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

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 this research, the evolution of damage and martensitic transformation of AISI 304, 316 and 321 stainless steels have been studied experimentally and numerically at room temperature. At first stage, the standard tensile tests have been performed and in the second stage the XRD has been tested and the available phases and the volume fraction of martensite have been obtained. The tensile tests have been performed for a variety of loading-unloading elongation. Damage is obtained using this loading-unloading slop. The graph of force-deformation has been achieved. The samples were cut by water jet machine and the X-ray diffraction tests have been performed on the stretched samples to obtain the volume fraction of martensite. Numerical simulations have been performed by the implementing the UMAT code in ABAQUS. In the simulations, the properties achieved from experimental tests are employed. The numerical simulations have two separated sections: damage-plastic and martensitic phase transformation. In this research, these two phenomena are considered simultaneously. This model can explain both phenomena at ambient temperature. The verifications have been compared simulation results with experimental data.

In the published studies, peridynamics has been used to simulate crack growth in brittle and quasi-brittle materials, simulation of plastic behaviour and solution of differential equations. With such achievements, the  extent of problems which can be solved using peridynamics is growing. Peridynamics was intended to analyze the impact and the wave propagation problems since its introduction. Due to existence of the characteristic length in peridynamics relations, it has been used to solve problems in various scales. Some researchers have also used peridynamics multiphysics models to analyze heat transfer, diffusion and fluid behavior problems. Analyzing the damage growth in composite materials and investigation of geomechanical problems are among other achievements of using peridynamics. In addition to all these cases, application of peridynamics to biological problems has also received attention in recent years. This paper reviews the studies on the applications of peridynamics in the solution of different problems.

Peridynamics is a nonlocal version of the continuum mechanics, in which partial differential equations are replaced by integro-differential ones. Due to not using spatial derivatives of the field variables, it can be applied to problems with discontinuities. In the primary studies, peridynamics has been used to simulate crack propagation in brittle materials. With proving the capabilities of peridynamics, the idea of using this theory to simulate crack propagation in quasi-brittle materials and plastic behavior has been proposed. To this end, formulations and models based on peridynamics have been developed. Meanwhile, the high computational cost of peridynamic methods is the main disadvantage of this theory. With the development of peridynamic methods and introduction of hybrid methods based on peridynamics and local theories, the computational cost of peridynamic methods has been reduced to a large extent. This paper introduces peridynamics and the models based on it. To this end, we first review peridynamics, its formulations, and the methods based on it. Then we discuss the modeling of quasi-brittle materials, simulation of plastic behavior and employing the differential operators in this theory.

Due to the need for orthopedic surgery, the mechanical behavior of the cortical bone in cyclic loading and physiological strain rate has been investigated. The emphasis is on developing a structural law that can establish the behavior during loading, unloading and reloading observed in experiments. These models will be formulated by combining rheological elements and energy principles. First, two one-dimensional models independent of the strain rate are formulated, one with one damage variable and the other with two different damage variables in tension and compression, are examined, and using laboratory data, the coefficients of each model are obtained. By comparing the simulation results and laboratory data, the necessary modifications have been made to the models. Finally, by combining the Bresler-Pister anisotropic yield criterion and the one-dimensional model independent of the rate associated with the two damage variables, the corresponding three-dimensional model was obtained. This three-dimensional model was implemented in the form of an explicit finite element method and the result showed acceptable compatibility with the simulation results of the one-dimensional model and experimental data. This three-dimensional model will be suitable for simulating complex geometries. The coefficient of determination for one-dimensional modelsRI ,RI± ,RI , RI±and  has been modified and the three-dimensional model has obtained values of 0.882174, 0.965665, 0.995508, 0.996279, and 0.984866, respectively.

