جستجوی مقالات مرتبط با کلیدواژه « residual stress » در نشریات گروه « مکانیک »

The significance of sheet metal drilling within industrial applications is widely recognized, particularly for its critical role in creating durable connections in sheet components. This process, especially when implemented through friction drilling, not only minimizes material waste by generating connecting flanges bush shape but also enhances the robustness of the connections. In this study, we used the ABAQUS finite element software (FEM) to simulate and design the friction drilling process on sheets of three different thicknesses, employing three rotational speeds and three feed rates. The analysis was conducted using Design Expert statistical software, which integrated response surface methodology (RSM) and data analysis to thoroughly investigate the process forces. Aluminum AA 7075 was selected for the study due to its extensive use in the automotive, manufacturing, military, and aerospace organizations, highlighting its industrial relevance. The findings suggest that optimizing the machining force leads to more favorable conditions within the process. The optimal parameters were determined to include the highest rotational speed, the lowest feed rate, and the smallest sheet thickness. These conditions promote higher temperatures that facilitate better material flow. An increase in thickness was also found to correlate with higher temperatures. Additionally, the study addressed the significant industrial challenge of residual stresses and their effects on component defects, a topic not previously explored through finite element simulations in this context. This research contributes new insights into the implications of residual stresses in various directions for friction drilling processes.

Wire EDM (WireCut) is a machining process utilizing electric discharge to gradually cut a workpiece with a spark-generating thin wire. It offers benefits like non-contact cutting, versatility with materials, and precise dimensional accuracy. However, it is important to note that surfaces cut by WireCut process may exhibit tensile residual stresses with significant values in the surface layers. Ultrasonic Peening Treatment (UPT) can be employed as a new approach to modify the surface structure of wire EDMed workpieces. UPT, which induces compression in the surface layers of the workpiece, generates compressive residual stress and enhances mechanical properties including hardness. In this study, UPT was performed on wire EDMed workpieces made of alloy steel Mo40 (DIN 1.7225) using an ultrasonic vibrating tool on a lathe, at 3 levels: finishing, semi-finishing, and roughing. The impact of parameters such as the number of passes, feeding rate, and the type of wire EDM cut on the constant amplitude of vibration were studied to understand their effect on the residual stress generated. The experiments conducted using the Taguchi method revealed that in every case, this process resulted in the conversion of tensile residual stresses into substantial compressive residual stresses in wire EDMed workpieces. The compressive residual stress also reached 1654 MPa. More passes lead to higher compressive residual stress, while a higher feeding rate leads to a decrease. Surface hardness increased by 30%, 31%, and 48% for the semi-finishing, finishing, and roughing workpieces in the initial pass. This trend continued with each subsequent pass.Key words: Residual stress - Surface hardness -Wire Electric Discharge Machining (WEDM) - Ultrasonic Peening Treatment (UPT) - Mo40 alloy steel

Shot peening is a cold working process that is used to increase the fatigue life of metal parts by creating compressive residual stress on their surface. The experimental investigation of the parameters of this process is very difficult and expensive, therefore, the finite element method is usually used to simulate and investigate the parameters of this process. However, most of the existing finite element models are not able to describe the real and random movement of the flow of particles and are also limited in terms of modeling the number of particles. Therefore, in this research, in the first stage, a finite element model of the shot pinning process is presented, which is capable of simulating the random and numerous states of this process, and in the next stage, the effect of different parameters of this process such as the size, velocity and angle of throwing the balls on the residual stress is investigated. the result of these investigations led to finding the optimal values of these parameters, so that the optimal value for the diameter of the shots was 1.5 mm, for the velocity of the shots was equal to 100 m/s and for the throwing angle of the shots was equal to 90 degrees. Also, the increase in the diameter of the shots had the greatest effect and the increase in the launch angle of the shots had the least effect in increasing the amount and depth of the residual stress. Also, we were able to numerically measure the important parameter " Shot peening intensity" which is usually measured experimentally by Almen gauge using this model. The obtained results were in good agreement with the experimental results obtained from the work of other researchers and therefore the validity of the presented model can be assured.

