Tearing Behaviors of Flexible Fiber-Reinforced Composites for the Stratospheric Airship Envelope

Tearing behaviors of the envelope material is important for the stratospheric airship because it directly decides the service life of the airship. Uniaxial tensile tests of the specimens with different initial damage were performed to study the tear propagation properties of a new kind of envelope material for the stratospheric airship. A Matlab program was developed to simulate the mechanical properties in the tensile process. The interaction forces between warp and weft yarns were considered, and the yarns in the transition region between deformed region and non- deformed region were assumed as hinges in the simulation. It is discovered that the interaction factor, which represents interaction force between different components of the material, and the frictional coefficient between warp and weft yarns are the crucial factors for the tensile curves of the material. The tear strength and the damage mode of the envelope material are determined by the number of the cutoff yarns and the declination of the initial cracks, respectively.     

Fatigue Damage Mechanical Model of the Envelope Material for Stratospheric Airships

As a major part of the stratospheric airship structure, the envelope material is used to contain lifting gas and keep the aerodynamic configuration. The main force on the envelope material comes from differential pressure between inside and outside the structure, which is cyclic stress because of the alternative temperature. Three different damage modes of the envelope material, including fracture damage of fabric yarns, cracking damage of resin matrix and functional membrane are investigated in this paper. A theoretical model to predict fatigue life of the envelope material under cycle load is developed base on the damage evolution properties of the material. The results indicates that the theoretical model can well predict the fatigue life. In addition, it can be seen from the results that the fracture of fabric yarns is the main damage modes for the material with off-axial angle of 0°and 90°, while the cracking damage of resin and functional membrane is the main damage modes for the material with other off-axial angles.     

含初始裂纹的平流层飞艇用蒙皮薄膜撕裂行为

为了研究含初始裂纹蒙皮薄膜的抗撕裂行为,选用Kevlar纤维增强的具有预制初始裂纹切口(PIC)的平流层飞艇用蒙皮薄膜,采用弹塑性断裂力学平面应力分析方法,考虑经纬向纱束之间的滑移摩擦效应,分析了薄膜裂纹尖端的撕裂强度与变形形式,建立了裂纹尖端撕裂物理模型和基于摩擦效应修正的等差法拉伸变形计算模型,获得了裂纹尖端应力集中区的应力分布。为验证计算模型的合理性,分别对PIC长度为10、20、30及40mm的试样进行了撕裂强度试验。结果表明:当PIC宽度比率不超过0.50时,模型计算值与试验值的误差小于4.12%;与改进的应力强度因子法相比,所建计算模型具有更高的计算精度,为平流层飞艇蒙皮撕裂强度的设计提供了一种有效的方法。     

临近空间飞艇蒙皮材料研究进展和需求分析

临近空间飞艇具有大范围不间断的高效信息获取与传输能力, 是近期国际上竞相发展的未来信息平台。蒙皮材料是临近空间飞艇的主要材料, 其质量和性能决定了飞艇的大小、 寿命和成本, 是发展临近空间飞艇的基础。本文中针对临近空间飞艇蒙皮材料选取和应用存在的若干困难, 从蒙皮材料工程需求出发, 对蒙皮材料的强度、 耐候性、 阻隔性和试验评价方法等方面的研究现状进行总结和分析, 讨论未来高性能临近空间飞艇蒙皮材料的研究发展方向。     

飞艇囊体薄膜材料的双向拉伸试验及结构仿真

飞艇囊体膜材的弹性常数是飞艇外形设计、结构分析、动力学分析的基础。本文根据正交异性平面应力模型假设,应用双向拉伸试验方法获得了飞艇囊体膜材HV130在不同载荷比条件下的经纬向弹性模量和泊松比。对飞艇囊体进行应力分析,得到囊体应力的理论解及环向应力和轴向应力的载荷比。根据飞艇囊体受力情况,选取对应的弹性常数,对飞艇进行不同压差条件下的静力学仿真分析。利用非接触应变测量系统测试不同压差条件下飞艇囊体的变形情况,验证了双向拉伸试验所得的弹性常数的准确性和仿真分析的可靠性。本文能为飞艇的外形设计和结构分析提供参考。      

