Papers by Keyword: Thin-Walled Structure

Abstract: Composite materials thin-walled structures are widely used as skin panel in flight vehicles in recent years. These structures will encounter severe complex loading conditions, which may be a combination of mechanical, aerodynamic, thermal and acoustic loads. Thin-walled structures subjected to this kind of loadings will exhibit nonlinear response; as a result, fatigue failure will occur. High temperature may cause large thermal deflection and stress, for some special conditions, may cause thermal buckling. Once the thermal buckling appears, the stiffness will change correspondingly, it will cause significant influence on the dynamic response and fatigue failure. Accordingly, it is important to research the nonlinear response of this kind of structures under elevated thermal environment. Nonlinear response and thermal pre-buckling/post-buckling behavior of a Graphite-Epoxy composite plate subjected to server thermal loading is numerically investigated in this paper. A composite laminated plate with clamped-clamped boundary conditions is chosen as simulated body, nonlinear finite element model is developed using the first-order shear deformable plate theory, Von Karman strain-displacement relations, and the principle of virtual work. The thermal load is assumed to be a steady-state with different predefined temperature distribution. The thermal strain is stated as an integral quantity of the thermal expansion coefficient with respect to temperature. Then the modes of the plate are analyzed, the nature frequencies and modal shapes are obtained. The critical temperature of buckling is calculated. The static nonlinear equations of motions are solved by the Newton-Raphson iteration technique to obtain the thermal post-buckling deflection. The Riks method is used to analyze static post-buckling behavior. In the numerical examples, four types of situations are studied, which include i) the buckling behaviors for different initial imperfections, ii) the buckling behaviors for different thickness to width ratios, and iii) The buckling behaviors for different width to length ratios; The critical temperature, the static thermal post-buckling deflection and the load to displacement relation are presented respectively. The influences of different boundary conditions on the buckling behaviors of the plate are achieved as well. The simulation method and results presented in this paper can be valuable references for further analysis of the nonlinear responses of thin-walled structures under complex loading conditions.

Abstract: Carbon-Carbon composite materials are widely used as the surface thermal protection systems (TPS) of advanced high-speed air-craft and spacecraft. The thin-walled structures with this kind of materials would exhibit large displacement response under high-level acoustic loads and possibly display buckling at elevated temperatures. Reliable experimental data are difficult to acquire because of the high costs and difficulties with instrumentation at high acoustic intensity and elevated temperatures. Thus, in the design process greater emphasis will likely be placed on improved mathematical and computational prediction methods. Among these researches, the simulation methods for nonlinear response of thin-walled composite panels under thermo-acoustic loadings are being developed emphatically .This paper presents a nonlinear finite element model for analyzing nonlinear random dynamic behaviors of Carbon-Carbon composite panels under the combined effects of thermal and random acoustic loads. The acoustic excitation is assumed to be a band-limited Gaussian random noise and uniformly distributed over the structural surface and the thermal load is assumed to be a steady-state with different predefined temperature distribution. Three types of motion: 1) linear random vibration about one of the two buckled positions, 2) snap-through motion between the two buckled positions, and 3) nonlinear random vibration over the two thermally buckled positions can be predicted. And the dynamic response behaviors of the structures are discussed. Based on this, the influences of sound pressure level (SPL) and elevated temperatures on the dynamic responses are analyzed emphatically.

Abstract: Thin-walled structures of future hypersonic flight vehicles will encounter complex loadings and exhibit obvious nonlinear responses. The thermal loads from high speed flow or engine jet flow can cause thermal buckling of thin-walled structures, such as Thermal Protection System (TPS). If the structures are loaded with intense acoustic loads simultaneously, large deflection nonlinear response, including snap-through, can be induced. Snap-through will give rise to large amplitude stress cycles and non-zero mean stress, which can lessen the fatigue life markedly. Starting from Hooker’s Law with thermal components, the large deflection governing equations of motion for simply-supported plate under thermo-acoustic loadings are derived. The partial differential equation (PDE) of motion which is difficult to solve is then transformed with Galerkin’s method to the system of ordinary differential equations (ODE) under modal coordinates. The displacement responses under different combinations of temperature increments and sound pressure levels are calculated by employing Runge-Kutta method. Typical thermo-acoustic responses are predicted: 1) random vibration around pre-buckled equilibrium position, 2) persistent snap-through between post-buckled positions, 3) intermittent snap-through, 4) vibration around one of the two post-buckled positions. By dividing the restoring force term in the equation into linear term and nonlinear one, the evolutions of each term are obtained to illustrate the mechanism of thermo-acoustic response and the contributions of each force, including shear force, thermal force and membrane force. Thus a further insight into thermo-acoustic response has been achieved.

