Modified hopkinson apparatus to investigate fluid cavitation as a potential source of injury

Abstract

Mild Traumatic Brain Injury (mTBI) has been recognized as an important issue for persons exposed to blast. Specifically, this injury has been associated with exposure to blast overpressure and more recently relatively large negative pressures have been identified as occurring at the posterior regions of the brain in experimental and in numerical studies of frontal blast exposure. These negative pressures are caused by the reflection of the incident bar stress wave from the free surface of the skull, and may be intensified due to focusing effects from the curvature of the skull. Under certain circumstances, this negative pressure is hypothesized to cause cavitation of cerebrospinal fluid (CSF) surrounding the brain, potentially resulting in injury to the brain. Unfortunately the cavitation pressure of CSF has not been directly measured, so the consequence of negative pressures in numerical head models exposed to blast cannot be accurately predicted. The cavitation pressure of fluids is highly variable, depending on the presence of impurities in the fluid and the presence of dissolved gasses. In this study, a modified Compressive Split Hopkinson Pressure Bar (CSHPB) apparatus incorporating a sealed confinement chamber was used to generate negative pressures in distilled water to investigate the cavitation properties of water as a surrogate for CSF. The negative pressures in the fluid were measured using a pressure transducer designed for compression and validated in comparison to the input signal on the modified Hopkinson bar apparatus, as well as verified by a numerical model of the experiment. The CSHPB apparatus was used to generate initial compressive waves ranging from 1.85 to 7.85 MPa to produce cavitation in distilled water. The experimental tests were simulated with good agreement and used to obtain water peak negative pressures ranging from - 1.32 to - 5.64 MPa. Future tests will be undertaken to investigate cavitation properties of CSF.