Assessing helmet impact damage
Dr Charlotte Meeks – QinetiQ
SAFE Europe, April 15th – 17th, Alicante
Overview
• Background
• Overview of helmet impact testing
• Review of UK and US aircrew standards used in study
• Overview of aircrew helmet structure • Database review – assessment of
performance
Background
• In 2006, the UK introduced Defence Standard 05-102, the ‘Military Aircrew Helmet Impact Standard’ (MAHIS) to replace previous aircrew helmet impact specifications that were based on helmet standards for road vehicle users
• The UK MOD is currently reviewing and updating the standard, utilising the latest information in the area.
• To support this process QinetiQ on behalf of the MOD in collaboration with USAARAL has performed a programme of work where UK and US aircrew helmets were tested to UK and US impact standards with the aim of standardising the performance of the different helmet types and (by interpretation and comparison with accident and injury rates) threat levels to which aircrew heads are exposed.
Background
• This presentation will explore some aspects of this recent collaborative work carried out between the UK Ministry of Defence and the US Army Aeromedical Research Laboratory • A large series of helmet impact tests were carried out in February 2012
• Testing carried out at USAARL Fort Rucker, Alabama and Southern Impact Research Center, Tennessee
• A number of UK and US helmet types were impact tested to both UK and US Rotary Wing helmet impact standards, as well as non-standard tests, to fully characterise their performance
• A large database has been formed from impact testing a current US aircrew helmet and two current UK aircrew helmets.
• Each type of helmet was assessed against the US (1680-ALSE-1012) and UK (Def Stan – 05-102) standards and against intermediate impact threats.
Overview of Helmet Impact Testing – Test Methods
• In general, helmet impact testing takes the following form:
• Helmet is fitted to rigid instrumented headform
• Helmet is dropped in a guided fall from height (For aircrew helmet standards), falling under gravity, and impacts a rigid anvil at a defined velocity
• Helmet decelerates headform, attenuating shock of impact by absorbing energy
• Test pass/fail limit is (in general) a maximum peak acceleration of the headform, below which the measured headform acceleration must remain to pass test
• Therefore, two of the critical test parameters are:
− impact velocity
− peak acceleration pass/fail limit
Impact test standards
• The two aircrew impact standards in this review were defined from historic protective headgear standards and largely from evidence provided from damage replication of accidents using existing helmets.
UK DefStan 05-102 Based on BS6658 and accident damage assessment by Glaister
7.5 m/s impact velocity with maximum 300G pass/fail criteria
US 1680-ALSE-101
Based on ANSI Z90.1 and accident damage assessment by Slobodnik
6.0 m/s impact velocity with maximum 150/175G pass/fail
Overview of Helmet Impact Testing
DefStan 05-102 RW
(MAHIS)
1680-ALSE-101
Impact velocity 7.5m/s (Flat Anvil),
7m/s (Hemi Anvil) 6m/s (Headband region), 4.88m/s (Crown) Pass/Fail limit 300G 150G / 175G location dependant
Anvil Types Flat and Hemispherical Flat
Repeat impact on same location Yes 5.3m/s (Flat Anvil), 5.0m/s (Hemi Anvil) None Environmental conditioned impacts
Ambient, Hot, Cold
and water immersion Ambient and Hot
Penetration Test
Yes
(Differs from 1680-ALSE-101)
Yes
(Differs from MAHIS)
7.5m/s drop height = 2.87m (9ft 5in) 6.0 m/s drop height = 1.83m (6ft 0in)
Overview of Aircrew helmet
Helmet fitting and comfort system: A system that allows the helmet to be correctly fitted to the wearers head. Foams used in the fitting system compresses readily..
Helmet Shell: Primarily, the stiff helmet shell is designed to protect against injury caused by penetrative events. In many cases the shell also 'spreads' the applied impact load onto a large area of liner thus making the threat easier to mitigate.
Helmet retention: system that secures the helmet to the users head.
Impact Attenuating Foam: The inner liner which mitigates injuries through energy dispersion and controlled deceleration of the head.
Helmet Attachments: Overs i.e visors, visor covers attachment points.
Impact velocity Peak G
Helmet A – Low stiffness shell design 6.0 m/s 174 G Helmet A – Low stiffness shell design 7.5 m/s 491 G Helmet B – High stiffness shell design 6.0 m/s 181 G Helmet B – High stiffness shell design 7.5 m/s 191 G
Database review – Assessment of helmet performance
Example result from the database showing the different peak G results for Helmets A (low-stiffness skin design) and B (high-(low-stiffness skin design) against a flat anvil on the crown
These helmets are designed to protect against one of the injury outcomes and can’t protect against the other in
this example.
Why can’t these helmets protect against both scenarios?
• Peak G measurements are not enough to understand why the helmets can not protect against the two head injury threats.
• Detailed data assessment, fractographic examination of the helmet damage and high speed video to understand the full performance of these helmets.
