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.     

Estimating interface bonding strength in clad sheets based on tensile test results

The bond strengths of the three-layered aluminum/stainless steel/aluminum composite clad sheets produced by cold roll bonding were estimated by the tensile test results. The developed procedure was based on the drops in flow stress after the maximum point of the stress–strain curves, which are related to debonding of interfaces and signify the effect of bond strength on the tensile behavior. The shape of the stress–strain curve of a specimen heat treated at 600 °C to form a brittle intermetallic layer at the roll bonding interface was found to be analogous to those obtained from the tensile test of conventional one-layer specimens, which was considered to be a good evidence of interface debonding during tensile test of roll bonded sheets that show sharp flow stress drops. An important contribution of this work is correlating the tension test to peel test results by simple calculations based on the principles of mechanics of materials.     

Double shear tests to evaluate the bond strength between GFRP/concrete elements

Externally bonded reinforced systems have been widely used in civil engineering. However, the problems associated with bond between structural elements are not yet fully solved. As a consequence, many researchers have been proposing tests and techniques to standardize procedures and reach better agreement for design purposes. In the present paper, an experimental program is described that was developed to characterize the glass FRP/concrete interface by double shear tests made on 15 cm side cubes with GFRP bonded on two opposite faces. The GFRP wrap had two layers applied by the wet lay-up technique and three classes of concrete were considered. With the support of the experimental program, cohesion and friction angle for GFRP-concrete interfaces were found leading to different envelope failure laws, based on the Mohr-Coulomb failure criterion for each concrete class, capable of predicting GFRP debonding. Results are discussed.     

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.     

碳纤维布加固RC梁中粘结性能的非线性有限元分析

碳纤维布加固钢筋混凝土(RC)梁中,碳纤维布与梁底混凝土的剥离破坏使碳纤维布的强度不能得到充分发挥。分析碳纤维布与梁底混凝土的粘结应力,是研究碳纤维布加固剥离破坏承载力的基础问题。根据4根碳纤维布加固RC梁的试验研究结果,采用商业有限元程序MSC.Marc建立有限元模型,进行了非线性计算分析。通过分离总粘结应力中的局部粘结应力,得到粘结延伸长度范围内的锚固粘结应力分布,并结合试验数据对其分布规律进行了研究。根据分析和试验结果,引入了“有效锚固粘结长度”和“锚固粘结应力”的概念,给出了极限荷载下锚固粘结应力的计算建议。     

Magnetic properties and microstructure of radially oriented Sm(Co,Fe,Cu,Zr)z ring magnets

Radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets are prepared by powder metallurgy with appropriate magnetic field molding, sintering process and aging treatment. The results indicate that radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets have obvious anisotropy of thermal expansion and sintering shrinkage, which easily lead to the splits and deformation of the ring magnets. So, slow heating, vacuum pre-sintering in sintering process and various quenching processes at different steps during quenching are adopted. The magnets have excellent magnetic properties: B_r = 10.8 kGs, H_cj = 27.6 kOe, BH_max = 28.1 MGOe. Besides, there is a uniform magnetization field on the surface of the ring magnets. The average surface magnetization field () is 1.502 kGs. The deviation from average (α) is only 4.2%. The microstructure of the magnets consists of a mixture of homogeneous cellular and lamella structures.     

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

Diffusion bonding between W and steel using V/Ni composite interlayer was carried out in vacuum at 1050 °C and 10 MPa for 1 h. The microstructural examination and mechanical property evaluation of the joints show that the bonding of W to steel was successful. No intermetallic compound was observed at the steel/Ni and V/W interfaces for the joints bonded. The electron probe microanalysis and X-ray diffraction analysis revealed that Ni₃V, Ni₂V, Ni₂V₃ and NiV₃ were formed at the Ni/V interface. The tensile strength of about 362 MPa was obtained for as-bonded W/steel joint and the failure occurred at W near the V/W interface. The nano-indentation test across the joining interfaces demonstrated the effect of solid solution strengthening and intermetallic compound formation in the diffusion zone.     

Surface modification of spherical NdFeB magnetic powders by a fluid-bed nickel electrodeposition

The spherical NdFeB magnetic powders were coated with Ni using a fluid-bed electrodeposition device. The oxidations of rich Nd phase or Fe phase were restrained after Ni coating. After annealing at 300 °C for 30 min, the magnetic properties of both coated and uncoated magnets were decreased. However, the Ni-coated powders showed better magnetic properties than the uncoated powders after the annealing. The compressive strength of bonded magnets improved after annealing due to enhancement of the adhesion between the adhesive and the surface of the magnetic powders.     

