Joint Resistance Research Articles

Numerical analysis and optimization of adhesively-bonded single lap joints by adherend notching using a full factorial design of experiment

The objective of this study is to carry out experiments using the experimental design methodology to determine the optimal geometric configuration which improves the resistance of a single-lap joint with notched adherends subjected to uniaxial tensile load. On the sides of the adherends in contact with the adhesive layer, a longitudinal rectangular notch is introduced. A complete experimental design (15) of three factors at two levels: the adhesive thickness (tA), depth (dN) and extended length (LE) of the notch was used to determine their effects on strength of an adhesive joint. The response or dependent variable used was the resistance of a single lap joint (SLJ). Finite element (FE) analysis was conducted to investigate the effect of different notch parameters on the strength enhancement of the SLJs. A quadratic regression model with interaction is developed to relate the strength of an adhesive joint and the variables (tA, dN and LE). The regression model was validated using various statistical approaches. According to the ANOVA results, the model is highly significant and in good agreement with the experimental results. The quadratic regression model suggested was used for optimizing the resistance of the joint. The maximum joint strength occurs when the adhesive thickness is the smallest (left end) and when the adhesive thickness is the largest (right end) of the validated domain. The strength of SLJ increases when the notching parameters (dN and LE) increase in the validation zone. The numerical results show that to improve the resistance of an adhesive joint with notched adherend, it is necessary to minimize the thickness of the adhesive layer and to use notch with the highest depth and extension equal or greater than the overlap. The results show that using the adherend notching can significantly improve the load bearing capacity of SLJs.

Modelling and design of cold-formed S960 steel brace-rotated tubular T- and X-joints

This paper presents detailed numerical investigation and design of cold-formed S960 steel grade brace-rotated (BR) tubular T- and X-joints. The BR tubular joint is one of the novel bird-beak tubular joint configurations, where the rotation of brace member(s) enhances joint resistance and aesthetic appearance. The numerical investigation was performed through finite element (FE) analysis. The tests carried out by the authors were used to develop accurate FE models of BR T- and X-joints, which in turn precisely replicated the joint resistances, load vs deformation curves and failure modes of test specimens. With an aim to broaden the data size, a comprehensive FE parametric study was performed using the verified FE models. The nominal resistances predicted from the literature and European code were compared to the joint failure resistances of 211 BR T- and X-joints specimens, including 192 FE specimens investigated in this study. The BR T- and X-joint specimens were failed by two failure modes, namely chord face failure (F) mode and a combination of chord face and chord side wall failure mode, i.e. combined failure (F + S) mode. It has been shown that the existing design provisions are unsuitable for the design of cold-formed S960 steel grade BR T- and X-joints investigated in this study. Hence, using three design approaches, accurate, less dispersed, reliable and user-friendly design equations are proposed in this study to estimate the joint failure resistances of cold-formed S960 steel grade BR T- and X-joints.

Fabrication and performance of Nb3Sn superconducting joints

Nb3Sn superconducting joints were developed, and the joint characteristics were investigated. The joints were prepared by powder metallurgy to sinter the precursor powders that were mechanically alloyed. The optimized parameters were confirmed through x-ray diffraction studies to confirm the stoichiometry of Nb, Sn, and Cu. First, the precursor powders after mechanical alloying were heat treated without the Nb3Sn wires. Electron backscatter diffraction was used to quantify the crystal morphology, phase distribution, grain boundaries, physical grain orientation, and so on. Small equiaxed Nb3Sn grains were formed after heat treatment. The high-angle grain boundaries in Nb3Sn indicated the complete solid-state diffusion reaction. After the Nb3Sn was successfully prepared, and the superconducting characteristics were measured, we introduce the Nb3Sn wires to prepare Nb3Sn joints and measure their electrical properties. To use the space as economically as possible, the joint was tested under different background fields to estimate the joint resistance under various locations in the magnet. The experimental method and the test results are reported in this work. The joint resistance was tested by using the current decay measure method. The electrical resistance of the Nb3Sn–Nb3Sn joints was on the 10−13 Ω level under the 5.5 T magnetic field.

