Research Problem:
Applying the
‘3 Millisecond Clip Acceleration’ Criterion
to
Playground Safety
Ian Currie, C.E.T., C.C.P.I.
Alert Property Inspections
Winnipeg, Manitoba
04 July 2010
Research Problem:
Problem:
Applying the ‘3 Millisecond Cl
Clip
ip Acceleration’
Acceleration’ Criterion
Criteri on to Playground Safety
Background
Throughout North America and other developed nations, playground protective surfacing materials are deemed to
protect against debilitating or life threatening injures to the head if the impact forces sustained during a fall do not
exceed 200 g and the Head Injury Criterion (HIC) does not exceed 1000 (a unit-less value). If one of the two thresholds
are exceeded (200 g or 1000, respectively) during impact testing the surface fails the test and is deemed to be unsafe.
The g-max and HIC thresholds were originally developed for automotive safety standards pertaining to
crashworthiness testing and have been ‘universally’ adopted by the playground safety industry, becoming a
requirement included in related standards (i.e. ASTM F1292, Standard Specification for Impact Attenuation of Surfacing
Materials within the Use Zone of Playground Equipment and CSA Z614, Children’s Playspaces & Equipment).
It is important to also note that many experts in the field of biomechanics have agreed that a human head / brain is
likely to become seriously injured if a force of 80 g’s or greater is sustained for a duration of 3 consecutive milliseconds
or more. Because of this agreed-upon conclusion, automotive safety organizations (world wide) use an additional
safety criterion known as the ‘3 ms Clip Acceleration’ (‘A-3ms’) criterion. This criterion is primarily found in
automotive impact tests that simulate head impacts against interior components such as dashboards and seatbacks.
An interior component fails an impact test if impact forces of 80 g’s or more are sustained for 3 consecutive
milliseconds (or longer) during the impact event duration. Federal Motor Vehicle Safety Standards (FMVSS) 201,
Occupant Protection in Interior Impact is just one example of an automotive safety impact standard that includes the
‘A-3ms’ criterion.
Conversely, FMVSS 208, Occupant Crash Protection, is an example of an automotive crash test standard that does not
include the ‘A-3ms’ criterion. In this crashworthiness standard, HIC is the only criterion reported with respect to head
injury potential. The reason for this is due largely to the fact that FMVSS 208 is used to test the ability for seat belts
and airbags to protect the driver and front seat passenger. Seat belt use, combined with air bag deployment (and
built-in crumple zones) is so effective at reducing impact forces that both g-max and ‘A-3ms’ are easily met; therefore,
are not reported (HIC must be reported and is the basis of the popular NHTSA crash test ‘star’ rating). The impact
waveform graphs from two crashworthiness tests are provided below. The one on the left shows the resulting g-max,
HIC and A-3ms results from a test where airbags deployed and the one on the right displays results from a test without
an airbag.
Greater impact forces are experienced when a head strikes an interior component (when compared to head-to-airbag
impacts); resulting in higher ‘A-3ms’ results; however, the increased impact forces still don’t typically result in g-max
or HIC scores that exceed their failure thresholds.
Based on the aforementioned, I have developed four separate hypothesis regarding playground safety standards
(described in detail on the following pages). The general concept of my hypotheses is that I believe that the ‘A-3ms’ =
80g criterion is the most applicable criterion when considering protective surface impact testing and therefore I
believe that the ‘A-3ms’ = 80g criterion needs to be included in all playground protective surface impact testing
standards and related playground safety legislation, guidelines and best practices. It is also my belief that the
exclusion of this criterion has resulted in (and will continue to result in) playground surfaces being deemed ‘safe’ (i.e.
unlikely to result in debilitating or life threatening injuries), even when a risk of debilitating or life threatening injuries
truly does exist. This oversight or misapplication of the automotive impact testing criteria is placing children in
unnecessarily dangerous situations since surfacing and equipment can be altered / adjusted in ways that will allow
playground falls to yield ‘A-3ms’ results that are below 80 g’s.
See the attached one page document I created that compares FMVSS 201, FMVSS 208 and ASTM F1292. Upon reviewing
the attachment you will notice that the test apparatus, test conditions and typical impact durations related to ASTM
F1292 more closely resemble FMVSS 201 than FMVSS 208. This comparison, in my opinion, supports my belief that the
‘A-3ms’ criterion needs to be included in the ASTM F1292 playground protective surface impact testing procedure.
Ian Currie, C.E.T., C.C.P.I. (Alert Property Inspections)
1 of 2
General Hypotheses
Hypothesis No. 1:
I believe that the 3 ms Clip Acceleration (‘A-3ms’) criterion will typically be the criterion that limits or defines a
playground protective surface’s critical fall height (instead of HIC or g-max) when 80 g is the 3ms threshold gforce (g) value. Similarly, approaching this hypotheses from a slightly different angle, I also believe that the ‘A3ms’ criterion will typically result in reducing a surfacing material’s critical fall height (compared with using gmax and HIC alone).
Hypothesis No. 2:
Since rubber surfaces (PIP, tiles and rubber mulch) produce ‘wider’ force-over-time impact waveforms (when
compared to other playground protective surfacing materials), I believe that rubber surfaces will be the material
most significantly effected by the 3 ms Clip Acceleration (‘A-3ms’) criterion.
