Decrease In Stiffness Research Articles

Effect of nano molybdenum disulfide (Nano-MoS2) on fracture behavior of Warm mix asphalt (WMA)

Warm mix asphalt (WMA) has been developed for producing cleaner products than other asphalt mixtures. However, fractures caused by structural factors (microcracks caused by insufficient mixing of bitumen and aggregates at lower construction temperature) and environmental factors (microcracks caused by freeze–thaw cycles (FTCs) and aging process (AP)) lead to the formation of low temperature cracking (LTC) and intermediate temperature cracking (ITC), which is intensified by the loading factor. The present study aimed to evaluate and compare the short and long-term fracture behavior of WMA (containing vertical and angular cracks) using semi-circular bend (SCB) geometries under mode I loading conditions at low and medium temperatures. In order to improve the fracture behavior of WMA, an eco-friendly additive called molybdenum disulfide nanoparticles (Nano-MoS2) was used. The results showed that the mixture reinforced with 0.3 and 0.6% nano-MoS2 had better fracture resistance (Fracture Energy (GF) and Fracture Toughness (KIC)), Fracture Flexibility (Flexibility Index (FI), Toughness Index (TI), and Cracking Resistance Index (CRI)), and fracture toughness (Tensile Stiffness Index (TSI) and Tensile Strength (TS)) than the sample without additives under short-term and long-term conditions at temperatures of −15 and + 15 °C. Under 3FTC, the reduction of three indices KIC, GF, and Pmax (in agreement with the hypothesis), decrease in stiffness (in agreement with the hypothesis), and increase in the flexibility of the mixtures (against the hypothesis) was achieved. In addition, the increase of the three indices KIC, GF, and Pmax (against the hypothesis), increase of stiffness (agree with the hypothesis), and decrease of flexibility of the mixtures (agree with the hypothesis) were obtained under 6AP. After the addition of nano-MoS2 to the WMA, a large part of the reduction in the fracture indices was compensated due to the application of FTD cycles. Therefore, the mixture reinforced with nano-MoS2 had better long-term performance under mode I at low and medium temperatures (compared to the sample without additives and at the beginning of the design year). On the other hand, the long-term performance results showed that SCB samples containing vertical and angular cracks were more affected by 3FTC than 6AP. This result indicated the use of FTD cycles to evaluate the long-term performance of WMA mixture to achieve a better design.

Performance Assessment of Multi-story Building

Any public infrastructure has a set service life limit. In the Indonesian case, the public building has a 50-year design life. This paper presents a performance assessment of a multiple-story building in Aceh-Indonesia. A multi-story Faculty of Engineering Universitas Syiah Kuala building was used in this study. This multi-story building structure has been used for more than 24 years since it was built in 1998. This structure is 29.4 m long, 13 m wide, and 14.4 m tall in geometric terms. This study reveals how this three-story building behaves, including displacement, base shear, and structure performance level per ASCE 41-17 criteria. The processes in this study are broken down into various stages, including pushover analysis and comparing the building's current natural frequency. ETABS software was used to model building structures. According to the study, the building's performance is still more or less similar to the initial plan. Pushover in the X-direction (Push X) is believed to be immediate occupancy (IO), and pushover in the Y-direction (Push Y) is believed to be life safety (LS). The pushover analysis results for Push X suggested that the structure is safe and retains rigidity. Push Y indicated minor damage within the life safety category after a service life of 24 years. This pushover analysis indicated that the structure has reduced its rigidity, making it less able to resist further displacement. Based on the base shear, which experiences a displacement that is significantly greater than the estimate at the original planning stage, it is known that stiffness decreases.

Experimental definition of lateral stiffness of a pneumatic wheel of the MTZ-82 “Belarus” tractor

