حسین خدارحمی

Multi-compartment structures are a sort of multi-layer armor widely used to protect naval equipment, due to their high resistance to underwater blasts, simple-tech/low-cost production, and repairability. The present article numerically investigates the contact UNDEX damages of conceptual-construction structures consisting of three metal layers and two mid-compartments. For this purpose, a set of experiments was designed and executed using AUTODYN. The numerical modeling process and outputs were validated with the results of an experimental test. The results showed that all the structures with the same mediums in compartments suffered rupture in their last layer. Structures with air-water mediums did not experience tearing in their last layer if had a St37 midplate. Benefiting from a St37 front plate led to a significant energy absorption increase of 62%. Also, all structures with water-air mediums had excellent resistance performances and presented a meaningful reduction of 49% in the last layer deflection providing a St37 midplate. Furthermore, the comparison of structural performances showed that the structure consisting of Al2024-T3, St37, and St37 layers respectively, and water-air mediums brings forward the most competitive altogether values for the minimum last layer's central deflection of 11.6 mm and maximum energy absorption per areal mass equal to 84.6 J.m2/kg.

In the present research, the experimental investigation of the female die forming of metallic plates with and without the use of venting hole in the central part of the plate has been performed. In order to apply the load to the specimen, repeated underwater explosive loading was used, so that in the 1st loading, the mass of the explosive charge was 4 gr and in the subsequent loadings, 8 grs were used. The results of the research showed that in the process of female die forming with venting hole, a protrusion or discontinuity is created in the central part, which leads to an increase of 28, 73 and 90% of the maximum thinning along the radial length of the plate in the 2nd, 3rd and 4th loadings in comparison to the 1st loading. Also, for the thinning in the center in the same situation, an increase of 27% and a decrease of 8% and 6% were observed, which was an unusual behavior caused by the presence of the venting hole. In this case, the filling rate of the specimen was 24%. This is despite the fact that in the process of female die forming without venting hole, the filling rate, maximum thinning and thinning in the center increased by 30%, increased by 6% and decreased by 10%, respectively. One of the important points is that the maximum amount of thinning in the primary bending region is lower compared to the center of the plate when using female die forming without venting hole, which is the opposite result obtained in the die forming with venting hole, so that in that case, the maximum thinning in the center of the area was higher than the region with initial bending. Therefore, it is very efficient to use the idea of female die forming without venting hole for sheet metal forming under repeated underwater explosive loading.

In this article, a new type of explosion energy absorbing sandwich panel has been introduced and studied experimentally. Its core is a combination of polyurethane foam and scoria mineral pumice with two different types of granulation. To study the behavior of this panels, 3 different types of tests have been performed. In the first series of tests, the matrix used in the core composition is selected. The selection criterion is the strength performance and energy absorption by the core of the sandwich panel. In the core matrix selection test, the performance of the core composed of polyurethane foam and mineral pumice is investigated under the load caused by free explosion. In the second series of experimental tests, material properties including stress-strain diagram and energy absorption efficiency of core, by pressure test and its results are presented. In the third series of tests, the mechanical behavior of the sheet and the maximum deflection of the back face have been evaluated. The composition of the new core has been studied, investigated and introduced, and it was approved according to the quality of energy absorption, lightness, strength, cost of preparation and easy production of the core with optimal energy absorption efficiency.

In this paper, using analytical, numerical and experimental methods, the range of ballistic limit thickness and fragmentation behind the metal target for the PELE projectile has been investigated. PELE projectile consists of two parts: hard shell with high density and soft core with low density. The soft core is compressed inside the hard shell and upon impact, the hard shell penetrates the target. There are limited models to analyze the penetration and fragmentation of this projectile, and few researchers have addressed this issue. Considering the effect of target characteristics (thickness and material) on the rate of fragmentation and the failure to consider these parameters in the models presented so far, establishing a relationship between the main parameters (target, impact velocity and projectile), using two shock theories and the dynamic expansion of the spherical cavity and their combination with the cracking model have been investigated in this article. In parallel, the experimental tests of the impact of the PELE projectile on the metal target (gas gun - extraction of ballistic limit thickness) and also 3D simulation of finite elements (Auto Dyna software - extraction of shrapnel number) have been carried out in the impact velocity range of 312-780 m/s. According to the appropriate results of the shock wave method compared to the results of the dynamic expansion of the spherical cavity (smaller difference compared to the experimental and simulation results), the final ballistic performance range (the number of shrapnel according to the target thickness and impact velocity) of this method for the velocity range impact rates of 100 to 1000 meters per second were extracted.

