飞机复合材料阶梯式胶接结构的疲劳损伤与寿命

随着复合材料胶接技术在飞机主承力结构上的广泛应用,阶梯式胶接形式由于在实际工程过程中易于实现而成为复合材料结构修理和连接的主要形式。目前文献多集中于研究金属、复合材料以及两种材料混合的单搭接结构的疲劳性能,但对于具有高效载荷传递的阶梯式结构的疲劳耐久性研究较少,其疲劳失效机制尚需厘清。本文对多级阶梯复合材料胶接结构进行了拉伸疲劳试验研究,应力比为0. 1。在疲劳试验过程中观测了宏观裂纹的起始与扩展。根据试验数据拟合的S-N曲线,发现疲劳寿命随应力水平的增加而线性降低。在发现目视可见裂纹后至完全断裂,复合材料胶接结构仍具有10%的剩余寿命,说明当出现目视可见裂纹时,应及时维修或更换部件。从试样受力及破坏形式可知,复合材料胶接结构的剪切破坏是引起疲劳损伤的主要原因。通过阶梯状断口形貌分析,发现了内聚破坏、粘附破坏、基体开裂和分层四种典型破坏形式。     

G／Epoxy复合材料胶接强度研究

使用G／Epoxy作为底材研究了垫板、结构胶黏剂厚度和底材表面处理对拉伸剪切强度的影响。使用光学显微镜观察了断口形貌。结果表明加垫板能减小试验过程中由于加载偏心引起剥离应力,测试结果较大;结构胶黏剂的厚度和底材表面处理对拉伸剪切强度影响十分明显,随着厚度的增大而减小,经打磨表面裸露出纤维的试样拉伸剪切强度很低。结构胶黏剂厚度较小时以内聚破坏为主,随着厚度的增加破坏模式转变为粘接破坏。     

激光表面处理提高铝合金胶接接头强度的研究

粘接作为重要的汽车轻量化连接技术之一,胶接接头的强度和性能是我们关注的重点,胶接接头的强度和性能完全取决于胶粘剂接触的表面类型,因此在粘接之前对基材表面进行一定处理是粘接工艺中最重要的环节之一。金属的表面处理包括溶剂擦拭、机械打磨、化学清洗和酸蚀。激光表面处理是一种新型绿色环保的表面处理工艺,它可以高速有效的清洁材料表面附着物,并且改变材料表面微观结构及材料表面自由能及浸润性。从而提高粘接接头十字拉伸强度、单搭接拉伸剪切强度和接头耐水性能。通过激光处理,所有接头的破坏形式由界面破坏转为内聚破坏。对铝合金环氧结构胶2098G胶接接头而言,十字拉伸强度、剪切强度和水浴剪切强度,激光处理后比溶剂擦拭分别提高了17.8%,133.8%,88.1%。对铝合金聚氨酯结构胶TS6015胶接接头而言,十字拉伸强度、剪切强度和水浴剪切强度,激光处理后比溶剂擦拭分别提高了698%,225%,223%。激光表面处理有效的使铝合金胶接接头的强度达到胶的本体强度的94%~100%,是铝合金粘接的有效表面处理方法。     

铝合金表面阳极化处理及其胶接性能分析

采用磷酸电解质对铝合金板进行了阳极化处理并测试了其胶接性能,测试了阳极化过程中铝合金板的基本力学性能,观察了阳极化处理后的铝合金板的表面形貌,分析了阳极化处理后铝合金粘接副的胶接界面、拉伸剪切失效模式.结果表明,铝合金板经过酸洗、碱洗和阳极化等过程后,其破坏强度、屈服强度、弹性模量和断裂延伸率等力学性能基本保持不变.阳极化后铝合金板表面形成了一层凹凸不平、多孔结构氧化膜,胶接时胶黏剂能渗透进入该氧化膜,形成一层胶接过渡层,阳极化处理提高了铝合金粘接副之间的拉伸剪切强度,其拉伸剪切强度最大可提高1.76倍.阳极化处理后的铝合金板粘接副之间的破坏模式为混合破坏,即存在胶黏剂的剪切破坏,同时存在粘接界面的剥离破坏.     

