- Email: [email protected]
Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading
Journal of Biomechanics xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
Short communication
Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading Ian D. Elgy ⇑, Graham H. Williams, Charlene S. Gibson, Daniel J. Pope Dstl Porton Down, Salisbury, Wiltshire SP4 0JQ, United Kingdom
a r t i c l e
i n f o
Article history: Accepted 6 January 2020 Available online xxxx Keywords: ATD Leg protection Vehicle protection Improvised explosive device IED Land-mine
a b s t r a c t Results are reported of small-scale explosive experiments with Hybrid III and Mil-Lx anthropomorphic test device (ATD) legs. The legs were subject to loadings from deforming metallic plates, driven by explosive loadings to replicate the movement of the floor of a protected vehicle subject to a land-mine strike. The forces measured by the legs are reported and compared between the two different leg types. The benefits of protective measures, including false-floors and commercially available footpads, are compared for their ability to reduce the forces measured in each leg type. It is concluded that the two leg types respond differently to different protective measures and hence cannot be used interchangeably. Crown Copyright Ó 2020 Published by Elsevier Ltd.
1. Introduction Buried explosive charges were a significant cause of injuries to mounted troops on recent military deployments in Afghanistan [Ramasamy et al. (2011)]. Injury outcomes from these explosive charge strikes vary but injuries to the lower extremities have been observed to make up a sizeable portion of the total injury types reported. A study into a sample of mounted combat injuries reported that 81% of injured UK personnel received lower extremity injuries making it the most common body region injured [Ramasamy et al. (2011)]. Other nations’ casualty rates show similar trends [Pasquier et al. (2011), Jørgensen et al. (2013)]. Of these lower extremity injuries, the most common regions injured were the heel, ankle and distal tibia. A number of approaches are common in improving armoured vehicle design to reduce these injury types. These include reducing vehicle structural deformation (which has been studied widely such as that summarised in the work of Anderson Jr. et al. (2011) or raising the feet above the floor of the vehicle with either footrests, or attenuating footpads manufactured from a variety of materials [Quenneville and Dunnning (2011)]. With internal space at a premium in armoured vehicles, vehicle designers seek to minimise the space taken up by these injury mitigating solutions whilst maximising the reduction in injury rates. Systems that raise the feet a long way from the floor can place troops in uncomfortable postures that can lead to fatigue and the development of chronic ⇑ Corresponding author. E-mail address: [email protected] (I.D. Elgy).
injuries (e.g. lower back pain) which can ultimately impact on military capability and performance. Finding an optimised solution to the trade-off between ergonomic usability [Ministry of Defence Defence Standard 00-250 (2008)] and protection provided is a subject of on-going research. To understand the benefits of different approaches without undertaking costly full-vehicle trials a small-scale model has been generated. This model allows Anthropomorphic Test Device (ATD) legs to be subjected to loadings representative of those experienced in protected vehicles during land mine strikes and the response studied. Examples of the model’s use are described to assess the benefits of a selection of different leg protection technologies. The model is also used to compare the dynamic response of two different ATD leg types, both of which are permitted in the same commonly used blast test standard [NATO (2011)], to determine if there is likely to be aystematic bias arising from selecting a specific leg. The data generated may also be of use to tissue researchers who may use the findings to validate their own match-pair testing.
2. Method There are a number of laboratory-based methods that are used to induce high forces in ATDs and Post-Mortem Human Specimens (PMHS) as summarised in [Mildon et al. (2018)]. These methods use impactors that drive a platen at velocities in the region of 5– 15 ms 1 into a leg. The velocity changes are generally lower than the, frequently citied, 12 ms 1 minimum value in vehicle buried
https://doi.org/10.1016/j.jbiomech.2020.109618 0021-9290/Crown Copyright Ó 2020 Published by Elsevier Ltd.
