Numerical simulations of low-velocity impact on an aircraft sandwich panel

The potential hazards resulting from a low-velocity impact (bird-strike, tool drop, runway debris, etc.) on aircraft structures, such as engine nacelle or a leading edge, has been a long-term concern to the aircraft industry. Certification authorities require that exposed aircraft components must be tested to prove their capability to withstand low-velocity impact without suffering critical damage.

This paper describes the results from experimental and numerical simulation studies on the impact and penetration damage of a sandwich panel by a solid, round-shaped impactor. The main aim was to prove that a correct mathematical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design an impact-resistant aircraft structure.

Part of this work presented is focused on the recent progress on the materials modelling and numerical simulation of low-velocity impact response onto a composite aircraft sandwich panel. It is based on the application of explicit finite element (FE) analysis codes to study aircraft sandwich structures behaviour under low-velocity impact conditions. Good agreement was obtained between numerical and experimental results, in particular, the numerical simulation was able to predict impact damage and impact energy absorbed by the structure.     

Optimization of composite sandwich panel against impact using genetic algorithm

The present paper deals with the optimization of composite sandwich panels subjected to low-velocity impact using advanced genetic algorithm (GA). Equivalent multi degree-of-freedom system is used to predict the contact force history and deformation response of the sandwich panels. Also, analytical solutions are used to determine the impact force and velocity at damage initiation. The GA's objective is to maximize the strength of panel by varying the ply angles of the facesheets. The results show that the stacking sequence of facesheets plays an important role in the strength of the composite sandwich panels subjected to impact loading.     

Fundamental frequency and design of the CFCF composite sandwich plate

The design efficiency of sandwich panels is often associated with the value of fundamental frequency. This paper investigates the free vibrations of rectangular sandwich plates having two adjacent edges fully clamped and the remaining two edges free (CFCF). The vibration analysis is performed by applying Hamilton’s principle in conjunction with the first-order shear deformation theory. The analytical solution determining the fundamental frequency of the plate is obtained using the generalised Galerkin method and verified by comparison with the results of finite element modal analysis. The approach developed in the paper and equations obtained are applied to the design of sandwich plates having composite facings and orthotropic core. Design charts representing the effects of the thickness of the facings and core on the mass of composite sandwich panel for a given value of the fundamental frequency are obtained.     

Large inelastic response of unbonded metallic foam and honeycomb core sandwich panels to blast loading

Sandwich panels constructed from metallic face sheets with the core composed of an energy absorbing material, have shown potential as an effective blast resistant structure. In the present study, air-blast tests are conducted on sandwich panels composed steel face sheets with unbonded aluminium foam (Alporas, Cymat) or hexagonal honeycomb cores. Honeycomb cores with small and large aspect ratios are investigated. For all core materials, tests are conducted using two different face sheet thicknesses. The results show that face sheet thickness has a significant effect on the performance of the panels relative to an equivalent monolithic plate. The Alporas and honeycomb cores are found to give higher relative performance with a thicker face sheet. Under the majority of the loading conditions investigated, the thick core honeycomb panels show the greatest increase in blast resistance of the core materials. The Cymat core panels do not show any significant increase in performance over monolithic plates.     

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

Low-velocity heavy-mass impact response of slender metal foam core sandwich beam

The objective of this work is to investigate the dynamic large deflection response of fully clamped metal foam core sandwich beam struck by a low-velocity heavy mass. Analytical solution and ‘bounds’ of dynamic solutions are derived, respectively. Also, finite element analysis is carried out to obtain the numerical solution of the problem. Comparisons of the dynamic, the quasi-static and numerical solutions for the non-dimensional maximum deflection of the sandwich beam with non-dimensional initial kinetic energy of the striker are presented for different cases of mass ratio, impact velocity and location. It is seen that the dynamic solution approaches the quasi-static one as the mass ratio of the striker to the beam is large enough, the quasi-static solution is in good agreement with the numerical results and both solutions lie in the ‘bounds’ of dynamic solutions. The quasi-static and numerical results for the impact force against the maximum deflection of the sandwich beam are obtained. It shows that the quasi-static solution can offer adequate accuracy to predict the low-velocity heavy-mass impact response of fully clamped sandwich beam.     

Deformation and failure of blast-loaded metallic sandwich panels—Experimental investigations

Metallic sandwich panels with a cellular core such as honeycomb have the capability of dissipating considerable energy by large plastic deformation under impact/blast loading. To investigate the structural response of sandwich panels loaded by blasts, a large number of experiments have been conducted, and the experimental results are reported and discussed in this paper. Quantitative results were obtained based on the measurement in the tests by a ballistic pendulum with corresponding sensors, and then the deformation/failure modes of specimen were classified and analysed systematically. The experimental programme was designed to investigate the effects on the structural response of face-sheet and core configurations, i.e. face-sheet thickness, cell size and foil thickness of the honeycomb, and mass of charge. The experimental data were then compared with the predicted data from finite element simulations, and the results show a good agreement between the experimental and computational studies.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

Transient response of a submerged cylindrical foam core sandwich panel subjected to shock loading

Predicting the dynamic response of submerged vehicles subjected to hydrostatic pressure and underwater shock loading is of great interest to many structural designers and engineers for improving material and configuration design in recent years. In this paper, the finite element method is used to evaluate the dynamic response of a submerged cylindrical foam core sandwich panel subjected to shock loading. The sandwich panel consists of a foam core surrounded by fiber-reinforced laminates. The effect of fluid–structure coupling is included in the finite element analysis whereas the fluid is assumed to be compressible and inviscid. Time histories of circumferential stress for different composite plies are presented in graphical form and the effects of core type on circumferential stress and velocity of stand-off point are also investigated. Additionally, the distribution of pressure in fluid domain and the deformation of cylindrical foam core sandwich panel are estimated. To the best of the authors’ knowledge, the specialized literature addressing the dynamic response of submerged cylindrical foam core sandwich panel to underwater shock loading is rather scanty. This work is likely to fill a gap in the specialized literature on this topic.     

等腰梯形蜂窝芯玻璃钢夹芯板的面内压缩性能

为研究等腰梯形蜂窝芯玻璃钢夹芯板面内压缩破坏机制, 利用材料试验机对夹芯板面内压缩性能进行了试验测试, 并开展了模拟研究。结果表明: 夹芯板的面内压缩破坏方式主要有面板折断、夹芯板屈曲失稳和夹芯板中面板与蜂窝芯脱粘3种类型。面板为夹芯板面内压缩的主要承载构件, 蜂窝芯对面板起到固支作用。面板结构参数与材料参数为影响夹芯板面内压缩抗压强度与抗压刚度主要因素, 多数蜂窝芯的结构参数与材料参数对夹芯板面内压缩抗压强度的影响微弱, 而个别蜂窝芯的结构参数对夹芯板面内压缩抗压刚度的影响比较显著。夹芯板体积一定时, 随着蜂窝芯胞体单元数量的增加, 夹芯板面内压缩的抗压强度与抗压刚度逐渐增大。      

全厚度缝合复合材料泡沫芯夹层结构力学性能研究与损伤容限评定

探索了全厚度缝合复合材料闭孔泡沫芯夹层结构低成本制造的工艺可行性及其潜在的结构效益。选用3 种夹层结构形式, 即相同材料和工艺制造的未缝合泡沫芯夹层和缝合泡沫芯夹层结构及密度相近的Nomex 蜂窝夹层结构, 完成了密度测定、三点弯曲、平面拉伸和压缩、夹层剪切、结构侧压和损伤阻抗/ 损伤容限等7 项实验研究。结果表明, 泡沫芯夹层结构缝合后, 显著提高了弯曲强度/ 质量比、弯曲刚度/ 质量比、面外拉伸和压缩强度、剪切强度和模量、侧压强度和模量、冲击后压缩(CAI) 强度和破坏应变。这种新型结构形式承载能力强、结构效率高、制造维护成本低, 可以在飞机轻质机体结构设计中采用。      

Structural behaviour of composite sandwich panels with plain and fibre-reinforced foamed concrete cores and corrugated steel faces

This paper studies the four-point bending response and failure mechanisms of sandwich panels with corrugated steel faces and either plain or fibre-reinforced foamed concrete core. Mechanical properties of both plain and polyvinyl alcohol fibre-reinforced foamed concrete were obtained, which are needed for the design of sandwich panel and numerical modelling. It is found that the fibre-reinforcement largely enhances the mechanical behaviour of foamed concrete and composite sandwich panels. Finite element code Abaqus/Standard was employed to investigate the influence of face/core bonding and fastening on the four-point bending response of the sandwich panels. It was found that face/core bonding plays a crucial role in the structural performance while the influence of fastening is negligible.     

