Modares Mechanical Engineering
Year:2021 | Volume:21 | Issue:6|Papers Count:6

Human ankle-foot gait is the result of a complex interaction between nerves and muscles. A significant number of prosthetic ankles-foots (passive, semi-active, active) have been designed to restore an identical function of a real limb. Excluding passive and semi-active prosthesis, that cannot generate any positive work, one of the biggest challenges in creating these prostheses is providing the needed power and energy during movement. Supplying this power and energy, requires a high-torque and high-power actuator having high weight, thereby causing a dramatic increase in the weight and size of the resulted prosthesis. In this paper, a combination of an active actuator (an electrical motor) and a passive stimulus (a spring) is utilized, which can decrease the needed power and energy. This design does not only support walking mode, but also provides running mode up to 2. 5m/s. Accordingly, the first stage of this article includes mechanical modeling of the ankle and evaluation of efficiency and power consumption in all presented models. Then, a combination of spring and series actuator is selected as the best and optimal combination, which is different from the previous structures. In this structure, power and energy consumption are dramatically reduced up to 58% and 26% in walking mode and 64% and 57% in running mode. Consequently, a lighter motor and battery can supply the required power, so the prosthesis’ s weight is decreased. Thus, in order to optimize power and energy consumption, it is necessary to determine the optimal spring stiffness based on design priorities (maximum power reduction). In the last step, the most suitable segments are developed for receiving the best performance, and then the designed prosthetic ankle is built. The results of simulations and experimental tests show that the produced prosthesis has all the desired performance, and it uses less energy and power while its user is moving.

Flatbreads have a small thickness and flat surface, unlike bulk bread, and there are only used in few countries such as Iran. Given the lack of research on this type of bread, the objective of this study was to numerically and experimentally investigate the porosity and surface brightness of flatbreads baked under different ascending step heat flux profiles. In order to investigate the effect of different heat flux profiles on the baking process of porous flatbread, the required bread was baked in a laboratory-scale device, and the equations of heat and mass transfer, porosity, and brightness were numerically modeled using the finite difference method in MATLAB. The results showed that there were suggestive of a good consistency between the experimental and numerical values. A total of 3 ascending step heat flux profiles with different time steps was experimentally and numerically investigated. The results also showed that the porosity of the bread baked under an ascending heat flux profile with 4 steps was more regular, and the average diameter and area of porosities obtained from the SEM imaging were higher in this profile compared to the other profiles. Moreover, based on the experimental and numerical results, the surface bright parameter (L*) was within the appropriate range for the bread baked under the ascending heat flux profile with 4 steps.

Offshore structures are widely used in the oil and gas industry. Since construction and maintenance costs in these structures are very expensive, therefore, it seems necessary to find cost-effective methods to repair and strengthen these structures, especially in the offshore industry. Ultrasonic peening is one of the mechanical stress relief methods used to increase fatigue strength and corrosion resistance in welded joints. In this paper, this method was implemented in both air and underwater atmosphere, and the treatment effect on fatigue strength was investigated, also surface hardness and surface cracks in X52-5L welded steel were investigated. The effect of peening on this type of steel has been compared in both underwater and outdoor conditions. The results of the fatigue test showed that the fatigue life of underwater peened specimens was improved by approximately %25. 2 and it changes from 42383 cycle up to 50083 cycle while in welded specimens with the same condition and peened in air condition, there is about %10. 5 increase in fatigue life. Surface hardness also increases by %14 in underwater peened samples and by %12 in air-peened samples. SEM microscopy and observation of surface micro cracks also showed a reduction of surface cracks after peening. Subsequently and loss of stress concentration was shown. It can be concluded that underwater peening increases the fatigue strength and hardness of X52-5L steel compared to air peening.

Although there have been many studies published in the field of simulating and predicting surface roughness of machining processes, they are chiefly related to turning and milling operations, and the number of studies concerning the internal turning process is excessively limited. Furthermore, the existing publications in this field have mostly implemented statistical approaches which not only lack generalizability, but also require a huge number of experiments. In the current research, a simulation method of surface roughness is developed by using kinematics and dynamics of the process. Despite the numerous applications of this approach in turning operations, it has not yet been used in the internal turning processes. The first stage of this approach is to measure the insert nose profile of the tool. Then, the surface profile consisting the periodical component of feed marks should get constructed. In the next step, the vibrations imposed by the long boring bar are measured by an accelerometer. In order to factor in the effect of vibration on the final profile, the recorded vibrations should then get twice integrated to attain the displacement signal and added to the periodical component of the roughness profile. Results obtained from internal turning experiments in order to validate this method revealed that the developed simulation approach has a maximum error of 19. 3% in estimating roughness parameters which can be presumed as accurate enough.

The aim of this study is to improve the strength properties of glass-aluminum multilayer hybrid composite using AA1050 aluminum sheets processed by the accumulative roll bonding (ARB) process. Also, the effect of different cycles of the ARB process on the strength properties of hybrid composite has been investigated. At first, the ARB process was applied on the AA1050 sheet. Afterward, the microstructure and tensile properties of the ARB deformed sheets were investigated. Then, the ARB processed AA1050 sheets were used to make glass reinforced aluminum laminate (GLARE). In the end, the tensile properties of the GLARE composite were examined. By the progress of the ARB process, the hardness and strength of the sheet increased. The elongation of the first cycle processed specimens dropped drastically. But, by increasing the process cycles, the elongation increased gradually. The use of the ARB processed aluminum sheet in the manufacture of GLARE composite significantly improved the tensile strength of the GLARE. In the GLARE made of annealed aluminum, most of the elongation of the aluminum layer occurred after the breaking of the glass fibers and in conditions outside the GLARE composite; as a result, the reduction of the sheet elongation during the ARB process caused the simultaneous failure of the metal layers and the glass fibers during the tensile test of the GLARE. Hence, this event did not reduce the ductility of the composite. In other words, the total energy absorption and fracture toughness of the aluminum layers occurred when the GLARE had not failed.

Nowadays, Cryogenic Machining has received a lot of attention due to the increasing demand for environmentally friendly production processes. Many studies show that the use of cryogenic machining using liquid nitrogen (LN2) leads to an increase in tool life, improved shear speed, and material extraction rate compared to other conventional cooling methods such as shear fluid and MQL. In this research, the effect of cryogenic machining is investigated on the roughness and hardness of AISI 440C stainless steel. The results showed that using cryogenic machining, the surface roughness of AISI 440C stainless steel was significantly reduced even at a high feed rate. The morphology of the chips showed that with the use of cryogenic machining, the surface burn and the accumulation of materials on the inner surface of the chips have been significantly reduced. In this study, it was found that in cryogenic machining, the tool wear pattern is the erosion pit on the chip surface and has increased the tool life by 400% compared to the dry machining.