Abstract in English:Abstract In this paper dynamical instability of three-layer micro-switch under DC voltage actuation has been studied. Recent studies have used the classical beam theory while leaving out the length scale parameter. In this paper dynamic behavior of the switch has been investigated based on couple stress theory and considering the length scale parameter. To this end, governing dynamic equation of the micro switch has been extracted and presented. Considering the nonlinearity of governing equation due to the existence of electrostatic force, Galerkin method has been implemented to overcome this nonlinearity and solve the mentioned equation and obtain the dynamic response. Dynamic response of micro switch has been investigated with and without considering the damping effects. Variation of dynamic pull-in voltage versus micro beam length and primary gap between micro beam and stationary electrodes have been studied using couple stress and classic beam theory and obtained results have been compared to each other. Also dependency of dynamic pull-in voltage to damping factor has been studied with considering two theories. Furthermore switching time of micro switch have been determined and compared using couple stress and classic beam theories.
Abstract in English:Abstract Rod structure has been widely used in aerospace engineering and civil engineering. Nondestructive testing is a very important method applied to detect unseen flaws in structures, ultrasonic wave nondestructive testing has been used in many areas. Finite Element Method is one of the most widely used numerical methods but would have a high cost when doing simulation on ultrasonic wave due to the requirement of small time interval and element size. Wavelet based finite element method could improve the spatial resolution with fewer elements needed but still needs very small time interval. Laplace transform could easily convert the time domain into frequency and then inverse to time domain. This paper presents an innovative method combining Laplace transform and B-spline wavelet on interval (BSWI) finite element method, which could not only decrease the element number but also increase the time integration interval. Moreover, this innovative method is applied to simulate the ultrasonic wave propagation in 1D rod structure as well as used for nondestructive testing of damages in rod structures.
Abstract in English:Abstract The efficiency of steel fibers for shear strength of reinforced concrete beams is assessed. Four beams were evaluated: control consisted of one beam without any steel fibers and three beams with reinforced concrete with steel fibers. All beams were reinforced for shear strength by a minimum reinforcement rate. The influence of fiber content added to concrete, at 0.5%, 0.8% and 1.0%, and the possibility of partial or total replacement of conventional shear reinforcement (stirrups) by steel fibers were evaluated. Results showed a significant increase in ductility and stiffness of the beams with steel fibers and, consequently, changes in the failure mode were observed, of shear (control beam) to flexure behavior (beams with steel fibers).
Abstract in English:Abstract This study focuses the light on the shear behavior of pre-cracked beams, and examines the feasibility of applying fracture mechanics concepts to widen the understanding of shear behavior and mechanisms. The experimental program contains ten beam specimens of high strength concrete (HSC) and steel fiber reinforced concrete (SFRC). Pre-cracks were embedded with different sizes and locations along the favorable path and orientation to shear failure. Three main scenarios of shear failure were reported with minute effect of pre-cracks. The geometrical effect is dominant with marginal effect of the material’s nonlinearity in case of severe pre-diagonal crack in HSC while the nonlinearity of the material is supreme to that of the geometrical effect for SFRC and shorter cracks. For verification, numerical simulation was conducted to examine the geometrical effect of the pre-diagonal tension crack in shear span on the structural behavior of RC beams. It is found numerically that, when the crack tip of the tensile crack is away from the tensile reinforcement, the closuring moment of tensile reinforcement increases, and as a result reduces the strain energy release rate. Therefore, the tensile cracks stop and the shear cracks keep propagating leading the failure mechanism to the end failure point.
Abstract in English:Abstract The behavior of the tuned liquid column damper (TLCD) is analyzed in the control of non-linear structures subjected to random seismic excitations. The structure is modeled as a system of one degree of freedom with incursion in the non-linear range. The Bouc-Wen hysteretic model is used to model the non-linear behavior of the structure. A stationary stochastic analysis is performed in the domain of the frequency. An equivalent statistical linearization was used for the analysis of the main system and the TLCD. The TLCD parameters considered for the optimization process were the frequency and the head loss coefficient. Two target functions were considered, (i) reduction of the main displacement of the system, (ii) reduction of the hysteretic energy. Two random processes were considered as seismic excitation, first a broad bandwidth process and secondly a narrow bandwidth process. The results show that for a broad bandwidth process, the TLCD tends to tune with the linear equivalent frequency of the system in the case without TLCD, while for the narrow bandwidth process, it tunes (TLCD) with the dominant frequency of the input. It is seen that the TLCD becomes detuned with regard to the frequency of the structure as the structure becomes more non-linear. It is also seen that the optimal tuning ratio of the TLCD is unsensitive to the mass ratio of the device and the main damping ratio of the system. It is also concluded that in case of flexible structures, the optimal head loss coefficient tends to be lower and increases with regard to its length ratio. It is seen that the effectiveness of the TLCD is greater for higher mass ratios of the device. In addition, it is found that the optimal TLCD becomes less effective as the structure enters the non-linear range, showing lower efficiency than what is seen in the literature for optimal TLCDs in linear structures.
