1. INTRODUCTION
Crash testing is simulation of actual accidents to bring in safety measures
in real life situation
In
U. S. , Europe more than 250000
car drivers and passengers were killed and further million were injuries in
road accidents these chilling figure shows how vital it is to access a car
performance in a serious crash. Further it is found that half of these fatal
and disabling road accident injuries could be saved if car could give best
protection in a car accidents it is unfortunately impossible to find out how
safe a car is, by simply looking at it in the showroom so new test methods have been utilized to
conduct a crash test and give consumer comparative safety data on new cars .
All
new car models by law must pass crash test before they are sold in the market all over the world along with
India. The car manufacturing companies carry out the crash test as per the
European standards .
Controlled
crash test provide a means of assessing the protection offered by the vehicle
in the crashes. This provides necessary
information for designers to develop and optimize a vehicle safety features as
well as results that can be compared with an established standard for
certification purpose .
Elements of Crash Test :
i)
Study of occupant safety
ii)
Study of energy impact on occupant.
iii)
Study of chassis structure changes
iv)
Design of new safety restraint
v)
Evaluation of exiting safety restraints.
2. GUIDELINES FOR CRASH WORTHINESS RATING SYSTEM
This
chapter explains rating systems used in crash test. It allocates rating of GOOD
ACCEPTABLE, MARGINAL or POOR for vehicle structure performance. occupant restraints and key injury
measurements. Depending upon rating in
these fields, it finally gives an overall rating for a car
Structure :
The
main item of interest is the loss of
occupant survival space. The
performance of the vehicle structure over the entire crash test event is
evaluated with the help of different measurements, technical reports,
photograph, and high speed film. Key
areas of interest are movement of the dash and steering column, reduction in
the width of the doorway opening, deformation of the firewell, pillers, roof
and floorman, movements of pedals, integrity of doors and seats, ease of
opening the door after the crash etc.
Depending upon these areas rating for structural performance is
decided.
Following helps to decide
the rating.
Rating For Structural Performance :
|
CRITERIA
|
GOOD
|
ACCEPTABLE
|
MARGINAL
|
POOR
|
|
A-pillar movement
|
Upto 50 mm
|
50-100 mm
|
100-200 mm
|
More than 200 mm
|
|
Steering wheel
Displacement
1. vertical
2.
horizontal
|
Less than 100 mm
Less than 80 mm
|
100-150 mm
80-130 mm
|
150-200 mm
130-180 mm
|
More than 200 mm
More than 180 mm
|
|
Footwell deformation
|
Less than 50 mm
|
50 –100 mm
|
100-200 mm
|
More than 200 mm
|
3. OCCUPANT RESTRAINT
The performance of the restraint system ( seat
belts, seats, air bag and head restraint ) over the entire cars event is
evaluated. Key areas of interests are
head, chest, and knee strikes, air bag deployments and occupants kinematics,
including rebound and partial ejection.
The table below sets some guidelines for assessments of occupant
restraints rating for a vehicle. The
worst of all four restraints is used for
the overall occupants restraints rating for vehicle.
Rating for Occupant Restraints
|
RATING
|
CRITERIA
|
|
Poor
|
Ø A door opens during the crash
Ø A seat becomes fully or
partially detached, slides forward on one or both sides of the seat or the
seat back falls
Ø An occupants slides off one side of air bag and makes a hard contact
Ø Seat belt failure
Ø Severe motion of steering
column
|
|
Marginal
|
Ø An occupants slides off
one side of air bag but does make hard contact
Ø An occupant seat tilts or
twists substantially
Ø The drivers head or chest
strike the steering wheel resulting in hard contact
Ø The passengers travels
forwards sufficiently to dash and its knees
|
|
Acceptable
|
Ø The drivers head or chest
strike the steering wheel but decelerations are low to moderate.
