finite element modeling

The stationary and rotating blades of jet engines and turboshaft turbines, particularly in the hot section, are subjected to extreme stresses at high temperatures. Consequently, they are constantly exposed to various forms of damage and failure, including hot corrosion, oxidation, creep damage, erosion, fatigue, foreign object damage, and cracking. A key challenge in turbine engine maintenance is the accurate estimation of the lifespan of critical components, particularly turbine blades. This study used finite element analysis in ABAQUS software to conduct a numerical analysis of a first-row turbine blade from the C20-250 engine, including its thermal barrier coatings. The stress and strain distribution over the operational period were determined, highlighting the areas most susceptible to creep failure. Based on the rupture strain of the turbine blade material, the creep life of the blade was estimated. The numerical results were compared with experimental data from three turbine blade samples to validate the estimated creep life. These findings provide crucial insights for predictive maintenance strategies, enhancing the durability and cost-effectiveness of turbine engines.

The distance of the secondary dendritic arms is an important microstructural feature in the dendritic solidification and has a great influence on the hardness of the alloy, and the main factor affecting it is the cooling rate. Due to the difficulty of its practical measurement, numerical methods should be used to predict it. In this study, first, 3 samples were produced with current of 95, 120 and 132. Then, using the results of the validated model, a 3D finite element model was developed to generate welding profiles and analyze transient heat transfer, and the cooling rate at different points of the weld metal was calculated. The model outputs (cooling rates, thermal gradient G and growth rate R) were used to describe the solidification structures. It was found that for the sample produced with current of 95 A, the cooling rate was between 650 K.S-1 to 800 K.S-1, but for the sample produced with a current of 132 A, the cooling rate was between 270 K.S-1 to 350 K.S-1. Then, using optical microscopy, the distance of the secondary dendritic arms in different regions was measured on the sample produced from the IN738LC superalloy, and the constants a and n in the mathematical relationship between the distance of the secondary dendritic arms and the cooling rate were obtained, which were equal to 34.62 and 0.3, respectively.

The inability of beam-column joints to play their key role in the seismic resistant system of the flexural frame is a subject that continues to be reported in significant damages and the threat of collapse of reinforced concrete buildings in recent earthquakes. Implementation violations and design of reinforced concrete buildings based on old regulations have caused non-seismic details to be observed in many existing connections. The operational and cost-effective technique of improvement by enlarging the connection area is one of the rare methods that, in addition to reviving the function of the connection susceptible to seismic damage, is able to enable access to high performance. In the current research, with the help of numerical modeling by the proposed method and available laboratory results, the effect of the size of the enlargement dimensions on the shear capacity of the joint area is determined. Considering the sliding effects of longitudinal beam reinforcements in the proposed method in modeling under narrow sided and cyclic loading, for non-seismic connections, 40% of stiffness and 5% of load such as yielding can lead to greater convergence with the experimental response. And this figure for reinforced joints is reduced to 35% for stiffness and 2.3% for load-like yielding. Also, intra-cycle deterioration is confirmed for unreinforced connections and non-deterioration for reinforced connections. On average, the presence of a transverse beam for non-seismic, seismic and reinforced connections causes an increase of 8.5%, 4.7% and an average of 2.15% of the force capacity of the connection, respectively

Traumatic brain injury is one of the most common injuries that threatens human life, which is also seen in football. In this study, a finite element model of traumatic brain injury is presented, in which the brain was defined as nonlinear hyper viscoelastic with transverse isotropy, based on the Mooney-Rivlin model. The 3D brain model was extracted from MRI images and finite element simulation was performed in LS-DYNA software. The results showed that in an impact from the lateral side, the pressure and the applied force spread faster and the brain suffers more severe damage. Moreover, it was demonstrated that if the impact direction is orthogonal to the orientation of the axon fibers, the brain tissue experiences higher stress. Furthermore, it was observed that in frontal impact, where the axon fibers are aligned with impact direction, due to the great deformation of the brain, the pressure at coup becomes positive and negative at the countercoup. In lateral impact which was perpendicular to the fibers, the pressure behavior at coup and countercoup was similar. 

