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One of the main issues in designing sandwich structures is selecting an appropriate core with optimal dimensions for the application. This study investigated the impact response of auxetic cores with re-entrant structures when subjected to low-velocity impact (LVI) loading. To this end, an analytical model was developed using high-order nonlinear theory, and the corresponding equations were extracted by applying the Ritz method and subsequently solved using the Runge–Kutta method. The analytical model was developed by replacing the porous auxetic core with an equivalent orthotropic solid layer. The numerical simulation demonstrated that the substituted layer exhibited behaviour similar to that of the original auxetic core. The validation of the analytical and numerical models was performed by fabricating several samples using fused deposition modelling (FDM); then, LVI tests were performed on the samples using a drop-weight apparatus. The comparison of the experimental and analytical results indicated that the proposed nonlinear model accurately predicted the behaviour of the auxetic core when subjected to LVI loading. The effects of the different geometric features – including re-entrant angle, strut thickness, core thickness, and impactor energy – were also examined in the parametric study in an effort to gain an enhanced understanding of the behaviour of the auxetic core. [ABSTRACT FROM AUTHOR]

In this study, thermoelastic stress analysis in composite metal cylindrical vessel is investigated with the help of two theories, first-order shear deformation theory (FSDT) and layerwise theory, using differential quadrature method (DQM). Hamilton principle and FSDT are employed for derivation of the principle differential equations. DQM and Newmark's methods are used for stress analysis of a composite metal cylindrical vessel subjected to internal dynamic pressure and thermal load achieved from experimental test. In order to more accurately estimate the thickness effect, the layerwise theory is applied to approximate the temperature field. In this solution method, DQM is used to discretize the resulted governing equations while Newmark's method is implemented to solve the problem in the time domain. It was observed that the fast convergence rate of the DQM method and its accuracy is confirmed by comparing the results with those obtained using ABAQUS software and also with the exact solutions. Furthermore, it was achieved significantly less computational efforts of the proposed approaches with respect to the finite element method. [ABSTRACT FROM AUTHOR]

Experimental studies are presented on the high velocity impact behavior of nanomaterial dispersed resin viz laminates made using E-glass fabric with epoxy resin. The nanomaterials used are silica nanoparticles and carboxyl functionalized multi-walled carbon nanotube (COOH-MWCNT) for polymer matrix composites. The composites' ballistic limit (Vbl) and impact energy absorbed (Eab) were determined by subjecting the material to impact loading of 85, 100, and 112 m/s by conical nose projectile. It was found that the high velocity impact response of epoxy composites improved when a nanomaterial was used as reinforcement. COOH-MWCNTs reinforced composites exhibited better energy absorption than silica nanoparticles composites. Moreover, the damage pattern for different types of materials studied is presented. It is observed that the damage size on the target around the point of impact decreases on addition of nanoparticles especially COOH-MWCNTs. Quantitative data are presented for high velocity impact behavior of the seven types of specimens studied. [ABSTRACT FROM AUTHOR]

Tandem projectile is one approach of the development of anti-structure concrete projectile, which is made of the forward shape charge and the follow-through projectile. In this paper, in order to study the performance of antistructure tandem projectiles, experimental and numerical analyses of the penetration process were performed for forward shape charge with conical copper liner hitting into the concrete targets. Having a well agreement, numerical and empirical results indicate that a forward shape charge with a conical copper liner will make appropriate tunnel diameter through the concrete targets. Following the validation of the applied numerical method, numerical simulations were conducted for the penetration of the follow-through projectile, by three different concrete targets: undamaged concrete, pre-drilled concrete, and damaged pre-drilled concrete. The parametric results show that the damaged predrilled concrete affects the penetration depth and residual velocity of the follow through projectile significantly. [ABSTRACT FROM AUTHOR]

In this research, effect of adding silica nano-particles on the mode II interlaminar fracture toughness of epoxy matrix composites reinforced with glass fibers was experimentally studied. Hand lay-up method has been used to manufacture nanocomposites withl8 layers of 2D woven glass fibers with 40% fiber volume fraction. The nano-epoxy resin system is made of diglycidyl ether of bisphenol A (epon 828) resin with jeffamine D400 as the curing agent. Nanosilica particles are dispersed with 0, 0.5, 1 and 3 wt.% (of epoxy resin) to study the effect of nanosilica content on fracture toughness. Also, a series of nanocomposites with 1 wt.% nanosilica content and containing 55 vol.% glass fibers were fabricated to investigate the effect of fiber volume fraction on results. End notch flexure (ENF) test was adopted for the measurement of mode II interlaminar fracture toughness. The results show that high loading of nanosilica has no significant effect on the interlaminar fracture toughness of nanocomposites while the addition of 0.5 wt% nanosilica enhanced the interlaminar fracture toughness about 36% compared to the neat composites. Decreasing fiber volume fraction improved interlaminar fracture energy. [ABSTRACT FROM AUTHOR]