Ø During rebound occupant
head strike a window frame B- pillar or roof
|
|
Good
|
Ø No head or chest strike,
decelerations are low to moderate, and rebound is well controlled with the
little traverse or vertical movement of occupant.
|
4. INJURY MEASURES
Injury measurements during the entire crash event is done
with help of dummies seated in the test vehicles. The dummies are fitted with sensors to
measure the forces and movement which would be experienced by a human
occupant. This measurements give an
indication of risk of injury
Rating for Injury Measure
|
Measurement
|
GOOD
|
ACCEPTABLE
|
MARGINAL
|
POOR
|
|
Head
injury measurement
|
Less than 750 |
751-899
|
900-999
|
More than
1000
|
|
Chest
compression
|
Less than
50 mm
|
50-59 mm
|
60-74 mm
|
More than
75 mm
|
|
Chest
acceleration
|
Less than
60 g
|
60-74 g
|
75-89 g
|
More than
90 g
|
|
Femur
axial force
|
Less than
7.3 KN
|
7. 3-9 KN
|
9.1-10. 8 KN
|
More than
9.8 KN
|
|
Lower leg
index
|
Less than
0.8
|
0.8-0.9
|
1.0-1.4
|
More than
1.4
|
Injury Rating for Each Body
Rating :
The worst of all injury measures is used in overall
injury rating for each body rating.
however greater weight is given to the drivers injury results, in
recognition of lower occupancy rate for the front passenger position. This is done by elevating the worst passenger
result by one level. For the purpose of
determining overall injury rating for a body region
For e. g.
The rating for the head are:
Driver/full frontal acceptable
Driver/offset good
Passenger/full frontal poor
Passenger/offset good
The poor
passenger rating is elevating to marginal and since this is the worst of four
rating, the injury rating for head is
marginal
Combined Injury Rating :
For the
purpose of determining and overall crash worthiness rating for the vehicle, a
combined injury rating is derived from the worst of head or chest rating. If head and chest rating is good and one or
more leg rating is poor then the combined injury rating is down graded to
acceptable, otherwise the leg rating does not have any influence on combined
injury rating
5. STAR RATING SYSTEM
In response
to consumer demand for a simple, non technical presentation of the results,
NHTSA ( National Highway Traffic Safety Administration) introduced the star
rating system for the crash test, based on the risk of the driver or front
receiving a serious or life threatening injury in a full frontal crash at 56
km/ hr. A short coming of this rating
system is that it does not account structural performance or occupant restraint
performance. It is possible for a test
to result in good head and chest injury measurements even though the structure
performs badly. In these case the risk
to life in a slightly different crash configuration, or slightly higher speed,
could result in a much higher risk of serious injury. Depending up on injury criteria, NHTSA finds
the percentage of life threatening injury that may occur to a occupant and
decide the star rating.
Full Frontal Crash Test
Rating Categories :
Chance of life threatening
injury
Less than
10%
* * * * *
10-19% * * * *
20-34%
* * *
35-45%
* *
more than 45% *
Side Impact Crash Test
Rating Categories :
Less than
6%
* * * * *
6-10%
* * * *
11-20%
* * *
21-25%
* *
more than 25% *
This rating
system is used in Euro-NCAP (European new car assessment program).
6. CLASSIFICATION OF CRASH TEST
Crash test
can be classified in two ways
Depending Upon Test
Procedure Adopted :
1. Moving Barrier Collision Test :
In this
test, the moving barrier shall impact the test vehicle while moving at
essentially a constant velocity. The
test vehicle is stationary with it’s parking brake of and transmission is
neutral. Side impact and rear impact adopt
moving barrier methodology
2. Barrier Collision Test :
In this
test, the test vehicle is made to impact on a fixed barrier made of concrete
and sometimes of aluminum honeycomb at the front surface of barrier to form a
deformable barrier. Full frontal and
offset crash test adopt barrier collision methodology
3. Car to Car Test
In this
test, two cars of same model, are made to move with constant velocity and
undergo a crash , Car to car test is carried out in order to assess the
difference between the barrier test and real world situation.
7. METHODLOGY
Crash test
results in high energy transfer. This
energy is absorbed by the crumble zones provided and less amount of energy or
force reaches upto the occupant. Crash test carried out within 30 seconds but
period for the preparation of crash test facility is large.
