Effects of adiabatic heating on the high strain rate deformation response of triaxially braided polymer matrix composites

At low rates of strain, plastic deformation is generally treated as isothermal; however, as the rate of deformation increases, conditions transition from isothermal to adiabatic. In the case of high strain rate impact events, adiabatic conditions prevail. Local temperature rises due to the conversion of plastic work to heat can cause thermal softening and subsequent localization, leading to a potentially substantial deviation from isothermal conditions. Thus, accurately modeling the effects of adiabatic heating is of utmost importance for the design and analysis of composite structures subjected to high strain rate impact loading, such as jet engine fan blade containment systems subject to blade-out. In this work, a strain rate, temperature, and pressure dependent state variable polymer constitutive model is used to model heat generation due to plastic deformation in the matrix constituent of braided polymer matrix composites. The constitutive model is embedded within the generalized method of cells micromechanics framework to investigate the effects of adiabatic heating on unidirectional composite response, which are demonstrated to be significant for transverse tensile and in-plane shear loading. A subcell-based approach is then used to discretize the mesoscale repeating unit cell of a representative T700/Epon 862 triaxially braided [0°/60°/-60°] composite material system into an assemblage of adjacent composite laminates, with ply layups determined from the braid architecture. A two-step homogenization procedure is applied to determine effective properties of the unit cell. Parametric studies are conducted to study how the effective in-plane elastic moduli of the triaxially braided composite vary with braid angle, strain rate, and temperature. Computed effective axial and transverse moduli agree well with available experimental data. The semi-Analytical nature of this model allows for the rapid determination of effective properties for various braid architectures for which there is no available test data and lends itself well to implementation into finite element codes and optimization routines.