Using Simulation Techniques to Reconstruct Accidents

Figure 1: Reconstructing a frontal collision using the reconstruction software, HVE.

Using virtual crash methods is a validated way to enrich an accident reconstruction and deliver a visual aid to better explain the events of a car accident. Virtual Crash 4 is an industry standard in forensic engineering capable of rapidly testing collision scenarios using the impulse-momentum calculation method. HVE is an alternative that precisely interprets the impulse duration and damage profile to capture the dynamic forces during a collision. Both methods are scientifically accurate and rely on physics to determine the resulting motion of vehicles. The dynamics of a simulation such as these can be used as input to a full-fledged visualization effort or as input to a biomechanical simulation. Biomechanical simulation can take the 3-dimensional dynamics from a vehicle impact simulation to predict injury mechanisms and occupant kinematics within the moving vehicle. Scientifically accurate visualizations can be visually appealing court admissible additions to an expert witness’s presentation and prove to be invaluable in accurately explaining the events of a car accident.

A Background in Vehicle Dynamics

The fundamentals of physics and mechanics are essential to the reconstruction of a vehicle collision. With a few engineering assumptions (e.g., the collision itself occurs over a short period of time ~1/10 of a second, the collision is assumed to occur within a closed system during this time, and the collision involves objects of similar masses), a two-vehicle collision can be broken down into a couple of simple physics problems. Using one of the simplest analogies for a collision, let us imagine that two vehicles are pool balls.

First, each moving pool ball has momentum. This is simply the velocity multiplied by the mass of the ball. A stationary pool ball has no momentum, while a high-speed pool ball will have plenty of momentum until it comes into contact with something. When a force is applied to a moving object (from a collision, for example), its momentum changes. In a collision between two pool balls, we know that the forces exerted by two objects on one another are equal and opposite (i.e., Newton’s Third Law). For example, if a moving pool ball gives up all of its momentum on contact with another ball, it will stop as the other ball rolls away at the original ball’s velocity. This concept is known as the conservation of momentum.

pool balls used to describe conservation of momentum
Figure 2: Classical representation of the conservation of momentum using pool balls.

This law holds true with larger objects as well. Say an expert was investigating a car collision and wanted to know the speed of a car before it made contact with a stationary car. First, he or she would need to quantify each vehicle’s mass. Second, the expert would need to investigate the change in speed of each vehicle after the collision. Most modern vehicles have an event data recorder (EDR) that records the changing velocity of the vehicle in the event that it detects a collision. With this information, the expert can begin calculating the estimated speed of the moving vehicle before impact.

While the foundations of pool ball momentum are fundamental to the study of accident reconstruction, vehicle collisions are a bit more complex. In the real world, energy is always conserved, but it can be lost to external forces. Friction between tires and the roadway or between two vehicles is lost as heat. Another external force that anyone who has seen a car collision can understand is deformation and crush. Cars, unlike pool balls, can crunch up. After steel is crumpled in a collision, just like a spring, the steel can spring back. The crush energy that is restored during this rebound phase is measured by the coefficient of restitution. Restitution compares the relative velocity before and after a collision between two bodies and helps in understanding whether a collision is more elastic or plastic. For example, a rubber ball bouncing on a tile floor is an example of an elastic collision where most of the energy is returned to the ball after impact. On the other hand, a high-speed vehicle impact will be very plastic, highly deforming each vehicle and sometimes interlocking the impacted vehicles together.

Figure 3: Using Virtual Crash 4 to demonstrate the conservation of momentum with restitution.

Virtual Crash and HVE

Reading this homage to Newton and basic laws of physics, you might imagine the thorough calculations needed to accomplish an effective accident reconstruction. While this method has been shown to be extremely accurate, it begs for a visual reference. A visual can be especially helpful to an expert while he or she explains the reconstruction to others. Virtual crashing software, such as Virtual Crash 4, is an accident reconstruction tool that integrates all of the aforementioned physical concepts into a 3-dimensionalsoftware. Virtual Crash 4 allows the user to input all case-specific variables, such as the subject-specific vehicle, velocities, masses, friction, and restitution. The software also allows the expert to prescribe driver inputs like braking, accelerating, or turning, and it can calculate for multiple collisions, complex rotational dynamics, and more. The Virtual Crash 4 software relies on a calculation method called the impulse-momentum model. This means that the simulation is based on traditional rigid-body dynamics and that the interacting vehicles exchange force impulses in an instant of time. This calculation method is highly accurate and allows for quick, real-time adjustments to the simulation.

An alternative collision reconstruction software is called HVE. In the same manner as Virtual Crash 4, HVE is a visual 3D environment specialized in reconstructing vehicle collisions using the conservation of momentum. However, this software differs from Virtual Crash 4 in that it uses a “penalty method” and calculates an impulse duration over which the collision occurs. This means that, unlike Virtual Crash 4, the impulse exchange of forces between vehicles does not happen all at once but rather over a period of around one-tenth of a second. The penalty method means that the simulation, in a step-by-step manner, balances the exchange of force between two interacting vehicles and exerts that force on each vehicle dynamically over the course of the impact. This method has the advantage of calculating the precise damage profile on each vehicle.

Figure 4: Reconstructing a complex intersection collision using Virtual Crash 4.

Accident Reconstructions, Visualization and Biomechanics at Explico –Conclusions

Explico utilizes validated vehicle dynamics and reconstruction software, such as Virtual Crash 4 and HVE, to analyze the physics and mechanics of a collision. The results of these analyses underly our fully physics-based visualizations, making them trial-ready forensic animations for use in litigation. These simulated vehicle dynamics can also be paired with a biomechanical simulation to predict occupant dynamics within the moving vehicle.

Contact an Explico expert to see the amazing analyses a reconstructionist can develop with modern virtual crash reconstruction methods.

simulation of a sideswipe collision, animated cars
Figure 5: Simulating a sideswipe collision using HVE and developing a crush profile to compare with physical evidence.


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