Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies

The analysis of stiffness and the identification of rupture mechanisms during and after static tests of sandwich panels and their components have been investigated. The sandwich panels, having cross-ply laminates skins made of glass fibre and epoxy resin were manufactured by vacuum moulding and subjected to three-point bending tests. Two polyvinyl chloride cores of similar type but with differing densities were investigated. The effect of core density and its thickness on the behavior and the damage was highlighted. In terms of stiffness and load at failure, the sandwich structure has better mechanical characteristics compared to its components. __________ Translated from Problemy Prochnosti, No. 2, pp. 88–98, March–April, 2007.     

On the impact response of sandwich composites with cores of balsa wood and PVC foam

This paper presents an experimental investigation on impact response of sandwich composite panels with PVC foam core and balsa wood core. A number of tests were performed under various impact energies. Damage process of the sandwich composites is analyzed from cross-examining load–deflection curves, energy profile diagrams and the damaged specimens. The primary damage modes observed are; fiber fractures at upper and lower skins, delaminations between adjacent glass–epoxy layers, core shear fractures, and face/core debonding. After visual inspection of the top and bottom face-sheets, initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning by an abrasive water-jet machine, of samples. In addition to the single impacts, repeated impact response of the samples is also investigated.     

Fatigue crack growth and life prediction of foam core sandwich composites under flexural loading

Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated. Sandwich panels were manufactured using an innovative co-injection resin transfer molding process. S2-glass fiber with epoxy resins was used as face sheets over a PVC foam core. Testing was performed in a three-point flexure mode utilizing a newly designed fixture such that the localized indentation damage was minimal. Extensive fatigue data were generated for the S–N diagram and crack growth was monitored to develop a model for life prediction. The first visible sign of damage initiation was a core–skin debond parallel to the beam axis. This debond propagated slowly along the top interface and eventually kinked into the core as shear crack and then grew in an unstable manner resulting in total specimen collapse. A fatigue model based on this crack growth has been developed and validated with experiments.     

Compression and impact testing of two-layer composite pyramidal-core sandwich panels

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.     

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。      

The influence of core height and face plate thickness on the response of honeycomb sandwich panels subjected to blast loading

This paper reports on an investigation into the behaviour of circular sandwich panels with aluminium honeycomb cores subjected to air blast loading. Explosive tests were performed on sandwich panels consisting of mild steel face plates and aluminium honeycomb cores. The loading was generated by detonating plastic explosives at a pre-determined stand-off distance. Core height and face plate thickness were varied and the results are compared with previous experiments. It was observed that the panels exhibited permanent face plate deflection and tearing, and the honeycomb core exhibited crushing and densification. It was found that increasing the core thickness delayed the onset of core densification and decreased back plate deflection. Increasing the plate thickness was also found to decrease back plate deflection, although the panels then had a substantially higher overall mass.     

Static and fatigue bending behavior of pultruded GFRP sandwich panels with through-thickness fiber insertions

This paper presents the findings of a research program that was undertaken to evaluate the static and fatigue characteristics of an innovative 3-D glass fiber reinforced polymer (GFRP) sandwich panel proposed for civil infrastructure and transportation applications. The research consists of analytical modeling verified by experimental results. A rational analytical model is presented and used to evaluate the effective elastic modulus, shear modulus and degree of composite interaction of the panels to resist one-way bending. The experimental program was conducted in two phases to study the static and fatigue behavior of the panels. In the first phase a total of 730 sandwich beams were tested to evaluate the effect of different parameters on the fundamental behavior of the panel. The parameters considered include the pattern and density of through-thickness fiber insertions, the overall thickness of the panels, and the number of FRP plies in the face skins. The study indicates that the shear behavior and degree of composite interaction of the panels is sensitive to the configuration of the panel core. The second phase of the experimental program included testing of 24 additional sandwich panels to evaluate the fatigue behavior. The results of the experimental program indicate that the panels with stiffer cores generally exhibited a higher degree of degradation than panels with more flexible cores. The findings of this study indicate that the proposed panels represent a versatile construction system which can be configured to achieve the specific design demands for civil engineering infrastructure applications.     

