1

CHAPTER 1

INTRODUCTION

1.1 WHIPLASH

In rear crashes the occupant will suffer a change in velocity through seat back, which

pushes the person forward. During this impulse the occupant’s head, held by neck

without being pushed in the impact exerts on the neck a whiplash effect that causes

neck injuries.

1.2 IMPORTANCE OF WHIPLASH STUDIES

In today’s world many safety features are available but still the risk of deaths and

injuries and particularly whiplash injuries as a result of road traffic accident which is now

acknowledged to be a global phenomenon, is high. All countries of the world concern

about the growth in the number of people killed and seriously injured on their roads

(World Health Organization 2009). Whiplash injuries have different causations which the

main critical factors are road and vehicle conditions, infrastructure aspects, driving

decisions, pedestrian behavior and impacts. Road traffic accident research has played a

key role in understanding the main causations of car accidents and would help to

improve the safety factors in order to decrease the number of accidents and possibly

reduce the whiplash injuries.

1.3 PROJECT METHODOLOGY

The objective of this project is to present an analytical and efficient approach to assist

engineers in analyzing the design parameters of the seat and head restraint system.

Also the goal is to minimize the injury criterion - neck forces, head restraint contact time,

and NIC values. The CAE simulation model with BioRID II dummy were simulated for

rear crash pulses using LS Dyna software to correlate to rear impact sled tests

according to Euro-NCAP whiplash protocol. The correlated model was adopted in

Design of Experiments (DOE) studies using HyperStudy software to explore all the

significant design parameters influencing occupant neck injuries.

2

1.4 DIFFERENT PHASES OF PROJECT

CHAPTER 3

Figure 1.1: Project Phases

Understanding of whiplash injury

Study of ENCAP Whiplash Protocol

Study of the CAE model using LS Dyna

Simulation of the CAE baseline correlation model

DOE basics study

Design variables and its ranges

DOE-L18 array using Hyperstudy with 6 design variables

Analyzing the critical parameters and its effects on various injury criterion

DOE-L27 array using Hyperstudy with 4 design variables

Optimizing the design using the DOE results

3

1.5 MS PROJECT PLAN

Figure 1.2: MS Project Plan

4

CHAPTER 2

LITERATURE SURVEY

2.1 REAR CRASH

Rear-end collision is a traffic accident wherein a vehicle crashes into the vehicle in front

of it. Common factors that contribute to rear-end collisions include by driver inattention

or distraction, tailgating, panic stops, and reduced traction due to weather or worn

pavement. Typical scenarios for rear-ends are sudden deceleration by the first car so

that the following car does not have the time to brake and collides with the first. A typical

medical consequence of rear-end crashes, even in case of collisions at moderate

speed, is whiplash. The rearmost passengers in minivans benefit little from the short

rear crumple zone, so they are more likely to be injured or killed in a rear-end collision.

2.2 STUDIES ON WHIPLASH

Michael Yuen et al performed development of an anti-whiplash seat which mainly

involved head restraint geometry modification. The head restraint on Prototype 1 was

positioned too close to the dummy head for the concept seat configurations with an

initial backset of 10mm. The results were minimized- accelerations and NIC.

Consequently, the head restraint mounting plates were redesigned to allow for a more

realistic head restraint position with an initial backset of 40mm for the concept seat. [3]

A research was carried out by Ravindra Jain regarding Head Restraints Backset field

measurements and Indian Regulation. Considering the distance between head C.G and

occipital condyles for an average human being as 55mm and limit angle value for neck

extension as 73deg, the total head movement towards rear(posterior) for just starting

hyperextension is approximately 70mm. This means 70mm is the limit value for

rearward movement of head to be safe. If within 70mm of rearward movement of head

if head restraint comes into contact with the head, it can be assumed that there will be

no neck injury. Therefore for further analysis backset was taken as 70mm. [4]

5

Rami Mansour and Douglas Romilly designed an energy absorbing foam to the seat

base to mitigate whiplash injuries in rear end collisions. Based on the selected foams,

the value of peak NIC was reduced for low, high, and severe crash scenarios. As NIC

quantifies the relative accelerations and velocities between the C1 and T1 vertebra,

these results indicate an overall trend toward injury reduction with the use of the

modified seat with energy-absorbing foam. Head restraint contact forces were reduced

with the use of energy-absorbing foam in the seat base, which indicates that the

dynamic loading of the head has been reduced. Seat back angle was also reduced with

the use of energy-absorbing foam which suggests that the probability of occupant

ejection should be reduced with application of the modified seat with energy-absorbing

foam. [5]

Koji Sano et al developed a whiplash injury reducing seat system using BIORID-II

dummy. The major modifications done are :

(1) To locate the head restraint as close to the head horizontally as possible to make

head acceleration rise earlier, while keeping comfort of a passenger.

(2) To soften the seat back cushion around the contacting height with T8 to make

T1’s acceleration rise gently.

(3) To remove hard structures inside the seat back that may cause bottoming when the

dummy’s back fully compresses the cushion in order both to avoid folding of the seat

back and to control the peak level of T1’s acceleration until head to head restraint

contact. [6]

2.3 DOE STUDIES FOR WHIPLASH

T. C. Weng and Yan Fu of Ford Motor Company conducted DOE to study the effect of

significant design parameters influencing occupant neck injuries for 10mph rear impact

tests using Madymo and Modefrontier software [1]. They found out that time for head

restraint contact can be improved by reduced backset and increased seat recliner

stiffness. They also increased the head restraint cushion stiffness and positioned the

head restraint higher than their baseline model to improve the injury criterion. In order to

6

meet overall ‘GOOD’ rating from the IIHS rear impact test the head restraint was

designed first which meets the static measurement criteria.

Based on the DOE study with the power seat track, the following factors were observed

which could improve the neck injury protection:

Time to head restraint contact:

• reduced backset

• reduced seat recliner rotation and head restraint rotation

• softer lower section of the seatback

T1 X-acceleration:

• softer upper section of the seatback

• combination of increased seat track sliding, reduced seat track rotation,

and reduced head restraint rotation

Neck shear force

• reduced backset

• reduced seat recliner rotation and head restraint rotation

• reduced upper neck rotation and lower neck rotation

• stiffer head restraint cushion

Neck tension force

• reduced height

• optimum head restraint contact surface (shape)

• reduced upper neck rotation and lower neck rotation with stiffer head

restraint cushion and reduced head restraint rotation.

Sung Chul Choi et al did design of experiments to understand design parameters of a

seat and active headrest(AHR) performance in relation to the rear-end collisions. They

also examined how the dummy position (including the height) affects the injury indexes.

The structural optimization is performed to obtain the optimal seat with AHR (Active

Head Rest) by using the TNO BioRID -II dummy and MADYMO software. They found

out that the headrest fore/after position and the upper/ lower position of the headrest as

the geometry parameter had the greatest effect on the output. The best combination set

of variables were found out which is capable of lessening the injury indexes. [2]

7

2.4 WHIPLASH AND ASSOCIATED DISORDERS (WAD)

Whiplash refers to the neck injuries caused by sudden neck distortion which mostly

occurs in cities due to low speed rear end collisions. In rear crashes the occupant will

suffer a change in velocity through seatback, which pushes the person forward. During

this impulse the occupant’s head, held by neck without being pushed in the impact

exerts on the neck a whiplash effect that causes neck injuries. The most common

symptom whiplash victims report is pain due to mild muscle strain or minor tearing of

soft tissue. Other injuries include nerve damage, disc damage, and in the most severe

cases, ruptures of ligaments in the neck and fractures of the cervical vertebrae.

Figure 2.1: whiplash mechanism

Generally, minor whiplash injuries are associated with pain and decreased range of

motion in the head and neck. These symptoms usually last only a short time, but

occasionally they last longer and include headaches, dizziness, and tingling in the arms.

People experiencing whiplash injuries report symptoms that last from a few hours to

several years with the vast majority experiencing short-term symptoms of pain.

The physical injury to create symptoms of whiplash is uncertain. It is suspected that the

biological cause of long-term whiplash symptoms is nerve damage while short-term pain

may be a minor strain or sprain.

8

Figure 2.2: neck injury due to poor restraint system

People can experience severe crashes with no neck injury if there is little or no

movement of the head relative to the torso. Consequently, neck distortion resulting from

sudden movement of the head relative to the torso probably explains most whiplash

injuries. Hyperextension of the neck, or distortion beyond its normal range of motion,

may explain many whiplash injuries, but experimental and field studies suggest that

nerve damage and its associated long-term symptoms can occur with milder levels of

neck distortion. One hypothesis that explains these nerve injuries is based on damage

to the nerves in the joints caused by motion of adjacent neck vertebrae during a crash.

Another hypothesis suggests that the nerve damage is caused by fluctuation in spinal

fluid pressure arising from neck distortions.

2.5 ANATOMY: CERVICAL VERTEBRAE

Cervical spine is divided into two functional regions: the upper (C0-C2) and lower (C3-

C7). By another meaning, the cervical spine is composed of seven vertebrae, including

three atypical and four typical vertebrae. The typical cervical vertebrae, (C3-C6) are

composed of a vertebral body and a vertebral arch and muscular attachment [17].

Different parts of the cervical spine are: Intervertebral Discs, Ligaments, Cervical Spinal

Canal, Spinal Cord, Nerve Roots, Vertebral Arteries, Muscle of the Cervical Spine.

9

The anatomy of the cervical spine is complex and allows for a high degree of motion

while serving as a protective conduit for the spinal cord and nerve roots. [17]

Figure 2.3: anatomy of the cervical spine [17]

10

CHAPTER 3

REAR CRASH ACCIDENT STUDIES

3.1 WHIPLASH INJURY CAUSATIONS

There are some crash causations which are the major causalities of whiplash injury

which could be divided into the road and vehicle condition, infrastructure, pedestrian

and driving behavior, impact with other vehicle and braking hard. Studies of the road

safety have shown that human error is the main cause in 57% of all accidents and is a

contributing factor in over 90%. In contrast, only 2.4% is due to mechanical fault and

only 4.7% is caused only by environmental factors [16]. The majority of car accidents

seem to be caused by bad driving behavior: driver inattention, failure to merge,

speeding, racing, aggressive driving and failure to exercise care in passing.