Sandwich structures with cores made of porous materials such as various types of metal and polymer foams are effective in absorbing the energy of explosive loads and due to their lightness can be used well in various industries. In this paper, deformation and energy absorption of sandwich panels with aluminum and steel face sheet and cores made of polyurethane foam, which is filled with two types of mineral pumice of different sizes (so-called chickpea size and almond size), under experimental free explosion loading. And numerical simulation has been studied with the help of Abaqus finite element software. In this study, the mechanical behavior of the core was first studied by making samples of polyurethane foam and mineral pumice under compression testing and the results were used in software behavioral models. After comparing the numerical results with the results of experimental test, and validating the results of numerical methods, numerical parametric studies were performed and the effect of pumice type, core thickness, face sheet material and thickness on the rate of deformation and energy absorption of sandwich panel components have been studied. The results show that the panel with a thicker back face sheet has a better performance in absorbing explosion energy. Also, as the strength or thickness of the face sheets increases, the role of the core in energy absorption decreases.

Circular plates are simple structures that deform under the action of blast energy. The plastic deformation of the plates is the most important factor of energy absorption in circular sheets. In these structures, which have symmetrical geometry and loading, the greatest amount of deformation occurs in the center of the sheet. Structures such as plates in military applications and even in civilian applications may be repeatedly exposed to explosive loads. In this paper, a new analytical method based on the balance of absorbed plastic work and initial kinetic energy, the behavior of the plate under free and uniformly repeated blast loading is investigated and compared with the experimental method. In this method, it is theoretically possible to investigate the response of the structure under any number of free explosion loads. Each plate is subjected to free blast loading in two consecutive stages. Loading at each stage of the explosion is the same and with the same charge mass and explosion distance. The results of the analytical solution are compared with the experimental results and there is a good agreement between the results.

Fiber Metal Laminates (FML’s) are being used in many applications ranging from aircraft, submarines and ships to pressure vessels and automotive parts. In this study, the theoretical and numerical analysis of fiber metal laminates (FML’s) subjected to time-dependent uniform pressure load have been investigated. For this purpose, the plate is modeled based on the Reddy’s higher order shear deformation plate theory and the effects of the von Kármán geometric nonlinearity are included in the derivation of the motion equations. The FML is assumed to rest on the Pasternak foundation and simply supported boundary conditions are considered for all edges of the plate. Then, Nonlinear Partial differential Equations (PDEs) of motion are separated by using of the Galerkin method and finally solved using the Runge Kutta method. The results of conducted theoretical analyses compared with the presented results in the literature and good agreement is found. Also, in order to investigate the effective parameters, the effect of aspect ratio, Pasternak foundation and type of pressure pulses on the dynamic response of the plate have been examined. According to the obtained results, by reducing the positive phase time of loading and increasing the waveform parameter, the effect of the negative phase of loading is amplified and leads to an increase in dimensionless displacement in the center of the plate. Also, it was realized that the linear stiffness parameter in comparison with the shear layer parameter has less effect on the dynamic response.

Resistance to explosive loads is one of the most important aspects of the design of critical structures due to the wide range of threats. With the widespread use of concrete in the development of explosion-resistant structures, there is much research on the response of concrete components against dynamic loads such as explosion and impact in the technical literature. This paper experimentally and numerically investigates the simultaneous effect of increasing the density of concrete and reinforcing it with hooked-end steel fibers against blast caused by the high explosive pentolite mixture (C-4). In this regard, a total of 150 cubic and cylindrical concrete specimens containing steel fibers in different volume fractions (0, 0.5, and 1%) were made to evaluate the physical and mechanical properties of normal-weight (NW) and heavyweight (HW) concrete. After examining the NW and HW concrete mix designs in the laboratory, blast field tests were conducted by directly attaching the explosive on a set of six concrete blocks with dimensions of 50×50×10 cm fabricated with optimal mix designs of both concrete types. Subsequently, numerical modeling of the concrete behavior against blast load was performed using LS-DYNA finite element software to validate the examined parameters, and then the modeling output was compared with laboratory and field results. The results obtained from the finite element modeling indicate an acceptable agreement with field observations and laboratory results

