Relative Slip Research Articles

Grouped rubber-sleeved studs–UHPC pocket connections in prefabricated steel–UHPC composite beams: Shear performance under monotonic and cyclic loadings

Prefabricated steel–ultra-high-performance concrete (UHPC) composite beams (PSUCBs) with grouped stud–UHPC pocket connections (GSUPCs) are a successful attempt of UHPC in bridge engineering, which offers significant advantages in accelerating on-site construction, enhancing structural performance, and improving environmental benefits. However, the uneven steel–concrete interfacial shear stresses distribution caused by the insufficient deformability of ordinary studs (OSs) in UHPC may result in the premature stud failure in large interfacial slippage regions. To address this issue, rubber-sleeved studs (RSSs) that wrapped rubber-sleeves around the stud roots were introduced to improve ductility and reduce the stiffness of connectors. However, the shear performance of grouped RSSs embedded in UHPC remains unclear, which hinders their application in practices. Against this background, systematic experimental and numerical studies were conducted to investigate the monotonic and cyclic shear performance of grouped RSSs–UHPC pocket connections. The results indicated that wrapping rubber-sleeve could increase the ultimate slip capacity and remained sufficient shear resistance, demonstrating great potentials of grouped RSSs–UHPC pocket connections in PSUCBs. Increasing the rubber-sleeve thicknesses and height effectively improved the ultimate slip capacity and deformation recovery, but led to the significant plastic deformation accumulation and cyclic stiffness degradation. Finally, design recommendation on rubber-sleeve configuration rationalization, ultimate shear resistance and load–slip relationship predictive models were provided.

Methodology for calculating load capacity reduction and slip relationship of clustered group nail connectors

Clustered group nail connectors are key connecting components for the full lifecycle construction and safe operation of steel–concrete composite structural bridges. To thoroughly investigate the stress mechanism of clustered group nail connectors in steel–concrete composite structures, this paper conducts a detailed numerical analysis on 100 sets of such connectors. It analyzes the stress mechanism of individual nail connectors and quantitatively calculates the group nail effect under the coupled action of multiple factors (nail spacing between layers, number of nail layers, concrete strength). Based on clarifying the force transmission patterns of nails and concrete in different layers during the loading process, this paper proposes a method for calculating the average bearing capacity reduction coefficient and the load-slip curve of single nails in clustered group nail connectors under the coupled action of multiple factors, which has been validated by experimental data. This research provides a theoretical basis for the design and calculation of group nail connectors in steel–concrete composite structural bridges.

Seismic performance and parametric study of precast reinforced concrete Shear Wall structure with indirect‐lapping joints

AbstractPrecast reinforced concrete (RC) shear wall structures (PRCSWS) are widely adopted by high‐rise buildings owing to their attractive advantages such as high construction quality and seismic resistance. A novel vertical joint for PRCSWS was proposed, featured by the indirect lap between connection cages and vertical distributed bars of the wall panel. The feasibility of this joint was verified by pseudo‐static experiments on precast shear wall specimens with different joint designs and a referential cast‐in‐place (CIP) specimen. To gain more insights into the failure mechanism of the PRCSWS with different joint designs, non‐linear finite element (FE) models were established and analyzed for the numerical study. The simulation showed good agreement with the experimental result regarding the load‐drift curves, cracking patterns, and failure modes, exhibiting a flexural‐shear failure with corner concrete crushing. The investigation into the failure patterns of four specimens indicated that anchorage deterioration of interfaces inside the joints was induced by cyclic tension, leading to both reductions in ductility and energy dissipation. This effect was more obvious when the connection cages (a design parameter) were less and more concentrated. The parametric study revealed that increasing the anchorage length of connection cage in wall panel and additional confinement reinforcements surrounding the reserved ducts could reduce the relative slip on the interfaces and enhance the anchorage of connection cages, which could further improve the seismic performance of PRCSWS.

