Abstract in English:Abstract The aim of this paper is to introduce a new buffering mechanism in impact reduction during assembly process of two space vehicles. Probe-cone mechanism, due to its computational easiness, is employed to study impact interaction between multibody systems. The proposed buffer with a compact structure includes a probe equipped with axial and torsional flexibilities during impact condition. Since all constraints in the problem are holonomic, the unconstrained form of Lagrangian approach will be used to derive the system’s equations of motion. The Lankarani-Nikravesh contact force model, because of its advantages in multibody systems, is applied to investigate dynamic behavior of vehicles during impact-contact event. A key factor in capture process of two vehicles is to determine the proper parameters of buffer. To do this, a strategy will be introduced to evaluate the coefficients of buffer. First, the theoretical model is solved and the obtained results are verified by those recently reported for special case related to impact analysis of two space vehicles carrying flexible probe. Second, the entire capture process will be built in MATLAB/Simulink’s SimMechanics multibody software to ascertain the correctness of theoretical model. Then, a simple ground-based experimental setup will be established to only prove the effectiveness of the proposed capturing system. The obtained results indicate that the proposed buffer leads to considerably reduce the peak value of impact force, to remarkably increase total contact time and to successfully perform capture process.
Abstract in English:Abstract In orthodontics, the CR is defined as the ideal point for the positioning of the bracket. This paper presents numerical analyses of a premolar tooth, which aim at searching the centre of resistance (CR) by applying several external torques. We built the geometry of the tooth by making a tomographic renderisation with the software MIMICS. The numerical simulation is performed in the software ANSYS WORKBENCH (ANSYS® Academic Research). The domain of the tooth is discretized using three-dimensional solid elements. The search for the position of CR is made through a heuristic analysis by applying external torques to the model and, afterwards, by seeking the intersection of the neutral planes (where there is no translation). The numerical results showed that the CR is not a point, but rather a region that strongly depends on the relative stiffness between the tooth and the Periodontium (PDL).
Abstract in English:Abstract The main aim of this paper is to employ an extension of two-variable Refined Plate Theory (RPT2) to determine Sound Transmission Loss (STL) across laminated composite plate in contrast to external excitation. It should be noted that RPT2 known as a second type of Refined Plate Theory which includes the extension term in displacement field. To achieve this goal, firstly, the lateral displacements are expanded by considering three parts, including shear, bending and extension term to provide an analytical model based on two-variable Refined Plate Theory without considering the effect of the shear correction coefficient in governing equations. Secondly, vibro-acoustic analysis is administered by incorporating the laminated composite plate equation beside the acoustic wave equation, simultaneously. Consequently, the power transmission through the structure is specified due to a plane sound wave. Beside, in order to illustrate the accuracy of the present formulation (RPT2), the obtained STL is compared with those available in literature. However, the application of present study (RPT2) is clarified in the acoustical designs due to presenting more precise results in comparison with Classical Plate Theory. Eventually, the numerical results are achieved to determine the influences of various properties on STL.
Abstract in English:Abstract In this work, we apply semi and fully-implicit time integration schemes to the damage and fatigue phase field presented in Boldrini et al. (2016). The damage phase field is considered a continuous dynamic variable whose evolution equation is obtained by the principle of virtual power. The fatigue phase field is a continuous internal variable whose evolution equation is considered as a constitutive relation to be determined in a thermodynamically consistent way. In the semi-implicit scheme, each equation is solved separately by suited implicit method. The Newton’s method is used to linearize the equations in the fully-implicit scheme. The time integration methods are compared and the results of damage and fracture evolution under the influence of fatigue effects are presented. The computational cost associated to the semi-implicit scheme showed be lower than the fully counterpart.
Abstract in English:Abstract In this study crashworthiness optimization of nested and concentric circular tubes under impact loading is performed by coupling Finite Element model, Response Surface Models and Genetic Algorithm. Specific Energy Absorption (SEA) and Crash Force Efficiency (CFE) are used in crashworthiness optimization since these criteria are important indicators for evaluating crashworthiness performance. Length and thickness of three concentric tubes as well as radius of one tube are adopted as design variables which are effective parameters on SEA and CFE. To reduce the computational cost of the optimization procedure, simple and computationally cheap Response Surface Models are created to replace finite element analyses in further calculations. The Non-dominated Sorting Genetic Algorithm –II (NSGAII) is applied to obtain the Pareto optimal solutions. Optimization results are presented for different selected designs that indicate relative importance of multi-objective functions. Results show that the total weight of the vehicles can be reduced by using nested tubes comparing to single tubes with identical masses. These designs can be adopted for use in practice.