Helmet A – 6 m/s impact onto a flat anvil
Database review – Assessment of helmet performance
•
Photos illustrate the differences in visible shell damage for Helmet A for
6m/s and 7.5m/s flat anvil crown impacts
Peak G – 174 G Peak G – 491 G
0 100 200 300 400 500 0 2 4 6 8 10 12 D e ce le ra tion ('g') Time (ms) 7.5 m/s impact 6.0 m/s impact
Helmet A – Time deceleration trace from impact
Helmet A – Deceleration versus deflection trace from impact
Database review – Assessment of helmet performance
0 100 200 300 400 500 0 5 10 15 20 25 30 35 40 D e ce le ra tion, G Deflection (mm)
crown, 7.61m/s, flat, ambient crown, 6.11m/s, flat, ambient
Peak G = 174
0 20 40 60 80 100 120 140 160 1 2
Energy remaining after 300 G limit reached
Energy absorbed below peak 175 G
En e rgy ab sorbe d , J 7.5 m/s impact 6 m/s impact 70J 0 100 200 300 400 500 0 2 4 6 8 10 12 Dec e le rati on ('g ') Time (ms) 7.5 m/s impact 6.0 m/s impact Helmet A
Helmet B – 6 m/s impact onto a flat anvil
Helmet B – 7.5 m/s impact onto a flat anvil
• Photos illustrate large differences in visible shell damage for Helmet B for 6m/s and 7.5m/s flat anvil crown impacts
− Very little visible shell or liner damage on 6m/s impacts, in contrast to large amount on 7.5m/s impact
Helmet A, Crown, 7.5m/s, Flat, 191G
Helmet A, Crown, 6.0m/s, Flat, 181G
101J 0 100 200 0 2 4 6 8 10 12 D e ce le ra tion ('g') Time (ms) 6.0 m/s impact 7.5 m/s impact
Database review – Assessment of helmet performance
Database review – Assessment of helmet performance
Helmet B – Deceleration versus deflection trace from impact
0 50 100 150 200 250 0 4 8 12 16 20 24 28 32 36 40 D e ce le ra tio n, G Defelction (mm) 7.5 m/s flat impact 6 m/s flat impact Peak G - 191 Peak G - 181
0 20 40 60 80 100 120 140 160 1 2
Energy absorbed below peak 300 G
Energy absorbed below peak 175 G
Energy absorbed below peak 150 G Energy abs orbed, J 6 m/s impact 7.5 m/s impact 13J 101J 0 100 200 0 2 4 6 8 10 12 Dece ler atio n ( 'g ') Time (ms) 6.0 m/s impact 7.5 m/s impact Helmet B
0 50 100 150 200 250 0 4 8 12 16 20 24 28 32 36 40 D e ce le ra tion, G Defelction (mm) 7.5 m/s flat impact 6 m/s flat impact Peak G - 191 Peak G - 181
0 50 100 150 200 250 0 4 8 12 16 20 24 28 32 36 40 D e ce le ra tion, G Defelction (mm) 7.5 m/s flat impact 6 m/s flat impact Peak G - 191 Peak G - 181
6.0 m/s impact onto a flat anvil
• Reduced contact area due to loading being applied from inner surface only
• Foam fails at a lower load
7.5 m/s impact onto a flat anvil
•
An example of the data and detail of understanding developed on current
aircrew helmets has been shown for just 4 or the 300 impacts performed.
•
The example showed the response of two different helmet designs to two
different injury scenarios
•
Helmet A – low stiffness skin/liner designed to prevent concussive threat at
lower impact energies.
•
At ‘low’ impact velocity the helmet absorbs the energy resulting in a
lower peak G through fracture of the skin and crushing of the liner from
both internal and external surfaces.
•
At ‘high’ impact velocity the helmet absorbs the energy through the
same mechanism but reaches a maximum compression then passes a
significant force to the headform.
Helmet B – high stiffness skin/liner designed to prevent skull fracture at higher
impact energies.
•
At ‘low’ impact velocity the shell does not fail resulting in the load being
absorbed on the inner surface of the liner only. This results in a reduced
contact area of loading causing local micro-failure of the liner at a lower load.
As the energy to absorb reduces the stiffness of the liner prevents any further
absorption and the load peaks pushing it above the limit for concussion.
•
At ‘high’ impact velocity the increased rate of loading causes fracture of the
helmet shell and then the liner is crushed from both the inner and outer
surfaces.
•
Helmet A was designed to prevent against concussive injuries at low impact
velocities, hence this helmet could not protect against higher velocity
threats.
•
A thicker liner would increase energy absorption and reduce ‘G’
however may detrimentally affect helmet mass and size.
•
Helmet B was designed to prevent skull fracture at high impact velocities.
Although some protection afforded at low velocities, the high stiffness of
the shell prevents efficient liner crushing from shell side - energy
absorption achieved by headform crushing inner surface of liner only.
•
A soft (or multi-layered) liner would allow shock attenuation at lower
‘G’, however this may detrimentally affect helmet mass and size to
meet high impact velocity requirements.
•
Comparison study has generated a significant volume of data. This data
has been gathered under controlled conditions eliminating any known
variances in test method and helmet set-up.
•
The database provides a useful reference on the damage produced for a
given velocity and the corresponding headform acceleration. The
understanding gained will support future accident investigations and will
improve the correlation between helmet test acceleration history and
head injury. This in turn will enhance future helmet standard
development.
Acknowledgements
•
This work was funded by the UK Ministry of Defence
•
