Mechanical Behavior Of Joints Research Articles

Experimental research on mechanical behavior and strengthening technologies of joints in hollow-core slab bridge

The joint is the connection part of a hollow-core slab bridge. Thus, it is necessary to study the joint systematically. In this paper, 15 beams with the joint were designed to study the mechanical behavior and the load transfer mechanism of the joints under flexure load, shear load, and flexure-shear load, and the corresponding failure mode was observed. The effects of pavement layer thickness, concrete surface treatment, joint concrete strength, and joint form on the mechanical properties of joints were studied. And the strengthening effects of CFRP strengthening technology and locally prestressed tendons strengthening technology were studied. The test results showed that the failure mode of all specimens was cracking at the interface between the joint and the beam segment. Cracks started from the bottom of the specimen and extended upward with the increase of load. The mechanical behavior of joints could be improved by increasing the pavement layer thickness, interface roughness, joint width, and the number of tensile rebars. However, the concrete strength of the joint had little effect on the mechanical behavior. CFRP could effectively delay crack development and improve mechanical behavior. The specimen with a locally prestressed tendon performed better structural integrity than the control specimen. Furthermore, according to the results obtained from the AASHTO and experiments, a factor with the aim to correct the calculated values of the bearing capacity in the code was proposed.

Numerical Analysis of Mechanical Behavior of Self-Centering Joint between CFDST Column and RC Beam

The existing connection between the concrete-filled double steel tubular (CFDST) column and the reinforced concrete (RC) beam is difficult to repair and reuse after damage. In this paper, a self-centering joint between the CFDST column and the RC beam is proposed. The self-centering of the joint is realized by prestressed steel strands, and the energy dissipation is realized by friction. The overall purpose of the research is to analyze the influence of steel strand and friction on the mechanical behavior of the joint. By comparing the envelope curve and the restoring force model of a numerical joint model with theoretical values, accuracy of the numerical model was verified. Then, joints with different parameters, including the friction, prestress of steel strands, and ratio of the resisting moment provided by steel strands to the resisting moment provided by friction in the opening moment of joints, were numerically analyzed. The results showed that the joints with greater friction and prestress of steel strands had higher bearing capacity. Increasing the friction could increase the energy dissipation capacity of the joint, but it would increase the residual deformation of the joint. To reduce residual deformation, the prestress of steel strands should be increased. When the resultant force of the pretension of steel strands was greater than friction, the steel head could be kept pressed on the connecting block, making the stress changes of steel strands and the self-centering performance of the joint stable.

Effect of Top Sheet Materials on Joint Performance of Self-Piercing Riveting

Three types of self-piercing riveting (SPR) joints, i.e., steel/aluminum, carbon fiber reinforced polymer (CFRP)/ aluminum, and aluminum/aluminum, were constructed using three different top sheet materials with the same aluminum alloy as the bottom sheet. The effects of the top sheet material on the joint quality and mechanical behavior were evaluated. The top sheet materials’ characteristics dominate the rivet piercing process and the consequent interlock distance. The high-strength steel top sheet requires a comparatively higher rivet setting force and induces early flaring of the rivet tail, resulting in a larger interlock distance. Though the CFRP needs the highest rivet setting force to penetrate the rivet through the CFRP fibers, the CFRP/aluminum joint exhibits the smallest interlock distance because of the SPR process-induced damages to the CFRP and subsequently less flaring of the rivet tail. In strength tests, the damaged CFRP sheet resulted in rivet head pullout of the CFRP/aluminum joints, which exhibited the lowest lap-shear and cross-tension strengths. In contrast, the steel/aluminum joints demonstrated the highest strengths because of their comparatively larger interlock distances. In addition to the experimental analysis, simulations revealed the rivet penetration and flaring mechanisms with various top sheet materials, and their respective joint quality and strengths.

