Fatigue crack growth in adhesively bonded composite-metal double-lap joints

This paper presents experimental and numerical investigations of the fatigue crack initiation and growth mechanism in metal-to-composite bonded double-lap joints. Fatigue tests were conducted under tension dominated loading, with crack lengths being measured optically. Examination of the fracture surface using scanning electron microscope revealed that fatigue cracks were near the interface between the co-cured adhesive and the first ply of the composite adherend. The finite element method has been used to determine the strain-energy release rate of a fatigue crack growing along the first ply of the composite. The effects of spew fillet size and crack initiation modes have also been studied by the finite element method. Comparison of the present experimental crack growth results with those measured using double-overlap joints, where the fatigue cracks were driven by pure mode II loading, indicate that the tensile mode loading has a overwhelming effect on the fatigue crack growth rates. The present results suggest that fatigue failure of metal-composite double-lap joints is mainly driven by tensile mode loading due to the peel stress.     

Contribution to Characterize the Properties of high-temperature brazed joints

Investigations performed to study the strength behaviour supply informations about the failure pattern under loading. Beside metallographic and fractographic investigations acoustic emission measurements are used for interpretation. The results obtained for brazed joints – Ni Cr 20 Ti Al/BNi-5- show by means of the analyzed acoustic signals the possibility to evaluate the quality of a joint after a short loading period. Furthermore, hints were given to reduce the testing expenditure being necessary until now by the application of the acoustic emission analysis remarkably.     

Bond strength of vacuum brazed Mg-PSZ/steel joints

Mg-PSZ/steel joints vacuum brazed by the aid of amorphous Cu-30 w/o Ti foils were prepared in the temperature range 883 to 990 °C. A maximum bond strength of 176 MPa (flexural strength, four-point bend test) was obtained after brazing at 930 °C/5 min. Fracture energy and fracture resistance data of the interface region and the adjacent dark-coloured, oxygendeficient zone of Mg-PSZ were obtained from experiments using notched specimens. A drastic decrease of fracture energy of Mg-PSZ from 178 J/m² for the as-received material to 60.2 J/m² for the blackened zirconia adjacent to the braze was observed. This effect is assumed to influence the bond strength of the brazed joints.     

Fatigue failures of bar-attachment brazed joints for implant-supported overdentures

Oral implants are increasingly being used to enhance the mastication ability of edentulous patients using removable dentures. The dentures are connected by clips to beams brazed to gold cylinders, which are screwed onto abutments attached to as many as three to six implants generally placed in the anterior region of the mouth. Cantilever beams some 7-12 mm in length support the extended molar portion of the denture. Clinical trials of these structures indicate that they usually fail at the brazed joint at the gold cylinder and the cantilever abutment brazed region some 2-3 years after placement. In this paper we investigate the nature of the clinical failure process and compare it with simple flexure and preliminary fatigue results of similar cantilever brazed joints.     

Microstructure and mechanical properties of brazed titanium/steel joints

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler.     

Improvements of the Si3N4 brazed joints with intermetallics

The bonding of Si₃N₄ ceramics with Ag–Cu–Ti, Ni and Ti was performed. The influencing factors on joint strength were investigated. Cu–Ni–Ti intermetallic particles formed in situ were observed in the joints. Scanning electron microscopy photographs show that the interfacial reaction layer is constituted of two layers. The intermetallic particles are homogeneously distributed in the matrix so that they could contribute to the decrease in the residual stresses and the improvement of the joint strength. When bonded with proper parameters, the joint shear strength can reach more than 200 MPa, with a peak experimental value of 215.33 MPa.     

Improving the strength of brazed joints to alumina by adding carbon fibres

The addition of short, bare, carbon fibres to a silver-based active brazing alloy (63Ag-34Cu-2Ti-1Sn) resulted in up to 30% improvement in the shear/tensile joint strength of brazed joints between stainless steel and alumina. The optimum fibre volume fraction in the brazing material was 12%. This improvement is attributed to the thinning and microstructural simplification of the alumina/braze reaction product (titanium-rich) layer, the softening of the brazing alloy matrix, the strengthening of the braze and the reduction of the coefficient of thermal expansion. The depth of titanium diffusion into the alumina was decreased by the fibre addition. The first two effects are due to the absorption of titanium by the fibres. This absorption resulted in less titanium in the brazing alloy matrix, a braze/fibre particulate reaction product (titanium-rich) on the fibres and the diffusion of titanium into the fibres. In contrast, the use of an active brazing alloy with a lower titanium content but without carbon fibres gave much weaker joints. This revised version was published online in November 2006 with corrections to the Cover Date.     