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
2
I.D. Elgy et al. / Journal of Biomechanics xxx (xxxx) xxx
explosive charge strikes [Wang et al. (2001)], and much lower than the transient velocities observed in studies of landmine attacks. Blast loading studies generally show the floor panels attaining velocities in the region of 100 ms 1 and reaching their maximum displacement of tens to hundreds of millimetres within 1–5 ms [Elgy et al. (2006), Park and Choi (2015), Pickering et al. (2012), Peles et al. (2008), Held et al. (2005), Fay et al. (2015)]. As a result of these discrepancies, it was decided that sub-system explosive tests were the most appropriate method to replicate the behaviour of ATDs in armoured vehicles. A test frame was constructed in the form of a hollow box (approximately 1.5 m 2.0 m footprint, 1.5 m tall) that allowed a Hybrid III leg or a Mil-Lx leg to be positioned above a mild steel plate, representing the belly of an armoured fighting vehicle floor. The mild steel plate was 3 mm thick and was bolted on the underside of a 0.8 m 0.8 m aperture cut into the floor of the box (Fig. 1). The legs were positioned atop an aluminium checker plate (to represent a false-floor in an AFV). The height of the checker plate was varied between tests. In some tests, commercially available mitigation mats (referred to hereafter as Mitigation 1, Mitigation 2 and Mitigation 3) were also placed upon the checker plate, under the ATD’s foot. A 250 g explosive charge was placed under the belly plate and detonated. This caused the 3 mm mild steel belly plate to deform upwards into the body of the box, simulating the floor deformation of an armoured vehicle. The steel belly plate is estimated to have deformed (from simulations of similar structured) by approximately 50–80 mm within 5 ms.
likely to exceed the 5.4 kN limit specified for the Hybrid III leg by NATO (2011). Also seen in Fig. 3 is the maximum lower tibia force measured when a commercially available mitigation material is added between the foot and the checker plate. The height of the foot above the ‘‘belly” plate was maintained by reducing the height of the checker-plate by the thickness of the mitigation. It can be seen that at a foot height of 97 mm, none of the mitigation schemes reduced the measured compression in the lower tibia from that seen in the unmitigated case. 4. Mil-Lx results Fewer tests were done with the Mil-Lx leg than the Hybrid III. Two tests were conducted with no mitigation system applied at a belly and checker plate separation of 97 mm. As shown in Fig. 4 the Mil-Lx upper tibia compression, the channel required by NATO (2011), is over 1 kN (30%) lower than the compression measured in the lower tibia of the Hybrid III. The same three commercially available mitigation systems were also trialled with the Mil-Lx leg as were trialled with the Hybrid III. Two tests were conducted on each mitigation type, each with a foot height of 97 mm. The maximum compressive forces measured in the upper tibia load cell are plotted in Fig. 5. No tests were conducted that allowed an inference of the level of improvement possible by applying a mitigation system while retaining the height of the checker plate. 5. Discussion
3. Hybrid III results Examples of the Hybrid III lower leg compressive forces, with no mitigating footpad used, are compared in Fig. 2. The mean and ± 1 standard deviation are plotted. The two plots in the figure show the effect on measured Hybrid III lower leg forces when the initial spacing between the belly and checker plate is reduced from 97 mm to 67 mm. Reproducibility between similar tests was good as indicated by the standard deviations being low. The force measurements (including those not plotted in Fig. 2) show two features. A short (2–3 ms) duration peak is seen to be heavily dependent on the initial spacing and is believed to be caused by contact between the belly plate and the checker plate. The more consistent, smaller and longer duration (around 5 ms) peak seen after 3 ms is believed to be caused by low amplitude, elastic deformation, flexure and recoil in the whole test frame. When the peak Hybrid III lower leg compressive forces are plotted against the height of the foot above the belly plate (Fig. 3 – ‘‘No Mitigation”) a bilinear relationship is observed with lower leg compressive forces exhibiting low sensitivity to foot heights exceeding 97 mm. Below 67 mm decreasing foot height correlates to a rapid increase in the compressive force measured. It appears that any contact between the belly plate and the floor plate causes forces
Angle secon sffening ribs
Checker plate Threaded bar Steel tube
“Belly” plate Fig. 1. Cross-section schematic (not to scale) of the lower face of the test rig.
