Joint Specimens Research Articles

Direct Shear Testing Apparatus for Saturated Rock Joints

Abstract A novel shear test apparatus has been designed and built to test saturated jointed rock specimens under normal and shear loading, capable of housing seismic transducers to monitor simultaneously the mechanical and geophysical response of the rock joints during shear. The system comprises a sealed pressure chamber and a biaxial compression frame. The internal dimensions of the chamber are 177.8 mm × 228.6 mm × 381.0 mm to accommodate a rock specimen with dimensions 152.4 mm × 127.0 mm × 50.8 mm. The chamber is made of aluminum to reduce its weight and is designed to sustain a maximum chamber pressure of 10 MPa, which is considered sufficient to be able to saturate a wide number of rocks. Structural calculations of the chamber are performed with the finite element method (FEM) software, ABAQUS, with the criterion of a maximum deflection of 1 mm at maximum chamber pressure, which is small enough to prevent the loss of seal between the loading shafts and the chamber. The rocks used in the study are Indiana limestone and Sierra White granite. B-value tests conducted on cylindrical specimens of the rocks placed inside the chamber show that the back pressures required to achieve saturation are 3.5 MPa for Indiana limestone and 5.0 MPa for Sierra White granite. The chamber performance has been evaluated by comparing the changes of volume of the chamber at different pressures, measured in the laboratory, with those predicted with ABAQUS. The successful completion of a number of repeatable direct shear tests, on tensile-induced rock joints in dry and saturated conditions specimens, has further established the correctness of the chamber design and its operation.

Investigation of Fatigue Mechanics and Crack Evolution Characteristics of Jointed Specimens Under Cyclic Uniaxial Compression

ABSTRACTNonpersistent joints are prevalent in engineering rock masses and are sensitive to cyclic loads induced by geological movements and engineering disturbances. Therefore, studying the fatigue mechanisms of rock masses with nonpersistent joints under cyclic compressive loads is crucial for ensuring the rational design and long‐term stability of rock engineering structures. Based on laboratory experiments, this study employed the discrete element method to create specimens with different nonpersistent joints, and uniaxial compressive cyclic loading tests were conducted on these specimens with different maximum cyclic stress levels. The results show that the joint inclination significantly affects the characteristics of jointed rock, such as deformation modulus, irreversible strain, energy evolution, and crack characteristics. Increasing the maximum stress in the stress path results in a rapid release of hysteretic energy in the jointed regions of the rock, which leads to an exponential decrease in fatigue life while an increase in initial irreversible strain, final irreversible strain, and hysteretic energy density. Additionally, the shear fracture zones on both sides of the model expand, and the propagation and merging of cracks between joints become more extensive and complex. The results are significant for studying rock fatigue instability and structure engineering design.

Study on the effects of laser shock peening on the microstructure and properties of 17-7PH stainless steel

To investigate the surface integrity of 17-7PH stainless steel welded structural components used in aviation, laser shock peening (LSP) with different power densities was applied to stainless steel welded joints. The microstructural morphology, structural features, full-width at half-maximum, microhardness, and surface roughness of the stainless steel welded joint specimens before and after LSP were characterized and measured using SEM, TEM, XRD, a microhardness tester, and a high-resolution confocal microscope. The effects of different laser power densities on the microstructure and properties of the stainless steel welded joints were explored. Results indicate that the stainless steel welded joints exhibit a typical BCC phase. Laser shock peening promotes grain refinement in the welded joints, leading to the phase transformation of residual austenite into martensite. The surface roughness of the specimen is positively correlated with laser power density. At a power density of 5.17 GW/cm2, the surface roughness increased to 1.919 μm, which is 117.08% higher than that of the non-peened specimen. The microhardness of the specimens shows a decreasing trend with increasing power density. When the power density is 2.79 GW/cm2, the microhardness of the specimen significantly increases to 462.94 HV0.5, which is 22.26% higher than that of the non-peened specimen.

