Resumo em Inglês:

Abstract In the present paper, a critique study on some models available in the literature for bending analysis of nano-beams using the gradient elasticity theory is accomplished. In nonlocal elasticity models of nano-beams, the size effect has not been properly considered in governing equations and boundary conditions. It means that in these models, because of replacing of the size effect with the inertia gradient effect, the size dependency has been ignored in bending analysis of nano-beams. Therefore, as the beam dimensions increase in comparison to its material length scale parameter, the obtained solution based on the gradient elasticity theory (either in the nonlocal elasticity theory or the strain gradient elasticity theory) should converge to the classical elasticity solution. Hence, satisfying of boundary conditions is a crucial point. In this paper, governing equations and boundary conditions are presented based on two gradient elasticity theories (i.e., nonlocal elasticity and strain gradient elasticity theories). Also, boundary conditions in strain gradient elasticity theory are modified based on a dimensional analysis approach. The results indicate that the strain gradient elasticity theory captures the size effect more sensitive in comparison with the nonlocal elasticity theory in bending analysis. In addition, modified boundary conditions in strain gradient elasticity theory can lead to converge the classical solution at large scales. To prove that the boundary conditions of nano-beam have the direct effect on mechanical behavior of structure, the size-dependent Young modulus of carbon nanotube (CNT) is investigated and the results show that the prediction of strain gradient elasticity theory with modified boundary conditions is in a good agreement with experimental results.

Resumo em Inglês:

Abstract An investigation on the effect of uniform tensile in-plane force on the radially symmetric vibratory characteristics of functionally graded circular plates of linearly varying thickness along radial direction and resting on a Winkler foundation has been carried out on the basis of classical plate theory. The non-homogeneous mechanical properties of the plate are assumed to be graded through the thickness and described by a power function of the thickness coordinate. The governing differential equation for such a plate model has been obtained using Hamilton's principle. The differential transform method has been employed to obtain the frequency equations for simply supported and clamped boundary conditions. The effect of various parameters like volume fraction index, taper parameter, foundation parameter and the in-plane force parameter has been analysed on the first three natural frequencies of vibration. By allowing the frequency to approach zero, the critical buckling loads for both the plates have been computed. Three-dimensional mode shapes for specified plates have been plotted. Comparison with existing results has been made.

Resumo em Inglês:

Abstract Composite laminates are made of glass woven roving mats of 610gsm, epoxy resin and nano clay which are subjected to projectile impact. Nano clay dispersion is varied from 1% to 5%. Impact tests are conducted in a gas gun setup with a spherical nose cylindrical projectile of diameter 9.5 mm of mass 7.6 g. The energy absorbed by the laminates when subjected to impact loading is studied, the velocity range is below ballistic limit. The effect of nano clay on energy absorption in vibration, delamination and matrix crack is studied for different weight % of nano clay and for different thickness values of the laminates. The natural frequencies and damping factors are obtained for the laminates during impact and the effect of nano clay is studied. The results show considerable improvement in energy absorption due to the presence of nano clay

Resumo em Inglês:

Abstract This study dealt with the dynamic model of composite cylindrical sandwich panels with flexible cores and simply supported boundary conditions under low velocity impacts of multiple large or small masses using a new improved higher order sandwich panel theory (IHSAPT). In-plane stresses were considered for the core and face sheets. Formulation was based on the first order shear deformation theory for the composite face sheets and polynomial description of the displacement fields in the core that was based on the second Frostig's model. Fully dynamic effects of the soft core and face-sheets were considered in this investigation. Impacts were assumed to occur simultaneously and normally over the top face-sheet with arbitrarily different masses and initial velocities. The contact forces between the panel and impactors were treated as the internal forces of the system. In this paper, nonlinear contact stiffness was linearized with a newly presented improved analytical method. Numerical results of the mentioned structures were compared with finite element model using ABAQUS code.