In the current research study, the large plastic deformations of similar and dissimilar single- and multi-layered metallic plates with the same areal density under repeated uniform impulsive loading have been investigated. The ballistic pendulum along a 200 mm length standoff blast tube was used to apply the blast load on the specimen. Forty testing specimen in three different layering configurations including single-, double, and triple-layered, and eight different arrangements were considered. For the same loading and experimental condition, 10 g plastic explosive was utilized and the dynamic response of structures was studied up to five consecutive loads. The experimental results indicated large plastic global deformation with thinning happening at the clamped boundary and also tearing for some experiments. The results also represented that the maximum permanent deflections of plates were increased by the number of blast loads and the progressive deflection of the plates at the center was decreased exponentially with increasing the number of blasts. Placing a thick front layer a thin back layer gives better blast performance to the tested specimen against repeated impulsive loading. The obtained result for multi-layered configurations with the same material is completely consistent with those structures made of dissimilar layers.

Sandwich panels are common structures for absorbing explosion energy and used as an explosion shield. In this paper, the performance and dynamic response of a new type of metal and circular sandwich sheet structures as blast energy absorbers with radial tube cores under blast load are investigated. Analytical and experimental methods have been used to evaluate the dynamic response of the sandwich panel structure. In the analytical method, the response of the structure is divided into three separate and consecutive time stages. The first stage includes the interaction of structure and fluid, the second stage is the compression and crushing of the core and the third stage is the dynamic response and bending of the whole structure of the sandwich panel. Using the basic laws of mechanics, such as the laws of mass and momentum conservation, the analytical response of the deformation and the governing equations have been formulated, and a closed form equation for the maximum deflection has been obtained. The experiments were performed by making sandwich panels under the blast load and by free blasting method in order to evaluate and validate the analytical results. The results are compared and there is good agreement between the results in analytical and experimental methods. The average difference between the results of analytical and experimental methods is 18%.

The sandwich panels are important structures for absorbing the explosion energy. Crushing and plastic deformation of the core along with the plastic bending of the faces are the main factors in absorbing the explosion energy in these structures. Structural components undergo significant permanent deformation after the explosion and its related energy absorption. In circular sandwich panels with symmetrical geometry and loading, the greatest amount of deformation occurs in the center of the back face. In this paper, the energy absorption of the structure and the deformation of circular metal sandwich panels with tubular core under explosion load have been studied analytically and experimentally. The tubes are arranged radially and symmetrically in the core constructions, which is a new configuration for the energy-absorbing sandwich panels in the literature. Experiments have been performed by making sandwich panels under free blast load in order to evaluate and validate the analytical results. The analytical solution is performed using the energy method by balancing the kinetic energy and the plastic work which is done by the different components of the sandwich panels. Maximum deflection, the amount of core crushing and the amount of energy absorbed by the whole structure and different parts of the structure are studied for different cases. There is a good agreement between analytical and experimental results.

Sandwich panels are common structures for absorbing explosion energy and used as an explosion shield. In this paper, the performance and dynamic response of a new type of metal and circular sandwich sheet structures as blast energy absorbers with radial tube cores under blast load are investigated. Analytical, experimental and numerical methods have been used to evaluate the dynamic response of the sandwich panel structure. In the analytical method, the response of the structure is divided into three separate and consecutive time stages. The first stage includes the interaction of structure and fluid, the second stage is the compression and crushing of the core and the third stage is the dynamic response and bending of the whole structure of the sandwich panel. Using the basic laws of mechanics, such as the laws of mass and momentum conservation, the analytical response of the deformation and the governing equations have been formulated, and a closed equation for the rise of the structure and its maximum value has been obtained. The experiment was performed by making a sandwich panel under the blast load and by free blasting method in order to evaluate and validate the analytical and numerical results. Numerical solution is performed in ABAQUS finite element software by generating pressure function by CONWEP method. The results are compared and there is good agreement between the results in different ways.

