Failure characterization of polypropylene block copolymer welded joints

The failure behavior of polypropylene block copolymer double-V welded joints was investigated. Joints were prepared using the hot-gas welding technique at varying gas temperatures in the range of 230–260°C. Uniaxial tensile tests, fracture mechanics experiments, several microscopy techniques, and complementary FEM analysis were carried out to assess the quality of filler rods and welding interfaces. The developed interfaces were weaker than the parent material as a consequence of polymer chains segregation during the welding process. The hot-gas temperature had a marked effect on the failure behavior of the welds. The highest interface toughness was attained at the highest gas welding temperature used at which, polymer chains were able to quickly diffuse into the parent material enlarging the distance of penetration and hence the micro-deformation capability of the joint. POLYM. ENG. SCI., 47:1062–1069, 2007. © 2007 Society of Plastics Engineers     

Fracture Mechanics of Linearly Welded Wood Joints: Effect of Wood Species and Grain Orientation

The energy release rates of beech, oak and pine wood specimens welded by linear friction were determined using double cantilever beam (DCB) tests. The influence of grain orientation both in welding along the wood longitudinal direction as well as in end-grain-to-end-grain welding to give butt joints was determined. The analysis of results was done with the experimental compliance method, based on linear-elastic fracture mechanics. The energy release rates obtained varied considerably according to wood grain orientation, wood species and welding cycle used. In many cases, the energy release rates obtained with the recently developed 150 Hz/faster weld technology were in the range as obtained for adhesive-bonded wood joints. Some cases also gave energy release rates higher than adhesive-bonded joints. Welding of butt joints by end-grain-to-end-grain welding was achieved, although the joints presented much lower energy release rates. In butt joints there appeared to be no significant difference in the energy release rates obtained for the three different timber species used.     

Application of response surface methodology for weld strength prediction in laser welding of polypropylene/clay nanocomposite

Welding as a fabrication process can be used to join materials, including composite and nanocomposites and laser welding process due to its advantages has found wide applications in this field. Its process parameters can play a significant role in determining the weld strength of laser-welded joints in polypropylene/clay nanocomposites. In this study, the effect of laser welding parameters, such as laser power, welding speed and focal position along with the clay content in a polypropylene/clay nanocomposite on weld strength were determined using response surface methodology. This methodology was applied for developing a mathematical model which can predict the main effects of the above parameters and their impacts on tensile strength of butt-welded laser joints in 2-mm thick polypropylene/clay nanocomposite sheets. The analysis of variance was performed to check the adequacy of the developed model. A comparison was also made between the predicted and actual results. The results showed that weld strength decreased when clay content was increased from 0 to 6 %, but welding speed increased from 30 to 60 mm/s. The above parameters were also optimized to achieve a high strength welded joint.     

Structure and mechanical properties of optimized extrusion welds

A knowledge of how welding parameters affect the mechanical properties of welds is important. However, the mechanical properties of welds cannot be characterized by nondestructive testing methods. Because of its sensitivity to process conditions, extrusion welding of polypropylene-homopolymer (PP-H) was used to investigate the effects of welding parameters on the resulting mechanical properties of welds. Overall optimization of the welding process to obtain stable conditions during welding, which required a redesign of the welding shoe and the welding geometry, resulted in improved weld properties through better build-up of critical weld areas and suppression of void formation. Investigation of material heating characteristics led to a new air nozzle design. The effect of air temperature and welding velocity on the temperature and thickness of the molten layer was determined. The effects of individual process parameters on the structure and mechanical behavior of welds were established, thereby making it possible to specify narrow limits on the values of the weld parameters for producing high-quality welds. The quality of these joints cannot be determined by short-time tests because, even with severe testing conditions, cracks occur in the bulk material. Polarized optical microscopy was used to correlate crack behavior with the build-up of a specific multilayer structure in the weld area. Long-term tests demonstrated that, in both the time-to-crack and crack behavior, the joining area is not the weakest link in an extrusion weld when the welding parameters are chosen correctly.     