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.     

Parameters affecting the debonding risk of bonded overlays used on reinforced concrete slab subjected to flexural loading

This research aims at better understanding the mechanisms involved in the cracking behaviour of bonded overlays used on reinforced concrete beams and slabs. The project involves the testing of reinforced concrete beams (1.8 m × 0.2 m × 0.2 m) repaired with concrete overlays using four different types of surface preparation and subjected to cyclic flexural loading. The concrete beam specimens were simply supported with a point load at midspan. Structural capacity (evolution of the apparent rigidity, maximum deflection, failure mode) and cracking behaviour (flexural cracking and interface debonding) were monitored. Technical data indicate that good adhesion alone is insufficient to guarantee bond durability. A total of 20 beam interfaces were prepared by scarification, sandblasting, chipping with a light jackhammer, and water jetting. A number of cores were tested to evaluate interface strength by direct shear and direct tension. Roughness was characterized quantitatively using a newly developed optometric method. The results indicate that cracking behaviour depends on the bond strength and on the surface roughness produced by a specific surface treatment. The relationships between adhesion, structural behaviour, and roughness were evaluated, and an updated debonding mechanism is proposed to take into account the influence of roughness. To achieve a monolithic behaviour, the surface treatment must generate critical adhesion and roughness levels. In addition to this roughness, the debonding risk decreases rapidly and monolithic behaviour is reached.     

Detailed investigation of ultrasonic Al–Cu wire-bonds: II. Microstructural evolution during annealing

Scanning and transmission electron microscopy were used to study the interface composition and morphology of copper wire-bonds heat-treated at 175 °C for 2, 24, 96, and 200 h in argon. Detailed morphological and compositional characterization of the Al–Cu heat-treated interfaces was conducted on site-specific specimens prepared by focused ion beam milling. Discontinuous intermetallic grains with varying size and morphology were found to grow in regions where they originally nucleated during the bonding process. The main intermetallic phase was Al₂Cu, which was found to grow via solid-state diffusion. In specimens heat-treated for 96 and 200 h, the Al₄Cu₉ phase was also detected. Void formation at the Al–Cu bonds heat-treated up to 200 h was not found to be a source of bond failure.     

各向异性Nd2Fe14B／α-Fe纳米晶复合磁体的制备和性能

采用声化学法、放电等离子烧结技术(SPS)和热变形工艺制备致密各向同性和各向异性Nd_2Fe_(14)B/αFe复合磁体,研究了软磁相包覆对磁体的结构和性能的影响.结果表明,软磁相α-Fe对各向同性Nd_2Fe_(14)B/α-Fe复合磁体的影响主要表现为增强两相间的交换耦合作用,从而提高剩磁.当α-Fe体积分数的数值适当(不超过2%)时,各向异性Nd_2Fe_(14)B/α-Fe磁体形成较好的c轴晶体织构,具有较高的磁性能.α-Fe体积分数为1%的磁体性能最高:B_r=1.367 T,H_(ci)=712 kA/m,(BH)_m=327 kJ/m~3.     

Evaluation of fatigue strength of adhesively bonded single and single step double lap joints based on stress singularity parameters

Most adhesively bonded joints have stress singularity points at the corners of the adhesive/adherend interface. Recently, stress singularity parameters, i.e. the intensity of stress singularity, K, and the order of stress singularity, λ, have been used to evaluate the strength of adhesively bonded joints. However, in many cases, stress singularity fields of adhesive joints cannot be formulated strictly by using constant values of K and λ. To apply these parameters to evaluate the strength of an adhesive joint, it is necessary to determine a key stress component and characteristic range for calculating the apparent singularity parameters K_app and λ_app. In this study, the endurance limits of adhesively bonded single lap, cracked single lap and single step double lap joints are evaluated using the stress singularity parameters. The results indicate that fatigue failure criterion for these joints can be obtained by using the apparent singularity parameters K_app and λ_app which are calculated by the least square method for the maximum principal stress distributions in the range from 0.05 to 0.5 mm from the singularity point.     