In the present study, the effect of different parameters including of tool rotational speed, feed rate and tool diameter on the damage caused by drilling operations near the drilling hole in the E-glass/epoxy composite laminates with twelve layers of plain woven fabric manufactured using hand lay-up method was investigated and the experimental tests conducted on them. Then, using the Sobol statistical sensitivity analysis method, the effect of the above-mentioned parameters on the induced damage near the drilling hole was investigated. The statistical and experimental results show that for analyzing the damage variable, the simultaneous effect of rotational velocity in conjunction with feed rate of the drilling tool should be taken into account in order to minimize the damage variable, so that in the low velocities, the reduction of feed rate along with the reduction of the rotational speed causes the damaged variable to be minimized, while at the same time, in the high diameters, the simultaneous increase of the two mentioned parameters, minimizes delamination induced damage in laminated composites. The Sobel sensitivity analysis shows that the diameter of the tool, the velocity, and the rotational speed will have the greatest effect on damage variable in laminated composites, respectively.

One of the common methods to resolve artery stenosis is the insertion of a shape memory alloy stent in the artery. The use of shape memory alloys in the manufacturing of self-expandable stent has expanded because of the superelastic behavior of this alloy. In this paper, we simulated a shape memory alloy stent insertion in an artery to explore stress and damage effects arising from the stent insertion on the artery by using ABAQUS software. To simulate the mechanical behavior of artery, we considered the effect of the inelastic arterial properties (stress softening and recoverable inelastic deformation) that activated under supraphysiological loading in the stenting process, and to simulate stent behavior, the Souza model is used. These two models were applied in ABAQUS by UMAT codes. By using the damage model, the amount of damage in the artery, which is one of the factors of restenosis, is investigated. Also, in this paper, we have used real diseased artery geometry, which was specified from high-resolution magnetic resonance images, therefore the analysis is more in line with reality and it is possible to determine the location of the maximum stress and damage in the artery.

The most important factor that threatens the health of a structure is the existence of imperfections that occur or appear in different contexts as damage. Therefore it is necessary to have adequate cognition of the effect of the presence of different types of damage on the mechanical response of the structure. In the present paper, the effect of the presence and characteristics of damage on the light aircraft wing composite spar as one of the most important components from the perspective of computing the variations of its mechanical response in the criterion points has been investigated.By determining the effects, the algorithm of the implementation of the health monitoring system of the structure will be identifiable. The computation and observation of changes in the mechanical properties of spar in a variety of models have been performed as the basis of the composite spar health monitoring process through FEM. The results represents that the mechanical behavior of composite spar in different loadings are in direct relation with the location of damage in which nucleated, damage growth process and sizes. The damage location represents more important effect than the damage size to the changes produced in the mechanical behavior of the composite spar.

The destruction of asphalt pavements is one of the biggest problems in road construction in the world, which incurs a huge amount of annual rebuilding. The healing process to recover damage is one of the effective methods to extend the life of these pavements, which has been presented by researchers in recent years. Self-healing polymers are a class of smart materials that, without the need for external stimulation, can repair part of the damage generated as microcracks and microvoids in their microstructures. Along with the purpose of the study, i.e., investigating the effect of healing in the lifetime of asphalt pavements, the explicit time-discrete form of the thermodynamically consistent model is presented in order to be utilized in a finite element ABAQUS software by VUMAT subroutine. The discretization method mentioned here has benefits such as low calculation costs and high reliability in response. Then, three-dimensional modeling of general vehicles' tire has been done using hyperelastic and viscoelastic materials in order to have the desired deformation on the pavement after applying the load. In the following, to evaluate permanent deformations (common failures in the asphalt pavement) and the effect of the healing process on its dilation, it is necessary to simulate its behavior in high-cycle loadings. In order to reduce the cost and time of computation, an alternative method has been used. In this method, in order to speed up the growth of damage and, also, healing effect, tire moving velocity has decreased. This may increase the time of loading on asphalt; therefore, rate of damage increase. In this simulations, the life of the asphalt pavement is compared with the effect of healing and without the effect. The results obtained from the simulations show that, by applying the effect of healing, the growth rate of damage and destructions caused by the permanent deformation of the pavement decreases, and the pavement lifetime can be increased up to more than 70%.