Fig. 3 demonstrates the kind of compromises that need to be made between the available cabin space and the lower leg protection afforded to occupants, in the design of military vehicles. The model, however, allows these phenomena to be investigated without full-scale vehicle testing. When tested with a Hybrid III leg, it can be seen that placing a commercially available mitigation material on the checker-plate has a benefit but this method of reducing the leg forces is less effective than simply raising the checker-plate. From this, it is concluded that raising a false-floor is more effective at reducing Hybrid III leg compression than utilising footpads for the same encroachment on available vehicle occupant space. However, urgent retrospective improvements to lower leg protection in vehicles may mean it is considerably more efficient and convenient to introduce a footpad as opposed to raising the false floor of the vehicle. In contrast to the Hybrid III results, the loading on the Mil-Lx leg was reduced, for a given foot-height, with the application of some of the commercially available mitigations. The forces measured when Mitigation 2 and Mitigation 3 (both recoverable systems) were used, were similar to the baseline with no mitigation (Fig. 5). Mitigation 1 (which is an irrecoverable, crushable system) appears to result in higher forces than the baseline with no mitigation. This distinction between the recoverable and irrecoverable systems may be caused by Mitigation 1 being thicker resulting in the checker plate being closer to the belly plate for the same foot height and therefore experiences a greater initial loading. This differentiation between different mat types is more pronounced in measurements made with the Mil-Lx leg than the Hybrid III. This distinction is likely to arise from the Mil-Lx leg’s compliant element, being softer than the Hybrid III leg, allows the leg to be more gently accelerated by the simultaneous compression of the mitigation and the compliant element. This effect is consistent with that reported by Quenneville et al. (2017) at lower velocity changes. It further raises the possibility that benefits can be gained by tuning the stiffness of the footpad to match that of the Mil-Lx’s
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
3
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Fig. 2. Comparison of the Hybrid III lower tibia compression for initial spacings between hull and checker plate of 97 mm (left) and 69 mm (right).
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compliant element. However it is uncertain how relevant these benefits would be to operational foot and ankle injuries. 6. Conclusions An experimental model has been developed to represent the response of occupants of vehicles subject to land-mine attack. Tests with a Hybrid III leg show the forces measured in lower tibia are heavily dependent on the proximity of the foot to the
deforming plate representing the floor of a protected vehicle. Even light contacts between the deforming floor and the Hybrid III ATD’s foot cause forces in the leg exceeding the accepted injury threshold of 5.4 kN. Commercially available footpad systems appear to reduce the forces experienced in the ATDs leg but these systems were less effective than simply elevating the foot to the same height with a false floor. Footpads appeared to be more effective when used with the Mil-Lx leg than the Hybrid III leg. This may be because the foot
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
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I.D. Elgy et al. / Journal of Biomechanics xxx (xxxx) xxx
Acknowledgements
NoMitigation_97mm: Average NoMitigation_97mm: Plus1SD NoMitigation_97mm: Less1SD
This research was funded by CSA S&T Portfolio via the Dstl Land Systems Programme. The authors are very grateful to Dr. Michael S. Neale (formerly of Dstl Porton Down), for conducting much of the testing during his time at Dstl. The authors would also like to express gratitude for the provision of samples by footpad manufacturers, including (amongst others) Exmoor Trim. Crown Copyright Ó [2019] Published by Elsevier Ireland Ltd. This is an open access article under the Open Government Licence
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Appendix A. Supplementary material
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Fig. 5. Mil-Lx maximum upper tibia compression with a foot height of 97 mm. The Hybrid III force (with error bars representing one standard deviation) at the same height is also included for comparison. The 2.6 kN limit specified by NATO (2011) is shown as a dashed line.
