Assessing helmet impact damage

Transcription

1 Assessing helmet impact damage Dr Charlotte Meeks QinetiQ SAFE Europe, April 15 th 17 th, Alicante

2 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 Concluding remarks

3 Background In 2006, the UK introduced Defence Standard , 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.

4 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 ) standards and against intermediate impact threats. Around 300 impact tests were performed.

5 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 Video

6 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. 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 criteria UK DefStan Based on BS6658 and accident damage assessment by Glaister 7.5 m/s impact velocity with maximum 300G pass/fail criteria

7 Overview of Helmet Impact Testing Impact velocity Pass/Fail limit DefStan RW (MAHIS) 7.5m/s (Flat Anvil), 7m/s (Hemi Anvil) 300G 1680-ALSE-101 6m/s (Headband region), 4.88m/s (Crown) 150G / 175G location dependant Anvil Types Flat and Hemispherical Flat 7.5m/s drop height = 2.87m (9ft 5in) 6.0 m/s drop height = 1.83m (6ft 0in) Repeat impact on same location Environmental conditioned impacts Penetration Test Yes 5.3m/s (Flat Anvil), 5.0m/s (Hemi Anvil) Ambient, Hot, Cold and water immersion Yes (Differs from ALSE-101) None Ambient and Hot Yes (Differs from MAHIS)

8 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. Helmet Attachments: Overs i.e visors, visor covers attachment points. Impact Attenuating Foam: The inner liner which mitigates injuries through energy dispersion and controlled deceleration of the head.

9 Example result from the database showing the different peak G results for Helmets A (lowstiffness skin design) and B (high-stiffness skin design) against a flat anvil on the crown Impact velocity Peak G Helmet A Low stiffness shell design 6.0 m/s 174 G These helmets are designed to protect Helmet against A one Low of stiffness the injury shell outcomes design 7.5 m/s 491 G Helmet and can t B protect High stiffness against shell the other design in 6.0 m/s 181 G this example. Helmet B High stiffness shell design 7.5 m/s 191 G But why and how do you get round this?

10 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.

11 Helmet A 6 m/s impact onto a flat anvil

12 Helmet A 7.5 m/s impact onto a flat anvil

13 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

14 Helmet A Time deceleration trace from impact m/s impact 6.0 m/s impact Deceleration ('g') Time (ms)

15 Helmet A Deceleration versus deflection trace from impact crown, 7.61m/s, flat, ambient crown, 6.11m/s, flat, ambient Peak G = 491 Deceleration, G Peak G = Deflection (mm)

16 m/s impact Energy absorbed, J Energy remaining after 300 G limit reached Energy absorbed below peak 175 G Deceleration ('g') J m/s impact Time (ms) Helmet A m/s impact m/s impact

17 Helmet B 6 m/s impact onto a flat anvil

18 Helmet B 7.5 m/s impact onto a flat anvil

19 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, 6.0m/s, Flat, 181G Helmet A, Crown, 7.5m/s, Flat, 191G

20 Helmet B Time deceleration trace from impact m/s impact 7.5 m/s impact 101J Deceleration ('g') Time (ms)

21 Helmet B Deceleration versus deflection trace from impact m/s flat impact 6 m/s flat impact Peak G Peak G Deceleration, G Defelction (mm)

22 Energy absorbed, J Energy absorbed below peak 300 G Energy absorbed below peak 175 G Energy absorbed below peak 150 G 13J 101J Deceleration ('g') m/s impact 7.5 m/s impact Time (ms) Helmet B m/s 1 impact 7.5 m/s 2 impact

23 m/s flat impact 6 m/s flat impact Peak G Peak G Deceleration, G Defelction (mm)

24 m/s flat impact 6 m/s flat impact Peak G Peak G Deceleration, G Defelction (mm)

25 Reduced contact area due to loading being applied from inner surface only Foam fails at a lower load 6.0 m/s impact onto a flat anvil

26 7.5 m/s impact onto a flat anvil

27 Database review Summary 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.

28 Database review Summary 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.

29 Database review Summary 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.

30 Conclusions 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.

31 Acknowledgements This work was funded by the UK Ministry of Defence Work in the US was supported by USAARL, specifically, the use of their test facilities, the generous support of their staff and supply of US helmets.