Under The Microscope: CRASH &'CRUMPLE'

 

Injuries and fatalities occur in all forms of transportation but numerically road traffic accidents account for the great majority worldwide. In developed countries, they are the most common cause of death below the age of 50 years, and in young men this trend is even more marked. The pattern of injury, fatal and otherwise, varies considerably depending upon whether the victim is a vehicle occupant, a motorcyclist, a pedal cyclist or a pedestrian. 

 

A number of elementary physical facts help to explain the complex pattern of traffic injuries, especially those sus-

tained by the occupants of a vehicle. 

 

o   Tissue injury is caused by a change of rate of movement. A constant speed, however rapid, has no effect whatsoever as is evident from space travel or the rotation of the earth. It is the change of rate that is traumatic - that is, acceleration or deceleration. 

o   Change of rate is conveniently measured in 'gravities' or 'G forces'. The amount that a human body can tolerate depends greatly on the direction in which the force acts. Deceleration of the order of 300 G can be sustained without injury and even 2000 G can be survived for a

o   short time, if it acts at right angles to the long axis of the body. The frontal bone may resist 800 G without fracture and the mandible 400 G, as can the thoracic cage.

o   During acceleration or deceleration the tissue damage produced will depend upon the force applied per unit area, just as a sharp knife penetrates more easily than a blunt one used with the same force. If a car driver is brought to rest from 80 km/hour by striking 10 cm2 of his head on the windscreen frame, the damage will be vastly more severe than if the same decelerative force was spread over 500 cm2 of a safety belt.

o    Between 60 and 80 per cent of vehicular crashes (either into a fixed structure or into another vehicle) are frontal, causing violent deceleration. Another 6 percent are rear impacts, which accelerate the vehicle and its occupants. Of the remainder, about half are sideswipes and the rest 'roll-overs'.

o   In the common frontal impact, there is never instant arrest of the vehicle, even when it runs into a massive, immovable structure. The vehicle itself deforms from the front so that there is always a deceleration distance and time, albeit small. In fact, much of the manufacturers' design research now goes into making deliberate provision for the crumpling or 'concertinaing' of the front and rear of the car, leaving a central rigid cell that comprises the passenger compartment. The object is to extend the stopping distance and time, so that the G value acting on the occupants is reduced.

For instance, travelling at 80 kmlhour runs into a stone wall that it penetrates for 25 cm, plus 50 cm crumpling of the front of the car, the deceleration would amount to about 33 G. If an occupant was rigidly belted into his seat (a practical impossibility), he would also suffer the same deceleration, which would be survivable. If, however, he was unrestrained, he would continue forwards momentarily at 80 km/hour and suffer massive G forces, the magnitude of which would depend on his deformation stopping distance (a few centimetres-of tissue compression) when he struck the internal car structures in front of him.

 

The type of vehicle (other than motorcycles) in theory makes little difference to the mechanism of injury, but most statistical surveys divide them into cars and light vans under 1.5 tonnes, on the one hand and heavier vehicles, such as trucks and buses, on the other, though the latter have different features more akin to passenger aircraft. Heavy goods vehicles naturally suffer less than cars and light vans in crashes because of their far greater mass and strength, and also due to their height above the ground. Structural damage from impact with other smaller vehicles is less and often sustained below the level of the driver. Given smaller deceleration forces, however, the cab occupants are vulnerable to the same injury patterns. Light vans are virtually identical to cars with respect to the front-seat occupants. In fact they may be more at risk, as modern vans tend to be flat-fronted and thus have little or no 'crumple' potential to increase the stopping time. Concentrating on cars, the most common vehicular casualty, the pattern of injury varies according to the position of the occupant.

Acknowledgements:

www.aived.nl    AIVD – @Erik Akerboom ©

www.politie.nl  Politiekorpschef  @Janny Knol©

www.politie.nl WEB Politie - @Henk van Essen©

 

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