服役低温老化对铝合金-玄武岩纤维增强树脂复合材料粘接接头力学性能的影响及失效预测

为了给铝合金-玄武岩纤维增强树脂(BFRP)复合材料粘接结构在汽车工业中的应用提供参考和指导,加工了铝合金-BFRP复合材料粘接接头。结合汽车服役中的温度区间,选取?10℃和?40℃的低温老化环境,对接头进行0、10、20、30天的老化。对老化后的粘接接头进行准静态拉伸试验和剪切试验,得到不同老化时间下铝合金-BFRP粘接接头的准静态失效强度。结合DSC和FTIR分析低温老化对BFRP复合材料的影响,并对粘接接头的失效断面进行宏观分析和SEM分析。结果表明:在低温老化环境中,胶粘剂与BFRP复合材料的化学性质受低温老化作用影响不大,BFRP中的官能团与玻璃化转变温度(T_g)没有发生明显的变化,接头的失效强度和失效模式主要受胶粘剂与粘接基材的热应力影响。对于拉伸接头,随着低温老化时间的增加,BFRP复合材料纤维与树脂基体间的结合力降低,铝合金-BFRP复合材料接头的失效断面中纤维撕裂的比例逐渐减少,拉伸接头失效强度逐渐下降。老化后剪切接头仍为内聚失效,BFRP复合材料的低温老化对铝合金-BFRP复合材料剪切接头的失效强度几乎没有影响,剪切接头失效强度的下降主要是胶粘剂与粘接基材热膨胀系数不一致引起的热应力的影响。采用二次应力准则公式对?10℃和?40℃低温环境下,拉应力、剪应力值随老化时间的变化规律进行了拟合,在此失效准则的基础上,根据响应面原理,建立接头失效强度随老化时间变化的三维曲面,为粘接技术在车身结构中的工程应用提供参考。      

胶粘剂在粘接接头内热分解动力学的计算

为了研究粘接接头内胶粘剂的耐热性能,采用X射线能谱分析确定了不同条件下粘接接头内胶粘剂的元素组成及其变化行为,利用X射线能谱计算了胶粘剂的热失重率,进而计算出聚酰亚胺薄膜粘接接头内胶粘剂的热分解动力学,并与空气环境下胶粘剂热分解活化能进行了比较.计算结果表明,粘接接头内胶粘剂的热分解速率低于空气环境下胶粘剂热分解速率,这种分析测试方法为原位表征粘接接头内胶粘剂耐热性能提供了一种新的分析方法.     

不同温度下胶粘剂粘接接头表界面元素变化行为的EDX分析

EDX 是一种研究粘接接头和聚合物基复合材料表界面性能的分析测试方法。利用EDX 对粘接接头表界面主要元素变化行为进行分析研究, 表明C/ C 复合材料从常温到900 ℃, Z 向的碳元素含量始终低于X 、Y向, 这是由C/ C 复合材料的性质决定的。粘接接头的破坏形式, 是由常温的C/ C 复合材料破坏转变为200 ℃及其以上温度的胶粘剂内聚破坏的。C/ C 复合材料和胶粘剂表面碳、硅和氧元素变化表明: 当温度低于550 ℃ 时,C/ C 复合材料和胶粘剂的热分解速率相似; 当温度高于550 ℃时, 胶粘剂的热分解速率明显快于C/ C 复合材料。      

高温老化对玄武岩纤维增强树脂复合材料-铝合金单搭接接头失效的影响

为了研究持续高温环境对车用新材料粘接结构力学性能的影响,加工了铝合金-铝合金（Al-Al）和玄武岩纤维增强树脂复合材料-铝合金（BFRP-Al）单搭接接头,在高温（80℃）环境下进行了0天（未老化）、5天、10天、15天的老化实验,并对胶粘剂和BFRP复合材料进行了DSC和FTIR测试,分析高温老化后胶粘剂、BFRP复合材料的玻璃化转变温度（T_g）和化学成分变化,通过准静态拉伸测试获得老化后接头的失效载荷,并对其失效模式进行分析。研究结果表明:高温环境下,胶粘剂会发生后固化及氧化反应,BFRP复合材料发生热分解及氧化反应;Al-Al接头的失效载荷随老化周期的增加而不断增大,老化前后的失效模式均为内聚失效,其性能变化主要由胶粘剂决定;BFRP-Al接头的失效载荷先增加后减小,不同老化周期的接头均发生内聚和撕裂的混合失效,其性能变化由胶粘剂和BFRP复合材料共同作用决定,且随着老化周期的增加,BFRP复合材料撕裂面积不断增大,BFRP-Al接头的失效模式越来越倾向于玄武岩纤维/树脂界面的破坏,BFRP复合材料老化对接头失效载荷的影响越来越显著。      

