Joint Specimens Research Articles

Seismic performance of HECC/RC beam–column external joint with no stirrup and reduced anchorage length in core region

In this paper, steel–polyethylene hybrid fiber-reinforced strain-hardening cementitious composites (HECC) are applied in beam–column external joint core region to form a novel HECC/reinforced concrete (RC) composite beam–column joint with reduced anchorage length of beam longitudinal rebar and eliminated transverse rebar, and thus to alleviate rebar congestion and simplify the construction process efficiently especially for precast RC frame structures. Five external beam–column joint specimens are constructed and tested under cyclic loading to illustrate the influences of the anchorage length of beam longitudinal rebar and longitudinal rebar ratio on the seismic performance of beam–column joint. The effects of design parameters on seismic performance, including hysteresis behavior, degradation of strength, energy dissipation capacity, and cracking patterns are discussed in detail. Experimental results indicate that the replacement of normal concrete with HECC in beam–column joint core region could apparently reduce the amount of stirrups in the joint core area while maintaining reliable seismic behavior. Remarkably, specimen with no stirrups in the core area exhibits nearly equivalent seismic behavior to that of the control RC specimen. Furthermore, the application of HECC in joint core area allows for a substantial reduction of the required anchorage length for the beam longitudinal rebar to 9d, which further simplifies the construction process considerably.

Static mechanical properties and failure behaviors of self-piercing riveted joints in aluminum alloy 5A06 after aging

This paper conducts an investigation on the static mechanical properties and failure behavior of self-piercing riveted joints in aluminum alloy 5A06 after being subjected to the aging process. The study involves three distinct categories of joint specimens: original specimens, 1-year aged specimens and 1-year aged specimens that have been additionally heat-treated at 200 °C. The research findings affirm that strain aging is responsible for a reduction in the peak strength of the joints. Furthermore, the weakest failure chain within the self-piercing riveted joint shifts towards the upper sheet due to a more significant reduction in internal stress experienced by the upper plate. This leads to a failure model characterized by upper sheet pull-off. Through Weibull distribution analysis, it has been established that the 5 % lower limit value for the strength of the SPR joint experiences an 86 % decline following 1-year aging. In practical terms, this means that for a vehicle structure with 7000 riveting points will lose an overall structural strength equivalent to the initial strength of 1000 riveting points within one year.

Load–slip behavior of steel bearing–square reinforced concrete column connection joints with adhesive bonding and self-locking

The connection joint between a steel beam and a reinforced concrete (RC) column is crucial for strengthening and renovating the existing buildings; hence, designing simple, safe, and reliable connection joints is necessary. This study proposed a novel steel bearing–square RC column connection joint with adhesive bonding and self-locking and investigated seven half-scale connection joints with variable joint configurations under vertical loads. The typical failure modes, load–slip behavior, interfacial shear stress, and slip stiffness were analyzed through a parametric study of the taper angle of the square tube, the height of the steel bearing (i.e. contact area between the tube and the concrete), the structural adhesive, and the type of steel bearing surrounding the RC column. The results demonstrated that for a steel bearing–square RC column connection joint with adhesive bonding and self-locking, the typical failure process included at least two load fluctuations with multiple peaks. Moreover, the presence of adhesive bonding substantially improved the slip stiffness of the connection joint specimen relative to specimens with only self-locking. Additionally, the taper angle of the square tube influenced slip stiffness; however, the influence of the taper angle decreased with increasing contact area. Finally, regardless of the surrounding type and taper angle, the first peak load increased with the larger contact area, and the type of steel bearing surrounding the RC column had an obvious influence on slip stiffness. Furthermore, a finite element model was proposed to simulate the working performance of the proposed connection joint based on experimental data. The simulation showed that joint failure occurred mainly owing to the buckling of the vertical stiffeners and the outward deformation or tearing of the square rings, and the tilting of the joint due to local falling off of the structural adhesive must be prevented.

