Experimental characterization of phenolic-impregnated honeycomb sandwich structures for transportation vehicles

In the present paper the main outcomes of an experimental characterization on phenolic impregnated honeycomb sandwich structures are presented. The experimental investigations addressed both the static and dynamic properties of novel sandwich material, manufactured expressly for transportation industry and both the structural and impact behavior of the sandwich configuration. Moreover in order to fulfill design requirements, the prediction of the material properties and structural behaviors of sandwich structures due to environmental degradation have been assessed using accelerated aging tests. The outcomes herein presented provide information for the modification of design parameters to minimize the influences of the environmental factors and the adverse effect of in-service impact events.     

Minimum-Weight Sandwich Structure Optimum Design Subjected to Torsional Loading

As one of the most valued structural engineering innovations developed by the composites industry, sandwich structures are now used extensively in automotive, aerospace and civil infrastructure due to the main advantage of lightweight. This paper develops a minimum weight optimization method for sandwich structure subjected to torsion load. The design process are identified for a sandwich structure required to meet the design constraint of torsion stiffness. The optimum solutions show that at optimum design the core weight accounts for 66.7% of the whole sandwich structure. To illustrate the newly developed optimum design solutions, numerical examples are presented for sandwich structures made of either isotropic face skins or orthotropic composite face skins. Agreement between the theoretical analysis and the examples results is good.     

基于蜂窝材料结构相似有限元模型的夹层结构抗爆炸冲击特性优化设计分析

根据蜂窝材料压缩力学理论,提出了一种蜂窝材料的结构相似有限元模型构建方法,并在蜂窝材料轴向压缩和夹层结构抗爆炸冲击2种分析工况下对结构相似模型计算分析的等效性进行了验证。结果表明,蜂窝材料的结构相似有限元模型不仅能够描述原蜂窝材料的宏观压缩力学特性,而且可以大幅地提高模型计算效率,为蜂窝材料夹层结构的抗爆炸冲击特性分析与设计优化提供条件。基于该结构相似有限元模型,以最小化蜂窝材料夹层结构的面密度为设计目标,利用自适应响应面方法对夹层结构的主要设计参数进行了优化,在结构的爆炸冲击响应速度峰值不增大的条件下提高了结构的轻量化水平。     

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.     

Sensitivity analysis of frequency response functions of composite sandwich plates containing viscoelastic layers

In the scope of structural dynamics, sensitivity analysis is a very useful tool in a number of numerical procedures such as parameter identification, model updating, optimal design and uncertainty propagation. In this paper the formulation of first-order sensitivity analysis of complex frequency response functions (FRFs) is developed for composite sandwich plates composed by a combination of fiber-reinforced and elastomeric viscoelastic layers, in arrangements that are frequently used for the purpose of noise and vibration attenuation. Although sensitivity analysis is a well known numerical technique, the main contribution intended for this study is its extension to viscoelastic structures, which are characterized by frequency- and temperature-dependent material properties and, thus, require particularly adapted analytical and numerical procedures. Due to the fact that finite element discretization has become the most used method for dynamic analysis of complex structures, the sensitivity analysis addressed herein is based on such models, being computed from the analytical derivatives of the FRFs with respect to a set of design parameters, such as fiber orientations and layer thicknesses. Also, a procedure for evaluating the sensitivity of the FRFs with respect to temperature of the viscoelastic material is suggested. After discussion of various theoretical aspects, including a parameterization scheme of the structural matrices with respect to the design variables, first-order response derivatives are calculated for a composite plate with inherent structural damping, and for a composite sandwich plate with a viscoelastic core. The results are compared to those obtained from first-order finite-difference approximations.     

Development of a High‐fidelity Experimental Substructure Test Rig for Grid‐scored Sandwich Panels in Wind Turbine Blades

This paper outlines high‐fidelity experimental substructure testing of sandwich panels which constitute the aerodynamic outer shell of modern wind turbine blades. A full‐scale structural experimental and numerical characterisation of a composite wind turbine blade has been conducted. The development of a full‐scale numerical model is detailed, and the necessary experimental set‐up is described. Further, the numerical and experimental results obtained are compared, and an idealised set of boundary conditions for a chosen blade substructure is presented. From this, the development of a test rig suitable for representing the established loading and boundary conditions is presented, and some preliminary experimental results are discussed. The work provides a road map for developing high‐fidelity experimental substructure tests, which in more generic terms are applicable to similar developments of substructure tests for composite wind turbine blades. Furthermore, recommendations on the use of grid‐scored sandwich structures in wind turbine blades are presented, which outline the sensitivity in terms of quasi‐static strength to the established loading conditions.     

