Methodology to Study Attenuation of a Blast Wave Through the Cranium

The purpose of the study was to quantify attenuation of open field shockwaves passing through the PMHS (Post Mortem Human Subject) cranium. A better understanding of the relationship between shockwave characteristics external to the cranium and insults experienced by the brain is essential for understanding injury mechanisms, validation of finite element models, and development of military safety devices for soldiers in the field. These relationships are being developed using experimental PMHS techniques. Our existing shock tube produced open field shockwaves by increasing input pressure behind a Mylar membrane using compressed nitrogen until the membrane burst. Increasing membrane thickness resulted in greater bursting pressure and peak shockwave pressure. Peak pressure decreased predictably with greater distance from the shock tube outlet. Input pressures between 1.6 and 3.2 MPa resulted in peak shockwave pressures between 45 kPa and 90 kPa measured between 40 and 60 cm from the shock tube exit. The experimental protocol consisted of obtaining a PMHS head, filling the voided cranium with Sylgard gel, and securing the head in front of the shock tube using a Hybrid III dummy neck. Pressure transducers were mounted on the external cranium surface on the ipsilateral side and on the internal cranium surface on the ipsilateral and contralateral sides. Because the specimen was tested in multiple orientations, the ipsilateral side referred to the frontal or temporal sides. Transducers were mounted prior to adding the Sylgard gel. Data from all tests indicated shockwave rise times less than 10 μs external to the skull and internal to the skull on the ipsilateral side. Therefore, the sampling rate was 10 MHz using a digital oscilloscope. Shockwave characteristics were quantified including peak overpressure, peak underpressure, and duration of positive phase. The results show peak overpressure attenuations between 14 and 26% from the external ipsilateral transducer to the contralateral transducers in frontal and lateral orientation. In addition, there was a 93–96% reduction in the rate of onset between those transducers. Each characteristic may affect injury type/severity. This setup can be used to understand injury mechanisms for blast-induced mTBI, to quantify effects of interventions (e.g., helmets) on attenuation of open field blast waves, and for validation of finite element models.