High speed atomic force microscopy (HS-AFM) is one of the widely used techniques in nanotechnology applications due to high resolution and the ability of 3D imaging. Despite its advantages and although it is known as a nondestructive technique, tip or sample damage can occur if maximum repulsive force is higher than the failure stress of the sample or tip, as a result of tip-sample interactions. Several studies in understanding the peak repulsive forces in tapping mode AFM have been carried out, but mostly in steady state situations. In transient situation when tip encounters a sudden steep upward step, the repulsive force can be much higher than that in the steady state situation and, consequently, damage could happen. Therefore, if appropriate parameters’ values are not tuned, the tip-sample stress may exceed yield stress of the tip or the sample. This paper presents the comparison of maximum transient interaction forces in time periods of net attractive and repulsive forces and the effects of important scanning parameters on maximum transient stress of compliant samples with the elastic modulus in the range of 2GPa together with lateral resolution and scanning speed diagrams, using theoretical analysis as a novelty of this paper, so that selecting cantilever stiffness in the range of 0.1-1N/m, free air amplitude 60-100nm, amplitude ratio 0.8-0.9, quality factor 50-100, tip radius 10-40 nm, and scanning speed 0.1-0.3mm/s relative to required lateral resolution indeed leads to safe high speed microscopy.

The aim of this study is to investigate the life-span according to the damage caused by the main mechanisms of damage development in turbine blades and to model the growth of the damage. For this purpose, the low cycle fatigue test on martensitic 410 stainless steel was immersed in tempered glass at 565°C in three strain gauges 0.8, 1 and 1.5 with a constant temperature of 500°C and 15 seconds per cycle. The effect of creep-fatigue interaction on life and also damage to turbine blade in different conditions was investigated. The results showed that with the variation of the strain amplitude from 0.8 to 1.5, the life of the piece varies from 205 to 65 cycles and this is while the level of failure of the samples varies. In the next step, the modified Coffin-Manson model was used to indicate the damage and its simultaneous effect on the life of the piece. The results showed that decreasing the number of grain boundaries and its effect on the cavities created in the piece decreases the damage and thus the life of the turbine blade increases. High-temperature tensile tests and low-tensile fatigue-temperature control were also performed in different tempering modes for 410 and 420 steel stainless steel and The results showed that, under the same conditions, the temperature increase from 200 to 565°C resulted in a decrease in life from 2218 to 1952 cycles.

One of the simple criteria for estimating the formability of sheet metals is use of forming limit diagram (FLD). Determining the amount of accuracy and the range of proper prediction of this criterion is one of the most important challenges of researchers and engineers. In the present research first, employing the forming limit diagram of St14 steel, number of practical experiments such as tensile test of standard, notched, and opened specimens, tube hydroforming and deep drawing processes are simulated, and possibility of crack initiation and fracture in them are predicted. Then, to evaluate the forming limit diagram damage criterion, the above experiments are also practically performed and the practical results are achieved. Finally, comparing the results of numerical simulations and empirical observations, the amount of accuracy and proper range for fracture prediction of sheet metals by the FLD damage criterion is revealed.

Due to the extreme increase in computational power over the recent years، numerical methods have gained the most proportion in analyzing composite structures and components because of the consideration complicated failure mechanisms such as delamination، fiber buckling and fiber breakage، matrix cracking، debonding ribs of skin and a combination of mentioned failure mechanisms. However exact three - dimensional modeling damages caused by impact phenomena is still a challenge. In present numerical work، the most advanced modeling techniques have been used to predict the behavior of composite structure under high velocity impact. The ribs and layers have been modeled using solid elements and a user defined material model with modified puck and Hashin (3D) failure criteria was implemented. Because these failure criteria do not exist in Commercial version of the Abaqus software، we have used Fortran software for writing these criteria so this capability was added to the software. Figures of velocity variations and force variations of projectile، damaged area، different mechanisms of fracture were reported as results and commented upon. In this study، The numerical results have been validated with experimental data and show very good agreement.