A. Mir, M. Aghaie-Khafri,
Volume 19, Issue 3 (3-2019)
Abstract
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.
H. Hashemi , S.h. Hashemi,
Volume 19, Issue 7 (7-2019)
Abstract
The API X65 steel (with a minimum yield strength of 65ksi equivalent to 448MPa) is one of the most common types of pipe steels in the transportation of natural gas in Iran. By studying the ductile and brittle fracture areas at the fracture surface of this steel, we can show the quality of this type of steel. In the present study, macroscopic fracture surface characteristics in three-point bending test specimen are studied (based on the geometry and standard notch of drop-weight tear test specimen). Test specimens were machined from an actual steel pipe of API X65 grade with an external diameter of 1219 mm (48 inches) and wall thickness of 14.3 mm. Due to the quasi-static test conditions and speed of the machine's jaw (0.1 mm/s), the test was carried out on base metal specimens with machine chevron notch of 15, 10, and 5.1 mm depth, respectively, that was controlled with changing location. By applying the test load, cracking initiated from the notch root in each specimen and continued without crack specimen (ligament). At the end of the test, test specimens were cooled by liquid nitrogen and were broken in a brittle manner. In this paper, after investigation of the failure mode and the crack expansion in the standard specimen, investigation of macroscopic fracture surface characteristics was conducted by optical microscopy. By observing the fracture surface, different features such as thickness variation, shear regions (ductile fracture), cleavage fracture, shear lips, inverse fracture, and brittle fracture were studied. Having above 85% shear area, the ductile fracture of specimen was confirmed.
A.a. Shami, S.e. Moussavi Torshizi , A. Jahangiri,
Volume 20, Issue 2 (1-2020)
Abstract
Superheater tubes are the most critical components of the power plant’s boiler. These tubes are subject to degradation such as creep and overheating, due to the hard operating conditions (exposure to high temperature and pressure for a long period). Therefore, it is important to diagnose and prevent these failures. The failure report in a 320-megawatt power plant indicates that most tube ruptures are concentrated in a particular region of the platen superheater (radiative superheater). The investigation of broken tubes shows that the temperature of the tubes in this area is higher than the other platen superheater’s regions. Three methods of metallography, oxide layer thickness measurement and thermal analysis using computational fluid dynamics were used to prove the existence of higher temperatures at the point of breakdown. All three methods provide the same results. The results of surveys confirm this significant temperature difference and show that the increase in the local temperature in the damaged tubes is due to the longer length of these tubes, which results in lower vapor mass flow rate, and absorb more heat due to the higher thermal surfaces of them.
F. Asnaashari, S.h. Dibajian , M. Mahmoudi ,
Volume 20, Issue 5 (5-2020)
Abstract
The amount of entropy generation during the fatigue loading is treated as an indicator of the damage accumulation in the material. Using the thermography technique and recognizing the temperature field distribution at a specimen surface under cyclic loading and calculating the dissipated energy and also considering the possibility of the specimen heat transfer with the environment, the net entropy production rate of the system can be computed. This research has been conducted to feasibility study and applicability of the methodology through numerical modeling and analysis. In this thesis, using the finite element numerical method and in the framework of Abaqus software, simulations of fully reversed bending are carried out on the standard specimens of aluminum (Al6061-T6) whose experimental test results are available in the literature. Based on results of the mechanical and thermal analysis, calculating the entropy production rate, fatigue fracture entropy, damage variable and remaining life assessment based on this variable are performed. The results obtained from the numerical simulation are compared and validated with the results of experimental tests. Also, a numerical analysis is carried out to estimate the temperature enhancement and fatigue self-heating phenomenon due to the cyclic loading based on the strain-life curve characteristics and dissipated energy on the axial specimen made of (AISI 4340). The results obtained from the research indicate that the infrared thermography technique as a non-destructive evaluation method in the low cycle fatigue range, is a suitable tool for the temperature field evaluation and subsequently, the accumulated damage estimation in material.
