含预裂缝复合材料缠绕圆柱壳轴压承载特性分析

为探究穿透裂缝对复合材料缠绕圆柱壳承载能力及失效模式的影响,首先开展不同壁厚含预裂缝复合材料缠绕圆柱壳轴向压缩试验。对于A系列厚壁圆柱壳,裂缝导致承载能力下降53.96%,失效模式由局部屈曲转化为裂缝扩展、脆性断裂;而B系列薄壁圆柱壳均发生局部屈曲,裂缝使承载能力下降12.59%。其次,采用有限元软件ABAQUS 6.14,基于非线性RIKS算法,建立轴压作用下含预裂缝复合材料圆柱壳极限承载能力计算模型,通过引入Hashin失效准则及损伤演化判据,预测结构渐进破坏模式及极限荷载。数值结果与试验数据吻合良好,最大误差为7.01%,验证了数值算法的可靠性。在此基础上,探讨裂缝方向、缠绕角度对含预裂缝复合材料圆柱壳极限承载的影响,可知:对于±55°螺旋铺层复合材料圆柱壳,随裂缝角度α增加,极限承载能力先升高再降低,当α=45°时,具备最大承载能力;对于含开缝角α=15°、45°、55°缠绕圆柱壳,随缠绕角θ增加,其承载能力呈先上升后下降趋势。且开缝角越小,缠绕角度对极限荷载的影响越大,当缠绕角θ=30°时,达到最大承载能力。     

复合材料波纹夹层圆柱壳设计及轴压性能

结构轻量化是航空航天发展的永恒主题, 波纹夹层圆柱壳作为常见的轻质结构形式, 在航空航天领域具有很大的发展空间。采用模具热压法, 制备出纵向和环向碳纤维复合材料波纹夹层圆柱壳, 其中芯子整体成型, 面板分瓣制备。采用经典板壳屈曲理论, 分析纵向和环向波纹夹层圆柱壳的轴压力学性能, 得到了欧拉屈曲、整体屈曲、局部屈曲和面板压溃4种失效模式下的极限载荷理论公式。绘制出结构的失效机制图, 直观显示出了失效模式与试件尺寸之间的关系。通过对纵向和环向波纹夹层圆柱壳的轴向压缩试验, 获得了结构的载荷-位移曲线及局部屈曲和面板压溃2种失效模式。结果表明:纵向波纹夹层圆柱壳的轴向承载能力及载荷/质量效率优于环向波纹夹层圆柱壳, 在一定范围内增加圆柱壳面板的厚度、减小圆柱壳的高度可提高结构的载荷/质量效率。      

Delamination buckling and postbuckling in composite cylindrical shells under combined axial compression and external pressure

A series of finite element analyses on the delaminated composite cylindrical shells subject to combined axial compression and pressure are carried out varying the delamination thickness and length, material properties and stacking sequence. Based on the FE results, the characteristics of the buckling and postbuckling behaviour of delaminated composite cylindrical shells are investigated. The combined double-layer and single-layer of shell elements are employed which in comparison with the three-dimensional finite elements requires less computing time and space for the same level of accuracy. The effect of contact in the buckling mode has been considered, by employing contact elements between the delaminated layers. The interactive buckling curves and postbuckling response of delaminated cylindrical shells have been obtained. In the analysis of post-buckled delaminations, a study using the virtual crack closure technique has been performed to find the distribution of the local strain energy release rate along the delamination front. The results are compared with the previous results obtained by the author on the buckling and postbuckling of delaminated composite cylindrical shells under the axial compression and external pressure, applied individually.     

格栅非均匀分布效应对复合材料格栅加筋圆锥壳体稳定性的影响

研究格栅非均匀分布效应对先进复合材料格栅加筋圆锥壳体稳定性的影响。首先,基于格栅间距沿母线方向的变化特征和等效平铺模型推导了格栅加筋圆锥壳体的等效刚度阵。其次,采用Donnell 型扁壳理论推导了在均布外压作用下格栅加筋圆锥壳体稳定性分析的总势能表达式,利用最小势能原理得到了该壳体总体稳定性的临界载荷值,所得计算结果与实验结果十分吻合。最后,通过典型数例参数讨论,说明格栅非均匀分布效应对先进复合材料格栅加筋圆锥壳体稳定性的影响将随底锥角增大而显著。该文将为先进复合材料格栅加筋圆锥壳体的参数优化设计提供一种高效和可靠的分析方法。     

Buckling Strength of Near-Cylindrical Shells

A procedure is proposed whereby critical loads for elastic near-cylindrical shells are determined with the use of equations in terms of displacements. An analytical expression has been derived for the assessment of critical stresses in momentless smooth shells. A contribution of individual components of the total potential energy of such shells to stabilization or destabilization of the strain state is analyzed graphically. Dimensionless critical stresses are assessed with respect to classical ones as determined for ideal cylindrical shells. The data obtained are compared with the known experimental results.     