Effect of beam-column depth ratio on seismic behavior of precast RC column-steel beam joint

To achieve the simultaneous horizontal casting of the joint and column in the factory, the authors proposed a new precast RC column-steel beam (RCS) joint. The overall behavior of the proposed joint has been investigated. However, the effects of various design parameters were unclear. This paper focused on the impact of the beam-column depth ratio on the proposed RCS joints. Three 2/3 scale interior RCS joint specimens were designed and tested under cyclic loading. Experimental results showed that the joint shear strength and ductility of the specimen improved as the beam-column depth ratio decreased. A large beam-column depth ratio could modify the bond condition of column bars. Moreover, a numerical study was conducted to further investigate the influential mechanism of the beam-column depth ratio. According to the distribution of compressive stress, the concrete strut was activated in the joint. The angle between the strut and the horizontal increased with the beam-column depth ratio, leading to less horizontal component to resist the joint shear force. The joint shear strength tended to be overestimated as the beam-column depth ratio increased by using current design methods. Ignoring the effect of the beam-column depth ratio in the design methods may cause significant errors in predicting the joint shear strength.

Stress, damage, and fatigue performance analysis of CFRP/Al double-sided countersunk riveted joints with variable rivet-hole clearance

The CFRP/Al countersunk rivet joints are one of the most fatigued dangerous points of the aircraft fuselage. To deeply investigate the failure mechanisms of the riveted joint structures, this paper utilizes electromagnetic riveting technology to prepare single-lap joint specimens and investigates the quasi-static tensile and fatigue failure behaviours of the joints under different rivet-hole clearances and stress levels. In addition, to assist this investigation, a three-dimensional finite element model, combining riveting and fatigue loading, is developed. This model takes into account the strain rate effects during riveting process and progressive failure behavior of various typical damage modes in CFRP laminates. The results indicate that Aluminum alloy sheets fracture dominates the fatigue failure of the joints. Increasing the clearance can reduce the stress amplitude on the shear plane and further enhance the fatigue life of the joints. Furthermore, all additional CFRP damage during the loading process is the growth of the initial riveting damage, and the damage-induced fibre bearing capacity decrease is considered the reason for the increase in stress amplitude of the Al sheet. Last, excessive clearance enlargement does not appreciably diminish the initial riveting damage, instead, it could result in a joint strength reduction.

Experimental and numerical investigation on the impact response of bolt-ball joints

The densely populated space lattice structure requires reliable performance under impact loads. The bolt-ball joint serves a crucial role in connecting members at various angles, significantly contributing to the overall safety of the structure. However, there is limited literature available on the impact resistance of the bolt-ball joint. In this study, the drop hammer impact test system was employed to conduct impact tests on 11 specimens of bolt-ball joints. Comprehensive records were maintained, including the final deformation states, video recordings of the impact process, and data on impact force, displacement, and strain. Additionally, numerical simulation analysis was conducted to expand the scope of the study. Five failure modes were observed: global bending deformation, bolt fracture, local denting, coupling deformation of local denting and global bending deformation, bolt and tube fracture. When the impact applied at the bolt ball or conical head and steel tube welding positions, the impact process was categorized into three stages: inertia, global bending deformation, and elastic recovery/bolt fracture. When the impact position was the steel tube, coupling deformation and elastic recovery/ bolt and tube fracture were observed. The energy absorption during the impact test was analyzed, revealing a positive correlation between the degree of specimen failure and the proportion of energy absorbed relative to the work done by the impact force. The effects of impact velocity, impact position, bolt dimension and impact angle on the dynamic response of the specimens were thoroughly analyzed.

Mechanical behavior and waterproof performance of longitudinal section of tunnel segment joint gasket

In the construction stage, due to construction errors and longitudinal differential settlement during tunnel operation, the amount of dislocation and opening at the segment joint increases, increasing the likelihood of water leakage. Therefore, it is necessary to conduct an in-depth study on the influence of the amount of dislocation and opening at the segment joints on the contact stress of the longitudinal section. Firstly, through theoretical analysis, this paper deduces that the waterproof performance of the gasket depends not only on its own contact area, linear compression stiffness, and Poisson’s ratio but also on the height of the segment joint specimen and the amount of joint opening caused by the sinking offset angle. Then, the effects of different openings and dislocations at the segment joints on the contact stress of the segment gasket section were compared using numerical simulation and model experiments. Through numerical simulation, it is found that the dislocation has a greater influence on the longitudinal left section. The average contact stress at 16 mm is 28.3% lower than that at 4 mm, and the influence of the opening amount on the sealing gasket section is greater than that of the dislocation. Combined with the test results, it is also shown that the influence of the opening amount of the waterproof performance at the segment joint is greater than that of the dislocation, and the waterproof rate of the segment gasket section joint is greater than 40% under the modified working condition.

