Inverse-freezing fluids as trauma attenuators for ballistic impacts

Inverse-freezing fluids, IFFs, are unique materials, which undergo a reversible, endothermic liquid-to-solid phase transition upon heating. Recent findings reveal that this transition can also be triggered by mechanical impact, wherein the impact's shockwaves enter the IFF and induce the liquid-to-solid phase change. This results in the attenuation of these shockwaves within the material. Although the precise mechanism of the attenuation remains to be fully elucidated, it is likely that a portion of the mechanical energy is absorbed via the endothermic phase transition. This energy-uptake route positions IFFs as promising candidates for integration into shockwave-mitigating systems, particularly as anti-trauma components in lightweight ballistic armor. The objective of the study reported herein is to examine whether IFFs can serve to attenuate trauma in ballistic scenarios. Anti-trauma units based on 20 %wt. aqueous methylcellulose, AMC, IFFs were prepared and, for the first time, tested in an accredited firing range. Their ability to attenuate blunt trauma, as measured by backface deformation depths, caused by impacts with stopped 7.62 × 51 mm FMJ NATO rounds was evaluated against U.S. National Institute of Justice, NIJ, performance standards. While anti-trauma units based on the AMC alone were borderline in their ability to meet the NIJ criteria, compositing it with ballistic fibers and thin aluminum sheets significantly enhanced the performance. The resulting composite systems exceeded NIJ requirements, with considerably smaller backface deformation depths. This is significant because these findings open a new mechanism for trauma attenuation – impact-induced phase transition. These results demonstrate the feasibility of combining IFF-based anti-trauma units into practical, field-relevant armor configurations.