Experimental and numerical analyses on the shear behavior of grouped single-embedded-nut high-strength bolts in steel–ultra-high-performance concrete composite slabs

Incorporating utilization of ultra-high-performance concrete (UHPC) slabs and single-embedded-nut high-strength bolts (SENHSBs) can optimize the structural performance and construction efficiency of steel–concrete composite slabs. Typically, SENHSBs are required to be densely arranged in groups to transfer sufficient interfacial shear forces in complex stress regions. However, the investigations focused on the performance of grouped SENHSBs embedded in UHPC slabs is limited. Against this background, 15 push-out tests were conducted, accompanied by comprehensive numerical analyses, to study the shear behaviors of grouped SENHSBs in steel–UHPC composite slabs. The test results demonstrated that the failure model was governed by SENHSB fracture. Specimens with the longitudinal spacing of twice the bolt diameter exhibited overlapping crushing areas in the UHPC slabs. The longitudinal spacing significantly influenced both the ultimate shear and ultimate slip capacities of the grouped SENHSBs, whereas the transverse spacing significantly affected both the shear stiffness and maximum slip of grouped SENHSBs. Additionally, the per bolt ultimate shear capacities of the specimens with multi-layer SENHSBs were significantly lower than those with single-layer SENHSBs. The SENHSBs located at various positions experienced different shear stresses, and the deviations became more significant as the longitudinal spacing decreased. Consequently, a formula was proposed to evaluate the strength reduction factor of the grouped SENHSBs. Furthermore, accurate models were developed for determining the ultimate shear capacity and load–slip relationship of grouped SENHSBs in steel–UHPC composite slabs.

Formation Mechanisms and Crack Propagation Behaviors of White Etching Layers and Brown Etching Layers on Raceways of Failure Bearings

White etching layers (WELs) and brown etching layers (BELs), formed on the upper layer of bearing raceways generally lead to surface crack initiation and propagation and significantly affect the stable operation of precision bearings. In this study, the microstructure features of WELs and BELs from two failure bearings have been characterized and analyzed. The BEL mainly consists of quenched martensite with higher hardness values. The bainite simultaneously exists at the boundary of the BEL/matrix. Owing to the relative roll and slip, temperature increases and rapid decreases are the main formation mechanisms of the BEL in this study (thermal-induced effect). The WEL can be only found on one sample, and elongated and coarse grains are found in this region. It can be speculated that the WEL may originate from the surface region temperature increasing again and then slowly decreasing. Cracks can initiate at the boundaries of the WEL/BEL/matrix. The crack propagation behaviors are significantly affected by the properties of the WEL and BEL microstructure. It is difficult for the cracks to propagate from a softer WEL into a harder BEL. Thus, the depth of cracks in the WEL is shallower. However, because of the brittle nature of quenched martensite in the BEL, cracks can easily propagate downward under contact stress. Thus, the depth of cracks can exceed 100 μm easily. The formation mechanisms of the WEL/BEL and crack propagation behaviors have been further proved and discussed in this study.

Effect of laser shock peening on fretting wear behaviour of AISI 304 stainless alloy