BACKGROUND: Due to the excessive vertical and angular vibrations of suspensionless vehicles used in agriculture and road construction that occur during movement, the velocity of their motion is limited, which, in the conditions of a constant traffic flow, reduces the road capacity. Therefore, the search for ways to improve the vibration-isolating properties of wheeled suspensionless vehicles is a relevant issue, the solution of which affects not only on traffic safety and driving comfort, but also on average speed and fuel efficiency.
 AIMS: To determine the lateral stiffness of the pneumatic wheel from the rear axle of the MTZ-82 Belarus tractor at different tire pressures, basing on bench tests.
 METHODS: On the basis of the hydraulic pulsation test rig of the Automatic Units department of the VolgGTU, a special-purpose movable supporting and measuring device was developed and assembled, the feature of which is that 4 rollers with flanges are installed on top of the hydraulic actuator force sensor, a flat rectangular base plate is supported on these rollers and connected with a vertical frame of the rig through a force-measuring device by means of a screw mechanism. The tested wheel is mounted on the base plate, the wheels axle is fixed on a vertically-moving traverse, on top of which weights creating the necessary vertical force are fixed. The test procedure consisted in measuring the lateral force and lateral displacement of the base plate until the tire slip occurred at the following tire pressures: at the recommended pressure of 0.16 MPa and at reduced pressures of 0.12, 0.08 and 0.04 MPa.
 RESULTS: Based on the test results, the elastic characteristic curves of the lateral stiffness of the tested wheel were obtained. These characteristic curves have a regressive form, as their slope significantly decreases with a significant reduce in tire pressure. Thus, when the tire pressure is reduced from 0.16 to 0.12 MPa, there is practically no difference in the obtained values of lateral stiffness, which reaches 112.5 kN/m. After a 2-fold tire pressure reduction, the lateral stiffness decreases by 7%, and after a 4-fold reduction, by 28%. At the same time, the static tire deflection increases from 22 to 32 mm, which significantly increases the contact patch of the tire with the ground.
 CONCLUSIONS: By means of the tests carried out, it is established that tire pressure reduction by 2550% of the recommended value does not have a significant effect on the loss of lateral stiffness of the pneumatic wheel of the rear axle of the MTZ-82 Belarus tractor, which is important to use in order to increase traction and improve ride comfort of wheeled tractors.

Evaluation of Rotational Stability and Stress Shielding of a Stem Optimized for Hip Replacements—A Finite Element Study

The natural distribution of stress in the femur is altered when total hip arthroplasty (THA) is performed. In fact, when a stem is inserted inside the femur, there is a variation in stress due to the difference in rigidity between the material with which the stem is made and the femur. This generates the phenomenon of stress shielding. The aim of this study is to design an optimized prosthesis that guarantees an excellent rotational stability and a reduced stress shielding. Methods: Through the finite element method (FEM), the mechanical behavior of the stem subjected to the loads described by ISO 7206-4:2010 is studied. Results: Through topological optimization, there is a reduction in stress shielding in the proximal zone of 31.46%. The addition of ridges on the dorsal side of the stem also improves rotational stability by 27.82%. Conclusions: The decrease in stiffness that is recorded with the optimized stem guarantees a greater distribution of stress on the bone. The presence of dorsal ridges also favors the corticalization of the bone as it loads the bone near the dorsal, ensuring further stability. The perforated prosthesis presented in this study shows an increase in primary stability and an improvement in rotational stability as there is also a bone regrowth inside the prosthesis.

On the use of transmissibility for the detection of damaged springs in the primary suspension of a locomotive

The condition monitoring of the suspensions of railway vehicles is of utmost importance, allowing the reduction of the maintenance actions and the increase in the operational safety. However, the available methods often require a simplification of the vehicle through linearised models, a high number of sensors, or the use of complex algorithms that disregard the mechanical phenomena that explain the vehicle dynamics. This work suggests the Localized Transmissibility Damage Indicator (LTDI), based on the existing Transmissibility Damage Indicator (TDI), to detect damage in the springs of a locomotive, using pairs of sensors placed in the bogie frame and the axle boxes. For that purpose, multibody simulations are used to simulate the dynamic behaviour of the vehicle in tangent tracks under nominal and damaged conditions. The results from multibody simulations allow the calculation of the LTDI values for different levels of damage and various operation conditions, as well as the study of the effect of the variability inherent to the railway operation. The results show that the LTDI is significantly sensitive to damage. However, depending on the use of the lateral or vertical response, the LTDI is more suitable to detect the stiffness increase or decrease, or even to locate the damage. In conclusion, the LTDI is a promising method for the detection of damage on suspension elements of railway vehicles.

