Finite Fracture Mechanics model for mixed mode fracture in adhesive joints

Up to now the failure load assessment of bonded joints is still not fully understood. This work provides a new approach for assessing the crack initiation load of bonded joints. A failure model for single lap joints is proposed that is based on Finite Fracture Mechanics. Only two basic fracture parameters are required: the tensile strength and the fracture toughness of the adhesive. A coupled stress and energy criterion proposed in 2002 by Leguillon is used to model crack initiation in the adhesive layer. The theory of this criterion is outlined in detail, its relationship to other failure criteria is discussed and an overview of applications found in literature is given. An enhanced weak interface model that predicts a linear variation of the shear stresses in the adhesive layer is utilized to model the single lap joint. To compare joint designs and to reveal the limitations of the given approach a dimensionless brittleness number for mixed-mode loading is proposed. Along with a detailed discussion of the results for exemplary joint designs a comparison to experimental results from literature is performed. The two necessary fracture parameters are each taken from standard test results published in literature. A good agreement of the failure load predictions with the experimental results is observed. A remarkable outcome is that the presented failure model renders the adhesive thickness effect correctly. The paper concludes with a discussion of the limitations of the approach and the effect of material parameters.     

Effect of adhesive thickness,adhesive type and scarf angle on the mechanical properties of scarf adhesive joints

The effects of adhesive thickness, adhesive type and scarf angle, which are determined as the main control parameters by the dimensional analysis, on the mechanical properties of a scarf adhesive joint (SJ) subjected to uniaxial tensile loading are examined using a mixed-mode cohesive zone model (CZM) with a bilinear shape to govern the interface separation. Particularly, the adhesive-dependence of the vital cohesive parameters of CZM, which mainly include initial stiffness, total fracture energy and separation strength, is introduced emphatically. The numerical results demonstrate that the ultimate tensile loading increases as the adhesive thickness decreases. Cross the ultimate tension, the joint loses the load-bearing capacity when adopting the brittle adhesive but sustains partial load-bearing capacity while selecting the ductile adhesive. In addition, for the joint with the ductile adhesive, the maximum applied displacement until the complete failure of it is directly proportional to the adhesive thickness, which is different from the case using the brittle adhesive. Taking the combination of the ultimate loading and applied displacement into account, failure energy is employed to evaluate the joint performances. The results show that the failure energy of the joint with the brittle adhesive increases as the adhesive thickness decreases. Conversely, the situation of the joint using the ductile adhesive is vice versa. Moreover, the effect of the adhesive thickness becomes more noticeable with decreasing the scarf angle owing to the variation of the proportion of each component of the mixed-mode. Furthermore, all the characteristic parameters (the ultimate tensile loading, the maximum applied displacement and the failure energy) that adopted to describe the performances of SJ increase as the scarf angle decreases. Finally, the numerical method employed in this study is validated by comparing with existing experimental results.     

基于同步辐射X射线相衬显微CT技术的竹木复合材料胶合界面特征研究

第三代同步辐射光源X射线相位衬度显微CT能获得样品内部结构的边缘增强图像,实现对低Z材料成像。利用上海同步辐射光源X射线相衬显微成像技术,实现了竹木复合材料中EPI和MUF胶合界面和胶黏剂渗透特征的无损探测,并基于这些特征分析了几种不同加工工艺对胶黏剂在木材和竹材中渗透的影响。该技术为人造板工艺解剖学研究提供了一种重要无损检测手段。     

原位计算水分在碳/碳复合材料粘接接头的扩散系数和扩散动力学

利用X射线能谱分析 (EDX)计算出粘接层胶粘剂水分含量的增加 ,从而计算出胶粘剂粘接碳 碳复合材料湿热老化后的扩散系数和扩散动力学 ,该方法不用将仅 0 1mm厚的胶粘剂从被粘材料表面除去再进行分析 ,因此相对简单 ,与元素分析测试方法相比误差较小 .比较了碳 碳复合材料表面不同处理方法粘接接头的水分扩散系数和扩散动力学 ,表明碳 碳复合材料经偶联剂处理的耐久性能要好于化学氧化和砂纸打磨处理 .     