Unlike many other surface treatment techniques, such as shot peening (SP), low plasticity burnishing (LPB), ultrasonic surface rolling process (USRP), ultrasonic nanocrystal surface modification (UNSM), laser shock peening (LSP) is a non-contact process. This means that LSP achieves surface modification without direct physical contact, thereby eliminating the risk of media contamination and undesired wear on the peening equipment. As a surface hardening technology, LSP tends to produce a hardened layer with a high gradient of residual stress (RS) in the near-surface, significantly enhancing wear resistance. The well-known wear theories applied for numerical prediction, whether Archard model or dissipated frictional-energy model, contain a multiplier for contact pressure and relative slip amplitude p·ds or shear stress and relative slip amplitude q·ds, respectively. These multipliers are sensitive to the mechanical properties near the surface. Additionally, the wear profile is governed not only by the wear effect but also by the deformation of materials. Therefore, to accurately evaluate the wear performance for surface-hardened materials, the influence of RS is hard to be overlooked. In this work, the finite element method (FEM) is utilised to construct a 3D numerical model, which ignores and considers RS, for both as-received and LSP-treated samples. Two distinct RS distributions induced by different scanning strategies (one- and five-time), are introduced in the finite element (FE) model. The numerical simulation results are then compared with the experimental data to verify the accuracy of the fretting wear model built in this study. On this basis, the impact of different RS distributions on fretting wear characteristics, including wear profile and volume, is extensively investigated. The findings reveal that higher amplitude RS can reduce the degree of fretting wear to a greater extent, highlighting the beneficial role of RS in enhancing wear resistance. Furthermore, the numerical models incorporating RS demonstrate superior predictive accuracy compared to those that omit RS. This naturally underscores the importance of integrating RS in numerical simulations of LSP and other surface-hardened materials.

Digital twin-driven graph domain adaptation neural network for remaining useful life prediction of rolling bearing

Remaining useful life (RUL) prediction is significant for the healthy operation of machinery. In order to accurately identify the bearing degeneration states, it is necessary to collect massive full lifecycle data. However, the bearing lifecycle data is insufficient for effectively training a RUL prediction model in engineering practice. In this paper, a digital twin-driven graph domain adaptation method is proposed. First, a full lifecycle dynamic twin model of bearings is constructed to generate abundant twin data, in which the surface morphology evolution and roller relative slip at different stages are simulated to generate vibration responses. Second, a novel multi-layered cross-domain gated graph convolutional network (MGGCN) is developed, in which a new graph domain adaptation model is designed to solve the problem that traditional domain adaptation methods are not effective in processing the non-Euclidean data. The spatial and temporal features are extracted by multiple nonlinear transformations and previous time-step hidden state incorporation, respectively. In addition, a graph Laplacian regularized maximum mean discrepancy (GLMMD) is designed and applied in the training of model to enhance the capability of discerning graph domain differences. The experimental results confirm that the proposed method can achieve effective performance even in scenarios with limited actual data.

Simulation of the degraded (steel - concrete) bond strength due to corrosion via modeling pull out tests

Steel corrosion is recognised as major degradation factor of structural capacity and durability of reinforced concrete (RC) structures, especially in marine environment. RC structures with corroded reinforcement present reduced performance due to loss of the cross-sectional area of reinforcement, cracks in concrete and loss of bond between steel and the surrounding concrete. In the present study, simulation of the degraded bond strength on corroded RC elements was elaborated developing a three-dimensional (3D) model through finite element analysis (FEA) in ABAQUS, taking into account the recommendations of fib Model Code 2010. As a basis for the model validation, an existing experimental study on the effect of corrosion and stirrups spacing on bond behavior was considered. In the abovementioned study the bond mechanism of two groups of corroded RC specimens in confined (stirrups Φ8/120mm) and unconfined (without stirrups) conditions were examined upon the completion of eccentric pull-out tests. In order to model the complex conditions of the corroded steel-concrete interface, the cohesive behavior was adopted by appropriately modificated parameters, for each corrosion level. The bond-slip curves extracted from the developed model were shown to be in a good agreement with the relevant experimental results. In particular, the bond strength of confined RC specimens deviates at about 10% among the analytical and experimental outcomes, for both uncorroded and corroded conditions up to 8.3% corrosion level. In case of the unconfined specimens, the bond strength prediction was satisfactory up to 5% corrosion level, whereas for greater corrosion level, large deviation was observed. Regarding the prediction of the relative slip between steel and concrete, further investigation is required as the development of slip is influenced by a plethora of factors.

