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

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

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

4

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

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

6

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

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

8

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]

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

10

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