Correlation of material constraint with fracture toughness of interface regions in a dissimilar metal welded joint

In this work, the constraint parameter A_p based on crack‐tip equivalent plastic strain was calculated by finite element analyses for the cracks located at different locations in two interface regions in a dissimilar metal weld joint (DMWJ). The capabilities of the parameter A_p for characterizing material constraint and establishing correlation of material constraint with fracture toughness of the interface region cracks have been examined. The results show that the parameter A_p can characterize material constraint effect caused by material mismatch and initial crack positions in the interface regions. Based on the A_p, the correlation lines and formulae of material constraint with fracture toughness of the interface region cracks in the DMWJ can be established, and they may be used for obtaining material constraint‐dependent fracture toughness for the interface region cracks. The results in this work combining with those in the previous studies indicate that the parameter A_p may be a unified constraint parameter that can characterize both geometry constraint (including in‐plane and out‐of‐plane constraints) and material constraint, and it may be used in accurate fracture assessments of welded components with different geometry and material constraints.     

Dynamic mechanical behavior of nickel-based superalloy metal rubber

The work describes the manufacturing and dynamic characterization of nickel wire-based metal rubber (MR) solids. The storage modulus and the loss factor of the nickel MR samples are measured over a frequency range between 0.1 Hz and 200 Hz, and at different levels of dynamic force and strain using a dynamic mechanical analyzer (DMA) technique. A sensitivity analysis about the effect of different static and dynamic testing parameters is initially carried out to identify suitable testing protocols for this metal porous material. DMA testing is then carried out over three different batches of samples (5 specimens each) with variable relative densities to identify the correlation between storage modulus and loss factors with frequency and dynamic force and strain levels. The results are discussed using a mechanical theoretical model relating the mechanical properties of MR solids to the contact states of the wire composing the microstructure. A comparison with analogous results obtained from cyclic tests at 1 Hz from a conventional tensile machine is also performed. The results from this benchmark highlight the necessity to use dynamic-based testing protocols to efficiently implement nickel-based metal rubber for vibration damping and energy absorption designs and applications.     

Interface-dominated mechanical behavior in advanced metal matrix composites

Metal matrix composites(MMCs)incorporate a reinforcing or functional secondary phase into a metal matrix to achieve specific properties.Of the parameters which may affect the mechanical behavior of MMCs,the structure and properties of the reinforcement/matrix interface play a crucial role.This article reviews recent developments in measuring the interfacial properties in advanced MMCs,with an emphasis on the use of micro-/nano-mechanical testing approaches.It is shown that,with the novel in situ and ex situ experimental capability,researchers can now obtain some of the critical interfacial properties as well as the effects of reinforcement/matrix interfaces on the composites’deformation and failure mechanisms that were unattainable previously by conventional methodologies.Moreover,the micro-/nano-mechanical testing platform allows for both fundamental and applied research on the composites’mechanical performance under service conditions,which is considered a promising and emerging research direction.     

Microstructure characteristics and mechanical properties of cold metal transfer welding Mg/Al dissimilar metals

AZ31B Mg alloy and 6061 Al alloy were joined by using cold metal transfer (CMT) welding with pure copper (HS201) as the filler metal. The microstructure of Mg/Al CMT weld joint was studied by means of Optical Microscopy, Scanning Electron Microscope (SEM), Energy Dispersive X-ray (EDX), X-ray Diffraction (XRD). Results showed that dissimilar metals of Mg/Al could be successfully joined by CMT under proper processing parameters. The bonding strength of the joint was 34.7 MPa. A variety of Al–Cu intermetallic compounds, i.e. AlCu, CuAl₂, Cu₉Al₄, presented in the fusion zone of Al side, and Cu based solid solution was generated in weld zone, while Cu₂Mg and Al–Cu–Mg ternary eutectic structure was formed in the fusion zone of Mg side. The micro-hardness in the both sides of fusion zones increased sharply, which were 362 HV in Mg side and 260 HV in Al side. The joint was brittle fractured in the intermetallic compound layer of the fusion zone of Mg side, where plenty of Cu₂Mg intermetallic compounds were distributed continuously.     

Fabrication of a single metal nanowire connected with dissimilar metal electrodes and its application to chemical sensing

We report a convenient method for the fabrication of a single metal nanowire connected with dissimilar metal electrodes and its application to chemical sensing. The method is based on a combination of atomic force microscopy nanomachining and conventional photolithography. The success of this integrated approach is confirmed by the linear current-voltage behavior of the created nanowires and comparable resistivities with those reported previously. The chemical sensing capability is demonstrated by the selective binding of a self-assembled monolayer onto a single Au nanowire connected with Ti electrodes and the subsequent resistance increase due to increased surface scattering effects after adsorption. It is found that the resistance increases by around 9% after the complete coverage of either octadecanethiol or dodecanethiol molecules onto a 20 nm thick Au nanowire. A theoretical explanation for the relationship between the resistance increase and the alkanethiol concentration is also given.     

