© M. K. Verma mkverma2002@gmail.com

TRENDS IN AUTOMOBILE SAFETY:

ANALYSES OF RECENT NCAP FRONT CRASH TESTS

This article discusses some findings about automobile safety in frontal crashes. As is

known, the NHTSA as part of their ‘New Car Assessment Program’ (NCAP) with the

stated intent of helping customers select vehicles that are ‘safer’ according to NHTSA’s

ranking of ‘more stars’. This rating system was revised some years ago for automobiles

of model years 2011 and later (http://www.safercar.gov/ staticfiles/safetytech/st_landing

_ca.htm). One of the interesting aspects of the new NCAP is that NHTSA publishes an

‘overall score’ for the vehicle’s crashworthiness by combining the scores from frontal

crash, side impacts and rollovers. Other significant changes in the NCAP are the addition

of a test dummy representing small occupants, the inclusion of new injury-related

parameters and significantly raised thresholds for achieving the highest (five stars) rating.

Since wide publicity is given to NCAP scores and all vehicle manufacturers try to

achieve the highest possible ratings for their new product, these ratings serve as mandates

in vehicle design.

NCAP Tests for Frontal Crashes: The test configuration for this mode is 35 miles per

hour impact of the vehicle into a fixed rigid barrier. This simulates crashes with fixed and

rigid objects on the road and also represents full

frontal crash of the car with another identical car.

In these tests, the driver is represented by an

anthropomorphic test device (ATD) simulating a

fiftieth percentile male whereas the front passenger

ATD simulates a fifth percentile female. The

measured parameters are the responses of head,

chest, femurs as well as the forces and moments on

the neck. These measurements are used to calculate

‘relative risk’ scores which are then translated into separate ‘stars’ for the driver and the

passenger. According to NHTSA, the following relationship exists between the ratings

(‘stars’) and the risk of injury in frontal crashes.

= 10% or less chance of serious injury

= 11% to 20% chance of serious injury

= 21% to 35% chance of serious injury

= 36% to 45% chance of serious injury

= 46% or greater chance of serious injury

ANALYSIS OF NHTSA’S TEST DATA

Background:

The figures below show a driver ATD in a test as well as its response parameters. All the

responses have been normalized (with different normalization factors as described in

NCAP test procedures) so that they can be compared in one plot.

35 mph

35 mph35 mph

35 mph

35 mph35 mph

© M. K. Verma mkverma2002@gmail.com

Several observations can be made from the above plots. The ‘time to zero’ for vehicle CG

occurs at approximately 70 milliseconds in this test, followed by a rebound. The initial

contacts of the driver ATD body segments (femurs, chest and head) occur close together

(~20 milliseconds). The maximum chest displacement (in x-direction) and the maximum

head acceleration (the x-component) occur in the 70 -80 millisecond timeframe. For the

neck bending moment My (extension or flexion) and neck force Fz (tension or

compression), both the positive and the negative maxima are used in the computation and

it is therefore important to consider the occupants’ motion during both the loading and

the rebound phases.

The exterior structure of the vehicle crushes to dissipate a portion of the crash-induced

kinetic energy and thereby improves the protection for properly restrained occupants. All

other factors remain constant, the larger the area under the curve, the more the

deformation of the front end.

Vehicle Structure Performance:

Shown below are some plots of velocity-versus-time measured near the vehicles’ CG

from tests of different vehicles. The legends associated with each curve designate the

tested vehicle by a letter (e.g. ‘C’) and also show its NCAP score, e.g. the first two

characters are the number of stars for the driver (‘3D’ means three stars for the driver),

the next two are the number of stars for the passenger and the last two show combined

frontal test score (‘3F’ means NHTSA’s combined rating of three stars for the vehicle’s

driver and passenger in frontal crash).

