车身铝合金板胶接接头固化度解析预测研究

为了预测铝合金胶接接头固化过程中固化度的变化规律.通过实验对5A02铝合金胶接接头固化过程中的传热特征进行分析,在忽略胶层所释放的内热的前提下,将胶接接头简化为一集总热源系统.根据热传导理论,获得了胶接接头固化过程中温度变化的解析预测公式,并结合胶黏剂的固化动力学方程,得到了胶接接头固化过程中固化度的预测方法.通过实验...     

封闭条件下胶接单搭接头固化过程的实验与数值模拟

汽车板胶粘件外部环境的封闭条件对胶接接头的固化情况影响很大。文中对封闭条件下单搭胶接接头的固化过程进行了实验研究,找出了钢板单搭胶接接头搭接区域温度以及胶粘剂固化度随固化时间的变化规律。为了预测封闭单搭接头固化过程中胶层的固化度变化规律,根据热传导以及固化动力学理论对其固化过程进行三维数值建模,利用实验对模型计算结果进行了验证,通过数值计算发现,固化过程开始时,接头升温速度较慢、且接头整体温度分布不均匀,搭接区域温度较低、两端温度较高,随着固化时间延长,接头升温速率加快,而接头总体温度分布越来越均匀。     

硅烷处理对有机涂层／金属胶接接头力学性能的影响

研究了硅烷处理的胶接接头的力学性能,分析了硅烷处理对涂层/金属界面力学性能的影响.对碳钢表面进行硅烷处理,用环氧树脂胶黏剂粘接.对疲劳后的胶接接头施加静态载荷,通过分析硅烷处理胶接接头的应力-应变曲线及接头破坏形式,得出疲劳破坏与静载破坏的区别.结果表明:硅烷处理的胶接接头经一定周次疲劳后能提高接头的粘接强度,接头的破坏形式虽然为界面断裂但却为韧性断裂;接头静拉破坏形式为界面破坏和内聚破坏的混和破坏.     

金属胶接接头残余应力计算及数值分析

利用Baker模型和弹塑性有限元方法研究了胶接接头搭接区残余应力的分布.结果表明,因胶粘荆的线膨胀系数比被粘物高得多,胶层固化时被粘物阻碍了胶层的收缩,故胶层中为残余拉伸应力,被粘物中为残余压缩应力,胶层中的残余应力远大于被粘物中的残余应力.利用Baker模型和有限元计算远离自由端的胶层中的残余应力,两者吻合.被粘物中的残余应力呈中心对称,等效应力经多项式拟合后呈抛物线分布.     

双因素方差分析阳极化时间和电压对胶接耐久性的影响EI

本文采用含交互作用的双因素方差分析方法,对磷酸阳极化工艺中选用不同电压和时间处理的胶接试样进行分析,用以评定阳极化时间和电压对胶接耐久性影响的显著性。     

基于纳米压痕法的民机复合材料胶接维修界面微区力学性能研究

胶接维修是民机碳纤维增强树脂基复合材料(CFRP)结构的重要维修方式,而胶接界面微观结构及微区力学性能几乎决定了胶接维修效果及耐久性。以共固化胶接维修试样为基础,以维修试样的界面微区为研究对象,采用扫描电镜(SEM)对界面微区形貌进行分析;采用纳米压痕仪对界面微区的微观力学性能进行测试,主要表征界面微区不同位置的微观硬度、模量及加载-卸载曲线。研究结果表明,胶接维修界面存在过渡区域,其宽度约为2μm;维修母体与补片中纤维的微观力学性能并无太大差异;但由于固化成型工艺不同(母体为热压罐工艺,补片及胶膜为真空袋工艺),母体树脂的硬度与弹性模量略高于补片树脂。     

铝合金表面特性对其胶接性能影响的研究进展

不同的表面处理方法会导致铝合金表面的表面理化特性发生改变,从而对铝合金板材与胶黏剂的界面结合强度以及胶接接头的耐腐蚀性能有很大的影响。本文从铝合金表面粗糙度、微观织构、表面氧化层和涂层化学特性等方面入手,对铝合金胶接接头的界面强度和耐腐蚀性能影响的研究现状进行了综述。探讨了铝合金胶接研究发展趋势,并认为铝合金表面理化特性的参数化表征以及表面特性与胶接性能的关系模型建立等方面是今后研究的重要方向。     

