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A novel geometrically nonlinear high order sandwich panel theory considering finite strains of sandwich components is presented in this paper. The equations are derived based on high order sandwich panel theory in which the Green strain and the second Piola-Kirchhoff stress tensor are used. The model uses Timoshenko beam theory assumptions for behavior of the composite face sheets. The core is modeled as a two dimensional linear elastic continuum that possessing shear and vertical normal and also in-plane rigidities. Nonlinear equations for a simply supported sandwich beam are derived using Ritz method in conjunction with minimum potential energy principle. After obtaining nonlinear results based on this enhanced model, simplification was applied to derive the linear model in which kinematic relations for face sheets and core reduced based on small displacement theory assumptions. A parametric study is done to illustrate the effect of geometrical parameters on difference between results of linear and nonlinear models. Also, to verify the analytical predictions some three point bending tests were carried out on sandwich beams with glass/epoxy face sheets and Nomex cores. In all cases good agreement is achieved between the nonlinear analytical predictions and experimental results.

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A novel geometrically nonlinear high order sandwich panel theory for a sandwich beam under low velocity impact is presented in this paper. The equations are derived based on high order sandwich panel theory in which the Von-Karman strains are used. The model uses Timoshenko beam theory assumptions for behavior of the face sheets. The core is modeled as a two dimensional linear elastic continuum that possessing shear and vertical normal and also in-plane rigidities. Nonlinear equations for a simply supported sandwich beam are derived using Ritz method in conjunction with minimum potential energy principle. After obtaining nonlinear results based on this enhanced model, simplification was applied to derive the linear model in which kinematic relations for face sheets and core reduced based on small displacement theory assumptions. A parametric study is done to illustrate the effect of geometrical parameters on difference between results of linear and nonlinear models. Also, to verify the analytical predictions some low velocity impact tests were carried out on sandwich beams with Aluminum face sheets and Nomex cores. In all cases good agreement is achieved between the nonlinear analytical predictions and experimental results.

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The fibers of date palms, which are widely available in the south of Iran, are a variety of natural fibers that can be used as a reinforcement in polymer composites. This work investigates the effect of alkali treatment on the mechanical properties of date palm fibers and their adhesion to thermoset polymer. The sodium hydroxide (NaOH) solution at three different concentrations (1 wt%, 3 wt%, and 5 wt%) was used to treat the fibers. The single fiber tensile test and fiber pull-out tests were performed to measure the mechanical properties and fiber/matrix interfacial share strength, respectively. Comparing the SEM images of the untreated and treated fibers showed that the 3% NaOH treatment could effectively remove non-cellulosic materials, i.e. lignin and wax, with minimum damage to the fiber surface. The experimental results showed a clear improvement of the mechanical properties and fiber/matrix adhesion after treatment. It was found that the 3% NaOH solution is the optimal concentration to achieve the maximum improvement in the fiber and bonding properties.