جستجوی مقالات مرتبط با کلیدواژه "ضربه" در نشریات گروه "فنی و مهندسی"

Improving the mechanical characteristics of concrete especially its tensile, bending and impact resistance, has long been the focus of various researchers. In this research, the effect of adding different steel fibers along with polypropylene fibers in different dimensional ratios together with commonly used mineral additives to improve the impact resistance of concrete has been investigated. The considered parameters included the percentage of polypropylene and steel fibers, the ratio of length to diameter of steel fibers (L/D), the percentage of fly ash, microsilica and epoxy additives, and the age of specimen. Experiments aimed at determining the impact, tensile, bending and compressive strength of the studied concrete specimens were carried out. To determine the impact resistance, a new laboratory method was introduced and used. The results of the strength of concrete specimens at different ages of 7, 28 and 90 days show the significant effect of the use of hybrid fibers on increasing the tensile and impact resistance and somewhat weaker on the compressive and bending strength of concrete. It was also observed that the use of mineral additives has a greater effect on increasing the flexural strength of concrete than adding fibers. Comparison of specimens with various L/D ratios of metal fibers showed that fibers with lower L/D ratios result in greater improvement of the mechanical properties of concrete. Finally, the positive effect of the simultaneous use of hybrid fibers with optimal L/D and mineral additives on the mechanical characteristics and especially the impact resistance of concrete was investigated.

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.

In light of the adverse ecological outcomes of cement production, researchers are exploring alternative materials that can replace cement in construction while maintaining environmental compatibility. An alternative strategy is the use of soil base materials containing aluminum silicate with alkaline activators and industrial waste. This study aims to evaluate the strength and durability properties of geopolymer mortar relative to cement and clay-based mortars. Eight mixing designs were employed, including two for cement and clay-based mortars for comparison, and six for geopolymer mortars. The geopolymer mortars tested comprised 0%, 30%, and 40% slag, with 0.1% polypropylene fibers used in three designs. A 12 M concentration of sodium hydroxide solution and a sodium silicate to sodium hydroxide ratio of 2.5 were utilized in this study. The samples were subjected to compressive and tensile strength evaluations at 7, 28, 54, and 90-day intervals. Additionally, water absorption, capillary water absorption, and impact resistance tests were performed after 28 days. The compressive strength test conducted after 28 days revealed that geopolymer mortars that contained either 30% or 40% slag, along with polypropylene fibers, achieved 53% and 73% of the compressive strength of the cement sample, respectively. This increase represented a 3.6 to 9-fold improvement over clay-based samples. The tensile strength of the geopolymer samples was found to be 53% and 63% of the strength of the cement, but 5 and 9.5 times higher than the clay-based sample, respectively. The results showed that the use of polypropylene fibers in geopolymer samples containing 30 and 40% slag reduced water absorption, but compared to cement samples, they absorbed 95 and 72% more water after 72 hours. The coefficient of capillary absorption in all geopolymer samples was high, but the use of fibers in the sample containing 40% slag resulted in a 1% reduction in this coefficient compared to the non-fiber sample. Cracking resistance tests were conducted on the geopolymer samples, which showed that the samples containing 30 and 40% slag with fibers achieved 40 and 73% of the cracking resistance of the cement sample, respectively. The destruction resistance of the sample containing 30% slag with fibers was approximately 46% of the cement resistance, while the model containing 40% slag with fibers exhibited a 15% improvement relative to the cement sample.

Man always tries to get inspiration from nature and create new structures. One of these new structures is cellular and lattice structures, which have attracted the attention of researchers due to their low weight and appropriate compressive strength. In this article, inspired by nature and the structures in it, these structures have been investigated and modeled on creatures such as turtles, snails and sea shells. For this purpose, two models of each pattern were designed and made of Petg material using the additive method. The samples were subjected to the test of two consecutive impacts with the impact weight of 2.2 Kg and at two heights of 5 cm and 10 cm. The results showed that the highest and the lowest impact force at a height of 5cm respectively correspond to one of the samples taken from a turtle and one of the samples taken from a snail with a ratio of 325% and at a height of 10 cm, respectively, it corresponds to one of the turtle samples and snail samples, with a ratio of 200%. The highest and lowest displacement in the maximum force resulting from the impact is at a height of 5 cm, respectively, related to one sample taken from a snail and a turtle with a ratio of 252%, and at a height of 10 cm, respectively, it corresponds to one of the snail samples and turtle samples, with a ratio of 238%

Concrete is one of the common materials in strategic structures, and because of it, fabric concrete (CC), which is also called fabric reinforced concrete (TCR), is considered one of the latest technologies in the world. According to the general applications of fabric concrete, the need to investigate and evaluate it in the field of military and defense becomes clear, and for this purpose, in this research, fabric concrete targets with dimensions of 120 mm x 120 mm and thickness of 10 mm are subjected to the penetration of steel projectiles with flat, hemispherical and conical noses. have been tested and the velocity of the ballistic limit of a specific fabric concrete sample and the maximum diameter of destruction of the front and back surfaces of the sample have been measured. The failure mechanism of fabric concrete samples has emerged in the form of brittle and fragmentation type failure, which have shown a relatively good performance against the penetration of projectiles, so that the area and diameter of destruction on the front surface of the samples was very small and sometimes as much as the diameter of the projectile. On the back surface of the samples, the amount of destruction is also acceptable and with the increase of the impact speed, the amount of destruction has also increased linearly, and the ballistic limit speed for steel projectiles with flat, hemispherical and cone noses is 124, 110 and 100 m/s, respectively. It is stated that it indicates the easy penetration of projectiles with a conical nose and the difficult penetration of projectiles with a flat nose. </em>

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%.

