Chapter 27 Investigation of Cavitation Using a Modified Hopkinson Apparatus Dilaver Singh and Duane S. Cronin Abstract Head injury, specifically mild Traumatic Brain Injury, has been identified as an increasingly common injury resulting from blast exposure. Advanced modeling has demonstrated the possibility of relatively high negative pressure at the posterior of the skull for frontal blast exposure, attributed to reflection and focusing of the stress waves due to curvature of the skull. It has been hypothesized that high negative pressures could lead to injury, possibly by cavitation of the cerebrospinal fluid (CSF). However, the cavitation pressure for CSF has not been measured directly in the literature, and thresholds are required for detailed numerical head models. Furthermore, the values for cavitation pressure of fluids in the literature vary widely, postulated to be due to varying levels of impurities and dissolved gases. In this study, a Split Hopkinson Pressure Bar apparatus was modified for tensile loading with a sealed confinement chamber and was used to investigate the cavitation properties of water. The modified apparatus was able to generate a tensile wave on the order of 3.4 MPa resulting in cavitation in the water sample. Future work will utilize this technique to investigate the cavitation pressure of CSF directly. Keywords Blast • Cavitation • Hopkinson bar • Cerebrospinal fluid • mTBI 27.1 Introduction Blast exposure has become a significant issue, due in part to the increased exposure to Improvised Explosive Devices (IEDs) in modern military conflicts. More specifically, brain injury due to blast has been recognized as a signature injury in recent conflicts [1], and there is a significant effort underway to understand the mechanism(s) of injury and to develop injury mitigation strategies, particularly with respect to mild Traumatic Brain Injury (mTBI). One possible injury mechanism that has been hypothesized is cavitation of the cerebrospinal fluid (CSF) surrounding the brain, due to large negative pressures resulting from primary blast wave interaction with the head [2, 3]. Cavitation of the cerebrospinal fluid as an injury mechanism for concussion has been postulated as early as 1948 by Ward et al. [4], and has recently seen a renewed interest in blast related brain trauma research. The hypothesized damage mechanism of cavitation is the generation of cavitation bubbles, which then collapse and create high pressure spikes in the surrounding tissue [5]. In fact, the collapse of cavitation bubbles is used by the medical community to erode kidney stones in shock wave lithotripsy (SWL) treatment, so the potential for cavitation as a tissue damage mechanism certainly exists. Although cavitation has been observed in both impact and blast experiments on surrogate heads [6–8], the presence of cavitation has not been directly observed in vivo for any load case, which is likely more indicative of the difficulties in measuring such phenomena in vivo, rather than the absence of cavitation altogether. To circumvent the limitations of experimental studies with human subjects, many authors have used finite element models of the head and brain to investigate blast injury mechanisms [2, 3, 9–11]. Furthermore, large negative pressures, on the order of several atmospheres, in the brain tissue and CSF resulting from blast have been reported in several of these numerical models [2, 3], underscoring the possibility of cavitation as a potential injury mechanism in blast (Fig. 27.1). Some numerical models have used threshold values of negative pressure to simulate cavitation of the CSF, but these threshold limits are unclear because the negative pressure limit for cavitation of CSF has not been measured directly in the literature. D. Singh (*) • D.S. Cronin Department of Mechanical Engineering, University of Waterloo, 200 University Ave W, Waterloo, ON, Canada N2L 3G1 e-mail: d3singh@uwaterloo.ca B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06995-1_27, #The Society for Experimental Mechanics, Inc. 2015 177
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