Seismic Behavior of Concrete Beam-Column Joints Reinforced with Steel-Jacketed Grouting

Joints in frame structures often fail before beams and columns in an earthquake and are a key part of reinforcement. In this study, to enhance the seismic performance of concrete frame structures, a steel-jacketed grouting composite reinforcement method is proposed by combining reinforcement technology, steel cladding technology, and eco-efficient materials from grouting technology. This method effectively utilizes the advantages of various materials, avoids major demolition and construction, and reduces waste and resource consumption. In order to verify the feasibility and effectiveness of the reinforcement method, one of the original joint specimens with a scale of 1:3 and one of the reinforced joint specimens were designed and tested. The experiments involved reversed cyclic testing of beam–column to measure its seismic behavior. The seismic performance indexes such as failure characteristics, hysteretic properties, and the energy dissipation capacity of the specimens were analyzed, and the corresponding finite element model was established. The influence of key parameters such as reinforcement range, steel plate thickness, and grout strength on its seismic performance was explored. The research shows that the method can effectively improve the seismic performance of the joints, and seismic performance indexes such as bearing capacity, ductility, and energy consumption of the specimens are significantly improved. The test results of the established finite element model are in good agreement. The variable parameter analysis of the finite element shows that the thickness of the steel plate has little influence on its bearing capacity. With the increase in the reinforcement range of the clad steel and the strength of the grouting material, the bearing capacity of the specimen increases. The research results can provide a reference for the reinforcement of frame structure joints.

Deformation ability of modular gymnasium steel inner sleeve all-bolt cross connection joint based on 3D-DIC method

Immense roof loads present a significant hurdle for the functionality of continuously supported modular connection joints in movable modular gymnasium structures. Consequently, we introduced leveraging inner sleeves and bolts to interconnect the modular units of the upper and lower columns, aiming to bolster the strength of inter-unit connections. Despite its noteworthy benefits, the deformation behavior and force distribution characteristics of this novel design remain underexplored. Hence, our study employs three-dimensional digital image correlation (3D-DIC) measurement to precisely capture the full-field displacement and strain distributions within the core area of this joint. Four distinct joint specimens were designed, and conducted comprehensive static tests, encompassing variables such as the inclusion or exclusion of inner sleeves, reinforcing ribs, and diverse loading directions. 3D-DIC detected subtle deformations and local buckling phenomena, undetectable by the naked eye, and revealed their occurrence patterns. Furthermore, the detections underscore the critical role of weld quality between beams and columns in ensuring the overall safety of the joints. The joint exhibited earlier yielding in beam bending compared to column bending. To delve deeper into this joint, we generated moment-rotation curves leveraging DIC results, deducing that this joint exhibits rigid-joint characteristics. The specimens primarily showed damage modes such as beam-root fractures or member buckling, while the core area remained intact. This observation reinforces the classification of the joint as a full-strength entity. These insights enrich our comprehension of this novel joint and serve as invaluable references and aids for designers.

Comparison of a Novel Posterior Integrated Transfixation Sacroiliac Joint Fusion Approach to the Posterolateral and Lateral Approaches: A Cadaveric Biomechanical and Computational Analysis of the Fixation, Invasiveness, and Fusion Area.

