芯材增强对夹层圆柱壳动力学性能影响的有限元分析

将表层、增强材料与芯材分开,应用有限元分析软件ANSYS,采用8节点SOLID45实体单元,对增强型夹层圆柱壳建立物理模型,进行自由振动及瞬态动力学过程分析。考虑树脂材性、尺寸以及分布等参数的变化,分析了点阵增强和齿槽增强对夹层圆柱壳动力学性能的影响,将两种增强方式进行了对比。结果显示,树脂柱及树脂齿槽均可改变圆柱壳的振动特性,对降低瞬态荷载下的动力响应有积极作用。其中树脂材性的影响较小,而点阵和齿槽的尺寸与分布对圆柱壳动力学性能的影响较为明显,分析显示,点阵增强对于提高结构固有频率比齿槽增强更好一些,而齿槽增强对于降低端部受冲击荷载时的动力位移比点阵增强更好一些。     

On attenuation of floating structure response to underwater shock

This paper presents an analysis on attenuation of floating structures response to underwater shock. An explicit finite element approach interfaced with the boundary element method is used for the shock-fluid–structure interaction. The bulk cavitation induced by underwater shock near the free surface is considered in this study. Two types of floating structural configurations are modeled: one is the two-layered panel and the other is the sandwich panel, both of which are extracted from the typical floating hulls—the former corresponds the single hull with coating material and the latter corresponds to the double hull with different material fillings. Their effective structural damping and stiffness are formulated and incorporated in the fluid–structure-coupled equations, which relate the structure response to fluid impulsive loading and are solved using the coupled explicit finite-element and boundary element codes. The cavitation phenomenon near free surface is captured via the present computational procedure. The attenuation effects of the floating structure response to underwater explosion are examined. From the results obtained, some insights on the improvement of floating structures to enhance their resistance to underwater shock are deduced.     

水下冲击波作用的铝合金蜂窝夹层板动力学响应研究

为研究铝合金蜂窝夹层板水下爆炸冲击波载荷作用的动态响应及抗冲击性能,利用非药式水下爆炸冲击波加载装置对气背固支5A06铝合金夹层板及具有相同面密度的单层板进行水下冲击波加载试验。利用高速相机结合三维数字散斑技术(DIC)对夹层板后面板动态响应进行实时测量,获得夹层板气背面受水下冲击波作用的动态响应历程及变形毁伤模式,比较分析铝合金蜂窝夹层板抗冲击防护性能。结果表明,较相同面密度的单层板,蜂窝夹层板受水下冲击波载荷作用的芯层压缩能有效减少气背面板的塑性变形,提高夹层结构整体抗冲击性能。     

基于比例边界有限元法和有限元法的水下结构瞬态分析方法

针对冲击波作用下水下结构与无限声学水域的流固耦合问题,建立了基于比例边界有限元法和有限元法的瞬态分析方法。无限水域用比例边界有限元法离散,而水下结构等有限域用有限元法模拟。该方法利用声学近似法将无限水域施加给水下结构的载荷分解成冲击波载荷和散射波载荷。冲击波载荷由水下冲击波理论确定,而散射波载荷由比例边界有限元法估值。为改善比例边界有限元法动态质量矩阵的计算效率,发展了动态质量矩阵的时域递推公式。数值算例分析结果表明了所发展的瞬态分析方法和时域递推公式的正确性。     

梯度铝泡沫夹层结构抗爆性能仿真与优化

采用动力显式有限元方法,以面比吸能和背板最大变形量为评价指标,研究了铝合金面板—梯度铝泡沫芯体—装甲钢背板夹层结构的抗爆性能。分析了芯体密度梯度排布对结构抗爆性能的影响,并与均匀密度铝泡沫夹层板进行了对比。同时,基于径向基函数建立了夹层结构抗爆性能预测响应面模型,在此基础上对夹层结构进行了多目标优化设计。结果表明,铝泡沫芯体相对密度排布顺序对夹层结构抗爆性影响明显;具有最佳芯体密度梯度排布的铝泡沫夹层结构的抗爆性能明显优于等质量的均匀密度铝泡沫夹层结构;多目标优化可进一步提高梯度铝泡沫夹层结构的综合抗爆性能。     

Analysis and interpretation of a test for characterizing the response of sandwich panels to water blast

Metallic sandwich panels are more effective at resisting underwater blast than monolithic plates at equivalent mass/area. The present assessment of this benefit is based on a recent experimental study of the water blast loading of a sandwich panel with a multilayered core, using a Dyno-crusher test. The tests affirm that the transmitted pressure and impulse are significantly reduced when a solid cylinder is replaced by the sandwich panel. In order to fully understand the observations and measurements, a dynamic finite element analysis of the experiment has been conducted. The simulations reveal that the apparatus has strong influence on the measurements. Analytic representations of the test have been developed, based on a modified-Taylor fluid/structure interaction model. Good agreement with the finite element results and the measurements indicates that the analytic model has acceptable fidelity, enabling it to be used to understand trends in the response of multilayer cores to water blast.     