N C A P F ro n ta l C rash Tes t - V eh ic le C G D ecele ra tio n

-5

0

5

10

15

20

0 0 .0 2 0 .04 0 .06 0 .08 0 .1

T im e (s ec)

V (

m/s

ec

)

A rea unde r the cu rve

equa ls veh ic le ’s fron t

c rush

S lope o f the tangent

is the dece le ra tion o f

veh ic le cg

N C A P F ro n ta l C rash Tes t - V eh ic le C G D ecele ra tio n

-5

0

5

10

15

20

0 0 .0 2 0 .04 0 .06 0 .08 0 .1

T im e (s ec)

V (

m/s

ec

)

A rea unde r the cu rve

equa ls veh ic le ’s fron t

c rush

S lope o f the tangent

is the dece le ra tion o f

veh ic le cg

N C A P F ro n ta l C rash Tes t - V eh ic le C G D ecele ra tio n

-5

0

5

10

15

20

0 0 .0 2 0 .04 0 .06 0 .08 0 .1

T im e (s ec)

V (

m/s

ec

)

A rea unde r the cu rve

equa ls veh ic le ’s fron t

c rush

S lope o f the tangent

is the dece le ra tion o f

veh ic le cg

7002 Sonata NCAP (F 4, D 5, P 4) ; Vehicle & Driver ATD Response

-10

-5

0

5

10

15

20

0 0.05 0.1 0.15 0.2 0.25 0.3Time (sec)

7002RS XM Vx m/sec0.005*7002D LF Load N0.5*7002D Chest D mm0.25*7002D Neck My N-m0.01*Neck Fz7002D Head X-aacn

Vehicle CG

Velocity Head X-acceleration

Chest X-Displacement

Neck MyNeck Fz

Left Femur Load

0

1

-0.5

Front NCAP Crash Test – Time history of Vehicle & Driver ATD Response

Initial contact

-Chest, Head, Knees

© M. K. Verma mkverma2002@gmail.com

Chest Compression:

The chest displacement of the ATD is a

function of its impact velocity, the force-

displacement-time properties of the

seatbelts and the shape, size and

deployment dynamics of the driver airbag.

To reduce the chest x-displacement, it

is necessary that will all safety-related

components e.g. seatbelts, airbag, knee

bolsters, seats, etc., be optimized as an

‘integrated system’. The plot below

shows chest x-displacements of driver

ATD from several crash tests. In calculating the NCAP score, the measured chest displacement

is compared to a reference value (63 mm for the driver, 52 mm for the passenger).

Similar results are observed for measured chest displacements of passenger ATD (not

shown here). Since this ATD represents a fifth percentile female for which a ‘base’ value

published by NHTSA (52 mm) is lower than that for the driver, it is obvious that the

passenger ATD has relatively larger influence on the vehicle’s NCAP ratings. Some of

the shown vehicles (C, D, J) with high values of chest compression therefore have a

relatively large ‘relative risk’ and lower star ratings.

Neck Injury Parameters:

The NHTSA defines [1] a parameter Nij which combines the

possible modes of neck loading - tension or compression combined

with either flexion or extension (forward or rearward bending):

Nij= Fz/ FCritical + My/ Mcritical

The values for Fcritical and Mcritical are defined differently for the two

ATDs. The neck injury probability calculation is based on

comparing Nij and the tension/compression to ‘base’ values.

Fz

My

Fz

My

NCAP Data for 2011 & 2012 Vehicles -

Analysis of Frontal Velocity-vs-Time History

-5

0

5

10

15

20

0 0.02 0.04 0.06 0.08 0.1Time (sec)

V (

m/s

ec

)

A (5D,5P,5F) RSXM V B (5D,4P,4F) RSXM V C (3D,2P,3F) RSXM VD (5D,4P,4F) RSXM VE(5D,4P,5F) RSXM VF (4D,3P,4F) RSXM VG(5D,4P,4F) R XM VH (5D,5P,5F) RSXM VJ(5D,4P,4F) RSXM V K(4D,2P,3F) RSXM V

NCAP Data for 2011 & 2012 Vehicles - Analysis of

Driver Chest X Displacement (mm)