Experimental and numerical investigations of seismic performance of stepped beam-column joints with different connection forms

The seismic resilience of prefabricated steel structures significantly depends on the design and construction of beam-column joints. To address the challenges related to the installation of prefabricated wall panels and overall system assembly, this study introduces a novel stepped beam-column joint. The effectiveness of different connection configurations on the seismic behavior of the joints was investigated. A series of cyclic tests were conducted on four joint specimens with different configurations, and their failure mechanisms, hysteretic curves, skeleton curves, stiffness degradation, load-bearing capacity, and energy dissipation capacity, were revealed. The experimental results showed that the stepped joint configuration offers favourable seismic performance, with notable asymmetry observed in the response patterns under positive and negative loading directions. Reducing the height of the stepped segment or the thickness of the end plate adversely affected the seismic performance, leading to early bolt failure. Consequently, a detailed finite element model of the stepped joint was developed and the model demonstrated satisfactory agreement with the experimental results. Furthermore, a parametric analysis focusing on variations in stepped segment's height was conducted, providing insights into optimization of the load-bearing capacity of the stepped joint. The difference in the performance between the stepped joints and traditional flush end plate joints is also highlighted.

Numerical Simulation Study on the Seismic Performance of Prefabricated Fiber-Reinforced Concrete Beam–Column Joints With Grouted Sleeve Connections

Based on the available experimental data, fiber models for four prefabricated fiber-reinforced concrete beam–column joint specimens with grouted sleeve connections are first developed in OpenSees software. Then, the simulated seismic performance of the specimens is compared with the experimental results. Finally, the effects of axial load ratio and shear-to-span ratio on the seismic performance of the specimens are further investigated numerically. The results indicate that Concrete02 material model and Reinforcing Steel material model can accurately simulate the constitutive relationship of concrete and reinforcing steel, respectively; the beam–column joint elements can accurately simulate different damage behaviors of the joint zone. Fiber-reinforced concrete can significantly improve the seismic performance of the specimens. The relative errors of the simulated seismic performance indexes are about 15%. It is recommended that the optimum value of shear-to-span ratio for prefabricated FRC BCJs is 2.0–2.5. The effect of axial load ratio on the seismic behavior of PBCJs-CM is very small, and can be negligible in the case that the prefabricated FRC BCJs has a moderate value of shear-to-span ratio. The fiber model developed in this article can provide a numerical simulation basis for subsequent studies of prefabricated fiber-reinforced concrete beam–column joint specimens with grouted sleeve connections.

Influence of different manufacturing techniques on GFRP flat-joggle-flat composite joints using multi-scale reinforcements for enhancing shear properties

Composite materials, particularly glass fibre-reinforced polymers, or GFRP are being used far more frequently. Airframes have been manufactured utilizing reinforced composites, including struts, frames, and flaps, employing raised epoxy-based co-cure technology. The current research describes a multi-scale approach to fortifying graphene nanoparticles (GNP) and carbon fibre Z-pins in order to strengthen the flat-joggle-flat composite joints with different manufacturing technique. Shear investigation showed that by adding GNPs and putting carbon fibre pins (Z-pins) in a crosswise position (perpendicular to the plane) to the joint's surface, concurrent reinforcement gives rise to greater shear characteristics with quasi-static loads. Specifically, there was a 45.6% improvement in shear resistance when contrasted with unreinforced co-cured FJF joints. The FESEM has been utilized to demonstrate the failure analysis of the specimens, which shows the clear failure mechanism of the FJF joint specimens. The FJF joint with multiscale reinforcement has a very high natural frequency of 685.1 Hz as compared to other configurations, according to the vibration analysis.

