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.     

High Velocity Impact Response of Composite Lattice Core Sandwich Structures

In this research, carbon fiber reinforced polymer (CFRP) composite sandwich structures with pyramidal lattice core subjected to high velocity impact ranging from 180 to 2,000 m/s have been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun are conducted to investigate the impact process and to validate the finite element (FE) model. The energy absorption efficiency (EAE) in carbon fiber composite sandwich panels is compared with that of 304 stainless-steel and aluminum alloy lattice core sandwich structures. In a specific impact energy range, energy absorption efficiency in carbon fiber composite sandwich panels is higher than that of 304 stainless-steel sandwich panels and aluminum alloy sandwich panels owing to the big density of metal materials. Therefore, in addition to the multi-functional applications, carbon fiber composite sandwich panels have a potential advantage to substitute the metal sandwich panels as high velocity impact resistance structures under a specific impact energy range.     

Sandwich structures with textile-reinforced composite foldcores under impact loads

The mechanical behaviour of composite sandwich structures with textile-reinforced composite foldcores, which are produced by folding prepreg sheets to three-dimensional zigzag structures, is evaluated under compression, shear and impact loads. While foldcores made of woven aramid fibres are characterised by a rather ductile behaviour, carbon foldcores with their brittle nature absorb energy by crushing, showing extremely high weight-specific stiffness and strength properties. The impact damage under low and high velocity impact loads tends to be very localised. In addition to regular single-core sandwich structures, a dual-core configuration with two foldcores is also investigated, showing the potential of a two-phase energy absorption behaviour. In addition to experimental testing, finite element models for impact simulations with LS-DYNA have been developed. Despite the high degree of complexity of the models due to the various skin and core failure modes that have to be covered, the results correlate well with test data, allowing for efficient parameter studies or detailed evaluations of damage patterns and energy absorption mechanisms.     

Use of piezoelectric as acoustic emission sensor for in situ monitoring of composite structures

In this paper, the influence of the integration of several sensors in composite structures is investigated. The plates and the structures in simple shapes, composed of laminated and sandwich materials, are considered. The mechanical behaviour, the acoustics activity and the location of damage sources in various structures with and without piezoelectric implant are compared. The analysis of results allowed a better identification of the influence of the impact of piezoelectric implant on the mechanical behaviour of different structures under different loads. Then, the analysis and the observation of Acoustic Emission (AE) signals led to the identification of the main acoustic signatures of different damage modes dominant in each type of composite materials (laminates and sandwich). Viewpoint comparison between integrated and non-integrated structures, acoustic activity is more significant in the case of integrated material. The location of the sources of damage has shown that acoustic events occurred far from the positions of integrated sensors.     

Effect of strain rate and density on dynamic behaviour of syntactic foam

The final objective of this study is to improve the mechanical behaviour of composite sandwich structures under dynamic loading (impact or crash). Cellular materials are often used as core in sandwich structures and their behaviour has a significant influence on the response of the sandwich under impact. Syntactic foams are widely used in many impact-absorbing applications and can be employed as sandwich core. To optimize their mechanical performance requires the characterisation of the foam behaviour at high strain rates and identification of the underlying mechanisms.Mechanical tests were conducted on syntactic foams under quasi-static and high strain rate compression loading. The material behaviour has been determined as a function of two parameters, density and strain rate. These tests were complemented by experiments on a new device installed on a flywheel. This device was designed in order to achieve compression tests on foam at intermediate strain rates. With these test machines, the dynamic compressive behaviour has been evaluated in the strain rate range up [6.7 · 10⁻⁴ s⁻¹, 100 s⁻¹].Impact tests were conducted on syntactic foam plates with varying volume fractions of microspheres and impact conditions. A Design of Experiment tool was employed to identify the influence of the three parameters (microsphere volume fraction, projectile mass and height of fall) on the energy response. Microtomography was employed to visualize in 3D the deformation of the structure of hollow spheres to obtain a better understanding of the micromechanisms involved in energy absorption.     

Modelling of composite sandwich structures with honeycomb core subjected to high-velocity impact

In this study the perforation of composite sandwich structures subjected to high-velocity impact was analysed. Sandwich panels with carbon/epoxy skins and an aluminium honeycomb core were modelled by a three-dimensional finite element model implemented in ABAQUS/Explicit. The model was validated with experimental tests by comparing numerical and experimental residual velocity, ballistic limit, and contact time. By this model the influence of the components on the behaviour of the sandwich panel under impact load was evaluated; also, the contribution of the failure mechanisms to the energy-absorption of the projectile kinetic energy was determined.     

