Ductility Of Joints Research Articles

Experimental and numerical investigation on strengthening mechanism of rib-reinforced round-over-round RCFDST column jointed with steel beam under cyclic loading

This paper involved both experimental and numerical investigations on the behaviour of twelve types of recycled-concrete filled double-skin steel tubular (RCFDST) structures subjected to the cyclic loading with reinforced ribs at the column–steel beam joints, of which the damage pattern and seismic performance were analysed by varying the ratio of beam–column flexural linear stiffness, axial compression ratio, hollow ratio, type and strength of the concrete, and the length of ribs. The results indicate that the specimens primarily exhibited the damage pattern of the plastic hinge at both ends of the beams, and that the horizontal load–displacement hysteresis curves with full loop at the top of the column were observed. Moreover, the structures composed of recycled concrete from 40 MPa to 60 MPa presented no reduction in ultimate load as well as the joint ductility to those of the ones with normal concrete. Regarding the performance of RCFDST, the optimal ratio of beam–column flexural line stiffness (ki) is found in the range of 1.0 to 1.5. This study demonstrated that the recycled concrete made from the industrialized aggregate formed by crushing waste concrete from road bases has potential application prospects in Concrete-filled double skin steel tubular (CFDST) structures due to its low cost and good seismic performance, including high ultimate load capacity and low energy consumption.

The Effects of Post-Weld Heat Treatment on the Microstructure and Mechanical Properties of 9% Ni Steel Weld Joint with Fe-Based Filler Material

A Fe-based filler material with the composition of Fe-20Ni-5Co-2.5Mn-0.2C was used for welding 9% Ni steel, and the effect of post-weld heat treatment on the microstructure and mechanical properties of the welded joints was investigated. Post-weld heat treatment of the welded joint was performed in the temperature range of 200 to 600 ℃. After the experimental selection, the microstructure changes of the weld metal and base metal were investigated in detail at heat treatment temperatures of 350 and 600 ℃. Grain morphology, size, and the location of precipitated cementite were studied using SEM and EBSD phase maps. The mechanical properties of the weld joint were characterized by tensile properties and micro-hardness. Based on the experimental results, this study provides key insights into the applicability of post-weld heat treatment to weld joints of 9% Ni steel prepared using Fe-based filler material to improve the mechanical properties, toughness and ductility of the weld joints.

A comparative experimental study on blind‐bolted beam connections to square steel tubular columns filled with polyurethane foam or concrete

AbstractIn the last two decades blind‐bolted connections have been developed to allow bolting of open section beams (e.g, I‐beam) to steel hollow section (SHS) columns where access from all sides of the column is not possible. Filling the tubular column with concrete in the vicinity of the beam‐to‐column connection has been shown to improve the anchoring mechanism of the blind‐bolted connection thereby increasing the strength and stiffness of the connection and reducing the column face deformation, however it adversely affects the joint ductility. This study introduces polyurethane foam filled steel tubular columns in combination with a new anchoring arrangement employing hollo‐bolts to develop a novel connection between the SHS columns and open section beams that provides a better combination of stiffness, strength and ductility. To investigate the behaviour of the proposed arrangement, monotonic pull‐out tests of blind bolted T‐stub connections anchored to foam‐filled steel tubular (FFT) and concrete‐filled steel tubular (CFT) columns using holloBolts were conducted. Benchmark tests on unfilled SHS columns were also conducted for comparison. The FFT in conjunction with the proposed anchoring mechanism was shown to provide a good stiffness as the CFT columns, and almost similar ductility to that of the unfilled steel tube blind‐bolted connection. Hence it is concluded that employing foam instead of concrete as an infill leads to favourable combination of strength and ductility, thus rendering FFT an attractive light alternative to CFT sections when ductility of the joint is an important design consideration.