Abstract: Current requirements for producing highly precise and ultra-smooth micro structured surfaces of small parts are proposed in certain situations. The following question arises: how to make a highly precise and ultra-smooth micro-structured surface with high efficiency and low cost Novel desktop lapping and polishing devices should be developed to satisfy these requirements. In order to improve the surface topography and remove the surface damaged layer of a highly precise and ultra-smooth micro thin-walled structure after milling with the width of 150 μm and the depth of 10 μm, a novel lapping desktop device is designed and developed. There are two key points in the design of the lapping desktop device: one is the vertical coupled macro-micro movement axis; the other is the fixture with a thin and flexible hinge structure, which has the capability of measuring both force and displacement as a double-feedback sensor to control both the micro lapping force and the depth of lapping. The experimental results show that the surface topography of the micro thin-walled structured surface is much improved after lapping, and that the three-dimensional surface roughness decreased from 329 nm to 82.2 nm.

Abstract: Based on the theory of structure impact dynamic and numerical optimization, application of SQP algorithm in nonlinear parameter inversion is demonstrated in this paper by combining the optimization software and finite element software. Parameter inversion of thin-walled cylinder subjected to axial impact load is studied. The material parameters of thin-walled cylinder are obtained from inversion; besides, the whole process of numerical optimization parameter inversed method is demonstrated.

Abstract: Advanced aircraft and spacecraft structures are exposed to increasingly severe operating environments, including a combination of mechanical, aerodynamic, acoustic and thermal loads. Such loading conditions can cause thin-walled structures to respond in a nonlinear fashion and exhibit complex response characteristics. This paper investigates the dynamic response of pre/post buckled thin-walled structure under high level random acoustic loading. Firstly, different orders of critical buckling temperatures and modal frequencies under alternative temperatures are obtained using Finite Element Method (FEM), and the modal frequency changes in a disorder fashion are discussed in detail. Then with coupled BEM/FEM method, the dynamic responses including transverse displacement, strain and stress of a stiffened rectangular plate under thermo-acoustic loading are simulated. By comparing the response characteristics of the plate in pre/post buckled conditions, some valuable conclusions are derived, which can be used to explain the response behaviours of thin-walled structures.

Abstract: Pier concrete is vulnerable to cracking when using concrete rapid construction after long off period. In order to solve this problem, crack mechanism and effects of several temperature control measures of thin-walled structure concrete is analyzed. Through analyzing the temperature and stress time-space variation using 3D finite element method, it can be found that stress is more sensitive to controlling concrete placing temperature than other measures. It also shows that under the condition of taking other proper temperature control measures, low placing temperature can not be spared.

Abstract: Explicit finite element method is adopted to simulate the crashworthiness performance of four types of typical thin—walled structures used in vehicle by software LS-DYNA. The structures with the same material、area and length are crash by a rigid body with 40km/h in10ms, The crash processes and crashworthiness characters are analyzed by a series crash parameters: deformation energy with unit displacement, impact force and deceleration to look for the optimal shape with crashworthiness. With comparing, the double caps section has ascendant performance than the others. The simulating methods of welded-joints are discussed to analysis their effects on crashworthiness simulation.

Abstract: To meet the requirement for the machining of the ultra-precision, ultra-smooth and micro-structure surface, an ultra-precision three axes micro milling machine was developed with the positioning accuracy better than ±0.25μm and the repetitive positioning accuracy better than ±0.2μm of all the three axes. The machine is proved to achieve the nanometer scale step response. Through milling experiments with micro-diameter tungsten carbide milling tool, the cutting performance has been further proved: the milling accuracy of 50μm-high step on the workpiece of aluminum alloy is better than ±0.3μm; and the 3D surface of pure copper workpiece is as smooth as mirror, with a roughness reaching 40nm. At last, the thin-walled structure of 10μm thickness on the workpiece of aluminum alloy is milled.

Abstract: A new stress-state dependent cohesive zone model for thin walled structures is proposed. The model incorporates the stress-state explicitly within the traction-separation law using basic elasticity-plasticity equations combined with a model parameter. The numerical implementation of the model is able to reproduce ductile fracture observed in a pre-cracked C(T) specimen as well as a notched plate specimen of the same material.