Assessment of Child Restraint Performance

CRASH AND SLED TESTS USING CHILD DUMMIES
Michael Paine
Vehicle Design and Research Pty Limited
Beacon Hill, NSW Australia
Julie Brown
Road safety consultant
Ashbury, NSW Australia
Keywords
child, neck injury, crash test
ABSTRACT: The child restraint designs used in Australia have been shown to provide
exceptional protection to child occupants in severe crashes. Cases of serious injury are likely to
involve misuse of the child restraint.
Dynamic tests of child restraints using sleds are conducted for Australian Standard compliance
and the Child Restraint Evaluation Program.
Child restraints are now included in new vehicle crash tests conducted under the Australian New
Car Assessment Program. These tests are conducted in accordance with protocols developed by
EuroNCAP. There are, however, concerns about the injury criteria set out by EuroNCAP. In
particular, a review of the history of the development of child dummies reveals that it is not
appropriate to base the assessment of child restraint performance on the injury measurements
from the P-series child dummies used by EuroNCAP. These dummies are not biofidelic and there
is no adequate link between dummy responses and the risk of injury in the human child.
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Introduction
This paper is in two parts. The first discusses
methods of assessing the performance of
child restraints (CRs) using child dummies in
dynamic tests. The second discusses the use
of injury measurements from child dummies
in the assessment of these tests.
moving barrier, fitted with a crushable
aluminium front, hitting the driver’s side of
the car at 50km/h.
Since the 1970s dynamic testing of child
restraints has been required under the
Australian Standard for child restraints.
In 1994 New South Wales introduced the
Child Restraint Evaluation Program (CREP)
to provide a guide to consumers. This
program includes sled tests of child restraints.
In 1999 Australian New Car Assessment
Program (ANCAP) aligned its testing
procedures with those of EuroNCAP. Offset
frontal and side impact crash tests of vehicles
now include child restraints in the rear seat.
Although data about child restraint
performance has been gathered during these
tests ANCAP has not released the results due
mainly to concerns about the EuroNCAP
assessment
protocol.
Furthermore,
EuroNCAP recently issued a proposed child
restraint assessment protocol that compounds
these concerns. We were asked by ANCAP to
review the proposed requirements in order to
provide feedback to EuroNCAP.
Dynamic Tests of Child
Restraints
New Car Assessment Program
NCAP assesses the crashworthiness of new
vehicles and provides a star rating for the
protection provided to front seat occupants.
Two types of crash test are used in the
assessment - an offset frontal crash test and a
side impact crash test.
The offset frontal crash is conducted at
64km/h. The vehicle hits a crushable
aluminium honeycomb barrier and the crash
forces are concentrated on the driver’s half of
the vehicle. The side impact involves a
ANCAP Offset Frontal Crash at 64km/h
ANCAP crash test procedures are based on
those used by the European New Car
Assessment Program (EuroNCAP). Under
these procedures two child restraints are
installed in the rear seat of the vehicle. The
child dummies used are TNO P1.5 and P3,
simulating 18 month and 3 year old children
respectively. In the offset frontal crash the P3
sits behind the driver and the P1.5 sits behind
the front passenger. The positions are
swapped for the side impact crash test.
The child dummies are instrumented with
head and chest accelerometers. Dummy
movement is recorded on high speed film and
is analysed to estimate the movement of each
dummy.
The EuroNCAP assessment protocol (Version
3) includes requirements for child restraints.
Due to the fundamentally different design of
child restraints in Australia ANCAP does not
currently apply the child restraint portions of
this protocol and hence does not report the
results of child restraint performance
(discussed in more detail below).
EuroNCAP child restraint assessment
There are separate methods for assessing the
offset and side impact crash tests.
Offset frontal crash test
Head
To obtain a ‘good’ result the P1.5 dummy
head upward acceleration (3ms Z) should not
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exceed 20g. No limit applies to the P3
dummy. Under the proposed changes, a 'poor'
result is obtained if the head upward
acceleration exceeds 40g (P1.5 only).
It is understood that the limit on vertical head
acceleration is intended as a surrogate for risk
of neck injury. We have serious concerns
about this assumption, as discussed later.
A ‘poor’ result (zero score) is also obtained if
the head excursion (relative to the CR point)
exceeds 550mm with either dummy. This is
for forward facing CRs. The limit is 600mm
for rearward facing CRs. It is a little difficult
assessing this head excursion due to the effect
of parallax and the difficulty of obtaining
good camera angles (EuroNCAP proposes to
introduce on-board cameras to assist with this
assessment. ANCAP has been using on-board
cameras for several years).