Experimental test setup design for ultimate limit state evaluation of ductile wrapped composite KK‐joints

AbstractComplex circular hollow section double KK‐joints are widely used in, e.g. offshore wind turbine jackets. As an alternative to conventional welded joints of the CHS multi‐membered structures, wrapped composite joints have shown superior structural performance in previous pilot tests. Experimentally characterizing the joint behaviour under uniplanar loading can be complicated, because of the ductile failure mechanisms of wrapped composite joints and second‐order bending moments. This paper presents the design of a test frame in combination with a conventional tensile 2.5 MN test rig. Ductile joint failure, joint imperfections and second‐order effects are considered in the design. Structural finite element software SOFiSTiK is used to analyse the effects of wrapped joint behaviour and imperfections on the test system and interaction of joint, frame and loading rig behaviours. Non‐linear joint behaviour and misalignments of the braces increased the second‐order effects significantly. Mitigation strategies have been explored to reduce additional bending moments in the KK‐joint. A complete test setup is proposed to perform static experiments on double KK‐joints in uniplanar axial loading conditions.

Study on plug-pin energy-dissipating joint with integrated mild steel dampers

Plug-pin joints are designed for easy assembly and fast construction. Full-scale experiments were carried out on three exterior beams-to-column joints subjected to reversal cyclic loading. It has been proved from experimental results that the proposed connection not only guarantees structural integrity but also moves the plastic hinge away from the column edge. Based on the experimental results, a novel plug-pin connection with integrated mild steel dampers is proposed. The energy dissipation joint fully adopts a dry connection method, and the joint's ductility is significantly improved by the integrated energy-dissipating steel plates. Finite element models are established to study the effects of different diameters of steel bars and thicknesses of energy-dissipating steel plates on structural response and mechanical properties under low cyclic reciprocating loads. Parameter studies were carried out, and the obtained results indicate that the proposed joints can bear a greater displacement load than that of cast-in-place joints and achieve greater energy dissipation. The maximum bearing capacity of the joint can be improved by increasing the thickness of the energy-dissipating steel plates, but the bearing capacity decline period of the joint will appear in advance under the same order of loading. If designed properly, structures installed with the plug-pin joint will be expected to adjust the order of plastic hinge development, thus achieving better seismic performance.

Theoretical analysis and numerical simulation of axial tensile behavior of pipe joints based on bi-linear cohesive bond-slip model

PurposeDue to its inexpensive production costs, low stress concentration and maintenance-friendliness, the adhesive bonded pipe joint is frequently utilized for pipe connection. However, further theoretical analysis is needed to understand the debonding failure mechanism of such bonded pipe joints under axial tension.Design/methodology/approachIn this study, based on the bi-linear cohesive zone model, the integrated closed-form solutions were derived by considering the axial stiffness ratio and failure stage to determine the relative interfacial slip, interfacial shear stress and relationship of tension–displacement in the bonded pipe joint.FindingsAdditionally, solutions for the critical bonded length and the ultimate load capacity were put forth. Besides, the numerical study was conducted to verify the theoretical solutions regarding the load–displacement relationship. The interfacial shear stress distribution at different failure stages was presented to understand the interfacial shear stress transmission and debonding process. The effect of bonded length on the ultimate load and ductility of pipe joints was also discussed.Originality/valueThe findings in this study can give a reference for the design of bonded pipe joints in their actual engineering applications.

Heterogeneous microstructure improves the strength and ductility of a flash-butt welded IN718 alloy joint

Higher strength and ductility are obtained in a flash-butt welded IN718 alloy joint due to an improved heterogeneous microstructure distribution by adjusting the upsetting deformation. With the increase in upsetting deformation, the microstructure distribution of the characteristic zones is effectively adjusted which contains a complete recrystallization zone (RxZ), a thermo-mechanical affected zone (TMAZ) and a coarse grain zone (CGZ). Finer grains and more residual stress are obtained in the heat affected zone (HAZ), which increases the strength and hardness. The improvement of the strength in the HAZ delays the concentration rate of local strain at the weld seam (WS) and enhances the coordination between the HAZ and the matrix. Therefore, the additional ductility is obtained by the coordination between the HAZ and the matrix, which results in a welded sample of UD0.86 achieving the same high strength and ductility as that of the base material.

