Abstract in English:Abstract This work deals with numerical simulation of the mechanical behavior of materials composed of heterogeneous ductile microstructures using a multi-scale approach considering plasticity processes and phase debonding. Due to few studies about yield surfaces of metal matrix composites (MMC) with weak interfaces presented in the literature, the major goal of this work is to propose yield surfaces for metal matrix composites reinforced by rigid inclusions. The yield surfaces are obtained for Representative Volume Elements (RVEs) of materials presenting perfectly bonded inclusions and phase debonding in the interface zone. The matrix is considered an ideally plastic material governed by von Mises model, whereas the interface zone is modeled by means contact and fracture constitutive models incorporated in a proposed finite element. Also, RVEs containing different distributions and volume ratios of voids are analyzed. Considering the phase debonding, for compressive loadings the RVE behaves like RVE with perfectly bonded inclusions whereas for tension loadings the RVE presents a behavior quite similar to the one with voids. On the other hand, the concentration of voids in the RVE decreases its mechanical strength.
Abstract in English:Abstract Low-velocity impacts have a relevant importance for safety of laminated and sandwich composite structures, because they are highly susceptible to damage. In this paper, a 3D cost effective zig-zag model is developed in order to efficiently simulate such impacts. It a priori fulfills the continuity of out-of-plane stresses at layer interfaces, the continuity of stresses under in-plane variation of properties across undamaged and damaged regions and it is suitable for general boundary conditions. Its main advantage is its capability to accurately predict stresses from constitutive equations at a low cost, along with being refined across the thickness keeping fixed the number of unknowns. A modified Hertzian contact law that forces the target to conform to the shape of the impactor and the Newmark’s implicit time integration scheme are used for computing the contact force time history. The progressive damage analysis is carried out using stress-based criteria. Non-classical feature, a continuum damage mesomechanic model is employed that accounts for the effects of local failures in homogenized form by modifying the strain energy expression. Fiber, matrix and delamination failures are predicted using stress-based criteria, and then the modified strain energy expression is employed for computing stresses. Such modeling options enable to account for the residual properties as they are in the reality, as shown by the comparison with the damage experimentally detected. As shown by the comparison with experiments, a closed-form solution by the Galerkin’s method, obtained as a series expansion of trial displacement functions, accurately simulates the contact force and the damage progressively accumulated. The results show the importance of in-plane stress continuity for obtaining accurate predictions.
Abstract in English:Abstract Thin walled multi-cell columns are highly efficient energy absorbers under axial compressions. Multi-cell features are usually obtained by placing stiffeners or ribs to get the required geometry features in the columns. In this study, the curved stiffeners with varying numbers in different configurations are used to support the circular single and bi-tubular metallic tubes and their shape, position and number effects are numerically investigated under dynamic axial crushing loads. Number of configurations can be classified by the number of tube/tubes and curved stiffened shells. This study can be divided in three parts; single tubes with cross-curved stiffeners with equal number of cells, single tubes with curved stiffened wall supports with ascending number of stiffeners, and bi-tubular tubes with ascending cross-curved stiffeners. The mass in each configuration is maintained as same by adjusting the thicknesses of the stiffeners. Energy absorption for all of the configurations are compared with published experimental literature of standard single tube and single tube with cross-straight stiffeners. Deformation modes and energy absorption characteristics are evaluated and discussed for all of the proposed configurations. Results indicate that the proposed configurations with curvy stiffeners are superior in energy absorption with increased mean crushing force and enhanced energy absorption along with a high value of crush force efficiency in comparison with the refernce configurations. Bi-tubular configurations with curvy stiffeners show a more stabalized response with the lowest peak crushing force in all of the proposed configurations.
Abstract in English:Abstract The comparison of the failure state for composite laminates under bending and tensile loads is a well-known issue which has been intensively discussed in the literature. The scope of the current work is to investigate the appropriate method of the flexural modulus of the composite laminates by the aid of experimental and numerical approach. The primary objective of this study is to compare the experimental measurements of the elastic modulus of the composite laminates under the tensile and flexural tests. The numerical study is performed through progressive damage analysis (PDA) approach which is verified by simulation of the tensile specimens. This procedure is then used to predict the failure of the beam specimens under three-point-bending (3PB) test. Both inter-laminar and intra-laminar damages are included in the developed models. The first type of damage is examined based on the continuum damage mechanics (CDM) approach and the second one based on the virtual crack closure technique (VCCT). The variation of the flexural modulus and ultimate strength of the composite beams is considerably depends on loading types and lay-up configurations. In this study, it is proven that the strain-based failure criterion can predict the realistic failure mode of the composite beams in consistency with the ultimate strains which was obtained from a simple tensile test.
Abstract in English:Abstract This paper investigates the dynamic response of rectangular prestressed membrane subjected to concentrated impact load based on multiple scale perturbation method. The governing equations of motion of nonlinear vibration are derived based on the Föppl large deflection theory and Galerkin method. By introducing different time scales to consider the process of vibration, the results of dynamic response are obtained by applying the multiple scale perturbation method. Furthermore, the effects of pretension force, velocity of load and dimension of membrane on the dynamic response of membrane are discussed. The present work studies the problem of the dynamic response of prestressed membrane subjected to concentrated impact load in different time scales, and provides a more accurate theoretical model for design of membrane structure.
