Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.     

丙烯酸酯改性EVA乳液胶黏剂的黏结性能研究

采用单一变量法,选择醋丙乳液、纯丙乳液、苯丙乳液、硅丙乳液分别改性EVA乳液胶黏剂,研究丙烯酸酯乳液与EVA乳液的共混效果,以及不同类型的丙烯酸酯乳液对EVA乳液胶黏剂黏结性能的增黏效果。热力学、差示量热扫描、红外光谱测试结果表明,丙烯酸酯乳液能够与EVA乳液互溶。剥离强度、高温黏结性能、低温黏结性能测试结果表明,经丙烯酸酯改性的EVA乳液胶黏剂的黏结性能整体得到提高,其中,经醋丙乳液改性的EVA乳液胶黏剂的综合性能最优,能够满足实际使用要求。     

Efficient 2D Analysis of Interfacial Thermoelastic Stresses in Multiply Bonded Anisotropic Composites with Thin Adhesives

In engineering practice, analysis of interfacial thermal stresses in composites is a crucial task for assuring structural integrity when sever environmental temperature changes under operations. In this article, the directly transformed boundary integrals presented previously for treating generally anisotropic thermoelasticity in two-dimension are fully regularized by a semi-analytical approach for modeling thin multi-layers of anisotropic/isotropic composites, subjected to general thermal loads with boundary conditions prescribed. In this process, an additional difficulty, not reported in the literature, arises due to rapid fluctuation of an integrand in the directly transformed boundary integral equation. In conventional analysis, thin adhesives are usually neglected due to modeling difficulties. A major concern arises regarding the modeling error caused by such negligence of the thin adhesives. For investigating the effect of the thin adhesives considered, the regularized integral equation is applied for analyzing interfacial stresses in multiply bonded composites when thin adhesives are considered. Since all integrals are completely regularized, very accurate integration values can be still obtained no matter how the source point is close to the integration element. Comparisons are made for some examples when the thin adhesives are considered or neglected. Truly, this regularization task has laid sound fundamentals for the boundary element method to efficiently analyze the interfacial thermal stresses in 2D thin multiply bonded anisotropic composites.     

Flip-chip assembly: is the bi-material model acceptable?

The bonding layer in flip-chip assembly designs is characterized, unlike in epoxy bonded assemblies, by a relatively high effective Young’s modulus of its composite material, which is comprised of high-modulus solder and low-modulus epoxy encapsulant (underfill). Simple, easy-to-use and physically meaningful tri- and bi-material analytical stress models are developed for the evaluation of the thermally induced interfacial shearing stresses, as well as normal stresses acting in the cross-sections of the assembly components. While in a tri-material model all the three materials, the chip, the substrate and the bonding layer, are treated as “equal partners”, in a bi-material model a significant simplification is made, assuming that the bonding layer is much thinner than the bonded components, the chip and the substrate, and/or that its effective Young’s modulus is significantly lower than the moduli of the chip and the substrate materials. In the carried out numerical example based on the application of the tri-material model, the highest shearing stress occurs at the chip-bond interface and is significantly, by the factor of about 2.45, higher than the stress at the substrate-bond interface, but even the latter stress is about twice as high as the maximum shearing stress predicted on the basis of the bi-material model. As to the normal stresses acting in the cross-sections of the assembly components, the tri-material model predicts that the highest stresses occur in the chip, the lowest—in the substrate, and that the stresses in the bond are rather high, about 59% of the stresses in the chip. The bi-material model, however, simply assumes that the normal stresses in the bond are zero. The normal stresses in the chip predicted on the basis of this model are only about 78% of the stress predicted by the tri-material model. The normal stresses in the substrate evaluated on the basis of the bi-material model are almost twice as high as the tri-material model predicts, but these stresses are low anyway: it is the state of stress in the chip and in the bonding layer, and the interfacial stress at the chip-bond interface that should be of the primary concern to the device designer. It is concluded that while a simple bi-material model can be successfully used for adhesively bonded assemblies, characterized by a thin and/or low modulus bonding layer, a tri-material model should be employed for flip-chip assemblies, when high-modulus solders are used. Future work should include finite-element analyses and experimental evaluations.     