Shear resistance strengthening of novel castellated key with embedded steel plates for dry joints in precast bridges

The shear performance of the precast segment concrete bridge depends largely on the shear resistance of joints between adjacent segments. However, the current joint structures have the limitation of shear resistance strengthening and the inconvenience of construction. To improve the shear capacity of the joints and achieve quick construction, a novel castellated shear key incorporating embedded steel plates is proposed. A total of fifteen single-keyed dry joints were designed, including two reference joints with a plain concrete key and a reinforced key and thirteen novel keys with embedded steel plates. Monotonic push-off tests of joints with or without shear keys were carried out. The influence of design parameters of the novel shear key, including height-to-thickness ratio, strength, steel ratio of the steel plates and bonding methods between concrete and steel plates on the shear performance of the joints were investigated comprehensively. The results indicated that the reinforcement of the shear capacity of the joints with the novel keys was 8.5%–33.9% higher than that of the joints with plain concrete keys due to the yielding of the steel plates. As expected, the embedded steel plates in shear keys could reach a shear-yielding state when the height-to-thickness ratio was larger than 25 or the yield strength of steel plates was lower than 355 MPa. A prediction formula of shear resistance capacity for the joints with novel shear keys was proposed considering the contribution of concrete and steel plates. It showed good prediction accuracy with an average value of 0.98 for the ratio of predicted to experimental values.

Numerical Investigation on the Performance of Exterior Beam–Column Joints Reinforced with Shape Memory Alloys

Upgraded design standards coupled with the damage caused by natural disasters have led to the development of smart materials with the potential to modernize current construction practices. This investigation proposes a nonlinear finite element (FE) model for evaluating the performance of beam–column joints (RC-BCJ) reinforced with shape memory alloys (SMA) and steel rebars. The model was validated based on accredited experimental data, followed by parametric analysis in ABAQUS to optimize the use of SMA bars for enhancing the seismic resistance of RC-BCJ without compromising their energy dissipation capacity. Parameters investigated include the (a) SMA–steel reinforcement ratio, (b) lengths of SMA bars, (c) elastic modulus of SMA, (d) compressive strength of concrete, and (e) axial load applied on the column. The finite element simulation results indicated that the model was capable of predicting the optimum length of SMA bars sufficient for relocating the plastic hinge away from the face of the column along the beam. Further, simulation results proved that the use of SMA bars in conjunction with steel reinforcement could be considered as an effective tool for enhancing the seismic performance of RC-BCJ joints. Among the parameters investigated, high-strength concrete was the most effective in improving joint resistance.

Low-resistance joints for YBCO-coated conductors with Ag nanoparticle paste

Due to the limited available piece length of YBCO-coated conductors (i.e. tapes or wires) and the different requirements for magnetic field, joints are inevitable for manufacturing high-temperature superconducting magnets. In this study, a sintering nano-silver (Ag) process was developed and used to connect YBCO tapes stabilized by anAg layer with low-temperature and short-time sintering of Ag nanoparticle (NP) paste. The thermodynamic characteristics of Ag NP paste were explored using a TG/DSC setup. The effects of sintering temperature, mechanical pressure and lapped length on microstructures and electrical properties of joints were comprehensively investigated. It is found that the pre-volatilization of low-boiling-point solvent in the paste is beneficial to improve the densification of sintered structure, thus contributing to increasing the critical current I c of the joint. With increasing sintering temperature, the I c of the joint will be close to that of the virgin tape, and the joint resistance experiences small fluctuations, but joint connectivity is enhanced. As the temperature reaches 205 °C, I c decreases to 84% of the virgin tape, and joint resistance increases obviously. In addition, the axial tension strength at room temperature is improved with the increase in mechanical pressure, while the resistance does not demonstrate distinct variation. Considering the electromechanical properties, the optimal joining process is determined as sintering at 180 °C and 30 MPa for 10 min. The joint with this technology possesses a closely connected interface and a well-sintered nano-Ag microstructure with pores. By further extending the lapped length, a YBCO joint resistivity as low as ∼10.56 nΩ cm2 is obtained, which is around a quarter of that of the soldering joint, and the process is much easier than that of the Ag diffusion joint.

