(83b) Shear Thickening Fluid (STF) – Nanocomposites for Improved Puncture and Hypervelocity Impact Resistance

Concentrated colloidal dispersions exhibit unique flow properties that depend on the applied rate of deformation, among these being the phenomena of shear thinning and shear thickening.  Shear thinning occurs at low shear rates and results in a reduction of the dispersion viscosity.  Contrarily, shear thickening manifests itself at high shear rates where the dispersion exhibits a reversible increase in viscosity.  For the most concentrated dispersions, a sudden reversible transition from a flowing state to a rigid, solid-like state.  In many industrial processes, shear thickening is often undesirable as it can damage equipment and limit flow rates for pumping.  However, when integrated with ballistic fabrics as STF-Armor^TM, this reversible fluid-to-solid transition makes these dispersions attractive for use as field-responsive protective materials such as soft body armor and correctional armor applications. 

            In low-earth orbit, astronauts performing extra-vehicular activities are exposed to the dangers of micrometeoroid and orbital debris (MMOD).  While generally less than a centimeter in size, these MMOD particles can travel at velocities on the order of tens of kilometers per second, rending them highly energetic and a threat to puncture the thermal micrometeoroid garment (TMG) lining the pressurized air bladder.  An equally dangerous threat to astronauts occurs as the result of MMOD impacts into the exterior of space vehicle.  These impacts result in the formation of craters with sharp raised lips.  These sharp surfaces are a threat to puncture the TMG and have resulted in the premature termination of spacewalks outside the International Space Station due to suit tears.  While the TMG is designed to withstand direct MMOD impacts, the record of suit tears during spacewalks suggests the need to enhance the resistance to cutting and puncture threats which involved drastically different energy and time scales than direct MMOD impacts.  As such, STF-Armor^TM is an attractive candidate composite material to incorporate within the TMG.

            We replaced the traditional neoprene-coated nylon absorber layers in the TMG with STF-Armor^TM and studied their resistance to quasi-static puncture and hypervelocity impact threats.  We used hypodermic needles to simulate the cutting and puncture threats associated with the sharp cratered surfaces on the International Space Station.  Under quasi-loading, TMG lay-ups containing STF-Armor^TM increased the peak force required to puncture the lay-up by 72% while the flexibility of the lay-up was unaltered and its areal density was reduced by 13%.  The hypervelocity impact resistance of the lay-ups containing STF-Armor^TM was investigated using spherical aluminum projectiles at velocities ranging from 4.5-7.5 km/s.  The ballistic limit of these lay-ups containing STF-Armor^TM is shown to provide meaningful hypervelocity impact protection.  On-going experiments concerning the effects of the real space environment on the puncture and hypervelocity impact resistance of lay-ups containing STF-Armor^TM and an International Space Station Flight Opportunity will be discussed.