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Failure mechanisms of polyurea under high strain-rate


This work addresses the failure mechanisms of polyurea when is loaded with an ultra-high strain pressure wave. This kind of loading is present when polyurea is used as a protective layer against projectiles in ballistics and explosions. A large amount of research has been done on this polymer at room temperature and at lower strain rates, but there is not a lot of information on the failure mechanism of polyurea when is subjected to ultra-high strain rate and low temperatures. Therefore, to understand the failure mechanisms that occur in these loading situations a noveler technique is necessary. Modifications were made to the Laser Spallation Technique in order to load structures under a single transient wave pulse, resulting in two different setups. This study characterized polyurea shock loaded at extreme pressures, strain rates and temperatures. By sandwiching polyurea between two right angle N-BK7 prism, the sample was subjected to a combined pressure shear stress at strain rates of 2.8 x 107 s-1. The stress at failure was determined by Abaqus, a finite element analysis software, that was loaded by interferometrically measured stress wave in the glass prism before it strikes the polyurea sample. A shear strength of of 90�9.9 MPa under a pressure of only 119�13 MPa was measured. Using a different testing setup, polyurea was loaded in tension-shear stress, the stress state at failure was calculated to be 27.71 � 2.43 MPa in shear and mean normal stress of and 57.13 � 5.02 MPa. This occurred at a calculated peak strain rate of 3.7 x 107 s-1. To explore an even more extreme loading environment, a specialized test setup was employed to load polyurea at temperatures ranging from room temperature to -100�C in an attempt to elucidate brittle behavior. Under these loading conditions polyurea failed in a different way, depending on the displacement constraints of the sample and the way that the stress wave was generated. In the spallation test setup, the spalled strength found through the FEM stress calculations is equal to 1.11 GPa with a strain rate of 6.4 x 107 s-1. Polyurea formed a Hertian cone crack when a certain combination of temperature and energy fluence was used, a failure mechanism not previously observed at room temperature using the maximum energy of the Nd:YAG laser (2.3J). Polyurea columns were loaded at different temperatures and energy fluence, showing wing cracks when the samples were loaded at -60�C with 490KJ/m2 fluence. Finally, the mechanical behavior of the polyurea was studied by using a modified high-strain rate punching test, creating shear bands at -20�C with a 512.6kJ/m2.

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