Microstructures and cryogenic mechanical properties of spray deposited 2195 Al–Cu–Li alloy with different heat treatments

In this study, 2195 Al–Cu–Li alloys with a remarkable layered structure were fabricated through spray deposition, hot extrusion, and three distinct heat treatment processes (namely, solid solution treatment (ST), artificial peak-aging (PA), and artificial over-aging (OA)). The samples subjected to varying heat treatments are revealed to exhibit an exceptional combination of strength and ductility at 77 K, in contrast to their counterparts at 298 K. Specially, the mechanical properties of the PA samples are enhanced to various degrees with decreasing temperature from 298 K to 77 K. It is found that the synergistic effects of dense dislocations and uniform T1 precipitates, as well as their extensive interactions, are responsible for the exceptional strain hardening capacity at cryogenic temperature. Particularly, multiple slip systems and the temperature-dependent relative slip distance are beneficial for the formation of dense dislocations and thus for enhancing ductility at 77 K. Furthermore, the observation of rare and small microcracks, along with intergranular delamination at 77 K, suggests that layered structures impede the plastic instability of the alloy and improve its ductility. These findings provide valuable insights into the deformation behaviors of high-strength aluminum alloys for cryogenic applications.

Study on Voids and Seepage Characteristics within Rock Fracture after Shear Dislocation Viewing from CT Test and Numerical Modeling

In rock mass engineering, stress balance changes often cause the relative slip of fractures along a wall surface, impacting the seepage behavior of fluid in the fractures. Using computer tomography (CT) scanning, spatial models of fractures with dislocations ranging from 0 to 10 mm were created to explore the relationship between changes in fracture dislocation and changes in fluid flow behavior, respectively. The spatial fractal dimension of cavity distribution within the fractures was calculated using a thin-plate filling approach to characterize the complexity of the fracture cavity distribution. The fluid flow within the dislocation fractures was then simulated using COMSOL, and the effect of cavity alterations in the form of dislocation on the fluid seepage behavior was analyzed using the spatial fractal. The results show that the values of mechanical aperture after dislocation of the fracture obtained by a CT test are normally distributed, the distribution range of mechanical aperture gradually widens with an increase in the dislocation distance, and the average mechanical aperture increases on a logarithmic curve. The relative spatial fractal dimension decreases gradually with an increase in dislocation distance, and the interconnected pathways within the fracture decrease; in addition, it is observed that the change in the relative spatial fractal dimension is closely correlated with the change in the mean mechanical aperture. Numerical simulations of dislocation fracture seepage found that the permeability increases nonlinearly with increasing dislocation distance. When the dislocation distance reaches 5 mm, nonlinear behaviors such as eddy currents occur, and the influence range of eddy currents gradually expands with the increase in dislocation distance under the influence of the boundary. Moreover, the inertia coefficient B in the Forchheimer equation and the critical hydraulic gradient Jc, which can describe the nonlinear seepage characteristics, show a power function decreasing trend with increasing dislocation distance, and the fluid in the fracture is more likely to produce nonlinear flow.

Strain-rate effect on the bond strength between concrete and reinforcing bars in dynamic pull-out tests

Static and dynamic pull-out tests were conducted to investigate the strain-rate effect on the bond strength between concrete and rebar and figure out the required dynamic development length. The concrete strength and loading rate were considered variables. The test results indicated that the bond strength increased at high strain rates, and the strain-rate sensitivity became more significant with the decrease in concrete strength. Moreover, the bond stress–slip relationship varied with strain rates. Based on these results, dynamic increase factor (DIF) models of bond strength were suggested. The required dynamic development length was evaluated by comparing the suggested bond DIFs to DIF of rebar yield strength. The dynamic development length was similar to or less than the static development length when rebars with yield strength above 400 MPa and concrete with compressive strength below 40 MPa were used. However, the dynamic development length was expected to be longer with concrete above 40 MPa than the static development length.