Joining stainless steel parts by means of weld bonding

Recent published works in the field of weld bonding present new results relevant to project and design of structural metal joints. However, it is still difficult to find technological data in the open literature to support and control manufacturing conditions. Gaps of knowledge are easily found in the set up of process parameters, in the selection of adhesives and in the understanding of the long term performance of weld bonded joints in hostile environments. The aim of this paper is to contribute to an extension of the actual knowledge on the weld bonding technology and to provide useful insights into its application to the joining of stainless steel. The research is accomplished by the weld bonding of stainless steel test specimens using three different commercial adhesives with varying working times and different exposures to water. The relative performance and quality of the weld bonded joints is evaluated against alternative solutions based on adhesives and conventional spot-welding. The experimental investigation is supported by finite element computer simulations with the purpose of establishing the welding parameters that allow spot-welding through the adhesives.     

Assuring successful bonding to carton boards

This paper considers the factors that will influence successful adhesive bonding to carton-boards. Although the paper assumes the use of carton-board within the packaging industry, the majority of the arguments may be applied to other bonding applications. The commonest adhesive types used within the packaging industry for carton-boards are PVA-type emulsion and hot-melts. Other adhesives used will include animal glues, starches and dextrines, acrylics etc. Although these adhesives vary considerably in properties they share the majority of requirements for successful bonding. The paper firstly considers bond structure and the basic modes of bond failure. It is surprising how often these are misunderstood or ignored when bonding problems occur. An inspection of the precise nature of a bond failure often suggests the cause of the problem, and at least eliminates a number of possible causes. The paper then discusses the properties required of boards, board surfaces (be they printed, varnished, coated or otherwise) and adhesives for the successful manufacture of bonds. Techniques for evaluating trial bond performance and, finally, the need for good 'housekeeping' are considered.     

SiC陶瓷表面处理工艺对SiC-AFRP界面粘接性能的影响

为提高SiC陶瓷-芳纶纤维增强树脂基复合材料(SiC-AFRP)的界面粘接性能,研究了陶瓷腐蚀工艺、偶联剂处理工艺、粘接剂种类对SiC-AFRP界面剥离强度的影响。结果表明:SiC陶瓷表面腐蚀工艺和偶联剂处理工艺能有效提高SiC-AFRP界面粘接性能。陶瓷经K3Fe(CN)6与KOH混合腐蚀液浸泡2h,使用乙烯基三乙氧基硅烷偶联剂偶联化处理后,SiC-AFRP的界面剥离强度由0.45kN/m提高至2.20kN/m;VA含量15%(质量分数)的EVA热熔胶膜是理想的界面胶黏剂。     

Delamination of filler-incorporated automotive epoxy adhesives from different steel substrates upon exposure to distilled water and NaCl solutions under applied bending stresses

Delamination of four filler-incorporated automotive epoxy adhesives from five different steel substrates was studied using a three-point bending device. The bent specimens were exposed to distilled water and NaCl solutions with different concentrations from 1–5 weeks. It was found that the degree of adhesive delamination increased with the concentration of the NaCl solution, and with the magnitude of the applied bending stresses. The harder the adhesives, or the higher the content of the fillers in the adhesives, the lower was the degree of the adhesive delamination. In addition, the rank of the steel substrates in terms of their bonding resistance against adhesive delamination was determined. It is believed that the adhesion between the adhesives and the steel substrates is governed by the surface roughness of the steel substrates.     

Analytical investigation of thermal and mechanical load effects on stress distribution in adhesive layer of double-lap metal-composite bonded joints

Significant thermal stresses are induced in the adhesive layers of a metal-composite bonded joint owing to the large temperature change associated with the difference in the coefficients of thermal expansions of metals and composite adherends. In this study, a theoretical analysis of shear and peel stresses in adhesive layers of a double-lap metal-composite bonded joint is carried out to evaluate the effects of thermal and mechanical loads on the stress distribution in the adhesive layer. The effects of temperature change and adhesive thickness on the shear and peel stresses in the adhesive layer of the bonded joint, with and without external forces, are examined based on the theoretical analysis. The results calculated for the condition involving a mechanical load application to the bonded joint and a decrease in temperature indicate that the absolute value of the shear and peel stresses peak at both ends of the adhesive layer, and that the absolute value of the peak stresses increases in the case of a thinner adhesive layer. When mechanical and thermal loads are simultaneously applied to a double-lap joint, shear and peel stresses synergistically increase at one end of the adhesive layer and decrease with an offset at the other end.     