Mitigating performance of elastic graded polymer foam coating subjected to underwater shock

The possible mitigating effect of elastic Density Graded Polymer Foam (DGPF) coating on the marine structure subjected to underwater shock is investigated. A 1-D unified nonlinear finite element model based on the updated Lagrangian frame is built to solve both the transient response of foam coated structure and dynamic cavitation of water near fluid–structure interface. The mitigating effect of DGPF coating with respect to design parameters such as average density, density difference (uneven density), gradient functions and load intensity is explored. It is illustrated that DGPF is superior in underwater shock protection to the equivalent uniform foam if the foam density is properly distributed while load density is not so high. Lower density foam in the water side is helpful to reduce the total impulse transmitted from water. But the total energy absorption capability may be discounted as the coating enters densification phase earlier.     

A new indentation model for sandwich circular panels with gradient metallic foam cores

A new analytical model is presented to predict indentation behavior of the sandwich circular panel with gradient foam cores under a flat-end cylindrical indenter. In the model, a displacement field of the upper face sheet of the sandwich panel is assumed to be a cosine function and plateau stress of the gradient foam core varies with the mass density along the thickness direction of the sandwich panel. The sandwich panel is modeled as an infinite, isotropic, plastic membrane on a rigid-plastic foundation. The explicit solutions of the relation between the indentation force and maximum plastic regions of the upper face sheet are derived based on the principle of minimum work. The analytical results are validated using the finite element code ABAQUS^®. The influences of the gradient foam core on the maximum plastic region, the indentation force and the plastic strain energy of the sandwich panel are also investigated.     

The dynamic response of end-clamped sandwich beams with a Y-frame or corrugated core

The dynamic response of end-clamped monolithic beams and sandwich beams has been measured by loading the beams at mid-span using metal foam projectiles. The AISI 304 stainless-steel sandwich beams comprise two identical face sheets and either prismatic Y-frame or corrugated cores. The resistance to shock loading is quantified by the permanent transverse deflection at mid-span of the beams as a function of projectile momentum. The prismatic cores are aligned either longitudinally along the beam length or transversely. It is found that the sandwich beams with a longitudinal core orientation have a higher shock resistance than the monolithic beams of equal mass. In contrast, the performance of the sandwich beams with a transverse core orientation is very similar to that of the monolithic beams. Three-dimensional finite element (FE) simulations are in good agreement with the measured responses. The FE calculations indicate that strain concentrations in the sandwich beams occur at joints within the cores and between the core and face sheets; the level of maximum strain is similar for the Y-frame and corrugated core beams for a given value of projectile momentum. The experimental and FE results taken together reveal that Y-frame and corrugated core sandwich beams of equal mass have similar dynamic performances in terms of rear-face deflection, degree of core compression and level of strain within the beam.     

Prediction of the dynamic response of composite sandwich beams under shock loading

Finite element calculations are reported for the dynamic shock response of fully clamped monolithic and sandwich beams, with elastic face sheets and a compressible elastic–plastic core. Predictions of the peak mid-span deflections and deflected shapes of the beams are compared with the previously reported measured response of end-clamped sandwich beams, made from face sheets of glass fibre reinforced vinyl ester and a core of PVC foam or balsa wood [1]. Good agreement is observed, and the maximum sustainable impulse is also predicted adequately upon assuming a tensile failure criterion for the face sheets. The finite element calculations can also be used to bound the response by considering the extremes of a fully intact core and a fully damaged core. It is concluded that the shock resistance of a composite sandwich beam is maximised by selecting a composite with fibres of high failure strain.     

