Numerical analysis and experiment of composite sandwich T-joints subjected to pulling load

This paper presents an investigation into the failure mechanism and alternative design of composite sandwich T-joints subjected to pulling load. Based on a conventional design of sandwich T-joint as the baseline, numerical modeling and analysis using finite element (FE) method was performed to assess the strength against pulling load. The effect of a cutout in the web panel near the joint has been considered. To validate the models, sandwich T-joint samples were manufactured and tested. Detailed FE analysis and inspection of the experimental results indicated that the failure was mainly due to the excessive stress in the adhesive between the cleat flange and the T-joint base panel. The manufacture defects, which reduced the strength of the T-joint test samples had also been investigated. This has been further demonstrated by experimental results of repaired T-joint samples. A very good correlation between the test data and FE results were obtained. An unconventional design of T-joint for simpler manufacture process was proposed. Based on the design, T-joint samples were modeled, manufactured and tested to demonstrate the manufacture process and evaluate the improved strength.     

Numerical analysis and experiment of composite sandwich T-joints subjected to pulling load

This paper presents an investigation into the failure mechanism and alternative design of composite sandwich T-joints subjected to pulling load. Based on a conventional design of sandwich T-joint as the baseline, numerical modeling and analysis using finite element (FE) method was performed to assess the strength against pulling load. The effect of a cutout in the web panel near the joint has been considered. To validate the models, sandwich T-joint samples were manufactured and tested. Detailed FE analysis and inspection of the experimental results indicated that the failure was mainly due to the excessive stress in the adhesive between the cleat flange and the T-joint base panel. The manufacture defects, which reduced the strength of the T-joint test samples had also been investigated. This has been further demonstrated by experimental results of repaired T-joint samples. A very good correlation between the test data and FE results were obtained. An unconventional design of T-joint for simpler manufacture process was proposed. Based on the design, T-joint samples were modeled, manufactured and tested to demonstrate the manufacture process and evaluate the improved strength.     

Effects of core machining configuration on the debonding toughness of foam core sandwich panels

Core machining is often applied to improve the formativeness of foam core and the manufacturing effectiveness of sandwich panels. This paper investigates the effects of core machining configuration on the interfacial debonding toughness of foam core sandwich panels fabricated by vacuum-assisted resin transfer molding process. Several machining configurations are conducted to foam core, and skin–core debonding toughness of fabricated sandwich panels is evaluated using double-cantilever-beam tests. The sandwich panels with core cuts exhibited higher apparent fracture toughness than the panels without core cut, specifically in the case of perforated core. The relationship between core machining configuration and measured fracture toughness is discussed based on the experimental observations and the numerical analyses of energy release rates.     

Kevlar短纤维增韧碳纤维/铝蜂窝夹芯板三点弯曲与面内压缩性能

介绍了碳纤维/铝蜂窝夹芯结构的Kevlar短纤维界面增韧方法。通过三点弯曲实验和面内压缩实验,对比增韧试件与未增韧试件的载荷位移曲线、破坏模式等特征,发现未增韧试件往往先发生界面分层破坏,继而面板和芯体分别发生局部破坏;而增韧试件通常发生整体破坏。实验数据显示,Kevlar短纤维界面增韧可以使碳纤维/铝蜂窝夹芯板的抗弯强度、压缩强度、能量吸收等力学性能分别至少提高14.06%、55.80%和61.53%。对破坏后界面的SEM观测发现:增韧试件并未发生界面脱粘,而是由于芯体撕裂造成面/芯剥离,揭示了Kevlar短纤维的界面增韧机制。对具有Kevlar短纤维界面增韧的碳纤维/铝蜂窝夹芯结构进行有限元建模,并分别对其在三点弯曲和面内压缩载荷下的力学行为进行数值分析,以指导该类夹芯结构的分析与设计。     

