Polymeric Hopkinson Bar-Confinement Chamber Apparatus to Evaluate Fluid Cavitation

Abstract

Mild traumatic brain injury associated with blast exposure is an important issue, and cavitation of the cerebrospinal fluid (CSF) has been suggested as a potential injury mechanism; however, physical measurements are required to evaluate cavitation thresholds. Modifications to a Split Hopkinson Pressure Bar (SHPB) apparatus were investigated with the aim to generate localized fluid cavitation and measure the cavitation threshold of fluids. The proposed design incorporated a novel closed cavitation chamber to generate localized cavitation resulting from a reflected compression pulse, which was generated by a spherical steel striker and Polymethyl methacrylate (PMMA) incident bar. A numerical model of the incident bar was developed and validated with 24 independent tests (cross-correlation: 0.970-0.997), and this was extended to a numerical model of the apparatus including the chamber, validated with 27 independent tests (cross-correlation: 0.921) to predict the tensile fluid pressure in the chamber. Tests on distilled water were performed with comparable numerical results for the chamber strain (R 2 : 0.875) and chamber end-wall velocity (R 2 : 0.992). The pressure in the chamber was determined from the model to avoid introducing a nucleation site via a pressure gauge, and was verified with a firstorder approximation showing good agreement (R 2 : 0.892). The 50% probability of cavitation for distilled water was −3.32 MPa ±3%, comparable to values in the literature. This novel apparatus, including a closed confinement chamber integrated with a polymeric SHPB apparatus was able to create localized fluid cavitation with loading comparable to blast exposure. Future studies will include the measurement of CSF cavitation pressure.

Non-Lethal Blasts can Generate Cavitation in Cerebrospinal Fluid While Severe Helmeted Impacts Cannot: A Novel Mechanism for Blast Brain Injury

“…However, the value of the predicted negative pressure is lower than that in the experiment. This is because when fluid cavitates, the pressure will not be further dropped, but kept at a level, called cavitation threshold or cut-off pressure ( Bustamante et al, 2018 ). Previous computational studies have used different cut-off pressures for the CSF material ( Panzer et al, 2012 ; Zhou et al, 2018 ; Yu et al, 2020 ; Yu and Ghajari, 2022 ).…”

“…The material models and properties of the CSF surrogate (distilled water) and brain (agarose gel) surrogate were identical to the properties of the CSF and brain in the 3D human head FE model. The skull surrogate (acrylic) was modelled with a hyperviscoelastic material model with properties reported in a previous study ( Bustamante et al, 2018 ). The pressure response of the human head FE model was validated against the experimental data from a previous study ( Nahum et al, 1977 ), which conducted impact tests on cadaver heads and measured the impact force and intracranial pressures (Supplementary Materials).…”

Investigation of blast-induced cerebrospinal fluid cavitation: Insights from a simplified head surrogate

“…The FE model of the head surrogate was developed by meshing its CAD model with hexahedral elements. The acrylic was defined as a hyperviscoelastic material model with properties reported in a previous study [14]. For the CSF simulant, the deviatoric response was represented by the dynamic viscosity at human body temperature and the volumetric response was modelled using the Gruneisen equation of state.…”

“…There have been several computational studies [9][10][11][12] and experimental studies [13][14][15][16][17][18] that have investigated cavitation in CSF. For example, using a three-dimensional detailed human head model, we predicted the onset and influence of CSF cavitation on the brain deformation during blast exposure [12].…”

“…Secondly, the mechanism of blast induced CSF cavitation is still unclear. Previous studies suggested that either skull elastic rebound induced by blast wave [13,15,21,22] or reflection of the pressure wave at the head/atmosphere boundary [14,19] may produce CSF cavitation. These mechanisms may be related to specific experimental model designs.…”