Abstract in English:Abstract This work presents a study of the behavior, performance and failure of thin-walled composite channel section columns. The study is carried-out using nonlinear finite element models of laminated columns, including geometric imperfections and material failure, as well as approximate analytical solutions based on the Classical Lamination Theory. The accuracy of the numerical model is assessed using experimental results available in the literature and very good results were obtained. The results show the influence of the column layup and wall thickness on the structural behavior of laminated columns, including the buckling mode and ultimate failure load. Finally, the numerical model is used to assess the accuracy of approximate closed-form expressions for evaluation of the local buckling loads of composite channel section columns.
Abstract in English:Abstract Nowadays, safety is a competitive advantage for automotive products and therefore receives considerable attention by automotive research centers. In this paper, a frontal crash test of sedan product of an under development platform is simulated and occupant head injuries are investigated based on ECE R94 regulation. First, an initial evaluation of the crash behavior of the sedan car is carried out and then airbag, dummy and seat belt are added to the model to study occupant head injuries under crash test. In this study, peak head acceleration and head injury criteria (HIC36) are considered as two output parameters based on ECE R94 regulation. Considering these two output parameters, sensitivity analysis and optimization are performed using Taguchi and analysis of variance (ANOVA) methods. In this way, airbag distance to dummy, trigger time, initial inflator gas temperature and tank pressure are considered as input parameters. Obtained results show that in all computer experiments designed by Taguchi responses values satisfy the requirements of ECE R94. Two different out-of-position conditions are considered by reducing the distance between dummy and airbag relative to the optimum design obtained by Taguchi. The worst case of design and its response values are also predicted using Taguchi. Finally, re-evaluating finite element analyses are performed based on the optimum and the worst cases. The results of these simulations show the validity of approach of this paper.
Abstract in English:Abstract In this paper the effects of deformation modes on martensitic transformation for TRIP steels which are composed of Ferrite, Bainite and Retained austenite are investigated in the view of the microstructural level. For this porpose the simulations are run for a synthetically generated microstructure which have 55% Ferrit, 35% Bainite and 10% Retained austenite. In the simulations tensile, biaxial and shear type deformation modes are considered. The results reveal that biaxially loaded microstructure has the maximum amounts of martensite phases at the end of the given deformation and the less martensite is occurred in the shear type loading condition.
Abstract in English:Abstract A numerical investigation is performed in this work in order to evaluate the aerodynamic performance of building cross-section configurations by using corner modifications. The CAARC tall building model is utilized here as reference geometry, which is reshaped considering chamfered and recessed corners. The numerical scheme adopted in this work is presented and simulations are carried-out to obtain the wind loads on the building structures by means of aerodynamic coefficients as well as the flow field conditions near the model’s location. The explicit two-step Taylor-Galerkin scheme is employed in the context of the finite element method, where eight-node hexahedral finite elements with one-point quadrature are used for spatial discretization. Turbulence is described using the LES methodology, with a dynamic sub-grid scale model. Predictions obtained here are compared with experimental and numerical investigations performed previously. Results show that the use of corner modifications can reduce significantly the aerodynamic forces on the building structures, improve flow conditions near the building locations and increase the Strouhal number, which may have an important influence on aeroelastic effects.
Abstract in English:Abstract This paper presents the fatigue behavior of Glass Fiber Reinforced Polymer (GFRP) composites at constant amplitude tension-tension loading conditions. A two parameter residual strength and fatigue life model has been proposed by accounting the effect of stress ratio when the structure undergoes continuous loading. A model is also developed to predict the fatigue life of GFRP composites based on fatigue endurance limit. Experiments were conducted on GFRP composite specimens to predict fatigue life and residual strength at various stress levels. Tests were also conducted to gain an understanding of the tensile behavior of GFRP composite specimens under different quasistatic strain rates. The lowest tensile strength resulting from strain rate studies has been used ultimately for conducting fatigue life and residual strength tests. Reliability of the proposed models has been verified with experimental results and with the models seen in literature.