Flexural fatigue characteristics of sandwich structures at different loading frequencies

The effects of frequency on the fatigue behavior of S2 glass fiber–vinylester reinforced sandwich composites with two different PVC cores have been investigated. Flexural fatigue tests were performed on sandwich beams with core densities of 130 and 260 kg/m³ at frequencies of 3 and 15 Hz, at a stress ratio, R=0.1 and at four different load levels viz. 90%, 85%, 80% and 75% of the ultimate load. S–N diagrams were generated and it was observed that the fatigue strength increased with core density, and the number of cycles to failure, N_f, increased with increase in frequency. In all cases failure was dominated by a primary shear crack in the core however, the crack path and crack propagation rates varied with frequency. The fatigue crack growth rate (FCGR) in the core of the H130 sandwich beams was subsequently investigated and the relationship between the crack growth rate, da/dN, and the cyclic stress intensity range, ΔK, was determined. It was found that crack growth rate decreased with increase in loading frequency.     

Static and Fatigue Behaviour of Hexagonal Honeycomb Cores under In-plane Shear Loads

Due to their high specific strength and high specific stiffness properties the use of honeycomb panels is particularly attractive in spacecraft structures. However, the harsh environment produced during the launch of a satellite can subject the honeycomb cores of these sandwich structures to severe quasi-static and dynamic loads, potentially leading to static or early fatigue failures. Knowledge of the static and fatigue behavior of these honeycomb cores is thus a key requirement when considering their use in spacecraft structural applications. This paper presents the findings of an experimental test campaign carried out to investigate the static and fatigue behaviors of aluminum hexagonal honeycomb cores subject to in-plane shear loads. The investigation involved carrying out both static and fatigue tests using the single block shear test method. These results are also discussed in relation to the observed damage and failure modes which have been reported for the statically tested specimens and for the fatigue tested specimens at various stages of fatigue life. As well as conducting tests for the more conventional principal cell orientations (L and W), results are also presented for tests carried out at intermediate orientations to investigate the variation of core shear strength with loading orientation. The results are further investigated using explicit non-linear finite element analysis to model the buckling failure mechanisms of the tested cores.     

夹层结构复合材料低速冲击试验与分析

设计了聚甲基丙烯酰亚胺(PMI)泡沫、 交联聚氯乙烯(X-PVC)泡沫、 NOMEX蜂窝、 缝合PMI以及开槽PMI泡沫等形式的玻璃布面板夹层结构复合材料, 研究了芯材种类和厚度、 面板玻璃布层数以及缝合和开槽等因素对夹层结构低速冲击性能的影响。结果表明, PMI泡沫芯较X-PVC泡沫芯和NOMEX蜂窝芯具有更高的冲击破坏载荷和吸收能量。随着泡沫密度及面板厚度的增加, 夹层结构复合材料的冲击破坏载荷和破坏吸收能量增大。合理的缝合和开槽, 能够增加PMI泡沫夹层结构的强度、 刚度及界面性能, 提高冲击承载能力。     

复合材料夹芯板低速冲击后弯曲及横向静压特性

对低速冲击后的复合材料Nomex 蜂窝夹芯板进行了纯弯曲和准静态横向压缩实验, 用X 光技术、热揭层技术和外观检测等对板内的损伤进行测量, 分析了被冲击面在受压情况下蜂窝夹芯板的弯曲破坏特点, 对比了横向静压与低速冲击所造成的板内损伤, 讨论了不同横向压缩速度时接触力P-压入位移$h 的变化规律和损伤情况。结果表明: 低速冲击可使蜂窝夹芯板的弯曲强度大幅度降低; Nomex 蜂窝夹芯板对低速冲击不敏感。      

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.     

Impact and post impact behavior of composite sandwich panels

Assessing the residual mechanical properties of a sandwich structure is an important part of any impact study and determines how the structure can withstand post impact loading. The damage tolerance of a composite sandwich structure composed of woven carbon/epoxy facesheets and a PVC foam core was investigated. Sandwich panels were impacted with a falling mass from increasing heights until damage was induced. Impact damage consisted of delamination and permanent indentation in the impacted facesheets. The Compression After Impact (CAI) strength of sandwich columns sectioned from these panels was then compared with the strength of an undamaged column. Although not visually apparent, the facesheet delamination damage was found to be quite detrimental to the load bearing capacity of the sandwich panel, underscoring the need for reliable damage detection techniques for composite sandwich structures.     