Localization and attempted quantification of various functional groups on pulpwood fibres

The distribution of different free chemical functional groups on wood and pulp fibres has been determined by means of atomic force microscopy (AFM) with chemically modified tips. Because these functional groups show a higher affinity to similar groups on the substrate surface during scanning, AFM images determined with an additional digital pulsed-force mode (DPFM) controller allow the distribution of the chemical components to be imaged and to a degree also to be quantified. The investigated tip coatings showed a different sensitivity towards the major chemical components present in wood fibres, determined on spin-coated films and on wood fibres. A clear distinction between cellulose and lignin was possible in both cases. This technique could therefore be used to differentiate between cellulose and lignin present on pulp fibre surfaces and confirm the successful removal of lignin by pulping.     

Constitutive behaviour of mixed mode loaded adhesive layer

Mixed mode testing of adhesive layer is performed with the Mixed mode double Cantilever Beam specimen. During the experiments, the specimens are loaded by transversal and/or shear forces; seven different mode mixities are tested. The J-integral is used to evaluate the energy dissipation in the failure process zone. The constitutive behaviour of the adhesive layer is obtained by a so called inverse method and fitting an existing mixed mode cohesive model, which uses a coupled formulation to describe a mode dependent constitutive behaviour. The cohesive parameters are determined by optimizing the parameters of the cohesive model to the experimental data. A comparison is made with the results of two fitting procedures. It is concluded that the constitutive properties are coupled, i.e. the peel and shear stress depend on both the peel and shear deformations. Moreover, the experiments show that the critical deformation in the peel direction is virtually independent of the mode mixity.     

A simple continuum damage model for adhesively bonded butt joints

The present work is concerned with the study of the damage behavior of adhesive joints consisting of an epoxy adhesive layer bonding aluminium alloy substrates. A model for butt joints, developed within the framework of Continuum Damage Mechanics, that accounts for the effect of the thickness of the adhesive layer on the strength of the system is proposed and analyzed. The predicted values of rupture stress for different values of the thickness of the adhesive layer are compared with experimental data, showing a good agreement.     

木质素活化改性制备酚醛树脂胶黏剂研究进展

木质素由于化学结构与苯酚相似,通过活化改性可部分替代苯酚制备木质素改性酚醛树脂胶黏剂。既可降低成本、达到生物质资源高效利用的目的,并且制备的木质素改性酚醛树脂胶黏剂有毒残余较低,具有环保意义,是合成制备生物质高分子材料的重要途径。本文综述了国内外研究人员在木质素活化改性制备酚醛树脂胶黏剂研究领域的最新进展,重点介绍了化学改性、物理改性、生物改性等木质素活化改性方法,比较了不同改性产物制备酚醛树脂胶黏剂的性能,并对影响木质素活化改性制备酚醛树脂胶黏剂实现工业化应用的主要因素进行了分析。     

仿贻贝黏附高分子的研究进展

贻贝具有极强的水下黏附能力,其分泌的黏附蛋白能够黏附在包括聚四氟乙烯（PTFE）在内的几乎所有基底材料上。贻贝不受水或者潮湿环境影响的特殊黏附性能有望指导制备新型水下黏附剂,因而引起了研究人员极大的关注。受贻贝水下黏附机理的启发,通过模仿贻贝黏附蛋白分子结构,研究人员设计制备了多种具有强黏附功能的高分子并探索了相关应用...     

Preparation of a Carbon Nanotube-Reinforced UV/Heat Dual-Curable Adhesive for Liquid Crystal Displays

Abstract

There is strong demand for narrow-bezel liquid crystal displays (LCDs) in the display industry. Adhesive materials for narrow-bezel LCDs require excellent adhesive properties in order to provide the same level of adhesive strength as conventional sealants, even when applied in small amounts. In this study, we prepared a previously unknown, highly adhesive, carbon nanotube-reinforced (CNT-reinforced) UV/heat dual-curable adhesive for narrow-bezel LCDs. Single-walled and multi-walled CNTs (SWCNTs and MWCNTs) were employed as fillers, resulting in superior adhesive properties; in particular, the inclusion of SWCNTs improved both adhesion and resistance to water permeability compared to a conventional adhesive.