نشریه مهندسی مکانیک مدرس
سال بیست و دوم شماره 7 (تیر 1401)

This paper focuses on 3-D dynamic modeling of a transport quadrotor with cable in the presence of load generating stochastic forces. In many practical cases, we are dealing with payloads that generate oscillating forces with random amplitude and frequency by their inherent dynamics or interacting with the environment. These forces can greatly affect the dynamics of the transport system. The innovation of the present study is to consider these forces and show their impact on quadrotor’s dynamics. Furthermore, in order to make the model more compatible with the real system, it is assumed that the connection point of cable and quadrotor is not located on the center of gravity of flying vehicle and the cable's inertia is considered in the modeling process. To this end, the systems’ equations of motion are derived based on both Euler-Lagrange and Newton-Euler methods. After obtaining equations of motion, the verification of the model is investigated in three steps. In the first step, by performing simulation, the validity of the nonlinear model is investigated and dynamical analysis is expressed. In the next step, by evaluating the structure of the model, it is shown that the equations of motion satisfy some structural features of a desirable dynamic model. Then, by setting some parameters and variables to zero, it is shown that simpler models provided in some references can be achieved from this comprehensive model. At the end, the conclusion of this work is presented.

In this study, the effect of fiber presence in continuous fiber reinforced Fused Deposition Modeling samples (FDM) on the stress and residual strain created during the process was investigated. The FDM process has become one of the most widely used Additive Manufacturing methods for layered prototypes from a three-dimensional model. One of the most important issues in this process is the distortion of parts produced during printing. The distortion created is mainly due to the rapid cycles of melting and solidification of the material, which produce residual stresses in the sample. The main objective of this study was to measure the residual strain rate of residual stress in unreinforced and reinforced PLA samples with continuous fiber using digital image correlation and hole drilling technique. Digital imaging is one of the novel non-contact optical methods for measuring displacements, detecting defects and investigating the properties of components. Among the various optical methods, digital image correlation is superior to other optical methods due to its low cost, high speed and no need for phase analysis. According to the results, the maximum strain in the fiber reinforced specimen in the x and y directions was 1.09 and 0.34%, respectively. The strains released in the reinforced specimen were higher than other specimen at all stages of drilling.

The microstructure characteristics of the material have a significant effect on the results of plastic deformation processes. In this research, the effect of the coarse and fine-grained microstructure on the microhardness and surface quality in the roller burnishing process has been scrutinized. To facilitate comparison of the results, the input parameters including the size of the workpieces, speed, feed rate, the number of passes, the penetration depth, and burnishing tool were selected the same in all experiments. The results revealed that before the surface devastation, the arithmetic average of surface roughness of the coarse-grained microstructure decreased more than the fine-grained microstructure. Moreover, the penetration depth of the burnishing tool in the coarse-grained microstructure is more than the fine-grained, which indicates its proper ductility. With increasing the number of passes in the roller burnishing process, the surface microhardness of coarse and fine-grained microstructures has gradually increased and in all cases, the microhardness of the fine-grained microstructure is higher than the coarse-grained microstructure. Measurement of sub-surface hardness values shows that the microhardness in coarse-grained microstructure has increased to a greater depth than the fine-grained structure.

In this paper, by using the Chaboche kinematic hardening model with the isotropic hardening law, the effect of temperature and bending moments is investigated on the strain accumulation behavior of carbon steel piping branch. Carbon steel branch junctions under internal pressure and temperature with dynamic bending moment are tested at five temperatures of 20, 50, 100, 150 and 200 ° C. The results obtained by numerical analysis show that the highest amount of ratcheting occurred near the branch junctions in the circumferential direction. The strain ratcheting occurred mainly because of dynamic moments and high temperatures. The results show that in all three samples, the amount of strain ratcheting increases with increasing of dynamic moment level and temperature. With increasing of the ratio of diameter to thickness in branch junctions, the onset of strain accumulation occurs at low moment levels. It can be concluded that initially, the rate of strain ratcheting is high and with the increase of loading cycles, this rate decreased due to the strain hardening phenomenon. The increase of strain ratcheting at high temperatures is due creep strain because of high temperature and mainly accumulated plastic strain under dynamic bending moments because of cyclic plasticity.

In the design of the combustion chamber, various parameters should be considered. These parameters include uniform temperature distribution at the outlet of the chamber, more flame stability, lower pollution, higher combustion efficiency, lower wall temperature, and lower pressure drop in the chamber. Regarding to the complex condition of the flow in the combustion chamber due to the various effects of turbulence and mixing of flows as well as the behavior of turbulent flames, predicting the performance of flow in the combustion chambers is very complicated. In this paper, it is tried to study and optimize the combustion chamber of Amirkabir University of Technology in terms of swirler. It is done by using the numerical method and finally the selected swirler in the numerical method is tested in the experimental setup to investigate optimization method .According to the studies, swirler with an angle of 60 degrees, 12 blades, and a thickness of 0.75 mm is selected as the final case. In the experimental results, the amount of CO pollution has significantly reduced. The output temperature, the pattern factor and unburned hydrocarbon have reduced in the final case. However, the temperature uniformity inside the chamber has increased.

In this paper, fracture prediction accuracy was evaluated by the GTN ductile fracture criterion and the effect of its evolution. To investigate the stress states, three calibration tests, including uniaxial tension, plane strain tension, and In-plane shear tension, were used to calibrate the failure criterion and determine the accuracy of fracture prediction. For simulation of the fracture behavior in Aluminum 6061-T6, the GTN ductile fracture criterion was calibrated using the combined experimental-simulation method. ABAQUS software was used to simulate the forming process, and fracture criteria were implemented to the software by the VUMAT subroutine. The force-displacement values and the fracture displacement in the experimental tests were used to validate the numerical results and evaluate the fracture criterion accuracy. According to the results, calibration using uniaxial tension and In-plane shear tension tests predicts failure with an average error of 6.17%. While the original GTN criterion cannot predict the fracture of the In-plane shear tension test, the error value in the plane strain tension test reaches 24%. A U- bending test was performed to investigate the fracture behavior of Aluminium 6061-T6 sheets and validate the calibrated fracture criterion in a more complex process other than tension tests. The Extended GTN criterion was found to predict the onset of fracture in the U-bending process with an error of 3%.