With the
help of high speed cameras, the entire crash event is captured and later on,
deep analysis is carried out.
Crash Test Facility :
Minimum Requirements :
Ø The test
site should encompass sufficient area to provide accommodations for the
barrier, location of various photographic equipment, a protected observer area
and facility to accelerate test vehicle to desired speed at impact.
Ø Approach
road of sufficient length should be available
Ø Immediate
crash site must be level
Ø A pit must
be installed in front of barrier to accommodate under vehicle photography.
Ø Allowances
should be made for after impact skidding
of both vehicle and moving barrier
Ø Crash
dummies should be available.
Crash Test Set Up :
Crash test
facility includes following essential items;
Rigid Barrier Surface :
It is
necessary to have a solid object into which cars can be rammed. The surface
must be very solid so that all the energy of collision is absorbed by impacting
vehicle, the barrier must not deform. Generally a heavy block of concrete is
used which is poured deep into the ground to provide an extremely solid object,
Deformable Barrier Surface :
In order to replicate crash characteristics of
another vehicle, a deformable barrier is some times attached to the front of
rigid barrier or a moving trolley, for side impact test. These are made of
aluminum honey comb and can be used only once. The stiffness of honeycomb is
arranged to duplicate the structures in a typical vehicle. This provide a more
realistic impact for certain type of test with a structure that is repeatable
for scientific comparisons and less expensive than using car to car testing.
Tow Systems :
In order to
guide a vehicle into a barrier surface at the precise speed and positions the
vehicle on a proper track, a towing system is used. This consist of very large electrical motor
driving a continuous cable which runs along the length of test facility. A clamping system is fixed to the cable and
attaches to a towing chain which in turn
is attached to the underside of the vehicle.
The clamp is designed so that the tow is released when the vehicle is
about one meter from the barrier and the tow system does not influence the test
results.
Lighting and Photography
Facility :
In order to
record the sequence of events in crash tests, high speed cameras are
use(capable of 1000frames/sec). These
allows detailed analysis of the the test and vehicle performance. High
intensity lighting is used to provide necessary light for the high speed
cameras. The vehicle is generally coated
with a mat finish to avoid glare and reflection on the film.
Dummy Calibration Facilities
:
Before each
test dummies must be calibrated. They must be kept at specified temp and are calibrated
by impacting with a range of suspended pendulums.
Data Acquit ion System :
Each test
is provided by a large amount of data These include accelerations and forces in
various parts of each dummy and in different parts of test vehicle. About 200 millisecond of digital data is generally recorded from a
test at a ate of upto 20 kHz. The data acquisition system is mounted in the
test vehicle and is connected to the test facility large umbilical cable to
allow rapid downloading and verification of results.
Vehicle Preparation :
Before the
vehicle is crash tested it must be prepared for exact specifications so that
the results of scientific value. The process include
Ø Seats and
steering column positioned to specification
Ø Dummies
positioned to specification
Ø Vehicle
mass and balance adjusted to specification
Ø On board
cameras are installed
Ø All fluids
drained and replaced with colored non inflammable liquids
Ø Data
acquisition system secured, connected and tested
Ø Emergency
backing system is installed
8. CRASH DUMMIES
In order to
provide a picture of likely injuries in
crash each test relies on having a driver
and passenger abroad. No ordinary
driver and passenger, these are steel-skeletoned, rubber skinned dummies packed
inside with sophisticated sensing equipments
Hybrid III
is a dummy used for frontal impact test and Euroside is a dummy used for side
impact. eurosid instrumentation is different from Hybrid III except head, as
Eurosid is designed to record accelerations and forces from side during test. Inside each dummy is a steel skeleton which
represents parts of human bone structure.
In test the dummi’s rubber flesh is clothed in order to reduce friction. Some sensing devices are wired upto computer
recording equipment which is carried in rear of the car during the impact. Occasionally dummy will emerge from a crash
with cuts but more extensive damage is rare. They are designed not to break
since each dummy cost$100000. After every few tests dummies are rectified.