Klebflächenvorbehandlung von Aluminiumoberflächen mit Haftvermittler

Surface pretreatment for adhesive bonding of aluminum with adhesive mediator The influence of three steps pretreatment degreasing, blasting and primer and the two steps pretreatment degreasing and blasting on the adhesive behaviour of aluminum alloy AlMg4,5Mn0,5 were investigated. The investigations were preformed using two cold hardening two‐components epoxy adhesives, one hot hardening one‐component epoxy adhesive and one cold hardening two‐components polyurethane adhesive. The three steps pretreatment indicate that the adhesion bonding strength by epoxy adhesives are higher than that by two steps pretreatment. On the other hand, the adhesion bonding strength was similar by using polyurethane adhesive independence of the pretreatment method. Leaving out the pretreatment step degreasing after corund blasting caused by using, PU, EP 2 and EP 3 adhesives a significant decreasing of the bonding strength in comparison with the only degreased specimen. This degreasing process was used to remove the residual blasting medium from the aluminum surfaces after blasting. The topographical structure of the surfaces after corund blasting was covered by further pretreatment with primer as a consequence of higher primer viscosity, which causes a decreasing in the surface roughness. The chemical composition of pretreated surfaces by three steps was different from that by two steps.     

可剥布对T300/Cycom 970环氧树脂复合材料胶接性能的影响

在商用飞机复合材料结构制造过程中,可剥布在复合材料共胶接和二次胶接的表面处理过程中应用越来越广泛,逐步替代传统打磨方式。然而在实际应用中,可剥布与复合材料树脂体系之间存在一定的匹配性,经不同可剥布处理后的复合材料表面胶接性能存在较大差异。为研究其对胶接性能的影响,选用四种航空用可剥布对复合材料进行表面处理,采用热压罐工艺制备T300/Cycom 970环氧复合材料层压板,选取同一种航空用胶膜进行胶接。按照ASTM剥离测试和单搭接剪切测试标准,对T300/Cycom 970环氧复合材料胶接结构的性能进行测试,采用接触角测试、SEM和X射线能谱测试(XPS)分别对复合材料层压板制件和可剥布织物的表面润湿性、表面形貌和表面元素进行测试与分析。结果表明:选用聚酯湿可剥布处理后的T300/Cycom 970环氧复合材料胶接性能最佳;可剥布织物及涂层的残留会影响复合材料胶接性能;可剥布处理能改善复合材料表面润湿性,提高表面能,但并不能保障胶接质量;可剥布编织形式直接影响复合材料表面形貌,决定其表面粗糙度,对复合材料胶接性能也有较大的影响。     

铝塑复合用聚氨酯胶粘剂的制备及性能研究

以聚酯、聚醚混合多元醇和异佛尔酮二异氰酸酯为主要原料,制备了一种低粘度无溶剂型聚氨酯覆膜胶粘剂。研究了聚酯、聚醚混合多元醇的配比对胶粘剂合成时间、粘度、抗水性能和粘结强度的影响;同时,探讨了催化剂的添加对胶粘剂综合性能的影响。结果表明:由混合聚酯、聚醚多元醇合成的聚氨酯胶粘剂粘度低、易涂覆且耐水性得以改善,但比例必须适宜,以2种多元醇所含羟基物质的量比为0.3:0.7为佳;催化剂的加入虽能显著提高固化速度,但降低了胶粘剂的剥离强度,不利于胶粘剂综合性能的改善,故应酌情适量添加。     

Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives

Single-walled carbon nanotubes (SWCNTs) were incorporated at low loading (up to ∼1 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability, as a route for development of a structural and conductive adhesive. At these low SWCNT loadings the tensile properties were maintained or improved, while strength decreased in a higher loading case. The structural bonding performance of composite-to-composite joints, evaluated in lap-shear and peel tests, was reasonably maintained for adhesives containing 0.5 wt% or 1 wt% SWCNTs. In the case of the 0.5 wt% SWCNT–adhesive, peel and lap-shear strength were unchanged while the addition of 1 wt% resulted in 30% increase of peel strength but the lap-shear strength was reduced by 10–15%. For 1 wt% SWCNT–adhesives, conductivities as high as 10⁻¹ S m⁻¹ and typically ∼10⁻³ S m⁻¹ were achieved. Joint electrical resistance measured between aluminum adherends was larger than predicted by the bulk conductivity, but was reduced by a post-treatment step resulting in apparent joint conductivities within one order of magnitude of the bulk samples.     

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.     