pad and the Mil-Lx’s compliant element compress in tandem allowing greater momentum transfer with smaller forces being applied. Because the effectiveness of commercial footpads differs with the leg used to make the measurement, the optimised solution will vary with the ATD type used. Statements in NATO (2011) standard test methods asserting that the legs can be used interchangeably are therefore difficult to justify. Declaration of Competing Interest
Anderson Jr., C.E., Behnerb, T., Weiss, C.E., 2011. Mine blast loading experiments. Int. J. Impact Eng. 38 (8–9), 697–706. Elgy, I.D., Pope, D.J., Pickup, I.M., 2006. A study of combined particle and blast wave loading of structures. J. de Physique IV 134, 467–471. Fay, S.D., Rigby, S.E., Tyas, A., Clarke, S.D., Reay, J.J., Warren, J.A., Brown, R., 2015. Displacement timer pins: an experimental method for measuring the dynamic deformation of explosively loaded plates. Int. J. Impact Eng. 86, 124–130. Held, M., Heeger, P., Kiermeir, J., 2005. Displacement device to measure the acceleration of the bulge of RHA plates under anti-tank mine blast. In Proceedings of 22nd International Symposium on Ballistics, Vancouver. Jørgensen, H.O., Heier-Madsen, K., Stokkebye, J.E., 2013. Casualty rates among danish soldiers in Iraq and Afghanistan. J R Army Med Corps, 2012 158, 10–13. Mildon, P.J., White, D., Sedman, A.J., Dorn, M., Masouros, S.D., 2018. Injury risk of the human leg under high rate axial loading. Human Factors Mech. Eng. Defense Safety 2, 5. Ministry of Defence Defence Standard 00-250, Human Factors for Designers of Systems, May 2008 NATO, 2011, Procedures for Evaluating The protection Level of Armoured Vehicles, AEP-55, Volume 2 (Edition 2) Park, J., Choi, H.J., 2015. Experiments and numerical analyses of HB400 and aluminum foam sandwich structure under landmine explosion. Compos. Struct. 134, 726–739. Pasquier, P., de Rudnicki, S., Donat, N., Auroy, Y., Merat, S., 2011. Epidemiology of war injuries, about two conflicts: Iraq and Afghanistan. Annales Francaises D Anesthesie Et De Reanimation 30 (11), 819–827. Peles, S., Touati, D., Azulay, I., Neuberger, A., 2008. Numerical simulation of mine detonation beneath a generalized add-on armor structure. . In Proceedings of. 24th International Symposium on Ballistics, New Orleans. Pickering, E.G., Yuen, S.C., Nurick, G.N., Haw, P., 2012. The response of quadrangular plates to buried charges. Int. J. Impact Eng. 49, 103–114. Quenneville, C.E., Dunning, C.E., 2011. Evaluation of energy attenuating floor mats for protection of lower limbs from anti-vehicular landmines. J. Battlefield Technol. 14 (3). Quenneville, C.E., Fournier, E., Shewchenko, N., 2017. The effect of anthropomorphic test device lower leg surrogate selection on impact mitigating system evaluation in low- and high-rate loading conditions. Military Med. 182 (9– 10), E1981–E1986. Ramasamy, A., Masouros, S.D., Newell, N., Hill, A.M., Proud, W.G., Brown, K.A., Hepper, A.E., Bull, A.M.J., Clasper, J.C., 2011a. In-vehicle extremity injuries from improvised explosive devices: current and future foci. Philos. Trans. Royal Soc. B: Biolog. Sci. 366 (1562), 160–170. Ramasamy, A., Hill, A.M., Phillip, R., Gibb, I., Bull, A.M., Clasper, J.C., 2011b. The modern ‘‘Deck-Slap” injury-calcaneal blast fractures from vehicle explosions. J. Trauma-Injury Infect. Critical Care 71 (6), 1694–1698. Wang, J.J., Bird, R., Swinton, B., Krstic, A., 2001. Protection of lower limbs against floor impact in army vehicles experiencing landmine explosion. J. Battlefield Technol. 4, 8–12.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
Short communication
Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading Ian D. Elgy ⇑, Graham H. Williams, Charlene S. Gibson, Daniel J. Pope Dstl Porton Down, Salisbury, Wiltshire SP4 0JQ, United Kingdom
a r t i c l e
i n f o
Article history: Accepted 6 January 2020 Available online xxxx Keywords: ATD Leg protection Vehicle protection Improvised explosive device IED Land-mine
a b s t r a c t Results are reported of small-scale explosive experiments with Hybrid III and Mil-Lx anthropomorphic test device (ATD) legs. The legs were subject to loadings from deforming metallic plates, driven by explosive loadings to replicate the movement of the floor of a protected vehicle subject to a land-mine strike. The forces measured by the legs are reported and compared between the two different leg types. The benefits of protective measures, including false-floors and commercially available footpads, are compared for their ability to reduce the forces measured in each leg type. It is concluded that the two leg types respond differently to different protective measures and hence cannot be used interchangeably. Crown Copyright Ó 2020 Published by Elsevier Ltd.
