Energy Absorption Behavior of Polyurea Under Laser-Induced Dynamic Mixed-Mode Loading

The energy absorption response of polyurea thin films is investigated under laser-induced dynamic mixed-mode loading. By exploiting the substrate geometry, the arrival times and the convenience of two simultaneously detected waves (longitudinal and shear), the transient shear loading of thin films is achieved under mixed-mode conditions. Polyurea films are fabricated on fused silica prismatic (90°–45°–45°) substrates by spin coating a polyurea mixture diluted with tetrahydrofuran solvent. A high amplitude shear wave loading of the polyurea films is attained by the mode-conversion of a laser generated compressive stress wave at an oblique face of the prism substrate. Mixed-mode dynamic shear experiments are conducted on 100 nm thick aluminum and 2 μm thick polyurea films over a range of laser loading fluences (energy/unit area). The free surface displacement, surface velocity and energy fluence histories are inferred from the shear portion of high-speed interferometric measurements. On comparing input–output velocities and fluence parameters, test results show velocity retardation and significant energy absorption (50–65 %) by the polyurea films. Postmortem spallation profiles reveal film wrinkling, tearing and a directional failure pattern, indicating shear as the dominant failure mode under the mixed-mode conditions.