Deformation behavior of a stiffened panel subjected to underwater shock loading using the non-linear finite element method

Given the superior strength-to-weight ratio, stiffened panels have been used extensively in the main structure of ships and underwater vehicles. The loads acting on a stiffened panel in a ship is in-plane compression or tension, resulting from the overall hull-girder bending moment or torsion, shear force resulting from the hull-girder shear force, and lateral pressure resulting from the external wave or shock loading. This work addresses the transient responses of a panel structure reinforced by ribs of different sizes to underwater shock loads using non-linear finite element code-ABAQUS. Verification of the reliability was made between the Ramajeyathilagam’s experiments results [Ramajeyathilagam K, Vendhan CP, Rao VB. Non-linear transient dynamic response of rectangular plates under shock loading. Int J Impact Eng 2000;24:999–1015, Ramajeyathilagam K, Vendhan CP. Deformation and rupture of thin rectangular plates subjected to underwater shock. Int J Impact Eng 2004;30:699–719] at several different locations on the plates. The shock factor is adopted to describe the shock severity. Additionally, the displacement–time histories under different shock loadings are presented which will be used in designing stiffened panels so as to enhance resistance to underwater shock damage.     

The response of clamped sandwich beams subjected to shock loading

The dynamic response of monolithic and sandwich beams made from stainless steel is determined by loading the end-clamped beams at mid-span with metal foam projectiles. The sandwich beams comprise stainless-steel pyramidal cores (with no axial stretch resistance), stainless-steel corrugated cores (with a high stretch resistance) and an aluminium alloy metal foam. High-speed photography is used to measure the transient transverse deflection of the beams. The resistance to shock loading is measured by the permanent transverse deflection at the mid-span of the beams for a fixed magnitude of projectile momentum and mass of beam. It is found that the sandwich beam with the pyramidal core was the weakest of the sandwich beams, but all sandwich beams had a higher shock resistance, then the monolithic beam. For each type of beam, the dependence of transverse deflection upon the magnitude of the projectile momentum is measured. A comparison of the measurements is made with analytical predictions for both impulsive and finite pressure loading. It is found that the impulsive loading analysis over-predicts the deflections of both the monolithic and sandwich beams. The finite pressure analysis, which considers the transient nature of the loading pressure provided by the foam projectile, can accurately predict the measured transverse deflection.     

浸没的圆柱壳在冲击波作用下动响应的Galerkin解法

研究了无粘、无旋和不可压缩流体中两端简支圆柱壳在给定冲击波作用下的动响应。圆柱壳的运动方程中考虑了流体动压力和冲击波压力的共同作用，通过将冲击波压力分布函数表示为Ｆｏｕｒｉｅｒ级数有限项形式，并利用Ｇａｌｅｒｋｉｎ方法对耦合方程进行数值求解，得到了圆柱壳在冲击波作用下的位移响应特性     

Non-linear indentation behavior of foam core sandwich composite materials—A 2D approach

Light weight high performance sandwich composite materials have been used more and more frequently in various load bearing applications in recent decades. However, sandwich materials with thin composite face sheets and a low density foam core are notoriously sensitive to failure by localized external loads. These loads induce significant local deflections of the loaded face sheet into the core of the sandwich composite material, thus causing high stress concentrations. As a result, a complex multiaxial stressed and strained state can be obtained in the area of localized load application. Another important consequence of the highly localized external loads is the formation of a residual dent in the face sheet (a geometrical imperfection) that can reduce significantly the post-indentation load bearing capacity of the sandwich structure.This paper addresses the elastic–plastic response of sandwich composite beams with a foam core to local static loading. The study deals with a 2D configuration, where a sandwich beam is indented by a steel cylinder across the whole width of the specimen. The ABAQUS finite element package is used to model the indentation response of the beams. Both physical and geometrical non-linearities are taken into account. The plastic response of the foam core is modeled by the ¹CRUSHABLE FOAM and the ¹CRUSHABLE FOAM HARDENING option of the ABAQUS code. The purpose of the numerical modeling is to develop correct 2D simulations of the non-linear response in order to further understand the failure modes caused by static indentation. In order to verify the finite element model, indentation tests are performed on sandwich composite beams using a cylindrical indentor. The numerical results show good agreement with experimental test data.     

圆柱壳在水下爆炸载荷下的流-固耦合响应分析

研究了无限域流场中两端简支圆柱壳受球形炸药爆炸冲击载荷作用的响应特性。运用Hamilton变分原理,推导出了考虑流固耦合作用圆柱壳受水下爆炸冲击载荷作用的运动方程,并分别展开成Fourier级数的形式,最后在时域上差分离散,用Galerkin方法求解,得到了在水下爆炸冲击载荷下圆柱壳的响应分析。文章着重就圆柱壳在迎爆面和背爆面的不同位置的变形位移和变形速度进行了对比分析,同时还比较了流体动压力对结构响应的影响。并将分析结果与有限元计算软件MSC.DYTRAN计算结果比较,验证了算法的可靠性。     