Laser welding of dissimilar metal combinations

The ability to manufacture a product using a number of different metals and alloys greatly increases flexibility in design and production. Properties such as heat, wear and corrosion resistance can be optimized, and benefits in terms of production economics are often gained. Joining of dissimilar metal combinations is, however, a challenging task owing to the large differences in physical and chemical properties which may be present. Laser welding, a high power density but low energy-input process, provides solutions to a number of problems commonly encountered with conventional joining techniques. Accurate positioning of the weld bead, rapid heating and cooling, low distortion, process flexibility, and opportunities for product redesign are its principal characteristics. The review describes the principles underlying laser welding of dissimilar metal combinations and highlights the above benefits in a number of practical applications. It is concluded that there is potential for its application in many industrial sectors.     

Determination of mechanical properties of Al–Mg alloys dissimilar friction stir welded interface by indentation methods

A series of friction stir welds was produced between heat treated Al–Mg–Si and strain hardened Mg–Al–Zn alloy sheets. Weld evaluation by transverse tensile testing showed a wide range of strengths and all the failures occurred along the weld interface. The formation of intermetallic compounds in the weld joints was investigated by X-ray diffraction, scanning electron microscopy imaging, and elemental analysis techniques. Micro and nanoindentation characterization methods were used to evaluate the mechanical properties at the interface, including the fracture toughness. The fracture toughness measurements by a Vickers indenter introduced Palmqvist type cracks at all four corners of the indents and cube corner indenter resulted in the intermetallic chipping. The fracture toughness (K _IC) calculation by both the micro and nanoindentation methods showed very low values, which is the primary reason for the brittle failure of the dissimilar weld joints and concomitant low tensile strengths.     

Microstructure and mechanical behavior in dissimilar 13Cr/2205 stainless steel welded pipes

This work aims to investigate the microstructure and the mechanical behavior of dissimilar 13Cr Supermartensitic/2205 Duplex stainless steel welded pipes. A wide variety of microstructures resulting from both solidification and solid state transformation is induced by the fusion welding process across the weld joint. The tensile tests show that the deformation process of the dissimilar weld joint is mainly controlled by the two base materials: the duplex steel at the beginning of the deformation and the supermartensitic one at its end. This is confirmed by the micro-tensile tests showing the overmatching effect of the weld metal. The fatigue tests conducted on dissimilar welded specimens led us to conclude that the weld metal is considered as a weak link of the weld joint in the high cycle fatigue regime. This is supported by its lower fatigue limit compared to the two base materials that exhibit a similar fatigue behavior.     

In-situ SEM study of short fatigue crack propagation behavior in a dissimilar metal welded joint of nuclear power plant

In-situ Scanning Electron Microscopy (SEM) fatigue experiments were carried out to study short fatigue crack propagation (FCP) behavior of various regions (weld zone, interface region and heat affected zone (HAZ)) in a domestic dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short fatigue crack initiation and propagation behavior was investigated with its influence on both material fatigue and structure fatigue analyzed. Considering material fatigue, in the weld region, crack grows along δ ferrites when propagating parallel to the dendrite, and deflects or branches along δ ferrite, γ austenite dendrite, δ/γ interface and grain boundaries when propagating perpendicular to the dendrite; in safe ends, the crack grows along slip lines and coalesces with secondary cracks; in A508 HAZ, the crack propagates or branches along martensite transgranularly. In terms of structural fatigue, the crack tends to deflect when propagating across the weld/A508 interface or weld/316 L interface with the influence of local microstructure, and the weld/A508 interface region has a resistance to FCP due to its high strength. The fatigue crack propagation rate of each region was compared and analyzed. The fatigue fractography was also characterized under SEM to analyze the crack propagation process.     

Friction stir welding of dissimilar metal joints

Friction stir welding is a solid‐state welding technology, which is suitable for joining dissimilar metals such as aluminium and copper. Because the solidus temperature is typically not exceeded, the formation of intermetallic phases can be reduced when compared to fusion welding processes. In friction stir welding, the intermetallic layer thickness, which determines the seam properties, is influenced by the welding temperature and is formed in correspondence with the Arrhenius law. It is typically in the range of a few hundred nanometers thick. In turn, the process temperature is determined by the process parameters, primarily the rotational speed and the feed rate of the machine tool. In this study, a temperature‐controlled friction stir welding process has been applied to lap joints of aluminium and copper. Welding experiments with various welding speeds and probe lengths were performed in order to assess the effect of the temperature‐time profile near the welding interface. The joints were investigated by tensile shear tests as well as optical microscopy and scanning electron microscopy.     