To concurrently assess and compare the fixation efficacy, invasiveness, and fusion potential of a posterior integrated transfixation cage system to the posterolateral threaded implant and lateral triangular rod systems, in a cadaveric model. Twelve (12) cadaveric sacroiliac joint specimens were utilized and tested within the single-leg stance multidirectional pure moment bending model. Each specimen was tested in the intact, destabilized, treated (using posterior, posterolateral, and lateral systems), and post-fatigue conditions by applying 0 to ± 7.5 Nm of moment in flexion-extension, axial rotation, and lateral bending while measuring the angular range of motion between the sacrum and ilium. Computational models were reconstructed from Computed Tomography (CT) scans and manufacturer surgical technique guides. The models were utilized to quantify the volume of bone removed during implantation and the surface area available for fusion. The posterior integrated transfixation cage system and the lateral triangular rods produced equivalent motion reduction in all motion planes (P > 0.583). The posterolateral cylindrical threaded implant produced less motion reductions than the posterior and lateral implants in flexion-extension (6% ± 3% vs 37% ± 10% and 33% ± 11%, respectively, P <0.05). The posterior system removed 22%-60% less bone volume from the sacrum and ilium (P<0.10), introduced 200%-270% more implant surface to the joint space (P<0.01) and decorticated 75%-375% more joint surface area (P<0.01). The posterior integrated transfixation single-implant cage system is superior to the posterolateral cylindrical threaded single-implant system. Its performance in osteopenic bone is equivalent to the lateral triangular rod system in healthy bone; however, the posterior integrated transfixation cage system requires a single implant, while the lateral triangular rod system requires three. The posterior implant removes the least bone volume and has the most surface area for fusion, providing a significantly better opportunity for robust sacroiliac joint arthrodesis.

Strength analysis due to thermal loading and tensile loading when metals are bonded by heat-curing adhesives

Heat-curing adhesives are widely used after being cured by heating to a temperature higher than room temperature. To evaluate the adhesive strength, therefore, it is necessary to consider both the thermal stress generated during heat curing and external loads such as tensile stress. Butt joint specimens are essential for evaluating tensile adhesive strength but also thermal strength. The interfacial strength can be discussed from the stress intensity factor (SIF) of a fictitious edge interfacial crack assumed at the interface end. This is because the SIF is controlled by the intensity of singular stress field (ISSF) at the crack-free interface end and a constant term associated with the thermal load. In this paper, a useful thermal SIF solution is proposed by superposing the SIF under tensile stress and the SIF under uniform interface stress associated with thermal loading. This general SIF expression provided under arbitrary material combination can be applied for predicting the tensile strength σc and critical temperature change ΔT without performing new FEM calculations. The usefulness of the expression is confirmed through the adhesive strength of Aluminum/Epoxy butt joint experimentally obtained. Once the critical SIF K1C can be obtained from the tensile strength σc and the temperature change ΔT, the adhesive strength can be expressed asK1C = constant of an assumed fictitious interface, and this can be used to predict critical σc for various temperature change ΔT and for various adhesive bondline thickness h.

Seismic behavior of exterior joint of CFDST column to steel beam with RC slab

To investigate the seismic performance of an exterior joint connecting concrete-filled double-skin steel tubular (CFDST) column to steel beam with reinforced concrete (RC) slab, four scaled-down joint specimens were designed at a ratio of 1:2. Quasi-static tests were conducted with different axial compression ratios were performed, and the failure mode, hysteresis curve, skeleton curve, bearing capacity, energy dissipation capacity, strength and stiffness degradation, and strain analysis of the composite joint were analyzed. A finite element method (FEM) model of the joint was constructed and validated against the obtained experimental results. Additionally, a parametric study was carried out to investigate the influence of various factors, including the hollow ratio of the CFDST column, the concrete strength of the slab, the thickness of the slab, and the width-to-thickness ratio of the outer steel tube on joint. Based on experimental studies and finite element analysis (FEA), a theoretical calculation method was proposed to evaluate the flexural capacity of the composite joints. The test results showed that the composite exterior joint specimens with RC slab failed at the beam end. Typical failure phenomena of fracture at the steel beam and horizontal end plate interfaces, steel beam flange buckling, and concrete crushing at the RC slab were observed in the experiments. While the interior joint specimen experienced significant plastic deformation and buckling at the column flange. The hysteresis curve of the specimen is elliptical and full without pinching. The presence of RC slab can effectively improve the ultimate bearing capacity, initial stiffness and energy dissipation capacity of the specimen. The FEA results show that the bearing capacity of the composite joints was improved significantly with the increase of the thickness of the slab and the width-thickness ratio of the outer steel tube. The theoretical calculation results of the flexural capacity of the joint are 86 %–112 % of the results of the finite element calculation. The stability and accuracy of the calculation method are excellent and can be effectively applied to calculate the flexural capacity in CFDST composite structural systems.

