Blast-Induced Acceleration in a Shock Tube

February 1, 2016

Army Medical Research and Materiel Command, Fort Detrick, Maryland

High-fidelity simulation of blast flow conditions can aid in developing strategies to mitigate blast-induced brain injury.

The prevalence of blast-induced traumatic brain injury (bTBI) has prompted an urgent need to develop improved mitigation strategies and advance medical care targeting casualties with bTBI. Despite considerable effort, the basic mechanisms of blast-induced brain injury are still undefined. Based largely upon computational modeling, several candidate mechanisms of nonimpact bTBI have been identified and include head acceleration. This work hypothesizes that explosion flow conditions can cause head acceleration sufficient to injure the brain, and that these inertial forces combine with other injury mechanisms to yield bTBI.

The primary innovation of this project is the development and utilization of a high-fidelity simulation of possible blast flow conditions. The goal was to replicate all key features of blast flow wave conditions, including the negative phase and secondary shock. Tight control of these components (notably acceleration and displacement), in combination with functional outcome measures, will greatly enhance understanding of the relation of the former to the latter. As the use of shock tubes has greatly expanded in recent years for biomedical research and TBI research in particular, it is critical that these experimental devices be used in a manner that most effectively simulates explosive blast conditions, recognizing that creating an injury does not constitute validation of an injury model.

An explosive shockwave is unlike any other conventional mode of loading, and will impart both an abrupt transient crushing action (i.e. static pressure) that envelops the head, as well as some aerodynamic drag (i.e. dynamic pressure creating blast wind). Controversies and confusion concerning the contributions of blast-induced head acceleration to bTBI have in great part resulted from laboratory studies in which blast was inappropriately simulated, and head acceleration was likely, in many cases, an experimental artefact uniquely associated with those particular exposure conditions. In particular, positioning experimental subjects at or near the mouth of the shock tube exposes them to endjet conditions; practically all flow energy is converted to a collimated jet at the shock tube exit, yielding extreme dynamic pressure and negligible static pressure as end wave rarefaction abruptly reduces static pressure and greatly accelerates flow. In addition, cylindrical shock tubes characteristically produce shock waves with flat tops and greater-duration positive phases, which will yield unscaled drag forces greatly exceeding those occurring with an explosion in the free field. Discerning the loading conditions and role of acceleration in blastinduced TBI requires careful monitoring and validation of the fidelity of the experimental model; as noted, creation of an injury does not constitute validation of an injury model.

As a first step toward understanding the head motion of soldiers exposed to a typical IED blast (<10 msec positive phase duration), high-speed video recording was utilized to record the motion imparted by the passage of an air shockwave in an advanced blast simulator (ABS) to various inanimate spherical objects of different areal densities, and to an articulated body represented by a 1-foot-tall wooden artist manikin of a human form. Test objects were carefully suspended in the test section of the ABS by a thread that immediately detached upon arrival of the shock front. Spheres ranged in size from 0.75" diameter steel ball bearings to a 10" Synbone head form ballasted by water to approximate the global shape and mass of a human head. Blast exposures were standardized to a 13 psi by 5 msec waveform.

Velocity of spheres as a function of acceleration coefficient areal density (i.e. total mass/surface area) presented to the oncoming shockwave is considered as the dominant factor affecting blast-drag studies, and its inverse is known as the acceleration coefficient. The blast-induced velocities of spheres with a wide range of mass and size were tracked as a function of these coefficients. In all cases, blast-induced motion was imparted almost immediately (<1 msec), and terminal velocities were reached long before the end of the positive phase of the shock wave, confirming that displacement was dominated by the diffraction phase and had no relation to the quasi-steady drag forces (i.e. dynamic pressure impulse and blast wind) as has been popularly accepted.

This work was done by Dr. Joseph B. Long of The Geneva Foundation for the Army Medical Research and Materiel Command. ARL-0186