Experimental Study and Numerical Analysis of a CFRP Butt-Joint between the Pipes of Dissimilar Materials

A butt-joint was formed between two pipes of dissimilar materials, steel and aluminum, by winding a wetted roving of carbon fiber with epoxy at ±45° angle. On the curing of the epoxy, a tight carbon fiber reinforced polymer (CFRP) sleeve was formed, joining the ends of the pipes. The CFRP butt-joint was characterized for two kinds of loads: tensile and bending. Based on the joint strength performance, the specimens were categorized into two groups, thin and thick CFRP sleeved specimens. In the tensile testing, the thin sleeved specimen failed through the breakage of the CFRP sleeve at the joint plane because the axial stress developed in CFRP sleeve exceeded the ultimate strength of the CFRP. However, the thick sleeved specimens resisted the axial load in the sleeve and the weaker adherend, the aluminum pipe, slipped out of the CFRP sleeve. In the flexural testing, the thin CFRP sleeved specimens also failed by failure of the CFRP sleeve at the joint plane while the specimens of thick CFRP sleeve failed by the formation of a plastic hinge near the edge of the CFRP sleeve.     

激光处理对CFRP与铝胶接性能的研究

碳纤维增强复合材料(CFRP)具有轻质高强的特点,被广泛应用于汽车、航空航天、建筑等领域。但CFRP表面惰性高,使得CFRP与其他异质材料复合时的胶接强度低,不能满足使用。采用激光(光纤激光器)对CFRP进行表面处理后,再与铝进行胶接测试。利用SEM、接触角测试和光学轮廓仪测试对不同脉宽的激光处理后的CFRP表面形貌、表面能、表面粗糙度进行研究。结果表明,随脉宽增加,处理后的CFRP表面树脂残留量减少,表面粗糙度增加,表面自由能也相应增加。对比激光处理与机械打磨两种方式与铝的胶接性能发现,激光处理的CFRP与铝的胶接强度比未处理提高了1.95倍,比打磨处理提高了1.02倍。对其胶接断面进行分析可知,激光处理试样的断裂模式主要为纤维撕裂破坏。     

Study on promotion of interface adhesion by ultrasonic vibration for CFRP/Al alloy joints

Abstract

Adhesively bonded CFRP/Al joints have been widely used in various engineering fields. However, the poor interface adhesion between the adhesive and the Al adherend limits its further use. In this study, ultrasonic vibration was applied to promote the interface adhesion, and the promotion mechanism was studied in detail. The vibration was exerted on the surface close to the bonding area after the adhesive was applied. According to the bonding strength test, this process improved the bonding strength and repeatability by approximately 32% and 48%, respectively. By comparing the failure behavior without and with ultrasound, it can be seen that ultrasound promotes interface adhesion of the adhesive/Al adherend significantly. Under the application of ultrasonic vibration, a tight microscopic bond was formed at the bonding interface, and a chemical reaction occurred to form chemical bonds. The opening of the epoxy group was promoted to allow Al to react with –O–C to form Al–O–C, because attack of electrophilic Al?+?on O– of the epoxy group was strengthened by high-frequency impact between the adhesive and the Al adherend at the interface caused by the ultrasonic vibration. It can be seen that the application of ultrasonic vibration during the adhesive bonding process can promote interface adhesion between Al and the adhesive in terms of physics and chemistry, thus significantly improving the performance of the adhesive bond.     

