陶瓷基复合材料结构固有频率与内部损伤分析

通过室温条件下的循环加卸载试验,研究编织SiC/SiC复合材料固有频率特性及内部损伤演化过程。结果表明:固有频率随循环加卸载过程中峰值应力的增大而减小,通过分别定义频率衰退参数Φ与模量衰退参数D表征了复合材料固有频率与弹性模量的折减程度。基于细观力学理论对编织陶瓷基复合材料循环加卸载迟滞回线进行理论计算,理论模拟结果与试验数据良好吻合。同时,分析计算结果还发现,随着循环加卸载过程中Φ的增加,材料弹性模量衰退参数D、界面脱粘比2l_d/l_c和纤维断裂概率q均呈明显上升趋势,当陶瓷基复合材料结构固有频率衰退1%时,基体出现开裂,界面发生脱粘;当固有频率衰退4.17%时,2l_d/l_c上升至1（即完全脱粘）,q增加到2.5%,这表明复合材料固有频率的变化可以反映出材料内部的损伤失效过程。     

Rolling consolidation of metal matrix composites

Transverse rolling of metal matrix composite precursor wires is proposed as a consolidation technique for making sheets. Rolling in the transverse direction to the fiber orientation is analytically shown to be feasible, and longitudinal rolling results in fiber breakage. Plasticity analysis is conducted using Hill's general yield criterion for an isotropic materials and the associated Levy-Miles equations modified for plane strain conditions. The slab method is used to calculate the stresses in the material, and the effects of rolling parameters on the principal stress ratio are investigated.At the microscopic level, an elastic-plastic finite element formulation and a computation procedure are presented. Individual fibers are modeled to determine the stress state around each fiber. The principal stress ratio is suggested as a parameter that determines the tendency for void formation due to debonding and fiber breakage; finite element analysis is used to determine the effects of the principal stress ratio on the fiber-matrix interfacial stresses in the micromechanics model. The analysis determines the deformed mesh, plastic zone propagation and the stresses at the interface as a function of volume fraction and principal stress ratio. Interfacial stresses are assumed to be responsible for debonding during the deformation of metal matrix composites. This assumption and the results of the analysis provide guidelines for defining, the level of the biaxial stress field in the plane transverse to the fibers during rolling that will minimize interfacial fiber-matrix stresses.     

Impedance-based wireless debonding condition monitoring of CFRP laminated concrete structures

Carbon fiber reinforced polymer (CFRP) laminated concrete structures are used widely in a range of engineering fields because of their many advantages. However they always carry the risk of structural collapse initiated from the debonding conditions that might occur between the CFRP and concrete surface. This study employed an electro-mechanical impedance-based wireless structural health monitoring (SHM) technique by applying PZT ceramic patches to identify the debonding conditions of a CFRP laminated reinforced concrete beam. In the experimental study, the CFRP-reinforced concrete specimens were fabricated and the impedance signals were measured from the wireless impedance sensor node according to the different debonding conditions between the concrete and CFRP. Cross correlation (CC)-based data analysis was conducted to quantify the changes in impedance measured at the PZT patches due to the debonding conditions. The results confirmed that an impedance-based wireless SHM technique can be used effectively for monitoring the debonding of CFRP laminated concrete structures.     

Effects of Fiber Volume Fraction on Transverse Tensile Properties of SiC/Ti-6Al-4V Composites

采用有限元法分析了在残余应力和外加横向载荷作用下纤维体积分数对SiC/Ti-6Al-4V复合材料横向拉伸行为的影响。通过弹簧连接纤维与基体界面的重合节点来模拟界面脱粘。结果表明,在界面结合强度一定时,界面脱粘应力(对应于应力-应变曲线上应变的跳跃)受0°方向界面径向残余应力影响较大;在界面脱粘先于基体屈服时,复合材料失效应力(对应于应力-应变曲线上的水平部分)主要取决于纤维体积分数,且体积分数越低,失效应力越高。     

A model for predicting micromechanical interfacial adhesion in polymer composites

The mechanical behavior of fiber-reinforced composites is largely determined by adhesion at the fiber-matrix interface. Thus, a fundamental understanding of the interfacial region and a quantitative characterization of the level of interfacial adhesion can contribute to an evaluation of the mechanical behavior and performance of composite materials. Among numerous techniques for interface characterization, the vibration damping method has attracted continually more attention because it provides sensitive and nondestructive detection of the interfacial region in composites. In the research presented here, a new optical system for measuring vibration damping was introduced, and a model for evaluating the interfacial adhesion between fiber and matrix from a damping parameter was developed. A quantitative relationship between the dynamic (vibration damping) and static (interfacial shear strength) adhesion measurements was established. The experimental data from glass-fiber-reinforced epoxy resin composites with different interfacial treatments showed good agreement with the theoretical model.     