Outstanding ductility of dissimilar laser Al/steel spot joint using a high entropy alloy interlayer

Laser spot welding (LSW) technology was used for the joining of low carbon steel to AA5052 Al alloy using a FeCoNiCrMn equimolar high entropy alloys (HEAs) interlayer for the first time. The addition of the HEA interlayer promoted the formation of a Fe-rich Fe-Al IMC with less brittleness and hardness. It resulted in an increase of the joint fracture load from 963 N to 1473 N and a dramatic improvement of the displacement increased from 0.66 mm to 3.32 mm. Another reason for the outstanding ductility of the dissimilar laser joint was the formation of a conical fusion zone in the joint, which realized the dual coupling of metallurgical bonding and mechanical interlocking at the interface.

Experimental investigation on the shear performance of UHPC large-keyed epoxy joints subjected to direct shear loading

The joints between precast segments in prefabricated ultra-high performance concrete (UHPC) segmental bridges are of great concern since reinforcements and fibers are disconnected at the interface of joints. To investigate the shear performance of UHPC large-keyed epoxy joints, a series of direct shear specimens were tested to consider three parameters, that is, the confining stress, the depth-height ratio of the shear key, and the steel fiber volume ratio. The cracking patterns, failure modes, and shear capacities of all the UHPC epoxy joints were systematically analyzed to explore the effect of the three parameters. Experimental results showed that the three parameters have significant impacts on the direct shear performance of UHPC large-keyed epoxy joints. Specimens with a depth-height ratio of 0.35 are more beneficial to the shear strength than those of the specimens of 0.2 and 0.5. This phenomenon is quite significant in the situations of low confining stress states. The increase of the confining stress and volume ratio of steel fiber substantially improved the shear strength and the ductility of UHPC epoxy joints, especially when the volume ratio of steel fiber increases from 1% to 2%. The initial crack occurred at 42% ∼ 84% of the ultimate shear load, and the initial stiffness and the residual strength increased greatly with the increase of the confining stress. In addition, a modified theoretical model was proposed to predict the shear strength of the UHPC large-keyed epoxy joint, which exhibited a relatively good agreement with the test data.

Study on the size effect of elastic modulus of rock considering the joint spacing

The relationship between the rock’s elastic modulus and deformation characteristics, affected by joint spacing and ductility, was not determined. This study investigates the size effect of the elastic modulus of rock with parallel joints using numerical simulation and regression analysis. The results showed an exponential relationship between elastic modulus and spacing of parallel joints and a negative exponential relationship between elastic modulus and rock size. The characteristic size of elastic modulus has a power function relationship with the parallel joints spacing, and the characteristic elastic modulus of rock has a power function relationship with the parallel joints spacing. The general and specific forms of these relationships are provided. Establishing these relationships allows for predicting and calculating the elastic modulus of the mine rock mass and serves as a reference for the deformation analysis of the rock.

An experimental study of the friction stir welding of Al 5083 H321 plates by using different process parameters

ABSTRACT In this study, 4 mm thick Al 5083 H321 plates were joined with Friction Stir Welding (FSW) method using different process parameters. In order to reveal the effects of different process parameters (Tool Shoulder Diameter, RPM, Direction of Rotation, Feed Rate), a total of 36 tests were carried out from 4 test parameters in a full factorial test setup. Tensile tests were performed with the test specimens cut from the welded plates, and their strength and mechanical properties were examined. In addition, the welding quality was subjected to an X-ray test, and quality control was passed. The ductility of the weld joints was demonstrated by the three-point bending test performed separately from the butt and root parts. Welded joints maintain their ductility without welding defects on bending tests by bending up to a 150° bending angle. On the other hand, it has been determined that the Al5083-H321 plate was pre-hardened by cold deformation; the heat generated during FSW causes recrystallization during welding, which resulted in a joint efficiency of 56%. The microstructural investigation and hardness tests proved the recrystallized regions.

