Numerical and experimental study of underwash effect and its role in blast-induced traumatic brain injury

The blast wave interactions with the helmet–head assembly can result in localized pressure amplification at certain locations around the head. The underwash effect is a typical example of such interaction in the gap between the suspension-type combat helmet and the head. There are hypotheses in the literature that suspect an increase in the severity of blast-induced traumatic brain injury due to combat helmet usage under blast loading. But the literature lacks concrete experimental visual evidence for the underwash effect and the cause–effect relationship between the underwash effect and brain injury. Firstly, in this study, shock wave interactions causing the underwash effect are visualized using the schlieren imaging technique. Secondly, a reasonable correlation between a significantly large, localized pressure amplification due to the underwash effect and the brain’s mechanical stress response was observed with an idealized helmet–head model in a coupled Eulerian–Lagrangian framework. But further studies are needed with more realistic models to prove their significance in the design of blast-resistant combat helmets.