结构对Sm_2Ce_2O_7-YSZ热障涂层残余应力及热冲击性能的影响

采用ANSYS软件分析了涂层结构对Sm_2Ce_2O_7-YSZ热障涂层热应力的影响。结果表明,在涂层表面存在较大的残余热应力和热冲击应力。径向残余热应力从试样中心至边沿逐渐降低,轴向和剪切残余应力在试样边沿明显增大。径向冲击应力与径向残余应力分布相似,但远大于径向残余应力。轴向冲击应力在试样边沿从压应力转变为拉应力,剪切冲击应力则急剧降低。涂层层数的增加有利于降低涂层的残余应力和冲击应力。     

碳纤维复合管-铝合金胶接接头拉伸性能研究

以直径为80 mm,壁厚为1 mm的碳纤维复合管-Al合金胶接接头为研究对象,通过整体拉伸试验比较了不同胶接长度、胶层厚度、外加紧固件对接头破坏载荷的影响。结果表明:胶接面的破坏载荷随着胶接长度的增大先增大后减小,随着胶层厚度的增大而降低。随着胶接长度和厚度的增加,胶接面由复合材料分层破坏和部分胶层的剪切破坏转向胶接界面剥离破坏。在胶接面上施加紧固件能够抵消部分附加弯矩引起的剥离应力,从而提高接头的破坏载荷。胶接长度为100 mm,胶层厚度为0.01 mm～0.02 mm,施加三条非均布紧固件的胶接面能够承受最大为255 kN的破坏载荷。     

胶铆连接性能研究

胶铆连接集中了胶接的连续密封性能、耐疲劳性能和铆接的抗剥离性能。本文选取SY－H2糊状胶粘剂,综合对比了胶接试样、铆接试样和胶铆试样的力学性能和耐疲劳性能。胶接能够提供连续的面连接,有利于减轻结构重量。胶接和胶铆试样的疲劳性能优于铆接试样,在相同频率下的破坏应力或循环周期都高于铆接试样。     

复合材料T型接头力学性能分析及优化研究

T型接头作为常见的复合材料结构连接型式,其力学性能直接关系到结构的安全性。建立了考虑胶层的复合材料T型接头有限元模型,将仿真结果与试验图像进行对比,其变形形式和破坏模式与试验吻合较好。研究表明,在垂向载荷作用下,T型接头呈现"S"型弯曲;在蒙皮折角处以及端部胶接处出现应力集中;胶层剪应力两端大,中间出现低应力槽型区,弯曲正应力、等效应力呈现出双峰值特征;T型接头最有可能出现的破坏形式为胶层与蒙皮之间剥离,而芯材则由于应力集中引起剪切破坏或拉伸破坏。在折角处倒圆、在胶接处光滑过渡可以明显消除应力集中。     

热-力载荷下复合材料/金属双面胶接接头界面力学模型

在温度和静拉伸载荷共同作用下,考虑胶层的材料非线性,建立了复合材料/金属双面胶接接头界面的力学分析模型,推导出弹性响应和塑性响应下胶层剪应力的分段表达式,使用胶层最大剪应变失效准则计算出胶层主导破坏的结构极限载荷,并与有限元数值结果进行对比和验证。分析表明,双面胶接接头应力分析理论模型与相关简化假设正确、合理。在此基础上,研究了复合材料/金属双面胶接接头在热-力载荷下的胶层剪应力分布特点、破坏模式和失效机理,为胶接结构的承载能力分析及结构改进设计提供理论依据。     

温度对芳纶纤维/EP复合材料与EPDM界面粘接性能影响研究

以芳纶纤维/环氧树脂(F-3A/EP)缠绕层作为固体火箭发动机用复合材料壳体、三元乙丙橡胶(EPDM)作为绝热层,采用适宜的胶粘剂制备壳体/EPDM、Φ480 mm壳体/EPDM和大尺寸壳体/EPDM胶接件,并着重探讨了固化条件(温度、时间等)对上述胶接件界面拉伸强度、剪切强度和剥离强度等影响。研究结果表明:3种类型的试样经130℃固化6 h后,其界面拉伸强度(3.02 MPa)、剪切强度(5.42 MPa)均明显提高,而Φ480 mm壳体/EPDM和大尺寸壳体/EPDM胶接件的剥离强度均达到5.84 N/mm,说明该胶粘剂及固化工艺完全满足壳体对EPDM绝热层的界面粘接要求;继续提高固化温度或延长固化时间,并不能显著提高胶接件的剥离强度,但对胶接件的成型工艺要求明显提高。     

Residual Stress Development in Adhesive Joints Subjected to Thermal Cycling

The effect of thermal cycling on the state of residual stress in thermoviscoelastic polymeric materials bonded to stiff elastic substrates was investigated using numerical techniques, including finite element methods. The work explored the relationship between a cyclic temperature environment, temperature-dependent viscoelastic behavior of polymers, and thermal stresses induced in a bimaterial system. Due to the complexity of developing a closed-form solution for a system with time- and temperature-dependent material properties, and time-varying temperature and coupled boundary conditions, numerical techniques were used to acquire approximate solutions.