Quasistatic fracture behaviour and defect assessment of brazed soft martensitic stainless steel joints

Brazed components are widely applied in industry and are often subjected to complex loading conditions. Even if such components often contain brazing defects, no failure assessment procedures for brazed joints are reported in the literature. In this work, the deformation and the quasistatic behaviour of brazed joints of the martensitic stainless steel X3CrNiMo13-4 were investigated. This includes the determination of stress–strain-curves as well as of the fracture toughness. In addition, the mechanical behaviour of components such as specimens with T-joint geometry under tensile loading were characterized. In order to consider the effect of brazing defects on the structural integrity, typical defects with different sizes and geometries were introduced in the brazing zone. The experimentally determined material parameters were used for additional numerical deformation analyses by means of finite elements (FE). Both the experimental and the numerical results were in good agreement with predictions according to the R6 method and provided a basis for the engineering defect assessment of brazed components based on failure assessment diagrams.     

Ceramic matrix composites containing carbon nanotubes

Due to the remarkable physical and mechanical properties of individual, perfect carbon nanotubes (CNTs), they are considered to be one of the most promising new reinforcements for structural composites. Their impressive electrical and thermal properties also suggest opportunities for multifunctional applications. In the context of inorganic matrix composites, researchers have particularly focussed on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material. Although there are now a number of studies published in the literature, these inorganic systems have received much less attention than CNT/polymer matrix composites. This paper reviews the current status of the research and development of CNT-loaded ceramic matrix composite (CMC) materials. It includes a summary of the key issues related to the optimisation of CNT-based composites, with particular reference to brittle matrices and provides an overview of the processing techniques developed to optimise dispersion quality, interfaces, and density. The properties of the various composite systems are discussed, with an emphasis on toughness; a comprehensive comparative summary is provided, together with a discussion of the possible toughening mechanism that may operate. Last, a range of potential applications are discussed, concluding with a discussion of the scope for future developments in the field.     

晶须增韧陶瓷复合材料

综述了晶须增韧陶瓷复合材料的制备方法和分类;讨论了影响增韧效果的各种因素及对陶瓷材料力学性能、抗热震性和耐磨性等方面的影响;并将近年来有关晶须增韧陶瓷基复合材料机理方面的研究进展,晶须在陶瓷材料中的应用及今后的发展趋势等作以介绍.     

The effect of processing variables on the microstructures and properties of aluminum brazed joints

Aluminum brazed joints are used extensively in the automotive and aircraft industries. In order to insure the integrity of the bond, the effects of processing variables on the quality of the bond must be understood. The effects of brazing period and joint thickness on the microstructure, tensile properties, microhardness and micromechanisms of failure of two aluminum alloy 3003 plates connected by a layer of 4047 aluminum filler material were investigated. It was found that the amount of aluminum-silicon eutectic microstructure in the reaction zone decreased with increasing brazing period and decreasing joint thickness. This was attributed to silicon diffusion from the joint material and dissolution of base metal and its entrance into the liquid joint. The amount of shrinkage porosity in the reaction zone was found to increase with increasing brazing period due to base material solutioning. The ultimate tensile strength of joints decreased with increasing brazing period and decreasing joint thickness. This was attributed to the joint microstructure and shrinkage porosity formed in the joint. Shrinkage porosity was found to be the primary cause of decreased joint strength. Joints with 10 minutes brazing period failed within the base material, while for brazing periods greater than 10 minutes, joints failed within the aluminum-silicon eutectic microstructure of the reaction zone. This indicated that the joint strength was greater than the base material for joint with brazing period of 10 minutes. Finite element analysis was performed to determine the effect of joint material yield strength and joint thickness on the stress and strain field in the brazed joint. Finite element analysis results supported experimental observations.     