Accidents can be attributed to specific causes apart from poor driving behavior itself

include: falling asleep; weather usually (Snow, Ice or Rain and fog); alcohol, drugs and

drunk driving; driver distractions from in-vehicle sources including mobile phones,

insects in the car, playing music, conversation with a passenger; collisions with other

vehicles and/ or animals on the road, braking hard usually due to unexpected barrier/

object/ car and/ or bad pedestrian behavior on the road, not maintained vehicle, vehicle

malfunction and infrastructure/ road condition normally with providing poor vision

system such as inappropriate lighting system at night and poor informative and visible

traffic signals and signs. [16]

3.2 IMPACT DIRECTION

Whiplash injuries occur in all impact directions such as rear- end, head- on and side

collision. Approximately 50% of the whiplash injuries occur in rear crashes, 30% in

frontal crashes and the rest in other types of accidents. Rear impact has been found to

be significantly less injurious than other impact directions in terms of long term outcome

for male. Females showed no consistent trend in this respect but in some cases their

average disability is non-significantly higher in rear impacts. [19]

11

3.3 VEHICLE FACTORS

3.3.1 Mass

Vehicle mass is one of the effective factor which has been investigated that there is a

strong link between vehicle mass and whiplash injury risks, with a fivefold difference in

risk factor between the best (large mass) and the worst (low mass) cars. However, there

are large differences between vehicles of similar mass, which they have difference in

vehicle structure and the seats [19]. The likelihood of whiplash injury claim are more for

heavier vehicles than the vehicle which has approximately equal or lesser mass.

However, drivers of rear-struck cars are more likely to claim a neck injury than drivers of

rear-struck SUVs. [18]

3.3.2 Speed

Impact speed is another most effective factor. The mass and speed of the involved

vehicles and/ or the change in velocity of the target vehicle are the primarily

determinants of delta V. The forces acting on an occupant are more a function of delta

V than the speed or mass of the vehicles. If the target vehicle is moving in the same

direction as the bullet vehicle prior to a rear-end collision, the difference in their speeds

will determine the effective impact velocity. The risk of injuries in rear-end impact tends

to increase with delta V. [17]

3.3.3 Seat Belt

Seatbelts are estimated to reduce the overall risk for serious injuries in crashes by 60-

70% and the risk for fatalities by about 45 percent. According to the studies of real

frontal crashes, wearing a seat belt increases the risk of whiplash injuries. But it is hard

to find clear evidence that wearing a seat belt in low severity rear end crashes increases

the risk of neck injuries. [19]

3.4 HEAD RESTRAINT SYSTEMS

Head restraint is an essential safety feature which prevents the excessive head and

neck kinematics and displacement during rear-end collisions. There are three common

systems in order to prevent/reduce whiplash which are becoming more common on

vehicles.

12

3.4.1 Active Head Restraint

An active head restraint is an adjustable device like standard head restraint and

includes the pressure plate and a pivot system in the seat back. It has other features

which provide comfort for occupant, such as adjustable shoulder and lumber support.

During rear-end impact, active head restraint optimize the anti-whiplash performance of

seat which the front section of the head restraint moves forward and upward to reduce

the distance between the head and the head restraint and a longer period of support for

the head. [20]

Figure 3.1: Active Head Restraint

3.4.2 WHIPS (Whiplash Protection System)

WHIPS seats have a fixed and integral head restraint, however, the entire seat back

has been designed in order to provide good geometry and protection for occupant from

whiplash injuries. During a rear impact, the seat moves backward and becomes reclined

and an expandable hinge at the base of the seat back which is designed to be used

once and should be replaced following the accident, controls its movement, so it keeps

the movement of the head and body together and increases the length of time that the

occupant is in contact with the head restraint. [20]

Figure 3.2: Whiplash protection system

13

3.4.3 SAHR (Saab’s Active Head Restraint)

According to the study, SAHR provided a 43% reduction in neck injury claims, 55%

reduction in claim rates for women and 31% reduction in claim rates for men. SAHR has

lumbar pad in the seat back which when is contacted by occupant in rear end impact,

causes a lever to move the head restraint forwards and upwards to support the

occupant’s head and neck. [21]

Figure 3.3: SAHR

3.4.4 Effective Head Restraints

Effective head restraints help to move an occupant’s head forward with the body in a

rear- end crash and decrease the likelihood of sustaining a whiplash injury. An effective

head restraint should have a good geometry which poses behind and close to the back

of an occupant's head to prevent a whiplash injury in a rear- end collision.

A restraint should be at least as high as the head's center of gravity or about 9 cm

below the top of the head. The backset or smallest horizontal distance between head

and restraint which must be 5.58 cm has been found that to have no effect on initial

disability for long term outcomes. Backsets should be as small as possible, due to the

distance of more than 100 mm have been associated with increased symptoms of neck

injury in crashes. [18]

14

The restraints are measured with the angle of the torso at about 25 degrees which

shows a typical seatback angle. Each restraint is classified according to its height and

backset into one of four geometric zones: good, acceptable, marginal, or poor. [18]

Figure3.4: Restraint performance

The head/ neck restraint should be high enough to prevent the head of the occupant

from acceleration over the restraint which a minimum height of the adjustable head

restraint in their lowest position must be 29.5 inches from an occupant's hip to the top of

a head restraint. Head restraints will not be required in rear seats, but if they are

voluntarily installed they must meet a height requirement. Fixed restraints in rear seats

must be at least 29.5 inches from an occupant's hip, and adjustable restraints cannot be

adjusted below 29.5 inches. There will not be a backset requirement for head restraints

installed in rear seats. [18]

A restraint system should have enough adjustability to accommodate a broad range of

occupant sizes and should lock in place. Adjustable head-neck restraints have been

found to be significantly better than either fixed restraints or no restraints for males in

rear impacts. However, studies indicate that fixed restraint is more effective for females.

[19]

A head restraint should be close enough to the back of the head and neck in order to

prevent extreme hyperextension of the neck and minimize the relative motion between

15

the head and torso. In addition, a head restraint should have a neck counter to enable

support of the cervical lordosis. Head restraints alone are not enough to prevent all

whiplash injuries. [19]

Figure 3.5: Adjustment of Head Restraints

Accident studies show that modern and strong seat without a head restraint and

eliminating elastic rebound from the seat without restraint has the highest risk of neck

injury. At lower severity rear- end crashes, some factors like the head restraint and seat

back geometry and cushion properties and at higher severity rear end crashes, the seat

force-deflection characteristics are more important. [19]

3.5 SEAT AND SEATING POSITION

Elasticity of seat back has been considered that collapse of seat back in a rear impact

has a beneficial effect on neck injury outcome which reduces the head rotation and

neck load. However, there is a critical correlation between disability and seat back angle

and seat back height. It has been shown that seatback with the better energy absorption

characteristics could provide higher level of protection for out of position occupants in

16

rear end crashes. However, stiffer seatbacks tend to increase loadings on the cervical,

thoracic and lumbar spine in rear- end impact. It is uncertain which seating position

exposes an occupant to the greatest chance of neck injury. Studies have demonstrated

that drivers have a higher risk of injury than passengers, due to they are prone to move

forward and away from the seatback as they reach for the steering wheel and observe

traffic around them, whereas passengers usually are more relaxed and lean further

back in their seats, with their heads closer to the restraint, however it has been found

that moving to the rear seat is nearly twice as effective as tilting a head restraint to the

front seat in reducing whiplash injury risk. [19].

3.6 HUMAN FACTORS

3.6.1 Gender

Females are 1.8 to 2.2 times more at risk of whiplash injury in all types of collisions than

men. A Swedish study found that females with whiplash injuries are more at risk of

developing long-term symptoms, due to they are more susceptible than male to neck

injuries. Fifty-five percent of females who sustained whiplash injuries are susceptible to

develop long-term symptoms compared with 38 percent of males. It has been shown

that the risk of disability for female drivers is three times higher than for males.

However, risk of injury for females in rear seats is four times higher than males due to

males have stronger neck muscles than females with approximately the same size of

head. [18]

3.6.2 Height

Height is one of most important risk factor particularly among females for neck injury.

Shorter people often are protected by unadjusted head restraints. Stature may not play

as significant role in the severity of injuries among males because many head restraints

are too low to protect even shorter males [18]. However, taller occupants who do not

adjust their head restraints are more likely to sustain whiplash injuries. [19]

17

3.6.3 Age

Children and old people are more at risk of whiplash injury.

3.6.4 Symptoms

Patients normally develop symptoms within 24 hours after a whiplash accident. The

symptoms of whiplash are dominated by pain in the neck and headache. The second

most common symptom is pain in the shoulder griddle, followed by weakness in the

upper limbs. In addition, dizziness, visual disturbances, concentration, cognition and

memory disturbances and tinnitus are less common and irregular symptoms. However,

approximately 10% of all whiplash injuries become long-term injuries in rear crashes

and about 5% in frontal crashes [17].

18

CHAPTER 4

WHIPLASH BIOMECHANICS

4.1 WHIPLASH INJURY MECHANISMS

Whiplash associated disorders are not considered life threatening, however they are

associated with long term consequences that cause human suffering and cost to

society. [22] Hypotheses for whiplash related neck injury mechanisms include the spinal

fluid pressure theory [10] and the facet joint damage theory [11]. While the actual cause

of whiplash injury is uncertain, researchers agree that the injury is likely to occur during

the initial phase of extension as opposed to the rebound phase.

Head and neck motion during extension in a rear impact can be categorized into four

main phases:

1. Initial position,

2. S-shape,

3. Extension

4. Hyper-extension.

Figure 4.1: Phases of head and neck motion during extension [23]

Phase 1 is the pre-impact initial position. Phase 2 is the s-shape phase where the head

translates with the lower neck in extension and the upper neck is flexion. Phase 3 is the

extension phase where the upper neck has changed from flexion to extension. Phase 4

is the hyper-extension phase that results if no head restraint contact is made.

19

During a rear impact the torso is accelerated forward by the seat back while the head

lags behind due to inertia. The head begins to translate with the lower neck in extension

and the upper neck in flexion. This phase is commonly described as the s-shape. Once

the translation is complete, the head begins to rotate back in extension. The head will

continue to rotate and reach a hyper-extension phase if no head restraint contact

occurs.

One hypothesis, suggested by Aldman et al, Chalmers University [10], predicts that the

volume changes inside the spinal canal may induce injurious mechanical loads to the

tissues of the intervertebral foramina. This is suggested as occurring mainly at the

beginning of the occupant motion in a rear end impact. This theory has been used to

propose a criterion, NIC (Neck Injury Criterion) as a measure for this specific injury

mechanism. [23]

Ono et al [24] performed a series of sled tests, using volunteers, in recording cervical

vertebrae motion by cineradiography, and suggesting the Facet Joint Injury Mechanism.

Due to initial torso straightening (ramp-up motion) the lower cervical vertebral segments

are extended and rotated prior to the upper cervical vertebral segments. Thus causing

concentration of loads on the cervical spine, even in low speed rear end impacts.Ono et

al suggest this as a cause of intervertebral injury. Sweden’s Folksam insurance group

suggests that the injury mechanism lying behind neck injuries, in both frontal and rear

end impacts, could be considered as a frontal mechanism for the occupant, occurring

when the occupant is moving forward and is restrained by the seat belt. In a rear end

collision, this would occur during the rebound phase.