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 requires the highest energy for perforation, which is 2.6 times more than blunt indenter's. [ABSTRACT FROM AUTHOR]

In this study, the numerical and experimental study of energy absorption and deformation of thinwalled tubes with square geometry and circular cross under impact loading is studied. The purpose of this study was to investigate the effect of geometry on the energy absorption of aluminum tubes and the effect of foam filled tubes to absorb more energy under transverse impact. In the experimental part, hollow aluminum tubes filled with solid polyurethane foam were prepared and then the quasi-static tests with static and dynamic loading rates were performed by drop hammer method on samples with different energy and the acceleration-time diagrams in each test were obtained. In the last part of this study simulation of the phenomenon of transverse impact on thin sections was carried out with the ABAQUS software. The discussion and conclusions of this study and the results of experimental tests carried out by the author of the thesis have been compared with the results of numerical analysis and show a good agreement (difference below twenty percent). Finally, it was concluded that with regard to material of structure, at high energies square tubes have 50 percent specific energy absorbed higher than circular tubes and filled tubes have 20 percent specific energy absorbed higher than hollow ones. Also, transverse displacement of the hollow tube and circular tube is always higher than the filled tube and square tube. [ABSTRACT FROM AUTHOR]

In this paper, the tensile properties of 2D woven glass epoxy composite reinforced by two different nanoparticles have been investigated and compared. Hand lay-up method has been used to manufacture nanocomposites with12 layers of 2D woven glass fibers with 40% fiber volume fraction. The nano-epoxy resin system is made of diglycidyl ether of bisphenol A (epon 828) resin with jeffamine D400 as the curing agent. The composites were reinforced by adding organically modified montmorillonite nanoclay (Closite 30B) and nanosilica (SiO2) particles. The nanoclay particles were dispersed into the epoxy system in a 0%, 3%, 5%, 7% and 10% ratio in weight with respect to the matrix, while the spherical nanosilica particles were dispersed into the epoxy system in a 0%, 0.5, 1% and 3% ratio in weight with respect to the matrix. The results show that low loading of nanoclay decreases the mechanical properties of nanocomposite, while significant improvements of nanocomposite mechanical properties are shown in low loading of nanosilica. Tensile strength and toughness of nanocomposite increase by 7% and 10% after adding 5 wt.% nanoclay. Loading of 0.5 wt.% nanosilica cause 10% and 27% improvement in tensile strength and toughness of nanocomposite. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Axial compression behavior of foam materials can be explained by two ideal deformation scenarios: discrete crush band process and progressive collapse. In this paper, a perfectly new model for strength assessment and quantitative/qualitative description of one-dimensional progressive collapse of aluminum foams under impulsive loadings is presented and its capability to split this way of crushing into two distinct regimes of shock wave and elastic- plastic wave propagation is highlighted. Then, using conservation relations and the new introduced model, the analytical solution of dynamic deformation of aluminum foams in the two mentioned regimes is developed. Regime 2 considers the case when the crushing front velocity is lower than the linear sound velocity of the foam; but remains higher than the effective sound velocity for a perturbation in which the amplitude lies in the so-called "plateau region" of the static stress-strain diagram. The physical difference between this regime and the first one entails not only the creation a shock front associated with the collapsing foam, but also an acoustic precursor in the case of second regime. Finite element simulation is also performed to validate the analytical procedure. The numerical prediction is found to be in very good agreement with the analytical results. [ABSTRACT FROM AUTHOR]

In this investigation, kinked crack path of friction stir Cu-Al7075-T6 alloy welded joints in fourpoint bending test conditions has been studied as well as the fatigue lifetime of the welded joints, numerically and experimentally. To do so, four-point bending and fatigue tests of welded specimens have been carried out and the experimental fatigue test data and the kinked crack angles in bending tests have been extracted. Maximum Tangential Stress (MTS) and (KII)min criteria have been used for estimating the kinked crack angles, and Paris law has been applied to predict fatigue crack propagation life of the welded specimens. Functionally graded materials concept has been employed for determining mechanical properties of different regions of welded joints. To do so, the mechanical properties of the weld region such as Young's Modulus and Poisson's ratio have been considered to be linear functions of the positions of the weld region points. It has been shown that, when the original notch is close to the material with the higher fracture toughness (Copper), the kinked crack angle becomes smaller. The results show good agreement between the experimental data and numerical estimations. [ABSTRACT FROM AUTHOR]