Conical pipe joints are widely used in pipeline systems of various aerospace vehicles, and the stability of their sealing performance directly impacts the reliability of the aircraft's operation. Engineering experience has shown that setting circular grooves on the conical surface can improve the stability of the sealing performance of conical pipe joints. However, there is currently a shortage of experimental data to support this viewpoint. This article takes a 74° conical pipe joint commonly used in aircraft engines as the object and demonstrates rotational bending fatigue tests that circular grooves can significantly improve the stability of the sealing performance of conical pipe joints under vibration condition. On this basis, this article verifies through finite element simulation that the edges of the annular groove can generate contact pressure concentration bands, which serve as a form of line sealing. Thus, a qualitative explanation is provided for the mechanism of improving the stability of sealing performance by the annular groove. This study provides a reference for improving the design of conical pipe joints and other forms of static sealing structures.

The Yellow River floodplain, covering an area of 250000 km² located in northern China, is a densely populated and economically developed region with a high level seismic background activity. In recent history, numerous earthquake-induced liquefactions occurred in this floodplain, however, relevant investigations and research are also lacking. So far, the knowledge of earthquake-induced liquefaction in the Yellow River floodplain is almost blank. In 2023, the Pingyuan M_S5.5 earthquake occurred in Shandong Province and induced soil liquefaction in the Yellow River floodplain. Through post-earthquake investigations, it is the first time that a case of real earthquake-induced liquefaction in the Yellow River floodplain has been analyzed in this paper. The characteristics of liquefaction induced by this earthquake have been obtained. The main points are as follows: this earthquake mainly induced soil liquefaction in silt and silty soils for 91% of sites in the total. The depth range of liquefied soil deposits is mainly of 7~12 m. The largest depth of liquefied soil deposit was confirmed at least 14 m, and possibly at an even deeper one. The depth of liquefied soil is beyond the common range of that in previous earthquakes. Liquefaction sites are mainly observed in the epicentral range of 6~12 km and are located in the seismic intensity 5 region and the boundary of intensity 6 region, which is significantly different from previous investigations. The abnormal distribution of liquefaction sites is closely related to the effect which is induced by the exclusive “highland-slope-lowland” microtopography in the Yellow River floodplain. In a moderate earthquake, the liquefaction in the Yellow River floodplain has induced moderate damage and has an especially different characteristic. It can be concluded that more severe liquefaction hazards may occur in large earthquakes. Therefore, it is urgent to develop a specific analysis procedure and evaluation method for soil liquefaction in the Yellow River floodplain. The work in this paper can improve the understanding of soil liquefaction during small and moderate earthquakes and provide firsthand information and new clues for research on soil liquefaction in the Yellow River floodplain.

The slopes of hydropower projects in southwest China are high and steep and located in high seismic intensity regions. Consequently, there is a risk of earthquake-induced slope instability. Taking the drainage building intake slope of Lawa Hydropower Station at Jinsha River as an example, the dynamic finite element method is used to simulate the slope under rare strong seismic conditions. Based on this, an analysis of the slope dynamic response is conducted. Then, the post-earthquake slope deformation, stress and plastic zone distribution are studied. Finally, the dynamic stability of the slope below the spillway structure is evaluated using the dynamic strength reduction method, which reveals its potential instability mechanism. The results indicate that the acceleration response of the slope shows an elevation amplification effect, a surface amplification effect and a structural surface amplification effect. The slope undergoes the maximum permanent deformation of 10.2 mm after the earthquake, creating new local tensile stress zones and plastic zones. In rare seismic conditions, the safety factor of the potential sliding mass on the slope is 1.80, and its potential failure mechanism is deformation failure with fault JF1 as the rear boundary surface and rock mass at the slope toe shear damaged. This study can provide reference for the dynamic response and stability analysis of complex high and steep rock slopes with favorable-dipping faults under seismic action.