Comparative Analysis of Controller Effect on Anti-lock Braking System Performance using MATLAB/ Simulink

Active safety features aim to avoid or reduce road accidents, in contrast to passive safety features, which are meant to protect occupants after a collision. An Anti-lock Braking System (ABS) is a category of active safety breaking that stops the wheels from locking during a braking action, keeping them in contact with the ground. Different researchers have proposed various control methods for developing the ABS controller. A controller is effective if it reduces stopping distance and time and maintains the desired slip ratio. In this research paper, a quarter-vehicle model is utilized to evaluate the effect of the controller on the Anti-lock Braking System (ABS) and study longitudinal braking dynamics. The Bang-bang controller, Proportional Integral Derivative (PID) controller and the Fuzzy Logic (FL) controller for the ABS were modelled in MATLAB/ Simulink software. A feedback controller that abruptly flips between the two states is called a bang-bang controller, also called an on-off, two-step, or hysteresis controller. PID (Proportional - Integral - Derivative) parameters were obtained using the Ziegler Nichols tuning Method. A comparative analysis of the controllers concerning braking distance, braking time, and normalized relative slip was performed. ABS with the Fuzzy Logic (FL) controller outperforms both the Proportional - Integral - Derivative (PID) and bang-bang controllers.

Effect of maximum applied cyclic stress on fretting fatigue stress distribution of flat-on-flat modified 9Cr-1Mo steel contact: Finite element analysis

Fretting fatigue experiments were conducted on a modified 9Cr-1Mo (P91) steel under flat-on-flat contact with maximum applied cyclic stress (σmax) levels of 450 MPa, 500 MPa and 550 MPa at a stress ratio of 0.3 and a contact pressure of 100 MPa. A decrease in the cyclic life was observed with an increase in σmax. Chaboche model with isotropic and kinematic hardening was employed in finite element analysis to evaluate the stress distribution near the contact pad and along the contact surface under fretting fatigue conditions. In addition, the contact-related parameters such as contact pressure, contact shear stress and relative tangential motion were also assessed. The relative tangential motion was found to increase with increasing σmax. Besides, the peak values of normal stress (parallel to the applied loading direction) and maximum principal stress were observed around the leading and trailing edges of the contact pad at the σmax. The amplitude of relative tangential motion and slip zone increases with σmax. The orientations of the principal plane and shear plane to the applied cyclic loading direction are −89.5° and −134.5°, irrespective of the σmax. The fracture surface of the failed specimen revealed that the direction of the crack was nearly perpendicular to the applied stress. Smith-Watson-Topper parameter was used for estimating the crack initiation life with σmax. It has been noticed that the fraction and dominance of crack initiation or propagation phase depend on the imposed cyclic condition for the steel.

Experimental study on bond performance between carbon/glass-hybrid-fiber-reinforced polymer bars and concrete

Coating GFRP bars with a layer of carbon fibers containing carbon/glass-hybrid-fiber-reinforced polymer (HFRP) bars has been proven to improved interlaminar shear durability. Thus, the bond performance of the HFRP bar-concrete interface depends on the behavior of the HFRP-reinforced concrete structure. In this context, the study investigates the effects of the compressive strength of concrete and the embedded length of HFRP bars on the bond behavior of the HFRP bar-concrete interface and compares the results with the data on the interfacial bond of GFRP bar to concrete. The results demonstrate that all the specimens fail at the FRP bar–concrete interface under the pullout. The bond strength and bond stiffness of the HFRP bar–concrete composites decrease with an increase in the embedded length of the HFRP bar from 4 to 10 times its diameter. Raising the compressive strength of the concrete enlarges the bond strength and bond stiffness of the HFRP bar–concrete composites. Furthermore, the outer layer of carbon-fiber on the GFRP bar decreases the impact of the embedded length of the HFRP bar on the bond strength and bond stiffness of the HFRP bar–concrete composites. Finally, the improved modified mBPE model can accurately predict the bond stress–slip relationship of GFRP/BFRP/HFRP bar–concrete composites.