Experimental study on the residual strength of butt adhesive joints subjected to systematic creep damage

AbstractUsing adhesively bonded joints is an effective strategy for achieving lightweight designs. However, understanding the long‐term performance of these bonded joints remains a critical concern. Creep is a time‐dependent phenomenon induced by sustained mechanical loads, which can lead to viscous strain in adhesive materials. This strain has the potential to cause cracking in bonded structures over time. This study aims to investigate the creep and post‐creep behaviors of adhesively bonded aluminum joints, focusing on two crucial parameters: creep load level and creep duration. Butt adhesive joint (BAJ) specimens were fabricated and exposed to varying creep loads and times. Subsequently, quasi‐static tensile tests were conducted to assess the residual mechanical properties. The results indicated that higher creep loads resulted in increased creep deformations at constant creep durations. Furthermore, a downward trend was observed in the residual tensile mechanical properties of BAJ specimens following creep. Specifically, higher creep load levels and longer times led to more pronounced reductions in joint strength, strain, and stiffness. These reductions in joint mechanical properties were corroborated through joint fracture surface analysis. Additionally, mathematical models were developed using response surface methodology (RSM) to predict the experimental joint residual mechanical properties under different creep loads and times.Highlights Investigation of creep and post‐creep behaviors in adhesive joints. Higher creep load levels result in greater joint permanent deformations. Impact of higher creep load levels and durations on joint failure strength. Decreasing in the joint deformability and stiffness after creep.

Behavior of steel-plate concrete wall-reinforced concrete slab joint with UHPC-strengthened lap-splice connection

The comprehensive integration of modular technology is the crucial pathway for advancing the upgraded Hua-long Pressurized Reactor (HPR1000). However, previous studies are limited and numerous challenges remain concerning the modular curved steel-plate concrete (SC) wall-reinforced concrete (RC) slab joint. This paper focuses on the behavior and design of the SC wall-RC slab joint through large-scale tests and numerical simulation. Two UHPC-strengthened non-contact lap splice (NS) connections, including versions with normal and headed rebars, were proposed to ensure effective load transfer and modular construction performance. The test results indicated that the four joint specimens subjected to cyclic loading did not experience lap splice failure but instead suffered interface shear failure and shear failure of the RC slab under shear-span ratios of 1.5 and 3.0, respectively. The excellent load transfer performance demonstrated by these two NS connections validates them as effective methods for this joint, with lap lengths of 21d for normal rebar and 13d for headed rebar deemed adequate. Besides, the developed three-dimensional FE model with non-linear spring element accurately simulated the test failure modes and F-Δ curves. The parametric study results recommended that the volumetric steel ratio of tie-bar should not be lower than 1.03 %. The tension of longitudinal rebars for normal and headed rebars can be fully transferred within 80 % of the designed lap length. Furthermore, a complete design method was established based on the load-transfer model, encompassing interfacial shear, lap length calculation and tie-bar configuration, and is applicable to SC-to-RC connections or joints.

Experimental study on shear mechanical properties of concrete joints under different unloading stress paths

