نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 6 (شهریور 1392)

Abstract- Materials which offer the combination of high damping and stiffness are attractive for many industrial applications. In this work the stiffness and damping properties of Polypropylene/Calcium-Carbonate (PP/CaCO3) nanocomposites have been studied. In order to improve the dispersion of CaCO3 nanoparticles in polypropylene matrix, nanoparticles were coated by monolayer of stearic acid. All samples containing 5, 10, 20 and 30 wt% nanoparticles were mixed in co-rotating twin screw extruder and were formed into sheet by hot press, then standard tensile specimens were provided by pneumatic punch. Tensile properties of samples were experimentally investigated. To examine dynamic mechanical properties we use DMTA and Morphology of samples was performed by SEM. SEM images showed a suitable dispersion of coated CaCO3 nanoparticles by monolayer of stearic acid. The results of tensile test showed that with growth of nanoparticles weight percent, tensile strength decreases and young’s modulus increases. According to the results of DMTA, damping properties of samples improve with the growth of CaCO3 nanoparticles.

Thin walled structures are among the main parts for several industrial machines. They usually expose variant stresses; therefore fatigue is their major failure mechanism. However experimental observations of the cross section of failed parts indicate that their fatigue fracture differs from what generally expected as high-cycle fatigue behavior. The mathematical estimations demonstrate that stress concentration due to surface roughness plays greater role compare to other surface metallurgical factors which may be predominant factor in the case of fatigue crack generation for thick walled structures. Generating several experimental data, an attempt is made in this study, to investigate fatigue failure of these particular parts. In addition the practical equation available in machine component design books is modified to implement the effect of manufacturing parameters properly. Application of developed equation makes it possible for designers to communicate with manufactures in a better way by proposing expected surface quality. The manufacturers are supposed to choose suitable fabricating parameters (particularly machining parameters) to achieve required quality. In addition this criterion may be inspected easily by quality control managers even via visual inspection.

In this paper, the effect of bandwidth and loading level parameters changes of random load histories on the fatigue crack growth is studied by the central limit theory. In this method, the life probabilistic distribution and probability of failure function and reliability function are derived by the central limit theory. The walker equation is used to account the stress ratio effect in the fatigue crack growth rate equations. Then, the theoretical results are verified by the test results in the three random loading conditions with the various bandwidths and loading levels. A good agreement among the theory and test results was observed. The probability of failure and reliability diagrams via number of cycles is presented too.

In this paper, the influence of nanoclay Closite 30B on ballistic impact behavior of 2D woven E- Glass/Epoxy laminated composite has been investigated Theoretical and experimentally. The structure of the hybrid nanocomposite is glass/epoxy/nanoclay laminate and is manufactured by hand layup method under pressure. The nanoclay is dispersed into the epoxy system in a 0%, 3%, 5%, 7% and 7% ratio in weight with respect to the matrix. Comparison of theoretical results and results of the ballistic impact test are shown a good correlation. The results have shown that optimal to increase in energy absorption is 10% in 3% nanoclay content. Howevere, in the impact velocities far than ballistic impact, maximum increasing in energy absorption is 20% in 10% nanoclay content.

Due to the energy concerns, it is always desired to run the mechanical elements under optimized operating conditions. Many lubricated mechanical elements such as gears, rolling element bearings, cam and followers etc. usually operate under mixed-lubrication regime. In this regime, both the lubricant film as well as the asperities contribute in carrying the load. The life span of these elements is divided into two regimes: running-in and steady-state. In this research an efficient model has been presented to predict the variation in asperities height during running-in. The predicted results are verified with experimentally-obtained published data. A parametric study on the effect of operating conditions such as load and speed as well as initial surface roughness and material hardness on the roughness variation during running-in has been conducted.

In this paper, a new analytical model has been presented for energy absorption of aluminum-foam sandwich panels under ballistic impact. The panels consist of foam core sandwiched between two aluminum skins. In analytical model two types of sticker including cylindrical projectile with flat and hemispherical ended have been considered. It is supposed that aluminum skins failure by mean resistive pressure. Also foam absorbed a partial of projectile energy by crushing. Energy absorption of aluminum-foam sandwich panel is calculated and energy balancing equation has been employed for determination the ballistic limit and residual velocity of projectiles. The results of ballistic limit and residual velocity computed by new model have good agreement with experimental results. Also the effects of projectile mass and diameter in energy absorption of sandwich panel has been investigated.

In this paper, the multi-objective optimization of twist extrusion process is carried out using the artificial neural network model and the genetic algorithm. the target purpose functions are equivalent plastic strain, strain distribution and extrusion force. the design variables are twist angle, friction factor and the loading rate. the FEM model of the process is first created and used to create training cases for the ANN, and the well-trained ANN is used as a quick and exact model of the process. Then the optimization of the design variables is conducted by an integrated genetic algorithm and the ANN model to create a set optimal solutions (pareto front).

In this paper, the role of isotropic and kinematic hardening models are discussed in cold forming of a U-channel considering the springback phenomenon. The effect of influential parameters on the springback is also studied. For this purpose, a cold roll forming machine was built using a milling machine. The effects of forming angle’s changes, sheet material, roll geometry and sheet thickness are studied experimentally and numerically. The results show that the isotropic work hardening model is more precise in prediction of the springback. 304 stainless steel and AISI 1015 are used in experimental verification. Comparison of the simulation results with experimental values demonstrates the accuracy of the modeling.