Head :
The dummy’s
head is made of aluminum and covered in a rubber flesh. We will find
accelerometers set at right angles, each providing information about the forces
and accelerations to which brain would be subjected in a crash.
Neck :
Features a
varity of measuring devices detect the bending, shearing and tension
forces on the neck as the head is thrown
towards and backwards during the impact.
Arms :
Neither arm
carries any instrumentation. In a crash test arms flail
around in a uncontrolled way, and although serious injuries are uncommon, it is
difficult to provide worthwhile protection against them.
Chest (front impact) :
The Hybrid
III dummy’s steel ribs are fitted with sensing equipments that records the
deflection of the rib cage in the front impact.
It is important that the loading on the chest area from the seat belt is
not too high.
Chest (side impact) :
The side
impact dummy, Eurosid has completely different chest area, with just three ribs
which are instrumented to record compression of the chest and velocity of the
compression
Abdomen :
Eurosid the
side impact dummy is equipped with sensors to record penetrating forces into
the abdomen area
Pelvis :
The Eurosid
has instruments fitted in pelvic girdle.
They record lateral loads on the pelvis that may cause fractures or hip
joint dislocations
Upper Leg :
In Hybrid
III, this area is made up of pelvis, femer and knee. Load cells in the femer provide information
in frontal impact tests on likely injury to all sections, including the hip joint which can suffer from fractures
and dislocations. To detect the forces
actually go through the knee joint if the impact is just below the knee,
instrumentation called ‘knee slider’is used to measure the forces transmitted
through the dummy’s knee joint.
Lower Leg :
Instruments
fitted inside the dummy’s leg measure bending, shear, compression and tension
at the top and bottom of lower leg, allowing the risk of injury to the tibia
and fibula to be assessed.
9. TEST PROCEDURE
Car to car test have been conducted by various
agencies to evaluate the crashworthiness of the vehicles. Agency should use same model for series of
tests. But the same model may not be present during the test, and so it becomes
difficult to compare the results of two models.
These car to car tests are more realistic but result in huge amount of
money so barrier test have been designed. The deformation resulting from
barrier collision is more sever than that produced by crushable vehicle. but
barrier is more readily reproducible than cars.
Same type of barrier can be produced by different companies so it is
easy to compare crashworthiness of two vehicles. Thus so as to establish collision tests,
barrier test have been designed.
Barrier :
A barrier suitable for impact testing of passenger
car should have following characteristics-
Ø Barrier
face should be at least 3m wide and 1. 5m high, but shall be large enough to
accommodate the entire frontal area of the vehicle
Ø Barrier
face should be normal to the final approach path.
Ø Effective
mass of the barrier can be achieved with reinforced concrete and compacted
filled
Instrumentation and
Equipments :
Adequate
means be provided to observe and record test results. it is essential that the recording system,
including transducers and mounting systems contain no resonant frequencies
within the freq response range of data
interest. It is desirable to record data
on magnetic tapes so that it can be readily filtered and computer processed.
Vehicle Acceleration
Measurement :
It is
measured by accelerometers located on floor pan, frame, body sill, body
components. These should be mounted in
areas of localized resonant vibrations or distortions such as seat belt
anchorages. For non perpendicular collision
approach angles, accelerometers on both sides of the car are recommended, as
well as multiple installations for purpose of back up are used.
Occupant loading :
Transducers
are fitted with the dummy to record load exerted on occupant. Conducting surfaces are installed on head
, chest and knee of dummy, so that time
history of their contact with instrument panel and steering wheel may be
recorded with respect to vehicle impact time.
Photographic Instrumentation
:
It is
desirable to provide comprehensive phographic coverage of each barrier crash
test. However in case, when this is not possible, following represent the
minimum coverage for meaningful information.