NDE of smart structures using multimode fiber optic vibration sensor

There are many conventional methods which may be used to monitor the health of structures — of which vibration monitoring is effective and can be used for special structures such as aircraft wings. A smart structure system based on fiber optic vibration sensors has been developed to monitor structural damage. A method based on the detection of spatial speckle of a multimode optical fiber is described. A multimode optical fiber with a diameter of 200/230 μm is used in the present experiment. The theoretical analysis and the experimental tests are described. The fiber optic sensors have been embedded in the carbon/epoxy composite specimens and surface mounted on the surface of aluminum specimens. Furthermore, experiments to examine the different types of damage to these specimens have been carried out. The results show that this kind of multimode fiber optic sensor can be used in the non-destructive evaluation of smart materials.     

Mechanical properties of bone-shaped-short-fiber reinforced composites

Short-fiber composites usually have low strength and toughness relative to continuous fiber composites, an intrinsic problem caused by discontinuities at fiber ends and interfacial debonding. In this work a model polyethylene bone-shaped-short (BSS) fiber-reinforced polyester–matrix composite was fabricated to prove that fiber morphology, instead of interfacial strength, solves this problem. Experimental tensile and fracture toughness test results show that BSS fibers can bridge matrix cracks more effectively, and consume many times more energy when pulled out, than conventional straight short (CSS) fibers. This leads to both higher strength and fracture toughness for the BSS-fiber composites. A computational model was developed to simulate crack propagation in both BSS- and CSS-fiber composites, accounting for stress concentrations, interface debonding, and fiber pull-out. Model predictions were validated by experimental results and will be useful in optimizing BSS-fiber morphology and other material system parameters.     

The mechanisms of interfacial failure for lateral force-sensing microindentation test: finite element analysis

The interfacial bond strength for coatings and composites can be quantitatively determined using a newly developed lateral force-sensing microindentation method. In this study, a finite element analysis was made to investigate the interfacial failure mechanisms for Cu–ceramic and Al alloy–ceramic interfaces. The model is validated by comparing obtained results of the finite element analysis with analytical solutions. Two different interfacial failure mechanisms, depending on material properties and microindentation positions, are proposed. As demonstrated, interfacial debonding may result from shear stress or a coupling of tensile stress and shear stress at the interface, corresponding to material “pile-up” deformation or “sink-in” deformation. In addition, the high sensitivity of the lateral force response to interfacial debonding, associated with two different interfacial failure mechanisms, is also examined.     

循环变形提高SiC纤维增强铝基复合材料强度及塑性Ⅱ.理论分析

采用断裂力学方法获得了纤维增强复合材料强度与脱粘长度、纤维临界长度以及纤维体积分数的定量关系。该公式较好地预测了纤维的临界长度以及强度与纤维体积分数的关系，并再现了复合材料混合定则。该公式也较好地解释了丝状复合材料强度随短期循环变形载荷与周次增加而增加的现象。其原因是在循环变形中，纤维与基体界面结合强度发生变化，导致纤维临界长度与脱粘长度发生变化。从而使复合材料强度增加，但这种增加是有限的和有范围的。循环变形的发展最终导致强度下降。     

采用顶出方法研究纤维增强钛基复合材料的界面断裂韧性

为了估计单向SiC纤维增强钛基复合材料的界面断裂韧性G_IIc,本文提出了一个关于单根纤维顶出试验的新模型,在本模型中,界面脱粘开始于试样的底端面。本文以断裂力学为基础推导出了G_IIc 的理论公式,并且讨论了几个关键因素对G_IIc的影响,如裂纹扩展所需的外加应力,裂纹长度以及界面的摩擦剪切应力。并且运用此模型预测了复合材料 Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834 and SCS/Timetal 21s 的界面断裂韧性,并与以前的有限元结果进行了比较。结果显示,对于脱粘起始于试样的底端面的顶出试验,本模型能较可靠地预测钛基复合材料的界面断裂韧性。     