Prediction of impact damage on sandwich composite panels

Sandwich structures are extensively employed in the aerospace and automobile industries. The understanding of their behaviour under impact conditions is extremely important for the design and manufacturing of these engineering structures since impact problems are directly related to structural integrity and safety requirements. This paper investigates the damage behaviour of composite sandwich panels with aramid paper honeycomb (NOMEX) and polyetherimide (PEI) foam cores under transverse impacts at high velocities. A numerical model was developed using the dynamic explicit finite element (FE) structure analysis program PAM-CRASH. For both sandwich structures numerical analysis reproduces physical behaviour observed experimentally in high velocity impact tests.     

Experimental and numerical evaluation of the mechanical behaviour of GFRP sandwich panels

This work deals with the analysis of the mechanical behaviour of a class of sandwich structures widely employed in marine constructions, constituted by fiber-glass laminate skins over PVC foam or polyester mat cores. In detail, a systematic experimental study and numerical simulations have shown that the theoretical prediction of the strength and the actual failure mechanism of these sandwich structures can be affected by significant errors, specially in the presence of prevalent shear loading. Moreover, because of the low shear stiffness and the elastic constants mismatch of the skins and core material, failure modes and strength are strongly influenced by eventual stresses orthogonal to the middle plane of the sandwich. In particular, for the sandwich structures with a PVC foam core, such a stress interaction leads to early skin–core delamination failure, whereas for those with a polyester core it leads to core shear-cohesive failure. By means of accurate non-linear simulations, accurate failure criteria, that can be used at the design stage in the presence of complex loading, have also been developed.     

体育场主看台悬挑屋盖表面风压的数值模拟

采用数值模拟方法对体育场主看台悬挑屋盖结构的表面风压分布进行了预测和分析。为验证数值模拟的准确性,首先对一具有风压风洞试验结果的主看台平屋盖结构进行了计算,计算结果与风洞试验结果作了比较。然后对浙江温州地区一主看台悬挑屋盖结构进行了数值模拟,考虑了屋盖倾角为0°和16°两种情况;通过对两个最不利风向角0°和180°的模拟结果的分析和比较,提出了这类屋盖表面风压的简化分布规律和计算方法,并与国外相关规范建议的公式作了比较。该简化计算方法可直接供同类结构抗风设计参考和应用。     

New peel stopper concept for sandwich structures

The paper addresses the damage tolerance of sandwich structures, where the prevention and limitation of delamination failure are highly important design issues. Due to the layered composition of sandwich structures, face–core interface delamination is a commonly observed failure mode, often referred to as peeling failure. Peeling between the sandwich face sheets and the core material drastically diminishes the structural integrity of the structure. This paper presents a new peel stopper concept for sandwich structures. Its purpose is to effectively stop the development of debonding/delamination by rerouting the delamination, and to confine it to a predefined zone in the sandwich structure. The suggested design was experimentally tested for different material compositions of sandwich beams subjected to three-point bending loading. For all the tested sandwich configurations the suggested peel stopper was able to stop face–core delamination and to limit the delamination damage to restricted zones.     

Reliability of inserts in sandwich composite panels

Inserts are commonly used to transfer loads to sandwich composite structures. Local stress concentrations due to inserts are known to cause structural failure, and experimental pull-out tests show that the failure load can vary by 20% between batches of sandwich panels. Clearly, uncertainty in the mechanical properties of the constituent materials needs to be addressed in the design and optimization of sandwich panel inserts. In this paper, we explore the utility of reliability analysis in design, applying Monte Carlo sampling, the First Order Reliability Method (FORM), line sampling, and subset simulation to a one-dimensional model of an insert in a homogenized sandwich panel. We observe that for systems with very low failure probabilities, subset simulation is the most efficient method for calculating the probability of structural failure, but in general, Monte Carlo sampling is more effective than the advanced reliability analysis techniques.     

Optimization of Sandwich Composites Fuselages Under Flight Loads

The sandwich composites fuselages appear to be a promising choice for the future aircrafts because of their structural efficiency and functional integration advantages. However, the design of sandwich composites is more complex than other structures because of many involved variables. In this paper, the fuselage is designed as a sandwich composites cylinder, and its structural optimization using the finite element method (FEM) is outlined to obtain the minimum weight. The constraints include structural stability and the composites failure criteria. In order to get a verification baseline for the FEM analysis, the stability of sandwich structures is studied and the optimal design is performed based on the analytical formulae. Then, the predicted buckling loads and the optimization results obtained from a FEM model are compared with that from the analytical formulas, and a good agreement is achieved. A detailed parametric optimal design for the sandwich composites cylinder is conducted. The optimization method used here includes two steps: the minimization of the layer thickness followed by tailoring of the fiber orientation. The factors comprise layer number, fiber orientation, core thickness, frame dimension and spacing. Results show that the two-step optimization is an effective method for the sandwich composites and the foam sandwich cylinder with core thickness of 5 mm and frame pitch of 0.5 m exhibits the minimum weight.     