In order to study the mechanical properties of ultra-high performance concrete (UHPC)-reinforced damaged specimens, a total of 10 UHPC-reinforced damaged reinforced concrete beams were designed. These beams were subjected to a four-point bending performance test to study the crack development, damage pattern, load carrying capacity, and displacement of UHPC-reinforced reinforced concrete beams under bending. The effects of different reinforcement thicknesses and different reinforcement methods on the bending performance of UHPC-reinforced damaged RC beams were analyzed. The experimental study shows that the load carrying capacity of UHPC-reinforced damaged RC beams is greatly improved, and the ultimate load is improved by up to 194%.The number of cracks that occur when damage occurs is more than that of the original beams, and the development is more complete; the ductility is greatly improved compared with the original beams, and the displacement ductility coefficients of the reinforced beams are considered to be increased by 49.77%~178.31% compared with that of the original beams. The calculation method and basic assumptions of the ultimate load of the UHPC-reinforced concrete beams are proposed, and the test parameters are substituted into the formula. The results are more consistent with the test values, indicating that the proposed formula can effectively predict the ultimate load of such reinforced beams.

In order to reveal the damage mechanism of ancient timber structures and to make a reasonable evaluation of the damage degree under earthquakes, this paper proposes a multi-scale seismic damage evolution analysis method for the ancient timber structure. Based on the generalized force-deformation relationship and performance-based seismic design, the generalized damage index is established through test data fitting analysis, thus multi-scale consistency damage indices are proposed. According to the multiple point constraint method (MPC), a multiple-scale model is developed for ancient timber structures, which can reveal macro mechanical properties and local failure details. The proposed damage evolution analysis method is validated by a cyclic loading test of mortise-tenon joint and a shaking table test of the multi-story timber framework, and the evolution process and results are also shown. The proposed multi-scale analysis method provides evaluation results of seismic appraisal for existing ancient timber structures, including assessments at the local material, component to overall structural levels. Furthermore, it elucidates the evolutionary relationships between each level of the whole structure, providing a scientific basis for reinforcement and repair work.

Environmental vibration is an important factor that affects the normal operation of various precision instruments and equipment. To solve the problem that the environmental vibration exceeds the vibration limit for the normal operation of equipment, the design of steel spring vibration isolation base is investigated. Through experimental verification and calculation analysis, the relationship between the vertical and horizontal stiffness of steel spring is derived, and the fuzzy problem of horizontal stiffness of steel spring is solved. A novel steel spring floating slab vibration isolation base is designed based on the requirements. The finite element method is utilized to study the vibration reduction and isolation effects of the pedestal under different steel spring stiffnesses. The deformation of the pedestal above the equipment with uneven mass distribution, the deformation of the pedestal with moving parts, and the vibration level of the pedestal are analyzed respectively. The analysis results show that the new steel spring floating slab vibration isolation base exhibits excellent horizontal and vertical vibration isolation effects. When the natural frequency of the base is 4.2 Hz, the maximum vibration reduction efficiency for the environmental vibration above 12 Hz can exceed 90%. The equipment with small moving parts on the base also demonstrates good stability. When the 50 kg moving parts move from one end of the base to the other, the vertical deformation of the base is less than 800 μm. When the disturbance force generated by equipment vibration is less than 1.5 kN, the base can still maintain the vibration level of VC-C. The research results can provide valuable references for the vibration isolation design of equipment in similar industrial plants.

The main task of the building structure array is to record the failure process of civil engineering structures in detail, and to provide structural response information for many related studies such as seismic design, seismic damage assessment, and earthquake safety alarm. However, due to the constraints of economic cost, field testing technology and data processing level, it is unrealistic to deploy sensor monitoring equipment on all floors of the entire structure, so how to obtain the most complete structural information with the least number of sensors is the purpose of optimizing the layout of structural array sensors. Considering the advantages and disadvantages of the effective independent method and the modal kinetic energy method, a unit stiffness energy-driving point retention method was proposed, considering the advantages and disadvantages of the effective independent method and the modal kinetic energy method. In this method, the unit stiffness modal energy is used as the information matrix, and the principle of effective independence method is used to screen the measurement points, so as to ensure that the high-energy measurement points maintain linear independence to the greatest extent. Finally, taking a steel frame as an example, the proposed method, the effective independence method, the modal kinetic energy method and the unit stiffness method are used to lay the sensors on the model respectively, and the modal assurance criterion and the Fisher information matrix criterion are used to evaluate the layout results of the four methods. The results show that, compared with the other three methods, the proposed method has the least number of sensors when the mode vectors are linearly independent, and the proposed method can obtain the most modal information with the same number of sensors.