A Drop-in High-Temperature Pb-Free Solder Paste that Outperforms High-Pb Pastes in Power Discrete Applications

Sn-based high-temperature lead-free (HTLF) solder pastes have been developed as a drop-in solution to replace the high-Pb solder pastes in power discrete applications. The pastes were designed to combine the merits of two constituent powders. A SnSb-based Ag/Cu-containing high temperature powder, with the melting temperature above 320° C, was designed to maintain a high-temperature performance. A Sn-rich SnAgCu-Sb powder, with a melting temperature around 228° C, was added to the paste to enhance wetting and improve joint ductility. In the design, the final joint will have the low-melting phase (the melting temperature >228o C) in a controllable quantity embedded into the high-melting SnSb matrix. HTLF-1, one of the designs, maintained the bond shear strength up to 15MPa, even around 290° C. Another design, HTLF-2, has a similar bond shear strength as Pb92.5/Sn5/Ag2.5 around 290° C, but exceeds substantially below 250° C. The power discrete components had been built with both HTLF solder pastes for both die-attach and clip-bond through the traditional high-Pb process, which demonstrated the drop-in processing compatibility. The components survived three additional SMT reflows (peak temperature of 260° C) and passed moisture sensitivity level 1. This confirmed that the maintained joint strength (comparable to or stronger than high-lead), helped to keep the joint integrity within the encapsulated components in the following SMT process, even with the controlled quantity of the melting phases above 228° C. Both HTLF solder pastes outperformed Pb92.5/Sn5/Ag2.5 in the resistance from drain to source when power is on (RDS(on)), even after 1000cycles of TCT (-55/175o C), which is attributed to the intrinsic lower electrical resistivity of Sn of both HTLF pastes. Microstructural observation had shown no corner cracks for both die-attach and clip-bond joints after TCT.

Experimental Investigation of Seismic Performance of Precast Concrete Wall-Beam-Slab Joints with Overlapping U-Bar Loop Connections.

In the era of energy conservation and environmental protection, as well as the industrialization of buildings, precast concrete (PC) structures have been developed and increasingly applied in construction industries due to their advantages of outstanding workability and ecofriendliness. In order to verify the reliability of overlapping U-bar loop connections and a modified form of these connections, and study the seismic performance of PC wall-beam-slab joints with these connection methods, three full-scale wall-beam-slab joints were designed and tested under low reversed cyclic loading, including one cast-in-place (CIP) specimen and two PC specimens. Based on the test results, the seismic performance of the PC joints was studied by comparing their damage process, hysteretic loops and skeleton curves, load-carrying capacity, ductility, equivalent stiffness, and energy dissipation with those of the CIP joint. After analyzing the experimental results, the following conclusions can be drawn: the overlapping U-bar loop connection and its modified form are effective and reasonable; the specimen with the modified connection form showed slightly better mechanical properties; the failure mode of the PC joints was consistent with that of the CIP joint; and the generation, distribution, and development of cracks in the PC specimens were similar to those in the CIP specimen. In addition, the stiffness of the PC joints was similar to that of the CIP joint, and the load-carrying capacity, ductility, and energy dissipation of the PC joints were better than those of the CIP joint. Moreover, the research in this paper can also provide some guidance for assembling wall-beam-slab joints in PC shear wall structures.

A Drop-In High-Temperature Pb-Free Solder Paste That Outperforms High-Pb Pastes in Power Discrete Applications

