مجله مهندسی مکانیک امیرکبیر
سال چهل و ششم شماره 2 (زمستان 1393)

The aim of the current study is to present a finite deformation shell theory incorporating interatomic potentials for single-wall carbon nanotubes (CNTs). For this purpose, a linkage between the strain energy density induced in the continuum and the interatomic potential is established by the employment of the modified Cauchy-Born rule. This theory, which considers the nonlinear, multi-body atomistic coupling and the CNT chirality, incorporates the important effects of bending moment and curvature for a curved surface. The theory is applied to extract the constitutive relations, which bypass the use of nanotube thickness and Young’s modulus, among stress, moment, strain, curvature and the interatomic potential. It is found that the chirality affects the mechanical behavior of the nanotube in tension and bending and this effect is less profound for the CNTs with higher radius at vanishing strain and curvature.

In this study, the experimental tests are designed and carried out to analyze the behavior of polyacetal ratcheting under both uniaxial loading and internal pressure. Experimental tests are performed by an accurate servo-hydraulic machine, Instron 8802. Three group tests were presented in which two of the stress amplitude, mean stress or internal pressure was constant to observe the effect of another factor. The effect of stress amplitude, mean stress and internal pressure on ratcheting strain and softening behavior of the polyacetal tube are investigated. Graphs of ratcheting strain, ratcheting strain range, hysteresis loops of group tests were drawn. By using these graphs, some results such as softening behavior obtained. A suitable hardening factor was presented and its relation with the polyacetal fatigue life was examined. In a specific loading cycle for the specimen, whose hardening factor is more than the others, a lower fatigue life predicted. Softening behavior of the material, increases with increasing the loading cycles. The results show that the specimen with the high ratcheting strain value has lower fatigue life.

Based on the results achieved using experimental data and related mechanical analysis, the magnitude of loads that a bicyclist can apply on the pedal at his extreme power, is a function of angular coordinates of pedals. Therefore, the resultant of torques which these loads apply at the axle of pedals has an oscillation nature. To avoid transferring the oscillations produced by torques to the bicycle’s propellant wheel which causes fluctuations in the bicycle acceleration, a special power transfer system should be used. Designing of such mechanisms is the objective of this project. Foresaid design is a optimized instance of ordinary systems of mechanical energy generation using the method of pedaling, in which, for transferring the power from the pedal axle to the chain an ellipsoid chair wheel is used.
Regarding to the properties of the new chain wheel, the way that it transfers the power, can decrease the frequent oscillations of torque at pedal axle. Thus, the procedure of accelerating of bicycle which uses this system of power transfer will be almost without oscillations.

Friction stir welding is a solid state welding processes due to the low temperature in the welding zone, prevent formation of defect that can occur during fusion welding. In this paper, the effects of three important parameters of rotational speed, welding speed and tool tilt angle on mechanical properties lap joint of aluminum alloy to austenitic stainless steel have been studied. The results showed that higher increasing the rotational speed or decreasing the welding speed can be cause the creation of defects such as created flash around the weld zone, cavity and sheet thinning defect which will strongly affect on the mechanical properties of the joint, So that maximum fracture load was obtained 7750N in the rotational and welding speed,1000rpm and 80mm/min, respectively which the rotational speed to 2000rpm led to reduce the fracture load of joint to 3350N. It also became clear that strength of the joint increases and then decreases with increasing tool tilt angle. Therefore, that with increases tool tilt angle from 1.5 to 2.5 degree, fracture strength reduce from 7750N to 4180N. Which due to increase of the heat input, create flash around the weld zone reducing the volume of material under the tool shoulder, and finally the formation of defects in the joint.

The excellent properties of super duplex stainless steels (SDSS) are mainly due to their balance of ferrite - austenite since this balance is highly likely to change during welding and undesirable phases can create. In this work gas tungsten arc welding (GTAW) process was used to join UNS S32750 SDSS to API X65 high strength low alloy (HSLA) steel. The properties of weldments were investigated by impact test, potentiodunamic and cyclic polarization tests. Zero resistance ammeter (ZRA) tests also were used for investigation of behavior of galvanic corrosion on weld metal – HSLA base metal. Results indicated that amount of austenite in weld metal was higher than SDSS base metal and did not created undesirable phases. Toughness of weldment was higher than base metals and corrosion behavior of weld metal was alike SDSS base metal and presented excellent corrosion resistance comparing HSLA base metal. The results of ZRA showed that HSLA eroded in galvanic couple weld metal – HSLA and mechanism of corrosion was pitting corrosion.

According to widespread and increasing efficiency of ceramics in different industries, identification their mechanical properties with minimum time and cost is important for the optimal design. In this paper, B4C nanocomposite samples with a volume of 10% of TiB2were prepared and the values of density, hardness and Elasticity modulus were determined and then the fracture toughness values were calculated by using various fracture toughness formula with Vickers indentation test method in loads of 100N and 150N and their results were compared to each other. The results show that the modified equation with using cracks leads high accuracy and efficiency compared to other relations. It also concluded that the growth mechanism due to different loads affects on the results of fracture toughness values.

This paper aims at investigating the torsional buckling behavior of the embedded multi-walled nanotubes (double- to five- walled) under combined loading based on the nonlocal continuum mechanics. The governing partial differential equations are derived according to Donnel shell model assumptions and Eringen elasticity theory. The effects of the number of layers of carbon nanotube, the existence of axial force, temperature change and the existence of the elastic medium on critical shear stress are studied. Results clearly reveal that at a fixed length, the carbon nanotube which has more layers can tolerate higher critical shear stress, although the existence of compressive axial force and/or temperature change at a high temperature environment decreases the load-bearing capacity of carbon nanotube. While the existence of elastic medium and/or tensile axial force increase the critical shear stress. It is also seen that with a rise in the number of half-wave, the effects of small-scale parameter on shear stress increase. The difference in predicting critical shear stress of multi-walled nanotubes between nonlocal and local continuum mechanics is investigated as well.

Previous investigations indicate that using the open crack model for vibration analysis of cracked structures may lead to incorrect results. Such a simple model can only be used as a rough approximation for predicting the dynamic behavior of the cracked structures. Therefore, in order to predict the nonlinear dynamic behaviour of the structures with a fatigue crack more accurately, one has to consider the nonlinearity of the crack. In this paper, the nonlinear behavior of the free vibration of a cantilever beam with a Fatigue Crack is investigated. To this end, first, the lateral vibration of the cracked beam in its first mode is modeled as an SDOF system with an equivalent mass and stiffness. Then, a new model is introduced for the bilinear stiffness of the beam with a breathing crack. Using this model, the governing differential equation of motion is converted to the standard form that can be analyzed by Lindstedt- Poincare’s method. The results show that the response is composed of two parts. The main part is the response of a system with the mean equivalent stiffness of the systems corresponding to the closed crack and the open crack cases. The second part is composed of the first and second order correction terms, which reflects the effect of opening and closing of the crack on the vibration response. In fact, the correction terms consist of the higher harmonic components of the spectrum. The results have been validated by the experimental tests.