جستجوی مقالات مرتبط با کلیدواژه « ضربه » در نشریات گروه « مکانیک »

In this study, the damage behavior of concrete under projectile impact is investigated. Thus, Johnson Holmquist brittle damage model, which is widely used to model penetration phenomenon, is chosen as the basic model. In order to more accurately predict the behavior of the material, a modified model is proposed by considering the new relationship for the effect of rate on the strength of the basic structural model. In the first step, the concepts of the basic model are expressed. Then, the formulation of the damage model is extracted and algorithm of the model for numerical simulation to solve it are presented. In order to implement and validate the algorithm, a subroutine is written in Fortran programming language based on the finite element method to simulate the damage process. In the following, using the developed model, the impact of the projectile on reinforced concrete is investigated. The model is verified by comparing the numerical and experimental results in the literatures. Numerical simulations using the proposed model show a good compatibility between simplicity and accuracy for calculations of concrete slabs under the impact of airborne projectile.

In this article, to investigate the severity of damage to the helicopter pilot under the crash impact, the numerical simulation of the actual helicopter cockpit seat has been done. To limit the complexity of the problem, a fully vertical impact was considered as a reference scenario for the assembly consisting of a collapsible helicopter seat, an energy absorption section, and a humanoid dummy. For this purpose, the three-dimensional model of the chair in the laboratory was studied in two states with and without the addition of the energy absorption mechanism. A honeycomb structure was used in the seat cushion section. The results of the research showed that the use of UPVC damper to test the crash impact with a certain mass based on biomechanical criteria, an average 80 % decrease in serious injuries to the skull and spine has been achieved. Also, based on the results obtained in this research, although the aluminum honeycomb structure meets the criterion of damage to the head and is within the permissible range, it has rejected the criteria of damage to the spine and permissible acceleration range. However, compared to the case without honeycomb, it has performed much better and has been able to absorb the energy of the crash impact.

The satellite system is subjected to shock during launch and separation. The shock causes damage to various parts, including the electronic system. In this article, the electronic board of the satellite guidance system has been analytically examined. This board is located on 5 legs that are connected to the body. The purpose of this study is to design the legs so that the least amount of shock is introduced to the electronic board. The electronic board is assumed to be a plate. Effective parameters in shock transmission, including geometrical parameters and the leg angle with the horizon surface, have been studied. The influence of the leg angle with the horizontal surface on the stiffness of the system has been investigated using analytical relations. Finite element simulation has been used to validate the equivalent stiffness. The results of this study show that changing the base angle causes the transmissibility of acceleration to change according to the excitation frequency. Acceleration transmissibility depends on the ratio ω/ω_n . For this reason, different parameters should be selected according to the excitation frequency range. If different parameters are not selected properly, the output acceleration will be higher than the input acceleration.

In this research, the mechanical properties including bending strength and impact resistance for steel composite beams with high performance concrete are discussed. In the construction of the steel part of the specimens, the shield cross-section grade 4 is used in the tensile part of the sheet with a thickness of 5 mm and two widths of 30 and 40 mm and a length of 70 cm. It should be noted that the compression wing of some specimens is made of concrete with Different steel reinforced fibers. Self-compacting concrete with 3 percent of different steel fibers, which are 0, 0.5, and 1 percent, was used in making the concrete of the specimens. The results of the impact tests showed that the amount of energy absorption as well as the initial maximum force increased with the increase in the fibers percentage that the highest energy absorption is related to the w4hc4sf1 beam, which has increased by 0.09% compared to the specimen without concrete. Regarding the results of the bending test, the resistance value increased intermittently with the increase of fibers. The w4hc4sf1 beam had the best performance, and its strength increase compared to the same specimen without concrete was equal to 204%.

Graphene sheets are subjected to various loads and events during the manufacturing process and their functional life, such as collisions with different masses or loads, which cause deformation, displacement, and vibrations in carbon atoms and sometimes rupture of graphene sheets. Graphene sheets with different lengths and impactors in different forms of selection and data collection tools are Lamps and Molecular Dynamics software. Simulation by Lamps software, which is a very accurate method and close to laboratory conditions; Based on the time step and the deviation of the central point of the page; The effect of some parameters such as mass, number of layers of graphene sheet, different shapes including cube, sphere and cone as well as different lengths of graphene sheet on sheet tearing was investigated. Therefore, with the increase in the mass of the spherical impactor and the increase in the length of the graphene sheet, the number of broken bonds increased, while the speed of breaking the bonds decreased with the increase in the mass of the impactor, and with the increase in the length of the graphene sheet, the speed of breaking the bonds increased. Then, by examining the effect of different impactor geometries (spherical, cubic, and conical), it was observed that the surface during the spherical impactor has the most displacement of the middle atoms of the plane and the conical impactor has the highest number of broken bonds compared to the two impactors. also with the increase in the number of layers, the amount of displacement of the atoms in the middle of the plane is less and the speed required to break the bonds has increased.