Axial compressive behaviors of circular steel slag concrete‐filled steel tube slender columns

AbstractThis study aims to investigate the feasibility of using steel slag aggregate in concrete‐filled steel tubes by conducting axial compression tests on five groups of 10 circular steel slag concrete‐filled steel tube (SSCFST) slender columns. The failure mode, load–deformation curve, load–strain curve, axial bearing capacity, initial axial stiffness and ductility of the columns are analyzed. The effects of the steel slag replacement ratio and slenderness ratio on the axial compressive behaviors of the specimens are also quantified. These test results are employed to verify the reliability of a finite element model of the circular SSCFST slender columns established using Abaqus software. Then, a parametric analysis is conducted to explore the factors affecting the axial compressive behaviors of circular SSCFST slender columns. Moreover, the prediction accuracy of the existing design methods for the axial bearing capacity of circular SSCFST slender columns is evaluated. The results demonstrate that the ultimate bearing capacity of the specimens decreases by 2.3% and 3.1% on average, the initial stiffness decreases by 2.8% and 3.5%, and the ductility index decreases by 1.2% and increases by 1.6%, respectively, when the steel slag replacement ratio increases from 0 to 50% and 100%. The ultimate bearing capacity of the specimens decreases by 11.1% and 37.1% on average, the initial stiffness decreases by 6.3% and 28.8%, and the ductility index decreases by 5.1% and 12.2%, respectively, when the specimen slenderness ratio increases from 40 to 60 and 80. The confinement factor and slenderness ratio play a significant role in determining the axial compressive behaviors of SSCFST slender columns. The accuracy of the formulas provided in TCECS 625‐2019 and by Han is demonstrated in predicting the axial bearing capacity of circular SSCFST slender columns.

Seismic performance of bolted T-stub Beam-to-Box column connection

Using internal continuity plates in box columns due to accessibility problems and complicated detailing of connection was perceived as fraught with shortcomings and limitations. To overcome this deficiency, the effectiveness of external continuity plates in conjugation with T-stub connectors in I-beam to box-column connections under cyclic action is investigated in this study. Three damage control strategies based on the fuse action of T-stubs (T-F), strengthening the connections (T-S), and using reduced beam section (RBS) connections (T-R) are pursued and are compared to a corresponding end-plate connection (E). Nonlinear finite element analyses are conducted using the ABAQUS/STANDARD software on the connections under the qualification cyclic loading conditions to elucidate all the main seismic behavioral aspects, including the strength, initial stiffness, energy dissipation, ductility, von-Mises stress, and equivalent plastic strain distributions of models. First, the accuracy of numerical models is validated by a series of available experimental results. Then, a parametric study is performed on the effects of the moment ratio of the beam to column, the number of continuity plates, stiffeners embedded on the beam, and using high strength materials on the seismic performance of the connections. From the results, it is found that the T-S model was the most efficient among the different connections studied, supplying the stiffness, strength, energy absorption, and ductility increases by 2%, 21%, −17%, and 22%, respectively with respect to the E model, satisfying the requirements of AISC Seismic Provisions for SMF. Increasing the beam-to-column moment ratio and the number of continuity plates, had limited effects by less than 10% on the seismic performance of the connections. In the absence of continuity plates, the connections exhibited worse inelastic seismic performance with decreases in stiffness, strength, and energy dissipation up to 67%, 44%, and 74%, respectively. The T-F connection gained a 13% strength increase and satisfied the AISC requirements when high-strength T-stubs are used. Based on the numerical observations, design strength and rotational stiffness of the proposed connections on the component-based joint design strategies, are finally outlined.

Bridge distributed stiffness identification of continuous beam bridge based on microwave interferometric radar technology and rotation influence line

Continuous beam bridges are a widely used and prevalent type of bridge. As material properties degrade over time, the bridge stiffness decreases, leading to a reduction in bearing capacity. Accurate identification of bridge stiffness is crucial for safe operation, but there are few studies on identifying the stiffness of continuous beam bridges. This paper proposes a novel method to identify regional stiffness distribution using microwave interference radar technology and rotation influence lines. Firstly, the regionally distributed multi-point displacement influence lines (DILs) are procured via an innovatively designed lightweight radar device and the multi-point rotation influence lines (RILs) can be subsequently derived through a difference method. Utilizing the derived multi-point RILs, the continuous beam is decoupled into multiple-span simple beams featuring additional moment couples. Then the moment envelope area (MEA) can be computed by integrating the superposition principle and the curvature envelope area (CEA) can be determined using multi-point DILs. Finally, the bridge distributed stiffness can be obtained by combining the MEA and CEA of the monitored area. To verify the validity of the proposed method, a laboratory experiment involving a continuous beam is conducted to identify its’ bridge distributed stiffness (BDS).