Study on the SPCC and CFRTP Hybrid Joint Performance Produced with Additional Nylon-6 Interlayer by Ultrasonic Plastic Welding

Due to the high degree of dissimilarity in physicochemical properties between metal and carbon fiber, it presents a tremendous challenge to join them directly. In this paper, cold rolled steel (SPCC) and carbon fiber reinforced thermoplastic (CFRTP) chopped sheet hybrid joints were produced with the addition of Nylon 6 (PA6) thermoplastic film as an intermediate layer by the ultrasonic plastic welding method. The effect of ultrasonic welding energy and preheating temperature on the hybrid joint microstructure and mechanical behavior was well investigated. The suitable joining parameters could obtain a strong joint by adding the PA6 film as an intermediate layer between the SPCC and bare carbon fibers. Microstructural analysis revealed that the interface joining condition between the PA6 film and the SPCC component is the primary reason for the joint strength. The crevices generated at the interface were eliminated when the preheating temperature arrived at 200 °C, and the joint strength thus significantly increased. The lap shear test results under quasi-static loading showed that the welding energy and preheating temperature synergistically affect the joint performances. At 240 °C, the joint strength value reached the maximum. Through the analysis of the microstructure morphology, mechanical performance, and the failure mechanism of the joint, the optimized joining process window for ultrasonic plastic welding of SPCC-CFRTP by adding an intermediate layer, was obtained.

Mechanical Behavior of Multi-Material Single-Lap Joints under High Rates of Loading Using a Split Hopkinson Tension Bar

In the presented research, a split Hopkinson tension bar (SHTB) was used to measure the mechanical response of multi-material single-lap joints in the high-rate loading regime. High-performance applications require high-quality measurements of the mechanical properties to define safe design rules. Servo-hydraulic machines are commonly used to investigate such small structures, but they are prone to produce oscillation-affected force measurements. To improve force–displacement measurements, an SHTB was chosen to investigate these joints. Three different kinds of joints were tested: multi-material bolted joints, multi-material bonded joints, and multi-material bonded/bolted joints. One substrate of the joints was made of aluminum (Al-2024-T3) and the other one was made of a laminated composite (TC250). A countersunk titanium bolt and a crash-optimized epoxy adhesive (Betamate 1496 V) were used to fasten the joints. A constant impedance mounting device was implemented to limit wave reflections and to improve the signal quality. Quasi-static experiments at a servo-hydraulic machine were performed to compare the data with the respective data from the high-rate loading conditions. The presented research shows that high-quality high-rate tests of multi-material single-lap joints can be achieved by employing an SHTB. With this high-quality measurement, a rate dependency of the mechanical behavior of these joints was identified. The dynamic increase (DI), which is the ratio of a high rate of loading over quasi-static loading, was measured for each of the joint types, where the dynamic increase in the max force was DI = 1.1 for the bolted, DI = 1.4 for the bonded, and DI = 1.6 for the bonded/bolted joints.

Mechanical behaviour of friction stir welded AA7075-T651 joints considering the effect of tool geometry and process parameters

ABSTRACT Obtaining high-strength aluminium alloy welded joints considering the effect of tool geometry and process parameters still needs attention. In this study, AA7075-T651 aluminium alloy plates were joined using friction stir welding (FSW). The mechanical behaviour of joints was investigated considering the effect of tool rotation, welding speed, and stirred pins having two different geometries (conical pin and conical threaded pin). The welded joints were characterised by their tensile strength, microhardness, microstructure, and percentage elongation. Welds exhibited superior tensile properties when conical threaded pins were used. The optimised FSW process parameters revealed higher tensile strength of 177 MPa, the microhardness of 136 HV, and surface roughness of 15 μm with the conical threaded pin at tool rotation and welding speed of 1740 rpm and 40 mm/min, respectively. No significant difference in the microhardness was observed with both tools. The highest joint efficiency and percentage elongation of 34% and 17.4% were observed with the conical threaded pin.

Effect of Rotational Speed, and Dwell Time on the Mechanical Properties and Microstructure of Dissimilar AA5754 and AA7075-T651 Aluminum Sheet Alloys by Friction Stir Spot Welding

In the current study, the effects of dwell time and rotation speed on the mechanical properties and microstructure of friction stir spot welded joints of dissimilar aluminum sheet alloys were investigated. Aluminum AA5754 and AA7075-T651 alloys were selected as the work piece. The joint quality, microstructural evolution and mechanical behavior of the welded regions were considerably affected by the welding parameters. The results obtained that rotation speed and dwell time play an important role on welding quality of aluminum sheet. The microstructure images showed the dwell time and rotation speed has great effects on pin penetration and hook deformation. Maximum tensile shear load 806.3 N was produced at 1000 rpm and 2 s dwell time, while the tensile shear load reduced around 25 % with longer dwell time 5 s and high rotation speed 1400 rpm. Moreover, the welding joint microhardness was improved by the decrease of dwell time and increase of rotation speed.