In a Volvo study using accident experience and a unique mathematical model with

segmented spine, measures such as forces between adjacent vertebrae were found to

be a good injury predictor for this specific model. For low injury risk, the relative interior

movements in the whole spine should be kept as low as possible.

There are several other studies which also discuss the mechanism of the injury. As long

as there is no single mechanism proven to be the only one valid, it is necessary to take

20

all possible theories into consideration. It is even possible that all mechanisms are valid,

for different persons and different impact types

Figure 4.2: Anatomy of Human Spine

The neck is a slender column which can be subjected to a variety of bending loads in

association with an axial load. But the most likely injury mode in rear crash is Tension-

Extension. The whiplash injury is result of the hyperextension of the neck, which is also

apparently in tension as the neck is stretched out over the headrest or seatback.

The cervical spine is initially placed in compression by the seatback pushing on the

thoracic spine. As the thoracic spine tends to straighten out, it pushes up on the cervical

spine and down on the lumbar spine, applying a compressive load on both spines in the

process. As a result, the ligaments and tendons holding the cervical spine together are

loosened while the vertebrae are required to transmit a large shear force from the torso

to the head. This is the mechanism causing soft tissue injuries of the cervical spine.

The injury occurs well before the head and neck go into hyperextension. Unpublished

data from cadaveric tests at Wayne State University show that there is stretch of the

facet capsules within 40 ms after the onset of the impact. In more severe impacts,

hyperextension injuries do occur. They include teardrop fractures of the anterior-

21

superior aspect of the vertebral body and separation of the disc from the anterior

vertebral endplate.

From a mechanical and structural point of view, the cervical spine is a very complex

mechanism. The human neck contains neurological, vascular and respiratory structures

as well as the cervical vertebrae and spinal cord [17]. Newton’s law can describe the

mechanism of neck injuries: a mass at rest remains at rest until acted upon by some

external force and a mass in motion remains in motion until acted upon by some

external force [25]. Rear-end collisions causes a sudden acceleration of the body which

in this situation, the head, being a mass at rest, remains at rest until acted upon by

some external force. The flexibility of the neck results in forceful hyperextension of the

neck as the body is accelerated forward. The head hits the head restraint and this

impact plus the reflex contraction of the neck muscles start the head in forward motion.

The head continues forward motion until it is acted upon by some external force such

as, contact with some stationary portion of the car and the restraining action of the soft

tissue structure which hold the head and neck on the body.

Sudden deceleration of a moving vehicle occurs in a head-on collision with another

vehicle or a stationary object. In this situation the body of the passenger is unsupported

from the front, it is thrown forward and it continues its momentum until it strikes some

stationary part of the car such as steering wheel, windshield or dashboard. The head

continues forward movement until it hits an immovable object, or until it is acted upon by

some external force, then it recoils in extension [25]. It is suspected that the biological

cause of long- term whiplash symptoms is nerve damage while short- term pain may be

minor strain and sprain.

Strain and sprain describe the Injuries of ligaments and muscles. Strain injury indicates

that the joint structures have been placed under stress or tension by sudden force, or

they have been stretched slightly beyond their usual elastic capacity. Sprain injury

indicates that the joint structures have been stretched beyond their functional capacity

and resulting in tearing of various degrees from their attachment to the bones which

22

produce a rending and forcing. The greatest amount of stress and strain on active

movement of the cervical spine in hyperextension occurs at the C4-C5 level and in

hyper flexion occurs at the C5-C6 level [17].

Figure 4.3: Vertebral Body Fracture

Studies suggest that nerve damage and long- term symptom can occur with the milder

level of neck distortion. One hypothesis is that nerve damage is caused by motion of

adjacent neck vertebrae during a crash. Another hypothesis suggests that the nerve

damage is cause by fluctuation in spinal fluid pressure arising from neck distortions. [18]

23

CHAPTER 5

EUROPEAN REGULATIONS FOR WHIPLASH

5.1 WHIPLASH TESTING PROTOCOL

The test is undertaken on a sled and uses the vehicle’s seat placed in a similar

geometric position as fitted in the test car. A BioRID dummy is used and is seated in a

standardized position restrained by a three point belt. Tests are done for low, medium

and high crash pulses. [12]

The test consists of following major activities:

1. Mounting the seat to the sled.

2. Seat belt mounting to seat.

3. Installation of H-point manikin and HRMD.

4. Measurement and recording of Reference Geometry for BioRID setup.

5. Installation of BioRID.

6. Instrumentation, Data Acquisition and Processing.

This test procedure enables the user to dynamically test a motor vehicle seat and head

restraint assembly to assess the extent to which they reflect best practice in preventing

soft tissue neck injuries. The test procedure is designed to evaluate all forward facing

front seats only. Protection in side facing or rear seats is not covered in this edition.

Whiplash, although officially classed as a minor injury, is the most commonly occurring

injury in motor vehicle crashes. Insurance data suggest 10% of all whiplash injuries are

long term and 1% of whiplash injuries having permanent impairment. Collision data

indicates that the majority of whiplash injuries, which are sustained in rear impacts,

occur at ∆Vs of 16km/h (10mph). However, insurance data also suggests that injuries

occur at higher and lower speeds. In light of this the Euro NCAP test consists of three

sled tests simulating a variety of rear crash scenarios at a variety of ∆Vs.

This test procedure features three pulses of low, medium and high severity. Real world

crash pulse recorder studies show that a variety of pulses and peak accelerations are

seen in real world crashes and form the basis for the low severity pulse. The medium

24

severity pulse was derived from Insurance Industry research featuring a number of car

to car tests. A high severity pulse is used to prevent long term injuries since these are

seen in more severe crashes. The test is undertaken on a sled and uses the vehicle’s

seat placed in a similar geometric position as fitted in the test car. A BioRID rear crash

dummy is used and is seated in a standardized position restrained by a three point belt.

5.1.1 Packaging

Euro NCAP reserves the right to refuse the sled testing of a seat where the

performance of the seat or head restraint could be influenced by the vehicle

environment or packaging. There should be no stiff structure in the vicinity of the head

restraint that could be contacted by the head in a rear impact or that could influence the

dynamic deflection of the seat back. There should be no additional support for the seat

back that is not present in the sled test set-up. The mid track seat position should allow

a seat back angle of 25 degrees adjustment in all cases. Where a bulkhead or similar

structure prevents this, the seat track shall be adjusted forward until 25 degrees is

achieved.

5.1.2 Seat Structure Reference Point

This is defined as a fixed point on the seat structure which stays in the same position

relative to the vehicle, independent of any seat adjustment. The seat structure reference

point shall be chosen such that the relationship of the seat to the vehicle floor can be

accurately reproduced on the sled.

Figure 5.1: Seat Structure Reference Point [12]

25

5.1.3 Toe Board

The toe board is defined as a simulated floor and toe pan, consisting of a horizontal

section sufficiently large to rest the dummy’s feet and connected to a section oriented

45° from the horizontal. When positioned for test, the gap between the front of the seat

and rear of the toe board shall be no more than 100mm.

Figure 5.2: Toe Board [12]

5.1.4 Heel Surface

The heel surface is defined as the horizontal plane of the toe board (i.e. sled floor or

movable footrest) on which the dummy’s heel rests. Its target position is determined

using the heel rest point location defined from the vehicle measurements, or from

information provided by the vehicle manufacturer.

5.1.5 Seat Mounting to Sled

The seat, including all of its adjustment mechanisms and hardware that normally

connects it to the vehicle floor (e.g. longitudinal adjustment rails), should be securely

fastened to the test sled platform. The attachment should be made so that the seat’s

orientation relative to the horizontal is the same as it would be in its vehicle as defined

26

by physical vehicle measurements or vehicle manufacturer data. The actual height of

the seat from the sled platform may be different from its height above the vehicle floor.

The toe board is also attached to the sled platform. The horizontal floor portion should

be mounted at the same height relative to the seat bolts/rails as the heel rest point. The

fore/aft position of the toe board should be adjustable. Seat mounts should be rigid and

non-deformable, and the seat mount interface to the seat should approximate that of the

interface to the vehicle floor.

Figure 5.3: Attachment of seat to test sled [12]

5.1.6 Initial adjustment of seat adjustment controls.

All seat controls should be set in sequence as follows:

Seat track should be in its most rearward locking position.

Seat height should be set to its lowest position.

Seat tilt should be set to the extreme of its range that puts the cushion angle

closest to zero (horizontal).

Cushion height should be set to its lowest position.

Cushion tilt should be set to the extreme of its range that puts the cushion angle

closest to zero (horizontal).

27

Lumbar support should be set to its most rearward or least prominent position.

Upper seat back, if separately adjustable from the lower portion should be

rotated fully rearward.

Cushion extension should be set to its most rearward or least extended

position.

Side bolsters should be set to the widest position.

Arm Rests should be set in the stowed position.

5.1.7 Seat Belt

A generic three point lap-shoulder seat belt equipped with an inertia reel should be used

during the test, placed in such a way that the belt, when worn by the ATD

(anthropometric test device), should lie across the torso, clavicle and pelvis, and must

always be routed above the pelvic angle gauge. For generic seat belts, where a seat is

equipped with anchorages or buckles, these may be used. The anchorages are the

points A, B and K. The tolerance on the position of the anchorage points is such that

each anchorage point should be situated at most at 50mm from corresponding points A,

B and K.

Figure 5.4: Generic seat belt anchorage mounting [12]

28

5.2 H-POINT MACHINE & DUMMY POSITIONING

5.2.1 H-point manikin installation

The lower legs of the HPM shall be adjusted to the 50th percentile leg length setting,

and the upper legs shall be adjusted to the 10th percentile leg length setting these are

the HPM settings closest to the Euro NCAP front and side impact protocol settings. The

knees should be 250mm apart. The centre line of the seat may be defined from features

such as the head restraint support tubes or seatback and seat pan side. The feet shall

be placed as far forward as possible, with the heels resting on the heel plane and the

feet positioned at 90° to the tibias. The toe pan shall be positioned sufficiently far away

so as to avoid any interaction with the feet during the HPM installation process.

5.3 HEAD RESTRAINT POSITIONS

5.3.1 Head Restraint Measurement Position Definitions

Down is defined as the lowest achievable position of an adjustable head restraint

regardless of other adjustments (e.g. tilt). The lowest position should be assessed from

the point of view of a seated occupant, and without using a third hand.

Up is defined as the highest adjusted détente position of an adjustable head restraint

Back is defined as the most rearward adjusted position of an adjustable head restraint,

or if this is difficult to ascertain, back should be taken as the position which results in the

greatest HRMD backset when set at the test height.

