Abstract in English:Abstract The dynamic response and failure mechanism of clamped thin aluminum alloy plates subjected to underwater impulsive loading are investigated by laboratory experiments and finite element (FE) simulations. The effects of plate thickness, impulsive loading and fluid-structure interaction on failure modes in clamped thin aluminum plates are comprehensively assessed in this study. The underwater explosive shock loading experiments were performed by underwater non-contact explosive simulator to identify failure modes of target plates under loads with different intensities. The 3D digital image correlation was applied to measure the real-time deformation of the specimens throughout the impulsive event. Depending on the loading intensity, the failure modes of thin aluminum plates were subdivided into three modes. The Scanning electron micrographs of the fracture surfaces show that the local failure mechanism was tensile necking in all cases. A calibrated FE model was adopted to predict the overall dynamic behavior of plates. The results indicate that the thickness of plates had no significant effect on the deformation modes. In addition, the quantitative relations of plate thickness, the effect of fluid-structure interaction and the failure of plate subjected to underwater shock loading were revealed by the combination of the experimental and simulation results. The results obtained in this research provide a potential guidance to enhance the impulsive resistance of underwater structure.
Abstract in English:Abstract As extension of the previous two-surface model in plasticity, a two-surface model for viscoplasticity is presented herein. In order to validate and investigate the performance of the proposed model, several numerical simulations are undertaken especially for structural steel under monotonic and cyclic loading cases, where experimental results and numerical results from the rate dependent kinematic hardening model are also provided for the reference. For all the cases studied, the proposed model can appropriately account for the rate-effects in both maximum stress and hysteretic shapes.
Abstract in English:Abstract One of the main drawbacks of Element Free Galerkin (EFG) method is its dependence on moving least square shape functions which don’t satisfy the Kronecker Delta property, so in this method it’s not possible to apply Dirichlet boundary conditions directly. The aim of the present paper is to discuss different aspects of three widely used methods of applying Dirichlet boundary conditions in EFG method, called Lagrange multipliers, penalty method, and coupling with finite element method. Numerical simulations are presented to compare the results of these methods form the perspective of accuracy, convergence and computational expense. These methods have been implemented in an object oriented programing environment, called INSANE, and the results are presented and compared with the analytical solutions.
Abstract in English:Abstract This paper focuses on robust optimal adaptive control strategy to deal with tracking problem of a quadrotor unmanned aerial vehicle (UAV) in presence of parametric uncertainties, actuator amplitude constraints, and unknown time-varying external disturbances. First, Lyapunov-based indirect adaptive controller optimized by particle swarm optimization (PSO) is developed for multi-input multi-output (MIMO) nonlinear quadrotor to prevent input constraints violation, and then disturbance observer-based control (DOBC) technique is aggregated with the control system to attenuate the effects of disturbance generated by an exogenous system. The performance of synthesis control method is evaluated by a new performance index function in time-domain, and the stability analysis is carried out using Lyapunov theory. Finally, illustrative numerical simulations are conducted to demonstrate the effectiveness of the presented approach in altitude and attitude tracking under several conditions, including large time-varying uncertainty, exogenous disturbance, and control input constraints.
Abstract in English:Abstract A new MEFP warhead with seven arc-cone liners which can form 7, 13 or 19 penetrators at different standoffs is designed. Dispersion patterns and penetration properties of MEFP are performed on five #45 steel targets of dimension 160cm x 160cm x 1.5cm at various standoffs (45cm, 60cm, 80cm, 120cm, 170cm). It reaches the conclusion that every surrounding liner is broken into three penetrators during the formation process of MEFP and a group of aimable penetrators consisting a central projectile surrounded by 18 penetrators is finally formed. Maximum divergence angle of surrounding penetrator is 9.8° and the damage area reaches 0.37m2 at 1.7m. A nonlinear surface fitting about perforations information on the targets at different standoffs provides a method of predicting the dispersion patterns of MEFP. Once initiated, damage probability for defeating light armor of MEFP warhead with seven arc-cone liners is significantly improved and the results provide important reference to the design and optimization of MEFP warhead in engineering.
