سال:1392 | دوره: | شماره: |تعداد مقالات:8

Ratcheting behavior of steel 304L cylindrical shell under cyclic combined and axial loading are investigated in this paper, numerically. Cylindrical shells were fixed oblique at angle of 20o and normal with respect to the longitudinal direction of the shell and subjected to force-controlled cycling with non-zero mean force, which causes the accumulation of plastic deformation or ratcheting behavior. Numerical analysis was carried out by ABAQUS software using nonlinear isotropic/kinematic hardening model. Numerical results compared to experimental data that was performed by an INSTRON 8802 servo hydraulic machine. Simulations show good agreement between numerical and experimental results. Also, the effect of length, angle of cylindrical shell and existence of cutout are studied with finite element method. Seen, the bending moment plays a strong role in increase of plastic deformation. It is observed that there is more plastic deformation for cylindrical shell under combined loading in comparison to cylindrical shell under uniaxial loading. Ratcheting behavior is sensitive to cutout and showed that creating the cutout increases the plastic deformation.

In the present paper, the problem of reflection and transmission of waves at an interface of elastic and micro stretch thermoelastic solids with micro temperatures has been studied. The amplitude ratios of various reflected and transmitted waves are functions of angle of incidence and frequency of incident wave. The expressions of amplitude ratios have been computed numerically for a particular model. The variations of amplitude ratios with angle of incidence are shown graphically to depict the effect of micro rotation. Some particular cases of interest have been also deduced.

Impact wear can be defined as the wear of a solid surface that is due to percussion, which is a repetitive exposure to dynamic contact by another solid body. It generally has the devastating effects on the mechanical elements and causes the equipments to shift away from their normal performance. Impact wear has not been studied as extensive as other wear mechanisms and as a result information on the causes and actual impact wear data is quite scarce. Knowing how the impact parameters affect the wear intensity would be helpful to have the more optimal designs. Having an experimental apparatus would be a reliable way for this aim. In the present work, a new impact tester was designed to explore the consecutive impacts between balls and a flat plate as a wearing specimen. Measurements of the plate mass loss after a number of impacts at the different impacting conditions revealed the effect of parameters on the impact wear. Design of experiment is carried out regarding the impact velocity, ball size and impact angle as the variables. An impact wear model is extracted based on the experimental data. The obtained results suggest that the model can be used as a predictive way to study the practical design problems and to explain some phenomena associated with impact erosion.

A model of the equations of two dimensional problems in a half space, whose surface in free of micro polar thermoplastic medium possesses cubic symmetry as a result of a Mode-I Crack is studied. There acts an initial magnetic field parallel to the plane boundary of the half- space. The crack is subjected to prescribed temperature and stress distribution. The formulation in the context of the Lord-shulman theory LS includes one relaxation time and Green-Lindsay theory GL with two relaxation times, as well as the classical dynamical coupled theory CD. The normal mode analysis is used to obtain the exact expressions for the displacement, micro rotation, stresses and temperature distribution. The variations of the considered variables with the horizontal distance are illustrated graphically. Comparisons are made with the results in the presence of magnetic field. A comparison is also made between the three theories for different depths.

The governing equations of a transversely isotropic dissipative medium are solved analytically to obtain the surface wave solutions. The appropriate solutions satisfy the required boundary conditions at the stress-free surface to obtain the frequency equation of Rayleigh wave. The numerical values of the non-dimensional speed of Rayleigh wave speed are computed for different values of frequency and initial stress parameter. The effects of transverse isotropy and initial stress parameter are observed on the Rayleigh wave speed.

In this paper, static indentation and low velocity impact responses of a fully backed composite sandwich plate subjected to a rigid flat-ended cylindrical indenter/impactor are analytically investigated. The analysis is nonlinear due to nonlinear strain-displacement relation. In contrast to the existed analytical models for the indentation of composite sandwich plates, the stacking sequence of the face sheets can be completely arbitrary in the present model. Furthermore, the effects of the initial in-plane normal and shear forces on the edges of the sandwich plate are also considered. Based on these modifications, an improved contact law (contact force – indentation relation) is derived. The low velocity impact analysis of the problem is performed using a discrete system of spring-mass-dashpot model. The characteristics of the equivalent spring and dashpot are identified from the derived contact law and by incorporating the effect of the dynamic material properties of the sandwich plate. Analytical predictions of the load-indentation response as well as the impact force history are compared well with the experimental results in the literature. The effects of various parameters on both indentation and impact responses of the sandwich plates are qualitatively and quantitatively investigated.

In this paper, surface effects including surface elasticity, surface stress and surface density, on the free vibration analysis of Euler-Bernoulli and Timoshenko Nano beams are considered using nonlocal elasticity theory. To this end, the balance conditions between Nano beam bulk and its surfaces are considered to be satisfied assuming a linear variation for the component of the normal stress through the Nano beam thickness. The governing equations are obtained and solved for Silicon and Aluminum Nano beams with three different boundary conditions, i.e. Simply-Simply, Clamped-Simply and Clamped-Clamped. The results show that the influence of the surface effects on the natural frequencies of the Aluminum Nano beams follows the order CC<CS<SS while this is not the case for Silicon Nano beams. On the other hand, the influence of the nonlocal parameter is opposite and follows the order SS<CS<CC. In addition, it is seen that considering rotary inertia and shear deformation has more effect on the surface effects than the nonlocal parameter.

In this study, the vibration behavior of circular and annular graphene sheet embedded in a Visco-Pasternak foundation and coupled with temperature change and under in-plane pre-load is studied. The single-layered annular graphene sheet is coupled by an enclosing viscoelastic medium which is simulated as a Visco- Pasternak foundation. By using the nonlocal elasticity theory and classical plate theory, the governing equation is derived for single-layered graphene sheets (SLGSs). The closed-form solution for frequency vibration of circular graphene sheets has been obtained and nonlocal parameter, in-plane pre-load, the parameters of elastic medium and temperature change appears into arguments of Bessel functions. To verify the accuracy of the present results, the new version differential quadrature method (DQM) is also developed. Closed-form results are successfully verified with those of the DQM results. The results are subsequently compared with valid result reported in the literature. The effects of the small scale, pre-load, mode number, temperature change, elastic medium and boundary conditions on natural frequencies are investigated. The non-dimensional frequency decreases at high temperature case with increasing the temperature change for all boundary conditions. The effect of temperature change on the non-dimensional frequency vibration becomes the opposite at high temperature case in compression with the low temperature case. The present research work thus reveals that the nonlocal parameter, boundary conditions, temperature change and initial pre-load have significant effects on vibration response of the circular nanoplates. The present analysis results can be used for the design of the next generation of Nano devices that make use of the thermal vibration properties of the graphene.