helmets and rotational acceleration

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.
They also render some previous comments ('no testing ever looks at
oblique impacts') untrue, which might require minor revisions of some
web sites (Guy?).

I'd need to pay for them (so have only read the abstracts), but
others with free-er access to these sorts of things might be
interested:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

Abstract

The performance of bicycle helmets was investigated in oblique impacts
with a simulated road surface. The linear and rotational accelerations
of a headform, fitted with a compliant scalp and a wig, were measured.
The peak rotational accelerations, the order of 5 krad s−2 when the
tangential velocity component was 4 m s−1, were only slightly greater
than in comparable direct impact tests. Oblique impact tests were
possible on the front lower edge of the helmet, a site commonly struck
in crashes, without the headform striking the ‘road’. Data
characterizing the frictional response at the road/shell and
helmet/head interfaces, were generated for interpretation via FEA
modelling.



Finite-element analysis of bicycle helmet oblique impacts

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1087-1101

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006

Abstract

Finite-element analysis (FEA) was performed for bicycle helmets making
oblique impacts with a road surface, to evaluate the linear and
rotational accelerations of the headform. Helmet rotation on the head
was considered, modelling the helmet and retention strap interactions
with the headform. The effects of frictional parameters on the
response were explored, and parameters selected to reproduce
experimental results. Predictions were made for two helmets, for a
range of impact locations and tangential velocities. The design method
for the peak headform linear acceleration was confirmed; it was hardly
affected by the tangential component of the impact velocity. The peak
headform rotational acceleration was investigated as a function of the
helmet geometry, impact sites and velocities and the contributing
mechanisms established.


regards, Ian SMith

--
|\ /| no .sig
|o o|
|/ \|

On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.
They also render some previous comments ('no testing ever looks at
oblique impacts') untrue, which might require minor revisions of some
web sites (Guy?).

I'd need to pay for them (so have only read the abstracts), but
others with free-er access to these sorts of things might be
interested:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

Abstract

The performance of bicycle helmets was investigated in oblique impacts
with a simulated road surface. The linear and rotational accelerations
of a headform, fitted with a compliant scalp and a wig, were measured.
The peak rotational accelerations, the order of 5 krad s−2 when the
tangential velocity component was 4 m s−1, were only slightly greater
than in comparable direct impact tests. Oblique impact tests were
possible on the front lower edge of the helmet, a site commonly struck
in crashes, without the headform striking the ‘road’. Data
characterizing the frictional response at the road/shell and
helmet/head interfaces, were generated for interpretation via FEA
modelling.



Finite-element analysis of bicycle helmet oblique impacts

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1087-1101

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006

Abstract

Finite-element analysis (FEA) was performed for bicycle helmets making
oblique impacts with a road surface, to evaluate the linear and
rotational accelerations of the headform. Helmet rotation on the head
was considered, modelling the helmet and retention strap interactions
with the headform. The effects of frictional parameters on the
response were explored, and parameters selected to reproduce
experimental results. Predictions were made for two helmets, for a
range of impact locations and tangential velocities. The design method
for the peak headform linear acceleration was confirmed; it was hardly
affected by the tangential component of the impact velocity. The peak
headform rotational acceleration was investigated as a function of the
helmet geometry, impact sites and velocities and the contributing
mechanisms established.

Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========

On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.

Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========

I would say you shouldn't conclude anything from the abstracts of two
technical papers. Which is why I'd be keen to hear the views of
anyone that does have access to the content. If it was someone I
trust to be able to read straight, it might even influence my opinion.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.
They also render some previous comments ('no testing ever looks at
oblique impacts') untrue, which might require minor revisions of some
web sites (Guy?).

I'd need to pay for them (so have only read the abstracts), but
others with free-er access to these sorts of things might be
interested:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

Abstract

The performance of bicycle helmets was investigated in oblique impacts
with a simulated road surface. The linear and rotational accelerations
of a headform, fitted with a compliant scalp and a wig, were measured.
The peak rotational accelerations, the order of 5 krad s−2 when the
tangential velocity component was 4 m s−1, were only slightly greater
than in comparable direct impact tests. Oblique impact tests were
possible on the front lower edge of the helmet, a site commonly struck
in crashes, without the headform striking the ‘road’. Data
characterizing the frictional response at the road/shell and
helmet/head interfaces, were generated for interpretation via FEA
modelling.



Finite-element analysis of bicycle helmet oblique impacts

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1087-1101

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006

Abstract

Finite-element analysis (FEA) was performed for bicycle helmets making
oblique impacts with a road surface, to evaluate the linear and
rotational accelerations of the headform. Helmet rotation on the head
was considered, modelling the helmet and retention strap interactions
with the headform. The effects of frictional parameters on the
response were explored, and parameters selected to reproduce
experimental results. Predictions were made for two helmets, for a
range of impact locations and tangential velocities. The design method
for the peak headform linear acceleration was confirmed; it was hardly
affected by the tangential component of the impact velocity. The peak
headform rotational acceleration was investigated as a function of the
helmet geometry, impact sites and velocities and the contributing
mechanisms established.

Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========

To me the second one seems to say, that linear impacts forces were not
affected by the helmet rotating. They also investigated rotational
forces, but do not have the results in the abstract.