Traverse and longitudinal analysis of AA5083/AA6082 dissimilar joint

This study investigates the mechanical and microscopic characteristics of the friction stir welded AA5083/AA6082 dissimilar joint. The welding process was performed at a rotational speed of 900 rpm, traverse speed of 60 mm min−1, and a tool tilt angle of 1°. Samples were extracted from the joint’s start, middle, and end positions in both the traverse and longitudinal directions. The analysis of these samples involved various tests, including microstructural analysis, bending, and tensile tests. The microstructural analysis revealed non-linear behaviour in both the longitudinal and traverse sampling directions along the joint. Specifically, there were notable differences in the microstructure between the different locations within the joint. In terms of mechanical properties, the ultimate tensile strength results of the traverse specimens were found to be higher compared to the specimens sampled longitudinally. This suggests that the joint’s strength may vary depending on the sampling direction. On the other hand, the flexural strength results showed a contrasting pattern, with the traverse specimens exhibiting lower strength values compared to the longitudinally sampled specimens.

Study on the Morphology, Microstructure, and Properties of 6082-T6 Aluminum Alloy Joints in MIG Welding

In this paper, metal inert gas (MIG) welding of 6082-T6 aluminum alloy with a thickness of 4 mm was simulated using a double ellipsoidal heat source. Based on the simulation results, the evolution of the microstructure, the strengthening mechanism of mechanical properties, and the corrosion characteristics of the welded joint were studied further. The thermal cycle curve of the welded joint was obtained through numerical simulation. When the heat input was 2.34–2.75 KJ/mm, the temperature of the welded joint reached the melting point of the material. With the increase in welding heat input, the weld metal (WM) organization changed from the dendrite to the cellular crystal transformation and presented a uniform distribution. The precipitation of the strengthened phase was inhibited at 2.75 KJ/mm. When the heat input changed from small to large, the tensile strength and toughness first increased and then weakened. Dimple distribution of tensile fractures was observed with a scanning electron microscope. When the welding heat input was 2.57 KJ/mm, the mechanical properties of the joint were the best. The tensile strength can reach 76.62% of the base material, and the elongation after breaking can reach 59.38% of the base material. However, it was concluded through studying electrochemical corrosion that the corrosion resistance of welded joints under this parameter was the worst. This may be caused by the presence of Cu, Fe, Si, Mg, and other compounds, and was proven to be Mg2Si through EDS analysis.

Design, Modeling and Experiments of a Variable Stiffness Soft Robotic Glove for Stroke Patients With Clenched Fist Deformity

Soft wearable devices for hand rehabilitation can assist activities of daily living. While existing soft robotic gloves can help with finger flexion, they are less suitable for patients who cannot extend their fingers due to poststroke increased muscular tension. In this letter, we proposed a variable stiffness pneumatic actuator that effectively increases the elasticity of the actuator to facilitate finger extension. The actuator consists of a fiber-reinforced actuator and a multi-stage articulated elastomer. It bends and extends by pneumatic actuation, with multi-stage articulated elastomer providing stiffness adjustment to accommodate varying joint resistance. The variable stiffness mechanism was achieved by the multi layered discrete metal plates which can gracefully fit the unique structure of human fingers. Analysis models were established to quantify the actuator's air pressure, vacuum pressure, and bending angle, also to predict the actuator's stiffness under different vacuum pressures. Experiments and finite element simulation were carried out to evaluate the bending performance of the actuator. Meanwhile, the output force and variable stiffness characteristic of the actuator were tested. The results show that the actuator can achieve 1.54 N fingertip output force and 2.4 times stiffness change, which satisfy the requirement of stroke patient. In addition, a clenched artificial hand with similar damper forces to human hand was applied to further verify the extending effect of the variable stiffness actuator. Finally, a rehabilitation glove integrated with multiple actuators was tested to assess typical rehabilitation gestures and the ability to grasp daily objects.

