Fabrication and mechanical testing of glass fiber entangled sandwich beams: A comparison with honeycomb and foam sandwich beams

The aim of this paper is the fabrication and mechanical testing of entangled sandwich beam specimens and the comparison of their results with standard sandwich specimens with honeycomb and foam as core materials. The entangled sandwich specimens have glass fiber cores and glass woven fabric as skin materials. The tested glass fiber entangled sandwich beams possess low compressive and shear modulus as compared to honeycomb and foam sandwich beams of the same specifications. Although the entangled sandwich beams are heavier than the honeycomb and foam sandwich beams, the vibration tests show that the entangled sandwich beams possess higher damping ratios and low vibratory levels as compared to honeycomb and foam sandwich beams, making them suitable for vibro-acoustic applications where structural strength is of secondary importance, e.g., internal paneling of a helicopter.     

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.     

缝纫泡沫夹芯复合材料细观纤维柱破坏行为

缝纫泡沫夹芯复合材料中的纤维柱在拔出过程中的破坏行为复杂导致结构承载性能难以预测。采用真空辅助树脂注射(VARI)工艺制备了缝纫泡沫夹芯复合材料,并使用层间拉伸试验(ITT)研究了缝纫泡沫夹芯复合材料中含有单根缝线纤维柱细观试件的破坏过程。讨论了不同破坏现象对缝线纤维柱拔出摩擦过程的影响,并分析了缝纫泡沫夹芯复合材料的破坏模式。分析了缝线粗细的变化对试件破坏过程中关键的力、位移等参数及能量吸收性能的影响。研究了由于成型工艺所导致的缺胶现象对缝纫泡沫夹芯复合材料性能的影响。结果表明:缝纫泡沫夹芯复合材料的能量吸收性能、关键位移参数及最大载荷都随着缝线变粗而增大。但是缝纫泡沫夹芯复合材料的破坏模式对其也有一定的影响,导致了变化趋势的波动;缺胶缝纫泡沫夹芯复合材料由于缺陷的存在,最大破坏载荷和能量吸收性能均有所下降。     

Development of rubberized syntactic foam

This study explored a novel hybrid syntactic foam for composite sandwich structures. A unique microstructure was designed and realized. The hybrid foam was fabricated by dispersing styrene–butadiene rubber latex coated glass microballoons into a nanoclay and milled glass fiber reinforced epoxy matrix. The manufacturing process for developing this unique microstructure was developed. A total of seven groups of beam specimens with varying compositions were prepared. Each group contained 12 identical specimens with dimensions 304.8 mm × 50.8 mm × 15.2 mm. The total number of specimens was 84. Among them, 42 beams were pure foam core specimens and the remaining 42 beams were sandwich specimens with each foam core wrapped by two layers of E-glass plain woven fabric reinforced epoxy skin. Both low velocity impact tests and four-point bending tests were conducted on the foam cores and sandwich beams. Compared with the control specimens, the test results showed that the rubberized syntactic foams were able to absorb a considerably higher amount of impact energy with an insignificant sacrifice in strength. This multi-phase material contained structures bridging over several length-scales. SEM pictures showed that several mechanisms were activated to collaboratively absorb impact energy, including microballoon crushing, interfacial debonding, matrix microcracking, and fiber pull-out; the rubber layer and the microfibers prevented the microcracks from propagating into macroscopic damage by means of rubber pinning and fiber bridge-over mechanisms. The micro-length scale damage insured that the sandwich beams retained the majority of their strength after the impact.     

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。      

短切纤维增韧泡沫夹芯复合材料梁界面断裂过程的物质点方法模拟

对短切纤维增韧泡沫夹芯复合材料梁界面韧性试验结果进行了分析讨论并基于物质点方法(material pointmethod, MPM)对试验过程进行了数值模拟。在MPM方法中,通过可视准则引入不连续性来处理裂纹问题,发展了包含裂纹的MPM 算法,模拟了泡沫夹芯复合材料梁界面断裂试验过程,数值分析结果和试验数据取得了良好的一致性。研究结果表明短切纤维增韧工艺能够显著提高泡沫夹芯复合材料结构的界面韧性和承载能力,同时表明该文推导的包含裂纹的MPM方法处理断裂问题的精确性和有效性。     

