缝纫泡沫夹芯复合材料板的稳定性分析

整体屈曲是缝纫复合材料夹芯板的一种重要失效模式。考虑到缝纫夹芯复合材料板一般较厚且面板与芯层厚度相差较大, 缝纫工艺对夹芯板刚度影响较大的特点, 基于高阶剪切理论, 编制了缝纫泡沫夹芯复合材料板稳定性分析的有限元程序。利用该程序对多个算例进行了计算, 所得临界屈曲应力与文献及试验结果吻合很好。同时, 讨论了不同边界条件下缝纫泡沫夹芯复合材料板稳定性随缝纫参数(包括针距、 行距和缝纫针半径)以及结构参数(包括面板铺层角、 芯层厚度和缝纫夹芯板边长)的变化规律。      

波纹腹板增强泡沫夹芯复合材料结构准静态压缩吸能特性

随着车船运输量与日俱增,由此引发的车船撞击结构物的事故频发,造成严重的生命财产损失与结构破坏,亟需为桥梁等结构物设置防护吸能装置。该文提出了一种新型波纹腹板增强泡沫夹芯复合材料吸能结构。该复合结构以聚氨酯泡沫为芯材,玻璃纤维增强复合材料(Glass fiber reinforced polymer,简称GFRP)为面板,在波纹型泡沫的间隙铺设双轴向玻璃纤维布,利用真空导入工艺成型。通过波纹腹板增强泡沫夹芯复合材料结构的准静态压缩试验,研究了波纹腹板与面板壁厚以及波长对夹芯结构破坏模式、承载能力以及吸能特性的影响。试验结果表明：腹板壁厚较大、波长较短的试件吸能效果最优。此外,对试验工况进行了有限元数值模拟,分析了腹板壁厚与泡沫密度因素对试件承载力的影响,为其在防撞领域的应用提供一定依据。     

海水浸泡对FRP筋-珊瑚混凝土粘结性能的影响

开展了30℃海水浸泡条件下玻璃纤维增强树脂基复合材料（GFRP）筋、碳纤维增强树脂基复合材料（CFRP）筋与珊瑚混凝土粘结性能的试验研究,分析了纤维增强树脂基复合材料（FRP）筋-珊瑚混凝土粘结滑移曲线特征、破坏形态及粘结强度变化。试验结果表明,海水浸泡后FRP筋力学性能和粘结性能均表现为不同程度的降低。随浸泡时间增加,GFRP筋表层树脂与纤维间的孔隙率明显增大,并逐渐出现脱粘现象,纤维本身遭受到侵蚀,而CFRP筋仅表面基体有少许损伤,其耐久性明显优于GFRP筋;FRP筋-珊瑚混凝土粘结强度呈现出先增加后减小的趋势,且后期下降速率逐渐变小,部分GFRP筋-珊瑚混凝土试件的破坏模式逐渐由筋被拔出转变为筋材断裂;增加保护层厚度能有效地减缓海水对GFRP筋的侵蚀,有利于保持GFRP筋-珊瑚混凝土间的粘结性能。     

等腰梯形蜂窝芯玻璃钢夹芯板的面内压缩性能

为研究等腰梯形蜂窝芯玻璃钢夹芯板面内压缩破坏机制, 利用材料试验机对夹芯板面内压缩性能进行了试验测试, 并开展了模拟研究。结果表明: 夹芯板的面内压缩破坏方式主要有面板折断、夹芯板屈曲失稳和夹芯板中面板与蜂窝芯脱粘3种类型。面板为夹芯板面内压缩的主要承载构件, 蜂窝芯对面板起到固支作用。面板结构参数与材料参数为影响夹芯板面内压缩抗压强度与抗压刚度主要因素, 多数蜂窝芯的结构参数与材料参数对夹芯板面内压缩抗压强度的影响微弱, 而个别蜂窝芯的结构参数对夹芯板面内压缩抗压刚度的影响比较显著。夹芯板体积一定时, 随着蜂窝芯胞体单元数量的增加, 夹芯板面内压缩的抗压强度与抗压刚度逐渐增大。      

湿热环境下泡沫复合材料夹芯板的Ⅱ型界面剥离

本文通过端部切口弯曲试验(ENF)对湿热老化后泡沫复合材料夹芯板Ⅱ型界面剥离进行研究。测定了湿热老化前后玻璃纤维增强复合材料面板、聚氨酯泡沫芯材的抗压强度和压缩模量。结果表明,由于后固化作用,GFRP面板抗压强度、压缩模量先增大后减小;聚氨酯泡沫芯材的抗压强度、压缩模量先减小后趋于稳定。Ⅱ型临界能量释放率(Gc)随着老化时间的增加呈现下降趋势。湿热老化90天后,Gc下降26.68%。     