Noise is unwanted sound in the ocean, since it generally, interferes with the operation of sonar or other underwater sound registration equipment. The principal sources of radiated sound from ships are (1) the propulsion system; (2) the propeller; (3) the auxiliary machinery; (4) the hydrodynamic effects and (5) the hull movements. One of the sources of artificial noise in the sea environment is the noise from the machines inside the vessels. This noise is caused by the inherent vibration of mechanical systems. The presence of small instabilities along with the forces and torques that produce the work creates vibrations that are eventually transmitted to the radiation surfaces and are therefore a source of noise. This type of noise is caused by the following 5 factors: mechanical unbalances, electromagnetic force fluctuations, impact sounds, piston sound in reciprocating machines and bearing noise. In this article, we only examine the noise generated by a floating machine and describe the frequency range as well as the noise level produced.

Composite structures under impact loading are prone to a variety of damage mechanisms such as delamination, fiber breakage, or matrix cracking. It is proven that the impact-induced damage mechanisms of composite materials are dependent on scaling (in-plane and out-of-plane) and layup configurations.  The present study has investigated the effect of scaling and layup configurations on the failure mechanisms of composite materials under low-velocity impact force using acoustic emission, C-scan and CT-scan tools. For this purpose, four samples with quasi-isotropic configurations of [45m/0m/90m/-45m]ns were manufactured, then they were loaded and acoustic signals were recorded. The three IS, PS, and SS samples were investigated based on D62624/D6264M ASTM standard test and the R sample had half of the in-plane dimension of them. The variables m and n vary according to the design plan. The obtained acoustic emission data were analyzed using sentry function, then C-Scan and CT-scan were utilized for damages’ size and location.   It was proven that scaling and layup configuration affect the type and intensity of damage mechanisms as well as mechanical behavior of the laminated composites. Furthermore, the acoustic emission method is shown as an indicator of scaling and layup configuration effects in glass/epoxy composite materials under the low-velocity impact.

This investigation for all specimens the concrete with self-contact is used. The specimen has expanded metal sheets layers. The behavior of sheets is analyzed for specimens.  For experimental method the acceleration parameter are measured with the acceleration sensor. Drop hammer test machine had been used for experimental tests. For numerical method the ABAQUS software is used. Abaqus is used for dynamic test loading and it is suitable for dynamic loading. For the specimens with two and free fixed of area are investigated on experimentally. For the specimens with thin thickness the free fixed is used. The specimens without expanded metal sheet are collapsed under impact loading. The specimens with expanded metal sheet have a strong resistance for impact loading. The displacement of the specimens without expanded metal sheet is more than of the specimens with expanded metal sheet. Expanded metal sheets are simulated in the specimens with experimentally and numerically loading.

In this paper an analytical model is presented to investigate the dynamic response of the nano-curved plate under impact loading of nanoparticles. Unlike the macroscale, long-range interatomic interactions, such as the Van der Waals (vdW) force, are considered at the nanoscales. The impact load on the nano- curved plate is considered as an interaction between the nanoparticle and the nano-plate. The vdW force between the carbon nanoparticle and silicon nano-curved plate is determined by the Lennard-Jones potential. The Love-Kirchhoff plate theory and Double Fourier series are used for determining the displacement field of the nano-plate. Also the governing equations of the nano-curved plate are derived by considering the residual surface stress, Gurtin and Murdoch relations and Hamilton's principle and are solved for a simply supported nano-curved plate by using the Rung-Kutta’s fourth order method in MATLAB. The analytical model results are validated with an analytical model that has investigated the dynamic response of the nanoparticle impact on a rectangular nano-plate. The effects of geometrical parameters such as curvature, thickness, mass and velocity are investigated. Also the surface effects of the nano-plate on the vdW force and the dynamic response of the nano-curved plate are studied. The results show that by increasing the radius of curvature, the maximum deformation at a constant curvature angle is decreased. Also, by considering the surface effect, the maximum displacement of the center of the nano-plate is reduced and the role of the surface effect on the maximum deflection of the nano-plate decreases with increasing nano-plate thickness.

