The role of interlayers in diffusion bonded joints in metal-matrix composites

Solid state bonded joints in a particulate metal-matrix composite containing 17 vol% SiC in Al-Li 8090 alloy had planar bond interfaces with particle-particle (P-P) interfaces and particle-matrix (P-M) interfaces aligned in and parallel to the bond interface. The insertion of a matrix interlayer into the MMC-MMC bond interface changed the type of particle interface and interface area fractions in the two new bond planes created. In these planes P-P interfaces became P-M interfaces and the P-M area fraction either increased or fell to zero depending on the particle symmetry with respect to the bond plane. The implications for the mechanical properties of diffusion bonded joints in MMCs are discussed.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

The shear strength () of overlap shear test pieces made by solid state diffusion bonding or by machining thin (2.5 or 4 mm thick) Al-Li 8090 alloy sheet has been determined for various overlap lengths (/). When / < 3 mm, was independent of / and equal to 188 to 202 MPa for the bonded joint and 199 to 209 MPa for the base metal sheet. The lower mean shear strength of the bonded joint was caused by the lower resistance of intergranular fracture in the planar grain boundary at the bond interface. The bond strengths were, however, greater than those previously reported for joints in 8090 alloy made by solid-state or liquid-phase diffusion bonding and about a factor of 7 greater than those for adhesive bonded joints.     

Effect of Ni interlayer on strength and microstructure of diffusion-bonded Mo/Cu joints

Mo and Cu were bonded successfully by means of diffusion bonding using a Ni interlayer. The tensile strength of the joint increases firstly and then decreases with the bonding temperature or holding time increases. Compared with 79 MPa which was the maximum value of Mo/Cu joint, the maximum tensile strength of joint with Ni interlayer was 97 MPa. The interfacial structure of the joints was studied by SEM, EPMA, EDS and XRD, the results showed that the different atoms diffused to each other in the bonding process and no intermetallic compound appeared. MoNi and NiCu solid solutions formed in the joint. The fracture of the joint had taken place in the Mo/Ni interface rather than in the Ni/Cu interface and the fracture way of the joints was brittle fracture.     

Microstructure and mechanical properties of diffusion bonded SiC/steel joint using W/Ni interlayer

This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W₅Si₃ and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni₄W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.     

Effect of carbon nanotube modified epoxy adhesive on CFRP-to-steel interface

The effects of carbon nanotube (CNT) modified epoxy adhesive on CFRP-to-steel interfaces were investigated using double strap joints. The bond behaviours studied were failure modes, bond interface at microlevel, bond strength, effective bond length, CFRP strain distribution and bond-slip relationships.For the first time, a novel type of failure in the CFRP-steel joint was discovered, attributable to weak bonding between woven mesh and CFRP fibres. This failure mode prevented exploitation of the full potential of the carbon fibres and the CNT modified epoxy adhesive. Joints bonded with CNT-epoxy adhesive had an effective bond length of about 60 mm, whereas that of joints bonded with pure epoxy was about 70 mm. The CNT-epoxy adhesive can transfer more load from the host structure to the bonded CFRP laminates, consequently modifying bond behaviour. It is therefore expected that CNT-epoxy nanocomposites will assist in the strengthening and rehabilitation of steel infrastructures using CFRP laminates.     

High-resolution interface analysis of SiC-whisker-reinforced Si3N4 and Al2O3 ceramic matrix composites

Whisker/matrix interfaces between -SiC whiskers and -Si₃N₄ or -Al₂O₃ matrices in composites were examined by high-resolution electron microscopy (HREM), and electron energy loss (ELS) and energy dispersive X-ray (EDS) spectroscopies. Most whisker/matrix interfaces were crystalline, with whiskers directly bonded to matrix crystals. Some whisker/matrix interface regions contained amorphous thin films and these occurred more often in the Si₃N₄ composite, which contained sintering additives, than in the Al₂O₃ matrix composite, which did not. No evidence for light element segregation at crystalline whisker/matrix interfaces was detected by ELS or EDS at 5 nm spatial resolution. Impurities were concentrated in glassy regions in matrix grain boundaries, triple junctions, or at infrequent whisker/matrix interfaces containing amorphous films.     

Analytical solution for the bond strength of externally bonded reinforcement

Interfacial behavior is critical to composite structures and materials reinforced by externally bonded reinforcement. Numerous empirical and semi-empirical models have been developed for evaluating interfacial bond strength. Analytical solutions have been derived for interfaces with infinite bond lengths, but no closed-form solutions have been derived for the bond strength of an interface with an arbitrary bond length. An analytical solution is derived in this work for the strength of a general externally bonded interface. With the analytical method, the interesting snapback phenomenon in simple pull-off tests is theoretically studied, and an invariant is identified as the condition for it to occur. Furthermore, a methodology is provided to evaluate the interfacial material properties based on a given empirical bond model.     

