مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 10 (دی 1399)

In this research, Inconel 738LC as the base metal was joined by wide gap brazing (WGB) using the mixture of Amdry718 as high temperature particle (HTP) and BNi-2 as low temperature particle (LTP). By using Amdry 718, the brazing temperature can be increased and porosity reduced significantly due to higher melting point temperature than base metal. The effect of mixture ratio on the microstructure and the mechanical properties was studied. The amount of the LPT in the mixture was 30, 40, and 50 percent. By decreasing the LTP ratio, the amount of eutectic phase and the blocky boride in the solid diffusion zone (SDZ) decreased markedly. The joint width increased by increasing the amount of LTP. There were two distinct regions in the joint/base interface. Due to the diffusion of B into the base, adjacent to the joint, the grain boundary melted and created a region including both the isothermal solidification zone (ISZ) and SDZ. By getting away from the joint, the content of B reduced by the grain-boundary diffusion. It was observed that aging did not have a significant effect on the amount and distribution of eutectic phases and blocky borides, however, the morphology of these phases changed slightly. Moreover, aging had a small effect on hardness making the hardness of different phases more uniform.

Guided waves have a wide range of applications in nondestructive testing of pipes. The generation of pure symmetric guided wave modes in a pipe simplifies the interpretation of results. Higher sensitivity to pipe geometric variations and discontinuities can also be achieved by using pure symmetric modes. Most pipes used in low-pressure fluid transmission lines are manufactured by cold rolling of a plate followed by resistance welding of the two edges of the plate. Consequently, these pipes have a straight weld seam along their length. In this article, Finite Element Method is used for simulating the propagation of symmetric guided wave modes L(0,2), L(0,1) and T(0,1) in straight seam welded pipes. First, a comparison is made between the propagation of these wave modes in seamless and seam welded pipes. Results indicate that the angular amplitude profile of the L(0,2) wave mode does not change much in the presence of the seam weld. However, the weld has a slight effect on the propagation of the T(0,1) mode. The effect of the presence of the straight seam weld can be observed in the propagation of the L(0,1) mode which creates an asymmetric angular profile such that the amplitude of the L(0,1) mode in the weld line zone is largely reduced. This phenomenon may reduce the sensitivity of this wave mode in detecting defects in the proximity of the weld line. As a result, the other two guided wave modes L(0,2) and T(0,1) are recommended for the inspection of straight seam welded pipes .

Flanged joints’ looseness is among the common causes for the failure of industrial structures with flanged joints such as wind turbine or transmission line pipes the timely detection of which can prevent the imposition of heavy, financial losses and in some cases property damage. Conventional methods for detect-ing this fault, such as torque control methods, have high error of measurement, or impedance-based measurement methods, have high expenses, or vibration or ultrasonic methods which lack accuracy due to the use of linear phenomena in fault detec-tion. Vibro-Acoustic modulation method is one of the nonlinear fault detection methods that detect and evaluate looseness of flanged connection with high precision through the measurement of the intensity of the vibrational and ultrasonic signals modulation applied to the structure. Although the published papers in recent years have often identified the cracking, delamination, or corrosion and decay of parts using this method, in this paper the efficiency of the Vibro-Acoustic modulation method in the detection and evaluation of flanged joints of simulated wind turbine towers has been numerically investigated by defining an index for modulation intensity and bond relaxation. Then, the effect of parameters such as ultrasonic and vibrational frequency, amplitude of ultrasonic and vibrational stimulation, sensors and actuators position, as well as preload force, on the method performance have been studied. Finally, in order to reduce the simulation time in ABAQUS software, the modeling of the neural network was performed using MATLAB software and the obtained results were compared with numerical results.

Damage mechanics is tool in mechanical engineerin, that is known as the complementarity for theory of plasticity and fracture mechanics. In this research, an experimental study of the fatigue damage growth process has been investigated for 316L stainless steel. For this purpose, using ASTM E466 fatigue special standard, testing samples with rectangular cross section, has been drawn and prepared. Uniaxial Low cycle fatige has been performed at similar stress amplitudes, similar frequencys and various loading cycles. After carrying out fatigue tests, in order to derivation damage development process, microhardness and Loading-Unloading tests –deriving relation of microhardness and chord modulus with strain- has been performd too. Experimental tests has been carried out under environmental conditions. Damage development process was achieved, Using results of fatigue, loading-unloading and microhardness tests. A method for estimating damage is purposed in this research. At the end, the results of the methods of the chord modulus, microhardness and purposed are compared with each other. The damage growth process of the purposed method has a good fit with the method of chord modulus. The obtained critical damage was 0.38 by using the chord modulus method and 0.41, by microhardness.

