JOURNAL OF MECHANICAL ENGINEERING AMIRKABIR (AMIRKABIR)
Year:2016 | Volume:47 | Issue:2|Papers Count:9

In this research, firstly, the vibrational behavior of a cracked short cantilever beam under the axial load is investigated, and then, an analytical approach for the crack identification based on the vibration analysis is proposed. The cracked section of the beam is considered as a flexible element, which divides the beam into two segments. Using the fracture mechanics theory, the local flexibility of the crack is modeled as a mass-less tensional spring. By applying the boundary conditions and the inner conditions at the crack location, and taking into account the effects of shear deformation and rotary inertia, the governing equation of motion for the cracked beam is derived. The influence of the axial load and the crack parameters on the vibration behavior of the cracked beam is studied by establishing and solving the corresponding eigenvalue problem, directly. Then in order to predict the crack depth and location through the known natural frequencies of the cracked beam, which are obtained by the experimental tests, the corresponding inverse problem is established and solved analytically. The Results have been validated by the experimental and theoretical data reported in the literatures.

Nowadays, fiber-metal laminate composites are highly used by designers due to their good mechanical properties and low weight particularly in aerospace industry. The Fiber Metal Laminate shows improvement over the properties of both aluminum alloys and composite materials. In this study, bending properties of these composites are investigated both numerically and experimentally. Since these multi-layers have a very limited formability and deformation of fibers is pure elastic, the fibers cause some kind of spring back that is known as a defect. In the current study, six types of specimens are prepared with variable fiber angles and thickness of layers for bending test. In addition, the bending test is optimized through the Taguchi design experimental method, until the results become independent of die and press parameters. The effects of design and fabrication parameters of fiber- metal composite on the spring back are investigated in detail. The results show that the spring back increases linearly with increasing punch radius and with decreasing pressure forming due to the reduction of the plastic deformation. On the other hand, by increasing the punch speed, the spring back increases slightly because forming by the high-rate punch speed increases the amount of elastic recovery in aluminum sheet and induces larger spring back. Moreover, when the fibers are parallel to the bending axis and the thickness of outer layers increase, the bend radius and spring back decrease. The obtained results are compared partially with the FE numerical results.

Wire electrical discharge machining (wire-EDM) is very important in the manufacturing, due to its growing industrial and research applications. Inaccurate geometry in corner and especially sharp-curved corner cutting is one of the major problems in this process. Creation of exact geometry and high dimensional accuracy has been subject of many research works. In the present research, actual cutting slots on straight and curved paths are studied in roughing operation. Experiments are designed using the Full Factorial Method by considering frequency of discharges, wire tension and radius of curvature as variables. By employing statistical techniques in analysis, influences of these parameters on width of the cut of slot, on both straight and curved paths, are evaluated. Practical results for straight side gap and curved corner average gap are achieved, for each set of parameters. Because of concavity of wall on inner arc and convexity of wall on outer arc, measuring errors appear in the measurement of slot width at the curved corner. In addition, depletion of material and accumulation of material are observed at entrance and exit of the curved path, respectively. By calculating variable side gap on a curved corner, practical wire diversion and machining error are obtained.

The most prominent feature of sheet material forming process is an elastic recovery phenomenon during unloading which leads to spring back and side wall curl. Therefore, evaluation of spring back and side wall curl is mandatory for production of precise products. In this research, the effects of friction coefficient, sheet thickness, yield strength of sheet and blank holder force on the spring back and side wall curl in U bending of DP600 dual phase steel sheets were investigated. These investigations were done by computer simulation. The simulations were done by ABAQUS finite element software and then, results were compared with experimental results. The finite element results have been validated with experimental results. MINITAB, a statistical software, was used to analyze finite element results. With the use of MINITAB, equations were obtained to predict spring back and side wall curl radius by friction coefficient, sheet thickness, yield strength and blank holder force.

In this paper, FGMs are used as non-uniform materials in high temperature environments. Different industries use them in thin and thick walled spherical pressure vessels. Based on governing equations, differential equation of stresses is obtained in plastic state that can be widely used in the study of reservoirs behavior in elasto-plastic state. The study has discussed on temperature distribution and stress - strain relationships in the tanks under internal pressure and temperature difference. Properties of these materials are considered as variable parameters function of radius. In this work, effects of these parameters have been investigated on yielding, yield temperatures and stress changes in thickness of the vessels. Furthermore, it is shown that vessels structure can be optimized by choosing appropriate parameters.

In this paper, the size effect on elastic properties of single- walled nanotubes is evaluated via the strain gradient elasticity approach. For this purpose, rod, torsion bar and Euler- Bernoulli beam models are used. The tension rod model is developed in the present study. The Euler- Bernoulli beam model is utilized and the boundary conditions are modified in the present research. In addition, a model for the rod under torsion is developed. Afterwards, by using the constitutive relation in strain gradient elasticity, the size- dependent elastic properties of carbon nanotube are achieved effectively. The results show that the length of the carbon nanotube is more effective on the Young modulus in comparison with that of on shear modulus and when the length of nanotube decreases, the Young modulus decreases similarly.

It is obvious that the natural frequencies of a submerged structure are less than those of in vacuum and these are due to the effect of added mass of water to the structure. for marine structures, fluid inertial (added mass) effects cannot be neglected. This paper focuses on the experimental, analytical and numerical solution of natural frequencies of submerged stiffened plate. The analytical solution based on the deflection equation of submerged stiffened plate, Laplace’s equation and Rayleigh' s method in vibration analysis. Considering small oscillations induced by the plate vibration in the incompressible and inviscid fluid. The velocity potential and Bernoulli’s equation are adopted to express the fluid pressure acting on the structure. The natural frequencies of the stiffened plate are obtained practically by using Fast Fourier Transformation functions (FFT) in experimental analysis. Experimental results demonstrate the validity of numerical and analytical solution and results. the experimental results validate the derived formulation for natural frequency of plate vibrate underwater.

Vibration analysis of post-buckled beam is investigated in this study. The governing nonlinear equations of motion for the post-buckled state are derived. The solution consists of static and dynamic parts, both leading to nonlinear differential equations. The differential quadrature method has been used to solve the problem. First, it is applied to the equilibrium equations, leading to a nonlinear algebraic system of equations that is solved utilizing an arc length strategy. Next, the differential quadrature is applied to the linearized dynamic differential equations of motion and their corresponding boundary and continuity conditions. Upon solution of the resulting eigenvalue problem, the natural frequencies and mode shapes of the beam are extracted. The investigation includes several numerical as well as experimental case studies on the post-buckled simply supported and clamped-clamped beams. The results show that the applied compressive load as well as the geometric imperfection largely affect the modal shapes and natural frequencies of the beam. Moreover, the study demonstrates the excellent accuracy and efficiency that can be obtained by applying the differential quadrature method to treat vibration of post-buckled beams.

In this research, polymer matrix composite filled with aluminum particles was synthesized and turned with different machining condition namely: cutting speed, weight fraction of particle, depth of cut and feed. Then, surface roughness was measured and two artificial neural networks models Multi-Layer Perceptron (MLP) and Radial Basis Function (RBF) were developed to estimate effects of four turning parameters on surface roughness. Correlation between training data and experimental data were shown that MLP network was better than RBF as a compatible network (correlations were 0.835 for MLP network and 0.542 for RBF network). Because of higher correlation for MLP network, this network was proposed as a model for investigation the effects of turning parameters on surface roughness.