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Mechanical Characterization and Finite Element Implementation of the Soft Materials Used in a Novel Anthropometric Test Device for Simulating Underbody Blast Loading

Understanding the mechanical behavior of components made from eight soft polymer materials is necessary to ensure the predictive capability of WIAMan FE models.

Anthropomorphic test devices (ATDs) have been used in automotive safety research since the 1970s to predict injuries. ATDs must repeatedly perform under a dynamic range of loading rates and reliably distinguish between injuries ranging from minor to severe.

Diagram of modified VALTS rig for isolated lower extremity tests. The WIAMan-LX is pictured and the five polymer components are labeled

Biofidelity is an assessment of a devices’ ability to replicate the kinetics and kinematics of a human subjected to identical loads. Automotive ATDs are suitable for impacts where the principle direction of force comes from the front, side or rear. However, during the recent military conflicts in Iraq and Afghanistan, improvised explosive devices (IEDs) accounted for the most death and injury to Coalition troops. Military vehicles were common targets of an IED attack because of their susceptibility to underbody-blasts and the potential to inflict multiple casualties. Current whole-body ATDs have been shown to exhibit poor biofidelity due to overly-stiff behavior when subjected to highly accelerative vertical loads.

To address the growing threat IEDs pose to vehicle occupants, the Warrior Injury Assessment Manikin (WIAMan) project was commissioned by the US Department of Defense. The WIAMan ATD must demonstrate biofidelity with respect to a human soldier subjected to varying severities of in-vehicle underbody blast exposure.

ATD biofidelity is strongly dependent on the viscoelastic properties of its soft components representing the flesh and bony structures. Polymeric components are often included in an ATD to simulate complex properties of human soft tissues. Polymer components were used extensively in the development of an advanced automotive trauma assessment device, the Test Device for Human Occupant Restraint (THOR), to achieve desired levels of biofidelity and durability. Updates to the six-year-old Hybrid-III pelvis involved softening of the surrounding vinyl flesh material to improve the ATD’s ability to assess child restraint systems. The military lower extremity (MIL-LX) (Humanetics, Plymouth, MI) surrogate has a compression absorber in the tibia shaft to improve biofidelity during vertical impacts. The current WIAMan design also includes numerous polymeric parts to meet biofidelic requirements.

The loading rates associated with under-vehicle explosions are not well established in the literature. In addition, models of polymeric materials used in current ATDs are not published, mostly due to proprietary information. A range of reported values (1-80.5 m/s) for vehicle floor deformation velocity show the uncertainty associated with blast tests of this nature. Initial simulations of full-body WIAMan ATD experiments within its intended loading environment indicate maximum strain rates in the polymeric components are in the range of 100-200 s−1. These rates were estimated in simulations where hyper-elastic material models were assigned to polymer components based on preliminary characterization tests. However, these findings have not been validated experimentally. It is therefore necessary to characterize the polymeric materials under a large range of strain-rates, which should include both static and high dynamic loading rates.

A finite element (FE) model of the WIAMan is being developed to be used in low cost simulation of underbody impact scenarios. To predict responses of the physical dummy, the FE model must be able to replicate the viscoelastic behavior of its polymer components. A thorough understanding of the mechanical behavior of these polymers is required to develop accurate computational models of the ATD.

In this study, eight polymer materials are characterized by uniaxial compression and tension tests at strain rates from 0.01 s−1 to approximately 1000 s−1 and implemented into an FE model of the WIAMan ATD. The material models for the eight polymeric materials have explicitly been characterized within the dynamic range necessary to model blast mechanics used for the under body blast simulation. The validated model can be used as a valuable tool for common FE tasks such as design of experiments and failure tests, which would be extremely costly to perform on a physical dummy.

This work was done by Wade A. Baker and Costin D. Untaroiu of Virginia Tech, Department of Biomedical Engineering and Mechanics; and Dawn M. Crawford and Mostafiz R. Chowdhury for the Army Research Laboratory. ARL-0208

Topics:
Aerospace Anthropometric test devices Child restraint systems Computer simulation Defense industry Materials Military vehicles and equipment Protective systems Restraint systems Safety testing and procedures Simulation and modeling Vehicle occupants Vehicles
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Mechanical Characterization and Finite Element Implementation of the Soft Materials Used in a Novel Anthropometric Test Device for Simulating Underbody Blast Loading

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    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2017 issue of Aerospace & Defense Technology Magazine (Vol. 2 No. 7).

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    Overview

    The document titled "Mechanical Characterization and Finite Element Implementation of the Soft Materials used in a Novel Anthropometric Test Device for Simulating Underbody Blast Loading" presents a comprehensive study on the mechanical properties of soft materials, specifically polymers, used in biomechanical devices. The research focuses on eight polymeric materials that are integral to the design of an anthropomorphic test device (ATD) aimed at assessing injuries from underbody blasts (UBB).

    The study emphasizes the importance of understanding the nonlinear viscoelastic properties of these materials, which mimic the behavior of biological soft tissues. This knowledge is crucial for informing design choices and developing accurate finite element (FE) models of human surrogates. The researchers conducted extensive tensile and compressive tests on the materials at varying strain rates, ranging from 0.01 s−1 to 1000 s−1, to characterize their mechanical behavior.

    The results revealed that the materials exhibited viscoelastic strain rate-dependent properties, with higher modulus polymers demonstrating rate-dependent, strain-hardening characteristics. The stress-strain relationships obtained from these tests were used to define hyper-elastic material models, which were then implemented into a commercial finite element solver. The performance of these material models was validated by simulating experiments conducted on the ATD's lower limb, showing a strong correlation between the simulations and experimental results.

    The study concludes that the validated material models effectively predict the forces experienced by the components of the ATD during UBB loading scenarios. This predictive capability is essential for ensuring the reliability of the WIAMan FE models, which are used to assess potential injuries in real-world situations. The research highlights the significance of thorough material characterization and validation in the development of biomechanical models, ultimately contributing to improved safety measures in military and civilian applications.

    The project was funded by the Army Research Laboratory, and the authors acknowledge the contributions of various teams involved in providing test data and feedback. The findings are intended to enhance the understanding of material behavior under dynamic loading conditions, thereby advancing the field of injury biomechanics and the design of protective devices. The document is a valuable resource for researchers and engineers working in biomechanics, materials science, and related fields.

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