Statistical analysis of particulate interface lengths in diffusion bonded joints in a metal-matrix composite

A statistical analysis has been made of the number and length of particle-particle (P-P), particle-matrix (P-M) and matrix matrix (M-M) interfaces present in solid state and liquid phase diffusion bonded joints in a particulate metal matrix composite (17 vol % SiC/Al-Li 8090 alloy). All solid state bonds had planar bond interfaces with the polished particle facets aligned in and parallel to the bond interface. The particle interface lengths in 400 m long sample lengths in the L direction in a bond interface could vary from zero to over 50% of the sample length, with typical values for a random sample of 8% and 22% for P-P and P-M interfaces, respectively.Liquid phase diffusion bonded joints contained a higher volume fraction of particles in the bond region, but the bond interface was non-planar and particle interfaces were not aligned.The significance of these results for the strength and processing of diffusion bonded composites is discussed.Vacation student, University of Surrey.     

Experimental study of diffusion welding/bonding of titanium to copper

In the present study, Ti–6Al–4V alloy was bonded to electrolytic copper at various temperatures of 875, 890 and 900 °C and times of 15, 30 and 60 min through diffusion bonding. 3 MPa uniaxial load was applied during the diffusion bonding. Interface quality of the joints was assessed by microhardness and shear testing. Also, the bonding interfaces were analysed by means of optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The bonding of Ti–6Al–4V to Cu was successfully achieved by diffusion bonding method. The maximum shear strength was found to be 2171 N for the specimen bonded at 890 °C for 60 min. The maximum hardness values were obtained from the area next to the interface in titanium side of the joint. The hardness values were found to decrease with increasing distance from the interface in titanium side while it remained constant in copper side. It was seen that the diffusion transition zone near the interface consists of various phases of βCu₄Ti, Cu₂Ti, Cu₃Ti₂, Cu₄Ti₃ and CuTi.     

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.     

An investigation on microstructure and mechanical properties of Al7075 to Ti–6Al–4V Transient Liquid Phase (TLP) bonded joint

Transient Liquid Phase (TLP) bonding of two dissimilar alloys Al7075 and Ti–6Al–4V has been done at 500 °C under 5 × 10⁻⁴ torr. Cu was electrodeposited on Al7075 and Ti–6Al–4V surfaces, 50 μm thick Sn–4Ag–3.5Bi film was used as interlayer and bonding process was carried out at several bonding times. The microstructure of the diffusion bonded joints was evaluated by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The eutectic and intermetallic compounds formation along Al7075 grain boundaries and Ti/Al interface such as θ(Al₂Cu), TiAl and Ti₃Al were responsible for joint formation at the aluminum and titanium interfaces. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. Hardness of the joints increased with increasing bonding time which can be attributed to the intermetallics formation at the interface. The study showed that the highest bond strength was 36 MPa which was obtained for the samples joined for 60 min.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Structural and Magnetic Properties of Nanocomposite Nd–Fe–B Prepared by Rapid Thermal Processing

Nanoscale permanent magnetic materials, which possess excellent magnetic and mechanical properties, thermal stability, and corrosion resistance, have become a research hotspot for permanent magnets. In reality, however, the obtained maximum energy product, (BH)_max, is not satisfactory in comparison with the theory limit, especially for exchange-coupled nanocomposite magnets. The construction of an ideal microstructure still remains a challenge in the synthesis and preparation of nanoscale permanent magnets. This work reported the impact of rapid thermal process (RTP) with electron-beam heating on the microstructures of Nd_12.5-xFe_80.8+xB_6.2Nb_0.2Ga_0.3 (x = 0, 2.5) nanocomposites. It was found that the crystallization time was greatly reduced, from 15 min under the conventional annealing conditions to 0.1 s under the RTP. For Nd₂Fe₁₄B single-phase materials, the crystallization temperature of the RTP ribbons decreased by about 248 °C compared with that of the ribbons produced by the conventional annealing method. A synergetic crystallization of the Nd₂Fe₁₄B and α-Fe phases was observed under the RTP, which restrained not only the shape, size distribution, and compositions of the hard and the soft phases, but also the interface between them. This modification effect became more obvious as the fraction of Fe increased. Due to the improvement in the uniformity of the Nd₂Fe₁₄B and α-Fe phases, and their grain size distribution, better magnetic properties were achieved using RTP in comparison with the conventional annealing method.