芳纶纤维增韧碳纤维泡沫金属夹芯梁压缩性能及界面性能

为研究芳纶短纤维对复合材料夹芯材料/结构的界面及性能的影响,对具有芳纶短纤维增韧界面的碳纤维-泡沫铝夹芯梁进行了试验和细观增韧机制研究.在夹芯梁制备过程中,在碳纤维-泡沫铝界面加入低密度芳纶短纤维薄膜,通过短纤维的桥联作用,提高夹芯梁的界面黏接性能.研究了芳纶纤维增韧对夹芯梁面内压缩性能和破坏模态的影响,采用非对称双悬臂梁(ADCB)试验测量了不同增韧参数条件下,碳纤维表板与泡沫铝芯体之间的临界能量释放率.试验结果显示:在相同增韧参数条件下,Kevlar纤维增韧夹芯梁的面内压缩性能和界面临界能量释放率均较好,而混杂长度Kevlar纤维的界面增韧效果最优.通过对试件断面的SEM观测,分析了芳纶纤维增韧的细观增韧机制.     

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.     

Strength of sandwich beams with interface debondings

The adhesive joint bonding the faces to the core material in a sandwich structure ensures that the loads are transferred between the components. However, debondings may arise either during the manufacturing process or due to overloading. These will reduce both the stiffness and the load bearing capacity of the structure. In the present paper, debondings in foam core sandwich beams are investigated assuming that cracks in the interface between the face and core are present. Stress intensity factors are found from an analytical model and compared to solutions from several finite element calculations. Fracture toughness values, determined from simple specimens, are used to predict the fracture loads for beams with simulated debondings subjected to four-point bending.     

加筋增强泡沫夹芯结构面内压缩与界面断裂性能试验

为解决复合材料泡沫夹芯结构面板局部屈曲与面芯脱粘的突出问题,提出了一种由筋条增强的玻璃纤维增强树脂基复合材料（GFRP）面板与泡沫芯层组合而成的新型夹芯结构。采用真空辅助树脂导入技术制备试验件,通过面内压缩与双悬臂梁试验,对比分析了加筋增强夹芯板与未加筋夹芯板的受力特性、失效模式和面芯粘结性能。面内压缩试验显示,与未加筋夹芯板相比,加筋增强夹芯板的失效模式由面板局部屈曲转化为面板压缩剪切破坏或整体屈曲,在GFRP材料使用量相同的情况下,试件长度为130 mm的加筋增强夹芯板平均失效荷载提高了40.87%,长度为190 mm试件提高了35.63%。双悬臂梁试验显示,加筋增强夹芯板的裂缝在发展过程中受到筋条与面板之间纤维丝搭接约束,改善了界面粘结性能,与未加筋夹芯板相比,其平均能量释放率提高了57.35%。     

Static indentation and unloading response of sandwich beams

This paper deals with analysis of foam core sandwich beams subject to static indentation and subsequent unloading (removal of load). Sandwich beams are assumed continuously supported by a rigid platen to eliminate global bending. An analytical model is presented assuming an elastic-perfectly plastic compressive behaviour of the foam core. An elastic part of indentation response is described using the Winkler foundation model. Upon removal of the load, an elastic unloading response of the foam core is assumed. Also, finite element (FE) analysis of static indentation and unloading of sandwich beams is performed using the FE code ABAQUS. The foam core is modelled using the crushable foam material model. To obtain input data for the analytical model and to calibrate the crushable foam model in FE analysis, the response of the foam core is experimentally characterized in uniaxial compression, up to densification, with subsequent unloading and tension until tensile fracture. Both models can predict load–displacement response of sandwich beams under static indentation and a residual dent magnitude in the face sheet after unloading along with residual strain levels in the foam core at the unloaded equilibrium state. The analytical and FE analyses are experimentally verified through static indentation tests of composite sandwich beams with two different foam cores. The load–displacement response, size of a crushed core zone and the depth of a residual dent are measured in the testing. A digital speckle photography technique is also used in the indentation tests in order to measure the strain levels in the crushed core zone. The experimental results are in good agreement with the analytical and FE analyses.     