In order to study the shear mechanical properties of rock joint under different unloading stress paths, the RDS-200 rock joint shear test system was used to carry out direct shear tests on concrete joint specimens with five different morphologies under the CNL path and different unloading stress paths. The unloading stress paths include unloading normal load and maintaining constant shear load (UNLCSL), unloading normal load and unloading shear load (UNLUSL), unloading normal load and increasing shear load (UNLISL). The results show that the peak shear strength, cohesion, internal friction angle, pre-peak shear stiffness and residual shear strength of concrete joints under CNL path increases with the increasing JRC and normal stress. Under the UNLCSL path, under the same initial shear stress τ1, instability normal stress σi decreases with the increasing JRC, and normal stress unloading amount Δσn increases with the increasing JRC. Under the same JRC, σi increases with the increase of τ1, and Δσn decreases with the increasing τ1. Under the same JRC and σi, τi is significantly smaller under the UNLCSL path than the CNL path. Under the same JRC, the cohesion under the UNLCSL path is less than the CNL path, and the internal friction angle is higher than that the CNL path. Under the same JRC and σi, τi is the largest under the path of CNL and UNLISL, followed by the UNLCSL path, and τi under the UNLUSL path is the smallest. Compared with the CNL path, the variation range of the specimen internal friction angle is within 3% while the average decrease percentage of the specimen cohesion reaches 37.6% under the UNLCSL path, UNLISL, and UNLUSL. Therefore, it can be inferred that the decrease in cohesion caused by normal unloading is the main reason for the decrease in joint instability shear strength. After introducing the correction coefficient k of cohesion to modify the Mohr-Coulomb criterion, the maximum average relative error after correction is only 3.5%, which is significantly improved compared with the maximum average relative error of 56.9% before correction. The research conclusions can provide some reference for the accurate estimation of shear bearing capacity of rock joints under different unloading stress paths, which is of great significance to the stability evaluation and disaster prevention of rock mass engineering.

Partial weight-bearing and range of motion limitation significantly reduce the loads at medial meniscus posterior root repair sutures in a cadaveric biomechanical model.

The aim of this study was to investigate the influence of medial meniscus posterior root avulsion (MMPRA) before and after surgical treatment on the biomechanics of the knee joint, including suture repair forces during daily and crutch-assisted gait movements. MMPRA were investigated in eight human cadaver knee joint specimens by a dynamic knee joint simulator with daily (normal gait, gait with additional rotational movement, standing up, sitting down) and rehabilitation-associated movements (crutch-assisted gait with limited flexion range of motion [30°] and 30% [toe-touch weight-bearing, TTWB] and 50% of body weight [partial weight-bearing, PWB]) with simulated physiologic muscle forces. Each specimen was tested in intact, torn and repaired (transtibial suture) state. The biomechanical parameters were: medial mean contact pressure and area, knee joint kinematics, medial displacement of the posterior meniscus horn and loading on the anchoring suture. Significant reduction of the contact area due to the avulsion was observed in all movements except for PWB and sitting down. MMPRA repair significantly increased the contact areas during all movements, bringing them to levels statistically indistinguishable from the initial state. MMPRA resulted in a medial displacement up to 12.8 mm (sitting down) and could be reattached with a residual displacement ranging from 0.7 mm (PWB) to 5.7 mm (standing up), all significantly (p < 0.001) reduced compared to the torn state. The mean peak anchoring suture load increased from TTWB (77 N), PWB (91 N) to normal gait (194 N), gait rotation (207 N), sitting (201 N; p < 0.01) and to standing up (232 N; p = 0.03). Surgical treatment of MMPRA allows restoration of physiological knee joint biomechanics. Crutch-assisted movements reduce the loading of the repair suture, thus likewise the risk for failure. From a biomechanical point of view, crutch-assisted movements are recommended for the early rehabilitation phase after MMPRA repair. Level V.

Chloride corrosion resistance of wet joints in manufactured sand reinforced concrete prepared in plateau environment

The durability of three types of wet joints in manufactured sand concrete prepared in simulated plateau environment was investigated. And the accelerated corrosion test on specimens in the chloride salt solution and the static tensile test on corroded steel bars in specimens were carried out by sequence. The durability of different types of specimens prepared in plateau environment was compared by analyzing the development of concrete cracks, the corrosion morphologies of internal steel bars, the degradation of mechanical properties of steel bars, and the concrete microstructure. The results demonstrate that the development of concrete cracks and the distribution of crack width are affected by wet joint. The nonlinearity of variation of average crack width with actual corrosion rate is enhanced by wet joint. The strength and elongation of steel bars in concrete decrease evidently by wet joint. The strength and elongation loss of steel bars after corrosion is effectively reduced by the punched treatment on joint interface. There is no clear superiority or inferiority pattern for the durability of wet joint specimens with untreated interface (UI) and chiseled interface (CI), and the durability of wet joint specimen with punched interface (PI) is better than that of UI and CI specimens.