Quasi-static and dynamic axial compression of glass/polyester composite hemi-spherical shells

An experimental investigation has been carried out to study the collapse mechanisms and the energy absorption capacities of the glass/polyester composite hemi-spherical shells under both quasi-static and drop hammer loading. The shells were made of randomly oriented glass fibre mats and polyester resin. Quasi-static tests were conducted at speed of 2 mm/min. and the impact velocities varied from 5 to 9 m/s. The radii of the shells varied from 53.5 to 106.1 mm and their thicknesses from 1.10 to 2.84 mm. Influence of these variables on the mechanism of deformation has been discussed. Experiments on 45 shells showed that their progressive crushing occurred due to the formation of successive zones of fracture. Based on these observations an analytical model has been developed for the prediction of load-deformation and energy-compression curves. The results thus obtained are found to match well with the experiments. It is seen that the ratio of the mean collapse loads recorded in impact and quasi-static tests for a given shell is greater than one but it decreases with the increase in thickness of the shell.     

Fragmentation of Spherical Shells under Internal Blast Loading

We present experimental data on the fragmentation, scattering, and penetration into barriers of fragments of thin-walled spherical shells made of soft steel or an aluminum alloy, which are loaded internally by blasting a sphere-shaped explosive charge. The speed, number, and average sizes of fragments formed of shells of different sizes as well as the maximum depths of penetration of fragments into barriers made of a number of homogeneous and heterogeneous materials have been determined. Some characteristic features of the fragmentation of spherical shells under internal blast loading have been elucidated. An attempt has been made to describe the characteristic (average) fragment size by a formula derived from the balance of elastic energy and the work of fracture. Thicknesses of fragmentation protective layers of some structural materials have been chosen, which enable catching soft steel or aluminum alloy fragments which are formed by the explosion of particular sphere-shaped explosive charges.     

Dynamic short-wave buckling of thin-walled cylindrical shells upon local action of an external pressure pulse

We present the results of experimental investigations of dynamic instability (buckling) of thin-walled cylindrical shells upon local action of an external pressure impulse. In order to create the local short-term loading, we used pulsed radiation from a CO ₂ laser. The test pieces were smooth and reinforced shells made from aluminum alloys. The nature of the buckled wave in the treated region and the magnitude of the critical impulse correspond to the short-wave type of buckling in the shell and are determined by the amplitude of the pressure pulse. Axial static compression of the shell leads to a decrease in the critical impulse and an increase in the number of waves in the meridional direction within the treated region.Translated from Problemy Prochnosti, No. 4, pp. 36–43, April, 1995.     

A comparative study on the elastic-plastic collapse strength of initially imperfect deep spherical shells

A finite difference method for the large deformation elastic-plastic analysis of spherical caps is applied to predict the collapse strength of initially imperfect deep spherical shells. Twelve uniformly loaded hemispherical shell models with flat spots at their apex are analysed. For each model, a number of shallow spherical regions containing the flat spot are selected from its domain. One of these selected shallow regions yields a minimum buckling pressure; this minimum value is taken as the theoretical buckling load for the shell model under consideration. Present solutions are in good agreement with existing experimental and empirical results. The good comparison suggests that initially imperfect deep spherical shells may be analysed by using a much simpler mathematical model—the spherical cap—and thus the analytical cost may be greatly reduced. This also demonstrates that the collapse of imperfect spherical shells is primarily a local phenomenon and therefore dependent on local geometry. Consequently, the presence of initial imperfections must be fully taken into consideration in any large deformation inelastic buckling analysis before such analysis can be expected to quantitatively predict the collapse strength of practical shell structures.     

Identification of reinforcement structures of composite constructions from the results of thermophysical experiments on steady-state temperature fluctuations

The problem of identification of reinforcement structures of thin-walled composite construction from the results of experiments on steady-state temperature fluctuations in them has been formulated. The particular case of shells of revolution with axisymmetric reinforcement structures has been investigated. The reinforcement structure of a cylindrical shell has been reconstructed under pseudoactual experimental conditions. Satisfactory accuracy of identification of the reinforcement structure and the effective thermal conductivities has been demonstrated.     