V. Hoseini, M. Shariati, M. Mahdizadeh Rokhi,
Volume 20, Issue 6 (6-2020)
Abstract
In this research, the behavior of conical and hemispherical shells, made of steel and aluminum, respectively, subject to impact loading has been investigated using experimental and numerical methods. The energy absorption capacity of these adsorbers has been calculated and the effect of foam injection on the collapse behavior and energy absorption capacity of aluminum hemispherical shells has been determined. The effect of geometrical parameters on the collapse behavior and adsorption capacity of steel conical adsorbers has also been investigated. Numerical simulations have been performed using the Abaqus finite element software and the results have been compared with the results of the experiments. In Numerical analysis, three damage models, Johnson-Cook, GTN, and modified Rousselier have been used. The Johnson-Cook damage model is available in Abaqus software but the GTN and the modified Rousselier damage models have been created through programming in Abaqus software. The results show that the modified Rousselier damage model is more accurate than the other damage models. Also, in this research, the effect of thickness of conical shells on their efficiency has been investigated and it becomes clear that increasing the thickness of absorber increases efficiency. In addition, foam injection does not a positive effect on the hemispherical absorber performance.
H. Gholipour, F.r. Biglari ,
Volume 20, Issue 8 (8-2020)
Abstract
The present study is devoted to experimental and numerical investigation of in-situ tensile tests to recognize the mechanisms of ductile fracture under different stress states. The GTN model, which is a micromechanical based damage model, has used for numerical simulations. The parameters related to this model for St12 steel were identified by response surface method (RSM) through minimizing the difference between numerical and experimental results of the tensile test on a standard specimen. The void related parameters of GTN model were determined 0.00107, 0.00716, 0.01, and 0.15 for ff, fc, fN, f0, respectively. After calibrating the damage model for the studied material, the tensile tests were carried out on the in-situ specimens with different geometries. The fractographic analysis was performed to identify the ductile fracture under a wide range of stress states and two failure mechanisms were observed. The calibrated damage model was applied to FE simulations of in-situ tensile specimens for numerical study of the experimentally observed fracture phenomenon. The extracted numerical results showed a good agreement with experimental observations comparing load-displacement plots with a margin of error within 5%. The location of fracture initiation, crack growth orientation, and the displacement at fracture zone in numerical studies also showed close correspondence with experiments.
A. Pazhouheshgar , A.h. Moghanian , S.a. Sadough Vanini,
Volume 20, Issue 8 (8-2020)
Abstract
The composites derived from the bioactive glasses, such as BG/polysulfone, have better mechanical properties than pure materials and their characteristics are closer to human bone. In this study, the unknown fracture behavior of 58s BG/PSF composite has been investigated. The extended finite element method (XFEM) was used, in order to model the fracture behavior of 58s BG/PSF composite with greater accuracy. The XFEM doesn’t require remeshing at each step and achieves the precise approximation of singularities by incorporating discontinuity behavior into the elements using the enrichment functions. The aim of using the XFEM was to obtain stress intensity factors, displacements, stress and strain around the crack tip, fracture toughness as well as strain energy release rate. Moreover, the 58s BG/PSF composite with 30% bioactive glass particles was synthesized using solvent casting method and the bending failure test was performed according to the relevant standard. Also, to demonstrate the quality of the interface between the glass particles and polysulfone, SEM investigation was performed on the fracture surface. The obtained fracture toughness was in the range of 1.4 to 1.6
, and the strain energy release rate was in the range of 1600 to 1900 J.m
-2, which was comparable to the same properties of natural human bone. Besides, the stress intensity factors and strain energy release rates were calculated by coding in MATLAB and modeling in ABAQUS, and the numerical results were validated with the analytical and experimental data and it was revealed that the numerical results were in great coordinance with the analytical and experimental results.