An adaptive fatigue crack growth model at the welded joints of steel bridge

Under the influence of random loads such as those from vehicles, the welded details in steel bridges tend to gradually develop into macrocracks, typically originating from manufacturing defects or stress concentration points. As the service life of the steel bridge increases, the continuous generation and propagation of fatigue cracks lead to a decline in the bridge's service performance, potentially resulting in catastrophic failures such as bridge collapse. Accurately analyzing the fatigue crack growth patterns at welded joints is a prerequisite for reliably assessing the structural service performance. The analysis of fatigue crack propagation depends on both the crack growth step size and the cyclic stress–strain response at the welded joint under loading. Considering the complexity of the material properties and stress concentration at the welded joint, which leads to a lack of analytical solutions for the cyclic stress–strain characteristics at the crack tip, numerical analysis was employed to obtain the cyclic stress–strain response at the crack tip of the welded joints. Using the size of the cyclic plastic zone at the crack tip as an adaptive crack growth step size, a model for fatigue crack growth was developed based on the plastic strain energy of the cyclic plastic zone. Fatigue test specimens extracted from the welded toe of cruciform welded joints were tested, providing cyclic stress–strain (Ramberg-Osgood material parameters) and fatigue damage characteristics (Manson-Coffin material parameters) of the material at the welded toe. High-cycle fatigue tests using cruciform welded joint specimens determined a good correlation between the crack propagation characteristic dimensions obtained from the adaptive numerical analysis model and the experimental results. The experimental findings confirmed the applicability of the established adaptive numerical analysis model for studying fatigue crack growth in cruciform welded joints. The parameters introduced in this model are physically meaningful, easy to obtain, and suitable for engineering applications.

Investigation of local mechanical behavior during the shear process of rough rock joints using DEM

Rough joints are prevalent in natural rock masses, and their shear mechanical properties are crucial for ensuring the safety and stability of these masses. Existing studies often neglect the contribution of various joint asperities to shear stress resistance. This study addresses this gap by eluci-dating the local shear mechanisms of different asperities in rough joints throughout the shear process. This was achieved through a combination of quantitative and qualitative analyses using the Discrete Element Method (DEM). Initially, Barton's ten standard roughness joint profiles were dig-itized, and rough joints were modeled using the Modified Smooth Joint Model (MSJM). Direct shear tests on joint specimens under constant nor-mal stress were performed using servo-controlled methods. Subsequently, the shear stress behavior was analyzed in relation to the test results. The failure modes of the joint specimens were examined in detail using crack tracking and force chain analysis. The study also explored the relation-ship between the number of cracks and shear stress. Additionally, variations in average stress across ten different segments of joints during shear were monitored using the measuring circle function. The shear resistance contributions of local joint segments were quantitatively assessed using three stress indices: maximum stress, upper limit of maximum stress, and the distribution range of maximum stress. The Joint Roughness Coeffi-cient (JRC) of joints was found to correlate well with these indices. Overall, the progressive failure of local joint segments was analyzed both qual-itatively and quantitatively. The study confirmed that the local shear mechanisms of different joint segments during the shear process are distinct.

Experimental study on eccentric compression of the prefabricated columns with a new type of column-column connection node