Brazing parameters determine the degradation and mechanical behaviour of alumina/titanium brazed joints

The main aims of the present study are simultaneously to relate the brazing parameters with the correspondent interfacial microstructure, the resultant mechanical properties and the electrochemical degradation behaviour of commercially pure titanium/alumina brazed joints. A filler metal on the Ag-26.5Cu-3Ti system has been used. Three different brazing temperatures (850, 900 and 950°C) and three holding times (0.3, 1.2, 2.4 ks) were tested, in order to understand the influence of each combination of brazing temperature holding times, over the final microstructure and properties of the joints. The mechanical properties of the M/C joints were assessed on the basis of bond strength tests carried out using a shear solicitation scheme. The fracture surfaces were studied morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The degradation behaviour of the M/C joints was assessed by means of electrochemical techniques. It was found that a brazing temperature of 850°C and a holding time of 2.4 ks, produces the best results in terms of bond strength, 130 ± 16 MPa. The mechanical properties obtained could be explained on the basis of the different compounds identified on the fracture surfaces by XRD. On the other hand, a brazing temperature of 950°C produces the best results in terms of corrosion rates (lower corrosion current), 3.44 ± 0.73 A cm^–2. However, the joints produced at 850°C using a holding time of 1.2 ks present the best compromise between mechanical properties and degradation behaviour, 122 ± 12 MPa and 7.59 ± 1.47 A cm^–2 respectively. The role of Ti diffusion from the metallic Ti to the Al₂O₃ surface is fundamental in terms of the final value achieved for the M/C bond strength. On the contrary, the Ti distribution along the brazed interface does not seem to play any relevant role in the M/C joints electrochemical performance.     

Adaptation of the DCB test for determining fracture toughness of brazed joints in ceramic materials

We have developed a test, based on a modified applied moment double cantilever beam specimen, to measure the fracture toughness of brazed joints between ceramics and ceramics and metals. Evaluation of samples directly brazed with experimental brazing filler metals showed that the brazed interfaces were generally as tough as the ceramics alone, with toughness increasing proportionally to the toughness of the ceramics. Although the specimen and techniques have so far been used only for direct brazes (no surface pretreatment of the ceramic), we suggest that they would also be valid for joints involving metallization of the ceramics prior to brazing.     

Effects of gap filler and brazing temperature on fracture and fatigue of wide-gap brazed joints

The influence of gap filler content on the fracture, fatigue crack initiation and propagation of AISI 316 stainless steel wide-gap brazed with nickel-based filler metal has been investigated. The brazed joints were found to consist of eutectic, intermetallic compound and solid solution. The volume of solid solution was observed to depend on the gap filler content and brazing temperature. Tensile tests with extra small strain gauge bonded at the centre of the joints showed that the strength and elongation of the brazed joints increased with brazing temperature, and the addition of gap filler was able to improve the load-carrying capacity of the brazed joints only when the brazing temperature was high enough. Fatigue crack initiation and growth under displacement amplitude control were also carried out. Crack closure in the brazed joints was determined by means of back face strain on the compact tension specimen used. The introduction of gap filler was able to increase the fatigue and fracture resistance of the brazed joints when a suitable brazing temperature was used. Crack deflection, branching and uncracked ligament bridging behind the crack tip were observable along the crack paths. Experimental results showed that gap filler was able to enhance the crack closure caused by roughness and ligament bridging.     

Microstructure and strength of brazed joints of Ti3Al-base alloy with NiCrSiB

Brazing of Ti₃Al alloys with the filler metal NiCrSiB was carried out at 1273–1373 K for 60–1800 s. The relationship of brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1323–1373 K, brazing time is 250–300 s. The maximum shear strength of the joint is 240–250 MPa. Three kinds of reaction products were observed to have formed during the brazing of Ti₃Al alloys with the filler metal NiCrSiB, namely, TiAl₃ (TiB₂) intermetallic compounds formed close to the Ti₃Al alloy. TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds layer formed between TiAl₃ (TiB₂) intermetallic compounds and the filler metal and a Ni[s,s] solid solution formed in the middle of the joint. The interfacial structure of brazed Ti₃Al alloy joints with the filler metal NiCrSiB is Ti₃Al/TiAl₃ (TiB₂)/TiAl₃+AlNi₂Ti (TiB₂)/Ni[s,s] solid solution/TiAl₃+AlNi₂Ti (TiB₂)/TiAl₃ (TiB₂)/Ti₃Al, and this structure will not change with brazing time once it forms. The formation of over many intermetallic compounds TiAl₃+AlNi₂Ti (TiB₂) results in embrittlement of the joint and poor joint properties. The thickness of TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) in the brazed joints of Ti₃Al alloys with the filler metal NiCrSiB are 349 kJ/mol and 24.02 mm²/s, respectively, and the growth formula was y²=24.04exp(−41977.39/T)t. Careful control of the growth of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) can influence the final joint strength.