奥氏体不锈钢的全自动气体保护焊焊接工艺试验

为了提高奥氏体不锈钢管焊接自动化程度,提高焊接效率,保证焊接质量,采用全自动气体保护焊接方法进行了焊接试验。通过试验分析、以及对焊缝的电镜检测,确立了一套适用的焊接工艺,实现奥氏体不锈钢管的全自动焊接。     

Application of lock-in thermography in nondestructive evaluation of adhesively-bonded aluminum joints

A major problem in industrial applications of structural adhesives is the quality assurance of manufactured joints. At present, for lack of a suitable nondestructive technique, production standards for adhesively-bonded aluminum joints are established on the basis of destructive tests and statistical inference. An experimental study was carried out to assess if lock-in thermography (LT) could be used as a tool for nondestructive evaluation of adhesively-bonded aluminum joints. Several samples were fabricated by varying the governing parameters such as nature of aluminum alloy, substrate thickness, surface treatment, adhesive type and bondline thickness. The effects of surface treatments on the loading capability of lap joints were evaluated through both destructive tensile tests and nondestructive evaluation with infrared LT. Tensile tests showed that the joint performance was not affected by the nature of the aluminum alloy but by the substrate thickness, the adhesive type and the bondline thickness. LT was capable of detecting imperfections such as scratches on substrates and foreign inclusions in the adhesive layer.     

Experimental Studies on Thermo-Mechanical Behavior of Ultrasonically Welded PC/ABS Polymer Blends

This research paper attempts to investigate the performance of blended PC/ABS joints using the ultrasonic material joining process. The key focus is on examining the thermal aspects during the joining of PC/ABS blends using ultrasonic welding and the subsequent mechanical testing to determine the strength of the weldments. Thermal behavior of the blends during welding may govern or alter the mechanical properties and integrity of the joints. Hence, investigations on thermal characteristics involved in PC/ABS blends when subjected to high vibrational heat generated during the ultrasonic welding process is imperative. DSC is used to measure the glass transient temperature (T_g) after subjecting it to welding. Mass loss is calculated with TGA. TGA and DSC results indicate change in T_g which are attributed to the molecular alignment occurring when the specimens are subjected to ultrasonic vibrations. Initially, two step mass losses occur that is contributed by ABS in which long single chains are associated and alters PC. SEM images reveal the absence in intermolecular compounds or impurities that tend to weaken weld joints. The diffusion of these molecules is uniform in the welded region. The amorphous nature enhances the integrity of weld joints. Molded part illustrates the higher strain rate in comparison with the welded specimens. The RSM model proposed is sufficient and has limited possibility for violating the independence or the assumption of constant variance.     

热塑性塑料激光透射焊接研究

针对混合20%玻纤的丙烯腈-苯乙烯共聚物(AGS)热塑性塑料,进行激光透射焊接试验,得到了线能量对AGS焊接质量的影响规律。在线能量为0.75~1J/mm之间时,剪切强度可达18MPa以上,焊接质量最优。     

Impact strength of joints bonded with high-strength pressure-sensitive adhesive

The impact strength of joints bonded with a double-coated high-strength pressure-sensitive adhesive (PSA) was experimentally investigated. PSA has recently been used to join parts of mobile devices such as smart-phones, which are often subjected to drop impacts. Consequently, the impact strength of PSA bonded joints has become important.Two types of specimens, butt joint specimens and double cantilever beam (DCB) specimens bonded with adhesives were utilized for the experiments. Quasi-static tests and impact tests of the specimens were carried out using a mechanical testing machine and an impact testing machine. The PSA layers in the specimens were observed using a high-speed digital camera. The deformation and strain distribution in the adherends of the DCB specimens were also measured using a novel high-speed digital camera with photoelastic imaging capability.Though the strength of the butt joints increased as the loading rate increased, the critical fracture energy of the DCB specimens decreased at high loading rates. This may be attributed to the transition to the brittle nature of the PSA in the loading range in which no cavitation occurred. To verify the critical fracture energy obtained with the DCB tests, finite element analyses (FEA) based on the cohesive zone model (CZM) were carried out, and the load–displacement curves of the DCB tests were simulated. The predicted results showed good agreements with the experimental results.     

