مهندسی مکانیک مجلسی
سال:1393 | دوره: | شماره: |تعداد مقالات:12

Upright balance control is an important human skill which can be impaired by aging and different disorders. Therefore, clinical and laboratory measures of balance assessment were established. The latest technology in the laboratory methods is a stabilometer with a computer or computerized dynamic posturography which assesses static and dynamic balance using body sways. A computerized dynamic posturographysystem consists of a computer and a stabilometer which is an unstable platform under patient's feet and calculates center of pressure on the platform to display it immediately on the screen. For these purposes, two protocols of "Limits of stability" (LOS) and "Postural stability" (PS) were designed and developed in the present study for a prefabricatedstabilometer with a max.of 20 degrees deviation from horizontal in all directions to achieve the mentioned purposes. The protocols calculate and display the person's functional characteristics that can be saved to the computer of computerized dynamic posturographyto demonstrate a diagram of the patient's functional progression. In addition to the balance assessment, the device improves balance and neuromuscular strength, and is useful in medicine, laboratory researches, physiotherapy and bodybuilding.

Shrink fit process is a useful technique in order to introduce beneficial residual stress in compound pressure vessels. In this paper, the effects of geometrical tolerances on residual stresses have been studied for a compound shrink fitted pressure vessel, practically. Three layers which are designed based on an optimum nominal thickness and overlap dimensions and tolerances, have been fitted by shrink fitting to obtain a multi-layered high pressure vessel with desirable residual stress distribution. But in the manufacturing process, variations of inner and outer diameter of each layer have been observed within the design tolerances. The geometrical tolerances considerably affect the residual stresses. In this work, experimental results for residual stress are obtained from measurements of inner diameter of innermost cylinder due to two stages of shrink fitting. Then, the residual stress distribution is compared with analytical solution and finite element method at the lower limit and upper limit of tolerance domains. It is shown that very small geometrical tolerance could have a significant effect on residual hoop stress distribution. Also, the experimental results have a good agreement with analytical and finite element results.

To achieve an excellent thermal-mechanical performance of CMCs, it is necessary to analyze and design the thickness of the multi-layered interphases for an optimized TRS distribution. An optimization was performed with a new version of the particle swarm optimization, the BSG-Starcraft Radius PSO linked to finite element software.

Superplastic forming (SPF) is a manufacturing process utilized in the automotive industry to produce complex geometry aluminum or magnesium alloy components which cannot be fabricated at room temperature. The die entry radius and the friction coefficient at the sheet-die interface greatly influence the metal flow during the SPF process. This paper investigates the effects of the die entry radius and the interfacial friction coefficient on the required forming time and thickness distribution of a superplastic formed vehicle closure panel. A commercial finite element software, ABAQUS/Implicit, is applied to simulate forming of AA5083 aluminum alloy into a license plate pocket panel. The results indicate that for a fixed friction coefficient, increasing the entry radius reduces the forming time and enhances the formed part quality, in terms of thickness distribution. It is also shown that the lower the friction coefficient, the higher the sensitivity of the forming time to the die entry radius variations.

The buckling analysis of annular composite plates reinforced by carbon nanotubes (CNTs) subjected to compressive and torsional loads are studied in this paper. The Mori-Tanaka method is employed to calculate the effective elastic modulus of composites having aligned oriented straight CNTs. The effects of CNTs volume fractions, orientation angles, boundary conditions and geometric ratio of plate are discussed. The results are calculated by analytical method based on classical plate theory and finite element methods (FEMs) using ANSYS software and third order shear deformation theory for moderately thick laminated plates. It is found that the stability of plate increases as the thickness or inner to outer ratio rises and when the CNTs arranged in the circumferential direction the highest buckling load is achieved.

 this study, the effect of carbon nanotube (CNT) on the buckling of the embedded sea lines is investigated. The sea lines are simulated with isotropic cylindrical shell subjected to thermal and mechanical loads. The sea line is reinforced by armchair CNTs, where characteristics of the equivalent nanocomposite being determined using Mori-Tanaka model. The elastic medium is modelled using Pasternak foundation. The governing equations are obtained based on strain-displacement, stress-strain and energy relations as well as Hamilton's principal. The influences of volume percent of CNTs in sea lines, geometrical parameters, elastic medium constants, temperature change and poison ratio on the buckling load of the system are investigated. Results indicate that the buckling load of the sea line increases with increasing volume percent of CNTs in sea lines.Hence, the CNTs are very useful as reinforcer for sea lines in order to increase the buckling load of the system.