环氧树脂湿热老化机理的正电子湮没实验

采用热失重(TGA)、热机械分析(DMA)、正电子湮没技术(PAL)对环氧树脂618/DDS体系湿热老化前后的性能进行测试,测试了体系湿热老化前后的吸水率和介电性能,对环氧树脂618/DDS体系的吸水机理进行了研究。结果表明,环氧树脂618/DDS体系的吸水率为3.5%,湿热老化后对体系的热失重温度没有影响,湿热老化后环氧树脂618/DDS体系损耗模量降低了250MPa,玻璃化温度降低了大约50℃,损耗峰由湿热老化前的一个峰变为两个峰。吸水后介电常数增大,介电损耗值增大。湿热老化后自由体积尺寸减小,自由体积浓度增大,体系中的水分起着增塑剂的作用,导致基体中形成更多的微裂纹。     

固体火箭发动机绝热层粘接性能影响因素分析

考察了胶粘剂、粘接工艺、硫化温度和硫化压力等因素对三元乙丙绝热层粘接性能的影响,发现不同胶粘剂对绝热层的粘接性能有显著影响,采用适当的胶粘剂(B201)可使粘接强度达到3.05MPa,在保证绝热层正硫化的条件下,降低绝热层的硫化温度以及提高气囊压力对提高粘接强度有利,在绝热层中加入带有极性的填料可以提高绝热层与发动机壳体的粘接强度。在150℃硫化温度、3.5MPa硫化压力,可使绝热层与壳体的粘接强度达到3.42MPa。     

膜组件浇铸用室温固化环氧树脂耐热胶粘剂的研究

针对耐高温膜组件的浇铸,研制了可室温固化环氧树脂耐热胶粘剂.探讨了环氧树脂、固化剂的种类以及固化剂和稀释剂添加量对胶粘剂性能的影响.考察了胶粘剂的粘接性能,并通过差示量热扫描仪(DSC)分析了固化反应过程中的放热情况及固化产物的耐热性.结果表明:双酚F树脂与A105固化剂配制的胶粘剂耐热性好,黏度低,粘接性能优良,当固化剂质量分数为28%时,固化物的玻璃化温度Tg可达91℃.     

聚酯镀铝膜的湿热老化机理

针对聚酯(PET)镀铝膜在湿热条件下表面腐蚀脱落的现象及其随老化时间的变化规律,分别从物理老化和化学老化角度研究了PET本体膜在湿热条件下的玻璃化转变、熔融行为、特征基团以及相对分子质量的变化。结果表明,PET本体膜物理老化显著,玻璃化转变温度在不同湿热条件下随老化时间出现不同程度的上升;而化学老化,即相对分子质量和特征基团(酯基)的变化并不明显。湿度是影响镀铝层腐蚀脱落的主要原因,而与温度相关的PET本体膜自身老化无关。扫描电镜结果显示,镀铝层腐蚀首先发生在缺陷部位,并形成腐蚀坑。通过光谱分析检测聚酯镀铝膜水热老化后铝离子浓度的变化,进一步证实铝层脱落的主要原因是铝在水热的作用下发生腐蚀。     

多频动态热机械分析法研究碳纤维复合材料湿热老化

采用多频温度扫描动态热机械(DMTA)方法研究了环氧树脂基碳纤维复合材料于65,95℃蒸馏水中的湿热老化性能,通过Arrhenius方程计算得到复合材料湿热老化前后的玻璃化转变表观活化能△Ea,分析了复合材料于两种温度下的吸湿性和静态力学性能变化.结果表明,不同树脂基复合材料湿热老化前后△Ea不同,随着湿热老化温度的升高,△Ea增加;湿热环境中复合材料的平衡吸湿率越低,△Ea越小;湿热老化后复合材料的力学性能保留率越低则△Ea越大.表观活化能可以用来表征树脂基复合材料的耐湿热性能.     