Broadside Cameras :
At least
one high speed camera should be located on each side of crash site. Locating
areas for precise positioning of cameras should be provided. The cameras should
be positioned so that the field of view is very large enough to include only
the test vehicle and is perpendicular to the path of that vehicle at the
instant of barrier contact. It should facilitate accurate micro motion analysis
of the film. The information include
total vehicle displacement, velocity, deceleration
Overhead Cameras :
Cameras may
also be placed directly over the crash site
Underneath Cameras :
It photographs the chassis and
component changes that can be visible only from beneath the vehicle.
Passenger Compartment
Cameras :
A suitable
high ‘g’ camera may be installed to view the passenger compartment to record
kinematics of front seat occupant.
In barrier collision test, car is brought on car track
and necessary arrangement is done. Car
is attached with towing system. Electric motor helps the car to attain constant
speed and made to crash with the fixed barrier.
Entire crash event is captured by
cameras and various readings are recorded by the computer. Results of crash
test is used to design new occupant restraint system.
10. OUTCOMES OF CRASH TEST
The most important outcome of crash test is the
design of new occupant system. following are some occupant restraint systems
which have bleb designed from 1950,
1. Belt Tensioners :
In case of frontal impact, the belt tensioners tighten
the belt of frontal seat occupant, pushing them deep into the seats, so that
upper body displacement is reduced drastically.
2. Wedge Pin Door Locks :
These prevents the car doors from opening during an
accident as this would pose a greater risk to the occupant. On the other hand it also allows easy opening
of doors after a serious collision.
3. Bumpers :
Front and rear sections are protected by bumpers with
integrated impact absorber made of shape
regenerating polypropylene. This prevent
damage to the body.
4. Air Bags :
Large air bags unfold within fractions of a second during
a major accident. Thus preventing head and chest from coming in contact with
steering wheel and instrument panel.
5. Crumple Zone :
These are zones that are provided with chassis structure
so that these zones absorb maximum energy during impact and less energy is
reached up to occupant.
6. Sandwich Floor Concept :
Larger the
car, more body to absorb impact energy
and smaller the car greater the risk of injury. So cars with short
bonnet do not have room for crumple zone deformation in an accident and there
is also potential danger that the engine could be pushed back into passenger
compartment. This resulted in a sandwich
floor concept. Here the engine and gear
box are moved at an angle partly in front of passenger cell, and partly
beneath it. These are designed to slide
away along an angled bulk head beneath the passenger compartment in a frontal
crash without harming occupants.
11. CASE STUDY
Crush test
is considered as secret test by the car manufacturing company. So, the test
result are not disclosed. As a result, various organization such as
international testing, Swedish national Road administration, FIA (Federation
International Automobile) Department of transport are cooperating together to
establish Euro-NCAP (European New Car assessment Programme). This Euro-NCAP
carries out the crash testing of various car models and discloses the results
for the sake of customer.
The Nissan
Micra was awarded two stars for protection in the frontal- and the side-impact
tests. However, with just a little improvement in performance, the car would
have been moved up into the three-star category. In the frontal-impact crash
test, the Micra failed to meet the new criteria for the left knee impact and
for protection of the right lower leg. Under side-impact conditions, it failed
to meet the abdomen requirements. On the other hand, the car did meet the requirements
that relate to the degree of steering wheel displacement. In the frontal-impact
crash test, the Nissan Micra's major problems related to intrusion,
particularly at knee and footwell level, although the passenger compartment did
remain stable. Improvements in safety performance are also needed in the
knee-impact area. For the side impact, improved protection is required for the
abdomen while care is taken not to transmit too much loading to the chest or
pelvis.
Frontal Impact :
In the
frontal impact, the Micra suffered moderate structural deformation, and the
passenger compartment maintained its stability. There was good control of
steering wheel intrusion – the wheel moved back into the cabin by 60mm – but
the test results showed there to be an excessive intrusion of the footwell. The
driver's door failed to transmit loads effectively, allowing a moderate
collapse of the door aperture and also intrusion of the facia. After the test,
the driver's door could not be opened – even with extreme hand force – and
tools had to be used. The passenger door opened normally.