Forward is defined as the most forward locking adjusted position of an adjustable head

restraint, or if this is difficult to ascertain, forward should be taken as the position which

results in the least HRMD backset when set at the test height.

5.3.2 Head Restraint Test Positions

The same head restraint position should be used for all three tests

Head restraint test position: The test position for the head restraint depends on

whether it is fixed or adjustable and, if adjustable, whether the adjustments lock.

5.3.3 Non-locking adjustable head restraint: The head restraint is first adjusted to its

lowest vertical adjustment position. If a non-locking tilt adjustment is available, this

29

should then be set to the most rearward horizontal adjustment position possible once

the head restraint has been set to its lowest position.

5.3.4 Locking adjustable head restraints, midrange positions: The head restraint

is adjusted to midrange of its vertical and/or horizontal adjustment positions. Only

locking adjustments are set to the midrange positions. For example, a restraint with

locking height adjustment and non-locking horizontal adjustment would be set to its

midrange vertical position and most rearward horizontal position. The head restraint

should first be set for the midrange vertical position. Midrange tilt position should then

be set where this adjustment has locking notches.

5.3.5 Setting of mid range height

Lowest position: Some head restraints can be lowered below the lowest locking

position and in these cases the bottom of the restraint may contact the top of the

seatback.

Highest Position: The highest position is considered to be the highest locking position.

If a restraint has a non-locking position above the highest locking position, then the

highest locking position is still considered as the highest position.

Midrange height position is determined by calculating the geometric midpoint between

the lowest position, and highest locking vertical adjustments, considering only the

vertical component of measurement. The test position will then be selected based on

the following conditions:

1. Place the head restraint at the geometric midpoint if a locking position exists

there.

2. If there is no locking position at the geometric midpoint, raise the head restraint

by up to 10mm. If a locking position exists within this 10mm of travel, that

position will be the test position.

3. If there is no locking position within 10mm above the geometric midpoint, lower

the head restraint to the next lowest locking position.

4. If there is no locking position before the lowest or stowed position is reached,

then the head restraint should be positioned fully down.

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5. Once the vertical test position has been determined, ensure the head restraint is

returned to rearmost tilt position.

Figure 5.5: Examples of HR test position with various locking configurations [12]

5.4 MEASUREMENT OF REFERENCE GEOMETRY FOR BIORID SETUP

The HRMD probe measurements used for the geometric assessment will be different

than the geometry recorded for use during the BioRID setup. This is due to the

curvature of the HRMD probes. Since the BioRID is set up based on reference

geometry recorded using the HRMD, there is a need to measure an equivalent feature

on both devices. The rear most point on the HRMD skull (i.e. the screw on the backset

probe) is equivalent to the rearmost point on the centreline of the dummy‟s skullcap.

This point can be found using a measuring tape that contours to the shape of the

skullcap: the point is 95mm from the top of the skullcap along the mid sagittal plane of

the skull. The BioRID reference backset is measured using CMM (This is the horizontal

31

distance between the rearmost point on the HRMD skull (i.e. the screw on the retracted

backset probe) and the identifiable point on the head restraint +15mm.

Figure 5.6: Measuring BioRID reference backset [12]

5.5 INSTALLATION OF BIORID

The seat should be set to give a torso angle of 25º ±1º measured on the H-point

machine fitted with HRMD. BioRID’s midsagittal plane should be aligned with the

centreline of the seat. The spacing of the legs is adjusted so that the centreline of the

knees and ankles is 200mm (±10mm) apart and knees should be level. The heel of

BioRID’s shoe should be resting on the heel surface. The tip of the shoe shall rest on

the toe pan between 230mm and 270mm from the intersection of the heel surface and

toe board, as measured along the surface of the toe board.

5.6 BIORID INSTRUMENTATION

The T1 acceleration should be the average of right and left side accelerometer

measurements. In order to retain sensitivity, CACs which are orders of magnitude

greater than the minimum amplitude should not be used.

32

Position Function Measurement CFC CAC

Sled X Pulse

Acceptance

Acceleration (g) 60 100

Pulse

Acceptance

Velocity (m/s) 30 NA

Rebound

Velocity

Displacement (m) NA NA

Head X NIC Acceleration (g) 60 100

Acceleration (g) 1000 100

Head COG X Rebound

Velocity

Velocity (m/s) 30 NA

Neck T1 X (LH and

RH)

NIC Acceleration (g) 60 100

Neck Force X Force (N) 1000 1400

Neck Force X My OC & Nkm Force (N) 600 1400

Neck Force Z Force (N) 1000 4500

Neck moment Y My OC Moment (Nm) 600 115

Head Restraint

Contact Time (T-HRC)

T-HRCstart &

T-HRCend

Time (ms) NA NA

Neck T1 X Force (N) 1000 5000

Neck T1 Z Force (N) 1000 5000

Neck T1 Moment Y Moment (Nm) 600 200

1st Lumbar X Acceleration (g) 60 200

1st Lumbar Z Acceleration (g) 60 100

Seat Belt Force

(lap section)

Force (kN) 60 16

Table 5.1: BioRID Injury Output Measurements [12]

33

5.7 DATA ACQUISITION AND PROCESSING

The measurement data shall be recorded according to ISO 6487 or SAE J211/1 at a

minimum sample frequency of 10kHz Table 2 specifies the channel frequency classes

for each necessary measurement. Measurement data shall be considered for evaluation

until the point in time at which the head rebounds from the head restraint or at 300ms

after T-zero, whichever occurs first. Prior to test all data channels shall be offset to zero,

where zero (acceleration/force/moments) is defined by the average quiescent channel

value over 100 samples at 10kHz (or equivalent) before time offset. This should be

recorded a significant duration prior to T0 such that the sled acceleration/deceleration

phase is avoided.

5.8 TEST SLED REQUIREMENTS

5.8.1 Acceleration or deceleration sleds

The dynamic test is intended to simulate a typical rear crash in which the rear-struck

vehicle is initially stationary or moving forward very slowly. Consequently, an

acceleration sled with the dummy seated facing the direction of motion is recommended

for these tests. A deceleration sled, on which the dummy is seated facing against the

direction of motion, and is initially moving rearward at the appropriate test speed and

then stopped, may be used if careful attention is paid to dummy positioning immediately

prior to (T=0). In either case, some sled motion is allowed at the initiation of the test

(T=0). The sled should not brake before 300ms from T=0.

5.9 WHIPLASH ASSESSMENT CRITERIA

1. Head Restraint Contact Time (T-HRC(start), T-HRC(end))

2. T1 x-acceleration (T1)

3. Upper Neck Shear Force (Fx) and Upper Neck Tension (Fz)

4. Head Rebound Velocity

5. NIC

6. Nkm

7. Seatback Dynamic Opening

34

5.9.1 Head Restraint Contact Time T-HRC(Start) T-HRC(End).

Head Restraint Contact Time T-HRC(Start) is defined as the time (calculated from T=0) of

first contact between the rear of the ATD head and the head restraint, where the

subsequent continuous contact duration exceeds 40ms. For the purposes of

assessment, T-HRC(start) shall be rounded to the nearest millisecond. For the

subsequent criteria, the end of head restraint contact must also be found; T-HRC(end).

This is defined as the time at which the head first loses contact with the head restraint,

where the subsequent continuous loss of contact duration exceeds 40ms.

5.9.2 T1 X-Acceleration

BioRID is fitted with twin accelerometers on the first thoracic vertebra (T1), one on

either side of the lower neck loadcell assembly. The data channels acquired from these

accelerometers should both be filtered to channel frequency class (CFC) 60 as defined

by SAE J211. An average channel, T1(t), should then be produced from the two filtered

signals, as follows:

𝑇1 𝑡 =𝑇1𝑙𝑒𝑓𝑡 𝑡 + 𝑇1𝑟𝑖𝑔𝑕𝑡(𝑡)

2

Where:

T1left(t) = Acceleration channel measured by the left hand T1 accelerometer.

T1right(t) = Acceleration channel measured by the right hand T1 accelerometer.

The maximum (T1max) should be generated from this average T1 channel, considering

only the portion of data from T-zero until T-HRC(end) as follows:

𝑇1𝑚𝑎𝑥 = max𝑇−𝐻𝑅𝐶(𝑒𝑛𝑑 )

[𝑇1 𝑡 ]

35

5.9.3 Upper Neck Shear Force (Fx) and Upper Neck Tension (Fz)

The upper neck loadcell of the BioRID records both shear and tensile forces. If the

instrumentation is configured in accordance with SAE J211, positive shear should be

indicative of a head-rearwards motion and positive tension should be associated with

pulling the head upwards, generating a tensile force in the neck. Firstly, both the Fx and

Fz channels should be filtered at CFC 1000.

Peak values, Fxmax and Fzmax, should then be determined for each of the forces,

considering only the portion of data from T-zero until T-HRC(end), as follows:

𝐹𝑥𝑚𝑎𝑥 = max𝑇−𝐻𝑅𝐶(𝑒𝑛𝑑 )

[𝐹𝑥 𝑡 ]

𝐹𝑧𝑚𝑎𝑥 = max𝑇−𝐻𝑅𝐶(𝑒𝑛𝑑 )

[𝐹𝑧 𝑡 ]

5.9.4 Head Rebound Velocity – Acceleration Sled Technique

The head rebound velocity (in the horizontal/X direction) should be determined using

target tracking. Ideally this should be performed using footage acquired from on-board

camera systems, however off-board systems can provide suitable views providing the

camera positioning is correct and compensation is made for the movement of the sled.

5.9.5 Time for occurrence of peak rebound velocity

Theoretically, the peak rebound velocity should occur due to the elastic energy release

from the seat assembly, after the peak sled acceleration has occurred. In the case of an

acceleration sled this should also be prior to the sled braking, which at the earliest

should occur from 300ms. It should be verified that there is sufficient time before the

onset of sled braking for the particular sled being used, and that any peak rebound

velocity analysis is not undertaken during the sled braking phase. The rebound velocity

of the ATD is usually generated due to the release of stored elastic energy within the

seat structure, suspension and foam. The time of occurrence of peak rebound velocity

36

should be the maximum horizontal component of head rebound velocity calculated

between T=0 and 300ms.

5.9.6 Determination of Rebound Velocity

Using a suitable ‘target tracking’ film analysis technique, generate traces as follows:

Head centre of Gravity target velocity.

Sled velocity.

Both traces should be offset adjusted then filtered at CFC30. Head rebound velocity is

then defined as the difference between the sled velocity and the head velocity. Rebound

velocity can be calculated as:

VRebound = VHead C-of-G (abs) – VSled (abs)

Where;

VRebound = Instantaneous rebound X-velocity of the head c-of-g, relative to the sled.