The first one says that extra rotational forces were negligible when the
rider hits the ground at 4m/s (about 9mph).

On Aug 11, 1:41 pm, Ian Smith wrote:
The peak rotational accelerations, the order of 5 krad s-2 when the
tangential velocity component was 4 m s-1, were only slightly greater
than in comparable direct impact tests.

Am I reading that correctly? 5000 rad per second per second in an 8mph
crash?

Surely we must be talking microseconds - which I'd have thought is as
likely to be an artifact of the modelling as a real acceleration. In
particular, I don't see how it's possible for those sorts of
accelerations to be transferred to the head from the helmet - or even
from the scalp to the head.

Oblique impact tests were
possible on the front lower edge of the helmet, a site commonly struck
in crashes, without the headform striking the 'road'. Data
characterizing the frictional response at the road/shell and
helmet/head interfaces, were generated for interpretation via FEA
modelling.

And here we seem to be talking about crashes where the head would miss
the ground other than for the helmet.

It certainly would be interesting to see the papers - although I
suspect I'm not qualified to critique them as FEA is something I've
never done.

Tim.

Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:

Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========

I would say you shouldn't conclude anything from the abstracts of two
technical papers. Which is why I'd be keen to hear the views of
anyone that does have access to the content. If it was someone I
trust to be able to read straight, it might even influence my opinion.

I would concur with Ian's analysis.

But, for the sake of argument, let's assume your take is a fair one.
The possibility of rotational injury aggravation by helmets has
primarily been a conjectured mechanism for how they might make things
worse. Even if you demonstrate that as a non-issue, it will not affect
in any way, shape or form the whole population studies that demonstrate
no clear advantage in terms of serious injuries to wearing a cycle helmet.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/

Ian Smith wrote:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

A quick search of the web also finds this:


http://www.asbweb.org/conferences/2004/pdf/357.pdf

Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.

Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========

I would say you shouldn't conclude anything from the abstracts of two
technical papers. Which is why I'd be keen to hear the views of
anyone that does have access to the content. If it was someone I
trust to be able to read straight, it might even influence my opinion.

Having scanned the two papers (which are related: the physical testing
generates paramenter values for the finite element analysis) the main
conclusion appears to be that the coefficient of friction between the
headform (with anatomically correct scalp + hair) and the helmet is
too low to lead to rotational forces in a 'typical impact' which are
large enough to cause brain damage.

I don't pretend to be either a materials scientist or somebody with
experience in analysing this kind of paper, so I'm not going to
critique their methodology. It seemed perfectly reasonable to me
froman 'outsider' perspective however.

They acknowledge that answering the question of whether the headform
stiffness makes a real difference to the measured outcomes will
require experiments with cadaver heads. Any volunteers?

Phil

--
http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt

On Aug 11, 2:48 pm, Martin wrote:
Ian Smith wrote:
Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

A quick search of the web also finds this:

http://www.asbweb.org/conferences/2004/pdf/357.pdf

Perpendicular impacts yielded an average
angular acceleration of –506 ± 419 rad/s2, with
the negative value denoting counterclockwise
acceleration. 30° LF and 30° HF yielded a
significantly higher (clockwise) angular
acceleration (p0.01) of 5844 ± 265 rad/s2 and
8846 ± 422 rad/s2, respectively (Figure 3).

Greater than 10x the angular acceleration using mid point figures.
Best case (closest to zero on the error bars) figures, 50x angular
accelerations and worst case, 5x

However, I'm not convinced even this test tells us anything useful as
written[1]. We need to compare the angular acceleration with and
without the helmet. I can't even begin to guess which would come out
higher. And then, we need to ask how many cases there are where the
only reason for a head impact (at speed) is due to the larger head
with helmet and this then depends on reflexes etc.

Tim.

[1] That wasn't the point of these tests which was in order to show
how these measurements could be made, not necessarily to get useful
data.

quote
However,
it effectively captured the angular acceleration
pulse generated during the initial impact. As
such, this test addresses the consistent request
for consideration of oblique impacts in helmet
testing (Mills, 1997) in a simplicity that may
be amendable for implementation into existing
test standards
/quote

On Mon, 11 Aug 2008 15:01:34 +0100, Phil Armstrong
said in
:

Having scanned the two papers (which are related: the physical testing
generates paramenter values for the finite element analysis) the main
conclusion appears to be that the coefficient of friction between the
headform (with anatomically correct scalp + hair) and the helmet is
too low to lead to rotational forces in a 'typical impact' which are
large enough to cause brain damage.

I think this is a bit of policy-based evidence making.

Dr Mills is a non-medical doctor with no relevant qualifications who
has given important (to one side) testimony in over 100 cases of
contributory negligence. So far as he is concerned, it appears that
helmets are always effective as the only factor of importance is the
1.5m a cyclist falls vertically when he's hit. That he might be
propelled forwards at the same time by a 50 mph car is totally
irrelevant.

Brian Walker and John Franklin, plus a few solicitors, are currently
largely warding off such nonsense but it could be more difficult if
he's now coming up with 'proof' that helmets guard against oblique
impacts. Note that the 'proof' in this case is largely dependent on
the assumption that he is right.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

85% of helmet statistics are made up, 69% of them at CHS, Puget Sound