Abstract in English:Abstract The most common method used to strengthening, rehabilitation or repairing of reinforced concrete (RC) members is to use external carbon fiber reinforced polymer (CFRP) sheets. CFRP can greatly improve the flexural and shear capacity of deteriorated members and therefore extends their useful life. The main problem of external CFRP is the debonding of the sheets from the concrete surface at some point of loading, which negatively affects the efficiency of strengthening and may consequently lead to an unanticipated failure of the strengthened members. The major reason for this early debonding is likely due to the low accuracy of the preparation and the high stress concentration at the flat contact area exists between CFRP sheets and the concrete. The problem has been extensively discussed in the literature and some CFRP application techniques such as “Externally Bonded Reinforcement on Grooves (EBROG)” and “Externally Bonded Reinforcement in Grooves (EBRIG)” have been proposed as alternatives to the conventional application methods. Although some research has been carried out, there have been few experimental investigations that provided quantitative discussion of the efficiency of the new developed techniques. This research was aimed to experimentally assess the efficiency of grooving techniques and to provide a quantitative data regarding the behaviour of bonding between CFRP and concrete. The effects of shape and direction of the grooves and CFRP layers on the load carrying capacity, mid-span deflection and failure mode of thirteen RC beams have been investigated and discussed. In general, CFRP has significantly improved the flexural capacity of strengthened beams especially when grooving technique has been employed.
Abstract in English:Abstract This paper presents the formulation of a two-dimensional numeri-cal model able to describe the fracture process in structural mem-bers of steel fibre reinforced concrete (SFRC) from the volume ratio of the fibres and the mechanical properties of the compo-nents: a concrete matrix and a set of steel fibres with a random orientation. The relationship between the stress and the strain fields of the composite material is obtained using the mixture theory with a compatibility strain of its component materials. The concrete matrix is represented with a scalar damage constitutive model with a softening strain and a different strength in tension and compression. The mechanical strain of an insulated fibre and the slip between the fibre and the matrix are simultaneously de-scribed with a one-dimensional plasticity constitutive model. The cracking of the composite material indicates a jump in the dis-placement field and non-bounded values of the strain field, which are represented by the Continuum Strong Discontinuity Ap-proach. The model has been implemented in the framework of the nonlinear analysis with the Finite Element Method, using con-stant strain triangular elements. Moreover, the fibres distribution and orientation change randomly in each finite element and each simulation or observation. The structural responses of the simula-tions are treated as curves and analysed by tools from the Func-tional Data Analysis. Confidence intervals for the structural re-sponse are built using bootstrap methodology. Finally, experi-mental tests of SFRC members subjected to tension and bending are simulated. The structural response and the cracking patterns obtained from the numerical simulation are satisfactory.
Abstract in English:Abstract In many engineering applications, there are systems that exhibit a series of collisions in a certain amount of time, for example the impact of floating ice in ships and the rubbing between stationary and rotary parts in turbomachinery. In many of these situations, it is of vital importance to know how the impacts affect the overall system behavior for the correct functioning or the prediction of faults in the system. This can be done by proposing models that can represent the system when it is subjected to sudden impacts in its oscillation movement. However, due to the nonlinear and non-smooth characteristic of the impact phenomenon, the implementation of impacts in mechanical systems may be a challenging task. In this manner, this paper aims in proposing a vibro-impact model of two adjacent portal frame structures, being one of them driven by an unbalanced DC motor positioned on top of it. The portal frames were modeled as continuous beams by means of the Euler-Bernoulli beam theory. In order to model the impact between the structures, three different models were considered: using the Hertz theory of elastic bodies to model the contact force using a nonlinear damping; using a spring and a damper in parallel, viscoelastic or spring-dashpot system, to model the force; and considering the impact to be instantaneous and using the coefficient of restitution to adjust the velocity after the contact. To validate the impact models, an experimental procedure was performed and the measurements compared with the simulations.
Abstract in English:Abstract The assessment of the differences in results obtained from various micromechanics homogenization schemes, as well as the implications of assuming different volume fraction profiles was carried out in the present work. The functionally graded composite chosen for the analysis was Al-SiC and comparisons were made in terms of stress and strain distributions along the wall of an internally pressurized hollow cylinder. Different micromechanics homogenization schemes were implemented into Abaqus as user-defined subroutines (UMAT). The numerical simulations were compared to a set of analytical solutions available on the literature. The obtained results varied substantially according to the homogenization scheme employed. It was also found that the type of function chosen to describe the volume fraction distribution plays a major role on the development of the hoop stresses. Additionally, the finite element analysis showed significant stress variation when the actual volume fraction distribution was used. These gradients did not appear when the same profile was approximated by smooth exponential functions. This paper points out some important issues related to common practices associated with the analysis of FGM composites and serves as an overture to a more in-depth discussion of such problems.