Sandwich panels which can be used as an explosion shield are important structures for absorbing explosion energy. Crushing and plastic deformation of the core with the plastic bending of the faces are the main factors in absorbing the explosion energy in this sandwich panel. Structural components undergo permanent deformation after explosion and energy absorption. In this paper, the energy absorption of the structure and the deformation of circular metal sandwich panels with tubular core under explosion load have been investigated analytically, numerically and experimentally. The tubes are arranged radially and symmetrically in the core constructions. The experiment have been performed by making sandwich panels under free blast load in order to evaluate and validate analytical and numerical results. The analytical solution is performed using the energy method by balancing the kinetic energy and the plastic work which is done by the different components of the sandwich panels. Numerical solution is performed in ABAQUS finite element software and the pressure function is generated by CONWEP method. The amount of energy absorbed by the structure and different parts of it is obtained. There is good agreement between the results in different ways.

In the current research, the plastic behavior of circular plates under dynamic loading was investigated. In the experimental section, a ballistic pendulum apparatus and aluminum alloy plates were used. To investigate the deformation profile and the failure pattern of the specimens, dynamic loads were applied in a wide range from 6.49 to 24.69 Ns. To test the behavior under successive loading, each experiment was continued up to 5 loadings. Experimental observations indicate a large plastic deformation of the structure along with the thinning of the test specimens at fully clamped boundaries and the rupture of some in the same area. The results showed that with increasing the number of explosions and the charge mass, the maximum deflection increases, but the progressive deflection decreases exponentially. In the modeling section, some closed-form relations were proposed using the dimensional analysis approach to predict the maximum permanent deflection plates. In these relations, the effect of various parameters such as the plate geometry, inertia of the applied load, and strain rate sensitivity of material was considered. Comparing the results of the model with the experimental results showed that there is a very good agreement between the experimental results and the predictions of the model.

In this paper, a series of experiments were conducted aluminum plates to investigate the large plastic deformation of single, double, and triple-layered plates under repeated uniform loading up to five times. In order to perform experiments, plates were mounted onto a ballistic pendulum. Generally, the tested plates represented large plastic deformation of dome-like shape along with thinning or tearing occurring at the boundary due to the uniform impulsive loading. The experimental results showed that the maximum permanent delfection of single- and multi-layered plates increases by the increase of mass charge and number of blasts. Furthermore, the progressive deflection was decreased exponentially because the test plate material undergoes work hardening after each blast load. The results also indicated that triple-layered plates made of similar materials may have a better blast performance compared to double-layered at the first low-impulse blast but their resistance decreases as the number of blast increases. However, this trend was not observed for high-impulse blasts and multi-layered plates have a different behavior.

Sandwich panels, due to high strength to weight ratio and energy absorption properties, are widely used in various industries including aerospace industries, marine and automotive industries. In this article several aluminum sandwich panels with polyurethane foam core with different densities were designed and tested using a shock tube facility. Some of blast tests were defined in order to determine the effects of foam density on transverse deflection of back face-sheet and energy absorption of sandwich structures. Also using the results of compression test performed on different foams, numerical simulation using Autodyn software were performed. There was a good agreement between experimental investigation and numerical results. Using experimental testes and parametric studies, it is shown that increasing foam density can lead to reducing the transverse deflection of back face-sheet of the sandwich panel, but the energy absorption of the panel also decreases. Also increasing the density of the foam, in addition to reducing the shape of the back face of the panel, lead to more uniform profile. On the other hand, the results show that by increasing the amount of explosives, the effect of increasing the core density on the amount of transverse posterior deformation grows. It can be concluded that if the sandwich panel is the main structure, it is advisable to use high-density foam, but if the panel is to be installed as a damping structure on another structure, a lower density foam should be used to reduce the pressure transferred to the back face of the panel.