Energy absorbers have various applications in vehicles, military industries, ships, trains, aerospace industries, etc. In recent years, with the development of science and technology, multiple absorbents have been invented to absorb impact energy and ensure the safety of structures against impacts. Among absorbents, foams have significant advantages such as lightweight and high energy absorption per unit mass. Synthetic foams have a higher energy absorption capacity than conventional metal foams; For this reason, in this study, the energy absorption of synthetic foams at high-speed impact is investigated numerically using Abaqus finite element software. Also, the force-displacement diagram, the amount of energy absorption per unit mass, the initial maximum force, and the samples average force are investigated. Models are low carbon steel synthetic foam, aluminum-6061, and aluminum-6063 synthetic foam. According to the results, it is observed that the amount of energy absorption, maximum strength, and the average strength of low carbon synthetic foam is higher than aluminum synthetic foams, but the amount of energy absorption per unit mass of aluminum synthetic foam-6063 is higher than others.

In this research, laboratory investigations and numerical studies are performed on flat and curved steel sheets under the influence of impact caused by the free fall of weights, with and without reinforcers of different transections. In this study, a flat sheet (infinite curvature) and a curved sheet with the curvature radius of 110 mm and a specific level of impact energy (a free fall height) are used for st.12 steel sheets of 1 mm thickness with the dimensions of 220 * 200 mm. The material and thickness of the energy absorbers are similar as the original sheet, and rectangular, cylindrical, half cylindrical, sinusoidal and triangular transection shapes, are considered. In the experimental method, the pickup acceleration is measured by the accelerometer sensor and the post-blow-out plate deformation is measured. The evaluated parameters include the amount of impact acceleration, the rate of permanent deformation, and the amount of energy absorption by the sheet. The Abaqus finite element software is used for numerical modeling. The results show that the curvature reduces the acceleration of the impact and increases the steady deformation and the energy absorption. The reinforcers in general, reduce the amount of crush length, but tape reinforcers reduce also the energy absorption of these sheets compared to plain sheets. Regarding the variety of the reinforcers, the cylindrical element performs better, because it has less impact acceleration and more energy absorption than orthogonal reinforcers, however, there is an increase in the amount of deformation of these sheets.

In this paper, the experimental, numerical, and analytical investigation ‎of a blunt rigid projectile to the steel targets is discussed, Therefore, in ‎the first part, the experimental tests are carried out to find the depth of ‎penetration and the projectiles are cylindrical in diameter 10 mm and 20 ‎mm in length with AISI 52100 and steel plates with dimensions of 120 × ‎‎100×10 are selected from AISI 1045. In the second part, the numerical ‎simulation of the impact model in the Abacus software is carried out ‎and the accuracy of the numerical model is verified with 8% error rate ‎of the numerical solution with the experimental results. In the third ‎section, presents an‏ ‏analytical model derived‏ ‏from the Chen analytical‏ ‏model and obtain the depth of projectile penetration. Finally, a ‎comparison between experimental, numerical and analytical results is ‎performed, the results obtained in three solutions (experimental, ‎Numerical and analytical) have acceptable agreement.‎

Application of porous structures whose voids are naturally or artificially contain fluids is common in several engineering fields such as plates with foams containing humidity or damping fluids, wooden plates subject to humidity, and bones. In the present paper, impact analysis of the poroelastic plates is accomplished for the first time. In this regard, the constitutive equations of the poroelastic materials are proposed based on Biot’s theory and then, the governing equations of the impact of the plate are derived based on the classical plate theory and the non-linear Hertz law. The non-linear finite element form of the governing equations is obtained based on Galerkin method and solved using a special algorithm. Newmark’s numerical time integration method is employed to consider the time-dependency of the resulting equations. Based on the written computer code, effects of various parameters are investigated. Results reveal that presence and flow of the trapped fluid can lead to behaviors that are in contradiction to those of the traditional plates. Furthermore, the contact force, indentation, central deflection of the plate and the bending moment due to the trapped fluid may increase by increasing the diffusion coefficient.

One of the main problems in the classical methods for analyzing crack is a discontinuity in materials and specific conditions at the crack tip. Peridynamic theory , which has been introduced in recent years, could be used to improve the analysis of cracked structures. In this theory, the points of a body whose displacement or displacement derivatives are discontinuous are not distinguished from other material points. In this paper, using the bond-based peridynamic theory, the crack growth in a beam with initial crack under the impact load has been investigated. The governing equation is developed and solved using Peridynamic theory and the results are validated using other investigations. Two models for the initial crack definition and two projectile shapes have been considered and in addition the effect of some peridynamic parameters on the results has been studied. The results demonstrate the ability of the peridynamic theory to model the crack growth in the studied problems.