Fe/Al异质金属接头界面组织演变、生长动力学及力学性能

采用真空扩散连接方法研究Fe/Al异质金属接头界面组织演变规律、金属间化合物(intermetallic compound,IMC)生长动力学及力学性能。结果表明：焊接温度为550 ℃时,接头界面无IMC生成,当焊接温度超过575 ℃时,界面由Fe₂Al₅及少量FeAl₃ IMC构成,且随焊接温度升高IMC层迅速长大。在120 min保温时间条件下,接头剪切强度随焊接温度的升高先增加后降低,当焊接温度为575 ℃时,接头剪切强度达到最大值37 MPa。在550~625 ℃范围内,基于热力学分析得出Fe₂Al₅的吉布斯自由能ΔG_Fe-Al最低,而FeAl₃的ΔG_Fe-Al次之,在接头界面处IMC生成顺序为Fe₂Al₅→FeAl₃。Fe/Al接头界面IMC的生长随焊接温度呈抛物线规律,其生长激活能为282.6 kJ·mol^-1。在575,600,625 ℃条件下,界面IMC的生长速率分别为1.13×10^-14,3.59×10^-14,1.21×10^-13 m²·s^-1。     

Boundary element analysis of dissimilar materials and interface crack

The highly-accurate BEM elastostatic program, which is especially useful for the analysis of dissimilar materials and interface cracks, is introduced in brief. By using this program, we can deal with the elastostatic poblems of isotropic or orthotropic dissimilar materials and also the bonded residual stress due to the mismatch of material constants. This paper shows some applications of the BEM program to the analysis of dissimilar materials and interface cracks considering the residual stress quantitatively, and also shows the method to evaluate the strength of dissimilar materials based on the interfacial fracture mechanics. Some experimental results and the evaluation on the strength of dissimilar materials are also presented.     

Stress intensity factor analysis of a three-dimensional interface crack between dissimilar anisotropic materials

A new numerical method to calculate the stress intensity factors (SIFs) of a three-dimensional interface crack between dissimilar anisotropic materials was developed. In this study, the M-integral method was employed for mode separation of the SIFs. The moving least-square method was utilized to calculate the M-integral. Using the M-integral with the moving least-square method, SIFs can be automatically calculated with only the nodal displacements from the finite element method (FEM). Here, SIFs analyses of some typical three-dimensional problems are demonstrated. Excellent agreement was achieved between the numerical results obtained by the present method and the corresponding results proposed by other researchers. In addition, the SIFs of a single-edge crack, a through crack, and a semi-circular crack between two anisotropic solids in three-dimensional structures were analyzed.     

Biaxial-load effects on isopachics for a crack at the interface of dissimilar media

Summary The plane problem of two dissimilar materials, bonded together and containing a crack along their common interface, which were subjected to a biaxial load at infinity, is examined by giving a closed-form expression for the first stress invariant of the normal stresses, which is equally valid everywhere, near to, and far from, the crack-tip region. This exact expression for the first-stress invariant is compared by constructing the respective isopachic-fringe patterns, to the approximate expression with non-singular terms, due to the biaxiality factor, for the same quantity. Significant differences between respective isopachic-patterns were found and their dependence on the elastic properties of both materials and the applied loads was demonstrated. The relative errors between the computedK _I - andK _II -components by using the approximate expression for the first stress-invariant and the accurate one, derived from closed-form solution along either isopachic-fringes or along circles and radii from the crack-tip have been given, indicating in some cases large discrepancies between exact and approximate solutions.With 7 Figures     

Steady state propagation of a mode III interface crack between dissimilar viscoelastic media

The steady state propagation of a semi-infinite crack between two dissimilar viscoelastic solids is considered. By means of the Wiener-Hopf technique, the stress intensity factor is found as a function of the crack tip velocity and the material parameters. Results for an interface crack between an elastic and a viscoelastic medium are obtained as a special case. Various limiting cases are examined as a check on the accuracy of the results. Finally, graphs are presented which examine the salient features of the stress intensity factor.     

Interaction of longitudinal wave with a penny-shaped crack at the interface of two bonded dissimilar elastic solids-II

The paper deals with the problem of finding the stress distribution near a penny-shaped crack situated at the interface of two bonded dissimilar elastic solids. The crack is opened by the interaction of plane harmonic longitudinal elastic wave, incident normally on the crack. The problem is first reduced to a set of simultaneous dual integral equations which are further transformed to a set of simultaneous singular integral equations. These are solved numerically by reducing them to a set of algebraic equations. The solution is used to calculate the stress-intensity factors and the size of the overlapping zones at the edge of the crack.

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Résumé Ce mémoire est relatif au problème de trouver la distribution des contraintes au voisinage d'une fissure circulaire à l'interface de deux solides élastiques dissemblables et collés. La fissure est ouverte sous l'effet d'une onde élastique longitudinale plane et harmonique, dont l'incidence est normale au plan de la fissure. Le problème est en premier lieu ramené à un système d'équations intégrales doubles, qui sont transformées ensuite en un système d'intégrales singulières. On résoud ces dernières par voie numérique en les réduisant à un système d'équations algébriques.La solution est appliquée au calcul des facteurs d'intensité de contraintes, et la dimension des zones de recouvrement aux bords de la fissure.