight bracket sectional force

These cross-sections have been highlighted in the figure below:

6) Accelerometers:

Accelerometer sensors have been setup at the base of B-pillar

7) Intrusions on the brake pedal:

2 Moving skews have been defined to account for intrusions on the brake pedal.

8) Control card definitions and Time history cards: 

As per the given guidelines, the run time and frequency for animation files to be printed are already set as 80 and 0.5 respectively. This is not changed. 

Another control card: ANIM/VECT is defined to animate displacement, stresses while post-processing:

Likewise, the time history card has been setup for the created sections and brake pedal nodes. 

9) Model checker:

HyperCrash model checker is used to check for errors and warnings in the model.

Around 2 errors and 8 warnings are used to were present in the system. 

These warnings can be ignored. 

The errors are due to ill-defined skew systems in the model. Error shown due to spring is unnecessary for the model and can be resolved.

After the errors are resolved, the model checker is run again.

Results:

Energy error:

The energy error is found well within the limits.

Displacement :

Von-Mises stress:

2) Plots:

Rigid-wall forces vs Time

Energy Plots vs time:

Sectional forces on the rail: Node 172427

A-pillar forces: 

Forces from bumper to headlamp bracket:

Resultant acceleration from Accelerometer:

Fender/Shotgun forces:

Intrusions on the dash wall:

Inference:

Von-mises stress and Displacement:

Graphs:

Rigid wall forces vs time		This graph is not as significant for the current simulation. However, the forces on the rigid wall keep on increasing till a point. This point is when the major impact happens and the car crashed into the rigid wall entirely.
Energy plots		The total energy of the system remains constant as can be seen. However, not all kinetic energy is converted into internal energy which suggests that the crash time should be over 80 ms. The Hourglass energy of the simulation is almost zero.
Shotgun forces		The sectional forces on the right side are higher than the left side
A-pillar		The sectional forces of the A-pillar increase to a maximum value and then decrease.
Headlamp bracket forces		The right hand side's peak value is higher than the left hand side.
Sectional Forces on and near the node 172427		Overall, the sectional forces on the right side are significantly higher than the left side
Intrusions on the brake pedal at the required node numbers		The intrusions on the brake pedal node are ultimately found to be higher than the acceleration pedal.
The accelerometer on the B-pillar rockers		From the accelerometer sensors, the resultant accelerations on the left and right are almost equal at the start of the simulation indicating equal deformation.  This behavior changes as the crash progress.

Conclusion:

Thus the frontal crash analysis of Dodge Neon is conducted as per the given guidelines.