Exploring the Effect of Asperity Order on Mechanical Character of Joint Specimen from the Perspective of Damage

The joint morphology is multiscale. The effect of each asperity order on the mechanical properties of joints is different. The shear mechanical properties of joint specimens are related to its surface damage characteristics. At present, there are still few studies on the effect of roughness on the shearing mechanical properties of joint from the perspective of damage of each asperity order. In this paper, the standard roughness profile was chosen as initial morphology. The standard roughness profile was decomposed into waviness and unevenness by the method combine the ensemble empirical mode decomposition (EEMD) and the cut-off criterion. Then, the joint specimen which contains waviness and unevenness and the specimen which only contains waviness were prepared by the 3D engraving technology. The 40 sets of joint specimens with different asperity order were subjected to direct shear tests under different normal stresses. Based on the 3D scanning technology and ICP iterative method, the damaged area and the damage volume were calculated. Based on the damage volume data and the acoustic emission (AE) data, the effect of asperity order to the joint mechanical behaviour was studied. The results indicate that (1) under low normal stress, the unevenness plays a control role in the failure mode of the joint specimen. Under low normal stress, the joint surface containing only waviness exhibits slip failure, and the joint surface with unevenness exhibits shear failure. With the increase of the normal stress, the failure mode of the specimen containing only waviness changes from slip failure to shear failure; (2) the unevenness controls the damage degree of the joint specimen. The damaged area, damage volume, AE energy rate, and accumulative AE energy of the joint specimen with unevenness are larger than those of the specimen with only waviness, and this difference increases with the normal stress increase; (3) the difference between the joint specimen with unevenness and specimen with only waviness mainly exists in the prepeak nonlinear stage and the postpeak softening stage. The characteristic parameters of acoustic emission generated in the postpeak softening stage of the joint specimen with unevenness are greater than those of the specimen with only waviness. This phenomenon can be used to explain the stress drop difference at the postpeak softening stage; (4) the AE b value can be used to evaluate the damage of joint specimens. Analysing the damage difference of each asperity order under different normal stresses is of great significance to the analysis of the influence of the morphology of the joint surface on the mechanical properties of the joint.

Analysis and determination of the behavioral mechanism of rock bridges using experimental and numerical modeling of non-persistent rock joints

Presence of rock bridges (RB) in natural non-persistent discontinuity sets is an effective factor on the stability of rock structures. Investigations show that the bearing capacity of jointed rocks is changed with variation of different joint parameters. Therefore, in order to investigate the effect of parameters such as contact area and number of rock bridges, normal load, angle, length, and number of joints on shear strength of non-persistent rock joints and also to recognize the interaction of these parameters on the mechanical behavior of joints, experimental design methods of Taguchi and central composite design (CCD) were used for the sample generation, testing and numerical modeling. The effective parameters on the shear strength of jointed samples were obtained based on the previous studies. Using the Taguchi method, 16 samples were tested at CNL condition and the effect of parameters such as normal stress, number, and area of rock bridges was investigated. To evaluate other joints parameters, 30 experiments were designed using CCD method and examined using numerical modeling. Using the analysis of variance (ANOVA), it was found that the obtained models are statistically significant. According to the results of ANOVA, the area of rock bridges and the angle of joints showed the highest and the lowest effect on shear strength of coplanar and non-coplanar jointed samples, respectively. The results displayed that the dominant failure for planar non-persistent joints is pure shear and a combination of tensile and shear cracks is created from the both tips of adjacent joints. Moreover, it was found that the dominant failure of non-coplanar joints is the combination of shear and tension. Bonded particle model (BPM) and smooth joint model (SJM) were also applied for numerical modeling. A new method was considered for applying SJ to the models, which solved the interlocking problem during the test.

