wrote:
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.

AIUI this was from the test result.

Absolute peak values for acceleration (in test or analysis) need to be used
very carefully. The peaks will be influenced by the mounting of the
accelerometer, data sampling rate and, most importantly, the filtering of
the signal. Often a test trace will be so noisy as to obscure the actual
result but careful application of filtering can reveal the true picture
without losing too much detail.

You are right about the effect of the high peak being dependant on its
duration. The usual way of quoting the maximum acceleration in biomechanics
is to use the 3ms clip, that is the acceleration level exceeded for more
than 3ms. This tends to discard the very short duration, non-damaging high
peak values.


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.


I would really like to do something like this in FEA, but unfortunately I
have a day job.

--

Nigel