Geodetic slip rate partitioning in the central Anninghe and Daliangshan faults: Effects of fault geometry, elastic heterogeneity, and viscoelastic rheology

Geodetic measurements can be employed to estimate the secular slip rates and locking depths of crustal faults, two key parameters which influence assessment of long-term seismic hazard presented by a fault. However, this process requires making assumptions about the rheological properties of the Earth's crust and lithosphere, as well as the fault geometry at depth. Previous geodetic studies have inferred widely varying slip rates and locking depths for the central Xianshuihe-Xiaojiang Fault System, located along the southeastern margin of the Tibetan plateau. In this study, we use updated GNSS measurements, including 13 new velocities, to explore the effects of assumed rheology and fault geometry on the inferred slip rates and locking depths of the subparallel Anninghe (ANH) and Daliangshan (DLS) faults in this region. We demonstrate that only models taking into account the viscoelastic relaxation of the lower crust can resolve the apparent discrepancies. Models assuming a fully elastic rheology suggest very deep locking depths (∼35 km) and a lower slip rate on the ANH fault (∼5 mm/yr) than the DLS fault (∼7 mm/yr). Neither the relative slip rates nor the locking depths agree with seismic or geologic observations. We also consider models assuming a heterogeneous elastic rheology, but find they are indistinguishable from the homogeneous models. Modifying the fault geometry can impact the relative slip rates of the two faults but does not produce a reasonable locking depth either. In contrast, viscoelastic models accounting for seismic cycle effects indicate a faster ANH slip rate (∼5–7 mm/yr), a slower DLS rate (∼4–5 mm/yr), and a shallower locking depth for both faults (∼18 km), all of which are more consistent with independent seismic and geological observations. Our study suggests that steady long-term motion on these faults is modulated by long-lasting viscoelastic effects over a relatively broad region.

Experimental Study of Reinforced Concrete T-Beam Retrofitted with Ultra-High-Performance Concrete under Cyclic and Ultimate Flexural Loading.

Structurally deficient bridges are commonly retrofitted using conventional methodologies, including reinforced concrete, steel jackets, and fiber-reinforced polymers. Although these retrofit methods aim to improve structural performance, exposure to aggressive environments may undermine the durability performance of the retrofit material. More recently, ultra-high-performance concrete (UHPC) has provided an alternative to conventional construction methods, with its superior material characteristics favoring its use in retrofit applications. In this study, a large-scale reinforced concrete (RC) T-beam is constructed and artificially damaged. The T-beam is then retrofitted with an external envelope of UHPC on all faces. Sandblasting is introduced to the surface, providing partially exposed reinforcement in the T-beam to simulate material deterioration. Additional reinforcement is placed in the web and flange, followed by casting the enveloping layer of UHPC around the specimen. The feasibility of this method is discussed, and the structural performance of the beam is assessed by subjecting the beam to cyclic and ultimate flexural loading. This paper presents the results of cyclic and ultimate testing on the RC-UHPC composite T-beam regarding load-displacement, failure mode, and strain responses. The retrofitted T-beam specimen is subjected to a cyclic loading range of 131 kN for 1.576 million cycles. Despite no visible cracks in the cyclic testing, the specimen experiences a 12.22% degradation in stiffness. During the ultimate flexural testing, the specimen shows no relative slip between the two concretes, and the typical flexural failure mode is observed. By increasing the longitudinal reinforcement ratio in the web, the failure mode can shift from localized cracking, predominantly observed in the UHPC shell, toward a more distributed cracking pattern along the length of the beam, which is similar to conventional reinforced concrete beams.

Experimental and finite element analysis for evaluating influence of contact pressure on fretting fatigue mechanisms of A319‐T6 cast aluminum alloy

AbstractThis paper evaluates the influence of contact pressure on the surface contact state, fretting damage degradation mechanism, crack initiation and propagation characteristics, and fretting fatigue life of A319‐T6 cast aluminum alloy in fretting fatigue test using bridge‐type fretting pad. For a certain range of contact pressure, the fretting fatigue life exhibits a trend of increasing and subsequently decreasing with increasing contact pressure. With finite element analysis, the fretting contact region can be divided into two characteristic zones according to the dynamic evolution of the contact status within one fatigue cycle. The contact pressure affects both the distribution of the two characteristic zones as well as the relative slip range, which results in distinct fretting damage mechanisms. The nucleation characteristics and propagation paths of fatigue cracks under three typical contact pressures were analyzed considering the effects of fretting damage and heterogeneous microstructures of the alloy, accounting for the contact pressure dependency of the fretting fatigue life.