Experimental study on acoustic emission sensing and vibration monitoring of corroding reinforced concrete beams

Reinforcement corrosion is a major issue in reinforced concrete structures with severe societal and economic consequences if not detected and treated in time. This paper presents an elaborate experimental study to effectively monitor chloride-induced corrosion damage in reinforced concrete beams. A combined method of acoustic emission sensing and vibration-based monitoring is developed to detect, characterise and localise damage. Additionally, crack measurements on the concrete surface are performed as a reference technique. Four beams are corroded with an accelerated corrosion set-up while two additional beams are used as reference specimens. Whereas these monitoring techniques have already been applied individually to assess corrosion in reinforced concrete beams, they are here combined, in order to verify their complementarity. This results in an elaborate and innovative data-set which allows us to draw new conclusions on the way the techniques provide information on the evolution of corrosion damage in time and space. It was found that acoustic emission sensing can accurately detect and localise damage before cracking of the concrete, especially after dedicated noise filtering through clustering of the AE signals. Vibration-based monitoring is less sensitive to early damage, yet natural frequencies contain absolute information about the stiffness decrease due to corrosion damage, and strain mode shapes can localise damage on a larger scale. In addition, results from both techniques are in line with the crack measurements. As a result, the combination of these various monitoring methods offers valuable and complementary insights about the corrosion process.

Structural Evolution in Glassy Carbon Investigated Based on the Temperature Dependence of Young's Modulus.

As a non-graphitized carbon material, possessing exceptional hardness and chemical inertness, glassy carbon (GC) is often synthesized through the pyrolysis method, which includes a compression procedure of powdered precursor materials, thus increasing the costs for production of glassy carbon at an industrial scale. Direct preparation of GC via pyrolysis of bulk precursors is a low-cost approach but encounters challenges arising from an insufficient knowledge of carbon structure formation. In order to solve this problem, a new analysis of the temperature-dependent variation in Young's modulus of GC obtained by the pyrolysis of phenolic resin at 1000 °C, utilizing the impulse excitation technique (IET), was performed. Our findings demonstrate that there is a critical temperature range of 500-600 °C where pyrolysis leads to the most significant density change and GC is formed as a result. When GC samples are heated again, a significant structural reformation occurs in the same temperature range. It causes a decrease in stiffness, especially at heating rates >3 °C/min, and an interesting restorative effect-increase in stiffness when a GC sample is annealed at temperatures of 500-550 °C. These results bring important implications for the direct formation of large amounts of glassy carbon using bulk precursors.

Nonlinear analysis of GFRP reinforced concrete beams using moment-rotation approach and conjugate beam method

The moment-rotation approach is a promising methodology aimed to predict the rotational capacity of reinforced concrete members. Recently, several authors have applied this method to evaluate the bending behavior of beams, replacing the moment–curvature approach. Considering the partial interaction phenomena, the moment-rotation approach can be used to develop parametric analyses and to assist in the formulation of rational equations for different material combinations. However, the techniques to integrate the moment-rotation approach into a nonlinear structural analysis have not been deeply explored, and the wider use of this methodology requires a rigorous validation. This study proposes and validates a modeling framework that integrates the moment-rotation approach into a reinforced concrete beam analysis based on the conjugate beam method. This model simulates the global stiffness decrease of beams as the cracking progresses. The numerical results were compared with experimental tests from literature performed on fiber-reinforced and plain concrete beams with glass‐fiber reinforced polymer (GFRP) bars. The experimental behavior of deflection, the crack-opening moment, and the moment–curvature were accurately simulated, especially for fiber-reinforced concrete beams. This accuracy indicates that the strategy adopted to integrate the moment-rotation approach into a structural analysis is definitely promising when modeling beams made of new materials.

