Finite Strain Behavior of Polyurea for a Wide Range of Strain Rates

Polyurea is a special type of elastomer that features fast setting time as well as good chemical and fire resistance. It has also good mechanical properties such as its high toughness-to-density ratio and high strain rate-sensitivity, so its application is recently extended to structural purpose to form sandwich-type or multi-layered plates. Those structures can be used for retrofitting of military vehicles and historic buildings, absorbing energy during structural crash. In order to investigate its behavior of hysteresis as well as rate-sensitivity, three different testing systems are used to cover a wide range of strain rates up to strain of 100%. In view of impact and blast events, the virgin state of polyurea is considered throughout the experiments. First, a hydraulic universal testing machine is used to perform uniaxial compressive loading/unloading tests in order to investigate its hysteresis behavior at low strain rates (0.001/s to 10/s). Second, two distinct gas-gun split Hopkinson pressure bar [SHPB] systems are employed to cover high strain rates: a nylon bar system (700/s to 1200/s) and an aluminum bar system (2300/s to 3700/s). Lastly, the rate-sensitivity for intermediate strain rates (10/s to 1000/s) is characterized using a modified SHPB system. The device is composed of a hydraulic piston along with nylon input and output bars. A finite strain constitutive model of polyurea is presented in order to predict the hysteresis and rate-sensitivity behavior. The 1-D rheological concept of two Maxwell elements in parallel is employed within the framework of the multiplicative decomposition of the deformation gradient. Model parameters are calibrated based on the uniaxial compressive tests at various rates. The corresponding algorithms is implemented as a user-defined material subroutine VUMAT for ABAQUS/Explicit, and used to predict the response of polyurea. The proposed constitutive model reasonably captures the experimentally observed asymmetric rate-sensitivity and stress-relaxation behavior: strong rate-sensitivity and large amount of stress relaxation during loading phase, but weak rate-sensitivity and smaller amount of stress relaxation during unloading phase. In order to validate the proposed model, various dynamic punching tests are performed, and their results are well compared with the model predictions during loading although the prediction of unloading behavior can be further improved. Thesis Supervisor: Tomasz Wierzbicki Title: Professor of Applied Mechanics, Department of Mechanical Engineering