Development of a satellite structure with the sandwich T-joint

In this study, a monocoque satellite structure composed of many composite sandwich panels, which consist of two carbon fiber/epoxy composite faces and an aluminum honeycomb core, was designed to reduce structural mass and to improve static and dynamic structural rigidity. To join composite sandwich panels with T-shape joints, a new I-shape side insert, which was fixed inside the composite sandwich panel edge with film adhesive, was suggested. The composite sandwich panels were assembled with bolts using the through-the-thickness insert and the I-shape side insert. The flatwise tensile and compressive tests of the composite sandwich panels were performed with respect to the bonding pressure between the composite face and the aluminum honeycomb core to achieve an optimal bonding pressure. To investigate the joint characteristics of the composite faces and the I-shape side insert, cleavage peel tests were performed with respect to the bonding thickness. Also, a finite element model of the composite sandwich T-joint with the I-shape side insert was developed from experimental results of the impulse response tests and composite sandwich T-joint static tests. From the finite element analysis, the structural reliability of the monocoque composite sandwich satellite structure was verified.     

Experimental,Theoretical and Numerical Investigation of the Flexural Behaviour of the Composite Sandwich Panels with PVC Foam Core

This study presents the main results of an experimental, theoretical and numerical investigation on the flexural behaviour and failure mode of composite sandwich panels primarily developed for marine applications. The face sheets of the sandwich panels are made up of glass fibre reinforced polymer (GFRP), while polyvinylchloride (PVC) foam was used as core material. Four-point bending test was carried out to investigate the flexural behaviour of the sandwich panel under quasi static load. The finite element (FE) analysis taking into account the cohesive nature of the skin-core interaction as well as the geometry and materials nonlinearity was performed, while a classical beam theory was used to estimate the flexural response. Although the FE results accurately represented the initial and post yield flexural response, the theoretical one restricted to the initial response of the sandwich panel due to the linearity assumptions. Core shear failure associate with skin-core debonding close to the loading points was the dominant failure mode observed experimentally and validated numerically and theoretically.     

The mechanical behaviour of corrugated-core sandwich panels

A series of experimental investigations and numerical analyses is presented into the compression response, and subsequent failure modes in corrugated-core sandwich panels based on an aluminium alloy, a glass fibre reinforced plastic (GFRP) and a carbon fibre reinforced plastic (CFRP). The corrugated-cores were fabricated using a hot press moulding technique and then bonded to face sheets based on the same material, to produce a range of lightweight sandwich panels. The role of the number of unit cells and the thickness of the cell walls in determining the overall deformation and local collapse behaviour of the panels is investigated. The experiments also provide an insight into the post-failure response of the sandwich panels. The results are compared with the numerical predictions offered by a finite element analysis (FEA) as well as those associated with an analytical model. Buckling of the cell walls has been found to be initial failure mode in these corrugated systems. Continued loading resulted in fracture of the cell walls, localised delamination as well as debonding between the skins and the core. The predictions of the FEA generally show reasonably good agreement with the experimental measurements. Finally, the specific compressive properties of the corrugated structures have been compared to those of other core materials where evidence suggests that these systems compare favourably with their more conventional counterparts.     

Dynamic models for low-velocity impact damage of composite sandwich panels – Part B: Damage initiation

Equivalent single and multi degree-of-freedom systems are used to predict low-velocity impact damage of composite sandwich panels by rigid projectiles. The composite sandwich panels are symmetric and consist of orthotropic laminate facesheets and a core with constant crushing resistance. The transient deformation response of the sandwich panels subjected to impact were predicted in a previous paper, and analytical solutions for the impact force and velocity at damage initiation in sandwich panels are presented in this second paper. Several damage initiation modes are considered, including tensile and shear fracture of the top facesheet, core shear failure, and tensile failure of back facesheet. The impact failure modes are similar to static indentation failure modes, but inertial resistance and high strain rate material properties of the facesheets and core influence impact damage loads. Predicted damage initiation loads and impact velocities compare well with experimental results.     

