碳/环氧复合材料层压板孔隙的形态研究

为深刻了解铺层为[(±45)4/(0,90)/(±45)2]S和[(±45)/04/(0,90)/02]S层压板内孔隙的尺寸、形状及分布情况,通过改变固化压力产生不同孔隙率,采用显微图像分析方法对复合材料层压板厚度方向内孔隙的形态进行了观察,并利用图像分析软件对孔隙的形状和尺寸进行了统计分析.研究表明,对于铺层为[(±45)4/(0,90)/(±45)2]S层压板,孔隙大多出现在层间树脂富集的地方,且孔隙均会沿层间发展;对于铺层为[(±45)/04/(0,90)/02]S层压板,孔隙率较小时孔隙主要出现在层间,而当孔隙率较大时孔隙开始在层内出现,且纵横比较小.对于这两种铺层,孔隙的长度、面积和纵横比均随着孔隙率的增加而增加.     

CFRP复合材料孔隙形貌特征的统计分析

采用光学显微镜观察和统计分析的方法研究了孔隙率(P)在0.03%～4.96%范围内的16组碳纤维增强树脂(carbon fiber reinforced polymer,CFRP)复合材料中孔隙的平均长度、平均宽度及平均宽长比等参数的变化规律。结果表明:CFRP复合材料的孔隙形貌在整体上呈现统计特性,当孔隙率0.03%≤P0.5%时,孔隙截面接近圆形,孔隙平均长度和平均宽度分别为10～50μm和10～30μm,平均宽长比为0.5～0.8;当孔隙率0.5%≤P3.0%时,孔隙截面逐渐趋向于椭圆形,孔隙平均长度和平均宽度分别为50～70μm和20～30μm,平均宽长比为0.5～0.6;当孔隙率3.0%≤P≤4.96%时,大多数孔隙截面呈现长条形,孔隙平均长度和平均宽度分别为50～100μm和30～40μm,平均宽长比0.35～0.5;同时,CFRP复合材料的孔隙形貌也具有随机特性,不同孔隙率试样之间和同一试样不同部位之间,其孔隙形状、尺寸及分布都可能存在明显差异。     

孔隙微观特征对碳纤维/环氧树脂复合材料横向拉伸强度的影响

通过有限元方法研究了相同孔隙率下孔隙的分布、尺寸和形状等微观特征对碳纤维增强环氧树脂复合材料单向板横向拉伸强度的影响。首先使用Matlab对复合材料微观图像进行处理,提取孔隙的半径分布。然后通过C++编写多种孔隙随机分布算法,包括可以生成不同分布孔隙、不同尺寸孔隙以及不同形状孔隙的随机分布算法。最后通过Python参数化生成代表性体积单元（RVE）,用有限元方法研究相同孔隙率下孔隙的分布、尺寸和形状对碳纤维/环氧树脂复合材料单向板横向拉伸强度的影响。研究结果显示,孔隙率相同时,碳纤维/环氧树脂复合材料的孔隙形状对横向弹性模量的影响较大,孔隙尺寸和形状对横向拉伸强度有较大的影响。     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

孔隙率对碳纤维/环氧树脂复合材料层合板湿热性能的影响

针对某飞机上应用的典型铺层[(±45)4/(0,90)/(±45)2]S和[(±45)/(0,90)2/(±45)]S,研究了孔隙对碳纤维增强环氧树脂基复合材料层合板的吸湿行为和层间剪切强度的影响。采用不同的热压罐固化压力制备了不同孔隙率的试样。采用显微图像分析技术对孔隙率和孔隙的微观结构特征进行了详细的分析。研究结果表明:孔隙主要分布于层间,且随着孔隙率的增大,孔隙的尺寸增大;2种层合板的吸湿率和最大吸湿量随着孔隙率的增加而增加;湿热老化和未经湿热老化的层间剪切强度都随着孔隙率的增加而下降。铺层[(±45)4/(0,90)/(±45)2]S未经湿热和湿热后的层间剪切强度随着孔隙率增加分别下降6%(孔隙率:0.6%~6.3%)和9%(孔隙率:0.4%~7.0%);铺层[(±45)/(0,90)2/(±45)]S未经湿热和湿热后的层间剪切强度随着孔隙率增加分别下降14%(孔隙率:0.4%~6.9%)和7%(孔隙率:0.2%~8.9%)。     