Optimization of parameters and formulation of numerical model employing GRA–PCA and RSM approach for friction stir welded Ti–6Al–4V alloy joints

In this work, an endeavour was made to determine the impact of the tool’s shoulder diameter, pin profile, rotational and traverse speeds on the mechanical properties of the friction stir welded namely Ti–6Al–4 V alloy joints. A total of 21 experiments were carried out based on the central composite design (CCD) concept of response surface methodology (RSM). A quadratic regression based numerical model was formulated to ascertain the relationship amidst the parameters of FSW process and the mechanical properties of the fabricated Ti alloy joints. Analysis of variance (ANOVA) was employed to confirm the importance of parameters and their interactive impacts. Optimized process parameter combinations were ascertained by grey relation based analysis (GRA) was coupled together with principal component analysis (PCA), a hybrid based approach. Single score of GRG based response was determined and GRG scores were ranked for all experiments. 1st rank was attained by the experiment no. 20 possessing a GRG score of 2.9989. Optimized values of 25 mm shoulder diameter having a taper cylindrical pin geometry employed at a tool traverse speed of 40 mm min−1, rotational speed of 1400 rpm resulted in the generation of flaw free Ti alloy joint possessing a largest tensile strength of 809.8 MPa, yield strength of 778.7 MPa and percentage of elongation of 6.9%. SEM based fractography of Ti alloy joint specimens announced that taper cylindrical pin geometry along with 25 m shoulder diameter of employed tool have an inevitable part in generating frictional heat in ideal volumes and a perfect stirring action during FSW process.

Shear Strength of Adhesively Bonded Joint with Toughened Epoxy Mussel Powder

Synthetic fillers were usually used to improve the shear strength of epoxy resin, but dependency on mineral-derived substances may increase the greenhouse effect. To surmount the problem, biofiller from household wastes mussel shells is proposed to improve the mechanical properties of epoxy due to its high calcium carbonate content. In this paper, the physical properties (X-ray diffraction test)and shear strength of toughened epoxy with mussel shell powder (TEMP) were investigated. Single lap joints (SLJ) specimens were tested for shear strength with incorporation of various TEMP volume fractions and over-lap length. The testing series includes TEMP volume fractions from 0% to 10% (by 2.5% increment) and overlap length ranging from 12.7 to 50.8 mm. The results demonstrated that the longest overlap length and 7.5% TEMP volume fractions exhibited a significant effect on the ultimate joint strength. From scanning electron microscopy (S EM), 10% TEMP was prone to particle agglomerations and gave less joint strength. The joint strength with 7.5% mussel shell powder was stronger compared with other volume fractions, with strength enhancement from 103.7% to 169.6% for the studied overlap lengths.

Effect of laser treatment on strength of moisture-degraded epoxy adhesive joints and microscopic observation of treated surface

ABSTRACT We examined the effect of laser processing surface treatment on the mechanical strength of adhesion. In addition to laser processing, double cantilever beam (DCB) and lap joint (LJ) specimens were prepared using an epoxy adhesive on aluminum plates that underwent acid etching, sandblasting, and acetone wiping. The open face method was employed to accelerate moisture degradation, and the tensile-shear strength and energy release rate of the specimens were measured. Laser processing resulted in remarkably high values of the measured energy release rate, even when the adhesive deteriorated. To examine the reason for the high strength, we performed a detailed fracture surface analysis and electron microscopic observation of the adhesive interface. When the interface of the fractured surface was observed, the laser treated surface was densely formed with fine filamentous protrusions that held the adhesive well. These filaments exhibited high oxidation resistance and achieved strong adhesion even when the adhesive and adherend deteriorated because of moisture. The laser treated adherend surface exhibited two strong anchoring effects owing to a significant increase in the surface area and the microfilamentous structure on the surface. However, the type of surface treatment did not have a significant effect on the LJ adhesion strength.