Pushover-based probabilistic seismic capacity assessment of RCFs retrofitted with PBSPC BRBF sub-structures

A new external retrofitting sub-structure is introduced in this paper, namely, precast bolt-connected steel-plate reinforced concrete buckling-restrained-brace-frame (PBSPC BRBF). The numerical model is proposed considering the joint mechanical behavior, postcast concrete effects, and interfacial shear performance, and the results are validated via an experimental study. The probabilistic seismic capacity assessments (PSCAs) are performed on the deficient reinforced concrete frames (RCFs) retrofitted with PBSPC BRBFs based on a pushover method. The structural and load uncertainties (e.g., the material parameter and load value) are analyzed, and the Latin hypercube sampling (LHS) method is adopted for the 1-span-3-story, 3-span-6-story and 5-span-8-story frames, respectively. The histograms, probability density functions (PDFs) and PSCA curves for all conditions are provided, and the average median values or logarithmic standard deviations (SDs) are recommended. The sensitivity analysis is also performed, and the tornado diagrams and stack columns are presented. The research provides the thresholds of damage measures for different limit states of the RCFs retrofitted with the external sub-structures, and the corresponding conclusions can be further combined with probabilistic seismic demand analyses to constitute the integrated fragility framework.

Role of plunge depth on the joint formation and mechanical behavior of Al6063‐AZ91 dissimilar lap joint produced by friction stir welding

AbstractDissimilar lap joint of Al6063 aluminium and AZ91 magnesium alloys was successfully produced by friction stir welding. Three different plunge depths (3.2 mm, 3.25 mm, and 3.3 mm) were adopted during welding. Similar Al6063–Al6063 lap joints were also produced along with the dissimilar Al6063–AZ91 joints for the purpose of comparing the joint formation. With the increased plunge depth, the width of the similar Al6063 ‐ Al6063 lap joint was increased. On the contrary, joint width was decreased for the dissimilar joint with increased plunge depths. The dissimilar joint was formed with a strong metallurgical bonding between the Al6063 and AZ91 alloys, which is attributed to the mechanical mixing of these alloys in the nugget zone. Additionally, the formation of intermetallics was also observed from the x‐ray diffraction analysis. The variations within the measured hardness values were higher at the joint interface due to the mixing of aluminium and magnesium alloys in the nugget zone. From the tensile shear tests, increased strength and decreased elongation were measured with the increased plunge depth. The results demonstrate the importance of the plunge depth on the lap joint formation between dissimilar Al6063–AZ91 alloys during friction stir welding.

Review on Aluminum and Steel Semi-rigid Connections Behavior Design Model

Several research reports agree on the influence of joint rotational behavior on the stability of space frames. One must therefore consider it in space structures study. Joint rotational behavior is generally considered in space structures study by means of its moment-rotation behavior curve. Models such as analytical, empirical, experimental, informational, mechanical and numerical mostly are used to determine joint mechanical behavior. This review paper presents an overview of available methods for the prediction of semi rigid connections behavior of under both static and dynamic loads. Advantages, disadvantages, and principal characteristics of each model stretched out. The modeling of joint behavior in studying space structures is associated with a mathematical representation model of the joint moment-rotation curve. Several models, linear, bilinear, multilinear and nonlinear representations are developed through the years to picture accurately the joints moment rotation behavior. The most precise representation applies continuous nonlinear functions, even though the multilinear representation is generally used for mechanical models. Using test data on aluminum and steel bolted connections conducted at Harbin Institute of Technology a simple stable quartic polynomial model is proposed to represent the behavior of the connections. In addition, Three others models are also proposed, including an Odd power Polynomial Model as proposed by Frye & Morris Model, a three parameter Power Model in accordance to the Kishi & Chen Model, and a four-parameter exponential model in line with the Yee & Melchers Model. These three models are compared with the simple quartic polynomial model proposed in this paper. As a result, the proposed connection design model, independent of test data, can be used directly by designers to assess semi-rigid, bolted connection behavior in Space Structures. The present work will give support to engineers for easy and accurate choice of the joint behavior prediction model and portrait correctly the behavior of the joints for best use in semi-rigid space structures construction.