缝纫泡沫夹芯复合材料失效强度的理论预测与试验验证

基于经典层板理论和细观力学桥联模型, 提出了缝纫泡沫夹芯复合材料失效强度的理论预测方法, 并进行了失效强度的相关试验验证。其中, 将缝纫复合材料面板看作单层组成的准层状结构, 采用经典层板理论进行逐层失效分析, 并同时考虑了局部皱曲的面板失效模式; 而对缝纫泡沫夹芯, 引入桥联模型计算其各组分材料中的应力, 并通过对各组分材料选取适当失效准则来建立失效判据; 对于缝纫泡沫夹芯复合材料采取逐级加载方式, 当面板或者夹芯失效时, 则认为其发生整体失效, 由此可以确定其在不同载荷形式下的失效强度。此外, 通过试验得到了缝纫泡沫夹芯复合材料板试件在平压、 侧压、 横向剪切及三点弯曲载荷形式下的失效模式及其失效强度, 并利用本文方法对缝纫泡沫夹芯复合材料的失效强度进行了理论预测, 所得结果与试验吻合, 证明了本文方法的有效性。      

缝纫泡沫夹芯复合材料的刚度预测与试验验证

基于材料细观结构,建立了缝纫泡沫夹芯复合材料的刚度预测模型,并进行了刚度性能的相关试验验证。其中,对缝纫复合材料层合面板部分,考虑了缝纫角对单胞尺寸和富脂区大小的影响,以及缝纫前后层合面板厚度的变化对复合材料面板纤维体积含量的影响,采用改进的纤维弯曲模型计算了缝纫复合材料层合面板的刚度;对缝纫增强的泡沫夹芯部分,把缝线树脂柱看作是泡沫基体中的增强相,将其简化为特殊的单向增强复合材料,提出了用串并联组合模型来预测其刚度。试验测试了缝纫泡沫夹芯复合材料板试件的刚度。应用本文模型对缝纫层合面板和缝纫泡沫夹芯复合材料板的刚度进行预测,结果均与试验结果吻合较好。采用理论模型系统研究了缝纫参数和结构参数对缝纫泡沫夹芯复合材料刚度的影响。     

碳纤维复合材料-泡沫铝夹芯板的冲击响应

采用泡沫金属子弹撞击加载的方式研究了T700碳纤维复合材料面层-泡沫铝芯体的夹芯结构动力响应。利用激光测速装置、高速摄像仪和位移传感器记录了泡沫子弹的撞击速度、子弹撞击夹芯板全过程和夹芯板后面板中心点的位移时程曲线。研究了加载冲量和芯层相对密度对夹芯板冲击响应的影响,得到了碳纤维复合材料-泡沫铝夹芯板的变形与失效模式。同时,采用ABAQUS有限元软件进行数值模拟,研究了复合材料面板铺层方式、面层厚度、芯层厚度和相对密度以及泡沫铝子弹的长度、速度和相对密度等参数对夹芯板冲击响应的影响。     

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。     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

The Effect of Load and Geometry on the Failure Modes of Sandwich Beams

Facing compressive failure, facing wrinkling and core shear failure are the most commonly encountered failure modes in sandwich beams with facings made of composite materials. The occurrence and sequence of these failure modes depends on the geometrical dimensions, the form of loading and type of support of the beam. In this paper the above three failure modes in sandwich beams with facings made of carbon/epoxy composites and cores made of aluminum honeycomb and two types of foam have been investigated. Two types of beams, the simply supported and the cantilever have been considered. Loading included concentrated, uniform and triangular. It was found that in beams with foam core facing wrinkling and core shear failure occur, whereas in beams with honeycomb core facing compressive failure and core shear crimping take place. Results were obtained for the dependence of failure mode on the geometry of the beam and the type of loading. The critical beam spans for failure mode transition from core shear to wrinkling failure were established. It was found that initiation of a particular failure mode depends on the properties of the facing and core materials, the geometrical configuration, the type of support and loading of sandwich beams.     

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.     