Magnetic Abrasive Finishing (MAF) is a nano-machining process; due to low machining temperature, this process is categorized as a cold forming process. Therefore, the machined surface is free from thermal damages such as micro cracks, phase changes, burnt area and etc. In this paper, the effects of machining parameters (machining gap, work piece rotational speed and abrasive particles’ type) on work piece surface roughness have been experimentally studied. To achieve this goal a series of experimental tests were conducted on a newly developed setup and workpiece surface roughness was measured. The results of experimental studies were then used to develop a mathematical model for work piece surface roughness using Response Surface Method (RSM). The results show that there is a good agreement between experimental results and model predictions. This model was then used to minimize workipece surface roughness. In the selected range of machining parameters the minimum value of surface roughness is achieved by workpiece rotational speed of 373.73 rpm, machining gap of 2 mm and using diamond particles as abrasive. In addition, it was shown that abrasive particles’ type is the most affecting parameter on workpiece surface roughness. Furthermore, Microscopic results of surface tissue specimens, Shows that magnetic abrasive finishing process the direction and grooves resulting from the grinding process have significantly eliminated and as well as a super-polishing and uniformity surface finish like a mirror to a range of 0.207 µm is obtained.

One of the most important and widely used tools to predict the behavior of sheets is the Forming Limit Diagram (FLD). The m-k model is one of the prediction methods, which can be combined with the phenomenological or the crystal plasticity equations to achieve the desired results. In this research, to predict the forming limit diagram, the direct combination of the m-k method with the crystal plasticity has been applied. The direct method is chosen due to the particular state of the mathematical equations associated with FLD. A Face-Centered Cubic (FCC) polycrystalline metal has been used here, so, the Taylor method for the polycrystals can be used. Although this method ignores the interactions between the crystals to describe plasticity, it can also reduce the computational cost by simplifying of the strain uniformity theory. In this study, polycrystal plasticity and dislocation method have been merged with a new way. Only the hardening process is modeled based on the dislocation density and its modifications, and the entire analysis is based on the rate-dependent crystal plasticity. For the first time, the forming limit diagram is plotted to take into account the effect of dislocation density, and the results have shown that considering the effect of the dislocation density on the shear strength changes, the forming limit diagram formulation becomes nearer to the experimental values.

Forming process of powders accompanies considerable volume change and complicated material flow. Understanding powder behavior and the ability to perform its deformation analysis may assist in reducing trial and error in die design process and providing more accurate predictions of forming loads and density distribution in the green part. Such analyses are feasible only based on the constitutive models for powder compaction. Modified Drucker-Prager cap (MDCP) mode, is a multi-surface yield model for description of the plastic behavior of powders during consolidation. In this research, the deformation behavior of the commercial ready to press alumina powder KMS-92 has been investigated using MDCP model. To this end, parameters of MDCP model as functions of density were obtained by means of experiments. The constants of the shear failure yield surface were obtained based on simple diametric and axial compressive loading cylindrical specimens with various relative densities. For determining the remaining parameters of the model, an instrumented die fitted with strain gage and load cell was designed and fabricated. Parameters of the cap surface were achieved based on the uniaxial die compaction experiments. Based on consecutive loading-unloading tests using the instrumented die, the friction coefficient and elastic moduli were derived from loading and unloading phases respectively. For finite element simulation of the uniaxial compaction, density dependent material parameters were employed in Abaqus. The variations of density were taken into account using USDFLD subroutine Simulation results prove a very good agreement with the experimental counterpart.