Marine Composites Based on Low Styrene Content Resins. Influence of Lamination Procedure and Peel Plies on Interlaminar Resistance

Delays between lamination of structures and secondary bonding are common in boatbuilding. This paper presents results from an experimental study of the influence of manufacturing procedure on the bond strength of unidirectional glass fibre reinforced polyester composites. Continuously laminated panels are taken as the reference and compared with panels overlaminated after delays of one week with and without peel ply protection. A standard polyester and low styrene DCPD formulation were tested. ILSS testing is shown not to be sensitive to the procedure but crack propagation tests show very large differences in mode I fracture energy. A small drop in fracture energy is measured for a delay of one week between lamination of the first and second halves of the specimen without peel ply. When a nylon peel ply is applied and removed just before overlamination of the second half very large drops in propagation energy, around 75%, are noted for both materials. Surface abrasion after peel ply removal enables fracture energy to be recovered for the standard polyester and to a lesser extent for the low styrene DCPD composite.     

Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

Exploiting Supramolecular Interactions from Polymeric Colloids for Strong Anisotropic Adhesion between Solid Surfaces

Adhesion occurs by covalent bonding, as in reactive structural adhesives, or through noncovalent interactions, which are nearly ubiquitous in nature. A classic example of the latter is gecko feet, where hierarchical features enhance friction across the contact area. Biomimicry of such structured adhesion is regularly achieved by top-down lithography, which allows for direction-dependent detachment. However, bottom-up approaches remain elusive given the scarcity of building blocks that yield strong, cohesive, self-assembly across multiple length scales. Herein, an exception is introduced, namely, aqueous dispersions of cellulose nanocrystals (CNCs) that form superstructured, adherent layers between solid surfaces upon confined evaporation-induced self-assembly (C-EISA). The inherently strong CNCs (E_A > 140 GPa) align into rigid, nematically ordered lamellae across multiple length scales as a result of the stresses associated with confined evaporation. This long-range order produces remarkable anisotropic adhesive strength when comparing in-plane (≈7 MPa) and out-of-plane (≤0.08 MPa) directions. These adhesive attributes, resulting from self-assembly, substantially outperform previous biomimetic adhesives obtained by top-down microfabrication (dry adhesives, friction driven), and represent a unique fluid (aqueous)-based system with significant anisotropy of adhesion. By using C-EISA, new emergent properties will be closely tied with the nature of colloids and their hierarchical assemblies.     

Elasto-plastic stress distributions in adhesively bonded double lap joints

Elasto-plastic stress analysis is an important solution for a used ductile adhesive in single or double lap solutions. After loading, the joint strength can be increased by residual stresses. In this study, an elasto-plastic stress analyses of the shear and peel stresses are carried out in a double-lap joint. The adhesive material is chosen as a ductile material which is DP460. Elasto-plastic solution of the shear stress is obtained in a simple and accurate form in one dimensional case. Then, the solution of the peel stress is performed by considering the shear stress in the elasto-plastic form. In this solution, the boundary conditions are satisfied by the Newton–Raphson method. A good agreement is obtained between these analytical and numerical (ANSYS 10) solutions. The values are calculated highest at the ends of the adhesive.     

Rehabilitation of Composite Steel Bridges Using GFRP Plates

The current study is a part of an extensive research program conducted to assess the use of Glass Fibre Reinforced Plastic (GFRP) sheets in enhancing the flexural capacity of steel beams. The properties of a heavy-duty adhesive system that can be used to bond GFRP sheets to the flanges of steel beams were experimentally determined in a previous study. The excellent performance of a W-shaped steel beam strengthened using GFRP sheets has encouraged the authors to assess the applicability of this technique to composite steel bridges.The dimensions and cross section properties of a real composite steel plate girder bridge are considered in a case study analysis. A detailed nonlinear numerical model is developed for the bridge before and after attaching GFRP sheets to the bottom flange of its steel girders. Nonlinear moving load analyses are first conducted to determine the critical truck locations that will lead to maximum GFRP axial stresses, and maximum adhesive shear and peel stresses. Using these configurations, nonlinear analyses are then conducted to assess the increase in the bridge capacity that can be achieved by bonding 38 mm GFRP sheet to the bottom flange of its steel girders.     

Improvement of thermal stability of UV curable pressure sensitive adhesive by surface modified silica nanoparticles

Pressure sensitive adhesives (PSAs) with higher thermal stability were successfully prepared by forming composite with the silica nanoparticles modified via reaction with 3-methacryloxypropyltrimethoxysilane. The acrylic copolymer was synthesized as a base resin for PSAs by solution polymerization of 2-EHA, EA, and AA with AIBN as an initiator. The acrylic copolymer was further modified with GMA to have the vinyl groups available for UV curing. The peel strength decreased with the increase of gel content which was dependent on both silica content and UV dose. Thermal stability of the composite PSAs was improved noticeably with increasing silica content and UV dose mainly due to the strong and extensive interfacial bonding between the organic polymer matrix and silica.