نشریه مهندسی مکانیک مدرس
سال بیست و یکم شماره 6 (خرداد 1400)

Human ankle-foot gait is the result of a complex interaction between nerves and muscles. A significant number of prosthetic ankle-foots (passive, semi-active, active) have been designed to restore an identical function of a real limb. Excluding passive and semi-active prosthesis who couldn’t 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 that prostheses. In this paper, a combination of an active actuator (an electrical motor) and an passive stimulus (a spring) is utilized, which decrease the needed power and energy, and in addition to walking mode can also support 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 the structure of Series Elastic Actuator differed with the previous structures is selected as the best combination.  In this opted structure, power and energy consumption is dramatically declined 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 chr(chr('39')39chr('39'))s weight is subtracted.

Flatbreads have a small thickness and flat surface and, unlike bulk breads, 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 breads were 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 were suggestive of the 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 that 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 breads 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 was 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 form 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 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 several research work  published in the field of simulating and predicting the surface roughness of machining processes, most of them are limited to turning and milling operations.  A few number of studies concerning the internal turning processes is very limited. Furthermore, the existing publications in this field have implemented statistical approaches which not only clearly lack in generality, but also require a huge amount of experiments. In the current research, the simulation of surface roughness has been investigated by using kinematics and dynamics of the process. Despite the numerous applications of this approach in turning operations, this approach has not applied in the internal turning processes. In order to implement the proposed approach, firstly the insert nose profile of the tool has been measured. Then, the surface profile consisting the periodical component of feed marks has been constructed. In the next step, excessive amount of vibrations imposed by the long boring bar have been measured by an accelerometer, which are then converted to displacements and added to the periodical component of the roughness profile. Results obtained from internal turning experiments show that the developed simulation approach has a maximum error of 19.3% in estimating roughness parameters which can be considered as a reasonably accurate results due to the complicated nature of surface roughness.

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 ARB process on the strength properties of hybrid composite has also been investigated. At first, the ARB process was applied on the AA1050 sheet. Afterwards, 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.

The high amount of nitrogen in the composition of AISI 440C stainless steel reduces the machinability of this steel due to the increase in hardness and generates high heat at the cutting edge and the chip-tool contact area. The use of new cooling technologies such as cryogenic machining using liquid nitrogen LN2, leads to increase the life of the tool and improve the cutting speed compared to other common methods. In this research, the effect of cryogenic machining is investigated on 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 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 has 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.