Abstract in English:Abstract This paper proposes a novel variable-length beam element that takes into account the effect of beam spinning. This is the first such beam element of variable length based on the absolute nodal coordinate formulation. In addition to the position and slope vectors, the angles of rotation around the element axis of cross sections that contain two nodes are introduced into the element coordinates to describe the spinning of an assumed Euler-Bernoulli beam with circular cross section. The material coordinates of the two nodes are also included in the arbitrary Lagrangian-Eulerian description used for previous element to describe the varying element length caused by mass transportation at the boundaries. The proposed element facilitates convenient and effective numerical modeling of the dynamics of a circular-cross-section beam with transportation boundaries and that is spinning. Numerical examples demonstrate that the proposed element can describe the dynamic behavior of a circular-cross-section beam effectively. In the field of engineering, this novel element could be used in the dynamic analysis of drill stems, the slender workpiece of a cylindrical shaft during the turning process, and the lead screw in a ball screw mechanism.
Abstract in English:Abstract Molecular dynamics simulations of the ballistic Taylor test are used to explore correlation between the largest fragment mass and the impact energy of a projectile as well as a set of selected state variables. Flat-ended, monocrystalline, nanoscale bars collide with a rigid wall with striking velocities ranging from 0.27 km/s to 60 km/s. The investigation emphasis is on two border regions of the emerging nonlinear phenomenological model identified with two transitions: the damage-fragmentation transition and the shattering transition. In between these two nonlinear regions, the maximum fragment mass is largely inversely proportional to the impact energy, and the maximum values of the pressure, temperature, and the square of the effective strain. A reverse-sigmoid phenomenological model is proposed to capture the unifying features of this nonlinear and saturable dependence. A crystallographic orientation dependence of the damage-fragmentation transition parameters is investigated.
Abstract in English:Abstract This article presents the vibro-acoustic modeling and analysis of un-baffled laminated composite flat panels subjected to harmonic point load under various support conditions. The frequency values of the panel are obtained by using simulation model through the commercial finite element package (ANSYS) via batch input technique. Initially, the rate of convergence of the current simulation results is established by solving different examples under various edge supports. Further, the natural frequencies and corresponding modes are computed and compared with available published and experimentally obtained values. Then, the modal values are exported to LMS Virtual.Lab environment for the computation of acoustic responses of the vibrating laminated plate structure. Further, an indirect boundary element approach has been adopted to extract the coupled vibro-acoustic responses. Subsequently, the radiated sound power of the structure and the sound pressure level within the acoustic medium are computed using the present scheme and compared with the published numerical results and in-house experimental data, respectively. Finally, a comprehensive study has been performed to highlight the effect of different structural parameters (thickness ratio, aspect ratio, modular ratio and lay-up scheme), support conditions and the composite properties on the acoustic radiation responses of the laminated plate structure.
Abstract in English:Abstract The estimation of the value and direction of post-liquefaction deformations is one of the most challenging issues in the modelling of liquefaction soil, due to the inherent and induced anisotropy. It is very important in the science of soil-constitutive models to present a simple and comprehensive model for the prediction of fabric anisotropy effects in pre- and post-liquefaction behaviour in granular soil. In the framework of the multilaminate method, 17 planes with pre-determined directions are defined, instead of defining all occurrences depending on the direction in three planes perpendicular to each other in a Cartesian coordinate system. As a result, calculation accuracy is increased in the point due to the effectiveness of the behaviours in different directions. In the present study, after modifying an advanced model by removing constants related to the fabric effect and using lower constants, the precision of model performance after the removal of constants was studied and compared with experimental results in different monotonic, cyclic, drained, and undrained loading conditions. After this, the formation of stress and strain in 17 planes was evaluated in terms of pre- and post-liquefaction, with monotonic and cyclic loadings. The study of the curves shows induced anisotropy in different directions of sandy soil and thus proves the capability of the model in this regard.
Abstract in English:Abstract A series of analyses are carried out to predict the structural crashworthiness of a ship during a collision. The numerical configuration is verified by structural simulations based on a laboratory experiment wherein a penetration test is considered as the experimental reference. Comparative observations of structural behaviour are carefully conducted to ensure the reliability of the present method for conducting large-scale collisions. At this stage, the proper procedure and configuration for structural calculations subject to accidental loads are determined. The second stage addresses the calculations for various side collision scenarios. The simulations consider a double hull Ro-Ro passenger ship being struck at various locations, and the overall behaviour of the side hull along the longitudinal axis is observed. The main study considers the target location and striking speed to obtain adequate data related to crashworthiness criteria, i.e. the internal energy and extent of damage. Finally, the criteria of various scenarios are summarized. Further calculations comparing the results with a safety factor are presented together with a consideration of structural behaviour to estimate the safety limit within the confines of strait territory.