An assembled building is defined by a construction methodology wherein prefabricated components are fabricated off-site and subsequently assembled at the construction site. This methodology confers cost efficiency, expedites construction timelines, and enhances the quality of the prefabricated components. Furthermore, it mitigates a range of environmental concerns inherent to conventional poured-in-place construction methods. This paper introduces a novel assembled column-column connection node that utilizes a new adhesive potted rebar lap joint. The research was divided into two principal components. Firstly, 18 specimens of adhesive potted rebar lap joints were tested under tension, with the lap length being the variable parameter. Secondly, eccentric compression experiments were conducted on one poured-in-place column and six assembled columns, varying parameters such as stirrup configuration and the length of the post-poured section. The failure modes of the columns, as well as their bearing capacities, deflection, rebar strains, and concrete strains, were analyzed and recorded. The test results revealed the existence of two different failure modes in the tensile tests of adhesive potted rebar lap joints. It was found that the reasonable lap length of the specimen should be no less than 14 times the diameter of the rebar (d). Eq. (15) was employed to estimate the suitable lap length for specimens with different sleeve and rebar diameters. In the failure sections of the prefabricated columns, all failures occurred outside the nodes. Moreover, the mechanical properties of small eccentrically compressed columns were comparable to those of the poured-in-place column. The use of spiral stirrups enhances the bearing capacity of the specimen, and an increase in the length of the post-poured section corresponds to a greater bearing capacity. The lateral deflection of the poured-in-place column generally follows a sinusoidal curve, while the lateral deflection of prefabricated columns in the later stages of loading does not exhibit a sinusoidal pattern. Both align with the assumption of a flat section. The stress performance of adhesive potted rebar lap joints in eccentric columns meets the strength requirements. The inclusion of tenons minimally affects the stress distribution in the node, fulfilling construction specifications. The stress performance of the assembled column nodes is reliable. The proposed adhesive potted rebar lap joint technology, along with the novel prefabricated column-column node, provides significant insights into engineering design and construction applications.

Seismic performance of new detailing RCS connection with different axial compression ratio and FBP thickness

Reinforced Concrete column and Steel beam (RCS) structural system has been extensively studied due to the effective use of concrete and steel material and cost efficiency. Bearing failure and constructability were two main issues of RCS joint in practical engineering. To improve these two deficiencies, the authors proposed a whole-section diaphragm type RCS joint. This study aimed to examine the impact of axial compression ratio and face bearing plate (FBP) thickness on the seismic behavior of the proposed joint. In this research, six RCS joint specimens were designed to achieve joint failure and tested under cyclic loading. Shear failure was identified as the typical failure mode. The results revealed that higher axial, compression ratio could lead to higher cracking loads, with improved load-carrying capacity, stiffness, and energy dissipation capabilities. Additionally, the joint bearing distortion reduced as the axial compression ratio increased. All specimens displayed spindle-shaped hysteresis loops while the specimen without axial compression exhibited pinching effect. The presence of parallel FBPs significantly improved the shear capacity. However, as the thickness of the parallel FBPs increased from 6 mm to 10 mm, the load-carrying capacity showed limited enhancement. Moreover, a comparative analysis of current calculation methods was conducted. The results demonstrated that the AIJ and CECS347:2013 formulas showed acceptable accuracy in predicting the joint shear strength. These findings validated the applicability of the existing methods for proposed composite joints, while the effect of the axial compression ratio required further evaluation.

A Theoretical and Experimental Study on Asperity Damage-Driven Strain Rate Dependency of Fractured Coal

AbstractAsperities within pre-existing fractures of coals can experience local damage during the fracture closure due to external loading. Previous research postulates that this local asperity damage can lead to strain rate-dependency without causing permanent deformation to the bulk of the coal specimens. This study aims to comprehensively investigate this behavior by developing a theoretical model that characterizes the strain rate-dependency driven by fracture asperity damage in coal. To achieve this objective, an initial series of micro-scale mechanical tests are conducted on joint specimens to establish a model for effective stress acting on asperities. Building upon this model, a theoretical foundation is further developed to describe the strain rate-dependent asperity damage evolution and resulting energy dissipation. These frameworks are subsequently incorporated into elasticity and damage mechanics to capture the strain rate-dependent stress–strain relationships. To validate the proposed model across multiple scales, additional triaxial tests on core-scale specimen and micro-scale mechanical tests on joint specimens are performed. The experimentally measured strain rate-dependency aligns well with the predictions of the proposed model, indicating a successful development of a robust model. The results of the model developed in this study reveal that the strain rate-dependency in fractured coals is governed by several factors, including asperity damage, mechanical properties of the coal specimens and effective stress acting on asperities of pre-existing fractures within the bulk of coal. Moreover, it is shown that the effective stress acting on asperities is significantly affected by both applied normal stress and joint roughness coefficient (JRC). The insights derived from this study demonstrate that the strain rate-dependency induced by micro-scale asperity damage of pre-existing fractures leads to observable strain rate-dependency in bulk specimens at core-scale and the proposed model can adequately capture this behavior.