M. Tazimi, S.h. Hashemi, S. Rahnama,
Volume 20, Issue 10 (10-2020)
Abstract
In this study for the first time, changes in the thickness of the fracture cross-section of the inhomogeneous sample (with horizontal weld seam) of the API X65 steel, using drop weight tear test specimen have been investigated experimentally. The fracture surface of the test specimen consisted of three zones of base metal, heat affected zone and weld metal with different microstructure and mechanical properties. The most thickness reduction was in the cleavage fracture area of the notch root. In the base metal zone, thickness changes were constant which indicated the stable crack growth in this area. In both heat affected zones before and after the weld zone, the thickness changed with a constant slope. Due to the high hardness and low fracture energy of the weld zone, the lowest percentage of thickness changes was in this zone. Thickness in the weld zone increased with a constant slope due to the stretching of the weld zone to the end of the crack growth path by the force caused by the change of fracture mode from tensile to shear. Also in the reverse fracture zone, due to the increased in compressive strain caused by impact of the hammer on the sample, the thickness increases with a significant slope and reached the maximum value.
Fatemeh Yaghoobi, Roohollah Jamaati, Hamed Jamshidi Aval,
Volume 20, Issue 11 (11-2020)
Abstract
In the present study, using a new method, dual-phase (DP) steel with high strength and good ductility was produced from plain carbon steel with 0.16% carbon. The DP steel with ferrite-martensite structure was obtained using austenitizing, quenching, asymmetric cold rolling, and intercritical annealing at temperatures of 770 and 800 °C and short holding times of 1 and 5 min. Due to the application of uniform shear strain through asymmetric cold rolling, a uniform distribution of the martensite phase was observed in the RD-TD and RD-ND planes. By increasing the holding time, the volume fraction of martensite increased from 8% to 12% at 770 °C and from 10% to 33% at 800 °C for the holding times of 1 and 5 min, respectively. Hardness and strength improved with increasing temperature and time of intercritical annealing. The sample produced at a temperature of 800 °C and a time of 5 minutes showed excellent mechanical properties such as 244 HV hardness and 1020 MPa strength and 12.5% ductility. In addition, due to the high volume fraction of martensite and the consequent reduction of its carbon content, the hardness of this phase decreased and as a result, it showed significant plastic deformation and high strain hardening. The fracture surface of all produced DP steels mainly included dimples, which indicates ductile fracture behavior.
Hossein Mamusi, Mohammad Bakhshi Jooybari, Hamid Gorji, Ramin Hashemi,
Volume 21, Issue 4 (3-2021)
Abstract
Forming Limit Diagrams (FLDs) are very useful measures for safe forming of sheet metals without failure due to necking or fracture under different loading conditions. This paper uses ductile fracture criteria to predict the formability of low carbon steel sheets to evaluate their accuracy in predicting the FLDs. In addition, the fracture forming limit curves (FFLD) and necking forming limit curves (NFLD) for St12 low-carbon steel have been extracted experimentally and numerically. In the experimental procedure, the Nakazima stretching test was used. In the numerical procedure, by defining six phenomenological ductile fracture criteria in ABAQUS / Explicit finite element software, the failure is predicted and compared with the experimental results. These criteria were calibrated using 6 tests namely as In-plane shear, uniaxial tensile test, circle hole test, notched tension test, plane stress test, and Nakazima stretching test. The results showed that the criteria, which include both the stress triaxiality (η) and Lode parameter (L), provide a more accurate prediction of failure. Also to predict necking during numerical simulation of Nakazima test and also to extract the NFLD, three criteria of the second derivative of major strain, the second derivative of thickness strain and the second derivative of equivalent plastic strain have been used.