软式平流层飞艇气弹模型相似参数分析

软式平流层飞艇具有较大的尺寸,基于风洞试验探讨其气弹响应特性时,需要缩比模型取代全尺寸模型。缩比模型的试验结果能否准确地预测原型结构的位移响应,与相似性设计直接相关。该文以设定的某软式平流层飞艇为原型对象,基于量纲分析法推导了模型的相似准则;建立软式平流层飞艇的有限元模型,对不同相似比对应的缩比模型进行了自振及风振响应分析,获得对应的振动频率与位移响应;统计确定了各相似准则对结构振动频率及位移响应的影响程度;对影响程度较大且无法满足相似条件的偏差进行了评估,且给出相应的补偿修正。结果表明:风速偏差的影响可以忽略;蒙皮面密度对位移响应的影响可以忽略;蒙皮张拉刚度及压差的相似偏差,对振动频率及位移响应都有较大的影响。     

Acoustic fatigue and damping technology in FRP composites

There is considerable interest in the use of composite materials in aerospace structures. One important area is to develop a stiff, lightweight composite material with a highly damped, high-temperature polymer matrix material. This paper concerns the application of such material, in the form used in thin skin panels of aircraft, and investigates of its fatigue properties at both room and high temperature. Flexural fatigue tests have been carried out at two different temperatures and harmonic three-dimensional finite-element (FE) analyses were performed in order to understand the dynamic behaviour of plates. Random acoustic excitation tests using a progressive wave tube, up to an overall sound pressure level of 162 dB, at room temperature and high temperatures were also performed in order to investigate the dynamic behaviour of panels made of the materials. Parameter studies were carried out in order to examine various methods for including damping in the structure, and conclusions have been drawn concerning optimal incorporation of a highly damped matrix material into a high-performance structure.     

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally.     

基于双轴拉伸试验的飞艇蒙皮材料非线性分析

飞艇蒙皮为多功能层压织物材料,力学行为呈现复杂的非线性,合理表征其非线性的力学模型对飞艇结构设计与分析至关重要。本文对飞艇蒙皮材料试件进行了7个应力比双轴拉伸试验,基于正交异性材料模型,采用应变残差平方和最小方法计算弹性常数,建立线性力学模型;根据试验数据建立应力弹性常数响应面,建立以应力为变量的完整三次式非线性力学模型。并根据此力学模型建立飞艇蒙皮材料有限元分析模型,通过数值模拟七种应力比下的双轴拉伸试验,分析结果的应力分布和分缝位移与试验现象一致。将分析得到的应力-应变曲线与试验结果进行对比,两者能很好的吻合,表明非线性力学模型及分析方法适用于飞艇结构分析。     

The effect of the measurement frequency on the elastic anisotropy of fibre laminates

Static tests and ultrasonic measurements (2.25 MHz) have been carried out on a series of composite laminates of glass fibres in a polypropylene matrix. A range of angle ply laminates were prepared for this study, with laminate angles θ of ±0, 10, 20, 30 and 40^∘. The high frequency measurements were made using the ultrasonic immersion technique, which allows the determination of a complete set of the elastic constants of a material. The relationship between the ultrasonically determined elastic constants of the angle ply laminates was found to be in excellent agreement with theoretical predictions, as previously validated for carbon fibre/epoxy angle ply laminates. A comparison between the ultrasonic and statically measured values was made for two of the angle ply laminates (θ = 0 and 20^∘). It was found that the static values were lower than those measured at ultrasonic frequency, particularly those constants that were more matrix dominated (for example the transverse moduli of the laminates). Measurements on a pure polypropylene sample at both testing frequencies confirmed that the change in matrix properties with frequency was the cause of this difference. The change in properties with test frequency is likely to be much larger in this system than in other composite materials because the glass transition temperature of polypropylene is close to ambient temperature. Dynamic mechanical tests (1 Hz) were carried out on a sample of pure polypropylene to assess this effect. We also give an appropriate method of estimating the dependence of glass transition temperature on frequency. The results for polypropylene are compared with those for other commonly used polymer matrix materials: epoxy resin, nylon and polyetheretherketone (PEEK): DMTA measurements were also made on these samples. The effect of test frequency on matrix properties, for the glass/PP laminates, was further investigated by examining the relationship of the Poisson's ratios with laminate angle using a mixture of ultrasonic experiments and theoretical predictions. Previously we have shown that the degree of anisotropy between the reinforcing fibre and the matrix phase is paramount in determining whether the material will show a negative Poisson's ratio at a critical laminate angle. The ultrasonic measurements carried out in this study on the glass/PP laminates showed a minimum in one of the Poisson's ratio at a laminate angle of 32°, but the value did not become negative. However, theoretical predictions showed that for a static frequency measurement (1 Hz), where the matrix is softer and hence the anisotropy of each laminate ply is higher, the laminate will show a negative Poisson's ratio with a minimum at a laminate angle of around 28°.     