Seismic performance of the ST-RC columns to RC beams exterior joint with assembled connection details

An innovative composite column type, the steel tube-reinforced concrete column (ST-RC), consists of a core of concrete-filled steel tube (CFST) and reinforced concrete (RC) encasement. This study aims to validate the reliability of two convenient and rapid assembly methods between RC beams and ST-RC columns. Four full-scale beam-column joint specimens were tested, comprising two prefabricated assembly joints of ST-RC column-RC beam frames, one integrally cast joint of ST-RC column-RC beam frames, and one traditional integrally cast joint of RC column-RC beam frames for comparison. The experiments were conducted based on the principles of a self-balanced system, and quasi-static tests under constant axial compression on the ST-RC columns were performed. Hysteresis curves, skeleton curves, failure modes, strength degradation, energy dissipation capacity, stiffness degradation, deformation capacity and the influence of connection method and joint form on mechanical performance were analyzed and obtained based on the test results. The results indicate that, compared to integral casting, the assembly surrounding method with steel sleeves reduces specimen energy dissipation by 19.8 % and damping ratio by 21 % at 4 % drift, with minimal impact on other mechanical properties. In contrast, reinforcement surrounded and connected by steel plates reduces initial stiffness and peak load-carrying capacity by 1.0 %, along with decreasing negative ductility. It also lowers total energy dissipation by 37.4 % and damping ratio by 45.4 % due to compressive effects from bottom steel plate connections. Despite these effects, its impact on negative ductility and load-carrying capacity remains minimal. Additionally, experimental observations and rebar strain data indicate higher stress on rebars at the base of steel plate connection beams, suggesting a need to strengthen transverse rebars at the beam’s base. The study confirms the feasibility of two convenient and rapid assembly methods for the joints of ST-RC column. Both methods meet current Chinese standards for MCE, with failure modes characterized by “strong columns and weak beams” in both cases.

Analysis of the Suitability of Ultrasonic Testing for Verification of Nonuniform Welded Joints of Austenitic-Ferritic Sheets.

The purpose of the presented research was to determine the suitability of using ultrasonic testing (UT) to inspect heterogeneous, from a material point of view, welded joints on the example of the joints of a ferritic steel element with elements made of an austenitic steel. The echo technique with transverse (SEK) and longitudinal wave heads (SEL) addressed this issue. Due to the widespread use of 13CrMo4-5 and X2CrNiMo17-12-2 steel grades in the energy industry, they were selected as the test materials for the study. The objects of the presented research were welded joint specimens with thicknesses of 8, 12, and 16 mm and dimensions of 300 × 300 mm, made using the 135 metal active gas (MAG) process with the use of the Lincoln 309LSi wire-a ferritic-austenitic filler material. The stages of the research task were (1) making distance-amplitude curve (DAC) patterns from the test materials; (2) preparation of specimens of welded joints with artificial discontinuities in the form of through-holes; (3) performing UT tests on welded joints with artificial discontinuities using heads with 60° and 70° angles for the transverse wave and angle heads for longitudinal waves with similar beam insertion angles; (4) selection, by radiographic testing (RT), of welded joint specimens with natural discontinuities in the form of a lack of sidewall fusion; (5) performing UT tests on welded joints with natural discontinuities, using heads as welded joints with artificial discontinuities. It was found that (1) the highest sensitivity of discontinuity detection was obtained by performing tests on the ferritic steel side, which is due to the lower attenuation of the ultrasonic wave propagating in ferritic steel compared to austenitic steel; (2) the best detection of discontinuities could be obtained using a longitudinal ultrasonic wave; (3) there is a relationship between the thickness of the welded elements, the angle of the ultrasonic beam introduction, and the effectiveness of discontinuity detection.