Effect of Surface Texture on the Adhesion Performance of Laser Treated Ti6Al4V Alloy

The growing demand in lighter and safer structures generates the requirement of lighter joining strategies, particularly for lightweight metal alloys, composites, and also joining dissimilar materials together. Titanium alloys stand out as the conventional choice for materials for light weight structures. Adhesive bonding of titanium is an appealing route for joint design, also for the possibility of joining it with dissimilar materials. The realization of a strong joint depends not only on the joint design and type of adhesive, but also on the preparation of the adhering surface. Laser texturing presents advantages compared to common surface preparation processes in terms of eco-compatibility, energetic efficiency, ease of manufacturing, and repeatability. This work presents a preliminary investigation on laser texturing of Ti6Al4 V alloy with a pulsed fiber laser source with the aim to increase surface adhesion for bonding. Particularly, different surface textures are proposed, and laser machining strategies are developed. The results showed that laser texturing provided up to eightfold and 30% higher shear strength compared to plain and sand blasted surfaces, respectively. Failure analysis showed that a margin of improvement is still possible by adapting the surface texture for better cavity filling and reducing surface damage caused by the laser treatment.     

Effect of surface cold ablation on shear strength of CFRP adhesively bonded joint after UV laser treatment

Ultraviolet(UV) laser treatment on the surface of the carbon fiber reinforced polymer (CFRP) laminate becomes an effective method to benefit the bonding strength of adhesively bonded joint in aerospace industries. In the present research, homomorphic CFRP laminates with different resin distribution on the surface are bonded into single-lap joints. Their shear strengths are tested to evaluate the effect of surface resin distribution on bonding mechanical performance. The different resin distributions on the surface of CFRP laminate are obtained by UV pulse laser with different laser scanning speeds. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are conducted to analyze the laser treated surfaces and fracture surfaces of tested joints. The experimental results indicate that the residual resin of CFRP surface increases with the increase of scanning speed. Compared with both the reference surface without laser pre-treatment and that with no-residual resin for bonding, the surface with partial residual resin results in an enhancement of the shear strength of bonded joint. Moreover, the shear strength of the reference sample is higher than that bonded by the surface with no-residual resin. The research lays foundation for understanding the relationship between surface resin distribution and bonding strength.     

Surface treatment of CFRP composites by Ar+ irradiation to improve bonding strength between aluminum and CFRP composites

The effect of surface treatment of carbon fiber reinforced plastic (CFRP) composites on the T-peel strength and the shear strength between CFRP and aluminum panels was studied. The surface of the composite panel was treated with Ar⁺ irradiation under oxygen environment. The optimal Ar⁺ ion dose was determined by measuring the changes of contact angle and surface energy as a function of ion dose. T-peel tests and SLS tests were performed using irradiated CFRP/aluminum specimens and unirradiated CFRP/aluminum specimens to determine the treatment effect by Ar⁺ irradiation under oxygen environment on the T-peel strength and shear strength of CFRP/aluminum composites. The results showed that contact angle on the surface of the composite panel was reduced from ∼80° to ∼8° and the surface energy increased from 31 ergs/cm² to 72.4 ergs/cm² with an ion dose of 10¹⁷ ions/cm². T-peel strength and shear strength are significantly affected by the surface treatment of composite panel. T-peel strength and shear strength improved 650% and 56%, respectively, when the treatment was made with an ion dose of 10¹⁶ ions/cm². SEM examination showed that the improvement of bonding strength was attributed to the uniform spread and fracture of epoxy adhesive.     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

Experimental study on bond behavior between corrosion-cracked reinforced concrete and CFRP sheets

Bond behavior between corrosion damaged reinforced concrete and carbon fiber reinforced concrete polymer (CFRP) sheets was experimentally investigated. Forty ordinary strength concrete blocks (150 × 150 × 200 mm) were reinforced at one side across the 200-mm-dimension using three conventional ?12 mm steel bars at a spacing of (30, 40, and 50 mm) at a concrete clear cover of 15 mm. Thirty blocks were subjected to a cyclic treatment in 3% chloride solution until corrosion initiated and resulted in three different global cracking widths of up to 0.90 mm. Both control and corrosion damaged blocks were attached to CFRP sheets over their steel reinforced zone at bond lengths and widths ranging from (90 to 150 mm) and (50 to 150 mm), respectively, with CFRP bond length-to-bar spacing ratio kept constant at 1/3. Near-end pull-off tests were carried out using a special setup, mounted on a Universal Testing machine. Corrosion cracking caused significant reductions in bond strength, and slippage at ultimate stress at (41 and 68%), respectively. Other bond characteristics such as stress at first slippage, and bond stiffness and toughness were reduced, as well, by as high as (83, 44 and 67%) of those of control specimens, respectively. Corrosion cracks were more detrimental for smaller bond length and width values; especially after first and second corrosion stages, where bond failure was categorized by concrete skin peeling-off.     