铝合金半刚性椭圆抛物面网壳静力稳定性分析

针对目前铝合金单层网壳结构中节点形式过于单一的局面,该文首先研发出了一种新型铝合金半刚性节点--柱板式节点。获得了柱板式节点绕强轴、弱轴和扭转三个方向的弯矩-转角曲线,并将其引入到网壳杆件单元模型中,建立了半刚性节点工字型杆件椭圆抛物面网壳的数值分析模型。在此基础上,考虑节点强轴刚度、节点弱轴刚度、节点扭转刚度及跨度和矢跨比等参数影响,对半刚性节点高强铝合金椭圆抛物面单层网壳进行了弹塑性全过程分析,得到了各参数对网壳极限承载力的影响规律,并详细地考察了网壳失稳时的变形形态。研究发现:网壳失稳时,网壳中的柱板式节点均处于弹性状态,说明柱板式节点有较好的刚度,网壳的失稳为伴随杆件扭转屈曲的局部失稳。     

Nanoindentations on conch shells of Gastropoda and Bivalvia molluscs reveal anisotropic evolution against external attacks

Nanoindentation method has been used to explore, at the nanoscale, the mechanical properties of four different representative types of conch shells belonging to the two biggest classes of molluscs, Gastropoda and Bivalvia, in order to compare nanohardness and Young's modulus with respect to the microstructural anisotropic architectures. For the experimental tests a Nano Indenter XP (MTS Nano Instruments, Oak Ridge TN) has been used. The mechanical tests have been carried out on the inner and outer surfaces of the shells, as well as on their cross-section, near to the inner/outer surfaces and in the middle layer. The results confirm the three layered anisotropic architecture of the investigated conchs. On each of these 5 surfaces, 2 x 5 indentations have been performed at different maximum depth: from 250 nm to 4 microm, with a step of 250 nm, for a total of 3200 tests. The numerous observations have been analysed applying an ad hoc modification of the Weibull Statistics, suggesting a natural evolution of the shells against external attacks.     

Dynamic elasto-plastic response of shells in an acoustic medium—EPSA code

A nonlinear, large deflection, elasto-plastic finite element code (EPSA) has been developed for the analysis of shells in an acoustic medium subjected to dynamic loadings. The nonlinear equations of shells are discretized with the aid of a finite difference/finite element method based upon the principle of virtual work. The resulting system of equations contains the nodal displacements as the generalized co-ordinates of the problem. The integration in time of the equations of motion is done explicitly via a central difference scheme. Shell strain-displacement relations are established by a two-dimensional finite difference scheme. The shell constitutive equations are formulated in terms of the shell stress resultants and the shell strains and curvatures. The fluid-structure interaction is accounted for by means of the doubly asymptotic approximation (DAA) expressed in terms of orthogonal fluid expansion functions. The analytically produced results satisfactorily reproduce available experimental data for dynamically loaded shells.     

On the prediction of the fatigue life of thin-walled structures based on a statistical model

We present experimental results of fatigue tests of AMg6 aluminum alloy model specimens-plates with a single notch, subjected to fully reversed and asymmetric cyclic axial loading. The experimentally determined lives of model specimens are used as the results of the base experiment for predicting the fatigue strength characteristics of large-scale cylindrical shells with circumferential ring-shaped stress raisers loaded by axial forces of a cyclic nature. A numerical study has been performed of the statistical model adopted for predicting the fatigue life of such shells. __________ Translated from Problemy Prochnosti, No. 3, pp. 112–122, May–June, 2006.     

Fracture mechanics analysis of pressurized cracked shallow shells

The prediction of the behaviour of pressurized cracked thin shells to static or fatigue fracture requires a definition of a crack parameter more general than the stress intensity factors, in order to take into account the local three-dimensional effects of the pressure which causes a local bulging in the cracked area.It is shown that the generalized strain energy release rate derived from an overall energetic balance is a parameter particularly suitable for this problem.Its expression for a non-linear elastic structure is derived and two experimental tests carried out on plates and shells cracked under pressure are described. This proof being obtained, this concept is used in an investigation of sub-structure of a cracked shell in order to estimate this effect of local bulging and to build-up a series of reference results.     

