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Motor Vehicle Accident Reconstruction & Biomechanical Physics – Part 3

April 13, 2009 Forensic Engineering 1 Comment

Aerial Crush Photography 

dsc_0088-largeOn-site aerial photographs taken with the aid of a boom and perimeter grid capture collision data. This system has proven to be extremely useful in addressing collision dynamics.A brief synopsis of this method reveals its usefulness to the accident reconstructionist and biomechanical expert. 

aerial-boom-being-raised-over-carHorizontal lines for the red car illustrate 1′ of interior crush to the impact side-bar beam in the door and 1.5′ of collapse in the vehicle roof structure.  Also, shown on the opposite side of the car is a general bowing of the vehicle structure showing how impact forces are transmitted throughout the vehicle.

COMPOSITE PHOTOGRAMMETRY 

pickup-truck-pulling-air-compressorAt night a westbound pickup truck was towing a 2700 pound job-site air compressor. The air compressor disconnected from the pickup, crossed the centerline of a two lane roadway and struck an eastbound 3,100 pound automobile. The speedometer of the car was “crushed” and read 55 mph. Delta V calculated at 108 mph.

9363-3-largePhotographs above are a composite photogrammetric assembly of the following:

Aerial Photographs Matched: Subject Vehicle and Air Compressor

Post Production Processing: Grid Lines of Subject Vehicle & Compressor Outline

VEHICLE CRUSH DAMAGE

car-interior-photoAnalysis of photographs reveals that the subject vehicle sustained 6.3 feet of crush as a result of the collision.

In photo to left, the roof has been pushed back to reveal the interior.  A 1’x1′ calibration square is placed on the driver’s side seat cushion.

92-pre-crashIllustrations are provided for a side view for vehicle structure collapse associated with the 108 mph closing speed in this fatality impact.

VEHICLE CRUSH DAMAGE – SIDE VIEW

These CAD illustrations show the vehicle side view before and after impact and the relative position of the driver inside the automobile. Post-collision vehicle deformation at right illustrates the vehicle contact with the driver. It can be clearly seen that this collision was not survivable.

29-post-crashCOLLISION SURVIVABILITY

Linear momentum calculations show a closing speed for this head-on collision of approximately 108 mph and Equivalent Barrier Speed (or EBS) of 59 mph. EBS is when a vehicle impacts a massive barrier which absorbs no energy of collision. It is a convenient concept to compare the energy absorbed in crushing vehicles. The National Highway Traffic Safety Administration annually releases its New Car Assessment Program (NCAP) crash test results for current model year vehicles. These tests give occupant injury criterion values for collisions at the 35 mph EBS. In the EBS crash of an exemplar Buick Century, approximately 2′ of uniform crush was sustained.

collision-crush-severityAs previously addressed, an increase in velocity at impact results in an increase in the energy of collision. The subject vehicle had an EBS of 59 mph which results in an increase of the energy involved in this collision of 184%, or almost three times as much energy being produced, as in the 35 mph crash tests. This massive increase in energy is translated directly into an increase in the forces making this collision unsurvivable.

compressor-car-impact-diagramThe air compressor has an overall width of 56 inches, 13″ less than the Buick, and weighed almost as much. It approached from the driver’s side at a slight angle of about 8°. The net result of these factors was primary damage and collision force concentration in an area directly in front of the driver.

Force concentration is shown in illustrations included in this report. The previous page however, best illustrates the total amount of intrusion into the driver’s side area. Measurements are shown of the distance from the driver to the left front of the vehicle both before and after collision. These measurements reveal that the initial 9.2 ft. of vehicle in front of the driver had been crushed to 2.9 feet. Thus, the area immediately in front of the driver sustained a total crush of approximately 6.3 feet.

Extremely high EBS loads coupled with massive collision damage concentrated directly in front of the driver, made this collision unsurvivable under any circumstances by the driver of the car.