VHead C-of-G (abs) = Instantaneous X-velocity of head centre of gravity, absolute.

VSled (abs) = Instantaneous X-velocity of sled, absolute.

Generate a third trace of head centre of gravity rebound velocity, relative to the sled.

The maximum value and the time at which this occurs should be noted. It should be

verified using the end of head restraint contact time, T-HRC(end), that this maximum is

during the rebound from the head restraint and is not generated within the sled braking

phase. Should higher peaks be generated in the sled braking phase, these should be

disregarded and the initial peak of rebound velocity which occurs at or very near to

initial rebound from the head restraint should be taken as the peak value.

5.9.7 NIC Calculation

The NIC is based on the relative horizontal acceleration and velocity of the occipital joint

relative to T1. To calculate NIC, two data channels are needed, which are the head x-

acceleration and average T1 x-acceleration. Each channel should first be converted

from ‘g’ to metres per second squared (m/s²), and the head x-acceleration should be

37

filtered at CFC 60. The average T1 channel (previously calculated) is the result of

combining two channels, both of which were filtered at CFC 60. The ‘relative x-

acceleration’ between head and T1 should be generated by subtracting the head x-

acceleration from the T1-x-acceleration.

This channel is calculated as follows:

The relative x-velocity (Vxrel) between head and T1 should be calculated, by integrating

the relative acceleration channel with respect to time, as follows:

The NIC channel is then calculated as a combination of relative acceleration multiplied

by 0.2, and added to the square of the relative velocity. The calculation is according to

the following equation:

The maximum overall NIC value (NICmax) should be obtained from the trace considering

only the portion of data from T-zero until T-HRC(end) as follows:

This maximum value should be noted, along with the time at which it occurs.

38

5.9.8 Nkm Calculation

The following definition is provided following the commonly accepted convention that

derives the ‘Anterior/ Posterior’ directions from the torso motion relative to the head.

Consequently, torso forward motion relative to the head would be referred to as

‘anterior’, and providing SAE J211 compliant instrumentation is used, would produce an

associated positive upper neck shear force, Fxupper .(Head rearward relative to the torso)

Conversely, the movement of the torso rearward relative to the head is referred to as

‘posterior’ and produces the opposite sign of shear force.

The Nkm criterion is based on a combination of moment and shear forces, using critical

intercept values for the load and moment. The shear force intercept value is identical for

anterior or posterior values, being 845N in both directions of loading. However, the

critical intercept value for the bending moment depends on the direction of loading,

having a value of 47.5Nm in extension (head rotation rearwards), but a value of 88.1Nm

in flexion (head rotation forwards). Two channels will be required to perform the Nkm

calculation, upper neck shear force Fxupper , in Newtons (N) and moment, My

upper in

Newton-metres (Nm).

Typically the shear force will be acquired in kilo-Newtons (kN), and so in those cases, a

conversion from kilo-Newtons (kN) to Newtons (N) will be required. Once it has been

confirmed that both shear force and moment are in the correct units, filter Myupper at CFC

600, according to SAE J211. To allow combination of the Myupper and Fx

upper channels,

another Fxupper channel should be produced, filtered at CFC 600. Due to the construction

of the BioRID, a correction must then be made to convert the actual moment measured

by the upper neck loadcell into the moment about the Occipital Condyle (OC). The

corrected moment, MyOC is equal to the upper neck shear force Fx

upper multiplied by a

constant, D, then subtracted from the measured moment, Myupper . Calculate the

Moment about the OC according to the following equation:

MyOC (t) = My

upper (t) − DFxupper (t)

where D=0.01778m.

39

The four components of Nkm are then calculated using the upper neck shear force

Fxupper and the corrected moment about the OC, My

OC .

Each channel first needs to be separated into it‟s positive- or negative-going

components by generating four new channels as follows:

Generate two new channels, Fxa and Fxp , based on Fupper force channel.

Generate two new channels, Myf and Mye based on the MyOC moment channel. Each of

the new channels should contain only selected positive or negative-going portions of the

respective Fx or My channels, with all unwanted data points being replaced by null or

zero value, as defined by:

Fxa channel contains only the positive portion of the Fxupper force channel as follows:

If Fxupper (t) > 0, then Fxa(t) = Fx

upper (t), else Fxa(t) = 0 .

Fxp channel contains only the negative portion of the Fxupper force channel as follows:

If Fxupper (t) <0, then Fxp(t) = Fx

upper (t), else Fxp(t)= 0 .

Myf channel contains only the positive portion of the MyOC moment channel as follows:

If M yOC (t) >0, then Myf (t)= My

OC (t), else Myf (t)= 0.

Mye channel contains only the negative portion of the MyOC moment channel as follows:

If M yOC (t)<0, then Mye(t) = My

OC (t), else Mye(t) = 0

The four components of Nkm are then defined as:

1) ‘Neck Extension Posterior’ ( Nep ) or the combined negative-going portion of the

shear force channel (Fxp) and negative-going portions of the moment channel

(Mye), as defined by:

Where: Fx-int = -845N, Mye-int = -47.5Nm

2) ‘Neck Extension Anterior’ ( Nea ) or the combined positive-going portion of the

shear force channel (Fxa) and negative-going portions of the moment channel

(Mye), as defined by:

40

Where: Fx-int = 845N, Mye-int = -47.5Nm

3) ‘Neck Flexion Posterior’ ( N fp ) or the combined negative-going portions of the

shear force channel (Fxp) and positive-going portions of the moment channel

(Myf), as defined by:

Where: Fx-int = -845N, Myf-int = 88.1Nm

4) ‘Neck Flexion Anterior’ ( N fa ) or the combined positive-going portions of the

shear force channel (Fxa) and positive-going portions of the moment channel

(Myf), as defined by:

Where: Fx-int = 845N, Myf-int = 88.1Nm

Each of the four components should be calculated as a new data channel, using only

the positive- or negative-going portions of the Fx and My channels as appropriate, and

the relevant critical intercept values. Maxima for each of the four components should be

calculated, considering only the portion of data from T-zero until T-HRC(end), as follows:

The Nkm value is taken as the maximum value reached by any one of the four

components Nea, Nep, Nfa, Nfp. It should be noted which component of the four reached

the maximum value and the time at which this occurred.

5.9.9 Seatback Dynamic Deflection

Seatback dynamic opening is measured using a suitable target tracking film analysis

technique.

- Define a line between the upper and lower seatback targets, ST2 and ST3.

41

- Define a second line between the forward and rearward sled base targets, B1 and B2.

Calculate the angle between these two lines at the T-zero position. The instantaneous

seatback deflection is defined as the instantaneous difference in angle between the

T-zero position and the deflected position. Track the change in instantaneous angle

between these two lines, throughout the dynamic test.

Figure 5.7: Video motion target placement description [12]

The Seatback Dynamic Opening is defined as the maximum change in angle achieved

at any time during the test between the T zero position and T-HRC(end).

5.10 SLED PULSE SPECIFICATIONS

5.10.1 Offset adjust the accelerometer

In order to make sure that there is no initial acceleration, which result in a non-zero

velocity profile, it is required to offset adjust the acceleration signal. It is assumed that

this step is a standard procedure for the participating laboratories and shall therefore

not be discussed into further detail.

42

5.10.2 Filter with CFC 60

To ensure that low level noise does not influence the results the acceleration signal is

filtered with a CFC 60 filter („endpoints‟-method in Diadem). The CFC 60 filter is used

according to SAE J211, for sled acceleration signals.

5.11 Description of Seats

5.11.1 Passive Seat

A seat that uses passive foam technology to absorb the energy of the crash

and allows the occupant to engage the head restraint without neck distortion.

5.11.2 Reactive Head Restraint

A head restraint that automatically moves up and forward during the crash,

actuated by the weight of the occupant in the seat.

5.11.3 Re-active Seat

An entire seat and head restraint that absorbs the energy of a rear end crash.

5.11.4 Pro-Active Head Restraints

A head restraint that automatically moves up and forward at the start of the

crash, actuated by crash sensors on the bumper or within the car.

5.12 DEFINITIONS

5.12.1 Definition of T0

T0 is defined as the time before the CFC60 filtered sled acceleration reaches 1.0g. The

relevant times for the low, medium and high pulses are 4.6ms, 5.8ms and 3.7ms

respectively.

5.12.2 Definition of T1

T1 is defined as the time when the sled acceleration for the first time is > 1g. Both the

initial onset of the pulse and specific low acceleration disturbances (< 0.5g) heavily

influence the behaviour at the start of the pulse. For that reason, it is in practice not

possible to identify the actual start of the pulse. Acceleration levels higher than 1g

however are unmistakably a direct result of the pulse on the sled. As such, the moment

in time when the sled acceleration crosses 1g can be uniquely and easily be found.

43

5.12.3 Definition of TEND

TEND is defined as the time when the sled acceleration for the first time is < 0g.

5.12.4 Definition of dT

dT is defined as the time span between TEND and T0,

dT = TEND - T0

5.12.5 Definition of dV

dV is defined as the difference between the maximum and minimum sled velocity

between T0 and TEND.

5.13 PROTOCOL TEST SETUP MEASUREMENTS

Criteria Measurement Remarks / Tolerance

1 Seat Back ANGLE 25 deg

2 Toe Board 45 deg wrt to horizontal

3 Tip of Shoe 23 – 27 cm Measured on Toe Board

4 Gap between front of seat and

back of Toe Board

<= 10 cm

5 Cushion Angle 0 deg ( close to 0 deg )

6 HPM Knees 25 cm apart

7 HPM feet position 90 deg To the tibias

8 Torso Angle 21 deg Before butt and chest weights

are added

9 H-Point on both sides of HPM + or – 2.5 mm Of each other

10 Torso Angle 25 + or – 1 deg Seatback angle should support

this torso angle

44

11 Vertical distance between

rearmost and topmost points of

skullcap

9.5 cm Measured along mid-sagittal plane

of the skull

12 BioRID Backset 1.5 cm + HRMD

Backset

13 BioRID Pelvis Angle 26.5 +or – 2.5 deg

14 H-Point of BioRID wrt HPM

( x- axis)

2 cm forward With a tolerance of 1 cm

15 H-Point of BioRID wrt HPM

( z- axis)

0 cm With a tolerance of 1 cm

16 Horizontal distance b/w centerline of knees and

ankles

20 cm + or – 1 cm

17 Spine Curvature-Occipital Condyle Angle 29.5 deg 0.5 deg

18 T2 angle 37 deg 0.5 deg

19 Neck Plate Angle 0 deg 0.5 deg

20 X-distance b/w H-point and O.C pin 15.6 cm 0.5 cm

21 Z-distance b/w H-point and O.C pin 60.9 cm 0.5 cm

Table 5.2: Protocol Test Setup Measurements

5.14 WHIPLASH SEAT ASSESSMENT

Criteria and Limit Values

The basic assessment criteria used for whiplash protection assessment, with the upper

and lower performance limits for each parameter are summarized below.