Adsorption of Direct Red 80 dye from aqueous solution onto almond shells: effect of pH, initial concentration and shell type

The adsorption of Direct Red 80 (DR 80) dye from aqueous solution on almond shells as an eco-friendly and low-cost adsorbent was studied. The effect of shell type (internal, external and mixture shells), pH and initial dye concentration were considered to evaluate the sorption capacity of almond shell adsorbent. The mixture type of almond shell showed to be more effective. The adsorption studies revealed that the mixture type of almond shells remove about 97% of the DR 80 dye from aqueous phase after 1h of the adsorption process in a batch system. Although, pH changes did not appreciably affect the adsorption process but the maximum adsorption capacity of different types of almond shells (20.5, 16.96 and 16.4 mg/g for mixture, external and internal shells) were obtained at pH 2. However, in order to have a better control on the experimental conditions, pH 6 was selected for conducting all adsorption experiments. Initial dye concentration was varied from 50 to 150 mg/L. Higher concentrations of dye in aqueous solution reduced DR 80 dye adsorption efficiency of almond shells. Equilibrium data were attempted by various adsorption isotherms including Langmuir, Freundlich and Brunauer-Emmett-Teller (BET) models. It was found that the adsorption process by mixture type of almond shells follows the Langmuir non-linear isotherm. Furthermore, the experimental data by internal and external almond shells could be well described by the BET and Freundlich isotherm models, respectively. The pseudo-second-order kinetics provides the best correlation of the experimental data.     

Calculation of Load-Carrying Capacity of Elastic Shells with Periodic Dents (Theory and Experiment)

The phenomenon of the local loss of stability (buckling) earlier discovered by the authors experimentally was simulated theoretically. An approach to assessing the load-carrying capacity of circular cylindrical shells with periodic dents is presented. The authors show that application of the linear theory or solution of the problem in the nonlinear formulation with few iterations (when calculating the subcritical state) results in large quantitative errors. To get more accurate and reliable numerical data, one needs to solve the problem of the nonuniform subcritical state in the nonlinear formulation with many iterations. The local buckling loads determined numerically are lower than the experimental values, i.e., the errors are on the safe side. The loads characterizing the overall buckling are either close to the experimental values or also lower.     

充液及内空圆柱壳在爆炸荷载下动力屈曲特性研究

采用实验与数值模拟相结合方法,对充液及内空圆柱壳在爆炸载荷下动力屈曲响应特性进行对比研究。将壁厚δ=2.0 mm、外径Φ=100 mm钢圆柱壳（内空及充水）置于75gTNT药柱、200gTNT药柱产生的爆炸场中进行冲击实验,获得不同工况下圆柱壳变形破坏模式。利用动力有限元程序LS-DYNA及Lagrangian-Eulerian流固耦合方法进行数值计算,分析壳壁屈曲变形过程及壳壁关键点速度、水介质内压力等动态参数。计算结果与实验结果一致性较好。研究表明,由于内充水介质的近似不可压缩性,承受冲击荷载时内压增大,因而参与对外界爆炸冲击载荷抗力作用,圆柱壳抗爆能力显著提高。     

Sensibility of shells to buckling disturbances in connection with parameters of critical loading spectrum

This paper considers the critical loading density in the vicinity of the first eigenvalue for the stability problems of both a structurally orthotropic longitudinally-compressed cylindrical shell and a spherical panel. For structurally orthotropic shells it is shown that the critical loading spectrum in the above-mentioned problems can begin with the condensation point only when definite relations between structurally orthotropic parameters take place. In connection with the presence of eigenvalue condensation points in the stability problems it becomes expedient to obtain approximate solutions for nonhomogeneous problems where the right-hand part of the equation describes disturbances. The linear systems of such a type show a strong selectivity as regards to initial deflections and to small external disturbances. The experimental results were treated. The process of the additional shell deflection development was analyzed. It was established by experiment that the shells with more rigid longitudinal stiffeners are less sensitive to initial imperfections and disturbances that explains a good agreement of critical loadings of these imperfect shells with Euler loadings. The number of equivalent forms for the shells with longitudinally stiffened, transversely stiffened and non-stiffened shells has the following order, respectively: 0(1), 0(10), 0(100).     

Range of the Applicability of the Wilkins Method to the Investigation of the Dynamic Stress-Strain State of Anisotropic Elastic Axisymmetric Shells

The exactness and stability of the Wilkins method in the investigation of the stress-strain state of axisymmetric anisotropic elastic shells, including thick-walled ones, have been studied for different values of artificial viscosity, pulse load rise and fall time, and different deformation process durations. A comparison of numerical calculations, performed by means of an application software package developed by V. A. Romashchenko on the basis of the Wilkins method, with experimental results, known numerical data, and with calculations using the computation kernel LS-DYNA 3D has been carried out.