Hypothesis No. 3:
I believe that the head injury criteria (g-max and HIC), when used without incorporating the 3 ms Clip
Acceleration (‘A-3ms’) criterion, are not sufficient to determine if a playground’s protective surfacing will
adequately reduce the probability of debilitating or life threatening head injuries (sustained from a head-first fall).
Hypothesis No. 4:
I believe that an unacceptable percentage of playground protective surfaces have passed (and surfaces will
continue to pass future surface impact tests) when using only the existing head injury criteria (g-max and HIC).
Therefore; I also believe that there are an unacceptable percentage of playground protective surfaces that will
likely result in debilitating or life threatening head injury should a child fall head-first onto such a surface (despite
the fact that the surface has been, or could be deemed CSA Z614 or ASTM F1292 compliant).
Experimental, Statistical Data Analysis
Independent Variables:
1. Drop / Fall Height
a. Critical Fall Height (CFH), CSA Defined Fall Height & CSA Loose-Fill Depth Recommendations
i. manufacturer’s tested CFH
ii. CSA Z614’s defined fall height
iii. CSA Z614’s loose-fill recommended depths
b. Specified drop test heights for the purpose of this experiment: Adopt the ASTM F1292 laboratory
drop height testing procedure for conducting 1 ft increment drops, starting at a height of one ft
and then stopping after ‘A-3ms’ = 80g or greater (to determine ‘A-3ms’ critical height).
2. Material type
3. Material thickness
4. Other material characteristics and site conditions (air temperature, material temperature, material age,
density, moisture content, contamination, base material type, etc)
Dependent Variables (expected results):
1. Drop / Fall Height
a. (i. – iii.) Surfaces installed to current specs will typically fail the ‘A-3ms’ = 80g criteria.
Regardless, drop testing at the CFH and CSA defined fall height will provide valuable data for
comparison purposes
b. Drop testing at 1 ft increments will allow determination of the critical height for all surfaces
tested, based on the ‘A-3ms’ = 80g criterion
2. Material type: Different material types will yield different ‘A-3ms’ criteria results. It is expected that
rubber surfaces, installed according to the equipment fall height, will exceed the ‘A-3ms’ criteria at lower
drop test heights than other material types also installed according to the equipment fall height (i.e.
wood chips, EWF, sand, pea gravel, etc).
3. Material thickness: With all other factors being equal, thicker layers of protective surfacing will yield
higher A=3ms g’s whereas thinner layers will yield lower A=3ms g’s.
4. Other material characteristics and site conditions:
a. Density: Denser surfacing material will yield lower A=3ms g’s than a less-dense surface of the
same material type
b. Material Temperature: When considering all protective surface materials (except for rubber
surfaces) minimal change in A=3ms g’s will be experienced when tested in a wide range of
different temperatures. Alternatively, when considering rubber surfaces, temperature changes
will have a significant impact on A=3ms results (lower temp = lower g’s and higher temps =
higher g’s).
Ian Currie, C.E.T., C.C.P.I. (Alert Property Inspections)
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Comparison of FMVSS 201, FMVSS 208 & ASTM F1292
Vehicle Interior (Instrument Panels, Seat Backs)
Playground Protective Surfacing
Vehicle Crashworthiness
Applicable Standard
FMVSS 201, Occupant Protection in Interior Impact
ASTM F1292, Standard Specification for Impact Attenuation of
Surfacing Materials within the Use Zone of Playground Equipment
FMVSS 208, Occupant Crash Protection
Test Apparatus
Description
Rigid Aluminum Headform:
External diameter of 6.5 inch in.
Total mass of 14.7 lb ± 0.2 lb
Rigid Aluminum Headform:
External diameter of 6.3 in ± 0.1 in.
Total mass of 10.1 lb ± 0.05 lb
Crash test Dummy:
Padded skull,
attached to neck and body.
Test Conditions
Rammed into interior component
via various methods that create an impact velocity of 14.7
mph ± 0.2 mph (23.7 km/h ± 0.3 km/h).
Released from defined fall height, free-falls onto surface.
Headform strikes surface at velocity determined by fall
height [e.g. a 7 ft drop equates to an approximate velocity
of 14.7 mph (23.7 km/h) at moment of impact].
Restrained in vehicle by seatbelt.
Vehicle strikes solid wall at 35 mph (56 km/h).
Dummy’s head strikes inflated airbag.
Typical Impact Duration
15 ms (based on one example from Honda R&D Americas,
Inc).
About 5 ms – 30 ms depending upon the drop height,
surface type, and surface characteristics.
100 ms or longer.
3 ms Clip Acceleration (A-3 ms)
The test deceleration shall not exceed 80g continuously
for more than 3 ms.
g-max: shall be 200 g or less
HIC: shall be 1000 or less
HIC: shall be 700, 570 or 390 (based on dummy ‘age’)
Test Apparatus Photos
Example Time vs.
Acceleration Graphs
(Impact Waveforms)
Criteria & Criterion
Ian Currie, C.E.T., C.C.P.I. (Alert Property Inspections)
04 July 2010
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