Insight into the shear behaviour of composite sandwich panels with foam core

While sandwich construction offers well-known advantages for high stiffness with light weight, the problem of designing the sandwich structure to withstand shear loading remains an important problem. This problem is more difficult with lower stiffness foam cores under high shear loading because the core is typically the weakest component of the structure and is the first one to fail in shear under the assuming of perfect contact between the skin and the foam core. In the present study, the shear response of the composite sandwich panels with Polyvinylchloride (PVC) foam core was investigated. The PVC H100 foam core is sandwiched between Glass Fiber Reinforced Polymer (GFRP) skins using epoxy resin to build a high performance sandwich panel to be investigated. Experiments have been carried out to characterise the mechanical response of the constituent materials under tension, compression and shear loading. Static shear tests for the sandwich panel reveal that the main failure mode is the delamination between the skin and the core rather than shearing the core itself due to the considerable value of the shear strength of the PVC foam. The Finite Element Analysis (FEA) of the sandwich structure shows that shear response and failure mode can be predicted, but that accurate predictions require a consideration of the non-linear response of the foam core. The results have a direct application in predicting the ability of the sandwich structure to withstand the shear loading.     

Response of metallic pyramidal lattice core sandwich panels to high intensity impulsive loading in air

Small scale explosive loading of sandwich panels with low relative density pyramidal lattice cores has been used to study the large scale bending and fracture response of a model sandwich panel system in which the core has little stretch resistance. The panels were made from a ductile stainless steel and the practical consequence of reducing the sandwich panel face sheet thickness to induce a recently predicted beneficial fluid-structure interaction (FSI) effect was investigated. The panel responses are compared to those of monolithic solid plates of equivalent areal density. The impulse imparted to the panels was varied from 1.5 to 7.6 kPa s by changing the standoff distance between the center of a spherical explosive charge and the front face of the panels. A decoupled finite element model has been used to computationally investigate the dynamic response of the panels. It predicts panel deformations well and is used to identify the deformation time sequence and the face sheet and core failure mechanisms. The study shows that efforts to use thin face sheets to exploit FSI benefits are constrained by dynamic fracture of the front face and that this failure mode is in part a consequence of the high strength of the inertially stabilized trusses. Even though the pyramidal lattice core offers little in-plane stretch resistance, and the FSI effect is negligible during loading by air, the sandwich panels are found to suffer slightly smaller back face deflections and transmit smaller vertical component forces to the supports compared to equivalent monolithic plates.     

An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners

An experimental investigation has been carried out to estimate the static and fatigue behaviour of specimens made up of steel plates and sandwich composite panels joined together by either blind or mechanical lock fasteners.A preliminary study was carried out in order to analyse the drilling operation of sandwich panels to determine the best values of parameters to use for fastener installation.A first set of pull-out and shear static tests was performed in 1992, using sandwich panels composed of a nomex honeycomb core between two laminates of glass/graphite/kevlar fibres in epoxy matrix.The investigation was completed in 1998. It consisted of performing a set of pull-out and shear fatigue tests on joints with blind fasteners, and of performing a new set of static tests on identical specimens with the same loading conditions as in 1992 so as to evaluate the possible ageing effect on mechanical proprieties of sandwich panels tested.     

Finite element analysis of sandwich panels with stepwise graded aluminum honeycomb cores under blast loading

This paper presents details and brief results of an experimental investigation on the response of metallic sandwich panels with stepwise graded aluminum honeycomb cores under blast loading. Based on the experiments, corresponding finite element simulations have been undertaken using the LS-DYNA software. It is observed that the core compression stage was coupled with the fluid–structure interaction stage, and the compression of the core layer decreased from the central to the peripheral zone. The blast resistance capability of sandwich panels was moderately sensitive to the core relative density and graded distribution. For the graded panels with relative density descending core arrangement, the core plastic energy dissipation and the transmitted force attenuation were larger than that of the ungraded ones under the same loading condition. The graded sandwich panels, especially for relative density descending core arrangement, would display a better blast resistance than the ungraded ones at a specific loading region.     

Mechanical behavior of the sandwich structures with carbon fiber-reinforced pyramidal lattice truss core

Sandwich panel construction with carbon fiber-reinforced pyramidal lattice truss is attracting more and more attention due to its superior mechanical properties and multi-functional applications. Pyramidal lattice truss sandwich panels made from carbon fiber reinforced composites materials are manufactured by hot-pressing. The facesheets are interconnected with truss cores, the facesheets and truss cores are manufactured in one manufacturing process without bonding. The buckling and splitting of truss member is observed in the compressive and shear tests and no nodal failure is observed. The predicted results show that the mechanical behavior of the pyramidal lattice truss core sandwich panels depends on the relative density of core and the material properties of truss members.     

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。