Microstructure and mechanical properties of superplastically joined yttria-partially-stabilized zirconia (Y-PSZ) ceramics

Joining of oxide ceramics has been recognized as a key issue in their successful utilization in a variety of demanding, high temperature applications. Superplastic joining of yttria-partially-stabilized zirconia (Y-PSZ) ceramics has been carried out to avoid the deleterious effects of joint interlayer materials. Microstructural characterization of superplastically formed joints indicate that the joint interface is indistinguishable from the bulk and that there is not grain growth across the interface. Compression tests conducted on double-notch specimens show that very good bonding was achieved in the superplastic joining process. From these experiments an underestimation of the shear strength was obtained as the specimens failed in the bulk. The room temperature flexural strength of joints measured had values up to 185 MPa.     

Study on the role of laser surface irradiation on damage and decohesion of Al/epoxy joints

In this work we investigate the effect of laser irradiation on the bond toughness of aluminum/epoxy bonded joints. The evolution of substrate surface morphology and wettability, for various sets of laser process parameters (i.e. laser power, line spacing, scan speed), was investigated by means of Scanning Electron Microscopy (SEM) and contact angle measurements. A proper combination of power, line spacing and scan speed was then selected and adhesive bonded Al/epoxy T-peel joints were prepared and tested. For comparison, similar samples were produced using substrates with classical grit blasting surface treatment. Finally, post-failure SEM analyses of fracture surfaces were performed, and in order to typify the increase in bond toughness of the joints, finite element simulations were carried out using a potential based cohesive zone model of fracture.     

聚乙烯及其复合管道安全检测与评价方法

聚乙烯及其复合管道广泛应用于油气输送、城市燃气等能源领域,其安全性至关重要。焊接接头的安全检测及评价是聚乙烯及其复合管道系统安全的关键技术。介绍了聚乙烯管道焊接接头的无损检测原理及方法、冷焊检测技术以及缺陷分类与失效模式三方面内容。对电熔和热熔焊接接头分别采用超声相控阵和耦合聚焦技术进行超声检测,并给出了缺陷剖切与检测结果的对比图。提出了物理概念清晰、工程应用方便的冷焊超声检测方法。将电熔接头中的缺陷分为熔合面缺陷、孔洞、结构畸变和过焊。分别对含不同类型和大小缺陷的电熔接头进行力学性能测试,发现电熔接头存在三种典型的失效模式,即沿电熔套筒壁贯穿裂纹失效、熔合面失效以及沿电阻丝所在平面贯穿裂纹失效。根据试验测试与理论分析结果,提出了相应的安全评定方法。所提出的方法,填补了国内外在聚乙烯管道安全检测与评价方法方面的技术空白,提高了燃气管道的本质安全性。     

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

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%).     

聚乙烯电熔焊接新方法及工艺优化研究

在电熔接头相控阵检测中首次发现接头内部存在一条由超声信号组成的特征线,进一步研究表明,可以根据特征线移动速度以及特征线与电阻丝的距离变化情况,估评聚乙烯（PE）电熔接头的焊接工艺是否合适。在PE电熔接头新产品开发中采用超声检测实时录像,通过特征线的移动情况对焊接工艺参数进行优化,从而可大幅降低电熔接头的开发时间和成本。     

Numerical simulation optimization for laser welding parameter of 5A90 Al-Li alloy and its experiment verification

Weight reduction is very important to aircraft. Laser welding technology is beneficial to reduce the weight of aircraft structure. However, due to the unclear laser welding properties of Al-Li alloy, the research on laser welding of Al-Li alloy is necessary. In current study, the temperature field, stress field and deformation of laser welding are simulated. The optimal welding parameters are obtained by simulation, which are used to weld the real Al-Li alloy sheet. Besides, the microstructure and mechanical property of welded joints are analyzed. It is found that the simulated optimal welding parameters are suitable for welding real Al-Li alloy sheet. And the simulation results are consistent with the experimental results.     