孔隙对碳纤维增强环氧树脂复合材料超声衰减系数及压缩性能的影响

孔隙对碳纤维增强环氧树脂（CF/EP）复合材料的力学性能和破坏模式有显著的影响,因此需要建立准确的孔隙率无损检测评估方法,并基于所评估的孔隙率提高CF/EP复合材料压缩性能预测的可靠性。本文主要研究了孔隙对CF/EP复合材料的超声衰减系数和压缩性能的影响,通过降低固化压力至0.7~0.2 MPa和延长预浸料室温贮存时间至30~180天的方法,制备了不同孔隙率的CF/EP复合材料层压板,通过金相验证其孔隙率在0%~3.0%之间,孔隙类型主要为层中孔隙和层间孔隙。通过理论和试验的方法,基于超声反射法建立了孔隙率与超声衰减系数的关系曲线,由孔隙引起超声衰减系数为α_v=1.08P_v2(P_v为孔隙率),与前人基于超声穿透法所得的超声衰减系数α_v=0.61P_v2较好地符合2倍声程的关系。对不同孔隙率的CF/EP复合材料层压板进行压缩测试实验,特别考虑了贴片和加载方向对测试结果的影响。从细观角度研究了含孔隙的CF/EP复合材料层压板的压缩破坏模式。结果表明:CF/EP复合材料层压板的压缩强度随孔隙率增加而下降,孔隙率增加至2.5%时,压缩强度下降13.7%,孔隙细观特征影响压缩破坏的形式,主要原因是孔隙诱发微裂纹的萌生和扩展,削弱了纤维与树脂间的结合力并引发纤维微屈曲。      

不同孔隙率CFRP层合板静态力学性能研究

为了研究孔隙率对织物碳纤维/环氧树脂复合材料层合板静态力学性能的影响规律,分别测量了孔隙率为0.33%至1.50%的CFRP层合板的弯曲强度和层间剪切强度,并进行有限元模拟.在适用于复合材料单向板的改进Hashin失效准则基础上,建立了适用于织物纤维增强复合材料静态力学强度的失效准则.通过引入复合材料基本强度参数预测不同孔隙率CFRP层合板的力学性能,结合刚度突然退化模型,采用ABAQUS软件建立了有限元模型.试验结果表明,随着孔隙率的增加,复合材料层合板的弯曲强度和层间剪切强度均呈下降趋势.有限元模型较为准确地预测了不同孔隙率织物碳纤维/环氧树脂复合材料层合板的弯曲强度和层间剪切强度.     

孔隙对碳/环氧层压板层间剪切强度影响的神经网络预测(英文)

采用一种新的方法研究了孔隙(孔隙率、孔隙形状和尺寸)对复合材料层压板[(±45)4/(0,90)/(±45)2]S,[(±45)/(0,90)2/(±45)]S和[(±45)/04/(0,90)/02]S的层间剪切强度影响的非线性关系。建立了了一个四层前馈型BP网络神经网络。采用真空袋和热压罐技术制备了含不同孔隙率的复合材料层压板。神经网络的算法为Levenberg-Marquardt算法。研究结果表明:Levenberg-Marquardt算法的预测能力较高,即,92%的B(决定系数)值大于0.9。而且,神经网络的结构会影响不同孔隙率的环氧树脂基复合材料的层间剪切强度的预测结果。     

CFRP中孔隙几何形貌与超声衰减系数关系的研究

通过金相和超声衰减系数表征CFRP层压板中的孔隙,研究孔隙形貌对超声衰减的影响。结果表明:超声衰减系数随孔隙几何形貌参数(平均长、宽、面积)的增大而增大;平均孔隙长度随孔隙率的增加程度大于平均孔隙宽度随孔隙率的增加程度;超声衰减系数随主要尺寸区域内的孔隙所占比例的增加,总体趋势减小;同一孔隙率下,大尺寸孔隙所占的比例越多,其超声衰减系数越大。总之,本试验中孔隙形貌对超声衰减有一定的影响且不能忽略。     

孔隙率对碳纤维复合材料超声衰减系数和力学性能的影响

复合材料的工艺过程对最终产品的力学性能具有决定性影响,但其机理十分复杂。通过设置一系列固化压力历程以产生不同水平的孔隙率,采用超声C扫描、显微镜分析和酸解法对孔隙的分布、形状、尺寸和体积含量进行了定性和定量表征。讨论了孔隙率对层压板层间剪切性能、弯曲性能和拉伸性能的影响规律,获得了性能下降的临界孔隙率值。结果表明:孔隙率对性能的影响只受孔隙含量、分布、尺寸和形状等的影响,而与产生方式没有直接联系。本文作者的研究将工艺参数、孔隙率和性能有机地联系起来,体现了复合材料设计、制备与性能一体化的特点。      