In this paper, the large inelastic deformation and failure mechanism of single and multi-layered circular plates under repeated uniform impulsive loading were studied. The ballistic pendulum was used to conduct a series of experiments (67 experiments) on aluminum alloy plates with different structural configurations. Three different layering configurations including single, double, and triple-layered plates made of the same material were considered and tested for the range of charge masses from 1.5g to 12.5g up to five times for repeated loading. The experimental results indicated large plastic global deformation with thinning happening at the clamped boundary and also tearing for some experiments. The results also represented that the maximum permanent deflections of plates were increased by the increase of the charge mass and the number of blast loads. On the other hand, the progressive deflection of the plates at the center was decreased exponentially with increasing the number of blasts. Furthermore, in the numerical modeling section, the Group Method of Data Handling (GMDH) neural network was used to present a mathematical model based on dimensionless numbers to predict the maximum permanent deflection of single and multi-layered circular plates under repeated impulsive loading. In order to increase the prediction capability of the proposed neural network for this process, the experimental data were divided into two training and prediction sets. Good agreement between the proposed model and the corresponding experimental results is obtained and all and 77% of data points are within the <10% error range for single and multi-layered plates, respectively.

One of the intrinsic properties of foams, porous materials, and granular materials, are their ability to mitigate the blast wave. The current research study investigates the capability of granular materials in the mitigation of blast wave both experimentally and numerically. In the experimental section, the explosion shock tube facility was employed to conduct a series of experiments. Sawdust and mineral pumice were used as granular materials in the sandwich panel core and their mechanical properties were obtained by performing quasi-static compression tests. Moreover, the commercial hydrocode AUTODYN was used to numerically simulate the process while the smooth particle hydrodynamics method (SPH) was employed to model granular materials. The obtained results indicate that mineral pumice leads to more mitigation of the blast wave compared to sawdust. Besides, using a low thick core made of granular materials is not considerably effective and using a high thick core leads to more mitigation of the blast wave.

Fiber metal laminates (FMLs) are hybrid structures composed of composite lightweight layers and aluminum layers that have high impact resistance and energy absorption along with low weight. In this study, by using of governing equation of non-linear behavior of FMLs, the effect of various parameter such as temperature and dynamic loading have been investigated. For this purpose, the geometric nonlinearity effects are taken into account with the von Kármán large deflection theory and the governing equations of motion for the plate are derived by the use of the virtual work principle. The simply supported boundary conditions are considered for all edges of the plate. Then, Nonlinear Partial differential Equations (PDEs) of motion by using of the Galerkin method are transformed to a single nonlinear Ordinary Differential Equation (ODE), which is solved analytically by the multiple time scales method, and an analytical relation is found for the nonlinear frequency of these plates. The results of conducted theoretical analyses compared with the presented results in the literature and good agreement is found. By using the validated theoretical model, the influences of changes in temperature change, elastic foundation and peak pressure values are investigated. The results indicated that increasing the peak pressure values would lead to an increase in deformation and a decrease in the frequency ratio of the system. The results also show that FMLs would be a good choice for structures under dynamic loads.

Sandwich panels, due to high strength to weight ratio and energy absorption properties, are widely used in various fields including aerospace, marine and automotive industries. This study has explored the blast resistance of sandwich structures with aluminum face-sheets and variable density polyurethane foam core (graded). In this article, several aluminum sandwich panels with polyurethane foam core having different densities is designed and have been tested. Using a blast shock tube facility, the effects of graded changes of core foam density and foam layers sequence with different densities on transverse deflection of back face-sheet are studied. Also using the compression test results performed on the foam, numerical simulations are performed using Autodyn software. The results show that there is a good agreement between numerical and experimental results with a maximum error of 13%. Experimental and numerical investigations indicate that deformation of back face-sheet of the sandwich panel with graded foam core, when the foam layer with higher density is placed in the blast side, is less than a single layer sandwich panel and equivalent mass. So it is better to use it as the main structure. Also, in case the foam layer with less density is placed in the blast side, the energy absorption of the panel increases and so the use of this panel is suggested as a protective structure.