Sn-based high-temperature Pb-free (HTLF) solder pastes have been developed as a drop-in solution to replace high- Pb solder pastes in power discrete applications. The pastes were designed with Indium Corporation’s DurafuseVR technology, to combine the merits of two constituent powders. A SnSb-based Ag/ Cu-containing high-temperature powder, with a melting temperature above 320 degrees C, was designed to maintain high-temperature performance. A Sn-rich SnAgCu-Sb powder, with a melting temperature around 228 degrees C, was added to the paste to enhance wetting and improve joint ductility. In the design, the final joint will have the low-melting phase (the melting temperature >228 degrees C) in a controllable quantity embedded into the high-melting SnSb matrix. HTLF-1, one of the designs, maintained the bond shear strength up to 15 MPa, even around 290 degrees C. Another design, HTLF-2, has a similar bond shear strength as Pb92.5/Sn5/Ag2.5 around 290 degrees C, but exceeds substantially below 250 degrees C. The power discrete components had been built with both HTLF solder pastes for both die-attach and clip-bond through the traditional high-Pb process, which demonstrated the drop-in processing compatibility. The components survived three additional surface mounting (SMT) reflows (peak temperature upto 260 degrees C) and passed moisture sensitivity level 1 (MSL1). This confirmed that the maintained joint strength (comparable to or stronger than high-Pb), helped to keep the joint integrity within the encapsulated components in the following SMT process, even with the controlled quantity of the melting phases above 228 degrees C. Both HTLF solder pastes outperformed Pb92.5/Sn5/Ag2.5 in the resistance from drain to source when power is on (RDS(on)), even after 1,000 cycles of temperature cycling test (TCT) under 255/175 degrees C, which is attributed to the intrinsic lower electrical resistivity of Sn in both HTLF pastes. Microstructural observation had shown no corner cracks for both die-attach and clip-bond joints after TCT.

Mechanisms associated with material deformation, and microstructural evolution during friction stir welding of pure titanium and Al alloy

Aluminium and titanium are important structural metals; however, their joining is difficult because of different microstructural responses and intermetallic compounds (IMCs). The present work underlines the mechanisms associated with friction stir welding of pure Ti and Al alloy. The results show no visible defect, while multiple particles are consolidated in the stirred zone (SZ). Chemical composition across Al-Ti interfaces is different and dependent on location and particle size. This affirms intensive mechanical mixing and thermal diffusion of Al-Ti substrates and IMCs formation. The UTS and ductility of the Al-Ti joint are ∼73% and 37% higher than Al base metal, respectively. Deformation of Ti in SZ is responsible for phase evolution and joint formation with better joint characteristics.

Comparative study on the seismic behaviour of assembled composite walls with vertical joint rigid connections and ductility connections

To investigate the efficient dry connection type of fully assembled composite (FAC) walls, two novel vertical joint connection types, including rigid connections (box connections) and ductile connections (U-shaped plate connections), are proposed. In the experiment, three specimens were constructed and tested under in-plane reversed cyclic loading, including specimens with box connections (FAC-B) and U-shaped steel plate connections (FAC-U) and a specimen with a horizontal cast-in-place assembled composite (CAC) wall as the reference specimen. In the reversed cyclic loading test, the failure mode, hysteretic behaviour, bearing capacity, stiffness degradation and energy dissipation capacity were obtained. The test results indicated that the failure mode of all specimens was shear failure. In addition, a finite element model (FEM) of FAC-U is subsequently established and validated to further investigate the seismic performance of the plate thickness, concrete strength, compression ratio and block strength on the bearing capacity and ultimate displacement.

Seismic behavior of steel frames with different joints: Shaking table test and finite element analysis

This paper discusses the effect of different joint types (common joint (CJ), reduced beam section joint (RJ), and cover plate joint (CPJ)) on the seismic behavior of steel frames. Three steel frames (1:2 scale) with different joints were tested under the shaking table. El Centro wave was selected as the input seismic wave. The failure modes, dynamic response, displacement response, strain response, and acceleration response of steel frame specimens under five peak ground accelerations (PGA) were measured and analyzed. The experimental results indicated that the seismic damage of the steel frame specimens was mainly concentrated at the beam-column joints on the first story. The plastic hinge could be achieved shift-away by reinforcing and reducing the flange at the beam end and significantly improving the structure's seismic capacity. In addition, the ductile joints used also affected the overall structural stiffness. The steel frame specimens' dynamic response (seismic shear coefficient, structural capacity curve, and fundamental frequency attenuation curve) was discussed. As the dynamic loading was applied, the seismic shear coefficients of CPJ and RJ steel frame increased continuously, and the structural capacity curves rose steadily, which indicated that the RJ and CPJ had excellent seismic performance and ductility. The finite element analysis was performed to elaborate further on the seismic performance and failure modes of steel frames. The experimental results were verified by the finite element analysis, and the finite element model could be used for the subsequent research on the steel frame.

