An investigation of the forces within the tibiae at typical blast loading rates : with different boots

Includes bibliographical references.Anti-Vehicular Landmines (AVLs), underbelly Improvised Explosive Devices (IEDs) or side-attack IEDs are some of the major threats to military vehicles and their occupants (Ramasamy et al., 2011). The lower extremities of the occupants are very prone to injury, mostly caused by underbelly detonation of AVLs or IEDs due to their spatial proximity to the rapidly deforming floor of a vehicle in response to the threat mechanism. Lower limb surrogate legs, such as a Hybrid III or Military Lower Extremity (MiL-Lx) legs, are used to quantify the impulse loading on the lower extremities when subjected to the forces of the rapidly deforming floor. These surrogate legs are also used in laboratories for simulated blast loading tests and scaled field tests to evaluate protection measures for the lower extremities. In this study, the responses of the HIII and MiL-Lx surrogate legs were evaluated at several blast loading conditions using the Modified Lower Limb Impactor. The impact tests were conducted using a lower limb impactor with the leg mounted vertically and attached to the knee of the Anthropomorphic Test Device (ATD). The MiL-Lx leg is a recently developed surrogate which has limited evaluation across the loading conditions. This work evaluated the MiL-Lx leg across a range of velocities from 2.7 – 10.2 m/s. The study also included the evaluation of the response of the surrogate legs when fitted with two different types of combat boot. The current study shows that the response of the MiL-Lx leg compares satisfactorily with a previous study of a simulated blast at 7.2 m/s and the Post Mortem Human Subject (PMHS) corridors conducted at Wayne State University (WSU), Michigan, U.S.A. The MiL-Lx leg force-time trajectories from both the lower and upper tibia load cell were found to have distinct features that can be related to the impactor dynamics. This observation implies that the response of the legs can be used to deduce the dynamics of the impactor or deforming floor. The MiL-Lx leg results measured by the lower tibia load cell shows that the combat boots mitigate the peak tibia force and delay the time to peak force. However, the results from the upper tibia load cell show that the boots did not reduce high-severity force, but only the delays the time-to-peak force. The upper tibia load cell did not show any potential mitigation capability of the combat boots. The HIII leg force-time trajectories from both the lower and upper load cells showed a similar bell shape and duration but different magnitudes. Both the lower and upper tibia load cells of the HIII leg showed that the combat boots had mitigation capabilities. This is the first time that the lower tibia response of the MiL-Lx leg has been tested and analysed at a range of loading conditions. This has resulted in better understanding of the response of the MiL-Lx leg and will ultimately lead to better protection measures of the lower extremities

Development Of Lower Extremity Injury Criteria And Biomechanical Surrogate To Evaluate Military Vehicle Occupant Injury During An Explosive Blast Event

Anti-vehicular (AV) landmines and improvised explosive devices (IED) have accounted for more than half of the United States military hostile casualties and wounded in Operation Iraqi Freedom (OIF). The lower extremity is the predominantly injured body region following an AV mine or IED blast accounting for 26 percent of all combat injuries in OIF (Owens et al., 2007). Detonations occurring under the vehicle transmit high amplitude and short duration axial loads onto the foot-ankle-tibia region of the occupant causing injuries to the lower leg. The current effort was initiated to develop lower extremity injury criteria and biofidelic biomechanical surrogate to evaluate military occupant injury during an AV (axial) blast event. Eighteen lower extremity post mortem human specimens (PMHS) were instrumented with an implantable load cell and strain gages and impacted at one of three incrementally severe AV axial impact conditions. Twelve of the 18 PMHS specimens sustained fractures of the calcaneus, talus, fibula and/or tibia. A tibia axial force of 2,650 N and impactor velocity of 8.2 m/s corresponds with a ten percent risk of an incapacitating injury. Currently available lower extremity biomechanical surrogates were shown to lack biofidelity when impacted at simulated AV blast severities. A THOR-Lx underwent a series of modifications intended to reduce the overall stiffness of the surrogate. Its tibia compliant element was doubled in length to enable additional clearance for compression. The modified surrogate, MiL-Lx (military lower extremity), was loaded axially at three simulated AV axial loading rates using a piston driven linear impactor. The diameter and compressive modulus of the tibia compliant element was varied until the axial force measured by the surrogate was equivalent to the PMHS non-injury response in magnitude and duration. The MiL-Lx surrogate was capable of distinguishing between incrementally severe loading rates using tibia axial force. The MiL-Lx improves the accuracy and sensitivity needed to evaluate blast mitigation technologies designed to reduce injury to occupants of vehicles encountering AV landmines. The use of the MiL-Lx shall result in the development of new standards for the testing of blast mitigation technologies including underbelly protection, floor board materials, and vehicle structure

Methodology For Performing Whole Body Pmhs Underbody Blast Impact Testing, And The Corresponding Response Of The Hybrid Iii Dummy And The Finite Element Dummy Model Under Similar Loading Condition

In recent wars, the use of improvised explosive devices and landmines has dramatically increased as a tactical measure to counter armored vehicles. These weapons not only deform and damage the vehicle structure but also produce serious vertical deceleration injuries to mounted occupants. The reported injury patterns largely differ from those in an automotive crash and are often more severe than those in other vertical loading scenarios such as pilot seat ejection, helicopter crash, parachute landing and fall from height. High kinetic energy predominately along the principal vertical (Z-axis) over a short duration makes the underbody blast (UBB) loading conditions unique compared to other vertical and blunt impacts. With the lack of biomechanical response corridors (BRCs), the non-biofidelic nature of the automotive dummies to Z-axis loading and the lack of a finite element dummy model designed for vertical loading make it difficult to evaluate occupant response and develop mitigation strategies for UBB impact conditions. An introduction to the development of the BRCs this study provides a detailed methodology to perform whole body cadaver testing under a laboratory setup. Two whole body PMHS UBB impact tests were conducted using a sled system. An overview of pre-impact parameters such as bone mineral density, instrumentation technique, and vertical impulse generation is presented. Post-test CT scans, response data, and possible injury mechanisms were investigated. In addition, to PMHS testing, the responses of the Hybrid III dummy to short-duration large magnitude vertical acceleration in a laboratory setup were analyzed. Two unique test conditions were investigated using a horizontal sled system to simulate the UBB loading conditions. The biomechanical response in terms of the pelvis acceleration, chest acceleration, lumbar spine force, head accelerations and neck forces were measured during the tests. Subsequently, a series of finite element analyses (FEA) were performed to simulate the physical tests. The material parameters of various components as well as the mesh size were updated based on the high strain rate loading conditions obtained from Zhu et.al (2015) study. The correlation between the Hybrid III test and numerical model was evaluated using the CORA version 3.6.1. The Cora score for WSU FE model was determined to be 0.878 and 0.790 for loading conditions 1 and 2, respectively, in which 1.0 indicated a perfect correlation between the experiment and simulation response. The original LSTC model simulated under the current loading condition became numerically unstable after 12 ms. With repetitive vertical impacts, the Hybrid III dummy pelvis showed a significant increase in the peak acceleration accompanied by rupture of the pelvis foam and flesh. The revised WSU Hybrid III model indicated high stress concentrations at the same location where the pelvis foam and flesh in the actual ATD showed rupture. The stress contour under the ischial tuberosities in the finite element model provides a possible explanation for the material failure in the actual Hybrid III tests

Adequacy of test standards in evaluating blast overpressure (BOP) protection for the torso

The blast wave emanating from an explosion produces an almost instantaneous rise in pressure which can then cause Blast Overpressure (BOP) injuries to nearby persons. BOP injury criteria are specified in test standards to relate BOP measurements in a testing environment to a risk of BOP injury. This study considered the adequacy of test standards in evaluating BOP protection concepts for the torso. Four potential BOP injury scenarios were studied to determine the likelihood of injury and the adequacy of test standards for appropriate protection concepts. In the case of vehicle blast, BOP injury is unlikely and test standards are adequate. In the scenario involving an explosive charge detonated within a vehicle, and the close proximity to a hand grenade scenario, test standards are not available. The demining scenario was identified as of importance as test standards are available, but do not mandate the evaluation of BOP protection. A prototype South African Torso Surrogate (SATS) was developed to explore this scenario further. The SATS was required to be relatively inexpensive and robust. The SATS was cast from silicone (selected to represent body tissue characteristics) using a torso mould containing a steel frame and instrumented with chest face-on pressure transducer and accelerometer. The SATS was subjected to an Anti-Personnel (AP) mine test and the Chest Wall Velocity Predictor and Viscous Criterion were used to predict that BOP injuries would occur in a typical demining scenario. This result was confirmed by applying the injury criteria to empirical blast predictions from the Blast Effects Calculator Version 4 (BECV4). Although limitations exist in the ability of injury criteria and measurement methods to accurately predict BOP injuries, generally a conservative approach should be taken. Thus, it is recommended that the risk of BOP injuries should be evaluated in demining personal protective equipment test standards

Experimental and numerical study of auxetic sandwich panels on 160 grams of PE4 blast loading

Mines, specifically as Anti-Tank (AT) mines are a significant threat for defence vehicles. While approaches such as v-shaped hulls are currently used to deflect the blast products from such threats, such a solution is not always usable when hull standoff is limited. As such the development of a low profile, energy absorbing solution is desirable. One approach that has potential to achieve these requirements are sandwich panels. While sandwich panel cores can be constructed from various materials, one material of particular interest are auxetics. Auxetic are materials that exhibit a negative Poisson’s ratio. This material has potential to be an efficient an impact energy absorber by increasing stiffness at local deformation by gathering mass at the impact location. This study investigates the effectiveness of novel auxetic core infills alongside three other panel types (monolithic, air gap, polymer foam sandwich) against buried charges. 160 grams of PE4 were buried in 100 mm depth and 500 mm stand off the target. Laser and High Speed Video (HSV) system were used to capture the deflection-time profile and load cell sensors were used to record the loading profile received by the panels. Experimental works were compared with numerical model. Explicit model were generated in LSDYNA software as ‘initial impulse mine’ keyword. The result found that the auxetic and foam core panels were effective in reducing peak structural loading and impulse by up to 33% and 34% respectively. Air-filled panels were the most effective to reduce the deflection of the rear of the plate, however variation between capture methods (HSV and Laser system) were reported, while numerical modelling provided comparable plate deflections responses. When normalised against panel weight, the air filled panels were experimentally the most efficient per unit mass system with the auxetics being the least effective

The blast pelvis

Decreasing the human cost of war is a vital role within the Ministry of Defence, and the Defence Medical Services. With the considerable improvements in care, from point of wounding to rehabilitation, it is possible that we have reached the ceiling of optimal management with available, deployed resources. Injury prevention or mitigation may therefore have a more important role than ever in improving survival rates. The current character of conflict, and certainly the recent conflicts in Iraq and Afghanistan have seen the Improvised Explosive Device used to devastating effect to personnel. These devices cause multisystem injuries, and have a high fatality. The lower extremity was most often affected in these recent conflicts, and many fatalities occurred. A greater understanding of lower extremity trauma biomechanics is likely to be key to preventing future fatalities due to injuries in this body region. This thesis focusses on lower extremity blast injury, performs a review of current understanding, and undertakes a casualty data analysis to further understand injury patterns and the cause of fatal wounding. This analysis finds that haemorrhage secondary to pelvic fracture is the key factor in fatal lower extremity injuries, and therefore an area of considerable research interest. Pelvic injury patterns were therefore analysed using measurement techniques to qualify injury patterns and understand the link between injury patterns and the presence of vascular injury. Subsequent physical and computational testing provided a platform to apply different loading conditions to the pelvis to replicate a blast injury, and understand the behaviour of the bony structures under high rate axial loading. This thesis concludes that the anterior pelvic ring at the pubic symphysis is key to pelvic integrity at high rates of loading. Disruption of the anterior pelvis can lead to subsequent posterior ligamentous rupture which, due to the proximity to major vessels, can lead to major haemorrhage and death. Preventing lateral disruption may be the key to maintaining pelvic integrity at these high loading rates, and preventing vascular compromise and fatality from lower extremity blast injuries.Open Acces

The visceral response to underbody blast

Blast is the most common cause of injury and death in contemporary warfare. Blast injuries may be categorised based upon their mechanism with underbody blast describing the effect of an explosive device detonating underneath a vehicle. Torso injuries are highly lethal within this environment and yet their mechanism in response to underbody blast is poorly understood. This work seeks to understand the pattern and mechanism of these injuries and to link them to physical underbody blast loading parameters in order to enable mitigation and prevention of serious injury and death. An analysis of the United Kingdom Joint Theatre Trauma Registry for underbody blast events demonstrates that torso injury is a major cause of morbidity and mortality from such incidents. Mediastinal injury, including those trauma to the heart and thoracic great vessels is shown confer the greatest lethality within this complex environment. This work explores the need for a novel in vivo model of underbody loading in order to explore the mechanisms of severe torso injury and to define the relationship between the “dose” of underbody loading and resultant injury. The work includes the development of a new rig which causes underbody blast analogous vertical accelerations upon a seated rat model. Injuries causes by this loading to both the chest and abdomen can be best predicted by the examining the kinematic response of the torso to the loading. Axial compression of the torso, a previously undescribed injury metric is shown to be the best predictor of injury. The ability of these results to translate to a human model is explored in detail, with focus upon the biomechanical rationale; that torso organ injuries occur through both direct compression and shearing of tethering attachments. Survivability of underbody blast could be improved by applying these principles to the design and modification of seats, vehicles and posture.Open Acces

Characterising material effects in blast protection

Higher strength grades of modern armour steel have promising applications in the blast protection system of armoured vehicles due to the combination of high strength, good energy absorption capacity and familiar fabrication techniques for vehicle manufacturers. While higher strength grades of armour steel are used regularly for ballistic protection and have been integrated into other areas of a vehicle armour systems, there is limited understanding of the response of this class of materials to localised blast loading and further their performance in a blast protection application is unclear. This thesis produces new knowledge and predictive tools regarding the deformation and fracture response of multiple grades of high strength armour steel subjected to localised blast loading. The response of four grades of high strength steel to localised blast loading was characterised through an extensive experimental investigation, providing significant new insight into the protective capacity provided by high strength, moderate ductility armour steels. The steels tested include three grades of armour steel: rolled homogeneous armour (RHA), improved rolled homogeneous armour (IRHA) and high hardness armour (HHA) as well as a high strength abrasive resistant steel (ARS) with a transformation induced plasticity (TRIP) strengthening phase. Quadrangular target plates were tested using cylindrical charges of PE4 plastic explosive at stand-off distances (SOD) from the target plate between 13 mm and 50 mm. The wide range of blast loading conditions produced localised bulging of the target plates through to rupture and wide-spread fracture propagation. Along with a thorough assessment of target plate deformation, the magnitude of blast loading required to rupture the four armour steels was isolated at a 13 mm and 25 mm stand-off distance. The rupture threshold of the armour materials was significantly higher than more ductile mild steel evaluated extensively in the literature. ARS, which possessed the highest rupture threshold of the armour materials, fractured at a charge mass 81% higher than the mild steel. Fractographic analysis showed that the high strength steels investigated initiated rupture via ductile shear fracture, as opposed to tensile tearing which is common in lower-strength steels. Cracks were propagated by a variety of ductile tensile and shear modes as well as a brittle radial crack propagation mode identified for the HHA steel. For the first time, the significant effect of SOD on the target response under free air blast loading was incorporated into a non-dimensional impulse parameter (NDIP) framework. The SOD impulse correction parameter was formulated to capture the more concentrated spatial distribution of blast loading and the contribution of a transverse shear response mode within the target plate, associated with reductions in SOD. The new SOD dependant NDIP produced significant improvements in the prediction of non-dimensional deflection across a large range of experimental programs and identified a characteristic NDIP at fracture for each armour material, which was unified across the SOD conditions tested. The large body of experimental blast results produced through this test program provides a level of insight into the deformation and fracture behaviour of this class of materials which has not been reported previously in literature. Comprehensive material characterisation was conducted for the four high strength steels to develop new constitutive models and understanding into the fundamental mechanical properties of each armour material. The plasticity and ductile fracture behaviour of each steel was experimentally characterised across a range of stress states and loading conditions, including elevated temperatures and strain rates. State-of-the-art constitutive models were generated for each armour material, capable of capturing the plasticity and fracture behaviour of 13 unique specimen geometries. Ductile fracture was modelled effectively by the Basaran fracture model, producing a level of fracture characterisation unseen previously for these grades of steel. The Basaran model was calibrated following inverse numerical modelling of each mechanical test and utilised a new time-dependant stress state calibration approach for history dependence in the ductile fracture process. Inverse numerical modelling of the high strain rate tensile experiments identified dislocation drag effects on flow stress at 2700 s-1. A novel two-stage exponential strain rate hardening term was developed and integrated into the constitutive model to capture the dynamic behaviour of each material accurately. A numerical modelling methodology was developed which significantly improved on the state-of-the-art approach for the prediction of deformation and fracture of the armour materials under localised blast loading. The loading from the explosive charge is modelled in an Eulerian representation and is coupled to a Lagrangian representation of the target plate, which deforms and fractures according to the constitutive models defined for each material. The model predicted the final deformation of target plates within 10% for 39 experimental test conditions and a gave good qualitative reproduction of the 3D scanned deformation profile of the experimentally tested target plates. The charge mass rupture threshold was predicted within 12% of experiments for both SOD conditions. Analysis of the spatial distribution of blast loading highlighted a significantly higher mesh dependence than for overall impulse transfer and a fine discretisation of the fluid domain was required to accurately capture fracture behaviour. The stress state evolution within the target plate approaching fracture was analysed and a shear failure mode was identified early in the target plate response. This shear mode is produced by the initial impingement of the blast product from close proximity explosive charges and plays a critical role is initiating strain localisation in the 13 mm SOD test conditions. An extensive numerical modelling study was performed providing a new understanding into the effect of various target plate mechanical properties on the deformation and plastic strain evolution under blast loading. High yield strength was highlighted as the most important target plate property for deformation resistance and minimising plastic strain development. While a high strain hardening capacity showed a smaller influence on deformation resistance, it significantly improved the ability of the material to resist the thermo-mechanical instability governing strain localisation thereby increasing the rupture threshold. The most critical property governing the onset of strain localisation and subsequent fracture of the target plate was found to be thermal softening behaviour (magnitude of strength loss at elevated temperatures). The results of this study provide guidance for new material developments and key parameters of armour materials. This highly accurate material characterisation and numerical modelling methodologies developed throughout this thesis can provide meaningful predictions of protective capacity without relying on extensive experimental blast testing

Heel biomechanics under blast conditions

The deformation of the floor above a vehicle attacked by an improvised explosive device transmits a high-rate, primarily axial, load to the lower limb of the occupants resulting in intra-articular, difficult-to-treat fractures of the foot and ankle. The aim of this thesis was to improve understanding of the biomechanics of the foot and ankle during this incident as a first step towards better design of efficient mitigation strategies. Due to the great potential of computational methods, the aim was achieved by developing and using a finite element model. A great challenge in developing FE models of biological systems regards the selection of appropriate material properties for biological tissues. The material properties of the heel fat pad are critical for the response of an FE model of the foot and ankle and yet ill-understood for high loading rates. The first objective of this thesis was to characterise the material behaviour of the heel fat pad across strain rates. This was achieved by importing data from tests performed on cadaveric heels to an inverse FE optimisation algorithm that was developed and used to quantify the non-linearly viscoelastic material properties of the tissue. An FE model of the foot and ankle was developed in this study to simulate high rate axial loading based on scans of a cadaveric specimen. The biofidelity of the response of the model was assessed against experimental data from various non-catastrophic tests; the good agreement between experimental and computational results allowed for a thorough analysis of the kinetic and kinematic response of all tissues which have allowed for a better understanding of the biomechanics of the foot and ankle in underbody blast. These findings offer an insight into the incident and can be used to facilitate the design of protective equipment and develop new ideas for mitigation strategies.Open Acces

Dismounted pelvic blast injury: mechanisms of injury, associated injuries and mitigation strategies

Explosive blast has been the most common cause of wounding and death in recent military conflicts. Where blast resulted in injury to the pelvis of an on-foot casualty, the mortality rate was high. The mechanism of injury by which this occurs is not known. The research presented in this thesis sought to understand the pattern and mechanism of this devastating injury, in order to develop protective strategies. An analysis was performed of battlefield data which identified pelvic vascular injury as the cause of death in these casualties. Furthermore, it showed displaced pelvic fractures, perineal wounding, and traumatic amputation to be associated with this lethal injury. Hypothesised mechanisms of injury were investigated using cadaveric animal models of blast. These investigations showed rapid outward movement of the lower limbs (‘limb flail’), caused by the blast wave, to be necessary for displaced pelvic fractures with vascular injury to occur. High velocity sand ejecta, as propagated by blast (‘sand blast’), showed correlation with increasing velocity and injury patterns of worsening severity across the trauma range. This included the associated injuries of perineal wounding and traumatic amputation. Following this research, lower limb flail and high velocity sand blast were identified as the mechanisms of injury of blast to the pelvis. Novel pelvic protective equipment was developed to limit lower limb flail in a cadaveric animal model of blast. This resulted in a reduction of pelvic fractures and elimination of pelvic vascular injury. Protective silk shorts were subsequently examined in a human cadaveric model and shown to markedly reduce the severity of injury from high velocity sand blast. Implementation of the protective strategies described in this thesis is suggested to reduce the severe injury burden and mortality rate associated with blast injury to the pelvis.Open Acces