Abstract- Hydrodynamics and Heat transfer of a gaseous flow in microchannels is performed numerically. Velocity and temperature at the channel inlet is uniform and the rarefaction effect is imposed to the problem via velocity slip and temperature jump boundary conditions, according to the slip flow regime. The channel is sufficiently long to reach fully developed flow at the outlet. The numerical methodology is based on the control volume finite difference scheme and discrete equations are solved using SIMPLE algorithm. Effects of various parameters such as viscous dissipation, rarefaction, axial conduction and thermal creep on heat transfer have been considered. The results indicate that the Nusselt number in microchannels has a different value than in conventional channels. Local Nu number is found to experience a jump by the presence of viscous dissipation. The magnitude of the jump is independ of the Brinkman number values. Heat transfer is affected in two opposite directions by rarefaction increasing. Also, as Peclet number increases, there is a weak increase in fully developed Nu number values but there is significant effect of Kn number on it.

Abstract- Damage of metals is a progressive physical process which finally leads to the failure of them. In this study, first, a coupled elastic- plastic- Lemaitre's ductile damage model combined with large deformations theory is developed and implemented as a subroutine into ABAQUS/EXPLICIT code. Then, by performing standard tensile and Vickers micro-hardness tests, mechanical and damage properties for St14 steel are determined. For validation of the damage model and also identified properties, cutting and fine cutting processes are simulated by the model in two cases of large and small deformation theories. Comparison of the numerical simulation results and experimental reports show that Lemaitre's ductile damage model combined with large deformation theory can accurately predict damage evolution, crack initiation, propagation, and ductile fracture in the metal forming processes with large deformations. Keywords: Lemaitre's Ductile Damage Model, Large Deformations Theory, Cutting and Fine Cutting Processes, Ductile Fracture.

In this paper, annealing processes for stress relaxation and removing residual stresses from polycarbonate sheet are studied and evaluated using experimental tests. Then, by making some alterations and modifications in the previous heating cycles thermal operations are performed on different samples in a try and error process. Appraising the obtained results, a modified thermal cycle is presented which can remove the residual stress from the polycarbonate sample, while reducing the time and cost of the annealing process. According to this process, the temperature is raised to 100˚C at the heating rate of 20˚C/h. The samples are kept at this temperature for 3 hours and then, the heating is continued up to 140˚C at a slower rate of 5˚C/h. Then, the specimens are heated at the rate of 1˚C/h until maximum temperature of Tmax = 156˚C. After reaching this point, the samples are cooled immediately, and the cooling rates are same as the heating rates.

In the present paper, Molecular Dynamics Simulation (MDS) is performed for Poiseuille flow of liquid Argon in a nanochannel by embedding the fluid particles in an external force with different potential functions. Three types of Lennard-Jones (LJ) potentials are used as interatomistic or molecular models for evaluations of interactions and density, velocity profiles across the channel are investigated. The interatomic potentials are LJ 12-6 potential, LJ 9-6 potential and LJ-Smooth potential. Density and velocity profiles across the channel are investigated. Obtained results show that hydrodynamic characteristics and behavior of flow depends on the type of interaction potential. It is shown that the LJ 9-6 predictions for velocity and temperature are larger than those of LJ12-6 and LJ-Smooth potentials. Also, applying LJ 9-6 results in further calculations time. The results show the effect of interaction force model on the understanding and analyzing of nanoscale flows.

The appearance of swirl in the fluid flow, without any external factors, is named self-rotation. There is no consensus among the researchers on the possibility of self-rotation in sink flow. In the present study, this phenomenon is studied experimentally and numerically. In the experiment, a new setup is proposed and the thymol-blue method is developed for velocity measurement. The experimental results show that the net value of circulation in the flow domain does not increase relative to its inlet value with increasing drain flow rate up to several times of claimed critical value. In other words, self-rotation does not happen in sink flow. In the numerical study, the three-dimension model of the experiment is simulated under the same conditions with the experiment. The simulation results have good agreement with the experiment ones and show that the self-rotation does not occur. In addition, the numerical results show that the swirl, or corresponding circulation, which was seen in the experiment, is due to asymmetry of input flow and by eliminating this asymmetry, the swirl can be removed completely.

An efficient reduced-order modeling (ROM) approach to predict unsteady behavior of partial cavity flows is proposed. The unsteady potential flow along with the cavity effects is analyzed using the boundary element method (BEM). Partial cavity flow is modeled based on the partially non-linear model without re-gridding with some modification. Proposed reduced-order model (PROM) is based on the fluid eigenmodes. The spatial iterative scheme that is usually used to determine the cavity extent is efficiently removed in order to construct the flow eigensystem. Eigenanalysis and reduced-order modeling of unsteady flows over a NACA 16-006 section are performed using the proposed reduced-order modeling approach. Numerical examples are presented to demonstrate the accuracy of the proposed method. Comparison between the obtained results of the proposed method and those of conventional ones indicates that the present algorithm works well with sufficient accuracy. Finally, it is shown that the proposed method is computationally more efficient than the conventional one for unsteady sheet cavitation analysis on hydrofoils.