One of the methods to increase the ductility and energy dissipation of structures is the use of metal-elastomer dampers in the moment connections. In this paper, a new metal-elastomer damper is introduced and its seismic behavior under cyclic load is evaluate numerically. The damper studied in this research includes an elastomer layer to control slight vibrations and a number of yielding bolts to dissipate energy in strong earthquakes. In the new configuration, placing two dampers at the top and bottom of the beam is considered as an alternative to the formerly studied connections with one damper at the bottom of the beam. The effect of different parameters such as the number, diameter and height of the yielding bolts as well as the thickness of the elastomer layer has been studied and compared to the connection without damper. Numerical models are created in Abaqus software and validated using results from previous studies. Based on the results, the stiffness and strength of the connections with dampers having few yielding bolts is very small compared to the rigid connection (stiffness about 25% and strength about 40%). As the number and diameter of the bolts increase, stiffness and strength of the connection increase reaching the corresponding values for the original connection. However, the maximum plastic strain in connection decreases by more than 50% which is a sign of improved behaviour. It is worthy to note that plastic stains in bolts of connections with two dampers are less than that of connections with one damper.

The volume filling ratio and type of filler significantly influence the rheological and mechanical properties of bituminous composites. This effect can stem from either chemical interactions between the filler and bitumen or simple physical phenomena. Understanding the influence of filler on asphalt mastic performance is crucial for comprehending the behavior of asphalt mixtures. This study employs both experimental and numerical modeling approaches. The rheological properties of asphalt mastic samples made with pure bitumen and limestone filler at various filler contents were determined using frequency sweep tests. The experimental results indicated that increasing the volume filling ratio in asphalt mastic leads to non-linear changes in the values of the complex shear modulus (G*) and phase angle (δ), known as the stiffening phenomenon. Considering the concept of the transitional zone between filler and bitumen, a parameter called the Effective Volume Filling Ratio (EVFR) was introduced to explain this phenomenon. To predict the viscoelastic behavior of asphalt mastic based on the mechanical properties of bitumen and filler, finite element method (FEM) simulations were utilized. The accuracy of these models was evaluated by calculating the relative difference between the experimental complex shear modulus (G*) and the complex shear modulus predicted by the model. The results of this evaluation indicated that incorporating the Effective Volume Filling Ratio (EVFR) into the numerical model can significantly enhance the accuracy of the predictions for the viscoelastic behavior of asphalt mastic samples.

In this paper, the effect of including SiC nanoparticles in the weld zone on the maximum shear strength of friction stir lap welded 7075 aluminum alloy is investigated, both experimentally and numerically. This objective is carried out by studying the effects of rotational and transverse speeds, tilt angle, the shape of the tool and the penetration depth. The numerical investigation is based on developing an FE model by means of Deform and ABAQUS to simulate the welding procedure, which results are verified by the experimental findings. Experimental procedure is designed based on Taguchi method. The verification of the developed FE model shows that the simulation results are in proper agreement with the experimental findings. In general, including SiC nanoparticles in the weld zone can increase the maximum shear strength up to 24%, compared to the case of where the specimens are welded without SiC. Furthermore, applying the threaded tapered tool leads to higher shear strength in comparison with the squared shape tool, i.e., the strength of the specimens welded by threaded tapered tool are 4 to 5% higher without SiC inclusion and 4 to 7.5% higher with SiC, compared to the same case welded by squared tool. In addition, while the rotational speed has the highest influence on the findings, the tilt angle does not affect the results that much.

