98 K.-S. Kim et al. Fig. 16.1 (a) A sharp-tip AFM tapping-mode image and (b) a blunt-tip AFM noncontact-mode image of Polyurea PU1000 16.2 Background Regarding the failure analysis, initiation of tensile failure in polyurea PU1000 under conditions of uniaxial strain has been observed through plate-impact, release-wave experiments [3], which are often used for spall tests. In these experiments, a thin flyer impacts a test-sample plate to create a compressive pulse that reflects from the rear surface of the polyurea and generates a tensile pulse after the reflected pulse passes through the oncoming compressive pulse. The dynamic plane-wave test revealed tensile failure stress of polyurea PU1000 approximately 105 MPa under conditions of uniaxial strain. However, polyurea did not spall at this stress level within the tensile-pulse duration. Instead, microvoids opened on the putative “spall plane,” which continued to transmit tensile stress. Knowing the failure initiations stress, actual constitutive relations of the full failure process in spalling require realistic fracture models involving void initiation, growth, and coalescence. Such failure processes are often characterized by cohesive failure models that provide a computational foundation for modeling failure in elastomers. The development of such cohesive models appears to require results of more well-characterized crack-propagation experiments than “spall” tests that involve heterogeneous nucleation and growth of voids. Such experiments, involving plane wave loading of plates with a single mid-plane crack have been previously developed and used to study crack growth in a metal, AISI 4340, under mode I as well as II & III loading conditions [4]. These plane wave experiments are attractive because of their simplicity in tracing the crack-tip loading conditions through measurements of the motion at the tractionfree rear surface of the pre-cracked target plate. However, all previous studies used interferometric techniques that could only measure velocity-time profiles at a single point on the rear surface of the plate. The single-point measurement was particularly troublesome in its accuracy to inversely assess the crack-tip loading profiles. 16.3 Analysis Now, a high-speed streak camera is available, and it is possible to monitor motion—along a line—instead of at a single point at the rear surface. Here, we present a line-image shearing interferometer (L-ISI) for plate impact experiments of dynamic fracture testing. The L-ISI optical circuit is shown schematically as red lines in Fig. 16.2a. In the optical circuit, a laser beam is modulated to synchronously switch the beam by an acousto-optic modulator (AOM) located at the focal plane of the spherical lens SL2, for the exposure duration of a high dynamic range streak camera (C7700). Then the laser light is collimated and aligned by two cylindrical lenses CL1 and CL2 to make a slit illumination on the mirror-finished rear surface of the specimen. The orientation of the illuminated slit is perpendicular to the crack-front line. Then, the slit image of the reflected light is projected on the image plane of the streak camera through the two spherical lenses SL2 and SL3. At the
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