Conventional plastics has a large impact in increasing the environment’s pollution. That’s why the interest has turned towards novel partially and completely biodegradable polymers. In this work, composites of polystyrene and starch with glycerol as plasticizer and compatibilizer were prepared in different ratios of starch and glycerol by melt mixing in twin screw extruder. To study the biodegradation properties, samples were buried in soil for 12 weeks and their weight changes were measured during two-week periods. Besides, the structure of prepared composites was studied using SEM analysis. To evaluate the impact strength and thermal properties of composites, we used Izod impact strength test, MFI (Melt Flow Index) and Vicat softening temperature tests. To examine the interaction between composite elements, thermal stability and degradation process, the samples were subjected to analysis using TGA. Biodegradation results and SEM analysis showed that the rate of biodegradation not only starch present in composites but also depends on the quality of starch particles dispersion. Notched Izod test showed the independence of samples impact strength to starch present. Due to the reduction of stress accumulation points, homogeneous dispersion of starch in the matrix leads to increase impact strength in unnotched Izod test. MFI test showed that addition of starch and glycerol leads to respectively increase and decrease in composites’ viscosity. Study of starch/glycerol ratio in obtained results showed that in an optimal amount of plasticizer, interaction between the composite components is in conditions that the best mode of starch particles dispersion is achieved in polystyrene matrix.

Among various manufacturing processes, the use of cold press- sintering has been considered with low cost, high efficiency, and the ability to produce components with the appropriate dimensional accuracy. In this method, at least two components of the alcoholic solution are used in granulation and milling steps. Removing the solutions is accompanied with energy consumption, as well as, an increment in probability of crack initiation, which may limited the widespread utilization of press-sintering process. Therefore, in the present study, the soluble component in the milling step was removed. In following just using PVA as a soluble component in the granulation step, the ceramic / ceramic composite was made. Doing this, samples based on NiO and YSZ were made with different weight percentages of the reinforcement component (25, 30 and 35% wt. zirconia-stabilized yttrium), and then their microstructure, density and micro-hardness were investigated. Microstructural studies indicate that the amount and distribution of porosity, surface quality and depth of cracks in the samples are depended on reinforcement weight. In detailed, at the surface of green tables, no deep crack was observed. In sintered specimens, the best distribution of the gas phase was observed in NiO- 30% wt. YSZ, which results in maximum micro-hardness. Also, the radiography results provided by the above samples indicated that deep crack in the discs is not visible. Therefore, it seems that the starch as a pore former displayed the beset role via porosity distribution and lessening crack nucleation in sample having 30% reinforcement.

Magnetorheological fluid is a suspension of magnetizable particles in a base liquid with stabilizer. The rheological properties of these fluids change significantly in the presence of magnetic field. In this study, nanomagnetic iron oxide particles were prepared by using co-precipitation method. X-ray diffraction and scanning electron transmission methods were used to characterize these particles. The magnetic properties of the particles were measured by using a Vibrator Sample Magnetometer. The sedimentation rate for a non-additive fluid was reported to be 70% and when the cellulose was added, it was reported to be 55%. The effect of samples contained nanoparticles iron core shell in various percentages of 0.5, 1 and 2% of nanoparticles with 3% cellulose were investigated. The results showed that almost all of the samples were completely stable after addition of the nanoparticles in the first three days. The results showed that the sample containing 5% by weight of cellulose and 1% by weight of nanoparticle core shell by cellulose had a high stability during more than a month. The increase in shear stress and yield stress in the optimized sample containing the nanoparticle was also significantly higher than the sample without any additive and it was about 10,000 Pasca.

Non-destructive test is one of the techniques that is used to ensure the health of concrete structures. In order to retrofit concrete columns, it is necessary to know the number of reinforcement steel bars embedded in concrete and their cover to the concrete surface. In this paper, for the purpose of obtaining the reinforcement cover to surface of concrete in concrete columns, finite element simulation is performed by using Maxwell software and a special type of measuring probe is designed and fabricated. When a conductor such as reinforcement steel bar approaches the coil and ferrite core, the reinforcement bars interacts with low frequency electromagnetic waves, and by measuring of these waves’ effects from the concrete surface, the location of the reinforcement can be estimated. The designed probe was tested on different reinforcement covers with various frequencies and the recorded results were evaluated. The reinforcement cover can be estimated from the inductance variation and the ohmic resistance of the coil while the frequency and voltage are kept constant. Effects of the reinforcement cover to the concrete surface and frequency on the ohmic resistance and inductance of the probe are evaluated by using three-level full factorial designs and a regression model from effective parameters is prepared. Finally, the neural network is employed to estimate the reinforcement cover with different values of frequency, the reinforcement cover to concrete surface and the ohmic resistance of probe.