Nowadays, due to the terrorist and hostile threats against the country and the urgent need to consider the principles of passive defense, it is essential to study some phenomena and subjects such as blast testing, blast effects on various targets, shock wave propagation in objects, evaluation of safety and  manufacturing of high quality safe equipment and other applications. One of these issues is the explosion test, the results of which,  can be obtained by having a safe chamber. To have such a place, it is necessary to design an explosive loading resistant chamber. For the design of the explosion test chamber, it is necessary to obtain the maximum shock pressure due to the internal blast on faces of the chamber structure. In this research, an internal blast in a chamber without opening with dimensions of 3 × 3 × 5 m is simulated with the help of AUTODYN software. But in that simulation, the obtained pressure is the pressure as exerted on a point, and the average pressure exerted on a given side of the chamber as a whole is not obtained. To solve this problem, the diagrams of pressure - time of gauges on a specified face of the chamber is made equivalent to a uniform pressure graph, with the help of the shape function of the configuration of gauges. Then, this graph is linearized with the proposed method, and is compared with the obtained graphs based on the UFC 3-340-02 regulation. The results of this study show that the maximum shock pressure obtained on the basis of simulation is larger than the results of the UFC 3-340-02 regulation in the internal blast. Also, the obtained equivalent uniform pressure graph based on the shape function is correct and can be used to design the chamber.

Research in field of explosions effects on hybrid and composite structures is expanding. One of the loads that defense structures needs to withstand and not lose their performance is the explosion wave. In this thesis, using hand layup glass / epoxy composite plate, fiber-metal laminates optimized for the best resistance to explosive load were made. Then, mechanical properties were obtained using standard tensile tests for composites. The explosion test was carried out using a Shock tube machine. Finally, the results of the empirical test were compared with the numerical simulation of these plates by the finite element software. It was found that experimental and numerical results are in good agreement. At the end, the results of the experimental test and finite element simulation were compared and it was observed that a good match between the results was observed. The results of the experiments have shown that these plates do not even in loading less than 10 grams of C4 (equivalent to a pressure of 28 MPa) inside the shuck tube, and they are delaminated and in Loading 20 grams ruptured. In all experiments, it can be seen that the back aluminum plate, due to the reflection of the compressive wave that converts to the tensile wave, is removed from the panel and deforms the plastic And makes the composite less damaged.

Sandwich panels, due to high strength to weight ratio and energy absorption properties, are widely used in various industries including aerospace industries, marine and automotive industries. This study explored the strength and performance of panels composed of low-density polyurethane foam core sandwiched between two aluminum skins. In this article several aluminum sandwich panels with polyurethane foam core having different thickness were designed and tested using a shock tube facility. Some blast test were performed in order to determine the effects of foam thickness on displacement of back face-sheet and energy absorption of sandwich structures. Also using the compression test results performed on the foam, numerical simulation using Autodyn software were performed. There was a good agreement between experimental investigation and numerical results. Using experimental investigation and parametric studies, it is shown that the amount of displacement of back face-sheet of sandwich structures is decreased and energy absorption is increased as foam and back face-sheet thickness is increased.

Performance improvement and increasing of penetration of Explosively Formed Penetrators (EFP) is one of the most considerable issues in the field of defense researches. There are various solutions to achieve this purpose, including innovative designs in the field of charge, linear, skin, explosive train, etc. In this paper, the effect of multi-point initiation on the formation and penetration of EFPs is investigated using by finite element method and Ls-dyna code. Furthermore, the simulated model was developed and exprimental tests were performed to measure the penetration depth and hole diameter caused by the impact of EFP to the target of steel plate. Comparison of simulations and experimental results shows good agreement. Also measurements shows the 40% increase of penetration depth in case of multi-point initiation in comparison with single initiation, which can be good result in performance improvement of EFP warhead.

Carbon nanotube-reinforced polymer nanocomposites have attracted great attention from research centers, due to their enhanced mechanical properties, and many studies have been performed for development of such materials. Many experimental and theoretical studies have investigated the effect of different parameters on the properties of these materials. However, there are some limitations associated with experimental methods, such as fabrication problems and high levels of costs. Hence, molecular simulations are growingly applied for study of properties and behavior of polymer/CNT nanocomposites. In this study, the molecular dynamics method was used to calculate the mechanical properties of CNT-reinforced epoxy nanocomposites. Since epoxy is a two-component thermoset polymer, the molecular dynamics method was utilized to create cross links between the monomers. Thereafter, cross-linked epoxy polymer and carbon nanotubes were used to construct the nanocomposite with containing 1-5 wt.% of CNTs. Finally, elastic constants of nanocomposite including Young’s and shear moduli were calculated using the constant-strain method. The results of simulations revealed that the mechanical properties of CNT-reinforced epoxy polymer were improved in comparison to those of pure polymer.