液态扩散焊制备泡沫铝夹芯板及其疲劳行为

采用超声辅助液态扩散焊接的方法制备冶金复合泡沫铝夹芯板,利用光学显微镜（OM）和SEM观察冶金复合样品的界面组织和结构,发现连接界面发生了侵蚀作用,接头均匀连续;EDS线扫结果表明,连接界面处焊接合金（Zn-10Al）和铝基体间的元素扩散现象明显,表明在超声作用下,基体材料表面氧化膜被破坏,枝晶在界面附着生长,形成良好的冶金连接。将制备的冶金复合样品和胶黏泡沫铝夹芯板样品进行三点弯曲疲劳对比试验,结果显示,冶金复合样品和胶黏样品的疲劳极限分别达到3 058 N和2 829 N。在相同载荷下,冶金复合样品的疲劳寿命（S-N）远远长于胶黏样品。两种样品的疲劳破坏方式完全不同,胶黏样品表现为面板和芯层黏接面的脱黏剪切破坏,冶金复合样品的疲劳剪切破坏出现在泡沫铝芯层,没有出现面板脱离现象。     

复合材料泡沫夹芯板局部连接拉脱破坏试验与数值仿真

对复合材料泡沫夹芯板局部连接拉脱破坏进行了试验研究,分析了接头的破坏模式、失效载荷和面板对接头的影响。采用ABAQUS有限元软件进行了数值分析,通过与实验结果对比验证其模型的可靠性,预测分析内部的破坏模式以及结构参数对接头破坏的影响,研究了泡沫芯体内部的渐进破坏以及面板和泡沫芯体之间的胶层脱粘破坏。结果表明:泡沫夹芯板预埋螺栓连接结构灌封胶边缘的泡沫先产生裂纹后向中间扩展,中间区域全部开裂时两端裂纹沿着45°方向向上扩展。胶层开裂的区域呈弧形条状,分布在螺栓紧固件的两侧,在面板宽度方向,开裂的区域贯穿两侧。随着预埋件深度的增加最大破坏载荷也在增加,随着预埋件直径的增加亚临界破坏载荷和最大破坏载荷没有比较明显的变化,但最大破坏位移在减小。      

Flexural behaviour of structural fibre composite sandwich beams in flatwise and edgewise positions

The flexural behaviour of a new generation composite sandwich beams made up of glass fibre-reinforced polymer skins and modified phenolic core material was investigated. The composite sandwich beams were subjected to 4-point static bending test to determine their strength and failure mechanisms in the flatwise and the edgewise positions. The results of the experimental investigation showed that the composite sandwich beams tested in the edgewise position failed at a higher load with less deflection compared to specimens tested in the flatwise position. Under flexural loading, the composite sandwich beams in the edgewise position failed due to progressive failure of the skin while failure in the flatwise position is in a brittle manner due to either shear failure of the core or compressive failure of the skin followed by debonding between the skin and the core. The results of the analytical predictions and numerical simulations are in good agreement with the experimental results.     

Compression and bending performances of carbon fiber reinforced lattice-core sandwich composites

To restrict debonding, carbon fiber reinforced lattice-core sandwich composites with compliant skins were designed and manufactured. Compression behaviors of the lattice composites and sandwich columns with different skin thicknesses were tested. Bending performances of the sandwich panels were explored by three-point bending experiments. Two typical failure mechanisms of the lattice-core sandwich structures, delaminating and local buckling were revealed by the experiments. Failure criteria were suggested and gave consistent analytical predictions. For panels with stiff skins, delamination is the dominant failure style. Cell dimensions, fracture toughness of the adhesives and the strength of the sandwich skin decide the critical load capacity of the lattice-core sandwich structure. The mono-cell buckling and the succeeding local buckling are dominant for the sandwich structures with more compliant skin sheets. Debonding is restricted within one cell in bending and two cells in compression for lattice-core sandwich panels with compliant face sheets and softer lattice cores.     