To study the dynamic response characteristics of submarine sedimentary layer under seismic action, this paper establishes a single-layer unsaturated porous medium seabed model, presents its governing equations, boundary conditions and wave field expressions, and obtains the analytical solutions for the steady-state responses of unsaturated seabeds under different bottom permeability conditions. Through numerical examples, the influences of different bottom permeability conditions, saturation degrees, permeability coefficients, incident wave frequencies and depths on the solid surface displacement amplification factor and pore water pressure are analyzed, and the following main conclusions are drawn: under low-frequency conditions, the influence of the saturation degree of the sedimentary layer on the displacement amplification factor and pore water pressure is relatively weak. While in the case of increasing frequency, the influence of the saturation degree on the displacement amplification factor and pore water pressure is significantly enhanced. Under low-frequency conditions, for the sedimentary layer with a permeable bottom, the displacement amplification factor and pore water pressure are relatively large. While under high-frequency conditions, for the sedimentary layer with an impermeable bottom, the displacement amplification factor and pore water pressure are relatively large.

To clarify the degradation law of the mechanical property of 6061-T4 aluminum alloy after elevated temperatures as recommended by the GB 50429—2007 code for design of aluminium structures, a total of thirty-eight 6061-T4 aluminum alloy specimens were designed, and the unidirectional loading tests and cyclic tensile loading tests at room temperature and elevated temperature were performed. The effects of cooling type and loading protocol on the failure characteristics, initial elastic modulus, strength, stress degradation, and energy dissipation were evaluated. The experimental results show that the yield platform and strain hardening behavior in the unidirectional cyclic tensile stress-strain curve of 6061-T4 aluminum alloy were not observed. When the heating temperature was in the range of 100~300 ℃, the surface of 6061-T4 aluminum alloy specimens became slightly darker but not significantly. The surface condition of the 6061-T4 aluminum alloy can not be used as an indicator to evaluate the damage degree after fire. The temperature had a slight effect on the initial modulus of 6061-T4 aluminum alloy, and the cooling method also slightly affected the mechanical property of 6061-T4 aluminum alloy. When the temperature was below 200 ℃, the temperature did not have a significant effect on the strength of the aluminum alloy. When the temperature was above 200 ℃, the strength of 6061-T4 aluminum alloy exhibited a significant decreasing trend with the increase in temperature. During unidirectional cyclic tensile tests at room temperature, the yield strain of aluminum alloy specimens was slightly lower than that of unidirectional loaded specimens. Due to the plastic accumulation damage during the cyclic tensile process, the ductility of the specimens deteriorated under unidirectional cyclic loading. With the continuous increase in temperature, the yield of the specimens tended to occur earlier, and the deformation capacity changed from worse to better. The energy dissipation capacity of 6061-T4 aluminum alloy gradually decreased with the increase of temperature, and the cumulative energy was positively correlated to aluminum alloy strength and cycle number.

In order to predict the bearing capacity of steel plate-concrete reinforced composite (SPRC) coupling beams more conveniently. In this paper, it is of great significance to study the bearing capacity prediction model of SPRC coupling beams by machine learning (ML) method. Firstly, the SPRC coupling beam database is established by collecting the existing experimental data. On this basis, six ML algorithms, including extreme learning machine (ELM) algorithm, back propagation neural network (BPNN) algorithm, support vector machine (SVM) algorithm, K-nearest neighbor (KNN) algorithm, random forest (RF) algorithm and extreme gradient boosting (XGBoost) algorithm were used for data regression training. Through the comparative analysis of model performance indicators, it is found that the prediction model based on XGBoost algorithm has the best robustness and generalization ability. Compared with the softened strut-and-tie model (SSTM), it has higher calculation accuracy and stability. A high-precision SPRC coupling beam bearing capacity prediction model based on ML method is proposed. In addition, the sensitivity analysis of the parameters affecting the bearing capacity of SPRC coupling beams is also carried out. The results show that the influence degree of each characteristic parameter on the bearing capacity of SPRC coupling beams is in descending order as follows: steel plate ratio (ρ_p), coupling beam section height (h), coupling beam section width (b), span-depth ratio (l_n/h), stirrup yield strength (f_vy), longitudinal reinforcement ratio (ρ_s), longitudinal reinforcement yield strength (f_sy), stirrup ratio (ρ_t), steel plate yield strength (f_py), concrete compressive strength (f_cu).