SIMULATING BIORESORBABLE LATTICE STRUCTURES TO ENABLE TIME-DEPENDENT STIFFNESS IN FRACTURE FIXATION DEVICES

AbstractAdditive manufacture (AM) enables a greatly increased design freedom owing to its ability to manufacture otherwise difficult or impossible geometries. However, design creativity can often present itself as a barrier to realising the advantages that AM could offer. In this study the use of AM, bioresorbable materials and lattice design is considered as a method of satisfying contradicting design requirements during fracture healing. Often, immediately after a fracture high stiffness fixation is required; contradictingly during the remodelling phase high stiffness can inhibit bone healing. This study proposes the use of a bioresorbable body centred cubic (BCC) or face centred cubic (FCC) lattice structure to meet the need for tailored variation in implant stiffness over time. To reduce computational expense of lattice modelling a method is outlined, including the use of homogenisation. Results show homogenised representations perform within 5.2% and 1.4% for BCC and FCC unit cells respectively, with a 95% reduction in computational expense. Using resorption rates from the literature, time-dependent change in unit cell geometry was also modelled, showing the way in which a decrease in stiffness over time could be achieved.

Assessing the liquefaction potential of seabed soils based on ocean ambient noise in the Yellow River Delta

Seabed soils can undergo liquefaction under cyclic loading, resulting in a rapid decrease in strength and stiffness, which may lead to the destruction of offshore structures. Therefore, the assessment of seabed soil liquefaction will become an important factor in disaster prevention and risk analysis in coastal and offshore engineering construction. In this study, the ocean ambient noise with low-frequency, long-wavelength, and wide-band characteristics was used to conduct and analysis noise based on the horizontal-to-vertical spectral ratio method. The shear wave velocity of the seabed soil was obtained by inverting the ocean ambient noise dataset. Then, we proposed a shear wave velocity threshold that can be used for liquefaction assessment of Holocene unconsolidated fine-grained soils by statistical analysis, and the liquefaction potential of the soils was evaluated according to 1-D shear wave velocity structures and 2-D shear wave velocity profiles. The results showed that the distribution of the shear wave velocity obtained by inverting ocean ambient noise was generally consistent with the measured shear wave velocity in the field, indicating that the inversion results have a certain degree of accuracy. A shear wave velocity threshold of 200 m/s was proposed for liquefaction assessment, determining that the soils within 0-10 m depth in the coastal area of Yellow River Delta have liquefaction potential. This result is in accordance with the assessment based on the critical shear wave velocity, indicating that this threshold is applicable to the assessment of seabed soil liquefaction in the Yellow River Delta. The in-situ observations of ocean ambient noise provide a more convenient, economical, and environmentally friendly method, which can help to investigate marine geology disasters and serve marine engineering construction.

Parametric analysis of the effective flange width of mass timber composite floor panels

Mass timber composites (MTCs), where the webs are typically spaced 750–2000 mm apart, have shown potential in overcoming span limitations of conventional flat slab mass timber floors. However, fundamental research investigating the non-linear stress distribution (i.e., shear lag) in the flanges is required. While this behaviour has been simplified in design for other materials using the concept of an effective flange width (EFW), no guidance exists in international design codes for mass timber composites. In the current study, a parametric analysis was conducted on a mass timber I-beam under positive bending where the effects of web spacing, beam configuration (edge or interior placement), shear connection stiffness, concentrated versus uniform loading, web depth, web width, flange thickness, and flange modulus of elasticity on the EFW, were investigated. The reference section in the finite element model consisted of a glued-laminated timber (GLT) panel for the flanges, a glulam beam web, and a fully composite glued connection between elements.The results have shown that existing design code provisions, even those meant for light-frame wood members, over-estimate the EFW of GLT-glulam I-beams. The web spacing, shear connection stiffness, loading type, flange thickness, and web width were observed to be the most influential parameters on the EFW of GLT-glulam MTCs. Between the upper and lower boundaries of connection stiffness, a sharp increase in the EFW was observed below the point of full composite action due to the section beginning to behave as three separate elements. Although, the decrease in composite action resulted in an overall decrease in the bending stiffness of the section. For GLT-glulam MTCs, the modulus of elasticity (MOE) of the GLT flange was not observed to significantly affect the EFW for high degree of composite action configurations. A small increase in the EFW was observed when the degree of composite action was relatively low. Ultimately, the development of equations for quantifying the EFW of MTCs is key to their increased adoption but requires the expansion of the current study to include other mass timber elements, as well as experimental data investigating the most significant parameters identified herein.