Mechanical behavior of sandwich panels with hollow Al–Si tubes core construction

A new type of lightweight sandwich panels consisting of vertically aligned hollow Al–Si alloy tubes as core construction and carbon fiber composite face sheets was designed. The hollow Al–Si alloy tubes were fabricated using precision casting and were bonded to the face sheets using an epoxy adhesive. The out-of-plane compression (i.e. core crushing), in-plane compression, and three-point bending response of the panels were tested until failure. The hollow Ai–Si alloy tubes core configuration show superior specific strength under crushing compared to common metallic and stochastic foam cores. Under in-plane compression and three-point bending, the buckling of face sheets and debonding of hollow cores from the face sheets were observed. Simple analytical relationships based on the concepts of mechanics of materials were provided for the compression tests, which estimate the sandwich panels’ strength with high fidelity. For three-point bending, detailed finite element analysis was used to model the response and initial failure of the sandwich panels.     

复合材料夹层结构自插式平接节点抗弯性能试验研究

采用复合材料连接型材的复合材料夹层结构连接节点具有轻质、防腐、无磁性等优点。该文试验研究复合材料夹层结构自插式平接胶结节点的受弯性能,重点考察连接型材的纤维铺设、节点搭接长度等因素对连接节点荷载-变形关系、应变发展规律和破坏模式的影响。试验结果表明,自插式平接节点的破坏形式主要表现为三类:节点连接区域的纤维面层与芯材的剥离破坏、搭接胶层剥离破坏、插入端变截面处纤维面层与芯材剥离破坏;连接型材纤维布的铺设方法与层数、胶结搭接长度以及节点区域的构造措施是影响节点承载力的主要因素。论文对平接节点的破坏机理进行了分析,研究成果对复合材料夹层板结构平接胶结节点的设计具有参考价值。     

复合材料双向波纹夹层结构力学性能

为改进传统单向波纹夹层结构横向力学性能较差的缺点,设计了一种新型复合材料双向波纹夹层结构。考虑复合材料双向夹层结构制备困难,研究了整套真空辅助成型工艺(VARI)工艺制备方案,实现双向波纹夹层结构的高效制备,以满足工程应用的需要。对制备出的复合材料双向波纹夹层结构与单向波纹夹层结构分别进行面外压缩、弯曲和剪切实验,分析了双向波纹夹层结构在不同载荷下的破坏模式及其失效机制,计算了该结构在不同荷载条件下的强度和模量,并将其与单向波纹夹层结构进行对比分析。结果表明,在压缩荷载作用下,玻璃纤维/环氧树脂芯子为主要承载部分,结构的失效主要体现在芯子的屈曲、断裂和分层;在弯曲荷载的作用下,由于纤维的抗压强度远小于抗拉强度,所以压头下方的上面板最先达到破坏荷载,结构的弯曲失效形式主要为上面板的断裂和脱粘;结构的剪切失效主要以泡沫与面板的脱粘和压溃为主,芯子和面板未见明显的破坏现象;与单向波纹夹层结构相比,双向波纹夹层结构力学性能显著提升。      

Suppression of initial failure at tapered end-closure sandwich panel joint by taper angle change

To examine the configuration of CFRP face plates/foamed plastic core sandwich panel joints, a tapered end-closure-type joint is selected and studied. In this type of joint, the sandwich panel is tapered to form a solid laminate comprising two face plates near the joint, and the two panels to be joined are mechanically fastened at the solid laminate with a splice plate. This type of joint may be suitable for aircraft panels because a flat surface can be obtained at the joint, which is advantageous from an aerodynamic viewpoint. However, in a previous study on such a joint, it was found that a delamination crack initiated from the tapered core end and propagated through the interface between the two face plates as an initial failure mode at a much lower tensile load than the final failure load in a tensile strength test. In this study, the angle of the tapered panel was focused on and the effect of changing the taper angle on suppressing the initial failure was investigated through experiments and numerical analysis. It was found that a smaller taper angle is more effective for suppressing the initial failure.     