Durability assessment of laser treated aluminium bonded joints

Structural adhesives are widely used in industrial applications dealing with the problem of assembly and bonding. Despite the several advantages brought by this technique, one of the main issue is represented by the need of the surface to be mechanically and/or chemically treated with the aim to make it suitable for the adhesive deposition. Previous works demonstrate how the surface treatment by laser ablation seems to enhance the joint strength with respect to the untreated material. In particular, the effect of pulsed Yb-laser ablated aluminium surfaces over the mode I energy release rate of Double Cantilever Beam (DCB) specimens was investigated and a significant growth of the fracture toughness compared with the untreated and the grit blasted joints was observed. In this work, an investigation concerning the durability of the mechanical properties of aluminium joints treated with several representative parameters configurations was conduced. These setting configurations were used in an experimental campaign with the aim of verifying their suitability varying the type of the test (fatigue tests) and the environmental conditioning of the specimens (quasi-static tests after ageing cycles). Concerning the fatigue behavior, the ranking of the laser parameters configuration according to the increase of toughness with respect the degreased and the grit blasted samples seemed quite consistent with the results obtained in the quasi-static test campaign. When an accelerated ageing cycle in control of temperature and relative humidity was applied, a general lowering of toughness affected every tested specimen. This effect was however more marked in the grit blasted sample than in the laser treated ones. Therefore, a relative improvement of the mechanical performance when using some laser ablation configurations instead of the grit blasting, under the same conditions, when the adhesively bonded joints were aimed to undergo some critical environmental exposure, was recorded.     

Laser transmission welding of poly(lactic acid) and polyamide66/sepiolite nanocomposites

Influence of sepiolite nanoclay on the properties of the resulting join between poly(lactic acid) (PLA) and different Polyamide66 (PA66)/nanoclay nanocomposites was studied in this work. Six different polymer nanocomposites based on PA66 were manufactured through a melt compounding process by adding a fixed 1.64 wt % of a commercial IR absorber additive and the respective weight percentages of sepiolite to the polymer matrix. Several nanocomposite/PLA joints were finally performed by means of the transmission laser welding technology and the resulting weldings were characterized in terms of mechanical properties by performing peeling and shearing tests. Furthermore, both welded and mechanically tested samples were also analyzed by scanning electron microscopy in order to study the morphology of the weld seam. The results of the performed tests show that the addition of sepiolite to the PA66 improves the welding performance only in those cases in which the percentage of sepiolite of the nanocomposites is higher than 5 wt %. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46638.     

Strength prediction of T-peel joints by a hybrid spot-welding/adhesive bonding technique

The need of joining methods that best meet the design requirements has led to the increased use of adhesive joints at the expense of welding, fastening and riveting. Hybrid weld-bonded joints are obtained by combining adhesive bonding with a welded joint, providing superior strength and stiffness, and higher resistance to peeling and fatigue. In the present work, an experimental and numerical study of welded, adhesive and hybrid (weld-bonded) T-peel joints under peeling loads is presented. The brittle Araldite® AV138, the moderately ductile Araldite® 2015 and the ductile Sikaforce® 7752 were the considered adhesives. An analysis of the experimental values and a comparison of these values with Finite Element Method (FEM) results in Abaqus® were carried out, which included a stress analysis in the adhesive and strength prediction by Cohesive Zone Models (CZM) considering failure simulation of both the adhesive layer and weld-nugget. It was found that the Sikaforce® 7752 performs best in the bonded and hybrid configurations. The good agreement between the experimental and numerical results enabled the validation of CZM to predict the strength of adhesive and hybrid T-peel joints, giving a basis for reducing the design time and enabling the optimization of these joints.