Impact absorbing body structures

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Contents

•The cost

•Accident statistics

•Vehicle safety

•Pedestrian safety

•Bumpers

•Bonnets

•Bumpers

•Towing eyes

•High impact crashes

•Crush and crumple zones

•Front impact

•Side impact

•Crash simulators

•Created by Harvey Heath 2005

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The cost of road accidents is huge and on going:-

• salvage and repair/replacement of motor vehicles

• repair/ replacement of other property

• emergency services

• hospital and medical expenses

• long term health services

• invalid rehabilitation

• loss of income

• loss of personal productivity

• loss of productivity due to transport interruptions

• legal expenses

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Statistics

Modern vehicles are travelling faster and faster and there are far more cars on the road than ever before

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What makes a vehicle safe in a crash?

answer- “stronger is best” used to be the approach by motor manufacturers and the general public and up to a point is still a philosophy that has some consideration when people buy motor vehicles

XNow the thinking is changing, manufacturers have been actively changing body and chassis design to make their vehicles safer by fitting energy absorbing sections to the body and cushioning devices inside to protect occupants

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Old vehicles constructed entirely for strength using full chassis or monocoque/subframe construction for overall strength.

This design has been identified as dangerous to both people inside and outside the vehicle

average records from the USA show:-

40% of all vehicle accidents are front to front impacts

these contribute to 40% of all motor vehicle related deaths

side impact is identified as the major impact causing death and injury

rear impact has less fatalities but creates more severe injury

roll over statistics show 25% increase in fatalities if ejected from the vehicle

low speed contact with pedestrians created high fatalities due to protrusions and non absorbing bodywork

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What makes a car safe?

trend 1997 2004

Safety features: air bags, seat belts 66% 75%

Crash prevention: ABS, tires, radar

19% 44%

Body strength: bumpers, chassis 54% 32%

Impact absorption 13% 20%

Vehicle Make: design, shape 6% 13%

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Developments to reduce injury when impacting with pedestrians

Crash analysis has shown that not only were older vehicles with bumpers designed to fend off other vehicles, dangerous, but also protrusions such as over riders, grille and bonnet ornaments, caused injury even at low speed.

The height of bumpers forced pedestrians under the bumper rather than over

Head injury caused by the momentum of the vehicle forcing the body over the bonnet and into the windscreen without any cushioning or absorption factor.

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Body shape has been identified as potential danger areas when impact pedestrians.

Bumper sections lowered to lift rather than force down.

Bumpers now must have cushioning sections and mountings

Bonnets must have “soft” sections to absorb head impact.

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In 1975 vehicles manufactured and imported into USA had to have impact absorbing bumpers in attempt to reduce pedestrian injury and also reduce vehicle repair costs involved in low speed impact.

1974 English TR6

1974 American export TR6

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Bumper mounting points are made to absorb impact by deforming or crushing in on themselves, or using hydraulic shock absorbers that move to absorb impact.

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All bumpers are designed to absorb the impact’s energy through a series of valves and air chambers or hydraulic chambers. Bumpers are also sometimes designed with built-in “crumple zones” which flexes on the impact.

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Impact absorption bumpers

steel mounting

Aluminium armature

Polycarbonate cover

polystyrene foam filling

They are molded in thermoplastic polycarbonate and tested to absorb impacts up to 8 Kph

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Impact absorption bumpers

Bumper side mountings are quick release type that are not bolted and break away on impact

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Impact absorption bumpers

Crash test dummies used to determine danger points on the front section that could reduce injury during pedestrian impact

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Impact absorption bumpers

This shock cone aluminum bonnet has been developed by Mazda to reduce impact, and tests show reductions in the extent of head injuries with vehicles fitted with this development.

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Low speed safety development in modern vehicles

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Front and rear towing hooks not only present a danger to pedestrians but also cause extensive vehicle body damage in low impact collisions

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The mismatch of bumpers presents a problem with energy absorption not starting in the correct place causes extensive body damage when low force energy absorption sections should have controlled the situation

Low high performance vehicles have poor bumper protection

4X4 MPV provide little protection to other vehicles

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Low speed impact appears to create a lot of damage but is created by the crumple zone working.