Damage to the structure during operation has always been possible, and therefore the issue of monitoring the health of important structures in order to control and manage safe operation has received attention in recent years. The process of extracting the parameters involved in identification the inherent characteristics of the target structural systems in order to monitor and detect damages is possible with different methods which have been introduced and investigated under the title of environmental modal analysis, which have been developed both in the time domain and in the frequency domain. Among the time domain methods, we can mention the stochastic subspace identification (SSI) method. The stochastic subspace identification method is one of the Output Only Modal Analysis (OOMA) methods that has been taken into consideration assuming the existence of uncertainty in modeling as well as noise in data observation and measurement, and system parameters are identification by applying statistical relationships to the output data. Due to the high accuracy of the covariance-drived stochastic subspace (SSI-cov) method which performs data monitoring by constructing the covariance function related to the recorded output data, therefore this sub-method has been used. Due to the limitation in the number of sensors that can be installed in real structures, the use of the Reference-based covariance-drived stochastic subspace identification (SSI-cov-ref) method will make it possible to identification the structure under investigation with a limited number of sensors and if there is damage, Its location can be recognized. The efficiency of the reference-based covariance-drived stochastic subspace identification method with a more limited number of sensors can also be presented. In order to damage detecting in the structure, the method of measuring changes in the modal strain energy of the members in healthy and damaged states has been used, and an index called modal strain energy has been defined and presented But in the case of damage in the structure, the undamaged members will also have strain energy due to the strain effect of the damaged members. Therefore, by using the probabilities approach, it will be possible to provide an index of the probability of structural member damage under a specific failure scenario using different information sources based on the mode shapes. In the following, using the genetic algorithm in the form of an optimization problem, the installation location for the available sensors for the target structure is optimized and presented. During the current research, after finite element modeling in MATLAB software and dynamic analysis of several samples of structure with the assumption of installing a limited number of sensors ,and the results obtained from the output of the reference-based covariance-drived stochastic subspace identification method using the changes in the modal strain energy of the members And the Bayesian strategy has been used in the damage detection in the affected members, and the efficiency of the proposed method has been discussed in the framework of the mentioned process. Based on the results obtained from the output of the proposed process, the affected members will be identified and distinguishable.
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Special Truss Moment Frame (STMF) is a type of steel moment frames that has high lateral stiffness due to deep girders and can be used over long spans. For an STMF subjected to cyclic loading, energy dissipation is provided through the yielding of the special part located in the mid-span. This paper presents a numerical study on the cyclic behavior of STMFs in which the special part is equipped with yielding damper as a means of energy dissipative device. For this purpose, it is aimed to investigate the effect of the parameters of X-shaped pipe damper (XPD) on the cyclic behavior of STMFs. The application of these dampers has been investigated in two configurations; once with clamped and again with pin-ended beams at the special part. The results of finite element analysis are compared based on the parameters of cyclic behavior (stiffness and strength), the amount of maximum cumulative plastic strain and the energy dissipation capacity of the specimens. Based on the results obtained from the analysis of the STMF with pin-ended beams at the special part, the use of proposed X-shaped damper increases the energy dissipation capacity of the frame by 70%. Increasing the thickness of the X-shaped pipe damper increases the yield and ultimate load bearing capacity of the frame. In addition, the stress and strain concentration in the frame members is reduced which in turn prevents the damage evolution in these members.

Previous studies have demonstrated the occurrence of brittle and early stage cracks in the connection between the link beam and the column, specifically in the vicinity of the groove welds  that connect  the flanges of the link beam to the column, when subjected to seismic activity. A similar type of rupture has also been observed in the end-plate connections of the link beam to the external link beam. This research explors the consept of utilizing a box-shaped link beam with a reduced cross-section in the Abaqus finite element software to magnitude the plastic strain demand generated at the flange ends of the box link beam. The objective is to prevent rupture in this specific region. The results indicate that the maximum equivalent plastic strain at the end of the link beam flanges can be reduced by up to 38% for short link beams, up to 41% for intermediate link beams, and up to 68% for long link beams. Consequently, this approach effectively prevents premature ruptures in the area adjacent to the groove weld connecting the beam flange to the end plate. The magnitude of the equivalent plastic strain at the end of the box link beam decreases as parameters "a" and "b" decrease, and parameter "c" (geometric parameters of the reduced cross-section) increases. Parameter "c" exhibits the most significant influence on this parameter, while parameter "b" has the least significant impact. Additionally, the adoption of a reduced cross-section in the box link beam results in an increase of up to 35% in the energy dissipation of the link beam due to enhanced participation of the flanges in energy dissipation for short link beams, up to 158% for intermediate link beams, and up to 250% for long link beams.

This study aimed to propose an NDE method based on the guided wave propagation for assessing the thickness loss (corrosion) in the metal pipes coated with composites. First, the finite element model of a steel pipe with the thickness of 4 mm and diameter of 200 mm coated with a layered composite material was developed, in which the composite coating constituted by the chopped strand glass fiber mat and woven roving glass fiber cloth layers. Then, the fundamental antisymmetric guided wave mode with the frequency of 100 kHz was propagated in the longitudinal direction of the structure and the phase velocity of the propagated wave was measured in specimens with different corrosion extent in the steel pipe. In the first step, a uniform corrosion was induced throughout the pipe, and in the next step, it was induced in a part of the pipe with a specific length and circumferential angles of 90, 180 and 360 degrees. It was shown that the reduction in the wave phase velocity in the corroded regions compared with the intact regions was between 9% to 33% for different corrosion extents. Besides, the detection of the corrosion in the steel pipe and its location and extent was properly performed using the guided wave propagation method. It was concluded that the simulated guided wave propagation method can be used as a virtual lab for the development of methods for nondestructive evaluation of pipes coated with composites and detection of the location and extent of corrosion in them.

