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Vertically-aligned carbon nanotube (VACNT) pads have recently received widespread attention for use as contact surfaces in material handling processes that involve the transfer of bare silicon wafers. Such processes will benefit from the strong friction force interactions and minimal adhesion force offered by these pads, allowing the wafer to be picked up, carried, and quickly placed, without encountering problems which may arise due to excessive adhesive forces. Despite these benefits, practical implementation has been hindered because VACNTs have nonlinear mechanical characteristics which are still not well understood. Consequently, significant attention has been devoted to fully understand and determine the behaviors associated with their nonlinear dynamic mechanical properties. Along this line, several experimental techniques are applied in this paper to further develop a comprehensive understanding of the mechanical behavior of these pads under compressive loading. It is important to note that the samples used in this testing are not standard VACNTs, but have been grown separately from the final substrate on which they are mounted during testing. After growth, the samples are turned upside-down and fixed so that the bottom ends of all VACNTs are planar and present an ultra-flat top surface for contact during manipulation. The tests performed in this research include a low energy impact test and position controlled load-displacement testing with both constant and sinusoidal velocity loading and unloading. Through these testing procedures, the dependencies of the VACNT material properties to compression depth and displacement rate are observed and an attempt is made to incorporate them into a continuous model. For this, the results from the low energy impact testing provide grounds to state the nature of the nonlinear behavior in our VACNTs. By interrogating the available data from each testing technique, a combination of information provided by the theoretical energy balance and the identified coefficients from the Levenberg-Marquardt curve-fitting algorithm is then applied to generate a parametrized phenomenological model of the VACNT pad behavior. The proposed identified model is continuous and reasonably accounts for the overall material behavior as seen in the experimental data. The validity of this model is shown by means of normalized vector correlation of over 99% between the results of the numerical simulations and the existing experimental data. The material behaviors observed in this research qualitatively support those of several earlier investigators who have previously recognized the complex dissipative behavior of VACNTs. The proposed work itself paves the road for developing a useful engineering model of VACNT pad dynamics which will enable their introduction to mechanical applications in industry. |