复合材料双波纹面外褶皱缺陷细观力学分析方法

复合材料由于其高比模量、高比强度和疲劳寿命长等突出的优点被广泛应用于航空航天领域,但在制备过程中易产生褶皱缺陷,并对复合材料层合板的刚度、强度产生负面影响。本文主要研究含有单波纹和双波纹外部褶皱的复合材料力学性能。提出一种解析方法计算单波纹和双波纹褶皱细观力学模型的等效材料属性,通过与数值模型结果的对比与分析,发现含褶皱层合板的等效模量随波纹比绝对值的增大而减小。当波纹比绝对值相同时,含凹陷褶皱的层合板等效模量比含凸起褶皱的层合板等效模量低。不同厚度的含有褶皱的层合板等效模量曲线存在交汇点,交汇点两侧等效模量随厚度变化的趋势相反,且双波纹褶皱与单波纹褶皱的交汇点不同。     

Thermal shock behaviour of angle-ply and woven dense ceramic-matrix composites

The behaviour of two Nicalon/calcium aluminosilicate ceramic composite laminates (a (±45°)_3s and a plain-weave woven) under conditions of thermal shock has been studied. Test specimens heated at various temperatures were quenched into room-temperature water. This was followed by detailed damage characterisation. In addition, post-shock mechanical properties were assessed by tensile tests (for the woven laminate) and flexural tests (for both laminates). Both materials were found to have comparable thermal shock resistance. Crack morphologies comprised matrix cracks of various orientations that exhibited similar characteristics to those described for thermally shocked cross-ply laminates with the same constituents, but cracking was found to be less widespread in the woven laminate. Fibre breaks were also detected on the woven material when high-temperature degradation of the fibre–matrix interface was present. A gradual reduction in properties (stiffness, proportional limit stress, fracture strength) of thermally shocked specimens was identified, which began at larger shocks than those at which thermal shock damage initiated. This was attributed to the extension of some matrix cracks into the bulk of material.     

Thin-Film Thermal-Conductivity Measurement on Semi-Conducting Polymer Material Using the 3ω Technique

Organic solar cells have gained increasing interest in recent years due to their promising low-cost processing possibility and high throughput compared to inorganic solar cells. Since the efficiency of organic solar cells is still low, further optimization has to be done. Reliable simulation of solar cell layout and performance strongly depends on correct input data of the electrical and thermal transport properties of the applied film materials. In many cases these material properties are only available for bulk material if available at all. Owing to the given film thicknesses on the order of tenths to hundreds of nanometer and to the preparation methods, the properties of the used system can differ from the bulk material values. For determination of the thin-film thermal conductivity, only a few measurement methods are known to provide accurate results with one of them being the 3ω technique. It allows the determination of the thermal conductivity of bulk materials as well as thin films down to a thickness of around 50 nm. This study is part of an investigation on the influence of local hot spots, generated by defects in the active layer of organic solar cells, and on the charge carrier mobility as well as the propagation of the hot spot due to the thermal conduction of the material. Applying the 3ω technique, the effective thermal conductivity of solution-derived poly(3-hexylthiophene) thin films of different thickness on a common glass substrate was investigated.     