Bond-slip model for FRP-to-concrete bonded joints under external compression

The influence of compressive stresses exerted on FRP-concrete joints created by external strengthening of structural members on the performance of the system requires better understanding especially when mechanical devices are used to anchor the externally bonded reinforcement (EBR). The numerical modelling of those systems is a tool that permits insight into the performance of the corresponding interfaces and was used in the present study, essentially directed to analyse the effectiveness of EBR systems under compressive stresses normal to the composite surface applied to GFRP-to-concrete interfaces. The compressive stresses imposed on the GFRP-to-concrete interface model the effect produced by a mechanical anchorage system applied to the EBR system. An experimental program is described on which double-lap shear tests were performed that created normal stresses externally applied on the GFRP plates. A corresponding bond-slip model is proposed and the results of its introduction in the numerical analysis based in an available 3D finite element code are displayed, showing satisfactory agreement with the experimental data. The results also showed that lateral compressive stresses tend to increase the maximum bond stress of the interface and also originate a residual bond stress which has significant influence on the interface strength. Also, the strength of the interface increases with the increase of the bonded length which have consequences on the definition of the effective bond length.     

Numerical modelling of the effects of elevated service temperatures on the debonding process of FRP-to-concrete bonded joints

There are many issues concerning the performance behaviour of FRP-to-concrete interfaces at elevated service temperatures (EST). At EST, i.e. slightly above the glass transition temperature (T_g), some properties associated with the FRP composites, such as the stiffness, strength or the bond characteristics, degrade. This is a crucial issue and there are only a few studies that take into account such effects on FRP-to-concrete interfaces at EST. This paper examines, through a numerical analysis, the performance of FRP-to-concrete bonded joints at EST using a new discrete model based on truss elements and shear springs. The External Bonded Reinforcement (EBR) systems subjected to EST are analyzed. The numerical discrete model was implemented in a MATLAB routine and the bond–slip curves of the interfaces at EST were obtained from a model found in literature. The numerical results revealed that the interface at EST behaves similarly to one with two equal mechanical loads applied at both ends of the FRP plate. The load–slip curves or bond stresses, strains or slippages along the bonded length obtained from several bond–slip curves at different temperatures were obtained. Two different single-lap shear tests were simulated at steady-state (steady temperature followed by load increase) and transient state (steady load followed by temperature increase). Regarding the influence of the temperature on the adhesion between the FRP and concrete, the results showed that an increase in the temperature at an earlier situation, i.e. during a period where temperature had no influence in the concrete deformations, leads to an increase in the effective bond length of the interface affecting the initial strength of the interface.     

Mechanical response of anchored FRP bonded joints: A nonlinear analytical approach

This article presents a nonlinear analytical solution for the prediction of the full-range debonding response of mechanically anchored, fiber-reinforced polymer (FRP) composites from the substrate. The nonlinear analytical approach predicts, for any monotonic loading history or bonded length, the relative displacements (or slips) between materials, the strains in the FRP composite, the bond stresses within the interface, and the stresses developed in the substrate. The load-slip responses of FRP-to-substrate interfaces with short and long bonded lengths are motives of analysis and discussion. The solutions obtained from the proposed approach are also compared with other experimental results found in the literature.     

Analytical modeling of the bond–slip relationship at FRP-concrete interfaces for adhesively-bonded joints

The bond–slip relationship is essential to the study of the macro mechanical properties of composite materials and structures. An analytical model is developed in this work to derive the bond–slip relationship at the reinforcement-substrate concrete interface (joint) for externally bonded reinforcement. The model is applicable to both long joints (infinite bond length) and short joints. When the bond length approaches infinity, the model reverts to a well-known existing analytical model. A bond–slip relationship for short joints with a limited bond length is derived for the first time. It is concluded from the modeling that the existing model for long joints is not applicable to short joints that have a bond length that is less than the effective bond length, or at locations in long joints that are closer than the effective bond length to the free end of the reinforcement.     