-60

-40

-20

0

20

0 0.1 0.2 0.3

Time

MM

A (5D,5P,5FD Chest D mmB (5D,4P,4F) D Chest D mmC(3D,2P,3F)D Chest D mmJ(5D,4P,4F) D Chest D mmH (5D,5P,5F) D Chest D mmF (4D,3P,4F) D Chest D mmG(5D,4P,4F) D Chest D mmD (5D,4P,4F) D Chest D mm

NCAP Data for 2011 & 2012 Vehicles - Analysis of

Driver Chest X Displacement (mm)

-60

-40

-20

0

20

0 0.1 0.2 0.3

Time

MM

A (5D,5P,5FD Chest D mmB (5D,4P,4F) D Chest D mmC(3D,2P,3F)D Chest D mmJ(5D,4P,4F) D Chest D mmH (5D,5P,5F) D Chest D mmF (4D,3P,4F) D Chest D mmG(5D,4P,4F) D Chest D mmD (5D,4P,4F) D Chest D mm

© M. K. Verma mkverma2002@gmail.com

Measured responses from several tests are shown below for the driver ATD. The first figure

below is plot of the moments (My) whereas the second figure shows the neck forces (Fz). The

peak values of the bending moment and of the tension/compression forces may occur at different

instants in the same vehicle. It should be mentioned that all of the vehicles shown in the plot

except ‘C’ and ‘F’ achieved 5-star rating for the driver.

Similar data are observed for passenger ATD in these tests. The reference values for neck

injury probability are relatively lower for the fifth percentile female and their measured

neck forces and moments have a relatively larger contribution to the relative risk

computations for the passenger-side ATD and to the NCAP rating.

COMMENTS: The changes in the NCAP test and measurement procedures described

above have led to several improvements in automobile designs. The development of more

efficient structures along with the optimization of safety systems in the vehicle are both

necessary for higher ‘star’ ratings. Possibilities also exist for utilizing pre-crash systems

[2] and pre-tightening the seatbelts to further improve performance in frontal crashes.

References

1. Eppinger et al ,“Development of Improved Injury Criteria for the Assessment of

Advanced Automotive Restraint Systems – II” , http://www.nhtsa.gov/ DOT/

NHTSA/NRD/Multimedia/PDFs/Crashworthiness/Air%20Bags/rev_criteria.pdf

2. Verma and Goertz, “Evaluation of Pre-Crash Safety Systems Effectiveness”, SAE

Paper 2010-01-1042.

NCAP Data for 2011 & 2012 Vehicles -

Analysis of Driver Neck Moment My (N-M)

-60

-40

-20

0

20

40

60

80

100

0 0.1 0.2 0.3Time

N-M

A (5D,5P,5F D Neck My NMB (5D,4P,4F) D Neck My NMC(3D,2P,3F) D Neck My NMJ(5D,4P,4F) D Neck My NMH (5D,5P,5F) D Neck My NMF (4D,3P,4F) D Neck My NMG(5D,4P,4F) D Neck My NMD (5D,4P,4F)D Neck My NM

NCAP Data for 2011 & 2012 Vehicles -

Analysis of Driver Neck Moment My (N-M)

-60

-40

-20

0

20

40

60

80

100

0 0.1 0.2 0.3Time

N-M

A (5D,5P,5F D Neck My NMB (5D,4P,4F) D Neck My NMC(3D,2P,3F) D Neck My NMJ(5D,4P,4F) D Neck My NMH (5D,5P,5F) D Neck My NMF (4D,3P,4F) D Neck My NMG(5D,4P,4F) D Neck My NMD (5D,4P,4F)D Neck My NM

NCAP Data for 2011 & 2012 Vehicles -

Analysis of Driver Neck Force Fz (N)

-1000

0

1000

2000

3000

0 0.1 0.2 0.3Time

Ne

wto

n

A (5D,5P,5F) D Neck FzB (5D,4P,4F) D Neck FzC(3D,2P,3F) D Neck FzJ(5D,4P,4F) D Neck FzH (5D,5P,5F) D Neck FzF (4D,3P,4F) D Neck FzG(5D,4P,4F) D Neck FzD (5D,4P,4F) D Neck Fz