Microstructures of modified RR2072 single crystal superalloys and their effects on LCF response at 950 8°C

Effects of microstructural modifications, that are a consequence of adding minor grain boundary strengthening elements (C, Hf), on the fatigue response of an experimental single crystal superalloy have been studied. Investigations show that in the modified alloys, as a consequence of the casting process, the population of pores is reduced, but the average pore size increases and larger pores occur close to the fatigue sample surface. Such porosity changes in the modified alloys are an important contributor to their low cycle fatigue (LCF) properties. At the high stress range of LCF tests undertaken, the growth of cracks initiates from pores and their location in the modified alloys are responsible for decreased fatigue lives. At lower stress ranges, crack initiation consumes a considerable proportion of fatigue life. Since crack initiation involves strain localisation at or near the specimen surface where there is interaction with the environment, the population of pores near the specimen surface and the increased length of the tests plays a significant role in governing fatigue lives. The modified alloys benefit from their lower density of pores in the vicinity of the free surface, and tend to have comparable fatigue lives to that of the base alloy at the lower stress level.     

Polymorphism of amyloid β peptide in different environments: implications for membrane insertion and pore formation

Amyloid-β (Aβ) peptides are thought to be involved in neurodegenerative diseases such as Alzheimer's disease and Down's syndrome. They form a large number of polymorphic structures, including heterogeneous ionic pores in membranes as well as different types of fibrillar and globular structures on surfaces and in solution. Understanding the origin of these structures and the factors that influence their occurrence is of great biomedical interest because of the possible relationship between structure and pathogenicity. Here, we use atomic force microscopy (AFM) and molecular dynamics (MD) simulations to demonstrate that at room temperature a truncated Aβ peptide which is generated in vivo and shown to be toxic in vitro forms fibrillar structures on hydrophobic graphite surfaces, but not on hydrophilic mica or lipid bilayers. Our results suggest that the toxic pores and fibrillar polymorphic organizations can be explained in terms of the U-shaped β-strand-turn-β-strand structural motif observed for full length Aβ and other amyloids, as well as the physicochemical properties at the interfaces. The interactions of the hydrophobic, truncated Aβ with its environment illustrate that the universal amyloid motif can provide a link between the pore and fibrillar structures and indicate that surfaces with different physicochemical properties can shift the polymorphic landscape toward other conformational states.     

Effects of amount, location, and character of porosity on stiffness and strength of ceramic fiber composites via different processing

The porosity dependence of ceramic fiber composite Youngs modulus, and especially tensile strengths, is reviewed. Though limited, data shows markedly different porosity dependencies for composite matrices derived from: (1) chemical vapor infiltration (CVI), (2) preceramic polymer pyrolysis, or (3) hot pressing of powders. CVI results in initially limited, then accelerating, rates of property decreases as porosity increases, as for typical monolithic ceramics. In contrast, hot pressing and polymer pyrolysis result in the opposite behavior, i.e., high initial then diminishing rates of property decreases. This markedly differing behavior is attributed to pores being rounded and especially away from the fiber-matrix interfaces in CVI while in hot pressing and polymer pyrolysis fiber-matrix interface, cusp/lenticularshaped pores (more difficult to remove and dominant at lower porosity levels) are more detrimental to properties, similar to grain boundary pores in monolithic ceramics. Competition between such interfacial pores and those totally in the matrix in both their elimination and the effects of those pores remaining in the processed composite is consistent with data differences and scatter. Implications for properties achievable by the above 3 types of processing, as well as for sintering of composites and possible use of porous layers at the fiber-matrix interface to limit oxidative embrittlement are noted.     

The effect of antimony on the interfaces of cast AlSi-SiC composites

When the matrix alloy of an AlSi7Mg-SiC composite has been modified with antimony, the wetting properties of the system deteriorate. Using transmission electron microscopy and microanalysis it was found that this is probably related to the formation of a film-like amorphous antimony-rich oxide at the matrix-silicon carbide interfaces. Associated with this compound are pores at the interfaces. The film-like compound is generally intermixed with magnesium-aluminium spinel crystallites at the interfaces and with the neighbouring matrix. No antimony-rich phase is found away from the interfaces.     