The vibration stress relief process is currently utilized as a suitable alternative to the thermal stress relief method to reduce the residual stress of various alloys. In this study, a theoretical model based on the analytical equations is presented. The proposed model including the frequency, amplitude and the process time have been revised compared to the previously proposed models to make it more comprehensible and applicable. Thus, the assumed parameters including the number of cycles (duration), the strain rate (loading frequency), and the amplitude (loading value) are embedded in the model. Experimental tests measuring residual stress distribution by X-ray diffraction method are conducted on specific spots to compare the results with the experiments. An acceptable range of error (below 10%) has been observed between the theoretical and experimental results. According to the obtained results, the model has an acceptable performance for residual stress calculation after the process. According to the results, vibration amplitude has been about 19% more effective than the other parameters. In addition, increasing the amplitude has increased the affirmative effect of the frequency parameter by 38% compared to the other parameters. To further examine the presented model, the variations of the stress rate has been analyzed versus each influential parameter.

Induction of compressive residual stress on the surface thick-walled cylinder is a way to increase the resistance to failure and increase the fatigue life. The shrink-fit process is one of these methods. In this paper, the three-layer shrink fit for the construction of the hot isostatic press cylinders has been analyzed using experiments, analytical modeling, and finite element simulation. The stresses created due to the shrink fit process in cylinders were investigated using analytical calculation and finite element simulation. To perform the shrink fit experimentally, three cylinders were made of VCN steel and the shrink fit operation was performed. Based on the results from the finite element simulation, the maximum radial displacement for the outer cylinder is 0.89 mm and the residual stress value is 39.9 MPa. Also, the comparison of simulation results and analytical method for the middle and outer cylinder shows that the differences were 2.3% and 2.8%, respectively.

Laser shock peening is a surface optimization operation that leads to the improvement of mechanical properties such as increasing fatigue strength, surface hardness and tensile strength. This improvement of properties can be attributed to the creation of residual stress and cold work done on the surface. In this new method, which is considered as an alternative to the methods of peening and ultrasonic peening waves, there are many influential factors such as power density, percentage of overlap, type of protective layer and number of shocks, etc. In this research, Al2014-T6 alloy, which is widely used in aviation industries, including aircraft wheels, has been subjected to shock peening in order to increase its strength and fatigue life. Single, triple and quintuple shock processes were applied on this alloy to investigate the effect of different shock modes. Next, the amount of residual stress was analyzed with the help of X-ray diffraction analysis and changes in surface roughness by atomic force microscope, and then fatigue, tensile and microhardness tests were performed on the part. The results of the experiments show that with the increase in the number of times of laser shock and under the condition of no melting and surface destruction, the residual compressive stress increases, which leads to a 300% increase in fatigue life and a 25% increase in microhardness of the surface. It was also found that in addition to optimizing the parameters, increasing the shock frequency can also change the surface roughness and tensile strength.

Additive manufacturing methods have become very popular in recent years due to their many capabilities, including the optimal use of materials and their use in constructing complex structures. One of the most important methods in these processes is the selective laser melting (SLM) process. In previous research, measuring the residual stress distribution of Inconel 625 samples under the SLM process has not been investigated. Therefore, in this research, the SLM process was carried out on the Inconel 625 sample using the finite element simulation method, and then the residual stress distribution in the sample obtained from the process was investigated in three main directions. Also, the formation of the molten pool, its dimensions, and temperature distribution were investigated. The results showed that tensile residual stresses were formed in the center of the layer and compressive residual stresses were more present at the edges of the sample. Also, the maximum of the residual stresses were formed in the axial direction and their minimum appeared in the layer thickness direction. The results related to the effect of process parameters on the residual stress distribution showed that with increasing laser power, tensile and compressive residual stresses increase in both axial and hoop directions, but decreasing the scanning speed does not give us accurate information about the increase or decrease of these stresses. For validation, the results of the finite element method were compared with the results of other researchers. The obtained difference was 13.97%. Therefore a good agreement existed between them.