Nondestructive evaluation of mechanical properties and health monitoring of composite structures is of great importance. The aim of this research was to propose a nondestructive evaluation method based on the propagation of Lamb waves to investigate the matrix cracking damage in laminated cross-ply composites. For this purpose, after developing the finite element model of a glass/epoxy composite, the Lamb wave behavior was studied in two antisymmetric (A0) and symmetric (S0) modes with different excitation frequencies. The dispersion curves of the A0 and S0 modes of the Lamb wave are then obtained by considering different crack densities. It was observed that the effect of matrix crack density and the loss of mechanical properties of the cross-ply composites on the phase velocity of the S0 Lamb wave mode was higher than that of the A0 mode. Furthermore, the detection of matrix cracking was better performed using the Lamb wave velocity at high frequencies close to the cut-off frequency. It was observed that the cut-off frequency decreased by increasing the crack density, and the drop in the Lamb wave velocity increased by increasing the number of 90° layers in the cross-ply composite. It was concluded that the Lamb wave propagation simulation method can be used as a virtual laboratory for nondestructive evaluation of composites and detection of matrix crack density in them.

Because of the development of wind turbine farms in seismic areas, such as East Asia, southern Europe, and the United States, as well as the importance of the foundation of the wind turbines in the design and implementation, it is necessary to study the seismic behavior of wind turbine foundations. In this study, the seismic behavior of monopile foundation of a simplified offshore wind turbine model under a specific earthquake record was evaluated by using finite element method in OpenSees software. At first, the proposed numerical model was validated by the results of a dynamic centrifuge modeling of a steel pile located in sandy soil. Then, the effects of various parameters such as dimensions of monopile (diameter and length), soil relative density, end bearing pile effect and wind load effect were evaluated. The results of this study showed that the changes of these parameters have significant effects on the seismic behavior of the monopile of offshore wind turbines. Increasing dimensions of monopile (diameter and length) and soil relative density improves the seismic performance of the structure and foundation system and reduces lateral displacement at the top of tower and reduces settlement and rotation of the monopile. Chang of behavior of the monopile from frictional pile to end bearing pile, improves the behavior of the wind turbine structure and foundation under seismic loading. Also, wind loading causes more displacement and rotation to the structure, and static wind load creates more critical conditions than the dynamic wind load.

The nickel-based superalloys high sensitivity to various types of cracks during fusion welding and post welding heat treatment, leads to limitations in the repair welding of damaged parts of this material. Strain aging crack is a type of prevalent crack in the IN738LC superalloys that occurs during the post-weld heat treatment, which was investigated in few studies. In this study, using numerical modeling, the susceptibility to strain aging cracking in IN738LC superalloy welded by TIG method without filler metal was investigated. The results of thermal-mechanical modeling as well as experimental evaluation of cross-section of the autogenous welded samples after post-heat treatment showed that the transverse plastic strain of (rather than longitudinal) plastic is a suitable preliminary criterion for evaluating the susceptibility of superalloy to the strain aging cracking and it seems that if the plastic transverse strain is more than 4.6%, the probability of this crack occurring after post weld heat treatment will be very high.

Auxetic materials with negative Poisson’s ratio, as a group of metamaterials, attracted significant attentions among researchers due to their interesting and remarkable mechanical properties. Rigid rotating structures are a subcategory of auxetic materials which can show the same behavior in various directions by tuning their parameters, but due to using rigid rotating blocks their relative density is high. As the rotating blocks are connected by weak joints, stiffness and strength of these structures are low and considering high relative density of these structures specific mechanical properties are even in worse condition. In this research, novel lattice structures based on rigid rotating structures but with remarkably lower relative density were presented. To reduce relative density of these structures, bar elements were used instead of rigid blocks. 3D printing method was used to manufacture samples with these structures and then tensile test was performed on the samples. Poisson’s ratios of the samples were measured by recording image of the structures before and during deformation. The behavior of the structures was predicted by finite element method and compared with experimental measurements. Both of the methods showed auxetic behavior of the structures. Then deformation mechanism of the structures and the effect of the structures shape on the auxeticity were investigated.