The strength of beam-to-RHS joints with welded studs

Construction industry is a significant contributor to global warming and other impacts on the environment. Demountable building may be a key to achieve sustainable construction as it facilitates the separation of components and materials for their recovery or even their reuse. However, when designing demountable bolted beam-to-column joints involving IPE-beams and hollow sections columns, difficulties associated with the inaccessibility of the interior of the tube can result in a problem during the execution process. To solve this problem, the use of joints with welded studs is great of interest since threaded studs can be attached to the column face without accessing the inside of the tube. The threaded studs can be used in combination with angle cleats to assembly demountable joints. In this paper, a new component-based analytical model to determine the resistance of beam to column bolted joints is presented. Firstly, analytical equations to calculate the resistance of the studs are proposed. Then their assessment with experimental tests is evaluated. Finally, the analytical strength and failure modes of the full joint are compared with the results obtained in a vast campaign of full-scale experimental tests, including single-side bolted beam-column joints and double-side bolted beam-column joints. The design parameters that govern the resistance of the full joints were also revealed. Drawing on this knowledge, guidelines to the design of joints based on their expected behavior have been proposed.

The effect of a prior eccentric lowering phase on concentric neuromechanics during multiple joint resistance exercise in older adults.

Aging involves a marked decline in physical function and especially muscle power. Thus, optimal resistance exercise (RE) to improve muscle power is required for exercise prescription. An eccentric lowering phase immediately before a concentric lift (ECC-CON) may augment concentric power production, due to various proposed mechanisms (e.g., elastic recoil, pre-activation, stretch reflex, contractile history), when compared with a concentric contraction alone (CON-Only). This study compared the effect of a prior eccentric lowering phase on older adult concentric power performance (ECC-CON vs. CON-Only) during a common multiple joint isoinertial RE (i.e., leg press) with a range of loads. Twelve healthy older adult males completed two measurement sessions, consisting of ECC-CON and CON-Only contractions, performed in a counterbalanced order using 20-80% of one repetition maximum [% 1RM] loads on an instrumented isoinertial leg press dynamometer that measured power, force, and velocity. Muscle activation was assessed with surface electromyography (sEMG). For mean power ECC-CON>CON-Only, with a pronounced effect of load on the augmentation of power by ECC-CON (+19 to +55%, 35-80% 1RM, all p < 0.032). Similarly, for mean velocity ECC-CON>CON-Only, especially as load increased (+15 to 54%, 20-80% 1RM, all p < 0.005), but mean force showed more modest benefits of ECC-CON (+9 to 14%, 50-80% 1RM, all p < 0.05). In contrast, peak power and velocity were similar for ECC-CON and CON-Only with all loads. Knee and hip extensor sEMG were similar for both types of contractions. In conclusion, ECC-CON contractions produced greater power, and velocity performance in older adults than CON-Only and may provide a superior stimulus for chronic power development.

Effects of ply thickness and interference-fit on the bearing strength of single-lap countersunk composite joints

The Countersunk composite joints are widely utilized in the load-bearing components of aerospace structures. However, hole-making causes local stress concentration, affecting joints’ bearing capacity. Therefore, a detailed experimental investigation was conducted on the strength under varying parameters including ply thickness, stacking sequence, and interference size in the single-lap countersunk composite joint. The installation force of the thin-ply joint was smaller than that of the thick-ply at the same interference. The initial damage of the thin-ply hole wall caused by the bolt-inserting process was less. The strengths of the two thick-ply joints improved by 10.12% and 9.09% (1.00% interference), respectively, compared with those of thick-ply neat-fit joints. The strengths of the two thin-ply joints increased by 18.18% and 21.78%, respectively, when the interference was increased to 1.44%. SEM imaging revealed that the damage extent of the thin-ply area is smaller with the same interference. Moreover, the fibre kinking angle of the 0° plies at the thick-ply section was more extensive, leading to fibre fracture. Using interference-fit technology to thin-ply laminates could improve the damage resistance of bolted joints and provide a novel idea for applying thin-ply laminates in thin-walled structural components.