Study of the tensile mechanical properties and failure mechanisms of CFRP screwed joints

Removable joint technology is commonly used in composite laminates for various load-bearing structures. However, existing research primarily focuses on bolted joints, there is relatively limited research on screwed joints in composite materials. This study investigates the influence of connected layer thickness and hole diameter on the tensile behavior of threaded joints in carbon fiber-reinforced polymer (CFRP) laminates. After fabricating different CFRP screwed joint specimens, tensile tests were conducted. The digital image correlation (DIC) technique captured the deformation process. The experiment results indicate a significant increase in load-bearing capacity with the increase in diameter. For instance, joints with an 8 mm diameter exhibited a load-bearing capacity of 10.82 kN. The increase in the connected layer thickness correspondingly enhanced the load-bearing capacity of the joint. The joint with a thickness of 7 mm had the highest load-bearing capacity of 8.83 kN. Besides, with the increase in the thickness of the connected layer, the failure mode transitioned from shear failure in the connected layer to screw pull-out. The tilt angle of the screw during the pull-out process also decreases with the increase in the connected layer thickness. Strain and out-of-plane displacement measurements under ultimate load conditions verify these observations.

Experimental investigation on shear behavior of PVA-ECC keyed joints

Keyed joints play a critical role in ensuring the overall behavior of a precast concrete segmental bridges. However, these joints can exhibit poor shear capacity and durability as well as brittle failure. Polyvinyl alcohol fiber-reinforced engineered cementitious composites (PVA-ECCs) exhibit high shear strength and ductility as well as excellent fracture performance, making them attractive for use in the joints of segmental bridges. This study fabricated 16 Z-shaped specimens with number of keys (one, two, or three), concrete materials (normal concrete or PVA-ECC), and types (monolithic, epoxy, or dry) to investigate their failure modes, cracking loads, shear capacities, residual capacities, and shear stress–vertical displacement curves. The results indicated that the keyed joints exhibited shear failure at the root of a shear key, and an increase in the number of keys increased the residual strengths and ductility of both the dry and epoxy joints, but it had a negative effect on the shear strengths. When PVA-ECC was applied instead of normal concrete, the cracking resistance index for all joints improved significantly and the shear strengths and ductility of the monolithic and keyed epoxy joints increased. Generally, the shear strengths of the epoxy joints were 6.3–29.1% higher than those of the corresponding dry joints; however, the ultimate displacements of the epoxy joints were 46.6–72.4% lower. Finally, a method for calculating method of the shear capacities of keyed PVA-ECC joints was proposed and verified to accurately predict the behaviors of previously tested joint specimens.

Experimental study on direct shear resistance of horizontal grouted joints in PC shear walls under compression and in-plane bending moment

The upper and lower panels of the prefabricated concrete shear wall are usually connected via horizontal grouted joints to maintain structural integrity. The horizontal grouted joints are typically under compression and in-plane bending moment. In the current design practice, it is generally considered that the direct shear resistance of the horizontal grouted joint is identical to that of the new-old concrete joint, and not affected by the bending moment if no bending failure occurs. In fact, as far as the authors know, both considerations lack experimental evidences. In this study, eight horizontal joint specimens were tested, including four grouted joints and four new-old concrete joints, to investigate their direct shear performances under constant compression and different in-plane eccentricities. Test results indicated that the ultimate direct shear resistance of the grouted joints was averagely 15.9 % lower than that of the new-old concrete joints. For the joints under small eccentricities associated with full shear plane in compression, the ultimate direct shear resistances remained unchanged compared to the cases under no eccentricity. However, for the grouted joints under large eccentricities associated with partial shear plane in tension, the ultimate direct shear resistances reduced up to 27.8 % compared to the grouted joint under no eccentricity. In addition, the shear transfer mechanism was investigated with the aid of the extended shear-friction model. Finally, recommendations were proposed for direct shear design of the horizontal grouted joints.

A model for the response of unreinforced and z-pin reinforced composite pi joints

Experimental data obtained from Narrow Element and Wide Element pi joints was used to motivate the development of a mechanics-based model to predict the quasi-static response of non-reinforced and z-pin reinforced composite pi joints. The progressive failure modeling approach used a novel cohesive formulation to include intra- and inter-laminar damage modes and their interactions. A smeared cohesive zone modeling approach was implemented in which effective fracture toughness and cohesive strength values were defined in the areas corresponding to the z-pin fields in the reinforced joints. Properties defining the cohesive responses were calibrated at the narrow element level and subsequently used in the wide element model to establish blind predictions prior to experimental testing. Unreinforced (unpinned) and z-pin reinforced pi joint specimens were tested in pull-off loading. Experimental and numerical comparisons were made in terms of load–displacement response, critical loads, damage modes, and failure progression. Comparisons between the experimental results and blind predictions for the unpinned joints were found to be in good agreement; however, discrepancies for the z-pin reinforced joint emphasize the challenges associated with physics-based simplified modeling techniques intended to capture complex z-pin behavior.

