نشریه مهندسی مکانیک مدرس
سال بیست و سوم شماره 1 (دی 1401)

The use of Additive Manufacturing (AM) techniques in medical science has resulted in a great change in this field, especially in bone tissue engineering. One of these techniques is the Fused Deposition Modeling (FDM) which is used to make bone scaffolds. From view point of bone tissue engineering, bone scaffolds must have acceptable mechanical properties in addition to the required biological properties. In this study, at first the printing parameters including layer height, printing speed and number of filaments in each row were determined and bone scaffolds were made with two different materials polylactic acid (PLA) and polycaprolactone (PCL) and were subjected to the compression tests. The results of Young’s modulus and yield stress analyzed in Design Expert software showed that increasing the layer height reduces the mechanical properties. Also, increasing the number of filaments in each row increases the elastic modulus of the scaffold. For example, for scaffolds made of PLA, the maximum modulus of elasticity belongs to 12 filament scaffolds with a layer height of 0.1, which is equal to 319 MPa, and the minimum elastic modulus belongs to 8 filament scaffolds with a layer height of 0.3, which is equal to 143 MPa. Printing speed for scaffolds made of PLA does not have a significant effect on the Young’s modulus and yield stress. But for scaffolds made of PCL, increasing the printing speed reduces the modulus of elasticity but it doesn’t have a significant effect on yield stress.

In this study, the effect of transverse steps location on hydrodynamic components and the longitudinal stability of the vessel has been investigated. The vessel studied in this research is a planning catamaran, each demi-hull with two transverse steps. At first, vessel resistance with a weight of 5.3 kg within a range of length Froude number of 0.49 to 2.9 in calm water has been calculated. Then, craft behavior was evaluated at displacements of 5.3, 4.6, and 4 Kg using the numerical method. The numerical simulation results have been validated with similar experimental results. The craft in 4 and 5.3 kg weights, in Froude numbers greater than 2.43 and 2.9, respectively, has a Porpoising instability. In order to improve the longitudinal stability of the vessel, the Taguchi test design has been used to determine the optimal location of the transverse steps. The results showed that by placing the transverse steps in the optimum location, the Porpoising instability in the vessel has been resolved. In planing mode, vessel resistance decreased by 12%, 9.5%, and 6.6% in the optimum state of transverse steps compared to the base state for the mentioned weights. In similar conditions, the lift force on the vessel increased by 15, 10, and 7 percent for the mentioned weights, respectively.

The electrical properties of nanostructured piezoelectric materials have attracted the attention of many researchers in the last decade. These features are used in piezoelectric micro-sensors. Mechanical propulsion is usually the result of contact between a piezoelectric surface and a foreign object. In this paper, the effect of mechanical propulsion using an air wave (sound) or vacuum on a silicon diaphragm is investigated. The local stresses created on the diaphragm due to the impact of an air wave have a significant effect on the peak-to-peak voltage of the piezoelectric sensor, which can be measured by measuring changes in this parameter. To investigate this, a micromachined diaphragm of silicon was examined and it was found that fabricating a piezoelectric sensor on a thin and patterned diaphragm could increase the peak-to-peak voltage by about 1.3 times. Detection of these stresses using piezoelectric material layered on the thin and formable diaphragm can act as a piezoelectric microphone or a barometer that the presence of microstructures on the diaphragm will increase their sensitivity.

Fatigue failure is the most common type of failure in structures under oscillatory loading. Fatigue damage in steel gas pipelines is very important due to internal pressure fluctuation. A large part of pipelines in oil and gas industry of Iran are made of thermomechanical steel of grade API X65, made by spiral submerged arc welding. In this study, the stress-life curve and fatigue limit of the spiral weld seam of this steel are determined by fatigue tests. For this purpose, 20 test specimens (12 specimens used in the limited fatigue life zone and 8 specimens used to estimate fatigue strength) according to ISO 1143 standard. All test samples were cut from an actual spirally welded pipe with 1219mm outside diameter and 14.3 mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. Mean curve, confidence interval, and characteristic curve of the results were obtained in the finite fatigue life range using Basquin fatigue model according to ISO 12107 and ASTM E-739 standards. In the fatigue resistance range ISO 12107 standard was used. The mean endurance limit of the seam weld of the tested steel was 258.5 MPa which is located in the conventional range of 0.4 to 0.6 of the ultimate tensile strength of this steel.

Energy absorbers are used to absorb the kinetic energy of objects and convert it into another form, the most important of which are cylinder thin-walled tubes. In a thin-walled cylindrical absorber, the three parameters of diameter, thickness, and length affect the amount of energy absorption. In this research, to obtain the necessary information for designing an inexpensive energy absorber with high absorption capability, thin-walled cylinders with air pressure inside which the air inside condenses when collapsing have been investigated. In the current study, dynamic and axial loadings were chosen to have a higher match with reality. The simulation and analysis of the problem have been done by the finite element method and by applying Johnson-Cook coefficients to model the material's behavior. In the following, the graph of the total work done with time is extracted as the output of the problem, and its correctness has been proved by experimental tests. Then, different samples were modeled and based on them, the method of design of the experiment was applied. Using the results of the variance analysis, the absorber's optimal parameters have been designed by using the time evolutionary optimization algorithm. The results show that it is possible to reduce the weight of the absorber by creating internal density without lowering the absorbency.

In this paper, the error of tool deflection in perpendicular to a milling process feed direction is investigated and novel compensation method to this error is proposed. While the end mill tool is machining, due to the existence of perpendicular disturbing force on the feed path while the endmill tool is machining, a deflection which reduces machining accuracy. If a compensation force is exerted to the middle of tool, the deflection can be reduced. By using a hydraulic actuator, the compensation force would be proportional to the disturbing machining force, in the opposite direction so that this error can be reduced. It is necessary to estimate disturbing forces and tool deflection magnitude in lateral side of machining precisely before applying the proportional force to the endmill. The first step endmill is modeled on SolidWorks and in the next step The machining operation is simulated to calculate the error causing force in which both the milling tool and the workpiece are 3D flexible. By obtaining the force results of the tool under different machining modes from Abaqus, a semi -analytical model is established on Simscape Multibody module of matlab software. By comparing Abaqus results the parameters of the lumped model are adjusted. The output force is extracted from Abaqus and put in tabular form the tool deflection is extracted from MATLAB SimScape which calculation speed is faster than the numerical model in this method. To find the compensating force, the beam theory obtains a factor of 3.2 times the machining force applied to the middle of the tool. This force is applied in open loop to the MATLAB model and the result indicates almost 70 percent reduction in the tool tip lateral deflection.