A finite strip analysis of cracked laminates

As a step towards the construction of a comprehensive model for damaged composites, the deformation of transversely cracked laminates is analysed. The approach is based on a generalised plane strain approach and uses the finite strip method which allows general lay-ups of laminates rather than just cross-ply to be considered. General loading is treated. In addition, the method permits a more flexible control to be exercised between accuracy and the extensiveness of the calculations than can be achieved using existing analyses. This method could be integrated with the finite element analysis of structures to allow for changes in the effective material properties of locally damaged regions. Results produced by the approach are compared with those available in the literature and good agreement is demonstrated.     

Design systems for gear elements made of cotton fiber-reinforced plastics

We look on the cotton fiber reinforced plastics as industrial gear materials, and have been developing design systems for industrial gears made of cotton fiber-reinforced plastics. In this report, we deal with a method estimating for tooth root stresses caused by bending movements under running conditions. The gear material used was cotton fiber plain woven cloth reinforced phenolic resin laminates. Paper-reinforced phenolic resin laminates, a commonly used material, was used as a control for comparison. The main dimensions of the gears were module 3–5 mm and tooth width 25 mm. First, accelerations of gears were measured under running conditions to estimate dynamic performance. Second, fracture tests of gear teeth were carried out under bending loads. Different fracture modes at tooth roots for cotton fiber-reinforced plastics and phenolic resin gears were observed. The fractures occurred at a high position from the tooth root in the case of the cotton fiber-reinforced plastics gear because the cotton fiber-reinforced plastics had excellent cleavage and shear strength. This gear also had higher strength for tooth bending loads. Third, the mechanical properties of the gears were researched by tensile, bending, and shearing tests. It was clear that the cotton fiber-reinforced plastics had excellent properties in cleavage fracture between laminates and shear strength. Finally, we proposed a design method for this gear, which considers the cleavage and shear strength.     

Design and Manufacturing of a Novel Shear Thickening Fluid Composite (STFC) with Enhanced out-of-Plane Properties and Damage Suppression

The ability to absorb a large amount of energy during an impact event without generating critical damages represents a key feature of new generation composite systems. Indeed, the intrinsic layered nature of composite materials allows the embodiment of specific hybrid plies within the stacking sequence that can be exploited to increase impact resistance and damping of the entire structure without dramatic weight increase. This work is based on the development of an impact-resistant hybrid composite obtained by including a thin layer of Non-Newtonian silica based fluid in a carbon fibres reinforced polymer (CFRP) laminate. This hybrid phase is able to respond to an external solicitation by activating an order-disorder transition that thickens the fluid increasing its viscosity, hence dissipating the energy impact without any critical failure. Several Shear Thickening Fluids (STFs) were manufactured by changing the dimensions of the particles that constitute the disperse phase and their concentrations into the continuous phase. The dynamic viscosity of the different STFs was evaluated via rheometric tests, observing both shear thinning and shear thickening effects depending on the concentration of silica particles. The solutions were then embedded as an active layer within the stacking sequence to manufacture the hybrid CFRP laminates with different embedded STFs. Free vibration tests were carried out in order to assess the damping properties of the different laminates, while low velocity impact tests were used to evaluate their impact properties. Results indicate that the presence of the non-Newtonian fluid is able to absorb up to 45 % of the energy during an impact event for impacts at 2.5 m/s depending on the different concentrations and particles dimensions. These results were confirmed via C-Scan analyses to assess the extent of the internal delamination.     