Functionally graded honeycomb with negative stiffness can be used extensively as an energy absorber because ‎of having two features of negative stiffness and being functionally graded. In this research, the effect of using ‎functionally graded honeycomb with negative stiffness in increasing energy absorption has been considered. In ‎functionally graded honeycomb the thicknesses of structure change gradually in each layer, as a result, each layer ‎has a different stiffness. Negative stiffness honeycomb is a type of energy absorber that absorb energy by ‎transition from one bucked shape to another and show snap through- like behavior .In this research, at first quasi-‎static tests have been carried out on negative stiffness honeycombs with constant thickness. Then finite element ‎model of negative stiffness honeycomb with constant thickness is simulated in ABAQUS software and the results of ‎simulation is compared with experimental results and a good agreement between numerical and experimental ‎results have been observed. Functionally graded honeycomb with negative stiffness then has been modeled ‎numerically. Energy absorption per unit mass of the functionally graded negative stiffness honeycomb has been ‎compared with conventional one. Based on the results, energy absorption per unit mass in functionally graded ‎honeycomb with negative stiffness increased by 1.57 times.‎‎

In this paper, the effect of differed loading conditions, as well as structure geometries on the behavior of sandwich panel structures under uniform blast loading, has been investigated. For this, a full 3D numerical model by using ABAQUS/Explicit commercial software was employed in order to simulate the dynamic response and plastic deformation of sandwich panel structures with a honeycomb core. The available experimental results were used to validate the numerical model. Afterward, in a rigorous parametric study, the influences of several effective parameters on the resistance of structure such as the front and back metallic layer thicknesses, the number of webs, and thickness of honeycomb core cell wall under three different mass weights of 0.5, 1 and 1.5 kg were studied. In the following, by using Response Surface Methodology, an appropriate equation was developed to predict the central permanent deflection of back and front layers. The obtained results showed that there is a well-agreement between experimental and numerical results predicted by the regression model. The high correlation coefficient between the studied parameters and the structure behavior (R2 = 0.99) indicates that the proposed model has great accuracy.

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.

Due to wide application of conical sandwich shells in the modern industries, it is nessesary to investigate the mechanical behavior of theses structures. In this paper, for the first time, by considering the flexibility of the core in the high order theory of sandwich shells, the the buckling behavior of the truncated conical sandwich shells which include a temperature dependent porous FG core and two temperature dependent homogeneous face sheets are investigated in various thermal conditions. The power law rule which modified by considering the two types of porosity volume fractions are applied to model the gradually variation of functionally graded materials. By applying the principle of minimum potential energy, considering the in-plane stresses in the core and faces, and nonlinear von-karman strains for both mechanical and thermal stresses, the governing equations are obtained under the axial in-plane compressive loads. A Galerkin procedure are used to solve the equations in a simply supported boundary condition. Uniform, linear and nonlinear temperature distributions are used to model the effect of the temperature changing in the sandwich shell. To verify the results of these work, they are compared with FEM results obtained by Abaqus software and for special cases with the results in literature. Critical load variations are surveyed versus the temperature changing, geometrical effects, porosities, and some others in the numerical examples.

In this paper the effects of position and shape of cutout on the buckling load of composite cylindrical panel under compressive load is investigated. The laminated cylindrical panel with an arbitrary cutout shape is simulated by the spline finite strip method. The first order shear deformation theory and Sanders shell nonlinear theory are considered in this study. Using a linear buckling analysis, an eigenvalue problem is solved to obtain the buckling load of the panel under the compressive axial load. A comparison between the results obtained from the finite strip, finite element and analytical methods were made to show the validity of the results obtained in this study. Several case studies are presented to investigate the effects of some parameters such as shape and position of cutout, central angle of panel, ply sequence of the composite layers and boundary condition of the panel on its buckling load. The results show that the position and shape of cutout have considerable effects on the buckling load of the panel. The buckling load of the panel reaches its minimum when the cutout center has eccentricity from the loading direction. The results also show that the buckling load with quasi-isotropic configuration is greater than those of cross-ply and angle-ply.

