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In this paper an analytical model for investigating of the ballistic impact behavior of two dimensional woven E-glass/epoxy composites is presented on the basis dividing the impact duration to several time intervals and calculating the energy absorbed during each time interval. The major components of energy lost by projectile during ballistic impact are identified, namely the cone kinetic energy formed on the back face of the target, the secondary yarns deformation energy, the tensile failure energy of primary yarns, the delamination and matrix cracking energy. It is assumed that the shear plug formation is not observed for glass reinforced composites and the energy lost in overcoming the frictional force between projectile and composite is negligible. Analytical formulations have been presented for calculating energy absorbed by each mechanism in each time interval. Finally, a good correlation has been observed, comparing the analytical model presented in this paper to the experimental results presented by others investigators.

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The bioactivity of the aqueous extracts of the leaf of Tetrapleura tetraptera against the wheat storage pest, Rhyzopertha dominica, on stored wheat grains was investigated in laboratory conditions. T. tetraptera leaf extracts were added to 20.0 g of grains at 25.0 mg/l, 50.0 mg/l, and 100.0 mg/l to assess contact toxicity, damage assessment, reproductive performance, and repellency ability. Results showed that the extracts were toxic to the insects. The leaf extracts applied at 100.0 mg/l significantly caused the highest mortality of 94% after 21 days. It also repelled almost 90% of the insects and inhibited adult emergence up to 98%. The 100.0 mg/l of the leaf extract further offered protection of nearly 99% of the wheat grains against insect damage compared to the control. This study revealed that leaf extracts of T. tetraptera can be used to control R. dominica in stored grains efficiently, and its incorporation into traditional storage pest management is strongly recommended in developing countries


 

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Aluminum foam structure is of great importance in aerospace, naval and automotive industries due to light weight and energy absorption characteristics. In this article several aluminum foam having different densities and thickness were designed and tested using light gas gun device. A series of ballistic test were defined in order to determine the effects of density, foam thickness and projectile velocity on energy absorption aluminum foam structures. The results of the experimental testes, it is shown that the amount of energy absorption of aluminum foam structures is increased as density, foam thickness and velocity of the projectile is increased.

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Sandwich panels(structures) of metal surface having aluminum foam core are of great importance in aerospace, naval and automotive industries due to high strength to weight ratio and high energy absorption characteristics. In this article several aluminum sandwich panels with aluminum foam core having different densities and thickness were designed and tested using light gas gun device. A series of ballistic test were defined in order to determine the effects of density, foam thickness and projectile velocity on energy absorption and ballistic limit velocity of sandwich structures. The material model used for metal foam was Deshpande- Fleck-Foam and coefficients were determined experimentally using foam and Matlab capabilities. Also, numerical simulation using LSDYNA software were performed. The results of the experiment and numerical simulation were compared and there was a good agreement between experimental investigation and numerical results. Using experimental testes and parametric studies,it is shown that the amount of energy absorption of sandwich structures is increased as density, foam thickness and velocity of the projectile is increased.

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In this paper, a new analytical model has been presented for energy absorption of aluminum-foam sandwich panels under ballistic impact. The panels consist of foam core sandwiched between two aluminum skins. In analytical model two types of sticker including cylindrical projectile with flat and hemispherical ended have been considered. It is supposed that aluminum skins failure by mean resistive pressure. Also foam absorbed a partial of projectile energy by crushing. Energy absorption of aluminum-foam sandwich panel is calculated and energy balancing equation has been employed for determination the ballistic limit and residual velocity of projectiles. The results of ballistic limit and residual velocity computed by new model have good agreement with experimental results. Also the effects of projectile mass and diameter in energy absorption of sandwich panel has been investigated.

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In this study, repeated low velocity impacts on aluminum plate are investigated experimentally and numerically. In order to investigate the failure mechanism, Lemitre's model of the continuum damage mechanics is used. Numerical simulation is carried out employing a Vumat subroutine in Abaqus FE package. Repeated impacts are performed on the plate with the same level of energy. Plastic deformation is observed on the plate in the first impact. During the subsequent impacts and prior to crack initiation, the effect of strain hardening on the aluminum plate is observed. After crack initiation, the stiffness of the structure decreases. As the impacts continue, stiffness further decreases and the damage area increases, finally perforation and penetration appear on the plate. Also, the present model is validated by the experimental results. Comparison of numerical with experimental results shows a good agreement for the force-time and force-displacement histories.

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In this study, experimental tests were performed to evaluate the effects of axisymmetric cylindrical projectile nose shapes and initial velocities on ballistic performance of laminated woven glass epoxy composites. Projectile initial velocity and nose sharpness changes, absorbed energy, delamination area, etc. are investigated by six blunt, hemispherical, conical and ogival projectiles. Hand lay-up method has been used to manufacture composite targets with 18 layers of 2D woven glass fibers of 45% fiber volume fraction. The epoxy system is made of epon 828 resin with jeffamine D400 as the curing agent. The results show that the maximum influence of projectile geometry on target behavior, occurs in ballistic limit area. In this range of initial velocity, ogival (CRH=2.5) and Blunt projectiles show the best and the worst ballistic performance. The delamination area decreases as the projectile nose sharpness increases or its initial velocity decreases. Ballistic curves for different projectiles show that the difference between projectiles behavior decreases in higher impact velocities. Because of target shear failure in blunt projectile impact, the amount of target absorbed energy for this projectile is less than other projectiles in higher impact velocities away from ballistic limit velocity.