Performance of low-loss demountable joints between CORC® cable-in-conduit-conductors at magnetic fields up to 8 T developed for fusion magnets

High-temperature superconductors (HTS) are promising candidates for use in the high-field magnets needed in particle accelerators and fusion reactors. HTS conductor on round core (CORC®) cables and wires wound from ReBa2Cu3O7-x (REBCO) coated tapes are being developed for use in high-field magnet applications including fusion magnets operating at currents beyond 80 kA, requiring them to be bundled into cable-in-conduit conductor (CICC) configurations. The use of HTS cables enable demountable superconducting fusion magnets that would allow easier access to the fusion machine for maintenance and parts replacement. Such demountable magnets require practical, low-resistance joints, capable of injecting current uniformly into the many REBCO tapes that make up different cable designs. Optimization steps on CORC® cables have resulted in high-current terminations and joints with a joint resistance measured between a pair of 30-tape CORC® cables of 51 nΩ at 76 K and 1.9 nΩ at 4 K. Demountable joints between CICCs consisting of six CORC® cables arranged in flat and round configurations were also tested and compared to joints between low-temperature superconducting (LTS) CICCs consisting of NbTi Rutherford cables. Samples were paired into two configurations (LTS-to-LTS and HTS-to-HTS) with a demountable joint between them that were each tested in series with currents up to 10 000 A in an applied background magnetic field of up to 8 T. The total loop resistance of the HTS-to-HTS sample, including their terminations and joint, was about 4 nΩ at 4 K in self-field with the resistance of the copper pressed joint being less than 1 nΩ. At 8 T, the total loop resistance increased to 6.9 nΩ with the pressed joint contributing 1.4 nΩ. These initial tests prove the feasibility of producing remountable (dry) joints with low resistance between superconducting magnet windings in future compact fusion machines.

Design of cold-formed high strength steel rectangular hollow section T-joints under post-fire conditions

A comprehensive numerical investigation looking into the static post-fire structural performance of cold-formed high strength steel (CFHSS) T-joints is presented in this paper. The braces and chords of T-joints were made of square and rectangular hollow section (SHS and RHS) members. The steel grade of SHS and RHS members was S960 with nominal 0.2% proof stress of 960 MPa. The static strengths of SHS and RHS T-joints were investigated corresponding to four post-fire temperatures, including 300°C, 550°C, 750°C and 900°C. Pandey and Young [1] carried out tests to investigate the post-fire residual strengths of cold-formed S960 steel grade SHS and RHS T-joints. The test results were used to develop an accurate finite element (FE) model. Using the validated FE model, a comprehensive FE parametric study was performed in this investigation. The validity ranges of critical geometric parameters were extended beyond the current limits mentioned in international codes and guides. The nominal resistances predicted from design equations given in EC3 [2] and CIDECT [3], using post-fire material properties, were compared with a total of 765 test and FE joint resistances, including 756 numerical data obtained in this study. Overall, SHS and RHS T-joint specimens were failed by chord face failure, chord side wall failure and a combination of these two failure modes. Generally, the current design rules in EC3 [2] and CIDECT [3] are quite conservative and largely dispersed. As a result, accurate, less dispersed and reliable design equations are proposed in this study.

Systematic investigation of a NbTi-Bi2223 hybrid low-resistive joint

In this study, a NbTi-Bi2223 hybrid low-resistive joint fabricated by the superconducting solder method was systematically investigated. The joint is a necessary component of gigahertz nuclear magnetic resonance (NMR) with Bi2223 as the insert coil. The joint resistance and current capability under different applied magnetic fields were measured, and the resistance exhibited a near-linear relationship with the applied field. Beyond the superconducting properties, the allowed mechanical criteria for the Bi2223 tape in the joint were also briefly studied. Based on the above two properties, a 1.3 GHz NMR design with a NbTi-Bi2223 hybrid low-resistive joint was shown as an application demonstration. Although the current resistance can fit the magnetic-field decay criteria, obtaining a lower resistance value was attempted by extending the joint length but failed. The main source of the resistance in NbTi-Bi2223 was studied. We attempt to show an engineering-feasible NbTi-Bi2223 hybrid low-resistive joint to help improve the development of gigahertz NMR.