45

5.14.1 Static Assessments

Head Restraint Geometry Assessment

The assessment is based on the worst performing parameter from

either the height or backset:

Higher performance limit:

Height: 0mm below top height of HPM & HRMD

Backset: 40mm

Lower performance limit:

Height: 80mm below top height of HPM & HRMD

Backset: 100mm

The geometric assessment will be based on the average height and backset taken from

at least 9 measurements obtained across all of the seats provided for assessment. A

minimum of three drops per seat shall be performed to ensure consistent

measurements are obtained on each individual seat. Where obvious outlying

HRMD/HPM measurements occur, further installations shall be undertaken on that seat

to ascertain whether differences are due to the individual installation or seat to seat

variability. Where a seat has a non-reversible head restraint and qualifies for a

geometric assessment in the deployed position, additional seats shall be provided by

the vehicle manufacturer for measurement. The geometry assessment has two points

allocated to it ranging from plus one to minus one. [13]

5.14.2 Dynamic Assessments

In the absence of a process to define representative vehicle specific pulses, the use of

generic sled pulses has been preferred. Instead of using a single sled pulse, Euro

NCAP has adopted three tests of different severity to avoid sub-optimization to a single

pulse and to ensure seat stability at a higher test severity. These pulses cover the range

of speeds at which the highest risk at short and long term injury is observed and at

which severe neck injury claims peak. Accident data suggests whiplash tests should

include crashes in the 16 km/h range (10 mi/h). The first pulse used is at 16km/h ΔV

pulse with a 5.5g mean acceleration, representative of one of the crash scenarios in

which whiplash associated injuries would occur. This pulse, originally double wave in

46

shape but simplified to a triangular pulse, has been used by IIWPG. The two other

pulses used are trapezoidal in shape and simulate a ‘low’ 16 km/h ΔV (peak 5g) and

‘high’ 24 km/h ΔV (peak 7.5g). The three pulses, shown in Figure 1, are termed ‘low’

(16km/h, SRA), ‘medium’ (16km/h, IIWPG) and ‘high’ (24km/h SRA) within the Euro

NCAP whiplash scheme.

Figure 5.8: Sled pulses [26]

A sliding scale system of points scoring shall be applied with two limits for each seat

design parameter, a more demanding higher performance limit, below which a

maximum score is obtained and a less demanding lower performance limit, beyond

which no points are scored. Where a value falls between the two limits, the score is

calculated by linear interpolation.

The maximum score for each parameter is 0.50 points, with a maximum of 3 points

available per test. For each of the tests, the score for each of the seven parameters is

calculated. The overall score for a single dynamic test is the sum of the scores for NIC,

Nkm, Head rebound velocity, neck shear and neck tension, plus the maximum score

from either T1 acceleration or head restraint contact time (T-HRC-start). The high

severity pulse will be subject to an additional seatback deflection assessment where a

47

three point penalty will be applied to seats with a rotation of 32.0° or greater. In the

medium term, seat translation may also need to be controlled but, for the interim

solution, only rotational control of the seat back is specified. The relevant performance

criteria for each pulse are detailed below.

Table 5.3: HSP performance limits [13]

Table 5.4: MSP performance limits [13]

48

Table 5.5: LSP performance limits [13]

5.15 WHIPLASH RATING SCHEME

5.15.1 Sliding Scales

The Euro NCAP assessment applies a sliding scale system of points scoring, which

involves two limits for each seat design parameter. Two performance limits (lower and

higher) are set at the 70th

percentile and the 5th

percentile values respectively of the

variable distribution observed in an earlier 31 car seat program undertaken jointly by

Thatcham, Folksam and Swedish Road Association. The more demanding ‘higher’

performance limit (HPL) below which a maximum score was obtained, and a less

demanding ‘lower’ performance limit (LPL) above which no points are scored. These

limit values, representing the range in performance of seats currently on the market, are

given in Table 1 for each of the seven measured variables for each test pulse. If the test

value recorded falls between the lower and upper limits, the points score is calculated

by linear interpolation.

5.15.2 Capping

For the first 5 variables in Table 1, the score is ‘capped’ at the 95th

percentile value (CL)

of the above variable distribution, meaning that if any single measured variable

exceeded the 95th

percentile limit, then a zero score is recorded for the complete test.

For T1 acceleration and head restraint contact time, a slightly more complex approach

is required. If both head restraint contact time and T1 acceleration were worse than the

49

lower performance limit and either one of these variables exceed the 95th

percentile,

then capping is applied and the score is also zero for that test.

The purpose behind capping is to avoid trade-offs between seat design parameters

where one or more parameters would be allowed to ‘max out’ while keeping others low.

This, for instance, would be the case where low Fx or NIC would be achieved by

allowing more seat back deflection thus raising Fz during extension. Capping therefore

encourages a proper balance between the seven seat performance criteria.

5.15.3 Whiplash Raw Score

The maximum score for each parameter is 0.5 points. For each of the pulses, the score

for each of the seven parameters is calculated. The scores for the NIC, Nkm, head

rebound velocity, neck shear and neck tensions are summed, plus the maximum score

from either T1 acceleration or head restraint contact time. There is a maximum possible

score of three points for each test pulse, hence 9 for the overall series of dynamic tests.

To calculate the raw whiplash score, the overall dynamic score is combined with the

result from the geometric assessment. The static assessment of design head restraint

position can either add or reduce the score with maximum one point, depending on how

well aligned the position is with respect to the head. In addition, for seats that score well

dynamically, per seat an additional 1/n points can be gained for the ‘worst case’

geometry or ease of adjustment (where n=the number of front seats).

Finally, the score can be reduced where excessive dynamic deflection of the seat back

was observed during the ‘high’ severity test (minus three points) or where there is

evidence of exploiting a dummy artefact (minus 2 points). These latter modifiers have

been introduced to prevent occupant ramping, which in extreme case can lead to

occupant ejection, or compromise of rear seat passenger space and to discourage seat

designs that intentionally misuse dummy features to enhance the performance. The

dynamic test points combined with the assessment and modifier points (whether

positive or negative) form the Whiplash Raw Score.

50

5.15.4 Scaled Points

The overall whiplash raw score is scaled to four points, which is the final score for the

seat and the maximum contribution of the whiplash test to the Adult Occupant

Protection score (maximum 36 points) of the overall rating of the vehicle. The points are

scaled to balance whiplash protection against the various other forms of protection

assessed in the other Euro NCAP tests. For the purpose of graphical representation,

the final four point score is divided into three coloured bands. A score of 0 to 1.49

scaled points is coloured ‘Red’ or ‘Poor’ (different from other assessments where ‘Red’

is zero points only), a score of 1.50 to 2.99 is coloured ‘Orange’ or ‘Marginal’, and finally

a score of 3.0 to 4.0 is coloured ‘Green’ or ‘Good’. The coloured bands are used as an

additional indicator to raise public awareness and aid understanding of whiplash

protection.

5.16 SCORING & VISUALIZATION

5.16.1 Raw Score

The protocol allows for a maximum score of 11 points as a result of carrying out the

three severities of whiplash test, assuming no negative modifiers have been applied.

This score is known as the raw score and its components are explained below.

Each severity of whiplash test pulse results in a maximum of 3 points being awarded

based on the measured criteria. Half a point is awarded for each of NIC, Nkm, Head

rebound velocity, Fx and Fy. A further half point is awarded on the basis of the best

score from either T1 acceleration or head restraint contact time (T-HRC).

If any of NIC, Nkm, Head rebound velocity, neck shear or tension exceed the capping

limit, no score is given for that pulse. Additionally, if both T1 and head restraint contact

time exceed the lower performance limit and either one also exceeds the relevant

capping limit, no score is given for the pulse. The sum of the scores from the dynamic

tests is then subject to the application of the modifiers. [13]

51

Table 5.6: whiplash scoring [13]

5.16.2 Scaled Score

The raw score is scaled to a maximum of 4 points by multiplication by a factor of 4/11.

Scaled scores less than zero are set to zero points. [26]

Figure 5.9: ENCAP whiplash scaled score [26]

52

5.16.3 Visualization

For whiplash, the protection provided for the neck of a front seat adult occupant is

presented visually using a coloured head and neck graphic. The colour used is based

on the scaled points (rounded to three decimal places), as follows:

Green 3.000 – 4.000 points

Orange 1.500 – 2.999 points

Red 0.000 – 1.499 points

53

CHAPTER 6

DESIGN OF EXPERIMENTS (DOE)

6.1 DOE BASICS

Design of Experiments (DOE) techniques enables designers to determine

simultaneously the individual and interactive effects of many factors that could affect the

output results in any design. DOE also provides a full insight of interaction between

design elements; therefore, it helps turn any standard design into a robust one. Simply

put, DOE helps to pin point the sensitive parts and sensitive areas in designs that cause

problems in Yield. Designers are then able to fix these problems and produce robust

and higher yield designs prior going into production.

Any scientific investigation involves formulation of certain assertions (or hypotheses)

whose validity is examined through the data generated fro, an experiment conducted for

the purpose. Thus experimentation becomes an indispensable part of every scientific

endeavour and designing an experiment is an integrated component of every research

programme. Three basic techniques fundamental to designing an experiment are

replication, local control (blocking) and randomization. Whereas the first two help to

increase precision in the experiment, the last one is used to decrease bias. DOE is a

systematic, rigorous approach to engineering problem-solving that applies principles

and techniques at the data collection stage so as to ensure the generation of valid,

defensible, and supportable engineering conclusions. In addition, all of this is carried out

under the constraint of a minimal expenditure of engineering runs, time, and money.

There are four general engineering problem areas in which DOE may be applied:

1. Comparative

2. Screening/Characterizing

3. Modeling

4. Optimizing

54

Comparative: In the first case, the engineer is interested in assessing whether a change

in a single factor has in fact resulted in a change/improvement to the process as a

whole.

Screening Characterization: In the second case, the engineer is interested in

‘understanding’ the process as a whole in the sense that he/she wishes (after design

and analysis) to have in hand a ranked list of important through unimportant factors

(most important to least important) that affect the process.

Modeling: In the third case, the engineer is interested in functionally modeling the

process with the output being a good-fitting (high predictive power) mathematical

function, and to have good (maximal accuracy) estimates of the coefficients in that

function.

Optimizing: In the fourth case, the engineer is interested in determining optimal settings

of the process factors; that is, to determine for each factor the level of the factor that

optimizes the process response.