The results indicate that residual stresses in an elastic-viscoelastic bimaterial system incrementally shift over time when subjected to thermal cycling. Potentially damaging tensile axial and peel stresses develop over time as a result of viscoelastic response to thermal stresses induced in the polymeric layer. The applied strain energy release rate at the ends of layered or sandwich specimens is shown to increase as axial stress develops. The rate of these changes is dependent upon the thermal cycling profile and the adhesive's thermo-mechancial response. Discussion of the results focuses on the possiblility that the increasing tensile residual stresses induced in an adhesive bond subjected by thermal cycling may lead to damage and debonding, thus reducing bond durability.     

Effects of moisture and temperature ageing on reliability of interfacial adhesion with black copper oxide substrate

The reliability of adhesion performance of bare Cu, as-deposited and surface-hardened black oxide coatings on Cu substrates was studied. The interfacial adhesion with a polyimide adhesive tape and an epoxy moulding compound was measured using the button shear and tape peel tests after hygrothermal ageing in an autoclave, high temperature ageing and thermal cycles. Moisture adsorption and desorption studies at different aging times suggested that the black oxide coating was effective in reducing the moisture adsorption. The bond strengths for all substrates remained almost unchanged after thermal ageing at 150°C for 8 h. Thermal cycling between ?50°C and 150°C for 500 cycles reduced by about 20% the button shear strength of the as-deposited black oxide substrate, but it did change much the bonding performance of the bare Cu substrate. Hygrothermal ageing at 121°C/100% RH in an autoclave was most detrimental to adhesion performance because of the combined effect of elevated temperature and high humidity. The reduction in button shear strength after the initial ageing for 48 h was 50–67%, depending on the type of coating. In all accelerated ageing tests, the residual interfacial bond strengths were consistently much higher for the black-oxide-coated substrates than the bare Cu surface, confirming a higher reliability of black oxide coating. Fracture surfaces analysis of tape-peeled bare copper substrates after 500 cycles of thermal loading revealed a transition in failure mechanism from interfacial to cohesive failure. In contrast, the failure mechanism remained unchanged for black-oxide-coated substrates. The observations made from the button shear and tape peel tests were generally different because of the different fracture modes involved.     

Avoidance of fabricational thermal residual stresses in co-cure bonded metal-composite hybrid structures

In this work, a smart cure cycle with cooling, polymerization and reheating was devised to nearly completely eliminate thermal residual stresses in the bonding layer of the co-cure bonded hybrid structure. In situ dielectrometry cure monitoring, DSC experiments and rheometric measurements were performed to investigate the physical state and the cure kinetics of the neat epoxy resin in the carbon fiber/epoxy composite materials. From the experimental results, an optimal cooling point in the cure cycle was obtained. Also, process parameters such as cooling rate, polymerization temperature and polymerization time in the curing process were investigated. Then, the thermal residual stresses were estimated by measuring the curvatures of co-cure bonded steel/composite strips and their effects on the static lap-shear strengths of co-cure bonded steel/composite lap joints were measured. Also, the effects of thermal residual stresses on the tensile strength, the interlaminar shear strength and the interlaminar fracture toughness of the composite material itself were measured using tensile, short beam shear and double cantilever beam tests. From these results, it was found that the smart cure cycle with cooling, polymerization and reheating eliminated the thermal residual stresses completely and improved the interfacial strength of the co-cure bonded hybrid structures, as well as the tensile strength of the composite structures.     

Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y-Mg-Si-Al-O-N system produced silicon nitride specimens with very similar micro- structures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and grain bridging.     

Effects of thermal annealing and Si incorporation on bonding structure and fracture properties of diamond-like carbon films

The effects of thermal annealing and Si incorporation on the structure and properties of diamond-like carbon (DLC) films were investigated. As-deposited DLC film (DLC) and Si incorporated DLC film (Si-DLC), both with and without thermal annealing, were analyzed for bonding structure, residual stress, film thickness, elastic modulus and fracture properties using Raman spectroscopy, wafer curvature, nanoindentation, four-point bend fracture testing, and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy clearly showed that thermal annealing of DLC films promotes more sp² bonding character, whereas Si incorporation into the films promotes more sp³ bonding character. Interfacial fracture energies, film hardness and elastic modulus, and residual film stress were all found to vary strongly with the degree of sp³ bonding in the DLC film. These changes in mechanical properties are rationalized in terms of the degree of three dimensional inter-links within the atomic bond network.     

