نشریه مهندسی مکانیک مدرس
سال بیست و سوم شماره 9 (شهریور 1402)

In present study, an improved severe plastic deformation process named improved tube cyclic expansion extrusion process has been introduced. The idea of this process is taken from the conventional tube cyclic expansion extrusion process, and in this novel process, it is tried to solve some important problems of the conventional process. Improved tube cyclic expansion extrusion process is capable of severe plastic deforming and improving microstructure and mechanical properties of tubular components. Also, this process can be considered for producing relatively long tubes. For this purpose, the improved tube cyclic expansion extrusion process was successfully performed on AZ91 magnesium alloy tubes, up to two passes. Then, the microstructure evolution and the mechanical properties improvement were scrutinized. The results showed that the microstructure and mechanical properties were improved considerably. In this way, after two passes of this process, an ultrafine grained (UFG) microstructure was formed, and the values of ultimate strength (UTS), hardness (Hv) and ductility (EL%) became 3.6, 1.83 and 1.8 times higher, respectively. Also, the comparison of the results of the improved tube cyclic expansion extrusion process with those of the conventional tube cyclic expansion extrusion process indicated that ultimate strength and hardness of the improved process were near to those of the conventional process, but the value of elongation to failure of the improved process is considerably higher than the value of the conventional process. This can be considered as one of the important advantages of the improved process over the conventional process.

In addition to the need for lightweight properties, the metallic bipolar plates in the PEM fuel cells should work in a humid and acidic environment. Due to its low density and excellent corrosion resistance, titanium is a proper candidate for manufacturing bipolar plates. In this paper, the manufacturing of bipolar plates made of commercially pure titanium with an initial thickness of 0.1 mm was investigated using the stamping process. A four-channel die with a parallel flow field was used in the experiments. To estimate the formability of microchannels of the bipolar plates, the response surface method, genetic algorithm, and adaptive neural fuzzy inference system were employed. Die clearance, stamping speed, and friction coefficient between the sheet and die were considered input variables, whereas the die filling rate was as output. The designed experiments using the response surface method were used to train the meta-heuristic techniques. The results showed that the regression model obtained from the response surface method predicts the die filling rate with acceptable accuracy. Furthermore, the coefficients of the equation obtained from the regression have been improved using the genetic algorithm and the error rate has been reduced by about 53%. Finally, an adaptive neural fuzzy inference system was used to predict the die filling. The results showed that the proposed system is very feasible and approximates the maximum filling rate with high accuracy.

In this research, energy absorption in sandwich panel structures with honeycomb core and skin made of glass-epoxy multi-layer composite, aluminum, and two-dimensional glass fabrics impregnated with shear-thickening fluid under low-velocity impact loading has been investigated. Polyethylene glycol 400 and hydrophilic fumed silica with nanometer dimensions were used to make the shear thickening fluid, and then a rheometer was used to verify the rheological properties of the fluid. The test was carried out at two heights of 100 and 500 mm with a drop weight device. The sandwich panel honeycomb core has been tested once filled with shear thickening fluid and again empty of shear thickening fluid. The results of the rheology test show that the viscosity value increases with the increase of the shear rate. The impact test results show an increase in energy absorption in the structure of sandwich panels filled with shear thickening fluid compared to empty sandwich panels. At the drop height of 500 mm, the absorption of specific energy in the sandwich panel structure with a skin made of two-dimensional glass fabrics impregnated with shear thickening fluid, the ratio of sandwich panel with a skin made of aluminum and composite increased by 27.93, and 9.47%, respectively, and energy absorption in The sandwich panel with composite skin has increased by 16.86% compared to the sandwich panel with aluminum skin.

  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. According to the comparisson of the results, in the optimum range of sample diameter, the initial crack length is suggested between 11.5 to 15.5 mm for PMMA.

Heart diseases are one of the most important causes of death in the world, and their treatment is very important from a medical and financial perspective. One of the effective ways that can be very useful in the treatment of cardiovascular diseases is computational modeling which can help medical professionals to better understand the human heart and provide more effective therapeutic approaches. The mechanical characteristics of the myocardium of human heart, known as a nonlinear and anisotropic tissue, are the most important part of the heart because it plays a key role in myocardial response to loading and unloading during heart cycle. In this study, the orthotropic hyperelastic and isotropic viscoelastic properties of the human heart were modeled by taking into account the effect of active stress on myocardium and using an idealized left ventricular geometry. Simulation results showed that the viscoelastic property cause the myocardium deformation to be damped and reduces the amount of torsion that experienced by the tissue. Also, myocardium tissue in viscoelastic case showed the hysteresis phenomenon which is found in clinical observations of heart mechanics. The Model is entirely implemented in COMSOL Multiphysics finite element software and can be used in heart electromechanical models in future studies.

Polymers are used in a wide range of industries. In this research, the mechanical behavior of polymers used in the glass industry has been studied. The investigated polymers included thermoplastic polyurethane (TPU), polyvinyl butyral (PVB) and sentry glas (SG). These polymers were subjected to tension and compression tests at different strain rates from 0.001 to 0.25 s-1. Also, the mechanical dynamic properties of the polymers were extracted using the mechanical dynamic analysis test at a constant frequency. The tensile test results showed that the mechanical behavior of polyurethane is not dependent on strain rate, but SG is highly sensitive to strain rate. Also, with increasing strain rate, the fracture stress of SG decreased drastically. The pressure test results showed that TPU can withstand more stress. The glass transition temperature of TPU was lower than the other two polymers. Overall, it can be concluded that among the polymers studied in this research, TPU had better mechanical behavior.