复合材料RTM十字型接头疲劳性能对比实验研究

测试了复合材料RTM十字型接头疲劳性能,分析对比RTM、缝纫RTM和共胶接3种不同成型工艺十字接头的疲劳强度,结果表明:RTM成型十字接头具有良好的疲劳性能,缝纫RTM接头仅为前者的1/4,而先固化后胶接接头最低.研究还发现,复合材料疲劳寿命对成型工艺非常敏感,导致试验结果存在严重分散性.RTM和缝纫RTM的接头破坏都是从根部富树脂区的尖端率先产生裂纹,然后裂纹沿着富树脂区和玻璃布的界面向下扩展,直至接头的整体破坏;先固化后胶接接头则表现为中间层板和T接头的胶层拉伸破坏.     

淀粉/酚醛预聚物共缩聚胶黏剂的制备

以玉米淀粉为原料,采用次氯酸钠进行氧化改性,并以聚乙烯醇为接枝剂进行接枝改性,再与酚醛预聚物进行共缩聚反应,制得淀粉/酚醛预聚物(S/PFO)共缩聚胶黏剂。讨论了淀粉/酚醛预聚物比例、共缩聚温度和共缩聚时间对S/PFO共缩聚胶黏剂固体含量、黏度、固化时间、干状胶合强度和湿状胶合强度的影响。结果表明,淀粉/酚醛预聚物比例为15/120,共缩聚温度为90℃,共缩聚时间为2.0 h时,所制得的S/PFO共缩聚胶黏剂综合性能最佳。采用同步热分析和扫描电子显微镜对S/PFO共缩聚胶黏剂的固化性能和胶合界面进行了表征。相比于酚醛树脂胶黏剂,S/PFO共缩聚胶黏剂固化温度和固化焓值均降低,能够有效降低生产能耗。S/PFO共缩聚胶黏剂能将木材表面的孔隙均匀填满,形成一层薄薄的且连续的胶膜,有利于提高其胶接强度和耐水性能。     

冲击作用下斜接修补CFRP层间分层和胶层的损伤机制及参数分析

首先,针对斜接修补CFRP抗冲击性能差的问题,分别使用基于接触的内聚力模型（SCZM）和基于单元的内聚力模型（ECZM）描述层间分层和斜接胶层破坏,研究CFRP层板的冲击响应和两种失效的演化规律。然后,分析了冲击能量、斜接角度和预拉伸作用对两种失效的影响。结果表明：层间分层起始时间早于胶层破坏,与冲击能量无关;分层和胶层破坏面积随冲击能量增加而增大,胶层破坏面积增加的更明显;斜接角度主要影响胶层破坏,对分层面积几乎无影响;预拉伸作用对两种失效均具有负面作用。最后,进一步讨论分层对胶层破坏的影响,通过与只考虑胶层破坏的情况进行对比,发现层间分层使胶层破坏的面积降低,延缓了胶层的最终失效。     

A Comparison of Curing Process-Induced Residual Stresses and Cure Shrinkage in Micro-Scale Composite Structures with Different Constitutive Laws

In this paper, three kinds of constitutive laws, elastic, “cure hardening instantaneously linear elastic (CHILE)” and viscoelastic law, are used to predict curing process-induced residual stress for the thermoset polymer composites. A multi-physics coupling finite element analysis (FEA) model implementing the proposed three approaches is established in COMSOL Multiphysics-Version 4.3b. The evolution of thermo-physical properties with temperature and degree of cure (DOC), which improved the accuracy of numerical simulations, and cure shrinkage are taken into account for the three models. Subsequently, these three proposed constitutive models are implemented respectively in a 3D micro-scale composite laminate structure. Compared the differences between these three numerical results, it indicates that big error in residual stress and cure shrinkage generates by elastic model, but the results calculated by the modified CHILE model are in excellent agreement with those estimated by the viscoelastic model.     

