A Novel In-Situ Nanoindentation Characterization of Phase Transforming Materials
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A Novel In-Situ Nanoindentation Characterization of Phase Transforming Materials A. Alipour Skandani1, R. Ctvrtlik2 and M. Al-Haik3 1
Department of Industrial Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA. Joint Laboratory of Optics of Palacky University and Institute of Physics of Academy of Science of the Czech Republic, Olomouc, Czech Republic. 3 Department of Engineering Mechanics, Virginia Tech, Blacksburg, VA 24061, USA. 2
ABSTRACT Materials with different allotropes can undergo one or more phase transformations based on the changes in the thermodynamic states. Each phase is stable in a certain temperature/pressure range and can possess different physical and mechanical properties compared to the other phases. The majority of material characterizations have been carried out for materials under equilibrium conditions where the material is stabilized in a certain phase and a lesser portion is devoted for onset of transformation. Alternatively, in situ measurements can be utilized to characterize materials while undergoing phase transformation. However, most of the in situ methods are aimed at measuring the physical properties such as dielectric constant, thermal/electrical conductivity and optical properties. Changes in material dimensions associated with phase transformation, makes direct measurement of the mechanical properties very challenging if not impossible. In this study a novel non-isothermal nanoindentation technique is introduced to directly measure the mechanical properties such as stiffness and creep compliance of a material at the phase transformation point. Single crystal ferroelectric triglycine sulfate (TGS) was synthetized and tested with this method using a temperature controlled nanoindentation instrument. The results reveal that the material, at the transformation point, exhibits structural instabilities such as negative stiffness and negative creep compliance which is in agreement with the findings of published works on the composites with ferroelectric inclusions. INTRODUCTION The concept of materials and structures with negative stiffness although not too recent but still requires further clarifications in terms of both definition and quantification. It is well established that a constrained bi-stable structure such as a buckled beam can exhibit negative stiffness under certain circumstances[1]. From negative stiffness one may expect the material/structure to displace opposite to the direction of the applied force. The importance of such materials/structures can be expressed in extremal vibrational damping [2], composites with elevated stiffness [3] and acoustical absorbers [4]. Even more application is being thought ranging from meta-materials to seismic protection of structures[5] and strings with negative stiffness and hyperfine structures[6]. The design and evaluation of bi-stable structures as negative stiffness elements has been touched in several cases whereas investigation for materials with negative stiffness remains limited to the proof of the concept by Lak
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