Diffusion mechanisms and reactions during reduction of oolitic iron-oxide mineral

Diffusion mechanisms with moving reaction interfaces involved in the reduction process of oolitic iron oxide, containing small goethite particles in a kaolinite matrix, are presented. Reduction was effected by means of CO gas at 950° C, with the oolite already transformed by dehydroxylation into haematite particles and a metakaolinite matrix. The haematite particle under the CO + O CO₂ reaction taking place at its external surface develops concentric layers with unreacted haematite at the core enclosed by magnetite wustite and metallic iron, in that order. In the matrix between particles, bridges of a two-phase mixture of hercynite and fayalite develop by diffusion of iron ions and reactive transport of oxygen (by means of CO₂ molecules), thereby permitting coarsening of the metallic particles. Detailed models are presented for the diffusion mechanisms and reactions involved, and the thermodynamical picture is brought out.Deceased November 1987.     

An interpretation of the internal length in Chang’s couple-stress continuum for bonded granulates

Theoretical investigation into bonded granulates is of great importance to modern geomechanics. This paper presents an interpretation of the internal length in Chang’s couple-stress continuum for bonded materials (Chang et al., Eng Fract Mech 69(17):1941–1958, 2002; Chang and Shi, J Eng Mech ASCE 131(2):120–130, 2005). First, a micro-model for bonds between particles is proposed, in which a pair of bonded particles is in contact over a width B and this bond contact width is continuously distributed with the normal/tangential basic elements (BE) (each BE is composed of spring, dashpot, bond, slider or divider). Second, a couple model for the bonds is established to describe the relationship between relative particle rotation and couple. Third, Chang’s couple-stress continuum for bonded material is analyzed in the case of a hexagonal packing structure of a monodisperse bonded material. The study shows that the internal length in Chang’s couple-stress continuum for bonded monodisperse material is equal to , while other usual elastic constants of the continuum can also be linked to B. In addition, those constants can be associated with a size-independent parameter proposed in this paper.     

Aligned microstructure of some particulate polymer composites obtained with an electric field

Alignment by an electric field was obtained for a variety of particles dispersed in photopolymerizable fluids. The particle shapes studied were irregular, spherical, rhombohedral, rod-like (fibres), and platelet. The sizes ranged from sub-micrometres to tens of micrometres, and the dielectric constants of the particles varied from less than that of the lquid matrix to very much greater than that of the matrix. Polymerization or hardening of the matrix was possible at room temperature, required only a few seconds, and the aligned structures obtained were able to be examined by both light and scanning electron microscopy after fracture or sectioning. Nominally equiaxed particles, containing a statistical proportion of non-equiaxed particles, could be completely aligned at 48 vol% concentration in a fluid having a viscosity of about 2.5 Pa s, but at 57 vol%, the mixture behaved as a paste, and only particle rotation and local rearrangements were possible. The rate of alignment seemed to depend generally on the magnitude of 1(a)2, where 1 is the relative dielectric constant of the liquid resin, a is the particle radius, and is the particle dipole coefficient given by (2–1)/(2+21), where 2 is the relative dielectric constant of the particles. 1(a)2 emphasizes the importance of particle size and the relative unimportance of the particle dielectric constant for alignment, except when 21. Platelets were more rapidly aligned than fibres.     

Microstructural characteristics of Au/Al bonded interfaces

Fracture characteristics at the interface of ultrasonic bonds between Au and Al were characterized by SEM following pull-testing to effect separation of the bonded joints. Vertical sections at the bonding point were produced by ion-sputter thinning, and were examined by TEM. Results show that the thickness of the Au/Al atomic diffusion interface was about 500 nm due to combined effects of ultrasonic and thermal energy. Ultrasonic vibration activates dislocations in the crystalline lattice and increases atomic diffusion. The fracture morphology on the lift-off interface was dimpled rupture. Tensile fracture occurred during the pull-test not at the bonded interface but in the base material; the bond strength at the interface was enhanced by the diffusion reactions that occurred across the interface due to the combined ultrasonic and thermal energy.     

Experimental study on CFRP-to-steel bonded interfaces

This paper presents an experimental study on the behaviour of CFRP-to-steel bonded interfaces through the testing of a series of single-lap bonded joints. The parameters examined include the material properties and the thickness of the adhesive layer and the axial rigidity of the CFRP plate. The test results demonstrate that the bond strength of such bonded joints depends strongly on the interfacial fracture energy among other factors. Nonlinear adhesives with a lower elastic modulus but a larger strain capacity are shown to possess a much higher interfacial fracture energy than linear adhesives with a similar or even a higher tensile strength. The variation of the interfacial shear stress distribution in a bonded joint as the applied load increases clearly illustrates the existence of an effective bond length. The bond–slip curve is shown to have an approximately triangular shape for a linear adhesive but to have an approximately trapezoidal shape for a nonlinear adhesive, indicating the necessity of developing different forms of bond–slip models for different adhesives.     