The flatwise compressive properties of Nomex honeycomb core with debonding imperfections in the double cell wall

Mechanical properties of Nomex honeycomb core are governed by not only its global dimensions, cell topology, material properties and proportion of the aramid paper and phenolic resin, but also possible manufacturing imperfections, such as the debonding between the two aramid paper sheets in the double cell wall. To account for the layered feature of the cell walls and the bonding conditions between aramid paper sheets, a three-dimensional unit cell model was proposed and developed in this study. The aramid paper sheets, the phenolic resin coating, the adhesive between the aramid paper sheets, and their bonding relationships were all explicitly modelled in accordance with their actual geometry and material parameters. The model was validated by comparing the predicted load-displacement curves and failure modes with the test results. The effects of representative bonding imperfections on both the collapse load and the related displacement of the honeycomb core under flatwise compression were evaluated. Through the analyses, it was found that the debonding imperfections have significant effects on the mechanical behaviour of the honeycomb core and that with the same debonding area the debonding at the outside edge of the adhesive printing line is the most critical. It was also found that debonding fracture may occur if adhesive is not strong enough or the debonding imperfection area is large.     

An experiment study on the failure mechanisms of woven textile sandwich panels under quasi-static loading

Quasi-static compression and three-point-bending tests were conducted to reveal the failure mechanisms and the energy absorption capacity of the woven textile sandwich material. The compression induces shear deformation due to the tilting of fiber piles within the core. The ductile load–displacement curves are featured by a long deformation plateau by plastic rotations of core piles. Densifications become apparent in the later stage of compression. In three-point-bending, skin crippling and shear failure dominate the load capacity of the thicker panels, while skin fracture dominates the thinner ones. After the initial failure, the progression of plastic hinges renders the panels residual load capacity in a long deflection plateau. The tests suggest that woven textile sandwich material is ideal to serve as an energy absorbing core.     

Mechanical behavior of carbon fiber reinforced polymer composite sandwich panels with 2-D lattice truss cores

Composite sandwich structures with lattice truss cores are attracting more and more attention due to their superior specific strength/stiffness and multi-functional applications. In the present study, the carbon fiber reinforced polymer (CFRP) composite sandwich panels with 2-D lattice truss core are manufactured based on the hot-pressing method using unidirectional carbon/epoxy prepregs. The facesheets are interconnected with lattice truss members by means of that both ends of the lattice truss members are embedded into the facesheets, without the bonding procedure commonly adopted by sandwich panels. The mechanical properties of the 2-D lattice truss sandwich panels are investigated under out-of-plane compression, shear and three-point bending tests. Delamination of the facesheets is observed in shear and bending tests while node failure mode does not occur. The tests demonstrate that delamination of the facesheet is the primary failure mode of this sandwich structure other than the debonding between the facesheets and core for conventional sandwiches.     

含面芯界面缺陷的蜂窝夹芯板侧向压缩破坏模式

为了对含面芯层间脱胶缺陷的蜂窝夹芯板在侧向压缩载荷作用下的典型破坏模式进行数值预报, 建立了基于蔡-希尔破坏准则和粘结模型的计算模型。该计算模型是建立在对蜂窝夹芯板的双悬臂梁(DCB)和单臂梁(SLB) 试验中所发现的一种新的破坏模式的分析基础之上的。对蜂窝夹芯板的侧向压缩破坏行为的数值预报中, 发现一种新的破坏模式: 位于脱胶区域的面板首先发生局部屈曲失稳, 随后面板内部靠近芯子的45°/0°层间出现分层, 与此同时最靠近芯子的45°铺层发生断裂, 伴随着45°/0°层间分层的扩展, 面板发展成为对称性整体屈曲失稳。与侧向压缩试验测试结果对比发现, 计算模型模拟中所预报的破坏模式在实验测试中也得到了很好的验证。