合成条件对单组分水性聚氨酯胶粘剂性能的影响

研究了合成条件对单组分水性聚氨酯胶粘剂性能的影响,并对聚氨酯乳液做了红外分析.结果表明,内加交联剂A和B与水性聚氨酯乳液发生了交联反应,从而提高了水性聚氨酯胶粘剂的粘接强度,两者混合使用效果较佳.聚酯聚醚多元醇复配能提高水性聚氨酯的综合性能,当羧基含量为1.2%(质量分数)、NCO/OH比值(摩尔比)为1.3时,水性聚氨酯胶粘剂粘接性能较好.适量加入小分子扩链剂双酚A或丁二醇,可提高水性聚氨酯胶粘剂的硬度和粘接强度,降低其吸水率.     

Effect of prepreg storage humidity on the mixed-mode fracture toughness of a co-cured composite joint

The present work investigated the effect of the level of prepreg moisture content on the mixed-mode fracture toughness of a co-cured composite joint. It was found that moisture was stored in the prepreg as either free or bound water. It was also shown that the prepreg stores moisture from high humidity environments as free water, while the level of bound water remains unaffected. The excessive moisture was shown to plasticise the adhesive, lowering the glass transition temperature. The fracture toughness decreased under mode I and mode II loading as the humidity level was increased. There was a significant increase in mixed-mode toughness under low humidity conditions. Although the mixed-mode toughness reduced with increasing levels of humidity, the values never fell below that of the joints fabricated using the as-received materials.     

Effects of low and high temperatures on tensile behavior of adhesively-bonded GFRP joints

The tensile behavior of adhesively-bonded double-lap joints composed of pultruded glass fiber-reinforced adherends and an epoxy adhesive was investigated under temperatures ranging between −35 °C and 60 °C. The load–elongation response was influenced primarily by the thermomechanical behavior of the adhesive and much less so by that of the adherends. For temperatures above the adhesive glass transition temperature, strength and stiffness decreased with the former being less affected than the latter. The failure mechanism changed with increasing temperature from fiber-tear to adhesive failure. The crack initiation loads were unaffected as long as the temperature remained below the adhesive glass transition temperature. However, the crack propagation rate was higher at low temperatures. Critical strain energy release rates for crack initiation and propagation consistently rose as temperature increased. Modeling results obtained using existing empirical models and FEA compared well to the experimental data in the examined temperature range.     

Environmental effects on fatigue behavior of adhesively-bonded pultruded structural joints

The fatigue response of adhesively-bonded pultruded GFRP double-lap joints has been investigated under different environmental conditions. Tests were performed at ?35 °C, 23 °C and 40 °C. A fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 °C) water. The tests were performed at 40 °C and at 90% relative humidity. Specimens were instrumented with strain and crack gages to record fatigue data. In addition to the S–N curves, stiffness fluctuations and crack initiation and propagation during fatigue were monitored. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the GFRP laminates. In the presence of high humidity, the failure shifted to the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its influence on the fatigue life and fracture behavior of the examined joints was apparent and was aggravated by the presence of humidity.     

Modification of polymer properties to minimize thermal stresses in adhesive joints

The adhesive behaviour of epoxy-glass microballoon composites was studied at 25 and 75° C for plate and tubular lap joints of various combinations. It was found that the adhesive bond for various joints is enhanced through addition of microballoons. High adhesive force is observed at higher temperatures for various joints combinations where the thermal stresses are lowered and the bond is strengthened. The obtained results for various joint combinations demonstrate that high strength can be achieved when significant thermal stresses do not exist. The filler effect on some physical properties such as density, glass transition temperature, thermal expansion and Poisson's ratio of the epoxy composites is also investigated. Simple analysis indicates the differential thermal expansion coefficient between the adhesive and metallic joints and bulk modulus of the adhesive dictates the stress state.     