Structure and strength of AlN/V bonding interfaces

AlN ceramics are bonded using vanadium metal foils at high temperatures in vacuum. Different bonding temperatures were used in the range 1373–1773 K with bonding times of 0.3–21.6 ks. The AlN/V interfaces of the bonded joints were investigated using SEM, electron probe microanalysis and X-ray diffraction. A bonding temperature of 1573 K was found to be suitable to activate both parts to initiate a phase reaction at the interface, because a thin V(Al) solid solution layer formed adjacent to the ceramic at 1573 K just after 0.9 ks, and a small flake-shaped V₂N reaction product formed inside the vanadium central layer. The formation of V(Al) and V₂N controls the interfacial joining of the AlN/V system at 1573 K up to 5.4 ks bonding time. The pure vanadium layer quickly changed to vanadium-containing V₂N. The diffusion path could be predicted for the AlN/V joints up to 0.9 ks at 1573 K following the sequence AlN/V(Al)/V₂N/V, while after 0.9 ks, the interface structure changed to AlN/V(Al)/V₂N + V by the growth Of V₂N into the vanadium. The AlN/V joints shovyed no ternary compounds at the interface. A maximum bond strength could be obtained for a joint bonded at 1573 K after 5.4 ks having a structure of AlN/V(Al)/V₂N + V. At 7.2 ks, nitrogen, resulting from AlN decomposition, escaped and the remaining aluminium reacted with V(Al) to form V₅Al₈ intermetallic, which is attributable to the decrease in bond strength.     

Effects of surface roughness on the diffusion bonding of Al alloy 6061 in air

The effect of surface roughness on the properties of Al6061 joints fabricated by diffusion bonding in air at 450°C was studied. It was found that rougher surfaces yield superior ultimate tensile strength and linearized bonded ratio. Joints with ultimate tensile strength comparable to that of bulk metal were obtained for holding times of 75 min using rougher surfaces; however, the maximum linearized bonded ratio obtained was only about 75%. Incomplete bonding was attributed to air entrapment along the bond interface. This was due to good bonding occurring at the periphery of the bonded specimens during the early part of the bonding process.     

Failure loads for model adhesive joints subjected to tension,compression or torsion

The Griffith fracture criterion has been applied to model adhesive joints subjected to tension, compression or torsion. Two model joints are considered: a rigid cylinder partly embedded in and bonded to an elastic cylinder (termed rod joint here), and an elastic cylinder inserted partway into, and bonded to, a rigid tube (termed sleeve joint here). Both types of joint have been constructed, using vulcanized rubber cylinders bonded to aluminium rods and sleeves.Measurements have been made of the failure loads under tension, compression and torsional loading. They were found to be in satisfactory agreement with the theoretical predictions except, in some instances, for rod joints subjected to tension or torsional loading when the failure loads were as much as three times the predicted values. This discrepancy is attributed to friction between the partially-detached rubber cylinder and the embedded rod, enhanced to a great extent by the tendency of the rubber cylinder to shrink in radius on stretching or twisting. A theoretical analysis of the effect of friction is presented. It predicts increasingly large pull-out forces or torques, as the depth of embedment increases, until frictional seizure occurs. Experimentally, frictional effects were limited by applying an internal gas pressure to the region being detached. All of the failure loads were then found to be in satisfactory agreement with the original theory, ignoring frictional effects. Thus, a simple fracture energy criterion is shown to govern the failure of adhesive joints under complex loading conditions, with or without friction acting at the interface.On leave from the Rubber Research Institute of Malaysia, Kuala Lumpur, Malaysia.     

Diffusivity of carbon in copper- and silver-based composites

Interstitial solid solutions could be formed at the matrix–fibre interface in the processing of metal matrix composites. Typically these solutions are of small concentrations and the solubility is usually diffusion-controled. To calculate the diffusivity, a model of the interstitial solid solution is used which gives the possibility of choosing whether octahedral or tetrahedral interstitial positions are occupied by the fibre atoms dissolved in the matrix. Analysis of Ag–C and Cu–C solid solutions allow a comparison of the occupation of the interstitial positions and its temperature dependences. The results obtained on the basis of non-empirical calculations predict the preferable occupation of octahedral positions up to T 1200 K. This confirms the structure of the interstitial solid solution. In the framework of this model we calculate the heights of diffusion barriers and the temperature dependences of carbon diffusion in silver and copper hosts.