In this case the shock absorbing sections have folded to take the energy out of the impact. The bumper has flexed and the mountings have collapsed to deform the front section. The side guards are not part of the shock absorption and push out of the way. The bonnet has a shock absorbing section which causes it to fold rather than move back into the windscreen or the passenger cage.

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The force created during a crash can be very high and absorption of vehicle energy takes time to dissipate. In cases like this, no amount of crumple zone can stop the vehicle destroying itself, but the passenger cage is remarkably intact though obviously some intrusion has taken place.

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FRONTAL 6.8% 5.2% 7.4% 5.1%

SIDE IMPACT 19.1% 19.8% 2.9% 3.7%

REAR IMPACT 0.8% 3.9% 0.5% 4.9%

Accident statistics

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A few statistics about vehicle crashes

84% of all road accidents are caused by human error

1.1% of road accidents are related to vehicle failures

6.5% accidents related to environmental factors

7.1% involve pedestrians

1.0% other factors

The three main driver related causes are

speed unsuitable for the conditions

failure to yield right of way

errors in leaving the traffic flow

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Newton’s first law.

“An object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force”

A crumple zone is built into the vehicle to absorb energy on impact. The purpose is to increase the amount of time it takes the vehicle to come to a complete stop in comparison to the object the vehicle is impacting with.

Having a crumple zone, the time it takes for the vehicle to come to a stop may be extended form 0.01 seconds, to 0.2 seconds. By taking more time to slow down using the crumple zone by a factor of 20 will decrease the acceleration and therefore the force factor.

Crumple zones yield during a crash changing energy into heat and sound, which in turn reduces chances that occupants of the vehicle being hurt.

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Time (MS)

Events

12 initial folding of longitudinal

16 initial folding of sub frame

21 1st buckling of rails upper in front of shock tower

35 engine contacts barrier

37 buckling rear of sub frame at fixture on extension longitudinal

50 rear end of longitudinal start to buckle behind reinforcements

51 wheels contact barrier

67 maximum dynamic deformation

The events as the crash happens

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0 ms Crash begins- bumper contacts impact object

15 ms Crash sensor determines to deploy air bag or not

20 ms The seat belt tensioners ignite

25–35 ms All front air bags break free and start to inflate

45 ms Drivers air bag is fully inflated

55 ms Passenger air bag fully inflates (usually larger than the driver’s air bag)

60 ms Driver’s head and chest contact air bag

70 ms Passenger’s head and chest contact air bag

100 ms Body impression into air bag is at their maximum and start to rebound

150 ms Body travels backwards until retained by the seat and head rest

( one millisecond = one thousandth of a second)

The vehicles safety features operating

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Vehicle manufacturers built in compression or crush zones into the front and rear of the vehicle to protect the passenger cage.

These zones act as time delays to absorb impact energy

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Bumper mounts absorb low speed impact Air bag sensors mounted here

Crash energy dissipated through bonnet and hinges

Cross car beam forms front of passenger cell as well as resist side intrusion

Reinforced doors with side intrusion bars

Rear floor pan bends upwards to keep intrusions away from the fuel tank

Rear absorption defuses energy with controlled crumpling

Bumper beam distributes point load across the body structure

Crush zones built into front panels to absorb energy

Severe impact drive train buckles under and directs engine under safety cage

Door hinges and latches withstand impact and resist opening under collision

Rigid floor pan transfers crash force through vehicle and protects passenger cage

A,B, and C posts support roof and form top section of the cage

Rear bumper integrated into body to help with impact dissipation and absorption

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Monocoque construction uses specially designed and positioned frames to absorb and distribute crash energy throughout the body

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Vehicle design must now incorporate safety features not only for the passengers inside abut also for pedestrians impacting on the outside