Vahid Gholami, F. Taheri-Behrooz, Behnam Memar-Maher,
Volume 21, Issue 4 (3-2021)
Abstract
The sandwich panel is a combination of a soft core and two stiff, high-strength facesheets. In many cases, the connection between the facesheet and the core is considered as a critical point that can damages the integrity of the sandwich structure. In this study, the debonding toughness between the facesheet and the core in sandwich beams with grooved cores made of Kevlar 49/polyester facesheets and polyurethane foam core has been measured experimentally. The values of the strain energy release rate obtained at the onset of crack growth for the tested specimens are in the range of 340 (J/Square meters) and increase with the crack growth up to 500 (J/Square meters). One of the innovations of the present study is to investigate the effect of grooving the core of the sandwich panel on the resistance of the structure to the growth of interfacial cracks. The results show that by placing the groove inside the core of the sandwich panel, the interfacial crack stops during growth by hitting each groove and requires higher force to restart its growth. This phenomenon increases the resistance of this type of structure against the growth of cracks in the face/core area. In this research, a model based on cohesive zone theory was used to simulate crack growth in the tested specimens. Comparison of load-displacement curves obtained from the analysis shows that the proposed model has a good ability to predict the behavior of the structure under similar loading conditions.
Hamidreza Zarei, Payam Shahnazar, Mohammad Meskini, Reza Sarkhosh,
Volume 22, Issue 5 (4-2022)
Abstract
Ultra-High molecular weight polyethylene (UHMWPE) fibers are among the strongest and lightest fibers available and are widely used in high-performance ballistic applications. Despite the great advancement of computational analysis in recent years, precise calculations have not been performed to identify the failure of these fibers due to the complexity of the material behavior to impact. In this research, using the most advanced finite element modeling method of composites (Abaqus-Explicit) has been used to study the composite behavior of these fibers subjected to high-velocity projectile impact. Fiber and matrix are designed using solid elements and 3D Hashin failure criterion was used to determine the behavior of the material. Since this criterion is not available in Abaqus, the VUMAT subroutine has been used to implement this criterion. Velocity diagrams and damage evaluation have been reported. To evaluate and validate this method, six samples of Ultra High Molecular Weight Polyethylene (UHMWPE) Composite panels, consisting of 20 and 45 layers, respectively, were experimentally studied by high-velocity projectiles at different velocities. The simulation results are in good agreement with the experimental results.
Maziar Khademi, Hassan Moslemi Naeini, Mohammad Javad Mirnia,
Volume 22, Issue 7 (7-2022)
Abstract
In this paper, fracture prediction accuracy was evaluated by the GTN ductile fracture criterion and the effect of its evolution. To investigate the stress states, three calibration tests, including uniaxial tension, plane strain tension, and In-plane shear tension, were used to calibrate the failure criterion and determine the accuracy of fracture prediction. For simulation of the fracture behavior in Aluminum 6061-T6, the GTN ductile fracture criterion was calibrated using the combined experimental-simulation method. ABAQUS software was used to simulate the forming process, and fracture criteria were implemented to the software by the VUMAT subroutine. The force-displacement values and the fracture displacement in the experimental tests were used to validate the numerical results and evaluate the fracture criterion accuracy. According to the results, calibration using uniaxial tension and In-plane shear tension tests predicts failure with an average error of 6.17%. While the original GTN criterion cannot predict the fracture of the In-plane shear tension test, the error value in the plane strain tension test reaches 24%. A U- bending test was performed to investigate the fracture behavior of Aluminium 6061-T6 sheets and validate the calibrated fracture criterion in a more complex process other than tension tests. The Extended GTN criterion was found to predict the onset of fracture in the U-bending process with an error of 3%.
Hosein Samadieh Labbaf, Sayyed Hashemi,
Volume 22, Issue 11 (11-2022)
Abstract
The Charpy impact test is an experimental method for determination of materials dynamic properties at different temperatures to investigate the ductile to brittle transition behavior of tested materials. The percentages of ductile and brittle fractures can be evaluated based on fracture area of Charpy specimen (according to API E23 standard) by visual techniques which do not provide exact percentages of these fractures. In this study, a method is proposed to calculate the exact percentage of ductile fractures using image processing, which makes it possible to quantitatively examine different parts of the fracture surface with high accuracy. All steps of image processing are described for eleven Charpy standard specimens of API X70 steel, tested at temperatures between +20 to -80 °C with a temperature increment of 10 °C. In this research, converting a qualitative image of fracture surface to a quantitative matrix is described for the first time. Prediction of the shape of ductile and brittle parts of the fracture surface at temperatures between +20 and -80 °C is one of the results of this study. The percentages of ductile fractures using image processing for temperatures of +20, 0, -20, -40, -40, -60 and -80 °C were obtained as 100, 100, 86, 53, 36 and 0, respectively. The transition temperature was -45 °C for this steel, corresponding of 50% ductile fracture.