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This paper, experimentally evaluates the effects of indenter geometry on quasi-static perforation process of laminated woven glass epoxy composites. Low loading rate tests were performed, using six indenters with blunt, hemispherical, conical (cone angle of 37˚ and 90˚) and ogival (caliber radius head of 1.5 and 2.5) nose shapes. Composite behaviors like energy absorption, contact force, failure mechanisms and friction force were investigated for different indenter shapes. Hand lay-up method has been used to manufacture composite targets with 18 layers of 2D woven glass fibers of 45% fiber volume fraction. The epoxy system is made of epon 828 resin with jeffamine D400 as the curing agent. The results show that the load displacement curve is divided to five areas. Some of these areas may have higher or lower magnitude, depending on indenter nose shape. The highest contact force is exhibited by unsharpened indenter. The lowest contact force and so the best performance is seen in ogival (CRH=2.5) indenter. Comparing absorbed energies shows that for an identical dent depth, the amount of absorbed energy is major for unsharpened indenters. The 37˚ conical indenter needs the highest energy for perforation, which is 2.6 times more than blunt indenter’s.

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In this paper, an analytical model has been developed for modeling high velocity impact on ceramic/nanocomposite targets. In this model, penetration resistance of ceramic is determined based on cavity expansion analysis and variables during perforation of projectile onto ceramic are considered. Also the force of ceramic-composite interface is modified. Ballistic performance of the ceramic/composite target is investigated with adding and dispersing of nano particles of zirconia (ZrO2) in the matrix of back up composite. Ballistic impact tests were performed to validate the analytical predictions. These tests were performed by firing 10 mm steel flat ended projectile onto ceramic/composite target. Front layer is alumina ceramic and composite laminates of back up made of E-glass/epoxy with and without nano-zirconia particle of 5 wt%. The effect of nano-zirconia dispersion in the matrix for different failure modes is discussed. Experimental results revealed an improvement in the ballistic performance of samples with nano-zirconia particle. The analytical predictions of ballistic limit velocity and residual velocity of projectile are found to be in good agreement with the experimental results.

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Considering broad applications of sheets, specially circular sheets in the industry and the widespread use of nanotechnology to pass from limitations of each branches of science, particularly mechanics of materials and also importance of vibration (or buckling) due to temperature changes or thermal loads, in this thesis, development of relative relations of circular single layer nanographene sheets’ vibrations due to temperature changes, were studied. Nonlocal thin plate theory of Eringen is employed to investigate effects of thermal environment on the behavior of circular single-layer graphene sheet freely vibration containing a circular perforation of arbitrary size and location. In order to analytically solve the equation of motion, the separation of variables method in conjunction with the translational addition theorem for Bessel functions is used. The results of changing various geometric and physical parameters and different kinds of restrains and boundary conditions on the natural frequency of a single layer circular graphene sheet were examined and discussed. In some cases, thermal buckling phenomenon was observed.

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Mechanical injuries in to the tissue of ready-to-eat fresh-cut ​​lettuce usually cause stresses which leads to increasing respiration rate. The shelf life of this product depends on the processing and packaging steps, the type of packaging film and the environmental conditions during storage. The purpose of this study was to evaluate the effect of storage time, storage temperature, immersion pretreatment and the number of perforations of packing film on the crispness coefficient and sensory evaluation (total acceptance) of fresh-cut packaged lettuce stored under modified atmospheric condition and also development of shelf-life kinetic model of this product. For this purpose, 250 g of the lettuce strips were washed and immersed into two different concentrations (1.5/0.5 and 1.5/0.1) of CaL/Cys, then packed and sealed in low-density polyethylene packaging films (LDPE, 42 μm thickness) with no perforation (N-MAP: 21% O2, 0% CO2) and 20 and 40 perforations per m², respectively. The study was carried out at two temperatures of 5 and 10 °C, for a storage time of 12 days. Indices of quality maintenance, i.e., crispness coefficient and sensory evaluation (total acceptance) were investigated. Furthermore, a mathematical model was developed to describe the kinetics of changes in crispness coefficient and sensory evaluation. Then, the shelf life of the samples was predicted based on sensory evaluation. The results showed that changes in selected targeted parameters could be best described by a zero-order. The prediction of shelf life of fresh-cut lettuce samples showed that samples treated with 1.5%CaL/0.1% Cys packed in 20-PM-MAP, stored at 5°C and 10 °C and samples treated with 1.5%CaL/0.5% Cys packed in 20-PM-MAP stored at 5°C, had a shelf life of 12.62, 12.59 and 11.59 days (with no significant difference at p < 0.05), respectively.
 

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