Thermoplastic Toughening of a Semiaromatic Polyamide with an Amino-telechelic Polyethylene.

Semiaromatic polyamides are used for metal replacement in advanced engineering applications to reduce weight and improve efficiency, but their range of application is limited by their inherent lack of ductility and toughness. Here, we combined semiaromatic polyamide poly(hexamethylene terephthalamide-co-isophthalamide) (PA6TI) with up to 30 wt % amine-terminated polyethylene (PE(NH2)2) by high-temperature melt compounding, which was suggested to lead to the formation of PA-PE block copolymers at the interface between the PE(NH2)2 and the PA6TI. This resulted in PA6TI/PE(NH2)2 blends with smaller, more uniform particle sizes than in PA6TI blended with nonfunctional PE or the commercial impact modifier, maleic anhydride-functionalized styrene-ethylene-butylene-styrene (SEBS) under the same conditions. The PA6TI/PE(NH2)2 blends and the corresponding glass fiber-reinforced composites consequently showed significantly greater increases in room-temperature tensile ductility and fracture energy with respect to unmodified PA6TI, as well as maintained mechanical stability at high temperatures, and only modest decreases in stiffness and strength, even at high PE(NH2)2 contents. These improvements were attributed to the crystallinity of the PE(NH2)2 particles and to improved morphological stabilization and matrix-particle adhesion, consistent with the presence of PA-PE block copolymer at the matrix-particle interfaces.

Discovering the causes for the change of the vibration characteristics of the core support barrel in PWR nuclear power plants: A combined investigation based on ex-core neutron noise analysis and numerical modal analysis

As a key component of pressurized water reactor (PWR) nuclear power plants, the motion of the core support barrel (CSB) has a great impact on the safety of the nuclear reactor. However, the vibration characteristics of the CSB have not yet been fully investigated and may change due to the high doses of radiation and high temperature in the reactor. In this paper, the ex-core neutron noise signals of two PWR units throughout the multiple fuel cycles were analyzed and the vibration frequency of the CSB beam mode was found, for the first time, to drop with time during the operation of PWRs. Then, the statics analysis of the CSB was conducted to preliminarily identify the potential causes. For the further clarification of such vibration frequency drop phenomenon, a numerical assembly model of PWR reactor internals, including the CSB, hold-down ring (HDR), fuel assembly hold-down spring (FAHDS), etc., was proposed with consideration of the frictional contact and fluid-structure interaction for the modal analysis of the CSB. The results showed that the decrease of the FAHDS stiffness is the main cause for the drop of the vibration frequency of the CSB beam mode in real conditions.

Structural design and analytical study of a new polysilicon tunable microactuator for measurement of different resonance frequencies

This paper presents a novel MEMS tunable resonator that draws inspiration from the folded beam structure in its comb drive resonator design. Conventional electrostatic comb-drive resonators commonly employ folded-beam suspensions to achieve linearity and reduce system stiffness. The proposed design incorporates eight zigzag-shaped meander springs to support the comb finger structure, featuring two central anchors. The objective of this design is to introduce zigzag-shaped nested-folded beam suspensions to the comb parts, thereby reducing the tuning voltage and sensor size. The governing equations are provided to calculate the support springs’ stiffness, and a comparative analysis is conducted to evaluate the stiffness of the proposed design against other parameters. The mathematical analysis demonstrates that Zigzag Nested-Folded beams result in a decreased resonance frequency and a softened electrostatic spring under applied DC voltage. The integration of zigzag-shaped nested-folded beam flexures enables MEMS resonator devices to exhibit reduced stiffness, leading to a lower pull-in voltage. The resonator design and simulation are performed using Intellisuite and SolidWorks software. Results indicate that the resonant frequency of the proposed tunable comb-drive resonator, equipped with 26 finger pairs and a movable part displacement of 21 μm, is reduced by 66% from the original frequency of 2.420 kHz when a tuning voltage of 59 V is applied. Simultaneously, the corresponding effective stiffness decreases by 66% from the initial value of 0.72 N m−1. This compact resonator occupies an area of approximately 1.058×278 (μm)2, demonstrating a smaller size compared to previous works.