泡沫铝夹层结构复合材料低速冲击性能

以泡沫铝为夹芯材料,玄武岩纤维(BF)和超高分子量聚乙烯纤维(UHMWPE)复合材料为面板,制备夹层结构复合材料。研究纤维类型、铺层结构和芯材厚度对泡沫铝夹层结构复合材料冲击性能和损伤模式的影响规律,并与铝蜂窝夹层结构复合材料性能进行对比分析。结果表明:BF/泡沫铝夹层结构比UHMWPE/泡沫铝夹层结构具有更大的冲击破坏载荷,但冲击位移和吸收能量较小。BF和UHMWPE两种纤维的分层混杂设计比叠加混杂具有更高的冲击破坏载荷和吸收能量。随着泡沫铝厚度的增加,夹层结构复合材料的冲击破坏载荷降低,破坏吸收能量增大。泡沫铝夹层结构比铝蜂窝夹层结构具有更高的冲击破坏载荷,但冲击破坏吸收能量较小;泡沫铝芯材以冲击部位的碎裂为主要失效形式,铝蜂窝芯材整体压缩破坏明显。     

具有短纤维增韧界面的复合材料夹芯梁断裂机制的实验和数值研究

对界面含短纤维的复合材料夹芯梁增韧特性进行了实验和数值研究。采用真空辅助树脂注射工艺制成复合材料夹芯梁, 并测定了含增强与未增强复合材料夹芯梁的界面断裂韧性。基于单根短纤维在界面裂纹扩展时其剥离和拔出过程产生的能量耗散, 建立了相应的细观模型。然后, 假设短纤维在界面内随机均匀分布, 得到在单位面积下短纤维能量耗散的宏观表达式。在宏观尺度上, 建立了用于界面裂纹扩展分析的双悬臂梁有限元模型, 通过引入非线性弹簧单元, 以反映界面短纤维所产生的桥联特征, 并采用虚裂纹闭合技术计算了裂纹尖端的能量释放率。通过典型复合材料夹芯梁断裂分析和参数讨论, 证实了本文中提出的随机分布细观模型预测短纤维耗散能量的有效性。实验及数值结果表明, 在复合材料夹芯结构界面中引入短纤维将是一种提高其界面断裂韧性的有效措施。      

Characterisation of a metallic foam–cement composite under selected loading conditions

An open-cell metallic foam was employed as an analogue material for human trabecular bone to interface with polymethyl methacrylate (PMMA) bone cement to produce composite foam–cement interface specimens. The stress-displacement curves of the specimens were obtained experimentally under tension, shear, mixed tension and shear (mixed-mode), and step-wise compression loadings. In addition, under step-wise compression, an image-guided failure assessment (IGFA) was used to monitor the evolution of micro-damage of the interface. Microcomputed tomography (µCT) images were used to build a subject-specific model, which was then used to perform finite element (FE) analysis under tension, shear and compression. For tension–shear loading conditions, the strengths of the interface specimens were found to increase with the increase of the loading angle reaching the maximum under shear loading condition, and the results compare reasonably well with those from bone–cement interface. Under compression, however, the mechanical strength measured from the foam–cement interface is much lower than that from bone–cement interface. Furthermore, load transfer between the foam and the cement appears to be poor under both tension and compression, hence the use of the foam should be discouraged as a bone analogue material for cement fixation studies in joint replacements.     

Shear fracture tests of concrete

Symmetrically notched beam specimens of concrete and mortar, loaded near the notches by concentrated forces that produce a concentrated shear force zone, are tested to failure. The cracks do not propagate from the notches in the direction normal to the maximum principal stress but in a direction in which shear stresses dominate. Thus, the failure is due essentially to shear fracture (Mode II). The crack propagation direction seems to be governed by maximum energy release rate. Tests of geometrically similar specimens yield maximum loads which agree with the recently established size effect law for blunt fracture, previously verified for tensile fracture (Mode I). This further implies that the energy required for crack growth increases with the crack extension from the notch. The R-curve that describes this increase is determined from the size effect. The size effect also yields the shear fracture energy, which is found to be about 25-times larger than that for Mode I and to agree with the value predicted by the crack band model. The fracture specimen is simple to use but not perfect for shear fracture because the deformation has a symmetric component with a non zero normal stress across the crack plane. Nevertheless, these disturbing effects appear to be unimportant. The results are of interest for certain types of structures subjected to blast, impact, earthquake, and concentrated loads.     

