Abstract
Segmented poly(urea) has been shown to be of significant benefit in protecting vehicles from blast and impact and there have been several experimental studies to determine the mechanisms by which this protective function might occur. One suggested route is by mechanical activation of the glass transition. In order to enable design of protective structures using this material a constitutive model and equation of state are needed for numerical simulation hydrocodes. Determination of such a predictive model may also help elucidate the beneficial mechanisms that occur in polyurea during high rate loading. The tool deployed to do this has been Group Interaction Modelling (GIM) – a mean field technique that has been shown to predict the mechanical and physical properties of polymers from their structure alone. The structure of polyurea has been used to characterise the parameters in the GIM scheme without recourse to experimental data and the equation of state and constitutive model predicts response over a wide range of temperatures and strain rates. The shock Hugoniot has been predicted and validated against existing data. Mechanical response in tensile tests has also been predicted and validated.
Highlights
Shock propertiesElastomers made by co-polymerisation of poly(urea) and poly(urethane) have been shown to be very beneficial in blast mitigation and in protection against bullet/fragment impact by being used as a rear face coating or in sandwich materials [1,2,3]
The segmented copolymer made from polymerising diphenyl methane, poly(tetramethyleneoxide)-diphenyl with urea linkages is a large molecule with the mer unit made from disparate functional groups
The phenyl and carboxyl groups are normally appended to the urethane chain as part of the soft segment but we have chosen here to consider it part of the hard segment due to its effective glass transition temperature
Summary
Elastomers made by co-polymerisation of poly(urea) and poly(urethane) have been shown to be very beneficial in blast mitigation and in protection against bullet/fragment impact by being used as a rear face coating or in sandwich materials [1,2,3]. Assuming pressure equilibrium between segments allows determination of the equation of state of the “composite” and prediction of the Hugoniot [8], Fig. 3. This prediction, required an assumption that the shock process suppressed either the free rotation expected of a long amorphous urethane chain or the extra degrees of freedom associated with a polymer above its glass transition. A comparison can be made with the longitudinal stress measurements This requires prediction of the C11 stiffness coefficient and needs to include a pressure dependence of the elastic modulus. This should be compared to measured bulk modulus at low rates of 2 GPa [5] and demonstrates the fewer relaxations available in the shock event
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