نشریه مهندسی مکانیک مدرس
سال بیست و یکم شماره 1 (دی 1399)

Slurry casting method is a novel process to produce metal forms, which makes it possible to produce a porous structure with open cell. In the present study, the microstructure and compressive behavior of aluminum foams produced by slurry casting method, under different number of immersion times were investigated. For the production of aluminum foams with different cell sizes, polyurethane preforms with characteristics of 45, 55 and 65 ppi were selected, and after immersing in a slurry having a solid mass of 88% and removing the excess semiliquid mixture, the samples were sintered at 630˚C. The size of polyurethane perform cell as well as the number of immersion times control the microstructure and compression performance of porous structures. The results of the study showed that the portability of porous aluminum increases by decreasing the size of preform cell or increasing the number of immersion times, which leads to thicker strut. In addition, the probability of crack existence, exactly at the corner of structures, decrease via enhancing the thickness of strut. Meanwhile, excessive increase in the number of immersion, i.e. third times, was associated with some closed-cells which results in strain localization and stress concentration. Therefore, the maximum plateau stress as well as the superior energy absorption capacity was observed in the sample having the minimum preform pore sizes which was immersed for two times in the aluminum slurry.

Kerf width (cutting width) is an important quality parameter in the laser cutting process, and if it is less, means higher cutting accuracy and lower cost of materials. In this study, using a fiber laser-cutting machine, which is one of the new generation lasers, the effect of all parameters affecting the kerf width has been investigated. These parameters include laser power (450 to 750 watts), cutting speed (30 to 130 mm/s), focal point position (5 to +5 mm), nozzle standoff (0.6 to 2.5 mm) and gas pressure (1.2 to 1.8 bar), for the cut of stainless steel 316L sheet with a thickness of 0.8 mm. After measuring the kerf width with a special imaging system and analyzing the results with ANOVA, it was found that laser power and gas pressure were directly related to the kerf width and the cutting speed and nozzle standoff were inversely related to the kerf width. Laser focal point position was also determined as the most effective parameter in the formation of the kerf width, which should be on the surface to minimize the kerf width. In this study, it was shown that with the correct adjustment of the parameters, material consumption and cutting accuracy are improved up to 70%. In addition, by using linear regression, the model of kerf width changes with respect to various parameters has been obtained and by comparing its response with the experimental results, acceptable model accuracy has been observed.

The falling and sedimentation of solid particles in liquids occur in many natural and industrial processes such as water and waste water industries, biotechnologies, environmental engineering, marine engineering, etc. This study represents the results of the experimental study of the falling velocity of steel balls in the water channel for different ball diameters (in the range of 8 to 25mm). The tests are done far from the channel walls. Moreover, as a case study, the wall effect on falling velocity of steel ball (i.e. diameter=12mm) is examined. A high-speed camera is used to determine the coordinate of a falling sphere and estimate the ball velocity and drag coefficients. In addition, a numerical method is used to solve the governing equations in comparison with experimental data. Comparing experimental and numerical results for transient and terminal velocities shows the maximum difference of 12 and 4.5% respectively. Experimental drag coefficients have good agreement with other published data. In addition, falling near the wall leads to a negligible effect on velocity but path diversion is observed.

In this paper, the non-linear dynamic behavior of immersed AFM micro cantilever in liquid has been modeled. To increase the accuracy of the theoretical model, all necessary details for cantilever and sample surface have been taken into account. As for the theoretical model, the Timoshenko beam theory which takes the rotatory inertia and shear deformation effects into consideration has been adopted. For modeling the vibrational system, cantilever thickness, cantilever length and breadth, the angle between cantilever and sample surface, normal contact stiffness, lateral contact stiffness, tip height, breadth taper ratio, height taper ratio, time parameter and viscosity of the liquids have been considered. Differential quadrature method (DQM) has been used for solving the differential equations. During the investigation, the softening behavior was observed for all cases. Here, water, methanol, acetone and carbon tetrachloride has been supposed as immersion environments. Results show that increasing the liquid density reduces the resonant frequency. Time variable does not have any considerable effect on the non-linear resonant frequency. Theoretical modeling has been compared for a rectangular AFM cantilever with experimental works in both of the contact and non-contact modes in air and water environments. Results show good agreement.

One of the measurement systems for the identification of modal parameters of the structure is digital video cameras. Modal analysis based on video measurements, despite the many advantages, is associated with some challenges due to its dependence on high contrast markers. In the present study, a new algorithm is presented to use only the measured full-field responses, without additional preparation of the structural surface. This algorithm is phase-based and is implemented using the blind source separation method and motion magnification technique. It uses a multi-scale pyramid analysis technique to extract the full-field spatiotemporal pixel phases. To validate this algorithm, the free and random vibration videos of two cantilever and simple beams with known modal parameters were reconstructed in MATLAB. The average difference between the values identified and the theoretical values for the frequencies of the first to fourth modes is less than 2% and less than 0/1 for damping. The results obtained in this section also confirm the ability of the algorithm to identification closely-spaced modes of the structure. Also, to evaluate the performance of the algorithm in laboratory conditions, a free and random vibration video of an aluminum cantilever beam, prepared in the laboratory using a high-speed camera, is examined. Comparing the results with theoretical values or case study reports shows that using the techniques introduced in this article is a suitable and promising solution to identify the modal parameters of the structure.

Metal/composite hybrid structures, which are a combination of low-density composites with low-cost metallic materials, have significant potential to provide cost-effective energy absorption devices for a variety of applications. In this research, an experimental study was performed in order to investigate the effect of overlapping composite layers on energy absorption and crashworthiness characteristics of aluminum/epoxy hybrid tube reinforced with glass fibers under quasi-static load.  Also, another experimental study is conducted to determine the crash performance of aluminum/composite hybrid tube under static axial crush force. The result is that Hybrid tubes consist of epoxy reinforced with E-glass fiberglass tape overlaps around aluminum tubes with different percentages of overlapping. Quasi-static crash tests are done on aluminum cylindrical and aluminum/composite hybrid cylindrical tubes with 5%, 50%, and 100% overlap and the amount of energy absorption, specific energy absorption, peak crushing force, mean crushing force, crush load efficiency and the percentage of their changes were obtained and compared. Finally, to validate the results in this research, the results of the performed tests were compared with the results of other references and literature in this context.