Temperature shift factor: Polymer mechanical properties above and below glass transition

A general relation between the temperature shift factor a_T and reduced volume ?, based on the Doolittle viscosity-volume equation, is considered. The shift factor and the specific volume are assumed to be continuous functions of the temperature T above, at, as well as below the glass transition region. A specific a_T(T) formula is obtained by using the Guggenheim-Lu-Ruether density equation; knowledge of the characteristic volume v¹ is not needed. Precise a_T(T) values for a structural adhesive obtained by Kenner, Knauss and Chai have been represented before by two W-L-F equations, one above and the other below the glass transition temperature. The same set of the shift factor values is now represented by a single equation with good accuracy. Accurate data on the temperature dependence of specific volume, v(T), have been obtained from a_T(T) values.     

盐雾环境中玻璃纤维/不饱和聚酯复合材料腐蚀深度对弯曲性能的影响

采用中性盐雾加速老化试验模拟海洋大气环境, 对玻璃纤维/不饱和聚酯复合材料在盐雾环境中的弯曲性能进行了研究。通过玻璃纤维/不饱和聚酯复合材料经盐雾加速老化后的吸湿率、玻璃化转变温度、巴氏硬度和弯曲性能的变化, 结合金相显微镜观测得到的腐蚀深度, 研究腐蚀深度对玻璃纤维/不饱和聚酯复合材料耐久性的影响。结果表明:老化初期玻璃纤维/不饱和聚酯复合材料的吸湿率随时间增长较快, 随后增长逐渐趋于稳定。玻璃纤维/不饱和聚酯复合材料的玻璃化转变温度呈现先增加后下降的趋势, 老化180 d后玻璃化转变温度增加了2.1%;老化180 d后玻璃纤维/不饱和聚酯复合材料的巴氏硬度与老化前相比降低了17.6%, 弯曲强度损失率为10%。基于金相显微镜分析得到老化后玻璃纤维/不饱和聚酯复合材料的腐蚀深度, 建立了腐蚀深度与弯曲强度之间的关系。      

Prediction of environmental degradation of closed adhesive joints using data from open-faced specimens

A framework was established to predict the fracture toughness of degraded closed DCB (CDCB) joints of a toughened adhesive-aluminum system using fracture data obtained from accelerated degradation tests on open-faced joints. The exposure index (EI), the time integral of water concentration, was calculated at all points in the closed joints using the water diffusion properties of the adhesive. The fracture toughness of the closed joints was then predicted from these calculated EIs by making reference to previously reported fracture toughness data from open-faced DCB (ODCB) specimens degraded to various EI levels. To verify the predictions, fracture experiments and analyses were carried out for closed DCB joints degraded at 60 °C-95% relative humidity (RH) and 60 °C-82% RH conditions. The failure mode of both closed and open DCBs was cohesive in the adhesive layer. Good agreement was observed between the predicted steady-state critical strain energy release rate (G_cs) values and the experimentally measured G_cs values for the degraded closed DCB joints. The results showed that the accelerated open-faced methodology, which significantly reduces the exposure time to reach a given level of degradation, can be used to predict the durability of degraded closed joints used in service conditions. It was also shown that at a given temperature, the knowledge of the degradation behavior at one RH level could be extended to other levels of RH with an acceptable accuracy using the fact that fracture degradation at a given temperature is a unique function of EI, independent of the RH exposure history that gives rise to EI. The results are applicable to other laminated systems where degradation of the bonding layer is a failure mode of concern.     

The effect of environment on the fatigue of bonded composite joints. Part 1: testing and fractography

In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap–strap joints was investigated. It was shown that the fatigue resistance of the lap–strap joints did not vary significantly until the glass transition temperature, T_g, was approached, at which point a considerable reduction in the fatigue threshold load was observed. It was also noted that absorbed moisture resulted in a significant reduction in the T_g of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of failure of the joints was seen to be highly temperature dependent, transferring from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the crack propagated along the lap–strap joint, the resolution of the forces at the crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces.