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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)
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