Vehicle design must involve impact absorption in a number of different directions

Frontal impact energy transferred through the frame around the cage

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Side impact strength resists crushing into cage but transfers energy around it into the other body members

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Crumple zones are created by using panels that have of varying thickness and shape to induce folding and bending at particular points

Panel compression is created in 3D computer software to determine pressure and distance

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Panels fold at particular points and absorb energy as the collapsing takes place

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Front side member deformation aids

By creating a crush initiator the designer ensures that when the front side member contacts the opposing structure at the point where force is introduced, namely the point of highest pressure, a conversion of energy begins immediately and this shockwave begins to counter the speed of the vehicle. When the pressure wave reaches the end of the rail, it is reflected and rebounds from front to rear several times.

MASSArea of high

impact pressure

counter wave formation

frontal shock wave against

velocity wave of material

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AB

C

D

E

F

G

Front side member deformation aids

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Impact absorption is controlled by panel shape and thickness.

The enclosed section with the separators clearly shows how panel deformation has occurred during impact and slows the speed of the vehicle while the energy is directed and redirected in the body sections.

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Using crash testing facilities safe structures can be created to absorb and transmit energy into the body structure.

“Computer crashing” allows simulations to reveal weaknesses and where structures are prone to failure. It also enables designers to add strength to the passenger cage only where necessary.

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Testing enables accurate data to collected as to how panels will react in known circumstances.

Absorption of crash energy can be determined using crush sections or deformation aids formed in the panel sections

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The research and development costs are huge, especially after all the theoretical and computer generated tests have been completed, manufacturers’ use real vehicles in the final testing stage.

This test undertaken by Mercedes to check the energy absorption of the small smart car against the large family saloon.

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The odds seemed stacked against the automotive welterweight, partly because of the enormous weight difference ratio of 1:2. The Smarts lack of a protective long front end was another source of doubt before the crash test began. Although the collision was to take place with both contenders driving at a speed of 50 Kph. The huge difference between their weights meant that the impact would have very different results on each of them. For the heavy Mercedes, the impact would be equivalent to crashing against a barrier at a speed of 33 Kph. The lightweight Smart, however, would have to absorb an impact corresponding to a crash against a stationary barrier at a speed of 67 kilometres per hour - double the speed of the Mercedes.

The measurements carried out on the four participating test dummies showed that potential occupants in the two vehicles would not have suffered any life-threatening injuries. The passengers in the S-Class would in all likelihood have walked away without a scratch, while the passengers in the smart would have suffered bruised ribs, and the driver would have also had some slight leg injuries.

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Crash simulators are used in the final stages of crash absorption development using controlled tracks, vehicles can be forced into barriers representing solid objects and other vehicles with shock absorbing capabilities.

By using crash dummies, potential injuries can be monitored to all passengers inside the cage.

High speed cameras are used to record all actions during the impact.

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Frontal impact is not always square on and may involve objects that have no impact absorption of their own.

This causes all vehicle momentum to be absorbed by the vehicle, especially when not all the vehicle crumple zone is exposed to the energy force.

Unequal force such as this will require the exposed crumple zone to account for all the vehicle energy unless the object being hit also has energy absorption facilities.

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Side impact is major cause of head and chest injury and door strength is important as well as being able to divert crash energy around the passenger cage.

Door intrusion bars or braces are constantly be testing both using computer simulation and actual vehicle testing.

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This graphic clearly shows how side intrusion is prevented by transferring energy through the controlling sections in the floor pan and leaving the passenger cage intact.

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Strength in the door frames is achieved by intrusion bars across the door frame. Frame shape causes the exterior of the door skin to ride over the pillar and keep the door shut under impact.

The door is foam filled between the steel frame and the trim panel which helps absorb impact and protect the pelvic section.

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Some vehicles are more prone to occupant injury regardless of crumple zones and side intrusion protection. Air bags and seat belt pensioners do help but body design also allows these protection devices to mounted in the most advantageous position.

48Smart and short!!