1. Introduction Buried explosive charges were a significant cause of injuries to mounted troops on recent military deployments in Afghanistan [Ramasamy et al. (2011)]. Injury outcomes from these explosive charge strikes vary but injuries to the lower extremities have been observed to make up a sizeable portion of the total injury types reported. A study into a sample of mounted combat injuries reported that 81% of injured UK personnel received lower extremity injuries making it the most common body region injured [Ramasamy et al. (2011)]. Other nations’ casualty rates show similar trends [Pasquier et al. (2011), Jørgensen et al. (2013)]. Of these lower extremity injuries, the most common regions injured were the heel, ankle and distal tibia. A number of approaches are common in improving armoured vehicle design to reduce these injury types. These include reducing vehicle structural deformation (which has been studied widely such as that summarised in the work of Anderson Jr. et al. (2011) or raising the feet above the floor of the vehicle with either footrests, or attenuating footpads manufactured from a variety of materials [Quenneville and Dunnning (2011)]. With internal space at a premium in armoured vehicles, vehicle designers seek to minimise the space taken up by these injury mitigating solutions whilst maximising the reduction in injury rates. Systems that raise the feet a long way from the floor can place troops in uncomfortable postures that can lead to fatigue and the development of chronic ⇑ Corresponding author. E-mail address: [email protected] (I.D. Elgy).
injuries (e.g. lower back pain) which can ultimately impact on military capability and performance. Finding an optimised solution to the trade-off between ergonomic usability [Ministry of Defence Defence Standard 00-250 (2008)] and protection provided is a subject of on-going research. To understand the benefits of different approaches without undertaking costly full-vehicle trials a small-scale model has been generated. This model allows Anthropomorphic Test Device (ATD) legs to be subjected to loadings representative of those experienced in protected vehicles during land mine strikes and the response studied. Examples of the model’s use are described to assess the benefits of a selection of different leg protection technologies. The model is also used to compare the dynamic response of two different ATD leg types, both of which are permitted in the same commonly used blast test standard [NATO (2011)], to determine if there is likely to be aystematic bias arising from selecting a specific leg. The data generated may also be of use to tissue researchers who may use the findings to validate their own match-pair testing.
2. Method There are a number of laboratory-based methods that are used to induce high forces in ATDs and Post-Mortem Human Specimens (PMHS) as summarised in [Mildon et al. (2018)]. These methods use impactors that drive a platen at velocities in the region of 5– 15 ms 1 into a leg. The velocity changes are generally lower than the, frequently citied, 12 ms 1 minimum value in vehicle buried
https://doi.org/10.1016/j.jbiomech.2020.109618 0021-9290/Crown Copyright Ó 2020 Published by Elsevier Ltd.