Fatigue-induced fracture assessment for duplex stainless steel protruding tube-to-tubesheet welded joints in multiple-tube test specimen

A novel fatigue testing result and its numerical discussions are presented for protruding tube-to-tubesheet welded joints. The welded joint specimen is made up of an innovative stainless steel grade for pressure vessels of urea production plant, i.e., duplex stainless steel (DSS). Multiple-tube test specimen in which a bundle of tubes fabricated to the tubesheet by welding, is employed. During the fatigue test, several initial cracking positions were observed from the central tube and surrounding tubes. The crack propagation (CP) phenomena, i.e., CP from the central tube to surrounding tube and from the surrounding tube to central tube, were characterized for the multiple-tube problem. Two types of fracture pattern were occurred. These distinctive cracking behaviors are studied using numerical analyses, i.e., FEM stress analysis and X-FEM CP simulation. The possible crack initiating positions as well as the fracture patterns are identified based on the stress distributions. The CP behaviors between the tubes are carefully examined using stress intensity factor (SIF). Additionally, fatigue life cycles and fatigue resistance of the specimen are discussed. The fatigue-induced fracture of the DSS specimen is rigorously investigated and presented using the experimental and numerical methods.

Experimental and simulative study on seismic performance of stiffener‐plate PEC beam‐column joint

AbstractAn innovative assembled stiffener‐plate PEC beam‐column joint (SP‐PEC) is proposed to improve better constructability. In this paper, 12 PEC beam‐column joint specimens were subjected to quasi‐static cyclic tests. The design parameters included the thickness of the stiffener‐plate (equal‐stiffness thickness at load standard combination, full‐stiffness thickness of the cross‐section, and 120% of full‐stiffness thickness of the cross‐section) and the beam‐column connection with or without the stub beam. The results revealed that the assembled SP‐PEC beam‐column joint exhibited better seismic performance than the conventional PEC beam‐column concrete joint. The strength degradation coefficients of SP‐PEC beam‐column nodes are all greater than 0.92, and the stiffness decreases slowly. The specimens had good bearing capacity, deformation ability, energy dissipation capacity, and stiffness under a reasonable stiffener‐plate thickness. Then, the bearing capacity calculation formulae of the novel joint with certain prediction accuracy were proposed. Besides, the finite element models of the test specimens were established in ABAQUS and validated against the test results. Finally, further investigations of the SP‐PEC joint configuration by finite element analysis and engineering design recommendations for the assembled SP‐PEC beam‐column joint were provided.

Laser welding of ultra-high strength steel rocket engine shell

The microstructure and mechanical properties of 30Cr3-30CrMnSiA ultra-high strength steel rocket engine shell welded joint were investigated in detail. The results indicated that the microstructure of the fusion zone was martensite of columnar and equiaxial due to the fast cooling rate of the laser welding. At the as-weld (AW) state, the average grain size of the 30Cr3 base metal was 4.63 μm, which was much smaller than that of the 30CrMnSiA base metal, which averaged 6.85 μm. After the post-weld heat treatment (PWHT), all the microstructures of the base metal, fusion zone and heat affected zone (HAZ) of 30Cr3 were tempered martensite. Meanwhile, at both AW and PWHT state, the tensile specimens of the welded joints of dissimilar materials were all fractured in the 30CrMnSiA base metal side at both 25 °C and 800 °C. After the PWHT, the tensile strength of 30CrMnSiA and 30Cr3 base metal at 25 °C were 1517.4 MPa and 1795.3 MPa, respectively. The welded joints of dissimilar materials by laser welding also showed excellent tensile properties at high temperature (800 °C), because of the small average grain size.