Concept-level analysis and design of polyurea for enhanced blast-mitigation performance

Polyurea is an elastomeric co-polymer in which the presence of strong hydrogen bonding between chains gives rise to the formation of a nano-composite like microstructure consisting of discrete hard-domains distributed randomly within a compliant/soft matrix. Several experimental investigations reported in the open literature have indicated that the application of polyurea external coatings and/or internal linings can substantially improve ballistic penetration resistance and blast survivability of buildings, vehicles and laboratory/field test-plates. Recently, it was proposed that transition of polyurea between its rubbery state and its glassy state under high deformation-rate loading conditions is the main mechanism responsible for the improved ballistic-impact resistance of polyurea-coated structures. As far as the shock-mitigation performance of polyurea is concerned, additional/alternative mechanisms such as shock-impedance mismatch, shock-wave dispersion, fracture-mode conversion, and strain delocalization have been suggested (without validation). In this study, an attempt is made to identify the phenomena and processes within polyurea which are most likely responsible for the observed superior shock-mitigation performance of this material. Towards that end, computational methods and tools are used to investigate shockwave generation, propagation, dispersion, and transmission/reflection within polyurea and the adjoining material layers as present in the case of a blast-loaded assembly consisting of a head covered with a polyurea-augmented helmet. The results obtained show that for effective shock mitigation, the operation of volumetric energydissipating/ energy-storing processes is required. Candidate processes of this type are identified and presented.