Behaviour of Stainless-Steel Fibre-Reinforced Exterior Beam-Column Joints under Reverse Cyclic Loading

The objective of this study is to evaluate the flexural behaviour of stainless-steel fibre-reinforced concrete beam-column (BC) joints under reverse cyclic loading. Based on the properties of concrete with various percentages of fibre, the optimized volume fraction was obtained as 0.75% of stainless-steel fibre. In the present work, two sets of beam-column joints with and without fibres were cast and tested under reverse cyclic loading. The beam-column joints were loaded up to five cycles, to study their behaviour, and examine the failure pattern of the joint. Based on test results, parameters such as ductility and the energy absorption capacity characteristics were evaluated. It is concluded that the inclusion of stainless-steel fibre improves the overall seismic resistance of RC beam-column joints.

Microstructure, strain hardening behavior, segregation and corrosion resistance of an electron beam welded thick high-Mn TWIP steel plate

In the present work, the microstructure, strain hardening behavior, segregation and corrosion resistance of a high-Mn TWIP steel joint fabricated by electron beam welding (EBW) method were investigated. It was observed that in the fusion zone (FZ), columnar dendritic structure and cellular structure were formed and the texture component of ⟨100⟩∥X-axis was developed. Annealing twinning events hardly occurred during the solidification of weld metal since few Ʃ3 and Ʃ9 grain boundaries existed in the FZ. The 0.2% yield strength (YS) of the joint is comparable to that of the base metal (BM). Deformation twinning played an important part in increasing the strain hardening rate. C, Mn and Si elements were found to segregate in the interdendritic regions, nevertheless, Al element was discovered to concentrate in the centres of the dendrites. The microsegregation of alloying elements and the formation of (Fe,Mn)3C cementites increased the corrosion sensitivity of interdendritic regions, which led to the occurrence of preferential corrosion in these regions and then reduced the corrosion resistance of the joint.

On enhancement of fracture resistance of adhesive joints by surface micropatterning using a femtosecond laser

This study focuses on the influence of surface micropatterns, including uniform and nonuniform grooves fabricated by selective removal of a designed volume from aluminum alloy substrates using a femtosecond laser, on the mode I fracture behavior of adhesively bonded interfaces. The morphology, wettability, chemistry and microstructure of the patterned surfaces have been analyzed. The mode I fracture behavior of adhesive joints was characterized by measuring the fracture resistance using a J-integral approach, and the fracture process in the joint was investigated numerically using a continuum damage model. The results show that the laser patterning has modified the surface roughness, wettability and surface chemistry such that the fracture resistance could be greatly increased. It also reveals the significance of patterning uniformity across the surfaces and the existence of a limiting effective patterning ratio (the ratio of the patterned area to the flat bonding area) on enhancing the fracture resistance. Local plastic deformation that occurred in the adhesive at the patterned structures due to stress concentration was found to be one toughening mechanism although it tended to induce crack growth close to one substrate-adhesive interface.

Calculation of Fit Tolerance with Clearance to Increase Relative Wear Resistance of Joints

Calculation of Fit Tolerance with Clearance to Increase Relative Wear Resistance of Joints

Miniaturized HTS linear flux pump with a charging capability of 120 A

Owing to the presence of joint resistance and flux creep, high-temperature superconducting (HTS) coils without a power supply inevitably suffer from current decay. A flux pump is a voltage supply that requires connections with smaller footprints and a lower heat load than traditional current leads. In this study, we explain the principle of the upper limit for the output current of the traveling wave flux pumps. Based on this principle, a miniaturized linear flux pump device was developed. With narrow and misaligned iron teeth, elaborate 3D geometry of the iron pieces, and optimized driving current waveform, the miniaturized flux pump can support more than 120 A output current with only a 10 mm wide HTS tape and a compact size of 4.6 × 4.6 × 3.4 cm. Our experimental results show that the critical current of the HTS tape has a significant effect on the flux pump output. An HTS tape with a larger critical current supports a higher maximum transport current, whereas an HTS tape with a smaller critical current requires less applied current for positive output. Finally, excitation tests on HTS coils were performed. Charge/active discharge and field supplement experiments were done on a maglev HTS racetrack coil of 0.4 H, where charging/field supplement capability of the miniaturized flux pump were demonstrated up to 46.8 A (close to the critical current of the coil). It has also been proved that the flux pump can work together with an external power supply with persistent current switch. The miniaturized flux pump can also independently charge an HTS coil of 60 μH to 91.6 A, which is the critical current of the coil at a low voltage criterion.