Residual Stresses in Short-Fibre Reinforced Injection-Moulded Thermoplastic Parts

Having the prediction of the warpage and shrinkage behaviour of injection-moulded short-fibre reinforced thermoplastics in mind, a model is developed to calculate the residual stresses. In accordance with the structure and the behaviour of these materials, a layerwise orthotropic, viscoelastic constitutive equation is used. As a cause for the stress development, a process determining the history of cooling under pressure is considered. The volume expansion of the matrix is described using a time-dependent viscoelastic free volume concept. From this, the viscoelastic functions for thermal expansion are derived as well as a time-dependent temperature shift factor. The material law of the composite is developed with the help of micromechanical considerations assuming viscoelastic matrix and elastic fibre behaviour. The residual stresses are calculated for a specially designed testing device which possesses regions of different fibre orientation. Measured fibre orientation tensors are used as simulation input. In the following, the calculated stress profiles were compared with measured values, which were determined using the layer removal technique. The calculated and measured profiles are in good agreement and show a strong influence of the fibre orientation state. The highest occurring stresses are in the same order of magnitude as the long-term strength of these materials.     

CURE-DEPENDENT VISCOELASTIC RESIDUAL STRESS ANALYSIS OF FILAMENT-WOUND COMPOSITE CYLINDERS

The residual stresses induced during processing of [0/90] Tcross-ply composite cylinders is examined. A cure-dependent viscoelastic material model is used to describe the development of material behavior during cure. A finite-element model is developed using a recursive formulation in order to overcome the large memory storage requirements and lengthy calculations. Both chemical and thermal strains are modeled. The geometry modeled includes a mandrel and Teflon separation film between the mandrel and the cross-ply tube. The mandrel was shown to hare a profound influence on the level of residual stress during cure. For example, the maximum hoop stress during cure with a mandrel is 154 MPa. When no mandrel is used the maximum hoop stress is only 26 MPa. Chemical shrinkage was shown to increase the final residual stress in all cases analyzed, since both thermal shrinkage (during cool down) and chemical shrinkage (during cure) are additive. To some extent the mechanism of residual stress development in cylinders is much different compared to laminated composites. For cylinders the geometric constraint of the cylinder itself plays an important role. For example, the outer 90^° layers in a [0/90]T cylinder effectively prevent free expansion and contraction during curing. The effect is to induce radial and hoop stresses during cure.     

Finer-Scale Extraction of Viscoelastic Properties from Nanoindentation Characterised by Viscoelastic–Plastic Response

Abstract:  Motivated by recent progress in viscoelastic indentation analysis, the identification of viscoelastic properties from materials exhibiting elastic, viscous and plastic material behaviour by means of nanoindentation is dealt with in this paper. Based on existing solutions for pure viscoelastic material behaviour, two methods allowing us to consider the effect of plastic deformation are presented: (i) the so-called double-indentation technique, with the second indentation characterised by pure viscoelastic material response and (ii) the use of spherical indenter geometries instead of commonly used pyramidal indenters avoiding plastic deformation at all. Both methods are applied to three different polymers, giving access to the model parameters of the fractional dash-pot which is used to describe the viscoelastic behaviour. The results obtained are compared with results from standard (single) indentation tests using a Berkovich indenter. Moreover, the influence of the maximum load, determining the amount of plastic material response, on the identified model parameters is investigated. Finally, the creep-compliance functions identified by nanoindentation are compared with the respective macroscopic creep-compliance functions obtained from bending-beam rheometer tests.     

A Stochastic Multi-Scale Model for Prediction of the Autogenous Shrinkage Deformations of Early-age Concrete

Autogenous shrinkage is defined as the bulk deformation of a closed, isothermal, cement-based material system, which is not subjected to external forces. It is associated with the hydration process of the cement paste. From the viewpoint of engineering practice, autogenous shrinkage deformations result in an increase of tensile stresses, which may lead to cracking of early-age concrete. Since concrete is a multi-phase composite with different material compositions and microscopic configurations at different scales, autogenous shrinkage does not only depend on the hydration of the cement paste, but also on the mechanical properties of the constituents and of their distribution. In this paper, a stochastic multi-scale model for early-age concrete is presented, which focuses on the prediction of autogenous shrinkage deformations. In this model, concrete is divided into three different levels according to the requirement of separation of scales. These levels are the cement paste, the mortar, and the concrete. A specific representative volume element (RVE) for each scale is described by introducing stochastic parameters. Different scales are linked by means of the asymptotic expansion theory. A set of autogenous shrinkage experiments on the cement paste, the mortar, and the concrete is conducted and used for validation of the developed multi-scale model. Furthermore, the influence of the type and the volume fraction of the aggregate on autogenous shrinkage is studied. Besides, a combined optimum of fine and coarse aggregates is determined. The analysis results show that the proposed model can effectively estimate the autogenous shrinkage deformations of concrete at early-age by taking the influence of the material composition and configuration into consideration.     