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

Large scale fiber reinforced polymer (FRP) composite structures have been used in highway bridge and building construction. Recent applications have demonstrated that FRP honeycomb sandwich panels can be effectively and economically applied for both new construction and rehabilitation and replacement of existing structures. This paper is concerned with impact analysis of an as-manufactured FRP honeycomb sandwich system with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components including: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, and (3) equivalent core material properties, are first introduced, and these properties for the face laminates and equivalent core are later used in dynamic analysis of sandwich beams. A higher-order impact sandwich beam theory by the authors [Yang MJ, Qiao P. Higher-order impact modeling of sandwich beams with flexible core. Int J Solids Struct 2005;42(20):5460–90] is adopted to carry out the free vibration and impact analyses of the FRP honeycomb sandwich system, from which the full elastic field (e.g., deformation and stress) under impact is predicted. The higher order vibration analysis of the FRP sandwich beams is conducted, and its accuracy is validated with the finite element Eigenvalue analysis using ABAQUS; while the predicted impact responses (e.g., contact force and central deflection) are compared with the finite element simulations by LS-DYNA. A parametric study with respect to projectile mass and velocity is performed, and the similar prediction trends with the linear solution are observed. Furthermore, the predicted stress fields are compared with the available strength data to predict the impact damage in the FRP sandwich system. The present impact analysis demonstrates the accuracy and capability of the higher order impact sandwich beam theory, and it can be used effectively in analysis, design applications and optimization of efficient FRP honeycomb composite sandwich structures for impact protection and mitigation.     

Pin-Contact Behaviour of Composite Sandwich Structures Under Compressive Bearing Load

The aim this research is to investigate the pin-sandwich contact behaviour of some sandwich composites structures when submitted to compressive bearing loads. A preliminary set of flatwise and edgewise compressive tests and three point flexural tests were performed to get information on the mechanical behaviour of the sandwich structures under different load conditions. The influence of two different manufacturing procedures on the bearing strength is evidenced. The experiments show that the bearing loads increase with the pin diameter, while the bearing stress depends in a different way of the pin diameter for the two kind of procedures employed. In addition a simplified numerical model is proposed to evaluate the stress/strain distribution in the sandwich structure under compressive bearing load, by employing a commercial code. The comparison of numerical results with experiments shows the accuracy of the model.     

The low velocity impact response of foam-based sandwich panels

This paper describes the results of a combined experimental/numerical study to investigate the perforation resistance of sandwich structures. The impact response of plain foam samples and their associated sandwich panels was characterised by determining the energy required to perforate the panels. The dynamic response of the panels was predicted using the finite element analysis package ABAQUS/Explicit. The experimental arrangement, as well as the FE model were also used to investigate, for the first time, the effect of oblique loading on sandwich structures and also to study the impact response of sandwich panels on an aqueous support.     

Transient Dynamic Response and Failure of Sandwich Composite Structures under Impact Loading with Fluid Structure Interaction

The objective of this study is to examine the Fluid Structure Interaction (FSI) effect on transient dynamic response and failure of sandwich composite structures under impact loading. The primary sandwich composite used in this study consisted of a 6.35?mm balsa core and a multi-ply symmetrical plain weave 6?oz E-glass skin. Both clamped sandwich composite plates and beams were studied using a uniquely designed vertical drop-weight testing machine. There were three impact conditions on which these experiments focused. The first of these conditions was completely dry (or air surrounded) testing. The second condition was completely water submerged. The final condition was also a water submerged test with air support at the backside of the plates. The tests were conducted sequentially, progressing from a low to high drop height to determine the onset and spread of damage to the sandwich composite when impacted with the test machine. The study showed the FSI effect on sandwich composite structures is very critical such that impact force, strain response, and damage size are generally much greater with FSI under the same impact condition. As a result, damage initiates at much lower impact energy conditions with the effect of FSI. Neglecting to account for FSI effects on sandwich composite structures results in very non-conservative analysis and design. Additionally, it was observed that the damage location changed for sandwich composite beams with the effect of FSI.     

Failure behaviour of foam-based sandwich joints under pull-out testing

Sandwich materials have been used widely in various fields. However, the failure behaviour of the joints that connect the sandwich structures is not fully understood. In this paper, foam-based sandwich joints were studied under pull-out conditions through experiments and numerical analyses. Two different types of joints, with and without inserts, were examined. The experiments showed that the failure modes of sandwich joints were a combination of different failure modes, such as core shear failure, the debonding of face and potting material, composite face failure, and local failure. A finite element analysis using MSC.MARC was also conducted to predict the failure load of sandwich joints. Two methods were applied to predict the first drop of the sandwich joints: progressive failure analysis and the damage zone method. The former resulted in an acceptable prediction for a preliminary design, and the latter was shown to predict the first peak of loading curves very well.     