Experimental Study on Seismic Behavior of Damaged Beam-Column Joints Retrofitted by Viscoelastic Steel-Enveloped Elements

In order to improve the seismic performance of damaged reinforced concrete joints, a new retrofitting method using viscoelastic materials and steel plates is proposed. A reversal cyclic loading test was carried out on four T-shaped RC joints to investigate the reliability of this method. Seismic performance, including the strength, stiffness, ductility, and energy dissipation capacity of the original non-damaged joint and different repaired and retrofitted joints are then compared. The results show that this new method can significantly improve the strength of the retrofitted joints, and the joints can be loaded to a larger displacement value. After retrofitting, the energy dissipation of the joints increases, and the stiffness degradation decreases. The test results indicate that the retrofitting method is effective in repairing seismically damaged RC joints.

Experimental study of plate-buckling type earthquake-resilient prefabricated steel beam-column joints with replaceable double flange cover plates

The damage control and post-earthquake repair of beam-column joints is a hot research topic in the seismic field. Therefore, a new type of earthquake-resilient prefabricated steel beam-column joint with replaceable double flange cover plates (RFCP-ERPSJ) was proposed. The yield load of this joint can be controlled by the buckling or slip of plates, so it can be divided into plate-slipping type and the plate-buckling type. Considering that the yield load of the plate-buckling type RFCP-ERPSJ is stable, firstly, a practical design procedure and design theory of the newly plate-buckling type joint is proposed in this paper. Then, to study the seismic performance and post-earthquake repairability of this RFCP-ERPSJ, a total of 6 full-size specimens are designed and subjected to a low-cycle reciprocating loading test and constant-amplitude low-cycle fatigue test. The effects of the thickness of FCPs, the material strength of FCPs, dog-bone width, amount of FCPs connecting bolts, and bolt spacing at the middle row on the seismic performance and energy dissipation performance of the joint are studied. The experimental results are also compared with the theoretical results. The research showed that the plastic hinge of the joint could be concentrated on the replaceable FCPs, so as to protect the beams and columns. By replacing damaged FCPs and bolts, the repaired joint had the same performance as the original joint. After comparison, the theoretical yield load is in good agreement with the experimental yield load. Besides, the small thickness and dog-bone width of FCPs can reduce the load carrying capacity and energy dissipation capacity of joints, and improving the material strength of FCPs will reduce the ductility of joints and increase the risk of plastic damage to the beams and columns. If the amount of FCPs connecting bolts is reduced, the specimens mainly dissipate energy by the slip of FCPs, and the load carrying capacity will descend. Excessive bolt spacing at the middle row will reduce the load carrying capacity and energy dissipation capacity.

Design of higher temperature copper brazing filler metals with reduced brittle phase content

Anneal-resistant copper alloys have been developed to address the need for stronger yet lighter fin and tube parts in heat exchangers, in order to facilitate greater efficiency and increased temperature capability. As such, brazing as opposed to traditional soldering has become the preferred joining method for these materials, due to the enhanced high-temperature properties of such joints. Copper-based filler metals are used extensively for the brazing of these materials, generally based on the copper-phosphorus system with alloying additions. In the present study, the microstructural issues arising from the current commercially available filler metals are discussed and addressed through CALPHAD-supported alloy design. Three selected novel compositions were used in the brazing of pure copper as a test of the design concept. Alloy compositions in weight percent of Cu(bal.)− 7Ni-4Sn-5 P and Cu(bal.)-10Ni-2Sn-6.5 P were found to result in a reduction of brittle Cu3P phase while increasing the fraction (0.78 and 0.77 respectively) of copper-rich solid solution in joints post-braze, comparing favourably to commercial Meta-Braze™ 077 under similar conditions (0.61). Furthermore, the copper-rich solid solution was leaner in tin, predicted to impart an increased melting temperature while microhardness profiles indicate increased joint ductility. The implications for mechanical properties of filler metals in this alloy family are discussed.