6.2 DESIGN OF EXPERIMENTS VIA TAGUCHI METHODS

The Taguchi method involves reducing the variation in a process through robust design

of experiments. The overall objective of the method is to produce high quality product at

low cost to the manufacturer. The Taguchi method was developed by Dr. Genichi

Taguchi of Japan who maintained that variation. Taguchi developed a method for

designing experiments to investigate how different parameters affect the mean and

variance of a process performance characteristic that defines how well the process is

functioning.

The experimental design proposed by Taguchi involves using orthogonal arrays to

organize the parameters affecting the process and the levels at which they should be

varies. Instead of having to test all possible combinations like the factorial design, the

Taguchi method tests pairs of combinations. This allows for the collection of the

55

necessary data to determine which factors most affect product quality with a minimum

amount of experimentation, thus saving time and resources. The Taguchi method is

best used when there are an intermediate number of variables (3 to 50), few

interactions between variables, and when only a few variables contribute significantly.

The Taguchi arrays can be derived or looked up. Small arrays can be drawn out

manually; large arrays can be derived from deterministic algorithms. Generally, arrays

can be found online. The arrays are selected by the number of parameters (variables)

and the number of levels (states). This is further explained later in this article. Analysis

of variance on the collected data from the Taguchi design of experiments can be used

to select new parameter values to optimize the performance characteristic. The data

from the arrays can be analyzed by plotting the data and performing a visual analysis,

ANOVA, bin yield and Fisher's exact test, or Chi-squared test to test significance.

6.3 PHILOSOPHY OF THE TAGUCHI METHOD

Quality should be designed into a product, not inspected into it. Quality is

designed into a process through system design, parameter design, and tolerance

design. Parameter design, which will be the focus of this article, is performed by

determining what process parameters most affect the product and then designing

them to give a specified target quality of product. Quality ‘inspected into’ a

product means that the product is produced at random quality levels and those

too far from the mean are simply thrown out.

Quality is best achieved by minimizing the deviation from a target. The

product should be designed so that it is immune to uncontrollable

environmental factors. In other words, the signal (product quality) to noise

(uncontrollable factors) ratio should be high.

56

The cost of quality should be measured as a function of deviation from the

standard and the losses should be measured system wide. This is the

concept of the loss function, or the overall loss incurred upon the customer and

society from a product of poor quality. Because the producer is also a member of

society and because customer dissatisfaction will discourage future patronage,

this cost to customer and society will come back to the producer.

6.4 TAGUCHI METHOD OF DOE

The general steps involved in the Taguchi Method are as follows:

1. Define the process objective, or more specifically, a target value for a performance

measure of the process. This may be a flow rate, temperature, etc. The target of a

process may also be a minimum or maximum; for example, the goal may be to

maximize the output flow rate. The deviation in the performance characteristic from the

target value is used to define the loss function for the process.

2. Determine the design parameters affecting the process. Parameters are variables

within the process that affect the performance measure such as temperatures,

pressures, etc. that can be easily controlled. The number of levels that the parameters

should be varied at must be specified. For example, a temperature might be varied to a

low and high value of 40 C and 80 C. increasing the number of levels to vary a

parameter at increases the number of experiments to be conducted.

3. Create orthogonal arrays for the parameter design indicating the number of and

conditions for each experiment. The selection of orthogonal arrays is based on the

number of parameters and the levels of variation for each parameter, and will be

expounded below.

57

4. Conduct the experiments indicated in the completed array to collect data on the effect

on the performance measure.

5. Complete data analysis to determine the effect of the different parameters on the

performance measure.

Figure 6.1: Taguchi Method of DOE

58

6.5 FRACTIONAL FACTORIAL DESIGNS

A factorial design is one in which every possible combination of treatment levels for

different factors appear. The two-way ANOVA with interaction we considered was a

factorial design. We had n observations on each of the IJ combinations of treatment

levels. If there are, say, ‘a’ levels of factor A, ‘b’ levels of factor B, ‘c’ levels of factors C,

then a factorial design requires at least ‘abc’ observations, and more if one wants to

estimate the three way interaction among the factors. This can get expensive when

experiments have many different factors.

To keep experimental costs in line, one approach is to use fractional factorial designs.

In these, one does not take measurements upon every possible combination of factor

levels, but only upon a very carefully chosen few. These few are selected to ensure that

the main effects and low-order interactions can be estimated and tested, at the expense

of high-order interactions. The scientific intuition is that it is unlikely for there to be

complex interactions among many different factors; instead, there are probably only

main effects and a few low-order interactions. Thus one might design the collection in a

fractional factorial so that all main effects and two-way interactions can be tested, but

not three-way or higher interactions.

A special kind of factorial design is the 2k factorials. In these, each of the k factors have

exactly 2 levels, so there are 2k different combinations of treatment levels. For a full

factorial, one would need a minimum of 2k observations, and even that would not allow

enough degrees of freedom for the error term.

2k-1 = k(2-1) + ( 𝑘2 ) (2-1) (2-1) + . . . + ( 𝑘

𝑘 ) (2-1) (2-1) . . . (2-1).

This is based on a standard combinatorial identity due to Pascal:

2k = 𝑘𝑖 𝑘

𝑖=0 .

Typically, one uses the highest order interaction as if it were an error term, or uses a

normal probability plot.

The following table shows the data for a full 23 factorial design. Note that the signs in

each interaction column can be found by multiplying the signs in corresponding main-

effect columns.

59

Figure 6.2: 23 factorial design

The main effect due to factor A is the average difference between the high and low

levels of factor A, or:

α = 1

2 (

𝑌2+𝑌4+𝑌6+𝑌8

4 –

𝑌1+𝑌3+𝑌5+𝑌7

4).

and similarly for the main effects of B and C. So, as per the signs in the table,

α = 1

8 (-Y1+Y2-Y3+Y4-Y5+Y6-Y7+Y8).

The AB interaction is half the difference between the main effect of factor A at the high

level of factor B and that at the low level of factor B, or:

(αβ) = 1

2

1

2(𝑌4+𝑌8

2 –

𝑌3+𝑌7

2) −

1

2(𝑌2+𝑌6

2 –

𝑌1+𝑌5

2) .

As per the signs in the table,

(αβ) = 1

8(Y1-Y2-Y3+Y4+Y5-Y6-Y7+Y8).

The ABC interaction is half the difference between the two-factor interaction AB at the

high and low levels of factor C, or

(αβγ) = 1

2

1

2(𝑌8−𝑌7

2 –

𝑌6−𝑌5

2) −

1

2(𝑌4−𝑌3

2 –

𝑌2−𝑌1

2) .

Note that

(αβγ) = 1

8(-Y1+Y2+Y3-Y4+Y5-Y6-Y7+Y8).

60

These relationships give us an automatic way to calculate the main effects (and,

implicitly, the sums of squares) for inference.

But 2k observations get expensive. Instead, one can carefully select half that number so

as to still permit estimation of main effects and low-order interactions.

Figure 6.3: Fractional factorial design

Consider the 23-1 fractional factorial design (The previous figure gave two illustrations)

Figure 6.4: 23-1 fractional factorial design

Note that because we have taken only half of the 8 observations needed for a full

factorial, some of the columns have identical entries. Columns that have identical

entries correspond to effects that are confounded or aliased.

In order to estimate the effects, note that:

1

4(Y1+Y2+Y3+Y4) = µ+ (α β γ)

61

1

4(-Y1+Y2-Y3+Y4) = α+ (β γ)

1

4(-Y1-Y2+Y3+Y4) = β+ (α γ)

1

4(Y1-Y2-Y3+Y4) = γ+ (α β)

Thus the estimate of the mean is confounded with the three-way interaction, the

estimate of the A effect is confounded with the BC interaction, the estimate of the B

effect is confounded with the AC interaction, and the estimate of the C effect is

confounded with the AB interaction. If one assumes that there are no interactions, then

one can make tests about the main effects, or use a normal probability plot.

Note that we write 2k-p to denote a fractional factorial design in which each factor has 2

levels, there are k factors, and we are taking a 1/2p fraction of the number of possible

factor level combinations. In order to construct a fractional factorial that deliberately

confounds pre-selected factors, one need to use a generator. The generator uses the

fact that squaring the entries in any given column gives a column of ones, which can be

thought of as an identity element I. If we want to confound the A effect with the BC

interaction, then that is equivalent to declaring A * BC = ABC = I. It follows that

B = BI = B * ABC = AC, so B is confounded with the AC interaction. Similarly, C is

confounded with AB, and the overall mean (I) is confounded with ABC.

62

CHAPTER 7

SOFTWARES

7.1 LS-DYNA

LS-DYNA is a general purpose finite element code for analyzing the large deformation

static and dynamic response of structures including structures coupled to fluids. The

main solution methodology is based on explicit time integration. An implicit solver is

currently available with somewhat limited capabilities including structural analysis and

heat transfer. A contact-impact algorithm allows difficult contact problems to be easily

treated with heat transfer included across the contact interfaces. By specialization of

this algorithm, such interfaces can be rigidly tied to admit variable zoning without need

of mesh transition regions. Specialized capabilities for airbags, sensors and seat belts

have tailored LS-DYNA for applications in the automotive industry. Adaptive remeshing

is available for shell elements and is widely used in sheet metal stamping applications.

LS-DYNA currently contains approximately one-hundred constitutive models and ten

equations-of-state to cover a wide range of material behavior.

7.2 ALTAIR HYPERSTUDY

Altair HyperStudy is a parametric study and a multi-disciplinary optimization tool for

robust product design. Specifically developed for design of experiments (DOE),

stochastic simulations and optimization techniques, engineers can:

• Gain insight into the physics of a design

• Assess the robustness of a design for variations in design parameters

• Optimize a design for multi-disciplinary attributes

HyperStudy has an easy-to-use interface that enables a process-based study set up.

Further, HyperStudy's deep integration in Altair HyperWorks provides direct accessibility

to a broad range of CAE solvers including linear, non-linear, fluid dynamics and other

multi-physics solver technologies.

63

• Provides an easy way to study effects of design changes for complex analysis events

• Minimizes time-to-market by providing clear design direction for complex designs

• Improve design performance by performing multi-disciplinary optimization studies for

different design attributes

• Perform system identification and correlation studies to expedite the design process

• Increases the return on CAE solver investments with added functionality and direct

interfaces

• Improve design quality and robustness

7.3 DOE USING HYPERSTUDY

HyperStudy provides comprehensive design of experiments capabilities for various

DOE types. Available DOE types include Full Factorial, Fractional, Taguchi, Box

Behnken, Plackett-Burman, Central Composite, Latin HyperCube, User-defined and

direct input of external run matrix. The study parameters can be continuous, discrete

numbers or character strings which can be either controlled or uncontrolled.