纸瓦楞-蜂窝复合夹层结构的跌落冲击缓冲性能研究

针对纸瓦楞与纸蜂窝的复合夹层结构在跌落冲击动态压缩条件下的缓冲防护性能,研究了纸蜂窝厚度对单面、双面复合形式的冲击加速度响应、变形特征和缓冲吸能特性的影响规律。结果表明,瓦楞夹层先压溃,其次是蜂窝夹层,而且较大的蜂窝厚度会引起纸蜂窝芯层的次坍塌行为。在相同冲击质量或冲击能量条件下,同一蜂窝厚度的单面复合夹层结构的单位体积吸能、比吸能和行程利用率较双面复合结构分别增加了7.94%、28.34%和8.47%,但总吸能较于双面复合结构降低了16.12%,单面复合夹层结构的缓冲吸能特性优于双面复合夹层结构,而双面复合夹层结构的抗冲击性能优于单面复合夹层结构。对于纸蜂窝厚度10 mm、15 mm、20 mm和25 mm的复合夹层结构,低冲击能量作用下蜂窝厚度的增加降低了结构的缓冲吸能特性,高冲击能量作用下蜂窝厚度的增加提高能量吸收能力。纸蜂窝厚度10 mm、15 mm、20 mm和25 mm的复合夹层结构的比吸能、单位体积吸能和行程利用率是蜂窝厚度70 mm的复合夹层结构的1 倍~3 倍,较低厚度的纸蜂窝更有利于复合夹层结构的缓冲吸能。     

Numerical Investigation of Dynamic Response and Failure Mechanisms for Composite Lattice Sandwich Structure under Different Slamming Loads

This paper mainly investigates the slamming dynamic response and progressive damage evolution of the composite lattice sandwich structure under different slamming velocities and deadrise angles. Based on the Coupled Eulerian–Lagrangian (CEL) method, an integrated numerical model of sandwich structures is developed to simulate the slamming process, in which the progressive damage evolution of composite material is considered with VUMAT subroutine. The reliability and accuracy of the corresponding numerical models are verified through the comparison between numerical and experimental results. Then, the typical slamming behavior of composite lattice sandwich structure is analyzed in detail, including hydrodynamic force, jet flow/water pressure distribution, progressive damage evolution and absorption energy. Subsequently, the influences of slamming velocity and deadrise angle on the hydrodynamic response and damage modes of the sandwich structures are investigated based on the developed numerical models. The results demonstrate that the slamming velocity and deadrise angle have significant influences on the hydroelastic behavior and damage modes of composite lattice sandwich structures. In the process of slamming, matrix damage and delamination damage are more prone to appear in the chine region, while the fiber damage more likely occurs under the higher slamming velocity and lower deadrise angle cases.

     

The bending and failure of sandwich structures with auxetic gradient cellular cores

We describe the bending and failure behaviour of polymorphic honeycomb topologies consisting of gradient variations of the horizontal rib length and cell internal across the surface of the cellular structures. The novel cores were used to manufacture sandwich beams subjected to three-point bending tests. Full-scale nonlinear Finite Element models were also developed to simulate the flexural and failure behaviour of the sandwich structures. Good agreement was observed between the experimental and FE model results. And the validated numerical model was then used to perform a parametric analysis on the influence of the gradient core geometry over the mechanical performance of the structures. It was found that the aspect ratio and the extent of gradient (i.e. the horizontal rib length growth rate or the internal angle increment) have a significant influence on the flexural properties of the sandwich panels with angle gradient cores.     

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.     

Face sheet debonding in CFRP/PMI sandwich structures under quasi-static and fatigue loading considering residual thermal stress

The face sheet debonding behaviour under quasi-static and fatigue loading in sandwich structures consisting of Carbon Fibre Reinforced Polymer face sheets and a Polymethacrylimid foam core is investigated. The sandwich structure is tested under global mode I and global mode II loading using the Single Cantilever Beam test and the Cracked Sandwich Beam test. Because of the different thermal expansion behaviour of the face sheets and the foam core thermal stresses occur already after the manufacturing process. The impact of these temperature loads on the crack propagation behaviour is investigated via evaluating the experiments numerically with Finite Element Analysis and Virtual Crack Closure Technique.     