Vibration Modeling of Multilayer Composite Structures with Viscoelastic Layers

This work deals with the vibration of orthotropic multilayer sandwich structures with viscoelastic core. A finite element model is derived from a classical zigzag model with shear deformation in the viscoelastic layer. The aim of the present work is to establish numerical models and develop numerical tools to design multilayer composites structures with high damping properties. To fulfill this purpose, a finite element model has been developed for vibration analysis of a sandwich plate (elastic orthotropic)/(viscoelastic orthotropic)/(elastic orthotropic). A numerical study from the variation of the damping properties of the structures was performed according to the faces materials fibers orientation.     

缝合泡沫夹芯复合材料低速冲击的多尺度数值方法

为了发展缝合泡沫夹芯复合材料低速冲击损伤的多尺度分析方法, 建立了缝合泡沫简化力学模型, 将缝合泡沫等效为缝线树脂柱增强的正交各向异性芯材, 其材料参数由各组分性能及所占体积分数根据均一化理论计算得出; 同时, 建立冲击试验有限元模型, 通过界面元模拟面板与芯材之间的层间分层。采用GENOA渐进损伤分析模块对缝合结构冲击动态响应过程进行数值模拟, 并将计算结果与试验记录进行对比分析。结果表明: 缝合可以减小面板破坏面积, 抑制面板与泡沫分层的扩展; 但缝纫会对结构造成初始损伤, 较高的缝合密度使芯材刚度增加, 不利于泡沫结构的缓冲吸能。数值模拟结果与试验记录吻合良好, 验证了多尺度分析方法的正确性。     

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.     

Experiment-Based Explicit Dynamics Analysis for Bird Strike Damage Prediction in Composite Structures

Bird strike analysis is a common type of analysis performed during the design and analysis of rotorcraft. These simulations are carried out in order to predict whether various designs will pass the necessary certification tests. In the past, the only way to determine whether forward-facing aeronautical composite structures could withstand bird strikes was with time-consuming physical tests. In the research of bird striking, the bird impact test is the most effective method. But the existing data of test results are highly disperse, so that they do less help for the design of aeronautical composite structures and also cost more. Tests usually needed to be repeated several times because components often failed and were required for each new design. There is a large variability in numerical bird models, composite modeling approaches and complexity of simulation processes to design the sandwich structures of an aircraft. This paper investigates the composite structures modeling for bird strike phenomenon by using state-of-the-art modeling tools capable of predicting the experiment-based composite structural damage, damage location, failure size and failure mode due to impact and addresses a critical review on analysis techniques. This paper also demonstrates the state-of-the-art bird strike simulation methodology developed, and the accuracy of modeling approaches available in explicit codes is discussed.     

A sub-laminates FEM approach for the analysis of sandwich beams with multilayered composite faces

A finite element approach based on the Hermitian ZigZag theory and on the sub-laminates concept is proposed for the static analysis of sandwich structures. The accuracy of the model is evaluated through a set of numerical results about the static response of sandwich beams with composite multilayered faces subjected to different boundary conditions and external loads; a comparison with three-dimensional exact elasticity solutions (when available) and high-fidelity FE models is also performed. An experimental tests campaign is also carried out to further assess the HZZ model accuracy. It is shown that the proposed approach is valid in determining global and local responses of sandwich beams.     

Analysis of Bird Impact on a Composite Tailplane Leading Edge

One of the main structural requirements of a leading edge of a tailplane is to ensure that any significant damage caused by foreign object (i.e. birdstrike, etc...) would still allow the aircraft to land safely. In particular, leading edge must be certified for a proven level of bird impact resistance. Since the experimental tests are expensive and difficult to perform, numerical simulations can provide significant help in designing high-efficiency bird-proof structures. The aim of this research paper was to evaluate two different leading edge designs by reducing the testing costs by employing state-of-the-art numerical simulations. The material considered was a sandwich structure made up of aluminium skins and flexcore as core. Before each test was carried out, pre-test numerical analyses of birdstrike were performed adopting a lagrangian approach on a tailplane leading edge of a large scale aircraft using the MSC/Dytran solver code. The numerical and experimental correlation have shown good results both in terms of global behaviour of the test article and local evolution of some measurable parameters confirming the validity of the approach and possible guidelines for structural design including the bird impact requirements.