Progress in development of SiC-based joints resistant to neutron irradiation

This study fills a knowledge gap regarding neutron-irradiation resistance of SiC joints for nuclear applications, by investigating high-dose neutron irradiation effects on the strength of selected joints and low-dose neutron irradiation effects on recently developed joints fabricated by state of the art processing methods. The joining methods used for the high-dose radiation study included pressure-assisted liquid-phase sintering (LPS) of SiC nanopowder, pressureless calcia-alumina glass ceramics joining, and reaction sintering of Ti-Si-C powders with hot-pressing. The joints were neutron-irradiated at 530 °C to 20 displacements per atom (dpa). Other joining methods included low-pressure LPS of cold-pressed SiC green body, pressureless reaction sintered Ti-Si-C powder joint, spark plasma–sintered Ti diffusion bond, and hot-pressed Ti diffusion bond, which were irradiated at ∼500 °C to ∼2 dpa. There was no notable degradation of torsional strengths of the joints following the high-dose irradiation. The irradiation-induced degradation at low neutron dose was highly dependent on joint type.     

Adhesive bonding of CFRP: a comparison of different surface pre-treatment strategies and their effect on the bonding shear strength

A promising joining technology for thermoset CFRP is adhesive bonding. However, the mechanical performance is influenced negatively by contaminants, like release agents, as well as an excess of matrix in the top layer. In order to generate most suitable surface qualities prior to the bonding process, carbon fibre reinforced plastics are treated with different – UV (355 nm) and IR (1064 nm) – lasers. The results are compared to commonly applied grinding surface preparation. The preliminary results on surface properties, e.g. energy and topography point towards high levels of free surface energy, as well as new originated surface structures. After the bonding process mechanical tests are subsequently carried out in Mode II, to characterize the final joint by its tensile shear strength. Finally, the individual fracture behaviour is examined in post mortem analysis. The results indicate that it is possible to increase the tensile shear strength of the joints to a maximum, which equals the interlaminar shear strength of the CFRP-laminate. Therefore, ultrashort pulsed IR-laser techniques show the best reproducibility and the highest tensile shear strength. In addition, IR-laser treatment is expected to have the highest potential for later applications.     

Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP–aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar⁺ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar⁺ ion irradiation treatment was carried out at Ar⁺ ion dose of 10¹⁶ ions/cm². Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

Experimental study of initial strengths and hygrothermal degradation of adhesive joints between thin aluminum and steel substrates

Growing usage of lightweight materials such as Al and Mg alloys and composites in automotive body manufacturing has come to a point that bonding of dissimilar materials is a realistic problem to address. A significant issue related to the bonding of dissimilar materials is that the differences in substrate surface conditions, substrate chemical and physical properties often lead to bond failure at strength levels far less than the bond strength established by the adhesive manufacturer for a balanced joint. This research experimentally studied several important factors influencing initial shear strengths and hygrothermal degradation of adhesively-bonded single lap shear (SLS) joints. The effects of surface treatments such as lubrication and zinc coating on the substrates were first investigated. It was observed that even a very small change in the amount of lubricant applied to an aluminum alloy affected the initial shear strength. On the other hand, varying the amount of mill oil on a galvanized steel surface had little effect. Next, the comparative study of the initial joint strength between electro-galvanized (EG) steels and hot dipped galvanized (HDG) steels revealed that the two coatings exhibited no difference in terms of the initial strength. Also, various combinations of aluminum alloys and steel substrates were studied to observe the effect of substrate materials. It revealed that the strength of a dissimilar joint constructed of a strong substrate and relatively weak substrate fell below the strength of the like-material joint made of the relatively strong substrate, and was closer to the strength of the like-material joint composed of the relatively weak substrate. Ageing tests were performed on SLS joints at various temperatures with and without humidity. The shear strength barely changed after 60-days of exposure at various temperatures with room humidity, but degraded significantly at high temperature with high humidity.     