GFRP复合材料-轻木夹芯梁弯曲疲劳性能试验

为研究真空导入成型的玻璃纤维增强树脂基复合材料-Balsa轻木（GFRP-Balsa）夹芯梁弯曲疲劳性能,进行了普通无格构、单格构增强、双格构增强三种类型共42根试件在不同荷载等级下的四点弯曲疲劳试验,得到夹芯梁的弯曲疲劳破坏模式、疲劳寿命和损伤演化规律,分析了三种类型夹芯梁在弯曲疲劳载荷下不同的损伤机制。研究结果发现,无格构夹芯梁的失效模式统一为芯材剪切和面板脱粘,格构增强夹芯梁的失效模式随格构设置及载荷等级变化,主要有上面板屈曲或压坏、下面板拉断等;采用指数经验模型拟合夹芯梁的疲劳荷载-寿命（S-N）曲线,得到三种类型夹芯梁的寿命预测公式;夹芯梁的位移演化历经"位移瞬降-平稳演化-损伤萌生至破坏"三个阶段,相对于无格构试件,格构增强试件在疲劳失效前有较明显预兆。     

Debonding in foam-core sandwich panels

The strain energy release rate is used to give a criterion for debonding in structural sandwich beams with isotropic faces and a foam core. The critical strain energy release rate of the interface is measured on double-shear specimens and the results of the debonding analysis are compared with experiments on sandwich beams with aluminium faces and foamed polyurethane cores. The analysis describes debonding failure well. Comparison of the load for bebonding with that for other failure modes shows that debonding occurs only if relatively large cracks exist at the interface between the face and the core.     

Debonding and crack kinking in foam core sandwich beams—II. Experimental investigation

Debonding and crack kinking in sandwich beams was experimentally examined, and also analyzed using the finite element method. Double cantilever beam (DCB) and shear fracture specimens employing aluminum facings bonded to a wide range of PVC and PMI foam cores using two types of adhesives were considered. It was found that the Young modulus of the core has a profound effect on the tendency of the facing/core interfacial crack to deflect (kink) into the core in DCB testing. In shear testing, crack kinking occurred for all core materials considered. The type of adhesive strongly influences the debond fracture resistance, but not the kink resistance and kink angle. The critical load for onset of kinking increased with increased core density. Finite element analysis of the fracture specimens enabled determination of mixed mode interfacial fracture toughness for the specimens that failed by debonding. For specimens that failed by kinking, interfacial stress intensity factors at the onset of kinking were determined. Measured kink angles compared favorably with kink angles calculated based on the interfacial stress intensity factors prior to kinking.     

Effect of sea environment on interfacial delamination behavior of polymeric sandwich structures

Sandwich structures are utilized in naval craft and thereby are exposed to sea water environment and temperature fluctuations over extended periods. The sandwich layup consists of a closed cell polymeric foam layer placed between thin carbon or glass fiber reinforced polymeric composite facings. Attention in this paper is focused on sea water effects on the interfacial mechanical response between foam and facing due to sustained sea water exposure using carefully controlled laboratory conditions. Pre-cracked sandwich composite samples are soaked in sea water for extended periods and interfacial fracture behavior compared against dry specimens. Results indicate that the delamination crack propagates close to the interface in the wet case, while it stays within the foam in the dry case. Significant reduction in fracture toughness due to sea water exposure is observed and needs to be considered in the design of ship structures. The effect of sea water on values of energy release rate are determined experimentally and predicted using the J-integral concept. A good agreement between data and predictions is achieved, indicating a reduction in fracture toughness by 30% due to sustained exposure to sea water.     

Face/core interface fracture characterization of mixed mode bending sandwich specimens

Debonding of the core from the face sheets is a critical failure mode in sandwich structures. This paper presents an experimental study on face/core debond fracture of foam core sandwich specimens under a wide range of mixed mode loading conditions. Sandwich beams with E‐glass fibre face sheets and PVC H45, H100 and H250 foam core materials were evaluated. A methodology to perform precracking on fracture specimens in order to achieve a sharp and representative crack front is outlined. The mixed mode loading was controlled in the mixed mode bending (MMB) test rig by changing the loading application point (lever arm distance). Finite element analysis was performed to determine the mode‐mixity at the crack tip. The results showed that the face/core interface fracture toughness increased with increased mode II loading. Post failure analysis of the fractured specimens revealed that the crack path depends on the mode‐mixity at the crack tip, face sheet properties and core density.