Free vibration of simply supported circular cylindrical shell made of Functionally Graded Material (FGM) under internal pressure was investigated. The effective material properties are assumed to vary continuously along the thickness direction according to a volume fraction power law distribution. First order shear deformation theory based on Love's first approximation theory was utilized in the equilibrium equations. The effects of FGM parameters such as material configuration and power law exponent, internal pressure as well as geometrical parameters such as thickness to radius and length to radius ratios on the vibration behavior were investigated. The validation of the results was achieved by comparing with those available in the literature. The results show that the vibration characteristics of Functionally Graded (FG) shells are greatly influenced by FGM parameters. Also, internal pressure and geometrical parameters considerably influence the frequency behavior regarded to different values of FGM parameters. پرونده مقاله

In this study, the strain rate effect on the bending properties of fiber reinforced composites for three types of polymer composites namely phenolic resin reinforced by woven basalt fibers, woven carbon fibers, and woven basalt/ woven carbon fibers at a total volume fraction of approximately 35% has been determined. Flexural tests have been conducted at low range of strain rates included 0.03 min-1, 0.06 min-1 and 0.09 min-1. Specimens with identical geometry have been used in all the tests. Experimental results showed that the strain rate has a significant effect on the material response in bending. Results showed that, both the flexural modulus and the ultimate flexural strength of the three types of composites are increased with the increasing in the strain rate. Also, the bending properties of composites reinforced with woven carbon fibers are very sensitive to the strain rate during the test.    پرونده مقاله

Changes in moisture content and temperature can perturb both stiffness and strength and as such the impact resistance of composite structures. In this paper, the pre-notched basalt fibre reinforced epoxy (BFRE) composite specimens are studied under Charpy impact loads in order to investigate the changes in impact energy absorption (fracture toughness) with different kinds of aging conditions. To create three types of aggressive environmental conditions, the specimens were frozen in dried air at T= -18 °C, frozen in tap water at T= -18 °C and kept in tap water at T= 30 °C and the results were compared with the specimen without aging influence (T= 25 °C). Water used for this examination has a high percentage of precipitation (70-80%) which causes high corrosion and consequent decrease in mechanical properties. The outcomes illustrated that the long-term exposure to tap water (T= 30 °C), dried air (T= -18 °C) and frozen in tap water (T= -18 °C) were all affected on energy absorption of BFRE composites, however, freezing effect in water was more intense. To study the nature of the failure mechanisms, all failed specimens were inspected by Scanning Electron Microscopy (SEM) photographs. پرونده مقاله

There exist simple theories to predict the strain of sandwich structures under a bending load and specific support conditions. Although theoretical predictions are highly effective, but the role of testing in validation of these theories and their progress is undeniable. The main goal of the present paper is to prepare a test instruction in order to minimize the error of data acquisition and their analysis. The effects of specimen thickness, place of strain gauge installation and kind of strain gauge was investigated. Characterization of mechanical properties of test specimens are another focus of the present study. Classic theories are used to validate the results of the tests. Based on the large number of tests conducted on the samples, the results show that an important source of error in between experimental and theoretical results may be incorrect installation of the strain gauge. A method to find this out, is to obtain the results of loading and unloading. The method of using the gauge factor of the strain gauge on the package gives better results. Using a circuit with three wires omits the error of wire resistance. The results of the tests on composite specimens obtained by TML Company strain gauges showed small differences with the results obtained by BLH Company strain gauges. The error between experimental and theoretical results in sandwich specimens increases by increasing the core thickness. The measurements obtained by strain gauge in compression is not reliable. An important result obtained was that a half-bridge installation of the strain gauge decreases the error so much. پرونده مقاله

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. پرونده مقاله

This paper presents the effect of centrifugal load in functionally graded (FG) hollow sphere subjected to uniform magnetic field. Analytical solution for stresses and perturbation of the magnetic field vector are determined using the direct method and the power series method. The material stiffness, the magnetic permeability and the density vary continuously across the thickness direction according to the power law functions of radial directions. Magnetic field results in decreasing the radial displacement, the radial and shear stresses due to centrifugal load and has a negligible effect on circumferential displacement and also small effect compared with the other quantities on the circumferential stress due to centrifugal load. Increasing the angular velocity results in increasing the all above quantities due to magnetic field. With increasing the power law indices the radial displacement, the shear and circumferential stresses due to centrifugal load and magnetic field all are decreased and the radial stress due to centrifugal load and magnetic field increased. پرونده مقاله