Size effect on strength of laminate-foam sandwich plates: Finite element analysis with interface fracture

Recent three-point bend tests of size effect on the strength of geometrically scaled sandwich beams of three types – with no notches, and with notches at the upper or lower skin–foam interface, which were previously evaluated using simplified sandwich beam theory and equivalent linear elastic fracture mechanics, are now reanalyzed more accurately by finite elements. Zero-thickness interface elements with a softening cohesive law are used to model fractures at the skin–foam interface, in the fiber composite skins, and in the foam. The fracture energy and fracture process zone length of a shear crack in foam near the interface are deduced by fitting an analytical expression for size effect to the test data. Numerical simulations reveal that small-size specimens with notches just under the top skin develop plastic zones in the foam core near the edges of the loading platen, and that small-size specimens with notches just above the bottom skin develop distributed quasibrittle fracture in the foam core under tension. Both phenomena, though, are found to reduce the maximum load by less than 6%. Further it is shown that, in notch-less beams, the interface shear fracture is coupled with compression crushing of the fiber–polymer composite skin. For small specimens this mechanism is important because, when it is blocked in simulations, the maximum load increases. The size effect law for notch-less beams is calibrated such that beams of all sizes fail solely by interface shear fracture.     

Wrinkling of corrugated skin sandwich panels

Compression wrinkling of composite sandwich panels with corrugated skins was investigated numerically, analytically and experimentally. Semi-circular and sine-wave shaped corrugations were studied. The corrugations significantly increased wrinkling strength when compared with equal mass flat panels. Semi-circular corrugations proved to be highly preferable to sine-wave shaped corrugations due to localized buckling in the latter. Over 40 fiberglass and foam core sandwich specimens were manufactured with semi-circular skin corrugations. These specimens were tested to failure, providing confirmation of the numerical and analytical results.     

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.     

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.     

Long-term hygrothermal effects on damage tolerance of hybrid composite sandwich panels

A sandwich construction, composed of hybrid carbon-glass fibre-reinforced plastic skins and a syntactic foam core, was selected as the design concept for a wind tunnel compressor blade application, where high damage tolerance and durability are of major importance. Beam specimens were prepared from open-edge and encapsulated sandwich panels which had previously been immersed in water at different temperatures for periods of up to about two years in the extreme case. Moisture absorption and strength characteristics, as related to time of exposure to hygrothermal conditions, were evaluated for the sandwich specimens and their constituents (skins and foam). After different exposure periods, low-velocity impact damage was inflicted on most sandwich specimens and damage characteristics were related to impact energy. Eventually, the residual compressive strengths of the damaged (and undamaged) beams were determined flexurally. Test results show that exposure to hygrothermal conditions leads to significant strength reductions for foam specimens and open-edge sandwich panels, compared with reference specimens stored at room temperature. In the case of skin specimens and for beams prepared from encapsulated sandwich panels that had previously been exposed to hygrothermal conditions, moisture absorption was found to improve strength as related to the reference case. The beneficial effect of moisture on skin performance was, however, limited to moisture contents below 1% (at 50°C and lower temperatures). Above this moisture level and at higher temperatures, strength degradation of the skin seems to prevail.     

泡沫夹芯复合材料梁界面裂纹曲折扩展实验与数值模拟

采用双悬臂梁(DCB)试验研究了具有不同密度的PMI泡沫芯体的玻璃纤维增强复合材料夹芯梁界面裂纹曲折破坏路径。基于包含裂纹的物质点算法(MPM), 建立了与试验研究相适应的MPM模型, 在不同的面板/芯体模量比下计算了界面裂纹裂尖模态比和曲折破坏角, 并结合曲折破坏准则模拟了界面裂纹曲折破坏路径。数值模拟结果和试验现象吻合良好, 说明了本文中数值分析模型和方法的有效性。研究结果表明, 面板材料和芯体材料模量失配越严重, 界面裂纹发生曲折破坏时的破坏角越大; 裂纹折入芯体后, 在 Ⅰ 型为主的加载模式的支配下以基本平行于界面的方向扩展。      

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.     

Dynamic response of clamped sandwich beam with aluminium alloy foam core subjected to impact loading

The dynamic response of clamped sandwich beam with aluminium alloy open-cell foam core subjected to impact loading is investigated in the paper. The face sheet and the core of the sandwich beam have the different thickness. And the sandwich beam is impacted by a steel projectile in the mid-span. The impact force is recorded by using accelerometer. The results show that tensile crack and core shear are the dominant failure modes. And the impact velocity and the thickness of the face sheet and the foam core have a significant influence on the failure modes and the impact forces. Combining with the inertia effect and experimental results, the failure mechanisms of the sandwich beams are discussed. The thickness of the foam core plays an important role in the failure mechanism of the sandwich beam. In present paper, the failure of the sandwich beam with a thin core is dominated by the bending moment, while the sandwich beam with a thick core fails by bending deformation in the front face sheet and the bottom face sheet in opposite direction due to the plastic hinges in the front face sheet.