Dynamic tensile strength of polyurea
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Dynamic tensile strength of polyurea is measured at an ultrahigh strain rate of 1.67 107 s1 by generating spall failures inside thick polyurea coatings bonded to steel plates using laser-generated stress waves of several nanoseconds in duration. Specifically, thick polyurea films were cast on a steel plate whose backside was provided with water glass–covered Al film. The Al film was melted by focusing a high-energy Nd:YAG laser pulse over 3-mm-diameter area. Exfoliation of the Al generated a compressive stress wave toward the polyurea coating, which turned tensile upon reflection from the free surface. At a threshold laser energy, the amplitude of the returning tensile stress wave exceeded the dynamic tensile strength of polyurea. The stress wave profile inside the steel plate was interferometrically recorded at the threshold laser fluence and was used in a wave mechanics simulation to calculate the peak tensile stress. The polyurea was modeled as a viscoelastic solid. I. INTRODUCTION
Recently, there has been a keen interest in measuring the high strain rate behavior of polyurea because of its potential use in developing advance armors. The latter was motivated by underwater blast experiments of Barsoum et al.1 who demonstrated that use of a mere 2- to 3-mm-thick layer of polyurea on top of a steel armor plate significantly improved its blast resistance. Specifically, the polyurea/steel plate failed through the formation of a single bulge or few large but intact petal tears in contrast to the complete fragmentation observed for an uncoated steel plate when subjected to the exact same shock conditions. To fundamentally understand these unusual observations and to further develop, optimize, and realize the full potential of polyurea for dynamic applications, it is necessary to measure its constitutive behavior and ultimate failure capacity (measured in terms of tensile strength and fracture energy) at high strain rates. By deforming thin polyurea layers using laser-generated stress waves of several nanoseconds in duration, our group has addressed all of these problems. Our results are provided in three articles. In the first article, presented here, we provide measurements of the dynamic tensile strength by adapting a previously developed laser spallation experiment to measure the tensile strength of thin film interfaces. In this experiment, a laser-generated pressure pulse on the back surface of the substrate reflects into a tensile wave from the free surface of the test coating deposited on its front surface. The returning tensile wave pries off the interface a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.405 494
J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012
http://journals.cambridge.org
Downloaded: 31 Dec 2014
at sufficiently high amplitude. Optical interferometry is used in conjunction with wave mechanics simulation to calculate the tensile strength of the failed interface. A number of previous publications provide results for several interface systems involving metal, cera
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