Effects of rotation speed and dwell time on the mechanical properties and microstructure of dissimilar aluminum‐titanium alloys by friction stir spot welding (FSSW)

AbstractIn this study, the effects of rotation speed and dwell time on the mechanical properties and microstructure of friction stir spot welded joints of dissimilar aluminum and titanium alloys were investigated. Aluminum AA6061 and titanium Ti‐6Al‐4 V alloys were selected as the work piece. The joint quality, mechanical behavior, and microstructural evolution in the welded regions were considerably affected by the welding parameters. The results of scanning electron microscopy showed the formation of Ti3Al intermetallic compounds near the thermomechanical affected zone, which significantly affected the properties of the welding joint. Maximum tensile shear load was produced at 1000 min−1 and 10 s dwell time. Moreover, the welding joint microhardness was improved with increasing the rotation speed.

Assessment of the stiffness of 3D cut welded connections with CHS columns and through I-BEAMS

A novel solution offered by the 3D Laser Cutting Technology, in the field of civil engineering, consists in the possibility of coupling hollow section columns with through I-beams. Such a kind of joint is alternative to the configuration with the I-beam simply welded to the external surface of the hollow profile for which Eurocode 3 part 1.8 provides specific rules for the strength calculation. The mechanical behaviour of joints with through I-beams is improved due to the higher flexural strength and stiffness of the connection. Nevertheless, no guidelines are currently available in national or international codes which allow to safely adopt this structural detail in practice. For such a reason, aiming at filling the knowledge gap about the structural behaviour of CHS to through I-beam joints, a research activity including experimental, numerical and analytical work, has been recently carried out at the STRENGTH laboratory of the University of Salerno. Specifically, the experimental phase has involved the execution of a cyclic and a monotonic test on a sub-assembly representative of the studied connection. The obtained results have been exploited to validate a numerical model developed in an FE software. This has allowed to simulate the behaviour, under monotonic loading, of other 29 connections, whose values of stiffness have been used to calibrate the coefficients of a design equation for the stiffness prediction based on the component method. The proposed equation has proved to be reliable in the stiffness prediction of such a kind of joints. Its accuracy in predicting the stiffness of joints with through-all beams is equal on average to 1, with a coefficient of variation equal to the 9%.

Prediction of the mechanical and failure behavior of metal-composite hybrid joints using cohesive surfaces

Friction Spot Joining (FSpJ) is an alternative technique developed to manufacture hybrid lightweight structures by joining metal to composites. This work has developed a finite element model to evaluate the failure behavior of aluminum alloy 2024-T3 and carbon-fiber-reinforced polyphenylene sulfide single spot joints produced by FSpJ. Cohesive surface behavior was applied to model the interface between aluminum and composite in the joint. The different bonding zones of the FSpJ joint were discretized in the model with a specific traction-separation law. The numerical and experimental force versus displacement curves have presented deviations of 8% for the ultimate lap shear force (ULSF) and 1.6% for displacement at failure. The evolution of the damage in the joint occurred preferably from the free edge of the composite due to the differential stiffness between aluminum and the composite. The influence of the edge distance on the mechanical behavior of the joints was also investigated using FEM. It has been observed that longer overlap lengths redistribute the stress in the bonding area more uniformly, thereby delaying the damage evolution in the bonding zones.

Processing and tooling considerations in joining by forming technologies; part B—friction-based welding

Solid-state welding is a variant of joining by forming technologies, in which metallurgical bonds are established as a result of severe plastic deformation at the interface of the workpieces. Solid-state welding is a promising tool to manufacture highly reliable joints of two or more metallic workpieces, either similar or dissimilar. However, some processes are also developed to perform solid-state welding on polymers and polymer matrix composites. Although solid-state welding methods are used to heat the workpiece, heat-affected zone (HAZ) is generally avoided or is considerably narrower compared to the conventional fusion welding and arc welding techniques, due to the more controlled heat input. Solid-state bonds can be established using frictional work, bulk metal forming, and severe energy of a high-speed impact. In this article, the processing conditions required to establish solid-state bonds by frictional work are reviewed. Several tooling and processing considerations are taken into account. Material-related aspects, process optimization techniques, joint mechanical behavior, and the influence of various processing and tooling parameters on mechanical behavior are the fields of concentration in this study.