Mehrdad Ghadami, Rahmatollah Ghajar,
Volume 22, Issue 12 (12-2022)
Abstract
Once a composite laminate is subjected to quasi-static tensile or fatigue loading, some damage modes initiate and propagate in the laminate. The first damage mode is the matrix crack that forms in the layers with an angle to the loading direction. Although not leading to breakage, these cracks reduce the equivalent mechanical properties of the composite laminate. In this paper, a new nonlinear analytical model is presented and used to predict the stiffness degradation of the cross-ply composite laminates. For this purpose, a new third-order polynomial function is proposed as the Helmholtz free energy of the composite, and the appropriate equations are derived. A microscopic experimental test is designed and accompanied by the analytical model to investigate the damage progression in a glass/epoxy cross-ply laminate. Also, finite-element micromechanical models with periodic boundary conditions (PBC) are proposed and used to determine the damage constants. The model is validated against the 3D micromechanical models and the quasi-static uniaxial loading-unloading experimental tests. The validation shows a very good agreement between the model and the experiments.
Mohammad Sajjad Khodayari, Rahman Seifi,
Volume 23, Issue 4 (3-2023)
Abstract
Thick-walled cylindrical vessels are specially used in oil, chemical, nuclear and military industries in order to withstand internal pressure. The presence of the compressive residual stress in the walls increases the bursting pressure and fatigue life. Autofrettage processes and radial interference in multilayer cylinders are among the conventional methods of creating residual stresses in the pressure vessels. In order to achieve higher strength and fatigue life, the combination of these processes is also considered. J integral method is a suitable criterion for evaluating the crack parameters in elastic and elastoplastic strain fields. In this research, distribution of the J integral along the semi-elliptical crack front on the inner surface of the interferenced two-layered cylinder with closed end has been studied. Inner layer was autofrettaged. Burst pressure was determined based on the fracture toughness criterion (JΙC). Also, the effects of the autofrettage percent, radial interference; depth, angle and aspect ratio of the crack on the J integral and burst pressure variations have been investigated. The inner and outer layers of the cylinder were made of 7075-T6 aluminum alloy. The periodic nonlinear hardening behavior of this alloy has been predicted using Chabooche model. The validity of the results and their accuracy were examined
Amir Kariman Mghaddam, Saeed Rahnama,
Volume 23, Issue 8 (8-2023)
Abstract
The proper method for jointing Carbon fiber reinforced polymers (CFRP) to aluminum, which causes uniform stress distribution, more suitable fatigue performance and weight reduction, is adhesive bonded joint. In adhesive bonding, the interface of adhesives and adherent are sensitive areas for the initiation and propagation of failure. In order to eliminate surface contamination, adherents must be surface treated. In this research, the effect of the functional pattern of laser surface treatment on the strength of aluminum/composite adhesive bonded joint in the mode I fracture has been investigated. At first, laser surface treatments were performed throughout the specimen in order to find the parameters of the laser device that increase the strength of the adhesive bonding by creating a suitable surface quality. After that, the functional pattern of laser surface treatment with the appropriate parameters for ablation and cleaning of the adhesive surface is done. The results show a 15.5% increase in the critical strain energy release rate of the mode I for the all-over laser surface treatment specimen compared to the sanding method. Meanwhile, with the functional pattern of laser surface treatment, the critical strain energy release rate of the mode I has increased by 5.9% and 22.4% compared to all-over laser surface treatment and sanding, respectively. Examining the fracture surface of the specimen shows the delay in crack growth in the specimen of the functional pattern with changes from the adhesive failure to the fiber tearing, which has improved the strength of the adhesive bonding.
