نشریه مهندسی مکانیک مدرس
سال بیست و یکم شماره 2 (بهمن 1399)

In this study, the mechanical properties of poly lactic acid samples produced by FDM 3D printing technique were investigated. The 3D printing process parameters were optimized using design of experiment (DOE) Taguchi approach for achieving the optimum mechanical performance. In this regard, infill percentage (at three levels of 30, 50, and 70%), raster angle (at three states of 0/90, -30/30, and -45/45 degree), and layer thickness (at three levels of 200, 250, and 300 µm) were considered as process parameters for optimization procedure. Their effects on density (as porosity degree), impact strength (as mechanical property), and specific impact strength (the impact strength to density ratio) were investigated. Analysis of variance (ANOVA) was utilized to find the most effective processing parameters. The findings revealed that the infill percentage was the most effective parameter on the density and the impact strength. The density and the impact strength were reduced with the decrease of the infill percentage. These decrements were in a way that their ratio, specific impact strength, was almost constant. The layer thickness had the most influence on the specific impact strength. The specific impact strength was improved by reducing the layer thickness due to the raster entanglement. The optimum conditions to achieve the highest mechanical performance were the raster angle of 30/-30 degree and the layer thickness of 200 µ. The optimum infill percentage depended on the application.

To determination of equal-channel angular pressing(ECAP) process on the stress-strain behavior of  steel core of steel/copper bimetal and also effect of Cu-shell thickness on the created surface stretch during ECAP, the bimetallic samples composed of steel rods with 8 mm diameter and copper shells with 0.75 mm thickness are prepared. The both bimetallic samples and steel rods with 9.5 mm are subjected to consecutive ECAP process using die with inner angle 90o and an outer curvature corner angle of 30o. The applied load and punch displacement are recorded during samples passing through an ECAP die. The tensile testing is carried out on both the initial and ECAPed series. Moreover, dependence of surface stretch to diameters, shell thickness and strength properties of constituents of core/shell bimetallic rods is analytically modeled. Then, the finite element method(FEM) is used to investigate the effect of Cu-shell thickness. The obtained results revealed that the ultimate tensile strength of bimetallic core and steel rods are improved approximately 60% and 108% by ECAP deformation, respectively. The applied punch load for passing of bimetallic sample through an ECAP die is 54% less than the ones for steel rod. According to the FEM results, the maximum value of surface stretch is linearly decreased with increasing the thickness of copper shell. The obtained results show a good agreement between the analytical model and FEM approach.

Memory-shaped polymers are thermally induced subsets of intelligent materials that require thermomechanical behavior to accurately understand their function. In this study, polymeric memory scaffolds were fabricated using polylactic acid by molten labeling method in three reticular honeycomb, rhombic and elliptical form. Parameters such as longitudinal dimensions and wall thickness of printed scaffolds compared to the designed scaffolds and solutions were presented to enhance the printing accuracy. Built-in scaffolds can be a good option for stent use. Formal memory properties experiments with 30% axial strain were performed to study the shape memory behavior of polymer on scaffolds. The results showed that the percentage of shape recovery in scaffolds with symmetric angular grid networks was higher than elliptical reticular stents. The minimum percentage of longitudinal recovery belonged to the elliptical reticular stent and was 74.5%. The percentage of longitudinal recovery of honeycomb and rhododendron reticular stents is approximately equal to 80.3%. The results of this study can be used to optimize the stent grid geometry to increase the retrieval force to resolving vascular clogging.

In this study, aluminum-to-copper welding was performed by friction stir welding (FSW) process and then the mechanical properties of the joints were evaluated and compared with the ones rolled to reductions of 30 and 60 percent. Tensile strengths (UTS) of the joints were 99 MPa, 143 MPa, and 132 MPa, for the initial weld, 30% rolling reduction, and 60% rolling reduction, respectively and in the non-rolled weld specimen, fracture occurred from the aluminum base material but in rolled welds, the fracture occurred precisely from the weld interface. Microstructural studies of the weld region and fracture surface of the specimens showed that the Al4Cu9 and Al3Cu intermetallic compounds, which are the most common intermetallic compounds in this type of dissimilar joining, formed in these areas. The presence of these compounds at the weld interface and propagation cracking during rolling has been one of the important factors in the failure of the weld interface in the rolled specimens. Results of the hardness test also confirmed the existence of these intermetallic compounds. By increasing the percentage of rolling reduction from 30% to 60%, the welding strength decreased due to the increase in the number of micro-cracks of the intermetallic compounds. Finally, it can be said that by choosing the optimal percentage reduction in the rolling process (30%), can be significantly increased (about 43%).  the ultimate tensile strength of dissimilar Al/Cu joints produced by friction stir welding (FSW).

Ultrasonic needle penning is a modern technique that enhances the surface properties of metallic components by imposing static and dynamic loadings. The efficiency of this technique dramatically is dependent on the process parameters. In this study an experimental and numerical investigation on ultrasonic needle penning was carried out. The numerically predicted residual stress profile was verified using X-ray diffraction measurement of residual stress. A 3D finite element model of ultrasonic needle penning was simulated by ABAQUS software. Moreover, a parametric study was performed to investigate the effects of needle diameter, amplitude, device moving speed and static force on residual stress distribution. In order to design of experiments and determine the optimized process parameters of ultrasonic needle penning, Taguchi’s method was implemented. Based on the results, needle diameter had the lowest impact on maximum compressive residual stress and residual stress increases by increasing amplitude and reducing device moving speed. The maximum residual stress was achieved for the needle diameter of 4mm, the amplitude of 16µm, the device moving speed of 1.5cm/s and the static force of 10N. For the optimum case, compressive residual stress was improved 24%.

Composites that withstand high temperatures are one of the parameters that always is the focus of attention. Especially when this type of material in sensitive parts flying craft is used. Generally, sensitive parts of flying craft should be also, radar transparency, have good mechanical strength at high temperatures. PTFE matrix composite is one of the options suitable for this purpose. PTFE is one of the polymers that have high heat resistance and low dielectric constant. This article processing preparation of glass fiber reinforced PTFE polymer matrix composites with conventional sintering methods. Then composite sheets made from E-glass woven fabric with PTFE have been produced with conventional sintering methods. To achieve optimal sintering processing of PTFE matrix composite, various cycles of time and temperature were selected and the optimal sintering cycle for such composite materials is obtained by the design of the experimental method. Then the mechanical properties of composites with different sintering processes were measured. To determine the mechanical properties, a tensile test was performed, and to determine the electrical and electromagnetic properties, a dielectric constant test in X-band was performed. The loss tangent, as well as composite products obtained. The maximum tensile strength and tensile modulus were achieved in this research respectively equal to 130 Mpa and 3.65 GPa. Constant dielectric and Loss tangent of the samples produced in the x-band are 2.37 and 0.096, respectively. Finally, to validate results in this research, the results of the performed tests were compared with the results of other references in this context.