Investigation of the interior RC beam-column joints under monotonic antisymmetrical load

The paper presents numerical findings of reinforced concrete interior beam-column joints under monotonic antisymmetrical load. The finite element models considered compression and tension damage were calibrated by test results in terms of the load-displacement, failure modes, and strains of longitudinal steel. The emphasis was put on studying the effects of hoop reinforcement ratio in joint core and the axial compression ratio on the responses of the joints. The results show that, in addition to the truss and strut-and-tie mechanisms, the confinement mechanism also existed in the joint core. A certain amount of stirrup is not only able to enhance the confinement in joint core, undertake a part of shear force and thus to increase the shear capacity, prevent the outward buckling of steel bars in column, improve the stress distribution in joint core, delay cracking and restrain the propagation of cracks, but also to increase the yield load, decrease the yield displacement of beam and improve the joint ductility. However, excessive horizontal stirrups contribute little to the joint performance. In a certain range, larger axial compression ratio is beneficial for the joint mechanical behavior, while it is negative when axial compression ratio is too large.

A Non-linear Viscoelastic Model of the Incudostapedial Joint.

The ossicular joints of the middle ear can significantly affect middle-ear function, particularly under conditions such as high-intensity sound pressures or high quasi-static pressures. Experimental investigations of the mechanical behaviour of the human incudostapedial joint have shown strong non-linearity and asymmetry in tension and compression tests, but some previous finite-element models of the joint have had difficulty replicating such behaviour. In this paper, we present a finite-element model of the joint that can match the asymmetry and non-linearity well without using different model structures or parameters in tension and compression. The model includes some of the detailed structures of the joint seen in histological sections. The material properties are found from the literature when available, but some parameters are calculated by fitting the model to experimental data from tension, compression and relaxation tests. The model can predict the hysteresis loops of loading and unloading curves. A sensitivity analysis for various parameters shows that the geometrical parameters have substantial effects on the joint mechanical behaviour. While the joint capsule affects the tension curve more, the cartilage layers affect the compression curve more.

Mechanical behavior of the joints between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics with YAS interlayer at high temperature

YAS (Y2O3–Al2O3–SiO2) glass ceramics were selected to join carbon/carbon (C/C) composites and Li2O–Al2O3–SiO2 (LAS) glass ceramics through vacuum hot-pressing sintering, due to the corresponding excellent compositional and structural stability at high temperature. Microstructures, mechanical properties and fracture features of the C/C-LAS joints were investigated. The joints with YAS interlayer exhibited more favorable results than the joints with MAS (MgO–Al2O3–SiO2) interlayer subsequently to shear testing at 300 and 600 °C. Strength retention of 70% for YAS joints was achieved at 600 °C, while for MAS joints the strength retention was only 28%. The joining properties amount decrease of YAS joints was due to degradation of the intrinsic properties of YAS and LAS glass ceramics. Regarding MAS joints, the poor structural stability of MAS interlayer at high temperature was the reason for the corresponding decrease. As a result, the mechanical behavior of joints at elevated temperatures was not only related to wettability and interfacial bonding, but it was also related to the intrinsic properties as well as to high-temperature stability of the interlayer and the joined substrates.

Mechanical integrity of friction-riveted joints for aircraft applications

The predictability of damage evolution is a challenge for mechanical joints of composite structures due to the highly nonlinear material behavior. In this study, friction riveting was investigated as an alternative joining technology for composite laminates by analyzing experimentally the joint mechanical behavior under different loading scenarios. The failure and fracture micro-mechanisms of composite laminate single lap joints were studied under quasi-static and cyclic loading. The joints failed mainly by rivet detachment from the composite hole, followed by adhesive/cohesive failure of the squeezed material, and rivet pull-through failure. Despite lower quasi-static strength of friction-riveted joints (6.2 ± 0.3 kN) compared to reference bolted joints (8.7 ± 0.2 kN), their fatigue life was higher by 88%. The main improving contributions were: the squeezed material, working as an adhesive between the composite parts and an additional fracture micro-mechanism, and the absence of clearance at the rivet-composite interface, which promoted an improved load transfer between the joined parts.