Experimental and Numerical Analysis of Damage in Woven GFRP Composites Under Large-deflection Bending

Textile-reinforced composites such as glass fibre-reinforced polymer (GFRP) used in sports products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. Such loading conditions cause high local stresses and strains, which result in multiple modes of damage and fracture in composite laminates due to their inherent heterogeneity and non-trivial microstructure. In this paper, various damage modes in GFRP laminates are studied using experimental material characterisation, non-destructive micro-structural damage evaluation and numerical simulations. Experimental tests are carried out to characterise the behaviour of these materials under large-deflection bending. To obtain in-plane shear properties of laminates, tensile tests are performed using a full-field strain-measurement digital image correlation technique. X-ray micro computed tomography (Micro CT) is used to investigate internal material damage modes – delamination and cracking. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus to study the deformation behaviour and damage in GFRP. In these models, multiple layers of bilinear cohesive-zone elements are employed to study the onset and progression of inter-ply delamination and intra-ply fabric fracture of composite laminate, based on the X-ray Micro CT study. The developed numerical models are capable to simulate these features with their mechanisms as well as subsequent mode coupling observed in tests and Micro CT scanning. The obtained results of simulations are in agreement with experimental data.     

The combined effects of curing and environmental exposure on fracture properties of woven carbon/epoxy laminates

This paper concerns a study of the combined effects of curing conditions and environmental exposure on the ultimate properties of two commercial woven carbon/epoxy laminates. Curing parameters (heating rate and applied pressure) were varied so as to obtain six different conditions for each material. Moisture saturation was also achieved by exposing some of the cured samples to environmental conditions of 70°C and 95% relative humidity. Four different tests (tensile, impact, Mode I and Mode II interlaminar fracture resistance) were therefore performed, and the results obtained on the different materials before and after moisture saturation compared. Neither curing pressure nor heating rate nor moisture absorption were observed to have any practical effect on tensile and impact properties. On the contrary, one noticeable effect was the interlaminar fracture resistance of the laminates. The results are discussed and interpreted in terms of damage formation and stress intensification mechanisms.     

含矩形边缘分层缺陷层合板的压缩性能

对含矩形边缘分层缺陷层合板的压缩性能进行试验研究和理论分析, 考察了层合板厚度(含铺层形式)、 分层位置、 形状、 面积以及环境等因素对压缩强度的影响, 并采用分层扩展以及软化夹杂两种模型对含分层层合板的压缩强度进行了计算和破坏机理分析。结果表明: 厚板对边缘分层缺陷不敏感, 中等厚度和薄层合板比较敏感; 缺陷的位置和形状对层合板压缩强度有一定的影响; 湿热环境改变了含缺陷板的压缩破坏机理, 并对中等厚度板和薄板的压缩强度有明显的影响。两种模型中软化夹杂模型效果较好, 可用于工程设计计算。      

Evaluation of the interfacial bond properties between carbon phenolic and glass phenolic composites

The effects of moisture and surface finish on the mechanical and physical properties of the interfacial bond between the carbon/phenolic (C/P) and glass/phenolic (G/P) composite materials are presented in this paper. Four flat panel laminates were fabricated using the C/P and G/P materials. Of the four laminates, one panel was fabricated in which the C/P and G/P materials were cured simultaneously. It was identified as the cocure. The remaining laminates were processed with an initial simultaneous cure of the three C/P billets. Two surface finishes, one on each half, were applied to the top surface. Prior to the application and cure of the G/P material to the machined surface of the three C/P panels, each was subjected to specific environmental conditioning. Types of conditioning included: (a) nominal fabrication environment, (b) a prescribed drying cycle, and (c) a total immersion in water at 71 C. Physical property tests were performed on specimens removed from the C/P materials of each laminate for determination of the specific gravity, residual volatiles and resin content. Comparison of results with Shuttle Solid Rocket Motor (SRM) nozzle material specifications verified that the materials used in fabricating the laminates met acceptance criteria and were respresentative of SRM nozzle materials. Mechanical property tests were performed at room temperature on specimens removed from the G/P, the C/P and at the interface between the two materials for each laminate. The double-notched shear strength test was used to determine the ultimate interlaminar shear strength. Results indicate no appreciable difference in the C/P material of the four laminates with the exception of the cocure laminate, where a 20% reduction in the strength was observed. The most significant effect occurred in the bondline specimens. The failure mode was shifted from the C/P material to the interface and the ultimate strength was significantly reduced in the wet material. No appreciable variation was noted between the surface finishes in the wet laminate.