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
2
I.D. Elgy et al. / Journal of Biomechanics xxx (xxxx) xxx
explosive charge strikes [Wang et al. (2001)], and much lower than the transient velocities observed in studies of landmine attacks. Blast loading studies generally show the floor panels attaining velocities in the region of 100 ms 1 and reaching their maximum displacement of tens to hundreds of millimetres within 1–5 ms [Elgy et al. (2006), Park and Choi (2015), Pickering et al. (2012), Peles et al. (2008), Held et al. (2005), Fay et al. (2015)]. As a result of these discrepancies, it was decided that sub-system explosive tests were the most appropriate method to replicate the behaviour of ATDs in armoured vehicles. A test frame was constructed in the form of a hollow box (approximately 1.5 m 2.0 m footprint, 1.5 m tall) that allowed a Hybrid III leg or a Mil-Lx leg to be positioned above a mild steel plate, representing the belly of an armoured fighting vehicle floor. The mild steel plate was 3 mm thick and was bolted on the underside of a 0.8 m 0.8 m aperture cut into the floor of the box (Fig. 1). The legs were positioned atop an aluminium checker plate (to represent a false-floor in an AFV). The height of the checker plate was varied between tests. In some tests, commercially available mitigation mats (referred to hereafter as Mitigation 1, Mitigation 2 and Mitigation 3) were also placed upon the checker plate, under the ATD’s foot. A 250 g explosive charge was placed under the belly plate and detonated. This caused the 3 mm mild steel belly plate to deform upwards into the body of the box, simulating the floor deformation of an armoured vehicle. The steel belly plate is estimated to have deformed (from simulations of similar structured) by approximately 50–80 mm within 5 ms.
likely to exceed the 5.4 kN limit specified for the Hybrid III leg by NATO (2011). Also seen in Fig. 3 is the maximum lower tibia force measured when a commercially available mitigation material is added between the foot and the checker plate. The height of the foot above the ‘‘belly” plate was maintained by reducing the height of the checker-plate by the thickness of the mitigation. It can be seen that at a foot height of 97 mm, none of the mitigation schemes reduced the measured compression in the lower tibia from that seen in the unmitigated case. 4. Mil-Lx results Fewer tests were done with the Mil-Lx leg than the Hybrid III. Two tests were conducted with no mitigation system applied at a belly and checker plate separation of 97 mm. As shown in Fig. 4 the Mil-Lx upper tibia compression, the channel required by NATO (2011), is over 1 kN (30%) lower than the compression measured in the lower tibia of the Hybrid III. The same three commercially available mitigation systems were also trialled with the Mil-Lx leg as were trialled with the Hybrid III. Two tests were conducted on each mitigation type, each with a foot height of 97 mm. The maximum compressive forces measured in the upper tibia load cell are plotted in Fig. 5. No tests were conducted that allowed an inference of the level of improvement possible by applying a mitigation system while retaining the height of the checker plate. 5. Discussion
3. Hybrid III results Examples of the Hybrid III lower leg compressive forces, with no mitigating footpad used, are compared in Fig. 2. The mean and ± 1 standard deviation are plotted. The two plots in the figure show the effect on measured Hybrid III lower leg forces when the initial spacing between the belly and checker plate is reduced from 97 mm to 67 mm. Reproducibility between similar tests was good as indicated by the standard deviations being low. The force measurements (including those not plotted in Fig. 2) show two features. A short (2–3 ms) duration peak is seen to be heavily dependent on the initial spacing and is believed to be caused by contact between the belly plate and the checker plate. The more consistent, smaller and longer duration (around 5 ms) peak seen after 3 ms is believed to be caused by low amplitude, elastic deformation, flexure and recoil in the whole test frame. When the peak Hybrid III lower leg compressive forces are plotted against the height of the foot above the belly plate (Fig. 3 – ‘‘No Mitigation”) a bilinear relationship is observed with lower leg compressive forces exhibiting low sensitivity to foot heights exceeding 97 mm. Below 67 mm decreasing foot height correlates to a rapid increase in the compressive force measured. It appears that any contact between the belly plate and the floor plate causes forces
Angle secon sffening ribs
Checker plate Threaded bar Steel tube
“Belly” plate Fig. 1. Cross-section schematic (not to scale) of the lower face of the test rig.