Shahrbanou Sayadi, Alireza Baghbanan, Morteza Javadi, Jordi Delgado-Martin, Hamid Hashemalhosseini,
Volume 23, Issue 9 (9-2023)
Abstract
Mode I fracture toughness (KIC) is one of the most important parameters in fracture mechanics, which represents the ability of a material containing a pre-existing defect to resist tensile failure. In this paper, the crack length effect on the mode I fracture toughness of an isotropic homogeneous material was investigated. For this purpose, several disc shaped PPMA samples were loaded in pure tension by performing pseudo-compact tension (pCT) tests. Digital image correlation (DIC) method was utilized to assess and monitor the distribution of the deformation field during the tests. DIC results were also used to compare the effect of crack length on the deformation field variation in samples. Very good agreement was found between the KIC values estimated in this study and those reported in the past for the similar material; indicating that the pCT method is convenient for the assessment of KIC. The experimental results also show that the initial crack length has a tangible impact on KIC, although the magnitude of its influence is closely related to material structure and type. According to the tests results, an increase in the initial crack length leads to increase the ultimate displacement at failure point, decrease the maximum load and the amount of absorbed energy until the moment of failure, and finally decrease the mode I fracture toughness of the material. Results of this study show that the pCT method configuration is useful for testing PMMA and may be useful for testing other materials suitable amenable of molding such as mortar, concrete and ceramics. According to the comparison of the results, in the optimum range of sample diameter, about 50 mm, the initial crack length is suggested between 11.5 to 15.5 mm for the PMMA.
Ali Reza Bahrami, Saeed Asil Gharebaghi,
Volume 24, Issue 1 (12-2023)
Abstract
An improved method for noise reduction from a time series obtained from a chaotic system is presented. This improved method is based on a noise reduction technique presented by Schreiber and Grassberger that has good performance and less complexity compared to other noise reduction methods from chaotic data. Here a global model created using a neural network has been used as a prediction model for chaotic time series. This global prediction model performs better compared to the local prediction model used in the original method. The improved method also takes advantage of the singular spectrum analysis reconstruction technique. Both of these improvements led to a more accurate noise reduction method while preserving the unique properties of the original. The improved method is applied to a time series obtained from the chaotic state of Lorenz equations that is polluted with Gaussian noise. The final results show a 33 percent reduction in mean absolute error values compared to the original method. Also, the error of calculating the correlation dimension from the data has been reduced to 2 percent after applying the improved method.
Mohammad Tavid, Sayyed Hashemi,
Volume 24, Issue 5 (4-2024)
Abstract
Thermomechanical steels are widely used in oil and gas pipelines due to their high toughness and high resitance against crak growth. A large part of the steel pipelines used in the oil and gas industry in Iran is made of API X65 steel. The fluctuations of internal gas pressure in steel pipes can cause fatigue failure and lead to gas leakage and explosion. So, the control of damage initiation and structural integrity of gas pipelines is of great importance. In this study, the S-N curve and the fatigue strength of the base metal of the API X65 steel were estimated by performing fatigue tests. For this purpose, 24 and 25 test specimens along the seam weld in the coil transverse direction, and perpendicular to the seam weld along the coil rolling direction were prepared according to ISO 1143 standard, respectively. All test samples were cut from an spirally welded pipe with 1219mm outside diameter and 14.3mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. The mean curve, characteristic curve, and confidence interval of the results were obtained both in the finite fatigue life range and in the fatigue resistance. The mean endurance limit of the base metal perpendicular to and parallel to the seam seam were 305 and 291 MPa, respectively which were in the range of 0.4 to 0.6 of material tensile strength and above the seam weld endurance limit (258 MPa).