Abstract in English:Abstract This study attempted to examine the ballistic resistance of sandwich structures with aluminum facesheets, polyurethane core and polyurethane foam reinforced with aluminum pins. The main focus was on the effect of variations in density of core foam and the effect of adding different percentages of aluminum pins on energy absorption and ballistic resistance of sandwich structures under the impact of blunt and conical nose projectiles at high velocities (170 to 260 m/s). The results firstly demonstrated that any increase in the density of foam led to greater energy absorption. Secondly, the ballistic limit of sandwich structures with composite core was more than the foam core by 18 percent. Thirdly, the use of aluminum pins not only enhanced core resistance, but also altered the shape of damage and energy absorption in the rear facesheet. Finally, the ballistic limit of blunt projectile was greater than that of conical nose projectile.
Abstract in English:Abstract Possibility of a water-transportation mode to experience numerous accidental loads is predicted limitless and always demand sustainable analysis. Impact phenomenon raises as main reason of environmental damage, for instance, grounding contributes in form of short-time impact and produces massive damage on the bottom structure of a marine vessels, e.g. ship. In this work, a series of numerical experiment is conducted with considering contribution of several parameters, namely impact location and seabed topology on the bottom structure of a tanker ship. The initial stage is conducted to obtain validation of the numerical method and configuration using impact test to ensure reliability of the present methodology which considers the finite element approach. In the second stage, proposed scenarios are calculated and overall evaluation of the damage extent is performed to observe resistance and behaviour of the bottom structure. Structural condition after grounding, rupture energy and crushing force are presented to observe history of grounding process. Contribution of the numerical parameter in form of meshing size is presented to provide adequate consideration in both of physical and numerical parameters. Finally, tendency of the target structure is summarized to provide prediction of further behaviour during the bottom structure experiences grounding with various scenarios. An alternative solution in respecting time cost is proposed and can be considered for other impact simulation and analysis.
Abstract in English:Abstract This research aims to study the effect of supplementary cementitious materials (SCMs) such as quarry dust limestone (QDL) and natural pozzolana (NP) on the performance of fiber reinforced self-consolidating repair mortars (FR-SCRMs). Based on previous optimization of QDL and NP replacement ratios, two mortar mixtures incorporating 10% QDL and 20% NP as cement replacements were prepared. The evaluation was based on both fresh (slump flow, flow time) and hardened (compressive and flexural strengths and adherence to old concrete) tests. In addition, the influence of three curing conditions, similar from those normally encountered in the field was evaluated on the compatibility between repair materials and substrate, under flexural strength test by using third-point’s loading beam test method. It is demonstrated that the FR-SCRMs is promising to be used in repair concrete structures class R4 (EN 1504-3) without reducing the adhesive strength.
Abstract in English:Abstract In the article, a new approach considering structural local failure for topology optimization of continuum structure is proposed. It aims at not only lowering the risk of local failure in the concerned structural regions, but also ensuring a good stiffness of the structure. The local failure may be caused by the structural uncertainties or possible structural fatigue. To this end, a criterion to evaluate the effect of one local failure on the structure is introduced. This criterion is minimized to reduce the probability of structural damage based on an initialized structure whose compliance is optimized. Solid Isotropic with Material Penalization (SIMP) method and Optimality Criteria (OC) method are combined to solve the design problem. The effectiveness of the proposed algorithm is verified by a series of numerical examples. Furthermore, experiments merging with additive manufacturing technique are taken to prove the practical ability of the method in actual engineering.
Abstract in English:Abstract Manufacturing defects in the sandwich structures have significant impact on their ultimate strength performance. It is essential to codify some criteria for the effects of such defects on the performance of the structure in order to investigate the quality of sandwich compo-sites and their load-bearing capabilities. By means of such criteria, one can predict the property degradation of the defected structure in comparison to a defect-free one, and assess whether the faulty part could be remained in the structures. Among all the potential defects in a sandwich structure they could be induced during the Vacuum Infusion Process (VIP), the dry area defect is the most significant and frequently occurring one. Hence, adequate tests should be conducted in order to examine the effect of dry area defect on the structure. For this purpose, a methodology is required for preparing the defected samples similar to the main structure. The current study is aimed at suggesting two accurate approaches to induce artificial defects in sandwich structures manufactured implementing the vacuum infusion process. These methods are used for making some defected samples and their responses are compared with those of the defect-free structures. All samples are subjected to the uniform inplane compression loading and their behaviors up to the fracture point are evaluated to assess the remaining strength. Infusion process.