Chest
To obtain a ‘good’ result the 3ms resultant
chest acceleration should not exceed 41g and
the vertical component should not exceed 23g
(3ms) . This applies to both dummies.
A ‘poor’ chest result is obtained if 3ms
resultant chest acceleration exceeds 55g or
the vertical component exceeds 30g (3ms).
This applies to both dummies.
The vertical limit on chest acceleration is
derived from ECE Regulation 44 for child
restraints. However, the ECE Regulation
specifies that the limit applies only to upward
acceleration, possibly with rearward facing
child restraints in mind. In the case of
forward facing child restraints the peak
acceleration is usually downwards and they
are likely to have great difficulty meeting the
EuroNCAP requirement. There appears to be
no justification for applying the limit in both
directions.
Head
3ms resultant head acceleration should not
exceed 80g.
In addition the dummy kinematics are
assessed for "head containment". Version 3 of
the EuroNCAP protocol states ”No part of the
head shall pass outside the forward projected
exterior surface of the child restraint”. It is
difficult to assess this requirement given the
awkward video angles that are available,
particularly with the EuroNCAP videos.
ANCAP uses two onboard cameras that give
a much better view of the child dummies than
the EuroNCAP videos but analysis is still
difficult.
Under proposed changes to the EuroNCAP
protocol the definition of head containment
would be tightened to "The child’s head is
considered to be contained if the energy
absorbing section of the side wing stays
between the side structure of the vehicle and
the child’s head during the impact, and the
supporting seat shell retains its integrity
This proposal encourages the use of headprotecting side wings on the child restraints.
ANCAP Side Impact Crash at 50km/h
Side impact crash test
Australian Standards
The protocol notes that “in the absence of
satisfactory child dummy and bio-mechanical
criteria for side impact, the criteria chosen
have been kept simple”!
Child restraints used in Australia must
comply with AS1754. The dynamic testing
for CRs is set out in AS3629.1. This specifies
sled tests for child seats (type B restraints) as
follows:
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•
a frontal impact at about 49km/h with a
peak deceleration of 24g and
•
a 90 degree side impact test with a peak
deceleration of 14g and an impact speed
of 32km/h.
•
a rear impact test with a peak deceleration
of 14g and an impact speed of 32km/h.
•
Inverted test at 16km/h to simulate a
rollover crash (rearward facing restraints).
Systems are assessed for:
•
retention of the CR
•
retention of the dummy
Assessment Criteria
•
separation of load bearing components
•
fragmentation of rigid components
•
adjuster slip
The CREP assessment criteria include those
covered under AS1754. The following
assessments are made Head acceleration
These assessments are made in all test
configurations.
Child Restraint Evaluation
Program
The Child Restraint Evaluation Program
(CREP) is operated by the NSW RTA,
NRMA and RACV in association with the
Australian Consumers Association. The
outcome is a buyers guide to CRs. The
assessments are based on the Australian
Standard but involve higher crash forces and
additional test procedures. In addition to the
AS1754 tests described above a frontal test at
56km/h and 34g is conducted.
CREP 45o Side Impact with Door Structure
•
Harness strap forces (frontal test)
•
Tether forces, harness forces and seat belt
forces (frontal test)
•
Head displacement (frontal test) –
including rebound – limits apply to
upward and rearward excursion (during
rebound) but not to forward excursion.
•
Head retention (containment) – side
impact tests
•
Retention of device and dummy
•
Adjuster slip
•
Buckle release force (frontal tests)
The side and rear impact tests are conducted
at the same speed as the AS but, in the side
impact test, a structure that is intended to
replicate the interior of a side door is added to
the test configuration.
A side impact test is also conducted at an
impact angle of 45 degrees.
With child seats a P6 dummy is used for the
frontal test and a P3/4 for the other tests.
CREP Frontal Impact Sled Test at 56km/h
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Note that chest decelerations are measured
for infant capsules (Type A) but not child
seats.
similar child restraint systems but they should
not be used for scoring child restraint
performance in NCAP tests.
With most criteria there are no limits set for
performance – the models of restraint are
simply ranked in order of measured values
and good performers tend to stand out in
these lists. There are specific reasons for
excluding child restraints from the ’preferred
buy’ list:
EuroNCAP uses vertical head acceleration as
a surrogate for neck tension. Tests by
Crashlab and others show there is poor
correlation between these parameters.