Abstract in English:Abstract In this paper, a nonlinear three-degrees-of-freedom dynamical system consisting of a variable-length pendulum mass attached by a massless spring to the forced slider is investigated. Numerical solution is preceded by application of Euler-Lagrange equation. Various techniques like time histories, phase planes, Poincaré maps and resonance plots are used to observe and identify the system responses. The results show that the variable-length spring pendulum suspended from the periodically forced slider can exhibit quasi-periodic, and in a resonance state, even chaotic motions. It was concluded that near the resonance the influence of coupling of bodies on the system dynamics can lead to unpredictable dynamical behavior.
Abstract in English:Abstract The penetration performance of projectiles is closely related to their nose shape as it influences the kinetic energy penetration of a rigid projectile. One new type of projectile, which is entitled U-shape-nose grooved projectile, is introduced in this study. A theoretical model, which describes the structural characterization of U-shape-nose grooved projectile, is constructed here to investigate the penetration characters into an aluminum target. Moreover, comparative penetration experiments were conducted to investigate the penetration performance of U-shape-nose grooved projectile. Effects of characteristic parameters and shape optimal conditions of the U-shape-nose grooved projectile on the penetration performance are analyzed with the theoretical model validated by the experimental results. It has been proved that U-shape-nose grooved projectile has a more excellent penetration performance than the ogive-nose projectile based on comparison between the results of experimental and theoretical model. The projectile with U-shape-nose grooves sharpens the nose of the projectile and changes the failure mode of target during penetration, which has advantages of decreasing penetration resistance and enhancing the depth of penetration.
Abstract in English:Abstract To investigate the deformation characteristic of deep rock under cyclic blasting, the static-dynamic loading experimental equipment was set up. The dynamic strain gauges and high-speed (HS) camera were adopted to obtain strain field and the crack propagation of rock under confining pressure of 0 MPa, 5 MPa and 10 MPa. Under the one-time loading from cylindrical charge, the cracks in crushed zone were generated by shear and tensile failure. Circumference compressive stress is formed around borehole by confining pressure, and it reduces the circumference tensile failure by blast loading. The number of radial cracks and broken radius reduce with confining pressure increase. The reflected stress wave drives the existing fissures further developing. When specimens subjected to cyclic loading from PETN Cord, the rock is controlled by elastic deformation and no damage appeared after the first loading. With the cycle-index increase, the accumulation of plastic strain is observably when the strain exceeds the elastic limit strength of rock. The cumulative damage is nonlinear increased under cyclic loadings. The existing flaws or radial cracks run through specimen when the cumulative damage exceeds the material's yield strength.
Abstract in English:Abstract This paper aims to explore crashworthiness performance of new designed foam-filled tapered structures under axial and oblique impact load conditions. The structures consisted of inner and outer tapered tubes connected together by four stiffening plates. They also included square, hexagonal, octagonal, decagonal and circular cross-sectional shapes with different ratios of a/b (ratio of the inner tube side length to the outer tube one). The ratio of a/b was assumed to be 0, 0.25, 0.5, 0.75 and 1. Crashworthiness criteria of specific energy absorption (SEA), peak collapse force (Fmax) and crash force efficiency (CFE) were obtained for these structures using the experimentally validated model in LS-DYNA. Based on the numerical results, semi-foam filled structures (i.e. those with the ratio of a/b=0.5) demonstrated greater crashworthiness performance than foam filled single-cell ones. Furthermore, according to the TOPSIS calculations, cross sectional shape of decagonal was found to be an outstanding potential profile for the automobile structural components.
Abstract in English:Abstract To homogenize lattice beam-like structures, a direct approach based on the matrix eigen- and principal vectors of the state transfer matrix is proposed and discussed. The Timoshenko couple-stress beam is the equivalent continuum medium adopted in the homogenization process. The girders unit cell transmits two kinds of bending moments: the first is generated by the couple of the axial forces acting on the section nodes, the other one is due to the moments directly applied at the node sections by the adjacent cells. This latter moment is modelled as the resultant of couple-stress. The main advantage of the method consists in to operate directly on the sub-partitions of the unit cell stiffness matrix. Closed form solutions for the transmission principal vectors of the Pratt and X-braced girders are also attained and employed to calculate the stiffnesses of the related equivalent beams. Unit cells having more complex geometries are analysed numerically. As a result, the principal vector problem is always reduced to the inversion of a well-conditioned (3×3) matrix employing the direct approach. Hence, no ill-conditioning problems, affecting all the known transfer methods, are present in the proposed method. Finally, comparing the predictions of the homogenized models with the finite element (f.e.) results of a series of girder, a validation of the homogenization method is performed.