碳纤维表面超声振荡辅助电镀镍涂层工艺及其参数的影响规律

碳纤维增强铝基复合材料具有诸多优良性能,在航空航天以及民用领域有广阔的应用前景。在碳纤维表面电镀镍涂层,可有效改善碳纤维与铝基体的润湿性,并抑制基体对纤维的损伤。镍涂层的质量与改善效果密切相关。通过预氧化处理并结合超声振荡辅助电镀的方法在纤维表面沉积镍涂层,解决碳纤维在电镀过程中因纤维束丝难以分散而出现的"黑心"问题。在此基础上,研究电镀工艺参数对涂层质量的影响规律,并对优化工艺参数后制备的镍涂层与碳纤维的结合强度进行评价。结果表明:使用添加剂可提高镍涂层与碳纤维之间的结合力,当电镀液pH值调整到3~4、电流密度大于0.3 A/dm^2、电镀液温度控制在20~40℃时,所制备的镍涂层均匀致密。     

Detecting debonding flaws at the epoxy–concrete interfaces in near-surface mounted CFRP strengthening beams using the impact–echo method

This paper discusses the feasibility of using the impact–echo method (IE method) to assess the debonding flaws at the epoxy–concrete interfaces of near-surface mounted carbon fiber reinforced polymers (NSM CFRPs). The effectiveness of NSM CFRP strengthening is influenced by whether epoxy bonds CFRP bars and existing concrete structures effectively and thoroughly, thereby mutually transmitting the stress in the reinforced zones with CFRP bars. Because of poor construction quality, aging, and deterioration, debonding occurs between epoxy and concrete surfaces, severely inhibiting the reinforcement effectiveness of NSM CFRPs. The IE method is based on the principle of stress waves and was adopted in this study as the nondestructive debonding flaw assessment technique. To clarify the characteristics of the stress wave propagation caused by impact forces on NSM CFRP structures, numerical analysis and experimental test were performed on the bar-like epoxy specimens with a cross-sectional size of 13×13 mm. The types of specimens used in this study included the bar-like pure epoxy specimen, bar-like epoxy specimens containing CFRP bars, and the concrete beams containing bar-like epoxy specimens with and without CFRP bars. For the concrete beams embedded with bar-like epoxy specimens, various debonding statuses at the epoxy–concrete surfaces were considered. The numerical analysis revealed that the impact responses in the bar-like pure epoxy specimen and bar-like epoxy specimens containing CFRP bars were dominated by the first few cross-sectional modes of vibration, and the IE test verified this result. The numerical and experimental results indicated that for the concrete beams containing bar-like epoxy specimens with and without CFRP bars and without debonding flaws, the impact response spectra each featured one high-amplitude peak at the fundamental mode frequency, referred to as the dominant frequency. When debonding occurred at the epoxy–concrete interfaces of the concrete beams containing bar-like epoxy specimens with and without CFRP bars, the impact responses showed that the dominant frequencies decreased significantly. When the dominant frequencies instead increased to levels nearly equal to those of the fundamental frequencies of the bar-like epoxy specimens, the epoxy thoroughly debonded from the concrete interfaces. According to the characteristics of the aforementioned impact responses, the IE method is capable of detecting the debonding flaws at epoxy–concrete interfaces in NSM CFRP strengthening.     

Monitoring fatigue cracks in gears

Vibration analysis is the most common means of gear monitoring and diagnostics. Gear vibration is affected by faults but the signal is usually picked up at the case, where it is also affected by the structural response. An appropriate filtering function is therefore proposed to recover the torsional gear vibration from the case vibration signal. The restored gear vibration can then be used with greater confidence than case vibration both for particular diagnostics purposes like crack detection and for more general objectives. This technique and its possible advantages in fatigue crack detection are illustrated in the paper.     

Macroscopic Mechanical Characterization of SMAs Fiber-Reinforced Hybrid Composite Under Uniaxial Loading

This paper presents an experimental and theoretical investigation on the macroscopic mechanical behavior of shape memory alloys (SMAs) fiber-reinforced glass/resin composite subject to uniaxial loading at ambient temperature. A series of unidirectional SMAs reinforced composite laminates is fabricated through vacuum-assisted resin injection. Scanning electron microscopy is conducted to evaluate the interfacial cohesive quality between SMAs fiber and matrix. A theoretical model is proposed based on the SMAs phase transformation model and rule of mixture. Uniaxial tensile tests are performed to study the effects of weak interface and SMAs fiber volume fraction on the effective modulus of composite. Failure morphology of composite is discussed based on the observation using digital HF microscope. Due to the effects of phase transformation and weak interface, the overall stiffness of SMAs composite at the second stage is on average 10% lower than theoretical results. The rupture elongation of experimental result is approximately 13% higher than theoretical result. The local interfacial debonding between SMAs fiber and glass/resin matrix is the main failure mode.     

