Material Selection and Layered Assembly for Shock Wave Attenuation
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ABSTRACT Theoretical models and experimental data taken primarily from the literature provide predictive capabilities of the sound speed in porous materials and hence their acoustic impedance can be determined. Applying the acoustic approximation (weak discontinuity) at the interfaces of the porous material along with an attenuation model describing conditions within the porous system good fits to shock tube data on an elastomeric foam and on layers of kevlar were obtained.
INTRODUCTION Evidence suggests that soft armor ballistic vests, worn to protect against fragments and small arms, may enhance injury to the lung that often results from the blast overpressure"-. In many cases the fragments and shock wave are created simultaneously and the question becomes are you safer with or without the protective vest. This research effort was initiated to address this question and to investigate the possibility of attenuating the shock overpressure by careful selection and arrangement of the materials used in the vest. The literature provided a vast array of data and models for the strength of blast waves, number, size and velocity of fragments, effect of overpressure on humans and
fragment casualty models. These data clearly showed that if fragments were present they were by far the most serious threat and that a protective vest should always be worn. The work then focussed on the interaction of shock waves with textiles and other soft materials. The conditions that exist across a normal shock and the conditions that occur behind a shock reflected from a solid wall are well known. However, when a shock wave strikes a soft flexible material, such as foams, textiles or the ballistic vest, it is not clear what conditions will prevail. Certain conditions must be satisfied at the interface, such as velocity and pressure matching, but beyond that it is unclear what path to follow. Several models exist for characterizing the thermodynamic properties of soft materials such as foams. 5,6 It proved possible to select, combine and modify these models to obtain a set of relationships that agreed quite well with literature data on a clothing system similar to the ones of interest in this work.
METHODS As mentioned above, the literature provided several models and much data to evaluate these models. Most models required the speed of sound in the material that is to attenuate the pressure, and various assumptions/approximations are made regarding the
269 Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
material to allow this property to be calculated. One model considered here assumes that a porous material (foam, etc.) can be approximated by a dense gas.5 The model shows how the gas constants are altered under this assumption and provides the thermodynamic properties of the material. A second model treats the material as a mechanical system and determines the speed of sound from the density and elastic modulus. A third model describes a modification to the mechanical properties of a solid material so that they r
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