Furthermore, there is considerable uncertainty
about injury risk to the neck - Australian
crash research indicates that children in
forward-facing child seats survive crashes
that are much more severe than the
EuroNCAP test with no serious neck or other
life-threatening injuries.
Proposed changes by EuroNCAP
•
Not passing requirements of Australian
Standard in all test configurations
•
Head excursion outside prescribed limits
in frontal test or rear impact test
•
Head contact with test rig during side
impact test
Discussion
Real world crashes
Top tethers, as used on all Australian child
restraints, are extremely effective at limiting
forward head excursion - considered to be the
most hazardous feature of child kinematics in
a frontal crash. Australian crash experience
shows that children correctly restrained in
forward facing child seats with top tethers can
withstand severe crash forces without serious
head, neck or chest injuries (Henderson
1994). "Restraint design should place a high
priority on the minimisation of excursion of
the upper body in order to prevent head
contact" (Henderson 1995)
EuroNCAP has proposed changes to the
assessment of child restraints and has issued a
draft protocol for discussion. This introduces
several new requirements.
Data from 13 offset crash tests conducted by
ANCAP since 1999 have been analysed.
Briefly the results for the 26 child restraints
are:
•
None reached the proposed upper limit on
resultant head acceleration of 88g (3ms) ,
although several peaks exceeded 80g
(usually associated with head to leg
contacts - in all P3 tests the dummy head
hit its legs but note that no head
acceleration limits apply to the P3). Three
exceeded the lower limit of 72g.
•
None was below the proposed upper limit
on vertical head acceleration of 40g (3ms)
meaning that all would score zero points.
The lowest value was 48g. With these
restraint configurations the maximum
(3ms) vertical head acceleration is very
similar to the maximum (3ms) resultant
head acceleration in the absence of head
contacts.
•
None reached the proposed upper limit on
resultant chest acceleration of 55g (3ms).
Seven exceeded the lower limit of 41g.
•
Nine were above the proposed upper limit
on vertical chest acceleration of 30g
(3ms), meaning they would score zero
points. Seven of these were P3 dummies.
Concerns about the use of child
dummy injury measurements
This issue is dealt with in more detail in a
separate paper (Brown 2001). In brief, there
are concerns about the lack of biofidelity in
the P-Series child dummy, the lack of
research linking dummy injury measurements
with injury risk in humans and the
assumptions about the risk of neck injury
with forward facing restraints.
Head and chest accelerations are not reliable
indictors of injury risk with the P-series child
dummies. These measurements may be useful
for comparing the relative performance of
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Head Acceleration Results ("Head Strike" means head hit legs)
Chest Acceleration Results ("Head Strike" means head hit legs)
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•
A further two P3s and 3 P1.5s exceeded
the proposed lower limit of 23g. In all
cases the acceleration was directed
downwards and so would not have failed
the ECE 44 requirement.
The proposed scoring system takes the worst
of scores for head acceleration (P1.5 only),
chest acceleration and head excursion
therefore most of the Australian child
restraints would score zero points. This is at
odds with crash experience in Australia that
shows children in crashes of greater severity
do not receive life-threatening injuries in the
absence of head contacts.
The danger from implementing the criteria
proposed by EuroNCAP is that vehicle and
restraint designers will need to build greater
compliance into the child restraint system in
order to reduce head and chest loads. This
will inevitably lead to greater excursion of the
child and much greater risk of life-threatening
head contacts (Webber 2000)..
Conclusions
The child restraint designs used in Australia
have been shown to provide exceptional
protection to child occupants in severe
crashes. Cases of serious injury are likely to
involve misuse of the child restraint or gross
intrusion.
A wealth of data about dynamic performance
of child restraints is being acquired through
the NCAP and CREP programs. This
information has potential for use in road
safety research in addition to its primary
purpose of rating the crash performance of
vehicles and child restraints.
There are serious obstacles to the use of
proposed EuroNCAP assessment procedures
for child restraints and application of these
procedures could result in less safe child
restraints in Australia.
Until several issues concerning the use of
child dummies for predicting injury risk in
children are resolved it is recommended that
child restraints are not rated on the basis of
dummy injury measurements. In particular,
the head and vertical chest accelerations
should not form part of the scoring system. At
this stage head acceleration measurements
should only be used to confirm undesirable
head contacts.
Implementation of the EuroNCAP proposals
is likely to be counter-productive in Australia
because it would probably result in greater
head excursion and therefore increased risk of
life-threatening head contacts.
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The paper is derived from a research report
prepared for Australian NCAP and the
assistance of the ANCAP Technical
Committee is gratefully acknowledged.
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