The present study provides the theoretical and numerical comparison and study of the dynamic behavior of a four-rod and four-elastic-plastic joint model under the inertia effects in both single and double cell modes. The theoretical study was carried out by solving nonlinear equations in MATLAB with dynamic equations of motion. Numerical analysis was also carried out with ABAQUS. The objective of this study is to derive the equation of energy absorption in terms of the inertia parameter to the expanded metal tube structure under impact loading and also to study the dynamic behavior of effective parameters and collapse mechanism of the structure in both single and double cell modes. Finally, the analysis of the effect of double cell absorber was carried out for the values of effective parameters in dynamic response. The results show that the collapse of the absorber will be symmetric in two directions.

At present paper, an equivalent model with different dimensions and also with different dimensions and material in comparison with main body for strain rate sensitive structures subjected to high rate loading is presented by using the novel finite similitude method. The finite similitude method provides performing a test on the model instead of the original sample. This method is used to obtain the properties of model and to reverse the obtained results for model to main body by using the principles of nature (the law of conservation of mass, the law of conservation of momentum, the law of conservation of energy and the law of conservation of entropy) which is always true for any system. The relationships for both pure dimensional and simultaneously dimensional/material scaling of strain rate sensitive structures are presented. To evaluate the efficiency of the proposed relationships, the numerical results are obtained for impacted circular plates. It should be mentioned that the numerical results are obtained by using the finite element software LS-Dyna in which the strain rate effects are considered into account by using the Cowper-Symonds and Johnson-Cook constitutive equations. The results indicate that the scaled plate to one tenth of its original dimensions and also made of different material in comparison with original plate predicts the response characteristics of the original plate with a very good accuracy.

In this research, the effects of arbitrary boundary conditions on a laminated composite plate response subjected to large mass and low-velocity impact has been studied. When the impactor is defined large-mass, its mass ratio to the target mass is greater than 2. For this purpose, the first-order shear deformation theory was considered as the displacement field of the composite laminated plate. Then, using the analytic method based on suitable algebraic polynomials and the Galerkin function, the motion equations for several types of different boundary conditions was solved. Also in this work, the interaction between the impactor and the composite laminated plate were modeled using a two degrees-of-freedom system, consisting of springs-masses. The results indicate that the arbitrary boundary conditions are effective on the natural frequency of the composite plate. These effects are remarkable on contact forces and displacements of the composite plate for clamped to simply support and free boundary conditions. Some of parameters like arbitrary boundary conditions, mass and velocity of the impactor in a constant impact energy level and radius of impactor are important factors affecting the impact process and the design of structures.

In this study, based on the third-order shear deformation theory the equations of motion are obtained to analyses the deformation of a long and slender composite beam. The beam has initial geometric imperfection and subjected to impact load. The impact procedures are applied by rigid body with a specific speed, off-center and at a certain distance from the beam's surface. Hamilton’s principle and the von-Karman nonlinear strain-displacement relationship are used to obtain the equations of motion that they are based on displacement and in a set of coupled nonlinear partial differential equations in dynamic mode. The generalized differential quadrature Method (GDQM) is used to discretize the obtained equations and convert them into a set of ordinary differential equations. Newton-Raphson iterative scheme is employed to solve the resulting system of nonlinear algebraic equations. Then, by solving the equations of the system, the effects of initial geometric imperfection on the beam’s deflection have been studied. Also the effects of mass and the initial velocity of the impactor on the beam’s deformation are investigated. The results of this research show that an increase in the amount of the initial velocity and mass of the impactor entail an increase in the beam deformation.

In this paper, the Wave Finite Element Method (WFEM) is developed for modelling of stress wave propagation in one-dimensional problem of nonhomogeneous linear, anisotropic micropolar rod of variable cross section. For this purpose, the WFEM equations are developed based on the micropolar theory of elasticity. Two kinds of waves with fast and slow velocities are detected. For micropolar medium, an additional rotational Degree of Freedom (DOF) is considered besides the classical elasticity’s DOF. The method proposed in this paper is implemented to solve the wave propagation and impact problems in micropolar rods with different layers. The results of the proposed method are compared with some numerical and/or analytical solutions available in the literature, which indicate excellent agreements between the results.