The preparation and performance of short carbon fiber reinforced adhesive for bonding carbon/carbon composites

A short carbon fiber reinforced adhesive for bonding carbon/carbon composites was developed. We found that when the thickness of the bonding layer was 80 μm, the concentration of short carbon fiber was 0.2 wt.%, and the heat-treatment temperature was 1000 °C, the adhesive could operate below 1700 °C and endure 20 times of thermal shock circles at 1500 °C. Finite element and micrograph analysis indicated that the bonding strength was larger than the interlaminar shear strength of carbon/carbon substrate, so that the fracture did not occur in the bonding layer but the carbon/carbon substrate. Weibull distribution analysis results showed that the Weibull modulus was 21.56 and the bonding strength was 11.43 MPa. We investigated that short carbon fiber could advance the tensile strength and thermal shock resistance of the adhesive, release residual stress and inhibit extension of micro-crack in the bonding layer.     

Variables that Determine the Fiber‐Matrix Bond Strength in Ethylene‐Type Ionomer Composites

The effects of some variables, namely, ion concentration, matrix tensile strength, matrix yield strength and the matrix tensile modulus on the fiber‐matrix bonding strength were determined for six ionomers (coded PEA‐1 to PEA‐6) bonded to surface‐modified poly(p‐phenylene terephthalamide) (PPTA) fibers. The results obtained show that the mean bonding shear strength of the ionomers correlates well with both their ultimate tensile strengths and their tensile yield stresses. However, correlation of the bonding shear strengths with the matrix yield stresses reveals that the bonding shear strength was about 1.1 times that of the matrix tensile stress. Failure criteria for all the materials predict maximum shear stress to be either 0.5 or 0.577 of the tensile yield stress, hence a value greater than unity cannot be interpreted nor theoretically justified. It was found that the bonding shear strength of the ethylene‐type ionomer PEA‐6 compared to carboxymethyl surface‐modified PPTA is about 20% lower than the bonding shear strength of this resin against sized PPTA fibers. The reduction of entanglements and/or ionic crosslinking across the bound polymer/bulk polymer interface leads to a weak interface with a subsequent decrease in the measured shear strength.     

The fracture toughness of alumina coatings plasma-sprayed at different in situ temperatures

Alumina coatings were prepared by atmospheric plasma spraying through controlling the surface temperature of the coatings during spraying. Both the polished and fractured cross-section microstructures of the coatings were characterized by scanning electron microscopy (SEM). The phase structures of the coatings and the feedstock were analyzed by X-ray diffraction technique (XRD). The microstructure and phase structure of the coatings prepared at different substrate temperatures were examined. SEM observations show that the intersplat bonding within the coatings was significantly improved by increasing the substrate temperature. The fracture toughness of the deposits was measured by indentation methods. For the coatings prepared at low substrate temperatures, the fracture toughness increased with the substrate temperature due to the improvement in the intersplat bonding. However, a significant decrease in the fracture toughness was found for the coatings prepared at high substrate temperatures. The change in phase structure of the coatings suggested that the residual tensile stress mainly resulted from phase transformation from γ-alumina to α-alumina at high substrate temperature should answer for the decline in the fracture toughness.     

真空热循环对T700/5224复合材料力学性能的影响

为了揭示T700/5224复合材料在真空热循环作用下的力学性能演化规律,本文在10-5Pa,-140～140℃条件下,对T700/5224复合材料的质损率和力学性能进行了表征,并采用SEM分析方法对试样表面形貌、断口特征及表层化学结构进行了研究。研究结果表明,随真空热循环次数的增加,T700/5224复合材料的质损率升高,经29次真空热循环后上升幅度降低,而经157次真空热循环后趋于平缓。随着真空热循环次数的增加,T700/5224复合材料90°拉伸强度首先降低,48次热循环后开始升高,198次热循环后趋于平缓;弯曲强度在48次真空热循环前变化不明显,48次真空热循环后开始上升,95次真空热循环后开始降低,经198次真空热循环后趋于平缓;层剪强度变化不明显。     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.