An analytical solution for the large deflections of a slender sandwich beam with a metallic foam core under transverse loading by a flat punch

The large deflections of slender ultralight sandwich beams with a metallic foam core are studied under transverse loading by a flat punch, in which interaction of bending and stretching induced by large deflections is considered. Firstly, a unified yield criterion for metallic sandwich structures considering the effect of core strength is proposed, which is valid for metallic sandwich cross-sections with various core strengths and geometries. This can reduce to the yield criterion for a solid monolithic cross-section and the classical yield criterion for sandwich cross-sections with a weak core, respectively. Then, analytical solutions for the large deflections of fully clamped and simply supported metallic foam core sandwich beams are derived under transverse loading by a flat punch, respectively. Comparisons of the present solutions with experimental results are presented and good agreements are found. The effects of the core strength, the size of loading punch and the boundary conditions on the structural response of sandwich beams are discussed in detail. It is shown that the axial stretching induced by large deflections has significant effect on the load-carrying and energy absorption capacities of sandwich structures in the post-yield regime, and the load-carrying and plastic energy absorption capacities of metallic foam core sandwich beams may be underestimated as the core strength is neglected in analysis, especially for the sandwich beams with a strong core.     

Modeling and characterization of fiber-reinforced plastic honeycomb sandwich panels for highway bridge applications

Fiber-reinforced plastic (FRP) composite decks have been increasingly used in highway bridge applications, both in new construction and rehabilitation and replacement of existing bridge decks. Recent applications have demonstrated that FRP honeycomb panels can be effectively and economically used for highway bridge deck systems. This paper is concerned with design modeling and experimental characterization of a FRP honeycomb panel with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components include: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, (3) equivalent core material properties, and (4) apparent stiffness properties for the honeycomb panel and its equivalent orthotropic material properties. A homogenization process is used to obtain the equivalent core material properties for the honeycomb geometry with sinusoidal waves. To verify the accuracy of the analytical solution, several honeycomb sandwich beams with sinusoidal core waves either in the longitudinal or transverse directions are tested in bending. Also, a deck panel is tested under both symmetric and asymmetric patch loading. Finite element (FE) models of the test samples using layered shell elements are further used to correlate results with analytical predictions and experimental values. A brief summary is given of the present and future use of the FRP honeycomb panel for bridge decks. The present simplified analysis procedure can be used in design applications and optimization of efficient honeycomb structures.     

Stability of the face layer of sandwich beams with sub-interface damage in the foam core

This paper addresses the effect of local indentation/impact damage on the bearing capacity of foam core sandwich beams subjected to edgewise compression. The considered damage is in a form of through-width zone of crushed core accompanied by a residual dent in the face sheet. It is shown that such damage causes a significant reduction of compressive strength and stiffness of sandwich beams. Analytical solutions estimating the Euler’s local buckling load are obtained for two typical modes of damage. These solutions are validated through experimental investigation of three sandwich configurations. The results of the analytical analysis are in agreement with the experimental data.     

Elastic structural response of anisotropic sandwich plates with a first-order compressible core impacted by a Friedlander-type shock loading

The foundation of the non-linear theory of asymmetric anisotropic sandwich plates with a first order compressible weak orthotropic core under a Friedlander-type explosive blast is presented. The equations of motion are developed by means of Hamilton’s Principle. Within the theory, the face sheets are asymmetric while adopting the Love-Kirchoff model. In addition, the core layer is assumed to be compressible (extensible) in the transverse direction thereby capturing any wrinkling or global instabilities. The theory is then simplified and applied for the case of sandwich plates with symmetric unidirectional fiber reinforced laminated composite facings with the axes of orthotropy not necessarily coincident with the geometrical axes. The governing solution is developed using the Extended-Galerkin method resulting in two coupled non-linear second order ordinary differential equations which are then solved using the 4th-order Runge–Kutta method for a system of differential equations.     