Static strength and fatigue life of optimized hybrid single lap aluminum–CFRP structural joints

Hybrid bolted/bonded joints are used to assemble structural components, commonly made by carbon fiber reinforced plastics (CFRP), with aluminum frames. Hence, they have become common solutions in a number of modern structural applications in the industrial fields, as well as civil constructions. Unfortunately, due to the lack of understanding of the relationships between the multiple parameters of influence that characterize their mechanical performance, only limited improvement have been achieved so far over classical bonding approaches, in terms of static and fatigue strength. As a result, further studies are needed in order to better exploit the potential of hybrid bolted/bonded joints and identify optimum joint configurations. This paper describes an optimization procedure of the joints, achieved through a systematic experimental analysis of hybrid single lap aluminum–CFRP structural joints. This, analyzing the effect of overlap length, stiffness imbalance, adhesive curing as well as of size, positioning and preload of the bolt, results in a significant rise of the strength, especially in presence of high cycles fatigue loading. Also, micrographic analysis and related numerical simulations have allowed to gain a better insight into the damage mechanisms occurring during the in-service tensile loading, corroborating the highest mechanical performance of the angle-ply lay-up proposed for the CFRP adherent.     

Joining aluminum sheets with conductive ceramic films by ultrasonic nanowelding

Directly joining metal electrodes with ceramic films is desired for many applications in electronic manufacture. In this work, ultrasonic nanowelding was used successfully to join aluminum (Al) sheet onto indium tin oxide (ITO) and aluminum-doped ZnO (AZO) ceramic thin films. The results show that the contact resistance of the nanowelded bonds is as low as that of the conventional silver paste joints, and the contact strength is enhanced dramatically after nanowelding. Compared with Al–ITO joint, the contact stength between Al and AZO is a little higher because of the different roughness at the joint interface. From the rupture interface of the joint zone after stress–strain measurement, domains of Al atoms have been observed, which suggests that a reliable bonding is formed between Al and the ceramic films.     

A method for predicting the curing residual stress for CFRP/Al adhesive single-lap joints

The purpose of this paper is to present a prediction model of the curing residual stress for carbon fiber reinforced plastic (CFRP) and aluminum (Al) adhesive single-lap joints. Based on the lamination theory and formation mechanism of the curing residual stress of composite structures, we have found that high curing temperature in joints will lead to a significant increase of the residual stress. In the CFRP/Al stacks, residual tensile stresses are found in the Al and glue layer, while compressive stresses in CFRP. Meanwhile, the residual stress in glue layer is smaller than that in the Al and CFRP. More comparisons are made among the prediction, finite element analysis (FEA) and experimental tests to prove the efficiently and validity of the predicting model.     

Pressure-less joining of alumina ceramics by the reaction-bonded aluminum oxide (RBAO) method

The present work demonstrates a pressure-less and reliable joining technique for alumina ceramics through a reaction-bonded aluminum oxide (RBAO) method. Effective joining relies on the RBAO mechanism, in which Al particles are converted to alumina through oxidation and bond with alumina particles from the parts to be joined upon sintering. Alumina ceramics in a green state were successfully joined with the use of an Al/Al₂O₃ powder mixture as an interlayer. The oxidation behavior of the Al particles was confirmed by thermogravimetry and X-ray diffraction analyses. Joining was performed in ambient air at 1650 °C for 2 h without applying any external pressure. Microstructural observations at the joining interfaces indicated a compact joining. The joining strengths were assessed by determining the biaxial strengths at room temperature, and the joined samples exhibited no fractures at the joining interfaces. Moreover, the joints had a strength of almost 100 % when compared with those of the parent alumina ceramics.     