Resumo em Inglês:

Abstract This paper introduces a method to identify damages in beam-like structures by analyzing the natural frequency changes of the first six transversal vibration modes. A correlation between the damage location and frequency change is established for each mode separately, by considering the modal strain energy stored in that location. The mathematical relation describing this correlation is used to characterize the dynamic behavior of a beam with a damage of known position and to derive its Damage Location Indicator (DLI) as a six-term vector. The method consists in comparing the vectors describing the damage at any possible location along the beam with the Damage Signature (DS), which is achieved from the measurements that compare the beam's frequencies in healthy and damaged state. A modified Kullback-Leibler Divergence is used to assess the damage location. In order to permit early damage assessment, an improved frequency evaluation algorithm was developed. It is based on signal truncation and consequent spectral lines rearrangement, in order to accurately find the strongest spectral components. The effectiveness of the proposed method is demonstrated by simulations and experiments.

Resumo em Inglês:

Abstract A parametric study devoted to assess the impact of increasing the structural redundancy in ductile reinforced concrete (RC) moment framed buildings is presented. Among the studied variables were the number of stories and the number of bays. Studied models were 4, 8, 12 and 16-story frames with a story height h=3.5 m (11.5 ft). Nonlinear static analyses were used to evaluate numerically redundancy factors. Based on the results of this research and previous studies reported in the literature, it can be concluded that it is justified to account directly structural redundancy in the design by using a redundancy factor, as proposed and done in some international building codes.

Resumo em Inglês:

Abstract A modified stress function approach is developed here to reconstruct induced stress, residual stress and eigenstrain fields from limited experimental measurements. The present approach is successfully applied to three experimental measurements set in surface peened plates with shallow shot peening affected zone. The well-rehearsed advantage of the proposed approach is that it not only minimizes the deviation of measurements from its approximations but also will result in an inverse solution satisfying a full range of continuum mechanics requirements. Also, the effect of component thickness as a geometric parameter influencing the residual stress state is comprehensively studied. A key finding of present study is that the plate thickness has no influence on the maximum magnitude of eigenstrain profile and compressive residual stresses within the shot peening affected zone while having a great influence on the magnitude of tensile residual stress and the gradient of linear residual stresses present in deeper regions.

Resumo em Inglês:

Abstract The dynamic response and energy absorption capabilities of clamped shallow sandwich arches with aluminum foam core were numerically investigated by impacting the arches at mid-span with metallic foam projectiles. The typical deformation modes, deflection response, and core compression of sandwich arches obtained from the tests were used to validate the computation model. The resistance to impact loading was quantified by the permanent transverse deflection at mid-span of the arches as a function of projectile momentum. The sandwich arches have a higher shock resistance than the monolithic arches of equal mass, and shock resistance could be significantly enhanced by optimizing geometrical configurations. Meanwhile, decreasing the face-sheet thickness and curvature radius could enhance the energy absorption capability of the sandwich arches. Finite element calculations indicated that the ratio of loading time to structural response time ranged from 0.1 to 0.4. The projectile momentum, which was solely used to quantify the structural response of sandwich arches, was insufficient. These findings could provide guidance in conducting further theoretical studies and producing the optimal design of metallic sandwich structures subjected to impact loading.

Resumo em Inglês:

Abstract The moving load problem is divided into two typical types: moving force and moving mass. The moving mass problem includes the time-varying mass effect. This work considers damage detection of the beam structure subject to a moving load including the inertia effect based on the only measurement data from strain gages and accelerometers without any baseline data. This experiment compares the feasibility of damage detection methods depending on the measurement sensors of strain gages and accelerometers. The measurement data are transformed to the proper orthogonal modes (POMs) in the time domain and the frequency domain, respectively. The magnitude of the moving mass and its velocity are also evaluated as test variables in this experiment. It is shown in the beam tests that the measured strain data can be more explicitly utilized in detecting damage than the acceleration data, and the mass magnitude and its velocity affect the feasibility of damage detection.

Resumo em Inglês:

Abstract In this paper, an active control is used to suppress the flutter vibration of a support excitation beam subjected to a follower force, using piezoelectric sensors/actuators. The beam is fixed to a motion support from one end and the other end is subjected to a follower force. The governing equations of motion are derived based on the generalized function theory and Lagrange-Rayleigh-Ritz technique, considering the Euler-Bernoulli beam theory. A robust Lyapunov based control scheme is applied to the system to suppress the induced flutter vibrations of the beam. The mathematical modeling of the beam with control algorithm is derived. Finally the system is simulated and the effects of the type of excitation, the magnitude of the follower force, instrument disturbances, and parameter uncertainties are studied. The simulation results show the applicability and robustness of the controller algorithm.