A semi-analytical finite element model for the analysis of cylindrical shells made of functionally graded materials under thermal shock

In the present work, a study of thermoelastic analysis of functionally graded cylindrical shells subjected to transient thermal shock loading is carried out. A semi-analytical axisymmetric finite element model using the three-dimensional linear elasticity theory is developed. The three-dimensional equations of motion are reduced to two-dimensional ones by expanding the displacement field in Fourier series in the circumferential direction involving circumferential harmonics. The material properties are graded in the thickness direction according to a power law. The model has been verified with the results of simple analytical isotropic cylindrical shells subjected to a transient thermal loading. Additional FGM results for stresses and displacements are presented.     

Thermo-mechanical load interactions in foam cored axi-symmetric sandwich circular plates – High-order and FE models

This work presents analytical and finite element analysis (FEA) results of the thermo-mechanical non-linear response of an axi-symmetric circular sandwich plates with a compliant foam core. The study investigates the load–thermal interaction response of a sandwich panel where the properties of the core are temperature dependent and degrade as the temperatures are raised. It presents briefly the governing equations for a sandwich plate based on the principles of the high-order sandwich panel theory (HSAPT) which incorporates the effects of the vertical flexibility of the core material as well as the effects of temperature independent/dependent mechanical properties of the foam core. The effects of the thermal degradation of core material on the thermo-mechanical non-linear response of a simply supported circular sandwich plate are studied through the analytical and FE models. The difficulties involved in non-linear geometrical FE modeling of sandwich panels with a compliant “soft” core with temperature-dependent mechanical properties are discussed. The HSAPT model predictions are compared very well with FE result. An important conclusion of the study is that the interaction between mechanical loads, temperature induced deformations, and degradation of the mechanical properties due to elevated temperatures, may seriously affect the structural integrity of foam cored sandwich plates.     

Foam core sandwich panels with interface disbonds

The influence of a face-to-core interface disbond on the strength of a foam core sandwich panel is investigated. The geometry considered is a square sandwich panel with a circular disbond subjected to a uniformly distributed pressure load. The disbond, located in the centre of the panel, is treated as a crack, and the finite element method is used to compute stress intensity factors. These are compared to fracture toughness data to predict the onset of crack growth and, thus, the reduction in load-bearing capacity. The analyses are verified with experiments on full-scale sandwich panels.     

Insight into the shear behaviour of composite sandwich panels with foam core

While sandwich construction offers well-known advantages for high stiffness with light weight, the problem of designing the sandwich structure to withstand shear loading remains an important problem. This problem is more difficult with lower stiffness foam cores under high shear loading because the core is typically the weakest component of the structure and is the first one to fail in shear under the assuming of perfect contact between the skin and the foam core. In the present study, the shear response of the composite sandwich panels with Polyvinylchloride (PVC) foam core was investigated. The PVC H100 foam core is sandwiched between Glass Fiber Reinforced Polymer (GFRP) skins using epoxy resin to build a high performance sandwich panel to be investigated. Experiments have been carried out to characterise the mechanical response of the constituent materials under tension, compression and shear loading. Static shear tests for the sandwich panel reveal that the main failure mode is the delamination between the skin and the core rather than shearing the core itself due to the considerable value of the shear strength of the PVC foam. The Finite Element Analysis (FEA) of the sandwich structure shows that shear response and failure mode can be predicted, but that accurate predictions require a consideration of the non-linear response of the foam core. The results have a direct application in predicting the ability of the sandwich structure to withstand the shear loading.     

A wave propagation model for the high velocity impact response of a composite sandwich panel

A solution methodology to predict the residual velocity of a hemispherical-nose cylindrical projectile impacting a composite sandwich panel at high velocity is presented. The term high velocity impact is used to describe impact scenarios where the projectile perforates the panel and exits with a residual velocity. The solution is derived from a wave propagation model involving deformation and failure of facesheets, through-thickness propagation of shock waves in the core, and through-thickness core shear failure. Equations of motion for the projectile and effective masses of the facesheets and core as the shock waves travel through sandwich panel are derived using Lagrangian mechanics. The analytical approach is mechanistic involving no detail account of progressive damage due to delamination and debonding but changes in the load-bearing resistance of the sandwich panel due to failure and complete loss of resistance from the facesheets and core during projectile penetration. The predicted transient deflection and velocity of the projectile and sandwich panel compared fairly well with results from finite element analysis. Analytical predictions of the projectile residual velocities were also found to be in good agreement with experimental data.