Experimental study on seismic performance and deformation damage of loose dovetail-tenon joints in ancient timber structures

A looseness is typical damage for mortise-tenon joints in ancient timber structures. The tenon of the loose dovetail-tenon joints is prone to be pulled out, leading to joint failure in the earthquake. Hence, it is necessary to investigate the seismic performance and deformation damage of loose dovetail-tenon joints. This study conducts pseudo-static and acoustic emission (AE) tests on three full-scaled dovetail-tenon joints to determine the four stress stages of each joint, including slight damage, medium damage, severe damage, and nearly failure. It examines the hysteretic and skeleton curve, stiffness and strength degradation, energy dissipation performance, and the variation raw of the amount of tenon pulled out at each stage. The deformation damage mode, damage degree, and damage evolution equation of the joint at each stage are obtained based on the AE characteristic parameters measured at each stage. The results indicate that as the rotation angle increases, each joint's stiffness and energy dissipation capacity decrease, while the joint bending moment rises and then reduces, and the amount of tenon pulled out increases. The greater the degree of looseness, the lower the bearing capacity of each joint, and the more pronounced the degradation of stiffness and strength. At each stage, the maximum decrease of initial stiffness is 85.75%, and the maximum amount of tenon pulled out is 93 mm. The failure modes of joints are mainly tenon pulled out failure, and extrusion deformation greatly influences the tenon pulled out failure. As the degree of looseness grows, joints are more prone to severe deformation damage under large rotation angles. At the slight and medium stages, the joints primarily suffer from tensile damage of material, but at the severe damage and nearly failure stage, they mainly suffer from shear damage of material. The damage value of each joint increases with the relative stress ratio, and looseness significantly impacts the damage degree of the joint.

Long-term static stretching can decrease muscle stiffness: A systematic review and meta-analysis.

Stretch training increases the range of motion of a joint. However, to date, the mechanisms behind such a stretching effect are not well understood. An earlier meta-analysis on several studies reported no changes in the passive properties of a muscle (i.e., muscle stiffness) following long-term stretch training with various types of stretching (static, dynamic, and proprioceptive neuromuscular stretching). However, in recent years, an increasing number of papers have reported the effects of long-term static stretching on muscle stiffness. The purpose of the present study was to examine the long-term (≥2 weeks) effect of static stretching training on muscle stiffness. PubMed, Web of Science, and EBSCO published before December 28, 2022, were searched and 10 papers met the inclusion criteria for meta-analysis. By applying a mixed-effect model, subgroup analyses, which included comparisons of sex (male vs. mixed sex) and type of muscle stiffness assessment (calculated from the muscle-tendon junction vs. shear modulus), were performed. Furthermore, a meta-regression was conducted to examine the effect of total stretching duration on muscle stiffness. The result of the meta-analysis showed a moderate decrease in muscle stiffness after 3-12 weeks of static stretch training compared to a control condition (effect size = -0.749, p < 0.001, I2 = 56.245). Subgroup analyses revealed no significant differences between sex (p = 0.131) and type of muscle stiffness assessment (p = 0.813). Moreover, there was no significant relationship between total stretching duration and muscle stiffness (p = 0.881).

Deep learning-based bridge damage identification approach inspired by internal force redistribution effects

Damage identification has always been one of the core functions of bridge structural health monitoring (SHM) systems. Damage identification techniques based on deep learning (DL) approaches have shown great promise recently. However, DL methods still need to be improved owing to their poor interpretability and generalization performance. The fundamental reason lies in the separation between physics-based mechanical principles and data-driven DL methods. To address this issue, this paper proposes a physics-inspired approach combining the data-driven method and the internal force redistribution effects to perform efficient damage identification. Firstly, the mechanical derivation of internal force redistribution is given based on a simplified three-span continuous bridge. Then, two types of typical damage scenarios including segment stiffness decrease and prestress loss are simulated to formulate the damage dataset with monitored field data noise added. Next, a modified Transformer model with multi-dimensional output is trained to obtain the complex dynamic spatiotemporal mapping among multiple measurement points from the intact structure as a benchmark model. Finally, the relationship between multiple damage patterns and the corresponding output regression residual distribution is studied, based on which the flexible combinations of the sensors are proposed as the test set to characterize the internal force redistribution due to damage. Validation on the extended dataset showed that this approach is effective to realize preliminary identification of damage patterns and resist interference from noise at the monitoring site.