Adherability and weldability of poly(lactic acid) and basalt fibre-reinforced poly(lactic acid)

The adherability and weldability of pure poly(lactic acid) (PLA) and basalt fibre-reinforced PLA were investigated in this research. The joining efficiency rate is introduced as a comparative parameter among different joining processes. In the case of adhesive bonding, 16 different adhesives were used to join specimens together. The highest bond strength and joining efficiency rate for both the pure (16 MPa, 78%) and basalt fiber-reinforced (18 MPa, 44%) adhesive-bonded specimens was achieved with acrylate-based two-component adhesives. The bond strength and joining efficiency rates of bonded specimens manufactured with four welding technologies (hot gas welding, friction stir welding, ultrasonic welding, laser welding) were also investigated. The highest bond strength for both pure PLA and basalt fibre-reinforced PLA specimens (51 and 125 MPa, respectively) was attained by laser welding. The highest joining efficiency rate for pure PLA specimens (85%) was attained by ultrasonic welding, while it was achieved by laser welding for basalt-fibre reinforced PLA specimens (70%).     

Joining of SiC ceramic by using the liquid polyvinylphenylsiloxane

ABSTRACT

The joining of SiC ceramic using the liquid polyvinylphenylsiloxane at the high temperature was investigated. The characteristic evolution of polyvinylphenylsiloxane during heating process, shear strength and microstructure of joint were especially discussed. The results show that active groups Si-OH and CH=CH₂ of polyvinylphenylsiloxane through cross-linking at low temperature (200°C) form the macromolecular structure, crosslinked polyvinylphenylsiloxane possess the higher ceramic yield and structure stability at high temperature. Shear strength of SiC joints increase with the joining temperature from 1000 to 1200°C, and then decrease when the joining temperature reaches to 1350°C. Combination with microstructure of fine grains of SiO₂ and SiC dispersion in the Si–O–C ceramic of the join layer and new phase SiC formation on the joint interface through the gas–solid reactions, the shear strength of joint achieves the maximum at 1200°C. The defects of joint increase with temperature higher than 1200°C, and the shear strength of joint begin to decrease.     

Bond strength of hybrid sol–gel coatings with different additives

The hybrid sol–gel coating on Al 2024-T3 was modified by adding polyaniline, TiO₂, or γ-Al₂O₃ nanoparticles in the formulation separately. The coating was then used as an adhesive to bond Al 2024-T3 alloys, forming a single lap joint. The bond strength of the sol–gel coating was investigated using a universal tensile test machine. The lap shear strength of the original sol–gel coating was about 1.38 MPa and it was increased up to 2.26 MPa after the modification by adding 0.05 wt% PANI microparticles in the sol–gel coating. The small increase in strength was attributed to an improvement in its adhesive flexibility because of incorporation of the long-chain organic polymer in its structure. Furthermore, the addition of different amounts of TiO₂ nanoparticles in the unmodified sol–gel coating also led to an increase in shear strength compared to the undoped sol–gel coating. Typically, a sol–gel coating containing 2.0 wt% of TiO₂ recorded the highest adhesive strength of about 4.0 MPa. A similar increase in strength was observed when doping γ-Al₂O₃ nanoparticles into the original hybrid sol–gel coating. Adding 0.5 wt% of γ-Al₂O₃ in the sol–gel coating increased the adhesive bonding strength up to 4.48 MPa. The fracture surface of the specimen was separately observed by SEM and Optical Microscopy in order to examine potential evidences of mechanism and nature of failure. The reason why the adhesive strength increased after the modification of the sol–gel coating is discussed in this article.