Journal of Stress Analysis
Volume:4 Issue: 2, Autumn-Winter 2019-20

Twist extrusion is a novel method for severe plastic deformation of materials. Severe plastic deformation in metals creates small and uniform grain size and therefore increases their mechanical strength. In this study, the effect of die angle in twist extrusion and cross-section of extruded parts on plastic properties and microstructures of Aluminum 7050 alloy was investigated using DEFORM 3D finite element software. Samples were simulated using dies with die angles of 20, 37, and 56 degrees with square, rounded-rectangular, and elliptical cross-sections. The aspect ratios of rectangular and elliptical cross-sections were also changed while keeping the cross-section area constant in order to investigate the effects of dimensions. Plastic strain distribution, gain size distribution, and the force needed for extrusion were extracted under all conditions. The results indicate that increase in die angle significantly reduces grain size and increases the force necessary for extrusion. Removing sharp corners in cross-section also results in more uniform plastic strain distribution and reduction in extrusion force. The elliptical cross-section with dimensions of 9×6mm which had the lowest dimension ratio can reduce grain size from 100ffm to 6ffm in a single pass and requires the lowest extrusion force.

Friction stir processing (FSP) in different pass number, accordingly one and four, was performed to AZ31 magnesium alloy. Optical and scanning electron microscopy (SEM) were used to investigate the effect of FSP and its pass number on the microstructure of FSPed samples. The hardness of thesamples was measured using microhardness measurement. Furthermore, wear behaviors of the samples, including wear rate and friction coefficient, were investigated using a reciprocal wear machine. To deduce the wear mechanism, SEM observations of the worn surface were carried out. Optical microscopy of FSPed samples showed grain refinement in the stir zone. Increasing FSP passnumber had a considerable effect on grain refinement. The average grain size of the as-received AZ31 base metal reduced from 11µm to about 4µm after four passes. Microhardness evaluations showed a substantial improvement by increasing FSP pass number, about 70% improvement. Wear tests results revealed enhanced tribological in FSPed samples. SEM observations of the worn surfaces indicated that the abrasion was the dominant wear mechanism governed in the samples.

The effect of pre-compaction on mechanical properties of Mg/SiC nanocomposites prepared through dynamic compaction was investigated. The dynamic compactions were carried out at two different loading rates using Drop Hammer (DH) and Split Hopkinson Bar (SHB). The quasi-static pre-compaction was performed under two different pressures of 50 and 100MPa and at 450◦C. The results show that the highest improvement in density, hardeness, and strength are obtained for the pre-compaction pressure of 50MPa. The reason is believed to be due to the discharge of the air packets trapped between the particles. For the pre-compaction pressure of 100MPa, however, density, strength, and hardness decrease. The reason is thought to be due to creation of cracks and faults in the specimens. The results indicate that there is an optimum for the pre-compaction pressure which varies depending on the type of matrix, reinforcing particles, and compaction loading rate.

Nowadays, composites have great applications in mechanical structures due to their proper ratio of the strength to the weight. Such application includes automotive and aerospace industries. These components may be affected by the creep phenomenon, when they work at high temperatures. Therefore, there should be appropriate creep behavior of materials for these parts. In this article, the Continuum Damage Mechanics (CDM) method was used to calculate the creep lifetime of various polymer matrix composites. For this objective, experimental data were utilized from creep tests in the literature, on standard specimens, at different temperatures. Then, therelation between the stress, the temperature and the lifetime was presented by the CDM approach, which was calibrated by experimental results. In addition, the Levenberg-Marquardt method was employed to optimize the creep lifetime equation and to find temperature-dependent material constants. Consequently, the obtained results showed that there was a good agreement between experimental and calculated creep lifetimes of composites.

In the current study, magnesium-matrix AZ31/ 1.5 vol.% (TiO2)p nanocomposites manufactured using stir casting underwent extrusion process. The as-cast ingots were extruded, then processed by multidirectional forging (MDF) up to 8 passes at constant temperature of 320◦C. Investigating microstructures showed that after the second pass, the size of matrix grains underwent a significant decrease. However, this decrease didn’t continue in subsequent passes and grain size increased at the fourth pass. In the sixth pass, grain size decreased again, resulting in the smallest microstructure in all the samples. However, in the last two passes, grain size increased similarto the case of the fourth pass. The results of shear punch and Vickers’ microhardness tests showed that changes in shear yield strength, ultimate shear strength, and hardness followed a similar trend. Furthermore, the results of these tests showed that the best mechanical properties are observedin the first two passes after which no further improvement is observed in shear strength and hardness of the samples while fourth, sixth, and eighth passes resulted in better mechanical properties compared to the extruded sample.

Continuous investigation and measurement of pore water pressure and arching depression play an important role in detecting the occurrence of hydraulic failure in earth dams. Increasing the pore water pressure during the initial impounding period reduces the effective stress and consequently decreases the shear strength of the earth dam core, which can overcome the water stress over the effective stress and consequently hydraulic failure. This research studies the hydraulic failure of Eyvashan earth dam under static loading conditions at the end of construction and the initial impounding period by Geostudio software with the Mohr-Coulomb behavioral model. The analysis shows that the values of the pore water pressure ratio (ru) and stress-strain values are acceptable and there is no stability problem for the dam. The highest percentage of arching (the lowest ratio of arching) is equal to 46%, at one-third of the lower height of the core. The critical arching ratio (maximum) is 0.44 and is in the normal range and hydraulic failure does not not occur despite the critical arching in the dam core.