Design guidelines for load introduction points at the boundaries of sandwich panels

In order to use sandwich structures effectively, load introduction points are required. Standardized inserts have been developed for this purpose and investigated in many scientific studies. However, these inserts are unsuitable for use directly at the sandwich edge. The aim of this paper is to develop design guidelines for the construction process of special inserts for load introduction at the sandwich edge. These are based on the theoretical failure modes of standardized inserts. The paper also describes the design and manufacture of a boundary insert on the basis of these considerations. It is demonstrated experimentally that these inserts can withstand higher loads compared with the standardized inserts that are placed directly at the edge of sandwich panels.     

Plastic collapse load prediction and failure assessment diagram analysis of cracked circular hollow section T‐joint and Y‐joint

This paper concerns the validation of standard safety assessment procedure given in BS 7910 for cracked circular hollow section T‐joint and Y‐joint, using the finite element (FE) results. A robust and efficient FE mesh generator is developed to produce the 3D models of the cracked joints and to calculate the elastic J‐integral (J_e) and elastic–plastic J‐integral (J_ep) values of the crack respectively. In order to verify its accuracy and convergence, the plastic collapse loads (P_c) obtained from experimental tests and FE predictions are compared; they agree very well with each other. It is also found from experimental tests that the plastic collapse loads (P_c) predicted using the BS 7910 reduction factor (F_AR) are safe and conservative. Subsequently, the failure assessment diagrams (FADs) of five cracked T‐joints and three cracked Y‐joints are constructed using the FE results, following the J‐integral method, which is classified as Level 3C in BS 7910. Thereafter, a comparison between the constructed FAD curves and the standard Level 2A curve is carried out, and it is observed that the safety assessment results using the standard Level 2A curve might be unsafe because some parts of the constructed FAD curves fall inside of the standard one. A penalty factor of 1.15 working on both the elastic–plastic J‐integral and plastic collapse load (P_c) is proposed to move all the constructed FAD curves just outside of the standard Level 2A curve.     

Multiscale design of a rectangular sandwich plate with viscoelastic core and supported at extents by viscoelastic materials

This work presents a multiscale model of viscoelastic constrained layer damping treatments for vibrating plates/beams. The approach integrates a finite element (FE) model of macroscale vibrations and a micromechanical model to include effects of microscale structure and properties. The FE model captures the shear deformation of the viscoelastic core, rotary inertial effects of all layers, and viscoelastic boundaries of the plate. Comparison with analytical and FE results validates the proposed FE model. A self-consistent (SC) model makes the micro to macro scale transition to approximate the effective behavior a heterogeneous core. Modal damping resulting from the presence of voids and negative stiffness regions in the core material is modeled. Results show that negative stiffness regions in the viscoelastic core material, even at low volume fractions, yield superior macroscopic damping behavior. The coupled SC and FE models provide a powerful multiscale predictive design tool for sandwich beams and plates.     

Fatigue crack initiation life estimation in a steel welded joint by the use of a two-scale damage model

This work deals with the fatigue behaviour of S355NL steel welded joints classically used in naval structures. The approach suggested here, in order to estimate the fatigue crack initiation life, can be split into two stages. First, stabilized stress–strain cycles are obtained in all points of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. This calculation takes account of: base metal elastic–plastic behaviour, variable yield stress based on hardness measurements in various zones of the weld, local geometry at the weld toe and residual stresses if any. Second, if a fast elastic shakedown occurs, a two-scale damage model based on Lemaitre et al. 's work is used as a post-processor in order to estimate the fatigue crack initiation life. Material parameters for this model were identified from two Wöhler curves established for base metal. As a validation, four-point bending fatigue tests were carried out on welded specimens supplied by 'DCNS company'. Two load ratios were considered: 0.1 and 0.3. Residual stress measurements by X-ray diffraction completed this analysis. Comparisons between experimental and calculated fatigue lives are promising for the considered loadings. An exploitation of this method is planned for another welding process.