Design of Experimental Methods to Test the Performance of Pads and Helmets under Blast Loading Conditions

Improvised explosive devices (IEDs) have become a primary weapon in conflicts against US and allied forces. Improvements in body armor and medicine have increased the survivability of such events. These factors have caused an increase in traumatic brain injury (TBI) and mild traumatic brain injury (mTBI) induced by primary blast waves. Injury mechanisms caused from primary blast waves are not clearly understood or defined. How primary blast waves interact with materials or between narrow gaps found between helmet pads is not known. Two novel test fixtures were developed to provide a basic understanding of these two issues. The first fixture was developed to examine the helmet-head subspace focusing on the so called “underwash” affect. All tests were carried out in the shock tubes at UNL. A relationship between the peak pressures on the forehead and crown of the head and the gap distance between pads was established. Based on these experiments, optimal gap distances were determined to be 1.6”, 2.1” and 2.9” for incident pressures of 30 psi, 20 psi and 10 psi, respectively. The second fixture was developed to investigate the blast mitigation performance of pads and other materials. Collaborative testing was performed with the Massachusetts Institute of Technology on sandwich samples filled with fluid or fluid-like materials. Results showed that below incident pressures of 20 psi the core material of the sandwich sample has little effect on the blast mitigation performance. Pad materials currently used in the US Army ACH helmet were tested with the blast mitigation performance fixture. Comparison of high speed video footage and pressure profiles taken behind the pads showed that the peak pressure occurs before maximum displacement of the pads into the system. Theoretical stress wave transmission times were compared to experimental values. Results confirm that stress wave propagation is the primary mechanism in blast pressure transmission, compared to dynamic loading caused from local deformation. In addition a novel device, AENID was designed to simulate an IED detonating under the floor of a vehicle such that occupant loading and kinematics can be studied in a repeatable fashion and is offered in the appendix. Advisor: Namas Chandr