It is known that friction has a significant effect on determination of the ballistic impact performance of woven fabrics. In this paper the ballistic behavior of fabrics woven from Twaron and Dyneema Aramid fibers against high velocity impact of a cylindrical projectile is investigated. This paper aims to numerically figure out the effects of inter-yarn friction performance including transverse deformation of fabrics, overall energy absorption and the forms of energy absorption. The numerical results show that increasing inter-yarn friction decreases the transverse deflection abilities of the fabrics and subsequently the response modes of them will transfer from a localized response to a globalized one. With the increase of inter-yarn friction, the energy absorption rate monotonously increases, while the failure time firstly decreases and then increases but further decreases again. Increasing inter-yarn friction also affect the forms of energy absorption. Near zero friction coeficient, strain energy is the dominant failure mechanism of a fabric. With the increase of inter-yarn friction, kinetic energy becomes the dominant failure mechanism. The frictional dissipation energy absorption is maximized for a finite inter-yarn friction. Experimental results were used to validate the results. The predicted values of the model show a good agreement with the experimental data. The correlation coefficient was 0.9426, which verify the accuracy of the simulation.

Laser peening is one of the life-enhancing process that by radiation of laser pulse with sufficient energy at very short time on the surface of the metal results in the penetration of shock waves inside the material and the formation of compressive residual stresses inside it. The purpose of this research is the multi-objective optimization of the laser peening process parameters. Finite element method is used for modeling, Taguchi orthogonal array for design of experiment and the gray relational analysis for multi-objective optimization. The diameter, pressure, time, and overlap rate between two adjacent laser pulses are considered as design factors that change in 4 levels and the Taguchi L16 orthogonal array is used for experiments layout. The average residual stress at the surface of first pulse, minimum and maximum residual stress and the mean of the residual stress depth in the center of two laser pulses were considered as optimization target functions. By performing a gray-relational analysis, the Gray relational grade for each experiment was calculated and the optimal level of each parameter was obtained. The results indicate that the optimal state of each parameters of diameter, pressure, time and the overlap rate between the two laser pulses are at the fourth, fourth, first and fourth levels, respectively are 8 mm, 4 GPa, 30 ns and 75%. Also, analysis of variance was performed on the results to determine the effect of each parameters on the output that the laser time with 58.87% is the most effective parameter on the results.

Growing the need to impact resistive structures, hybrid laminates have been absorbed much attention. To reach appropriate strength, high stiffness and good energy absorption, the interfacial adhesion between different layers is important. The present paper is an attempt to assess the adhesion between different layers in a hybrid laminate consists of natural rubber, glass/epoxy composite and two layers of aluminum under high and low velocity impact and quasi static three-point bending conditions. In order to minimize the debonding, three kinds of specimens were made by three different adhesives including Chemosil 222 and its primer, Bylamet S2 and Cyanoacrylate. High and low velocity impact tests were carried out by a gas gun and drop hammer apparatus using a 10 millimeter hemispherical striker. Finally, quasi-static three-point bending tests were accomplished by a universal tension/compression apparatus. Damage and failure mechanisms and mechanical properties of the samples were studied and the best adhesive for each interface was determined. The results showed a better performance of Bylamet S2 adhesive in almost all types of loadings.

The aim of this research is to propose a procedure for predicting creep-fatigue life of gas turbine blade. Components in gas turbine hot section such combustion chamber and turbine gets damaged under effect of thermomechanical loadings. These damages developed via fatigue and creep together, thus it is necessary to identify their interaction on each other. To achieve this goal Chaboche’s elasto-viscoplastic model which is an appropriate model for simulating the behavior of nickel alloys being used. Damage evolution rules for fatigue and creep which introduced by chaboche and kachanov respectively added to viscoplastic model and implemented in Abaqus by a UMAT. After verification of Generated subroutine by a simple model, a rotor blade of last stage of a low pressure turbine was used in the process of creep-fatigue life assessment. Predicted life has proposed in three ways as the operating hours, number of starts and the equivalent operation hours which is the common ways of announcing gas turbines life. Comparison the gained results with common gas turbines life represent that this procedure’s capability and practicality.