Inverse Opal Scaffolds with Gradations in Mineral Content for Spatial Control of Osteogenesis

The design and fabrication of inverse opal scaffolds with gradations in mineral content to achieve spatial control of osteogenesis are described. The gradient in mineral content is established via the diffusion‐limited transport of hydroxyapatite nanoparticles in a closely packed lattice of gelatin microbeads. The mineral‐graded scaffold has an array of uniform pores and interconnected windows to facilitate efficient transport of nutrients and metabolic wastes, ensuring high cell viability. The graded distribution of mineral content can provide biochemical and mechanical cues for spatially regulating the osteogenic differentiation of adipose‐derived stromal cells. This new class of scaffolds holds promise for engineering the interfaces between mineralized and unmineralized tissues.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.     

Room-temperature gas-sensing ability of PtSi/porous Si Schottky junctions

Gas-sensing ability of n-type PtSi/porous Si Schottky junctions is investigated at room temperature. These junctions exhibit a breakdown-type current-voltage (I-V) curve at low reverse bias voltages (5-15 V) due to very large electric fringing fields (10/sup 5/-10/sup 6/ V/cm) developed at the sharp edges and the bottom of the pores. Experimental results for selected gases are presented. Gases with inherent dipole moments tend to decrease the breakdown voltage. Gases without dipole moments do not affect the I-V curve directly, but they can replace gas content inside the pores and decrease the dipole moment of the ambient gas, which results in an increase in breakdown voltage. Based on these experiments, the possible application of this structure as a gas sensor at room temperature is discussed.     

Bond-slip model for FRP-to-concrete bonded joints under external compression

The influence of compressive stresses exerted on FRP-concrete joints created by external strengthening of structural members on the performance of the system requires better understanding especially when mechanical devices are used to anchor the externally bonded reinforcement (EBR). The numerical modelling of those systems is a tool that permits insight into the performance of the corresponding interfaces and was used in the present study, essentially directed to analyse the effectiveness of EBR systems under compressive stresses normal to the composite surface applied to GFRP-to-concrete interfaces. The compressive stresses imposed on the GFRP-to-concrete interface model the effect produced by a mechanical anchorage system applied to the EBR system. An experimental program is described on which double-lap shear tests were performed that created normal stresses externally applied on the GFRP plates. A corresponding bond-slip model is proposed and the results of its introduction in the numerical analysis based in an available 3D finite element code are displayed, showing satisfactory agreement with the experimental data. The results also showed that lateral compressive stresses tend to increase the maximum bond stress of the interface and also originate a residual bond stress which has significant influence on the interface strength. Also, the strength of the interface increases with the increase of the bonded length which have consequences on the definition of the effective bond length.     

Equilibrium of partially dried porous media influenced by dissolved species and the development of new interfaces

The equilibrium state of partially dried porous media is modeled using continuum thermodynamics as a framework for identifying the equilibrium state. Entropy of the interfaces within the porous material are taken into account in the thermodynamic analysis of the system. Additionally, the effect on the equilibrium condition of increasing concentration of dissolved species in the fluid within the pores caused by moisture loss to the surroundings is examined. Both the presence of dissolved species in the fluid within the pores and the development of new interfaces during the drying process suppress internal vapor pressure, indicating that the equilibrium internal vapor pressure within the porous media will be lower than that of the boundary at equilibrium.     

Statistical analysis of particulate interface lengths in diffusion bonded joints in a metal-matrix composite

A statistical analysis has been made of the number and length of particle-particle (P-P), particle-matrix (P-M) and matrix matrix (M-M) interfaces present in solid state and liquid phase diffusion bonded joints in a particulate metal matrix composite (17 vol % SiC/Al-Li 8090 alloy). All solid state bonds had planar bond interfaces with the polished particle facets aligned in and parallel to the bond interface. The particle interface lengths in 400 m long sample lengths in the L direction in a bond interface could vary from zero to over 50% of the sample length, with typical values for a random sample of 8% and 22% for P-P and P-M interfaces, respectively.Liquid phase diffusion bonded joints contained a higher volume fraction of particles in the bond region, but the bond interface was non-planar and particle interfaces were not aligned.The significance of these results for the strength and processing of diffusion bonded composites is discussed.Vacation student, University of Surrey.