Bond and Cracking Characteristics of PVA-Fiber-Reinforced Cementitious Composite Reinforced with Braided AFRP Bars

Easy maintenance and high durability are expected in structures made with fiber-reinforced cementitious composite (FRCC) reinforced with fiber-reinforced polymer (FRP) bars. In this study, we focused on the bond and cracking characteristics of polyvinyl alcohol (PVA)-FRCC reinforced with braided AFRP bars (AFRP/PVA-FRCC). Pullout tests on specimens with varying bond lengths were conducted. Beam specimens were also subjected to four-point bending tests. In the pullout tests, experimental parameters included the cross-sectional dimensions and the fiber volume fractions of PVA-FRCC. A trilinear model for the bond constitutive law (bond stress–loaded-end slip relationship) was proposed. In the pullout bond test with specimens of long bond length, bond strength was found to increase with increases in both the fiber volume fraction and the cross-sectional dimension of the specimens. Bond behavior in specimens of long bond length was analyzed numerically using the proposed bond constitutive law. The calculated average bond stress–loaded-end slip relationships favorably fitted the test results. In bending tests with AFRP/PVA-FRCC beam specimens, high ductility was indicated by the bridging effect of fibers. The number of cracks increased with increases in the fiber volume fraction of PVA-FRCC. In specimens with a fiber volume fraction of 2%, the load reached its maximum value due to compression fracture of the FRCC. The crack width in PVA-FRCC calculated by the predicted formula, considering the bond constitutive law and the fiber bridging law, showed good agreement with the reinforcement strain–crack width relationship obtained from the tests.

Influence of the slip boundary on square cylinders with lattice Boltzmann method

In this investigation, two-dimensional flow past square cylinders with slip boundary have been studied with the lattice Boltzmann method. Three modes, which are a single cylinder, an oblique cylinder, and side-by-side cylinders, are investigated with Reynolds numbers from 25 to 200 while relative slip length ranges from 0 to 0.1. It can be concluded that both the flow state and the slip boundary have a great effect on the drag reduction rate. For a single square cylinder, drag forces decrease with larger slip length while the Strouhal number is almost constant. For an oblique cylinder, the slip length also has effects on the stability of the flow except the drag and lift forces. Vortex separation delays with a slip wall of oblique cylinder. For side-by-side cylinders, the jet between two cylinders is not conducive to the drag reduction rate of the slip boundary. Moreover, the application of slip boundary may also lead to additional drag force as vortex separation intensifies, which is extremely different from simple channel flows. Studies have shown that the slip boundary does not always reduce the drag in some complex flow fields. It can be concluded that the drag reduction effect of slip boundaries is more effective in uniform flow.

Bolt looseness identification using a knocking acoustic signal

As a common failure form of the bolted joint, bolt looseness can lead to relative slip between the connected surfaces. Under the action of shear external forces, the bolt experiences fatigue failure and fracture, which can affect the service performance and connection strength of the equipment, and even cause major safety accidents. At present, good results have been achieved in detecting the tightness of bolt connections through Structural Health Monitoring (SHM) technology. However, most of them are still carried out under laboratory conditions and cannot be applied in engineering. Therefore, this article proposes looseness recognition of bolts by knocking acoustic signals. This method converts the acoustic time series signal into a Mel-Frequency Cepstral Coefficient (MFCC) frequency spectrum and then it recognizes the bolt looseness status through the Mobilenetv3 deep neural network. The results show this method can identify loose bolts. Meanwhile, it has a strong engineering application background.