Fatigue Crack Growth is an important physical phenomenon and one of the damage mechanisms in engineering materials. This damage mechanism is intensified in the presence of tensile residual stresses. In contrast, compressive residual stresses delay the fatigue crack growth process. In this study, The effects of tensile residual stresses are studied on fatigue crack growth in the titanium alloy Ti-6Al-4V specimens. Mechanical residual stresses are induced in the samples using the 4-point bending method and measured using the hole drilling technique. Fatigue crack growth tests are conducted on SEN(b) specimens according to the ASTM-E647 standard, using samples with and without residual stresses. The commercial finite element software ABAQUS was used to study the distribution of residual stresses, the plastic zone sizes at the crack front and fracture mechanics parameters. The finite element results were in good agreement with the experimental results. The results reveal that the presence of tensile residual stress may accelerate the fatigue crack growth rate in the specimens. This increase in fatigue crack growth rate can reduce the fatigue life of samples up to 50%.

In the present study, in addition to reviewing the severe plastic deformation (SPD) processes and their mechanisms in the microstructural evolution, previous researches on the corrosion behavior of various metallic materials that have undergone SPD are investigated. Grain size, as an important metallurgical parameter, affects a broad range of mechanical, chemical, physical and electrochemical properties of metallic materials. As mostly reported, the reduction in grain size is in line with a considerable improvement in the mentioned properties. In recent years, SPD methods have been nominated as an ideal way to reach nano- and ultrafine-grains. Among the mentioned properties, the corrosion performance of ultrafine-grained materials produced by SPD methods is still controversial for researchers. The purpose of this study is to review the SPD methods, the mechanism of grain refinement during them, and their effects on the corrosion behavior of various metals and engineering alloys such as steels, aluminum, titanium, copper, and magnesium.

Welding is very important in the aerospace industry and is widely used in aerospace structures. One of the problems that most industries are facing is created residual stress by the welding process. Residual stresses in the surrounding areas of welding can cause cracks and crack growth so identifying and evaluating residual stresses in the welded structures is necessary. There are different methods for determining residual stress. In this paper, laboratory and numerical methods were presented for determining the residual stress. Then, the welding process of two aluminum sheets of 6061-T6 alloy has been done and the residual stresses have been obtained by drilling method. Welding is done in two passes and by spot welding, the first, the end and the middle of the weld line are connected to prevent the sheets from moving. Also, the welding process of the two aluminum sheets was simulated in 3D in the ABAQUS finite element software and the residual stresses were extracted. All conditions in the finite element analysis are similar to the welding conditions in the laboratory. Results show high accuracy in the modeling of finite element processes in the welding process. Finally, in the finite element method, thermal stress is modeled and optimal parameters are extracted for the above operations.

In this paper, the combination of digital image correlation and hole drilling methods has been used to investigate and measure the residual stress of polymer components. The bending stress was created by a three-point bending system and measured by the provided equipment. These stresses are considered in theory to be equivalent to the residual stresses within the component. Digital image correlation method utilized as a novel method to measuring released strain. In this regard, Plexiglas polymer materials in various thicknesses were analyzed under the different values of 3-point bending test. Finally, the obtained results from the introduced method in this paper, with the results of Finite element analysis and simulation have been compared and validated. Unlike the samples that have an unknown amount of residual stresses, the residual stresses in these specimens have a certain amount that can be compared and verified with the simulation and analytical results. Polymer materials have a higher released strain range than metals and it is possible to investigate the strain released by digital image correlation. The results show that the combination of digital image correlation and hole drilling techniques as innovations presented in this paper are capable of measuring residual stresses with a high accuracy less than 10% and also because of their major advantages. The method of using strain gauge would be a suitable alternative to the hole drilling whit strain gauge method for measuring residual stress in polymer samples.

Refineries and petrochemical industries, due to the presence of all-metal equipment and acidic environment, have high corrosion. This study investigated the effect of ultrasonic shock on the corrosion behavior of 316 stainless steels in order to the increasing use of stainless steels in this industries. The aim of this study was to investigate the effect of ultrasonic stress relief method on increasing the strength of parts in corrosive environments. It should be noted that the residual stress in the samples is created by welding. In this study, first the considered sample is De-tensioning using ultrasonic vibration at a frequency of 20 kHz, then the obtained results are compared with the results of samples without De-tensioning operation and also thermal De-tensioning operation. XRD method used to measure the residual stress. The obtained results show that in ultrasonic method and heat treatment, the amount of residual stress is reduced by 58.7% and 54.3%, respectively. It has also been observed that the use of ultrasonic waves has increased the life of the sample in a corrosive environment.