基于二维激光扫描的复合材料层合板热振试验装置研发与应用

研发了一套基于二维激光扫描测试装置的复合材料层合板热振试验装置。利用PLC控制器、触摸屏、滑台型导轨、步进电机、激振器、功率放大器、激光测振仪、加热及温控装置、振动控制仪及数据采集分析装置等,建立了满足层合板高温测振需求的硬件系统。基于PLC梯形图语言编写了二维激光扫描测试装置的控制软件,并详细介绍了手动、自动控制模块对应的操作界面与测控优势。以HF10碳纤维/树脂基层合板试件为例,对该装置进行了研究。结果表明:该装置可显著提高热环境下层合板结构模态振型的测试效率,且有利于研究不同温度对结构振动特性的影响。研究发现热环境对层合板固有频率和阻尼性能有着复杂影响,且随着温度的不断升高,模态阻尼比呈现减速增大的趋势。     

TiO2纳米管薄膜的脱层现象（英文）

分别通过曲率法和基体拉伸应变测试方法研究TiO2纳米管薄膜的残余应力和脱层行为。结果表明:TiO2纳米管薄膜的内残余应力为-54MPa。TiO2纳米管薄膜室温样、250°C退火样和400°C退火样的脱层出现点的应变依次为2.6%、5.1%和8.6%,半径依次为27.5、17.1和19.4μm。TiO2纳米管薄膜室温样、250°C退火样和400°C退火样的真实临界脱层应力为220.4、394.5和627.9MPa。在脱层条件下,对界面剪滞模型进行修订,并对TiO2纳米管薄膜界面剪切强度进行多项式拟合。由于拟合结果与裂纹密度的分析结果能很好吻合,因此界面剪滞模型的修订方程和多项式拟合方程均为可信的。     

3种材料在柔性机械臂振动抑制中的特性对比

在所设计的柔性机械臂的基础上,利用SolidWorks Simulation对合金钢、铝合金、碳纤维增强环氧树脂基复合材料3种结构材料的机械臂进行线性动态分析,分别从谐波分析和瞬态振动分析两个方面对机械臂进行仿真。结果表明,碳纤维增强环氧树脂基复合材料对机械臂具有明显的减振作用,其振动衰减速度及振动衰减效果要优于传统合金材料,为机械臂的设计及振动抑制提供了参考。     

Novel ancillary device for minimising machining vibrations in thin wall assemblies

Milling thin wall structures is challenging due to their low stiffness and hence consequential vibration problems. Most of the research in this area was focussed on minimising chatter vibrations—either through generation of stability lobes or by employing targeted damping solutions such as piezoelectric damping; such solutions are suitable only for damping resonant vibrations. However, most of the thin wall structures also get poor surface finish due to forced vibrations either at tooth cutting frequency or tool natural frequencies. In this work, relying on the importance of improving mass and stiffness for greater vibration reduction in milling circular thin-wall components, an innovative articulated device pre-tensioned by torsion springs is proposed. The concept is novel in the sense that it is compact and light-weight and can be used on any shape of thin wall structure. Employing such a device does not alter the nature of dynamic characteristic of the structure thus ensuring better control of achieved dimension of structure. Significant (8 times) vibration reduction was observed using proposed device.     

Integrity evaluation of glass-fiber composites with varied fiber/matrix interfacial strength using acoustic emission

An advanced acoustic emission analysis method was used to study the dynamics and sequence of microfractures in uni-directional glass-fiber reinforced plastics (UD-GFRPs). UD-GFRPs (60 wt% fiber) with different interfacial qualities were prepared by adding a small amount (4 or 8 wt%) of paraffin wax into vinylester matrix without affecting much the mechanical properties of the matrix. The orientation dependence of both the P-wave velocity and attenuation were accurately measured by the laser ultrasonic method, and incorporated into the acoustic emission analysis of visco-elastic media. The analysis iteratively compares the acoustic emission signals (measured as out-of-plane displacements) with theoretical waveforms calculated assuming the time history and type of an acoustic emission source. The waveform simulation allows us to estimate the fracture dynamics and the sequence of the Mode-I fiber fracture (Type 1), Mode-I debonding (Type 2) and Mode-II disbonding (Type 3). The fiber fracture in UD-GFRPs without wax started at 0.1% strain. The initiation strain increased to 0.25 and 0.3% in the specimens with wax. The specimens with lower interfacial strength showed rapid succession of microfractures at higher stresses (above 100–200 MPa). Fracture dynamics showed distinct differences by wax addition. Using the present analysis method, we can determine quantitatively effects of interfacial quality on the microfracture processes and assess the integrity of UD-GFRPs.     

Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.