Fig. 3 demonstrates the kind of compromises that need to be made between the available cabin space and the lower leg protection afforded to occupants, in the design of military vehicles. The model, however, allows these phenomena to be investigated without full-scale vehicle testing. When tested with a Hybrid III leg, it can be seen that placing a commercially available mitigation material on the checker-plate has a benefit but this method of reducing the leg forces is less effective than simply raising the checker-plate. From this, it is concluded that raising a false-floor is more effective at reducing Hybrid III leg compression than utilising footpads for the same encroachment on available vehicle occupant space. However, urgent retrospective improvements to lower leg protection in vehicles may mean it is considerably more efficient and convenient to introduce a footpad as opposed to raising the false floor of the vehicle. In contrast to the Hybrid III results, the loading on the Mil-Lx leg was reduced, for a given foot-height, with the application of some of the commercially available mitigations. The forces measured when Mitigation 2 and Mitigation 3 (both recoverable systems) were used, were similar to the baseline with no mitigation (Fig. 5). Mitigation 1 (which is an irrecoverable, crushable system) appears to result in higher forces than the baseline with no mitigation. This distinction between the recoverable and irrecoverable systems may be caused by Mitigation 1 being thicker resulting in the checker plate being closer to the belly plate for the same foot height and therefore experiences a greater initial loading. This differentiation between different mat types is more pronounced in measurements made with the Mil-Lx leg than the Hybrid III. This distinction is likely to arise from the Mil-Lx leg’s compliant element, being softer than the Hybrid III leg, allows the leg to be more gently accelerated by the simultaneous compression of the mitigation and the compliant element. This effect is consistent with that reported by Quenneville et al. (2017) at lower velocity changes. It further raises the possibility that benefits can be gained by tuning the stiffness of the footpad to match that of the Mil-Lx’s
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
3
I.D. Elgy et al. / Journal of Biomechanics xxx (xxxx) xxx
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Fig. 3. Hybrid III maximum lower tibia compression with increasing foot height above the belly plate. The 5.4 kN limit specified by NATO (2011) is shown as a dashed line.
compliant element. However it is uncertain how relevant these benefits would be to operational foot and ankle injuries. 6. Conclusions An experimental model has been developed to represent the response of occupants of vehicles subject to land-mine attack. Tests with a Hybrid III leg show the forces measured in lower tibia are heavily dependent on the proximity of the foot to the
deforming plate representing the floor of a protected vehicle. Even light contacts between the deforming floor and the Hybrid III ATD’s foot cause forces in the leg exceeding the accepted injury threshold of 5.4 kN. Commercially available footpad systems appear to reduce the forces experienced in the ATDs leg but these systems were less effective than simply elevating the foot to the same height with a false floor. Footpads appeared to be more effective when used with the Mil-Lx leg than the Hybrid III leg. This may be because the foot
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618
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Acknowledgements
NoMitigation_97mm: Average NoMitigation_97mm: Plus1SD NoMitigation_97mm: Less1SD
This research was funded by CSA S&T Portfolio via the Dstl Land Systems Programme. The authors are very grateful to Dr. Michael S. Neale (formerly of Dstl Porton Down), for conducting much of the testing during his time at Dstl. The authors would also like to express gratitude for the provision of samples by footpad manufacturers, including (amongst others) Exmoor Trim. Crown Copyright Ó [2019] Published by Elsevier Ireland Ltd. This is an open access article under the Open Government Licence
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jbiomech.2020.109618.
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References
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Fig. 4. Mil-Lx upper tibia compression with no mitigation.
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Fig. 5. Mil-Lx maximum upper tibia compression with a foot height of 97 mm. The Hybrid III force (with error bars representing one standard deviation) at the same height is also included for comparison. The 2.6 kN limit specified by NATO (2011) is shown as a dashed line.
pad and the Mil-Lx’s compliant element compress in tandem allowing greater momentum transfer with smaller forces being applied. Because the effectiveness of commercial footpads differs with the leg used to make the measurement, the optimised solution will vary with the ATD type used. Statements in NATO (2011) standard test methods asserting that the legs can be used interchangeably are therefore difficult to justify. Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Please cite this article as: I. D. Elgy, G. H. Williams, C. S. Gibson et al., Response of hybrid III and Mil-Lx ATD legs to explosively driven vehicle floor loading, Journal of Biomechanics, https://doi.org/10.1016/j.jbiomech.2020.109618