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Accident Reconstruction Goals of Accident Reconstruction – What happened? • Investigation of product/environment needs Data Collection • Characterise/document crashes for data processing – Litigation • Assessing the cause of injury and collision cause Accident Reconstruction Data Collection / Accident Investigation – Accident type, vehicles, injuries – Gross motions of vehicles and occupants • Reconstruction – Calculate severity – Detailed simulation of system dynamics Organisations Road Designers – Highway Department Vehicle Designers – Manufacturers, Safety Engineers Medical / Human Factors Researchers – doctors, ergonomics Terminology Crush : Quantity of Vehicle Damage Yaw - Vehicle Rotation or Combination of rotation/forward motion Delta () V: Change of velocity EBS, EES : Equivalent Barrier (Energy) Speed PDOF : Principal Direction of Force • Overlap • Underride/Override Point of Impact (POI) / Point of Rest (POR) Overlap Crush Location Direction Terminology Haddon’s Matrix Pre Crash Crash Post Crash Human Vehicle Environment

Accident Reconstruction

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Page 1: Accident Reconstruction

1

Accident Reconstruction

• Goals of Accident Reconstruction– What happened?

• Investigation of product/environment needs– Data Collection

• Characterise/document crashes for data processing

– Litigation • Assessing the cause of injury and collision

cause

Accident Reconstruction

• Data Collection / Accident Investigation– Accident type, vehicles, injuries– Gross motions of vehicles and

occupants• Reconstruction

– Calculate severity– Detailed simulation of system dynamics

Organisations

• Road Designers– Highway Department

• Vehicle Designers– Manufacturers, Safety Engineers

• Medical / Human Factors Researchers– doctors, ergonomics

Terminology• Crush : Quantity of Vehicle Damage • Yaw - Vehicle Rotation or Combination of

rotation/forward motion• Delta (∆) V: Change of velocity• EBS, EES : Equivalent Barrier (Energy)

Speed • PDOF : Principal Direction of Force• Overlap• Underride/Override• Point of Impact (POI) / Point of Rest (POR)

OverlapCrush

Location

Direction

Terminology Haddon’s Matrix

PreCrash

Crash PostCrash

Human

Vehicle

Environment

Page 2: Accident Reconstruction

2

Crash Parameters

• Delta V:– is NOT the speed of impact– described the speed change during

a crash phase– has been a historical measure of

accident severity

Example of Vehicle Accelerations

Crash History Case 1

0

10

20

30

0 0.1 0.2 0.3

Acc

eler

atio

n [g

]

0

5

10

15

20

Vel

oci

ty [m

/s]

∆V

Crash History Case 2

0

20

40

60

80

0 0.05 0.1 0.15

Time [s]

Acc

eler

atio

n [g

]

0

5

10

15

Vel

oci

ty [m

/s]"Braking""Crash"

∆V

Delta V

V1V2

V1

V2 ∆V

Principal Direction of Force

• PDOF represents the line of action for the force exerted on a vehicle during a collision

PDOF

Why Delta V?

• Historical - Unbelted Occupants– Correspondence to occupant

impact with vehicle interior• Practical

– Can be easily calculated• Biomechanical - Response

– acceleration * duration

Energy

• EBS / BEV / EES / ETS– equivalent impact speed for a rigid

barrier test– Energy Absorbed in vehicle

damage is expressed as an equivalent speedE= 1/2 MV2

V=EBS/BEV/EES/ETS

Page 3: Accident Reconstruction

3

Energy Equivalent SpeedWhen are Delta-V and EBS Different?

1) When the crush is not equally distributed over the 2 collision partners2) When the vehicle does not come to rest at the end of the crash phase

Scene Data Complete Reconstruction

• Requires information for all three phases

• Critical information– Point of Impact– Point of Rest

Pre-Crash• Response time = Perception-Decision Time +

Reaction Time• 85th percentile of drivers = 1.6s + 1.5s = 3.1s• distance for response time

– 100 km/h => 28 m/s => 87 m• Braking and Steering: Driver Reactions

Time

Spe

ed

0

Responsetime

Ped

al A

ctio

nB

rake

ac

tivat

ion

Braking

Braking• Passenger Cars

– assume brakes are capable of locking all wheels

– f=mu*g– if not locked wheel, brake efficiency factor η

also known as lockup factor• Trucks

– Brakes may not be capable of locking wheels

– long hills may produce overheating– out of adjustment brakes

Page 4: Accident Reconstruction

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Vehicle Handling/Vehicle Dynamics

• What did the vehicles do prior to collision?