The driver's head protection was good and the head's contact on the airbag was stable. Neck protection was also found to be good. Seat belt loading of the chest was measured as adequate but this was down rated to a score of marginal because of the intrusion of the facia. The left knee impacted on the steering column cover, bent the column adjuster and then hit the rigid steering column and its mounting bracket. The dummy's right knee hit the car's facia and pushed it on to a tube supporting the steering column, distorting a bracket which was mounted to it. For both knees, there were stiff structures which could concentrate loads on part of the knee and further penetration into the facia would have resulted in sharply increased loads. The right knee would also have received greater loading if it had impacted the facia in a slightly different position horizontally. The left knee protection was poor on the basis of dummy instrumentation and could not be down rated, but the right knee was down rated to weak to account for these points. The excessive footwell intrusion led to poor protection ratings for the right lower leg and for feet and ankles. Protection of the left lower leg was rated as marginal.
The driver's head protection was good and the head's contact on the airbag was stable. Neck protection was also found to be good. Seat belt loading of the chest was measured as adequate but this was down rated to a score of marginal because of the intrusion of the facia. The left knee impacted on the steering column cover, bent the column adjuster and then hit the rigid steering column and its mounting bracket. The dummy's right knee hit the car's facia and pushed it on to a tube supporting the steering column, distorting a bracket which was mounted to it. For both knees, there were stiff structures which could concentrate loads on part of the knee and further penetration into the facia would have resulted in sharply increased loads. The right knee would also have received greater loading if it had impacted the facia in a slightly different position horizontally. The left knee protection was poor on the basis of dummy instrumentation and could not be down rated, but the right knee was down rated to weak to account for these points. The excessive footwell intrusion led to poor protection ratings for the right lower leg and for feet and ankles. Protection of the left lower leg was rated as marginal.
On the front passenger side of the car, the Micra's
protection of the head, neck, upper and lower legs and feet was good. Seat belt
loading resulted in the chest protection being adequate.
| Good |
|
Marginal
Marginal
Poor |
Side Impact :
Protection
from injury in the abdomen area was poor under side-impact conditions because
of excessive loads which were put on the body. The degree of head protection
afforded by the Nissan Micra was found to be good, while the protection offered
to the chest and pelvis areas was rated as adequate.
Child Restraint :
There was a
warning label on the Micra which advised against the use of a rearward-facing
child restraint in the front seat, even though the car was not fitted with a
passenger airbag. A forward-facing Romer King child seat was fitted, as
recommended by Nissan for use in the rear seat of the car. The forward movement
of the child seat during the frontal impact test was poorly controlled and
there was found to be insufficient restraint of the child's upper body,
allowing a large forward movement of the head. During the side-impact tests,
the lateral movement of the child restraint was found to be poor, with the
upper part of the restraint moving nearly to the mid line of the car. Under
these conditions he child's head then moved well beyond the sides of the child
restraint.
Pedestrian Protection :
Child Head Impact :
Three of
the six test points gave better-than-average protection. Poorer areas on the
bonnet were above the battery, a metal bracket on the air intake and a
suspension turret.
Upper Leg Impact :
Upper Leg Impact :
Two test
points gave better-than-average protection. Poorer protection was provided at
the location of the bonnet latch.
Adult Head Impact :
One out of
three test points gave better-than-average protection. The two poorer areas
were on the scuttle panel ahead of the windscreen and on the bonnet above the
hinge.
Leg Impact :
Leg Impact :
Two of the
test points provided protection better than that required for proposed
legislation. These were at the centre of the bumper and in line with the inside
edge of the headlight. The third test point which gave worse-than-average
protection was on the bumper in line with the towing eye.
12. CONCLUSION
Crash
test have resulted in cars with more safer system than in the past. Now, we can
say that, nowadays cars are manufactured with a view point of safety rather
than outlook. But, still the possibility of injury in accident depends upon the
driver i.e. in which mode the driver (i.e. safer mode or in unsafe mode),
drives the car.
13. REFERENCES
1. “SAE Handbook”, 1982
2. Internet Site-
a) www.crashtest.com
b) www.fia.com
c) www.nishanmicra.com
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