Hypervelocity impact on CFRP: Testing,material modelling,and numerical simulation

This paper describes the derivation and validation of a numerical material model that predicts the highly dynamic behaviour of CFRP (carbon fibre reinforced plastic) under hypervelocity impact. CFRP is widely used in satellites as face sheet material in CFRP-Al/HC sandwich structures (HC = honeycomb) that can be exposed to space debris. A review of CFRP-Al/HC structures typically used in space was performed. Based on this review, a representative structure in terms of materials and geometry was selected for study in the work described here. An experimental procedure for the characterisation of composite materials is documented by Riedel et al. [ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003.]. The test results from the CFRP of the current study allow for the derivation of an experimentally based orthotropic continuum material model data set that is capable of predicting the mechanical behaviour of CFRP under hypervelocity impact. Such a data set was not previously available. In the work by Riedel et al. [Hypervelocity impact damage prediction in composites: part II – experimental investigations and simulations. International Journal of Impact Engineering, 2006;33:670–80.] an orthotropic material data set was used for modelling HVI on AFRP (aramid fibre reinforced plastic), which shows relatively high deformability before failure. The enhancements of the modelling approaches in previous studies [Riedel W, Harwick W, White DM, Clegg RA. ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003. Hiermaier S, Riedel W, Hayhurst C, Clegg RA, Wentzel C. AMMHIS – advanced material models for hypervelocity impact simulations. Final report, EMI report E 43/98, ESA CR(P) 4305, Freiburg; July 30, 1999.] necessary to model brittle CFRP are specified. An experimental hypervelocity impact campaign was performed at two different two-stage light gas guns which encompassed both normal and oblique impacts for a range of impact velocities and projectile diameters. Validation of the numerical model is provided through comparison with the experimental results. For that purpose measurements of the visible damage of the face sheets and of the HC core are conducted. In addition, the numerically predicted damage within the CFRP is compared to the delamination areas found in ultrasonic scans.     

Finite Element Analysis of Dynamic Mechanical Responses of Aluminum Honeycomb Sandwich Structures Under Low-Velocity Impact

Honeycomb sandwich structures, composed of many regularly arranged hexagonal cores and two skins, often show excellent impact performance due to strong energy absorption ability under impact loads. This paper studies dynamic mechanical responses of aluminum honeycomb sandwich structures. Parametric geometry modeling using UG software and finite element analysis using ANSYS explicit dynamics module are performed. Finite difference algorithm based on time-stepping integration is used to get the impact displacement, and stress and strain with time. Effects of different impact velocities, core length and wall thickness on the distributions of plastic stress and strain are also explored. Results show that thinner honeycomb side length and thicker wall thickness lead to stronger impact resistance. This research provides theoretical support for promoting optimal design of lightweight structures against impact loads.     

蜂窝金属夹芯板重复冲击动态响应研究

蜂窝金属及其夹芯结构是一种物理功能与结构一体化的新型轻质高强结构,广泛应用于结构轻量化与碰撞冲击防护领域。采用ABAQUS非线性有限元软件建立了蜂窝金属夹芯板(honeycomb sandwich panel,HSP)结构动态冲击数值仿真模型,数值仿真计算结果与文献实验结果吻合较好,验证了数值仿真模型的正确性。在此基础上,开展了重复冲击载荷作用下蜂窝金属夹芯板结构动态响应研究,得到了重复冲击力时程曲线、动态变形时程曲线、冲击力位移曲线以及最终挠度,分析了冲击能量、蜂窝壁厚以及上、下面板厚度分配对蜂窝金属夹芯板结构重复冲击动态响应的影响规律。研究结果表明,重复冲击载荷作用下蜂窝金属夹芯板结构上、下面板弯曲变形以及蜂窝芯层压缩变形逐渐积累,蜂窝芯层薄壁结构逐渐达到密实化,结构抗弯刚度逐渐上升,变形增量逐渐减小,结构整体能量吸收率下降。通过调节蜂窝壁厚和上、下面板厚度分配可以显著调节蜂窝金属夹芯板结构重复冲击动态响应与能量吸收性能。     

Non-linear finite element analysis of inserts in composite sandwich structures

In aeronautics, sandwich structures are widely used for secondary structures like flaps, landing gear doors or commercial equipment. The technologies used to join these kinds of structures are numerous: direct bonding or joining, tapered areas, T-joints, etc. The most common is certainly the use of local reinforcement called an insert. The insert technologies are numerous and this study focuses on high load bearing capacity inserts. They were made with a resin moulded in the Nomex™ sandwich core. Such structures are still designed mainly empirically and the lack of efficient numerical models remains a problem. In this study, pull-out tests were conducted on a representative sample and the non-linearities and the types of failure were analysed. Core shear bucking, failures of the potting and perforation of the composites skins are the main modes of failure. For each mode, local experimental and numerical analysis was carried out that led to the identification of the independent non-linear behaviour of each component. Including the results in a global non-linear finite element model gave good prediction of the failure scenario and an acceptable correlation with the tests.