• Begin with tire-road interface– traction circle

Y

X

Friction Force Lat

Friction Force Long

Tire Behaviour

Slip

Force longitudínal slip 20%lateral slip angle 15 deg.

longitudinal slip(v-r*w)/v

α

Tires velocity

ωv

locked / sliding wheel

Yaw

• vehicle on threshold of spinning

• maximum cornering• critical cornering speed• upper bounds on vehicle speed

Critical Speed

• Speed to produce onset of yaw

• Assume:– constant speed– vehicle point

mass– constant friction

ρ

mcrit gV µρ=

Yaw Analysisρ ρ´

28

2 mmC+=ρ

m

chord

Average radius

Example

Page 5: Accident Reconstruction

5

Brake Marks on Site Example

• Slide to stop distance– straight line, locked wheel braking 23 m– road friction measured µ = 0.72

• Speed at start of skid?

brakei

ibrake

fibrakebrake

brake

gdv

mvmgd

vvmdF

KEW

µ

µ

2

2/1

)(2/12

22

=

=

−=

= ∆

v = 18 m/s

ABS Brakes Post-crash

• similar to pre-crash BUT …– tires disabled– vehicle mass distribution changed

• spinout– equivalent friction value

Spin Out

Front Wheels

Rear Wheels

C of G

Each wheel has:- different speed- different distance- different slip angle

Spin Equivalence

Angular Velocity, ωLinear Velocity, V

Actual

Idealised

Time

µµωµµ <⇒= effeff KVf ),,,(

ω ,V

Page 6: Accident Reconstruction

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Yaw mark Skid markYaw mark from a test

Equations of Motion

• estimate effect for each wheel

∑= Fxmrr

&&

∑ ×= rFIrr

αr

F

Crash Dynamics

• Two Approaches– Momentum– Energy

• Options– FEM– Lumped Mass Models

Application Restrictions

• Momentum– Accurate scene evidence– Vehicle Masses– preimpact speed

• Energy– Vehicle Stiffness data– limited crash speed range (20-70

km/h)

Examples

A

B

A

B

Impact

2

2imv

2

2kxE =

V∆1v'iv

time

2

2fmv

Page 7: Accident Reconstruction

7

Crush Model• Background

– Determine severity of accident from vehicle damage

– discovered linear relationship between crush and vehicle impact speed for barrier tests

c

c

V

Energy Approach

• Formal model of energy dissipated in damaged vehicles

Residual Crush

Forc

e/w

idth

A

B

G=Area

A2/(2B)

Energy Approach

• Formulate model of energy dissipated in damaged vehicles - linear force/deflection

Residual Crush

Forc

e/w

idth

A

B

dF

dx

w

A=> N/m, B=>N/m2

BdcdxAdxdF +=

dc

Example

• RenaultA= 45400 N/mB=296900 N/m/mG=3470 N/m

• DamageC1=0.7 mC2=0.44 mL=1.6 m

• RoverA= 67160B= 870000C=2592

• DamageC1=0.35C2=0.2L=1.67

C1

C2

L

Energy Calculation

Residual Crush

Forc

e/w

idth

A

B

G=Area

A2/(2B)

∫ ∫∫

∫ ∫∫++=

+=

=

dxGdxxcB

dxxcAE

dcdxxBcdxxAcE

FdE

2)(

2)(

)()(

dFw

dc

Accident Severity

• Equivalent Barrier Speed (EBS)• Equivalent Energy Speed (EES)• Kinetic Energy change for

vehicle is equal to energy absorbed

mE

v

Emv

B

B

22

2

=

=

v=> EBS, EES

Page 8: Accident Reconstruction

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Calculation of Energy

kmhVkmhV

MM

M

EEV

MM

M

EEV

kmhEESkmhEESME

EES

EE

GLLCCB

LCC

AE

2247

)1(

)(2

)1(

)(2

2833

2

6330086200

)21(22

)21(

21

1

22

212

2

11

211

21

2

1

22

=∆=∆

+

+=∆

+

+=∆

==

=

==

++++

=

Energy Calculation

C1

C2

C3

C4

C5

C6

L2 L3 L4 L5 L6

Crush Measurements

∫∫∫ +=cll

dldcBdlAE

Damagewidth

DamageArea

Crush Measurements Numerical Solution

2

2)()(

BA

G

GareaBwidthAE

=

++=

Area is numerical integration of crush measurements

Crush Coefficients

• Define Vehicle Stiffness, A & B for vehicles

• Defaults based on wheelbase• Individidual vehicle values are

better• Values are becoming available

for side and rear structures

Load Cell Walls

Load cell walls can provide force-deflection information

Load Sensors

0 200 400 600 800 1000Displacement [mm]

400

800

600

200

Forc

e [k

N]

Page 9: Accident Reconstruction

9

Error Sources

• Measurements - accuracy of field measurements

• Stiffness Coefficients -– data specific to vehicles under

investigation– application of rigid barrier tests to car-

car crashes– angled impacts may not deform vehicle

under similar conditions used to generate stiffness data

Error Sources

• Vehicle damage only reflects static crush, elastic rebound not incorporated in formulation

• Vehicle damage only partly describes collision, pre & post crash data needed for complete damage

Validation of CRASH Algorithm to Crash Recorder Data (Default Stiffness

Data)Percent Error

Angle of ImpactsΦ

Vehicle Measurement

• Damage types– Direct – Induced

Actual Case Information

• Problems in the field– No scene data– Missing / unavailable vehicles– Complex impact configurations– Measurement of vehicles

Page 10: Accident Reconstruction

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Crush Measurement

•Exterior Damage Profile

•Undamaged Exemplar Profile

•Crush Profile

Crush can not be measured directly

Crush Measurement Side

Side Impacts

Pole Impact Car-Car

Software Tools• CRASH 3• WinCrash (CRASH)

– Damage and Momentum

Damage Analysis Fundamentals

Estimate crush energy

of both vehicles

Estimate weights

Ma and Mb

Calculate Closing Speed

Calculate vehicles’Delta-V

1

2

3

4

Estimate vehicle speedsfrom closing speed using additional info

Optional