Abstract in English:Abstract Nonlinear analyses using an updated Lagrangian formulation considering the Euler-Bernoulli beam theory have been developed with consistency in the literature, with different geometric matrices depending on the nonlinear displacement parts considered in the strain tensor. When performing this type of analysis using the Timoshenko beam theory, in general, the stiffness and the geometric matrices present additional degrees of freedom. This work presents a unified approach for the development of a geometric matrix employing the Timoshenko beam theory and considering higher-order terms in the strain tensor. This matrix is obtained using shape functions calculated directly from the solution of the differential equation of the problem. The matrix is implemented in the Ftool software, and its results are compared against several matrices found in the literature, with or without higher-order terms in the strain tensor, as well as the Euler-Bernoulli or Timoshenko beam theories. Examples show that the use of the Timoshenko beam theory has a strong influence, especially when the structure has small slenderness (short members). For high axial load values, the consideration of higher-order terms in the strain tensor results in larger displacements as expected.
Abstract in English:Abstract In this paper a most promising scope of the exact quasiconvex energy envelope in modeling the granular materials with microstructures is presented. This study shows that it is possible to observe both the extended microstructures and localized deformations in granular materials using a variational model based on the mathematical relaxation theory. The variational model is derived within the framework of Cosserat continuum. The computational algorithm based on finite element method is used to carry out numerical computations. The features of the proposed model are studied for three representative examples: the Couette shear cell, the rectangular specimen in compression and the indentation of a granular structure. The obtained results demonstrate on the possible applications and features of exact quasiconvex energy envelops in modeling the granular materials.
Abstract in English:Abstract The present paper proposes a new procedure to determine the optimal parameters of a dynamic vibration absorber (DVA), considering both damped and undamped primary system. The DVA design is formulated as an optimization problem in which the objective function is constructed based on Den Hartog's equal-peak method. The DVA parameters are selected to minimize the response of the primary system when it is subjected to harmonic force or base motion. Firstly, we propose a numerical strategy based on Frequency Response Curve (FRC) in which the parameters of the absorber are updated by minimizing the objective function. The results are presented for a set of reference parameters, which demonstrate the feasibility of the proposed method for determining the optimal parameters of the absorber for both excitations. Taking into account the system response with respect to reference parameters, the bilinear interpolation technique was employed in order to obtain explicit formulas of the damping and frequency ratios of the DVA.
Abstract in English:Abstract This paper aims at analyzing the sensitivity of the structural response of flexible rotors subjected to uncertain interval parameters. The interval approach encompasses both an uncertainty and sensitivity analyses. The uncertainty analysis consists in computing the interval uncertain structural responses by using the global optimization method. The sensitivity analysis computes the normalized relative sensitivity indices that quantify the degree of importance of each uncertain interval parameter on the uncertain structural responses of the flexible rotor. The uncertain structural responses and the sensitivity of the flexible rotor subjected to interval parameters are analyzed in terms of the Frequency Response Function (FRF), orbits, Campbell diagram, and run-up operating condition. Numerical simulation results illustrate the interval approach conveyed.
Abstract in English:Abstract In this paper, a nonlinear H∞ state feedback control is designed for both orientation and altitude of a flying robot system in the presence of external disturbance. An analytical solution is proposed for Hamilton-Jacobi-Isaac (HJI) equation. According to the quadrotor's orientation and altitude, a suitable storage function is considered and the appropriate robust control law is derived. The controller coefficients are tuned from Hamilton-Isaac-Jacobi inequality. The closed-loop nonlinear system with the proposed controller has L2-gain less than or equal to γ, and guarantee its asymptotic stability closed-loop nonlinear system with external disturbance. Simulations are provided with the model uncertainties and external disturbance to verify the robustness of the proposed controller. Simulation results confirm the effectiveness of the desired robust controller.
Abstract in English:Abstract Roadway deformation and coal bumps are major challenges for underground engineering. Taking the Lu'an mining district in Shanxi Province as an example, the failure mechanism of deep high-stress roadway was studied. Numerical simulations and the similar material test were performed to investigate the effect of the roof presplitting and rock mass filling on the stability of roadway surrounding rock. The change laws and distribution features of stress and plastic zone, and the fracture characteristics of roof strata were analyzed. Under conditions where the roof was not presplit, the roadway was in a high-stress environment due to the massive suspended roof. The failure process of the high-stress roadway began with tensile crack in the shallow roof and then extended to the two ribs, resulting in the yield of a coal pillar. Under the condition that the roof was presplit, the peak value of the vertical stress at the coal pillar was decreased from 18.2 to 9.8 MPa while that of the virgin coal rib was decreased from 15.8 to 13.5 MPa. Both the similar material test and numerical results showed that the Roof outside the Coal Pillar (RCP) was cut off along the presplitting lines and was fractured into different sizes of masses. The broken rock mass filled in the goaf and supported on the overlying strata, which reduced the load on the coal pillar. The field monitoring data indicated that implementing the roof presplitting and rock mass filling processes successfully controlled large deformations of the roadway and coal bumps, which improved the stability of surrounding rock.