KINETIC ENERGY CALCULATION 

One important principle in accident reconstruction is a calculation of the forces and energy involved in the collision. This has a direct effect on the amount of damage to the vehicles involved and the severity of the injuries sustained. A technical description for the calculation Kinetic Energy is provided in AR Software, AI Tools Equations Advanced Module, 1992:

KINETIC ENERGY

In vehicle dynamics, the most common form of energy is Kinetic Energy or KE. There are two important forms of kinetic energy, Linear and Rotational. If a car is spinning it has rotational kinetic energy as well as linear kinetic energy. The equations of this module require that if there is rotation of the vehicle it is constant. As long as there is no change in rotational kinetic energy relative to the linear kinetic energy the energy equations can be used. The definition of linear kinetic energy at some point in time or at some state is given by: KE = 1/2 mV²  — where m = the mass of the vehicle, and  V = the speed of the center of mass of the vehicle.

An 8 pound heavy-truck driveshaft yoke on the roadway is thrown into the air by a passing car.  Vehicle at 60 mph impacts yoke and KE is calculated at 962 ft-lb.  Vehicle occupant sustained injury.

An 8 pound heavy-truck driveshaft yoke on the roadway is thrown into the air by a passing car. Vehicle at 60 mph impacts yoke and KE is calculated at 962 ft-lb. Vehicle occupant sustained injury.

Kinetic energy is always positive. It does not matter whether the speed is positive or negative, its square is always positive. Since the kinetic energy is a state condition it is always changing as the speed changes. Therefore, during an event what we are interested in is the change in kinetic energy or DKE. The change in kinetic energy is defined as DKE = KEf – KEo = 1/2m (Vf² – Vo²)

Note: If the speed has increased during the event, the kinetic energy has increased. As the equations assume no change in rotational KE, the Linear Kinetic Energy will be referred to as the Kinetic Energy.

Thus, the change in Kinetic Energy of a collision increases with the square of the change in velocity. Since the mass remains constant, the percent increase (or decrease) in the Kinetic Energy involved for a change in speed from V1 to V2 can be calculated as: % increase in KE = [(V2/V1)² – 1.0] x 100%

For example, an increase from 30 to 35 mph increases the energy of the crash by 36%. Similarly, increasing the speed of the collision from 35 to 59 mph produces a 184% increase in the energy involved as illustrated in the yellow and red graphic above right.

SUMMARY AND CONCLUSIONS

Accident reconstruction and biomechanical analysis with their respective mathematical interpretations provide the basis to assess vehicle damage and calculate injury loads sustained by the occupants.

Vehicle damage photographs taken at ground level and with the aid of an aerial boom are important in biomechanical analysis and accident reconstruction. These photographs permit detailed analysis of crush and principle direction of force. Post production photographic techniques can be used to compare the damaged subject vehicle with an undamaged exemplar vehicle.

An accident reconstructionist and biomechanical investigator should keep in mind their job is to record facts necessary to mathematically analyze an accident. Some data or clues are perishable and must be obtained or photographed early. Each occupant has a unique level of injury and experiences unique crash loads depending on their location and other accident factors. The purpose of the biomechanical analysis is to determine means and rationale for injuries and improve the chances of survival in future collisions.

REFERENCES

1. Daily, John, “Fundamentals of Traffic Accident Reconstruction”, Institute of Police Technology and Management, Jacksonville, FL, 1988.

2. Boddorff, Thomas C. and Ian S. Jones, “Simple Overhead Photography Techniques for Vehicle Accident Reconstruction”, S.A.E. Paper No. 900370, Society of Automotive Engineers, Warrendale, PA, 1991.

3. Baker, J.S., Traffic Accident Investigation Manual, Northwestern
University, Evanston, IL, 1975.

4. AI Tools, AR Software. Trantech Corporation, Redmond Washington.

5. McElroy, Robert C., “Aerial Crush Photography & Analysis For Accident Reconstruction” Special Problems in Traffic Accident Reconstruction, IPTM, University of North Florida, 1994.

6. Fox, Roy G., “Helicopter Crash Survival Investigation”, Proceedings of 23rd International Seminar of the International Society of Air Safety Investigators, Dallas, TX, 1992.

Currently there is "1 comment" on this Article:

  1. Anonymous says:

    Hi, good post. I have been pondering this issue,so thanks for sharing. I will definitely be coming back to your site. Keep up the good work

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