The Study of Hot Spots as a Cause of Circumstantial Initiation in Energetic Materials in order to Increase Work and Storage Safety of these Materials is of Special Importance. Hot Spots, by Converting Different Types of Force like Mechanical Forces, Friction, Heat, and electro static discharge into Heat, Cause the Temperature to Rise Exponentially by about 700 Kelvin in 10-3 to 10-5 Seconds at that Point. If This Temperature Higher than Energy Threshold Required for the Initiation of Energetic Materials, it will Operate Combustion or Explosion Reactions at that Point, and then the Bulk of Energetic Materials may Initiate. Several different Mechanisms for the Formation of Hot Spots (such as Impact, Adiabatic Cutting, Adsorbed Gas Heating, Sparks, etc.) Are Investigated. The Type of Mechanism Involved in Creating the Hot Spot Depends on the Type of Energy Entering, the Nature and Components of the Mixture, the Physical Properties of the Material as well as the Environmental Conditions. Depending on the Conditions, one or more Mechanisms at the same Time may be Involved, and the Effect of each Mechanism is Different in Various Situations.

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.

Low compressive and tensile strength, high water absorption of the AAC leading to excessive absorption of the mortar’s water as well as low impact resistance, are the technical and executive drawbacks of using the ACC in the construction projects. To address these issues, in this paper, it has been attempted to replace cement with Nano-Graphene (NG) at ratios of 0.2, 0.4 and 0.8% and study mechanical properties of the modified NG-incorporated AAC, using the compressive and tensile strength tests (cylindrical specimens with diameter and height of 10 and 20cm, respectively), impact resistance and water absorption tests (cubic specimens with size of 15cm) as well as SEM and XRD tests. Based on the results, replacement of cement with NG, could enhance the compressive, tensile strength and impact resistance by 45, 81 and 130% compared to the control specimen. Moreover, it was observed that inclusion of the NG reduced the water absorption up to 61%. Finally, the SEM results revealed that incorporation of the NG strengthened the bonds between the compounds and the particles were downsized by 30%.

In this study, the potential of progressive collapse of a four-story steel structure due to the collision of an 8-ton truck with the ground floor columns was investigated. For this purpose, a 4-story steel structure ( with intermediate steel moment frame system and special concentric steel bracing in the x-direction and intermediate steel moment frame system in the y-direction ) was modeled and designed in ETABS software. ABAQUS finite element software was used to simulate the structure under collision scenarios. In this research, 6 scenarios of truck collision with adjacent columns of the structure have been considered by performing nonlinear dynamic analyzes. The vehicle speed is assumed to be 20 meters per second. The results showed that corner columns are more vulnerable to impact than perimeter columns. The presence of braces reduces the collapse and deformation in the desired scenario. The horizontal displacement in the impacted column in the direction of the moment frame was about 3 times that of in the direction of the dual system. this shows the positive effect of bracing system in reducing the structural responses to impact.

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.

Over recent decades, regarding the spread of unusual events such as fires, explosions, and vehicle collisions, the study of the behavior of structures subjected to abnormal loadings has been taken into account by the researchers and structural engineers. In this study, 2 and 5-story steel moment-resisting frame structures have been considered under the heavy vehicle collision impact. They are modeled in OpenSees software two-dimensionally with regarding to uncertainty in materials and applied loads and the sensitivity based reliability analysis of the studied random parameters is performed in Matlab software. Then, the effect of each of the variables is investigated using simulation-based methods according to limit state functions based on the maximum permitted beam rotation of damaged bay of frames. The results of this study presented that the random variables such as mass and velocity of vehicle and yield strength of material were the most influential parameters in calculating the failure probability in representative structures. Also, the control variates-based subset simulation (CSS) method compared to Monte Carlo Simulation (MCS) approach estimated the failure probability with permissible error rate, less sample number and the minimum computer processing time duration. Furthermore, the results of this research indicated that by increasing the number of stories of the building, the probability of its failure due to vehicle collision increased. According to the results of this study, the value of beam rotation of damaged bay parameter of the 5-story frame compared to 2-story one has increased by 55, 18 and 33% under the LSF1, LSF2 and LSF3 functions, respectively

Pounding of adjacent buildings during earthquake causes local and total damages in buildings. One of the ways to reduce the damage in pounding, is creating a separation gap between two adjacent buildings. Park-Ang's damage index computes the amount of damage to the structure, according to the amount of deformation and the amount of energy absorbed by the earthquake reciprocating forces. In this study, estimates the effect of far-field and near-field earthquakes, on the Park-Ange damage index in pounding of two adjacent frames and separation gap is equal to the value proposed by Standard 2800.The frames are arranged in three, regular and irregular soft story and very soft story frames on the first floor. These frames are analysed under 5 far-field records and 5 near-field earthquake records. Accordingly, three steel moment frames are placed in pair proximity states and Park-Ang's damage index is calculated using dynamic analysis of the nonlinear time history with the OPENSEES software. Non-linear viscoelastic element is considered as a link element. The results indicate that in the regular frames, the average damage magnitude in the near-field earthquakes is about 12% more than the far-field earthquakes. In the soft story and very soft story frames on the first floor, the mean values of the damage index in the near- field are about 5% and 3% more than the far-field earthquakes.