Manufacturing of an optical lenses demand a specialized technology and thus needs sophisticated precision manufacturing process. PMMA is one of the most used optical polymer for manufacturing of optical lenses, the shaping of which into the required precision and accuracy is a challenging task. The focus of the present paper is to understand the effect of machining parameters on the surface characteristics especially flatness obtained in single point diamond turning (SPDT). The experiments were conducted according to Taguchi L9 design. The machining parameters chosen are feed rate, spindle speed, depth of cut and cutting environment. Analysis of results reveals that the spindle speed is more influential on the surface flatness generated by the SPDT. All the flatness measurements were done by the non-contact type of measurement. It is observed that the minimum PV value produced is 0.837 mm.

There are mainly two methods of deep drawing analysis; experimental and analytical/numerical. Experimental analysis can be useful in analyzing the process to determine the process parameters in producing a defect free product.The analytical/numerical modeling can be used to model and analyze the process through all stages of deformation. This approach is less time consuming and more economical. Sheet metal forming often includes biaxial in-plane deformation with non-proportional strain paths. In deep drawing of cylindrical cup, the deformation in the flange is dominated by pure shear deformation, while it changes to plane strain when the material is drawn into the die. This paper deals with the analysis of deep drawing of circular blanks into axi-symmetric cylindrical cup using numerical modeling. The blank drawability has been related both theoretically and experimentally with the initial diameter of the blank and deep drawing parameters. The strains in the radial and circumferential directions have been measured. A correlation on the flange thickness variation by taking into account the work hardening with the analytical and experimental values also has been searched.

This research deals with monitoring tool wear through the chip formations, forces, and edge temperature of drill while drilling in superalloy plate to optimize effective parameters which lead to facilitate machining process to improve tool life, and enhance productivity. Inconel 718 superalloy material, and cemented coated carbide tool were selected in this study to investigate tool wear mechanism. Mathematical models were deduced by Minitab software to display the influence of the main cutting variables such as cutting speed, feed rate and tool diameter on tool wear. A wear process model of twist drill is established based on finite element method. The 3D FEA model established here provides a new approach to study the mechanism of drill wear. The predictive models in this study are believed to produce values of tool wear close to those readings recorded experimentally with 95% confidence interval, verified using ANOVA. The simulation results were in accordance with experimental and predictive values from RSM with error rate of 4-6%, proving the ability of the tool wear model to correctly forecast it. In addition, the experimental results demonstrated that cutting speed as the main parameter followed by feed rate; contribute significantly the tool wear of drill bit.

In this paper, the effects of transverse cracks on rotating shaft are studied. According to earlier works, crack shaft failure decrease the critical speed of the shaft and also exhibit a nonlinear vibration behaviour leading to increases the amplitude of 1X and 2X harmonics (first and second order of rotational speed in torsional vibration). This suggests that the application of order analysis to be very effective. Moreover, vibrations arising from resonance frequencies of the system can be differentiated from other ones, by using this method. For this purpose in this article, first, un cracked and cracked shaft were mathematically modelled and numerical analysis were carried out to obtain the vibration response for both healthy and faulty shaft. Then order analysis method was applied on the vibration signals obtained from the model. Finally, an experiment was also carried out on a laboratory scale test rig to verify effectiveness of the order analysis in practice. The results indicate that the vibration orders arising from torsional vibration can be clearly sensed by the transducers installed on the supports which primarily measure the bending vibrations.

In this paper, due to the importance of incoming flow turbulence intensity into combustion chamber, tripping wire effect on the flow wake has been experimentally investigated within a linear compressor cascade. Besides, effects of changes in the attack angle of compressor blades on the characteristics of wake are investigated. Investigation on the velocity, turbulence intensity and vortex frequency in the wake of blade in the attack angles of -10, 0 and 10 is done. In previous work the effects of a smaller angle of attack has been examined. Therefore, to better understand the flow in the wake, more essential investigations appeared to be necessary for more angles of attack. Thus, two wires were implemented along each blade and their effects on average velocity, turbulence intensity and vortices frequencies were accurately considered.Increasing the angle of attack, leads toincrease inthe domain of wake and frequency at maximum amplitude, and the maximum Strouhal number.To do this, single channel hot wire anemometer was used to measure the wake parameters.

The nonlinear bending analysis of thick functionally graded plates subjected to mechanical loading is studied in this paper. The formulation was derived based on the third-order shear deformation plate theory and Von Kármán type non-linearity. Young’s modulus is assumed to vary according to the power law distribution in terms of the volume fractions of the constituents. The principle of virtual work was used to obtain the weak form of the governing differential equations. The most important advantage of employed numerical solution in this work is that the whole plate was considered as one element and the components of displacement field were interpolated over the entire domain, then a hierarchical finite-element scheme was developed. The validity and accuracy of the method was verified by comparisons made with other solutions. In addition, the effect of numbers of interpolation functions on the accuracy of results was studied. It is concluded that accurate results are obtained even by few numbers of interpolation functions.