树盘干缩异向性引起应变的测算及分析

研究探讨了干燥过程中树盘受抑制干缩应变、自由干缩应变、弹性应变、黏弹性蠕变应变以及机械吸附蠕变的图像解析测算法;运用该方法测算了白桦树盘常规缓慢干燥(含水率分布均匀)过程中干缩异向性引起的弦向各应变,分析了干燥过程中不同含水率阶段的应力状态及应力与各应变的关系。结果表明:应变的图像解析测算法可满足精度要求;树盘干燥至fiber saturation point(FSP)以下,弦向首先受拉伸应力作用,随着干燥的进行,拉伸应力转变为压缩应力;应力方向与各应变对应关系不同,与黏弹性蠕变应变无明显对应关系,与机械吸附蠕变基本对应。     

Estimation of piezoelastic and viscoelastic properties in laminated structures

An inverse method for material parameter estimation of elastic, piezoelectric and viscoelastic laminated plate structures is presented. The method uses a gradient based optimization technique in order to solve the inverse problem, through minimization of an error functional which expresses the difference between experimental free vibration data and corresponding numerical data produced by a finite element model. The complex modulus approach is used to model the viscoelastic material behavior, assuming hysteretic type damping. Applications that illustrate the influence of adhesive material interfaces and viscoelastic parameter identification are presented and a few simulated test cases aid the interpretation of results.     

Discrete element modeling and experimental investigation of hot pressing of intermetallic NiAl powder

This paper presents the numerical and experimental analysis of hot pressing of NiAl powder with an emphasis on the best possible representation of its main stages: initial powder compaction and pressure-assisted sintering. The numerical study has been performed within the discrete element framework. In the paper, an original viscoelastic model of hot pressing has been used. In order to ensure that the applied values of material parameters in numerical simulations are appropriate, the reference literature has been reviewed. It produced the relations and equations to estimate the values of all required sintering material parameters of the considered viscoelastic model. Numerical simulations have employed the geometrical model of the initial dense specimen generated by a special algorithm which uses the real grain distribution of powder. The numerical model has been calibrated and validated through simulations of the real process of hot pressing of intermetallic NiAl material. The kinetics of compaction, sintering and cooling stage indicated by the evolution of density, shrinkage and densification rate have been studied. The comparison of numerical and experimental results has shown a good performance of the developed numerical model.     

基于分数导数模型的粘弹性桩振动分析

研究成果表明混凝土桩具有粘弹性性质,为了准确分析粘弹性桩的振动特性,必须建立准确的本构模型。在分数导数理论、粘弹性理论、应力波理论基础上建立了基于分数导数模型的粘弹性桩的振动方程,利用Zhang-Shimizu分数导数数值积分法得到基于分数导数模型的粘弹性桩的振动方程数值解。分析结果表明分数导数微分算子的阶数和粘弹比对粘弹性桩桩端速度衰减的快慢和衰减周期等有很大的影响。     

Stress and strain in a plate subjected to intensive drying

The study was carried out to investigate the development of a stress-strain state in a thin porous plate subjected to intensive drying. A narrow evaporation zone (an evaporation front) divides the plate into two regions with different structures and rheological behavior. The complete saturation area is described by Kelvin-Voigt's viscoelastic model, and the dry area, by Hooke's elasticity law. Shrinkage is suggested to be a function of the evaporation zone velocity. The influence of variable shrinkage on the stress distribution across the plate is studied.N. G. Chebotarev Research Institute of Mathematics and Mechanics, Kazan State University, Kazan. Translated from henerno-Fizicheskii Zhurnal, Vol. 63, No. 2, pp. 237–241, August, 1992.