A validated numerical model of a lower limb surrogate to investigate injuries caused by under-vehicle explosions

Abstract

Under-vehicle explosions often result in injury of occupants׳ lower extremities. The majority of these injuries are associated with poor outcomes. The protective ability of vehicles against explosions is assessed with Anthropometric Test Devices (ATDs) such as the MIL-Lx, which is designed to behave in a similar way to the human lower extremity when subjected to axial loading. It incorporates tibia load cells, the response of which can provide an indication of the risk of injury to the lower extremity through the use of injury risk curves developed from cadaveric experiments. In this study an axisymmetric finite element model of the MIL-Lx with a combat boot was developed and validated. Model geometry was obtained from measurements taken using digital callipers and rulers from the MIL-Lx, and using CT images for the combat boot. Appropriate experimental methods were used to obtain material properties. These included dynamic, uniaxial compression tests, quasi-static stress-relaxation tests and 3 point bending tests. The model was validated by comparing force-time response measured at the tibia load cells and the amount of compliant element compression obtained experimentally and computationally using two blast-injury experimental rigs. Good correlations between the numerical and experimental results were obtained with both. This model can now be used as a virtual test-bed of mitigation designs and in surrogate device development.

Introduction

Under-vehicle explosions cause high-rate axial loading of the lower limbs of occupants; this injury mechanism has been particularly prevalent in recent conflicts in Iraq and Afghanistan and is likely to continue to be common in future conflicts and in post-war humanitarian efforts (Ramasamy et al., 2009). This injury mechanism often results in severe lower limb injuries that are associated with a high amputation rate and very poor clinical outcomes (Ramasamy et al., 2012, Ramasamy et al., 2011).

Anthropometric Test Devices (ATDs) are mechanical representations of the human musculoskeletal system, designed to be biofidelic in terms of anthropometry, articulations, and structural response. Their designs have been developed predominantly for use in the automotive industry and are generally agreed to be robust, reliable testing tools (Crandall et al., 2011). Current NATO standards (STANAG-4569) recommend that vehicle design is assessed through the use of either the Hybrid-III or the MIL-Lx ATD, both of which have built-in tibia load cells. The output of the tibia load cells can be used to provide an indication of the risk of injury to the lower extremity through the use of injury risk curves derived from cadaveric experiments. The response of the Hybrid-III leg, however, has been shown to be stiffer than that of a human lower limb in simulated under-vehicle explosions; the load rate in the Hybrid-III has been measured to be more than double that measured in cadaveric specimens, rendering it unsuitable for assessing mitigation technologies below the foot (Bailey et al., 2013, Newell et al., 2012b). The MIL-Lx was developed specifically to improve the biofidelic response in the under-body blast scenario (McKay, 2010). It differs in design to the Hybrid-III in that it has a straight tibial shaft and a 70 mm length rubber compliant element in the mid-tibia.

Numerous experiments have attempted to replicate the loading transferred to the lower limbs of occupants during under-vehicle explosions. Bailey et al. (2013) used a blast rig consisting of two independent sleds (the hammer sled and the carriage sled) mounted on a track to impact both cadavers and ATDs. The cadaver/ATD is positioned on the carriage sled and impacted at both the seat and foot pan by the hammer sled at velocities between 7.6 and 14.2 m/s, corresponding to accelerations between 234 and 686 g. The shape of the input acceleration is adjusted through the use of blocks of polyurethane. The peak forces measured by the MIL-Lx lower tibial load cell ranged between 3.77 and 6.46 kN, while the Hybrid-III measured between 6.96 and 14.4 kN. The force in the tibia of the cadaver was measured using a bone cell (array of strain gauges bonded around the tibia, calibrated to measure load); the resultant peak forces were between 3.3 and 6.3 kN.

Newell et al. (2012b) also tested the MIL-Lx and Hybrid-III legs using an anti-vehicle under-body injury simulator (AnUBIS) in which cadavers/ATDs rest on a 42 kg plate that is pneumatically accelerated upwards (Masouros et al., 2013). The compression of the combat boot observed with the MIL-Lx matched closely with that observed with the cadavers, whereas that observed under the Hybrid-III was much greater, thus suggesting that the response of the Hybrid-III deviates from that of the human limb in an under-vehicle explosion.

While a number of physical experiments have been undertaken using ATDs to investigate the loading observed during under-vehicle explosions, this has not been the case numerically. Finite element (FE) models offer a cost-effective alternative to expensive experimental setups, allowing the simulation of multiple scenarios through alteration of modelling input parameters. One numerical investigation of note is that of Dong et al. (2013) who developed an FE model of a lower extremity to predict the minimum axial force required to cause fracture. Their model was used to determine the critical maximum velocity of the floor plate to cause tibia fracture at a range of knee flexion angles. They found that, as the angle increased, the critical velocity increased. Aside from the long run time, one limitation of Dong et al.׳s model is that it is subject specific. The development of a surrogate lower limb model, whose behaviour in comparison to a large number of cadaveric results is known, may provide a more powerful tool in predicting injury, since it can be used in conjunction with injury curves that have been developed through analysis of a large number of cadaveric tests.

This study, therefore, aims to develop an FE model of an ATD able to predict its response in simulated under-body blast. Recent findings have shown that the MIL-Lx ATD may be more biofidelic than the Hybrid-III ATD and, to the authors׳ knowledge, there are no published FE models of the MIL-Lx. Hence, the MIL-Lx is the focus of this study. The accuracy of the model is assessed by comparing the computational results with those obtained experimentally in two blast-injury experimental rigs.