HyperStudy's extensive post-processing capabilities include an advanced approximation

module to create response surfaces as well as tools to diagnose and measure their

fidelity. Response surfaces can be used for performing robustness and optimization

studies.

64

CHAPTER 8

SIMULATION OF BASELINE MODEL

8.1 CAE SEAT DESIGN MODEL

Seating assembly mainly contains following subassemblies.

1. Head rest.

2. Back panel.

3. Recliner mechanism.

4. Cushion panel.

5. Lock for height Adjustment.

6. Four bar link and connection.

7. Sliding Track used for position seat in forward and rear direction.

Figure 8.1: CAE seat model with head restraint

The CAE seat model together with the BioRID-II dummy was simulated for all the three

crash severity pulses according to ENCAP protocol. The acceleration pulse was given

to the sled for 200ms in such a way that the sled is pushed in forward direction so that

the dummy is ramped into the seat and then rebounded within this time.

65

Figure 8.2: Sled movement

The CAE simulations were then correlated to actual sled tests and the results were

`found to be very identical.

Figure 8.3: Correlation to actual test

8.2 MEASUREMENT OF WHIPLASH INJURY CRITERION

Accelerometers and load cells which are placed at various locations of BioRID dummy

records the accelerations and forces respectively. This helps in the measurement and

calculations of whiplash injury criterion.

66

Figure 8.4: BIORID load cells and accelerometers

The geometrical measurements like seatback dynamic deflection are calculated using

the LSPre postprocessor from femd3plot files while neck forces and acclerations are

calculated from binout files. Head restraint contact time can also be known from

femd3plot animation.

Table 8.1: Measurement of whiplash injury criterion

67

8.3 ASSESSMENT OF WHIPLASH INJURY CRITERION

Table 8.2: injury criterions measured in simulation

8.4 ENCAP HIGH SEVERITY PULSE WHIPLASH RUN- CORRELATION MODEL

At 0 ms;

Figure 8.5: initial position of dummy

68

At 90 ms;

Figure 8.6: head restraint contact

8.5 CORRELATION MODEL- INJURY CRITERION PLOTS FOR HSP

8.5.1 Upper Neck Shear Force

Figure 8.7: Baseline model upper neck shear force

69

8.5.2 Upper Neck Tension

Figure 8.8: Baseline model upper neck tension

8.5.3 Neck Injury Criterion

Figure 8.9: Baseline model Neck Injury Criterion

70

CHAPTER 9

DOE FOR WHIPLASH HIGH SEVERITY PULSE

9.1 DOE RUN-1 DETAILS FOR HIGH SEVERITY PULSE

Software used: HyperStudy v11.

DOE method used: L18 array Taguchi.

Number of Variables: 6

DOE Class: Fractional Factorial.

Number of Runs: 18.

Simulation Time: 200 ms.

Altair e-compute Run Time (for 1 Run): 17 hours 50 mins (16CPU)

: 5 hours 50 mins (32 CPU)

Figure 9.1: DOE-1 L18 matrix

71

9.2 WHIPLASH VARIABLES CHOSEN FOR DOE-1

1. HR Backset

• Horizontal distance between rear of BioRID head and head restraint.

• Translated the head restraint cushion in –X direction for DOE.

Actual Backset (mm) Distance moved from current position (mm)

92 0

66 26

40 52

Table 9.1: Backset modifications for DOE

2. HR Height

Increased the height of HR cushion by two levels : +20mm and +40mm.

Figure 9.2: HR Backset and height modifications

2. HR Foam Stiffness

Changed the material (MAT 057) stiffness curve using scale factor by ±30%.

4. Seat Back Cushion Stiffness

Changed the material (MAT057) stiffness curve using scale factor by -30%.

72

5. HR Rod Stiffness

Increased the HR Rod Stress-Strain curve by two levels: +30% and +60%.

6. Seat Recliner Stiffness

• Increased the seat recliner Young’s Modulus (E) value by +30% and +60% for

changing the recliner spring stiffness.

• Recliner spring is modeled as beam element in the CAE model.

9.3 RESULTS FOR DOE-1 HIGH SEVERITY PULSE

9.3.1 Set of Factors affecting the Injury criterion

1. Factors affecting Time for Head Restraint Contact (T-HRC)

• HR Backset

• HR Height

Figure 9.3: improved head restraint contact

73

2. Factors affecting T1-x-acceleration

• HR Backset

• HR rod stiffness

• HR foam stiffness

• HR Height

Figure 9.4: T1-x-acceleration response surface

3. Factors affecting Upper neck shear force

• HR backset

• Seat Back cushion stiffness

• HR rod stiffness

74

Figure 9.5: upper neck shear force response surface

Figure 9.5 shows that the upper neck shear force is getting reduced with decreased HR

backset and decreased seat back cushion stiffness.

4. Factors affecting Upper neck tension

• HR Backset

• HR Height

• Seat Recliner Stiffness

• HR foam stiffness

• Seat Back cushion stiffness

Figure 9.6: upper neck tension response surface

75

5. Factors affecting Rebound Velocity

• HR Backset

• HR Height

• HR Rod stiffness

• HR foam stiffness

Figure 9.7: Rebound Velocity response surface

6. Factors affecting NIC

• HR Backset

• HR Height

• Seat back cushion stiffness

76

Figure 9.8: NIC response surface

9.4 DOE RUN-2 FOR HIGH SEVERITY PULSE

The critical output parameters (injury criterions) to be controlled were found out to be

head restraint contact time, NIC, and the neck forces. Practically altering all the design

variables is not feasible and will result in higher cost factor and more time. So another

DOE setup was executed with less number of variables. An L27 Taguchi array was

selected for the same to get more interactions between design variables.

Software used: HyperStudy v11.0-130.

DOE method used: L27 array Taguchi.

DOE Class: Fractional Factorial.

Number of variables: 4

Number of Runs: 27

Simulation Time: 200 ms.

Altair e-compute run time (for 1 Run): 17 hours 50 mins (16CPU)

: 5 hours 50 mins (32 CPU)

77

Figure 9.9: DOE-2 L27 matrix

9.5 WHIPLASH VARIABLES CHOSEN FOR DOE

1. HR Backset

• Horizontal distance between rear of BioRID head and head restraint.

• Translated the head restraint cushion in –X direction for DOE.

78

Actual Backset (mm) Distance moved from current position (mm)

92 0

66 26

40 52

Table 9.2: Backset modifications for DOE-2

2. HR Foam Stiffness

Changed the material (MAT 057) curve using scale factor by ±30%.

Seat Back Cushion Stiffness

Changed the material (MAT057) curve using scale factor by ±30%.

4. HR Height

Increased the height of HR cushion by two levels ie +20mm and +40mm.

9.6 RESULTS FOR DOE-2 HIGH SEVERITY PULSE

9.6.1 Set of Factors affecting the Injury criterion

1. Factors affecting Time for Head Restraint Contact

• HR Backset

• HR Height

79

Figure 9.10: improved head restraint contact time

2. Factors affecting T1-x-acceleration

• HR Backset

• Seatback cushion stiffness

• HR foam stiffness

• HR Height

80

Figure 9.11: DOE-2 T1-x-acceleration response surface

Figure 9.11shows that T1-x-acceleration decreases with increased HR foam stiffness.

3. Factors affecting Upper neck shear_Fx

• HR backset

• HR foam stiffness

• HR Height

Figure 9.12: DOE-2 upper neck shear force response surface

81

4. Factors affecting Upper neck tension_Fz

• HR Backset

• HR Height

• HR foam stiffness

• Seat Back cushion stiffness

Figure 9.13: DOE-2 upper neck tension response surface

Figure 9.13 shows that upper neck tension reduces with decreased HR backset and

height.

5. Factors affecting Rebound Velocity

• HR Backset

• HR Height

• HR foam stiffness

82

Figure 9.14: DOE-2 Rebound Velocity response surface

6. Factors affecting NIC

• HR Backset

• HR Height

• Seat back cushion stiffness

Figure 9.15: DOE-2 NIC response surface

Figure 9.15 shows that NIC decreases with lower seat back cushion stiffness.

83

CHAPTER 10

RESULTS AND DISCUSSION

10.1 SELECTION OF BEST DESIGN VARIABLE SET

The injury criterions were calculated using Hyperstudy from the binout and femd3plot

files. The DOE-2 results showed that Run9 with decreased seatback cushion stiffness

and reduced backset and heights have the lowest injury levels.

Seatback Cushion Stiffness

HR foam stiffness HR Height HR Backset

Baseline model

- - 76mm 92mm

DOE-2 Run 9

decreased by 30% Same as baseline model

36mm 40mm

Table 10.1: modifications done to DOE-2 Run9

10.2 BASELINE MODEL VS DOE–2 RUN9 INJURY CRITERIA COMPARISON

1. Reduction in Upper Neck Shear Force (Fx) by 44% compared to the baseline model.

Figure 10.1: improved upper neck shear force

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2. Reduction in Upper Neck Tension Force (Fz) by 55% compared to the baseline model.

Figure 10.2: improved upper neck tension

3. T1-x-acceleration is getting increased by 10% compared to the baseline model.

Figure 10.3: increase in T1-x-acceleration

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4. Reduction in Rebound Velocity by 7%

Figure 10.4: improved rebound velocity

5. Reduction in Neck Injury Criterion (NIC) by 31% compared to the baseline model.

Figure 10.5: improved Neck Injury Criterion

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10.3 CONCLUSIONS

Simulation for high severity sled pulse and evaluation of rear impact whiplash

injuries was successfully done using Ls Dyna.

Various seat design parameters and its effects were studied through DOE using

Hyperstudy and LsDyna.

DOE using Hyperstudy was found effective to know the interactions of whiplash

design variables and their effect on injury output criterions.

An optimal set of design parameters which gives least whiplash injuries were

identified after running two DOE setups and analyzing all the simulations with

different design variable combinations.

It was found out from first DOE setup that the HR backset and HR height are

having the highest effect on all injury criterions.

Seat back cushion stiffness has the highest effect on NIC after HR backset and

HR height.

To reduce NIC levels seat back cushion stiffness was chosen as a design

parameter for the second DOE setup.

By the modifications done to the seatback cushion stiffness, head restraint height

and backset the injury levels were significantly reduced.

Decreasing the seat back cushion stiffness helped to reduce the Neck Injury

Criterion. The head restraint contact time (T-HRC) was reduced by 30ms.

The upper neck shear force (Fx) was reduced by 44%.

The upper neck tension (Fz) was lowered by 55%.

However the T1-x-acceleration was increased by 10% but the values are within

the performance limits.

7% reduction in Rebound Velocity.

Neck Injury Criterion was reduced by 31% compared to baseline model.