In this article, the friction stir welding of dissimilar AA6061-T6/AA7075-T6 aluminum alloys was studied experimentally. The joining process was implemented with and without the addition of the TiO2 nanoparticles. To infer the resulting quality, tensile tests were carried out and the microstructure of the welded samples was investigated by the optical microscope. Furthermore,the samples were welded using gas tungsten arc welding (GTAW) to provide further comparisons with the FSW process. The ultimate tensile strength and maximum elongation increased by 12.3 and 12.5% respectively by adding TiO2 nanoparticles. Microstructure observation shows that equiaxed grains formed in the FSW process and no precipitation aging occurred in the melting zone-however, precipitation particles can be observed in the heat-affected zone. Coarser grains can be obtained by adding TiO2 nanoparticles, resulting in good dispersion at the stir zone and retarding the dynamic recrystallization (pinning the grain boundary movements). The sample welded by the GTAW processshowed very weak strength compared to the samples welded by the friction stir welding process.

Ensuring the safety of passengers as much as possible is essential in automobile and airplane accidents. In this study, an open-cell aluminum foam was introduced as an energy absorber. Analytical equations of absorbed energy were extracted. The analytical results had acceptable agreement with numerical and empirical ones. Based on the graded nature of natural impact absorbers, graded designed was used for the helicopter seat impact absorber. Optimization methods including genetic algorithm and sequential quadratic programming algorithm were used to create an optimum graded impact absorber. Satisfying standard requirements of the JAR-27 air standard was used as a design goal for impact absorber. The designed impact absorber was then modeled inABAQUS software to calculate the absorbed energy, acceleration, and the force applied to the passenger and HIC for the protected passenger. According to the results, the graded foam satisfies all requirements for helicopters during emergency landing. The derived analytical equations can be used to study the energy absorption of other foams.

In this paper, the static response of functionally graded piezoelectric plates under mechanical, electrical, and thermal loads is studied using a meshless method. The Radial Point Interpolation Method (RPIM) is used to create the shape function to approximate field variables. Given that RPIM shape functions pass Kronecker delta condition, boundary conditions can be applied directly. The First-order Shear Deformation Plate Theory (FSDT) is used to model the behavior of the plate. Power law distribution through the thickness is considered for all of mechanical, thermal, and piezoelectric properties. Effective parameters on deflection and stresses of Functionally Graded PiezoelectricMaterial (FGPM), including different electrical and mechanical loads, thermal loads, thickness, and different boundary conditions are studied. In this paper, the effect of power law index on the deflection and stresses of the functionally graded piezoelectric plate under external loads is investigated and different results are obtained in each case of mechanical, electrical, and thermal loading. By analyzing the results of this paper, the effective structure design and sensor/actuator behavior of the plate subjected to thermal and electrical loading could be obtained.

In this research, Constraint Groove Pressing (CGP) process, which is one of the most important and effective methods of severe plastic deformation processes has been studied. Ultrasonic assisted CGP (UCGP) process has been conducted to investigate and compare the effects of applying ultrasonic vibrations on the residual stress with the conventional method. Contour method was applied to measure the residual stresses distributions in the CGPed and UCGPed samples. Pure copper sheet samples were tested both with and without ultrasonic vibrations up to 2 passes. The measured values of the residual stresses indicated a relative reduction of stress in the presence ofultrasonic vibrations. By investigation of residual stress normal to the surface in thickness direction, it was observed that residual stresses are compressive on the edge and tensile in the middle of the thickness of the sheet. This reflects the self-balancing feature of residual stresses. In all conditions for both passes, residual stress reduced about 20MPa while using ultrasonic vibrations compared to traditional CGP method.

In this paper, the effects of two parameters named width of the roller and exerted force on it in direct and indirect rolling, on residual stresses in Friction Stir Welding (FSW) process of SU304 steel have been studied. FSW numerical modeling has been performed by ABAQUS. In both direct and indirect rolling, five levels have been considered for each variable. Based on the results, it has been shown that both variables have significant effects on the pattern and maximum of residual stresses. In general, in both direct and indirect rolling, by increasing the rolling force, residual stresses decrease intensely. In direct rolling, tensile residual stresses decrement happens locally by using relatively narrow rollers and increasing the rolling force. While in wide rollers, the decrement in tensile residual stresses occurs constantly. Based on the results, using direct rolling causes more decrement in welding tensile residual stresses in comparison with indirect rolling. In direct and indirect rolling, the minimum tensile residual stresses take place when the width of roller is equal to diameter and half of the diameter of welding tool, respectively. In this situation, the maximum of tensile residual stresses decreases 97.4% for direct rolling and 57.3% for indirect rolling.

Machining in the scale of nanometer and investigating its behavior is premier in the field of machining. Molecular dynamics is a new robust tool to investigate controlling mechanisms in atomic scales, complex dislocation, and grain-boundary in severely deformed workpieces with the submicron dimensioning; consequently, process simulation was performed by molecular dynamics method. In this study, some useful parameters of tool geometry in orthogonal cutting of monocrystalline copper were investigated. With this end in view, relief angle, rake angle, and tooltip radius were considered as influential geometrical parameters of orthogonal cutting of monocrystalline copper. By using the Response Surface Method (RSM), the variation effect of input parameters was studied on the cutting output parameters like cutting force, temperature, and hydrostatic stress all in nanometer precision. Furthermore, with mathematical modeling using a second-order linear regression equation fitted to the process outputs, single objective and multi-objective optimizationof the cutting process was followed.