Impulse current under short-circuit condition is the key factor for the structural reliability of the testing transformer. Therefore, the structural reliability analysis model of transformers was proposed based on the probability density evolution theory. Firstly, the basic principle of probability density evolution theory was introduced, and the analysis method of electromagnetic force field for windings was given considering the coupling effects of magnetic and electric fields. On the basis，the numerical analysis model of transformers structure was constructed by using the Abaqus software finite element analysis method. Taking an AGF 20 kV testing transformer as an example, the above model was validated. The longitudinal Mises stress of windings was chosen as the control variable, and the stress distribution and probability density evolution characteristics were given. Then, the structural reliability index was calculated, and the extreme mechanical response and the corresponding distribution zones of windings and iron cores were discussed. The results show that the stress of windings increases significantly under the action of short-circuit impulse current. The maximum value of windings’stress reaches 82%of the threshold, and plays the key role in structural reliability, the increase of impedance has obvious influence on the reliability of transformers.

In order to ensure the safe and stable operation of gas turbine units, and grasp the heat transfer mechanism of the ventilation system of the mainframe housing, a study on the ventilation and heat dissipation of the main engine box of gas turbine units was carried out. Based on the principle of field synergy, the internal flow field, temperature field and heat dissipation performance of key equipments in the mainframe cabinet were analysed, and an optimized scheme for adding a flow-guiding device was proposed. The results show that the volume of the high-temperature area inside the mainframe housing is 7.6%, mainly concentrated near the gas turbine shaft component, which is the main heat source inside the mainframe housing, with an average external surface temperature of 86.94 ℃. After the installation of a 50° flow-guiding device, the volume of the high-temperature area inside the mainframe housing is reduced to 5.1%, and the average external surface temperature of the gas turbine shaft is reduced to 81.98 ℃. The heat dissipation effect is significantly improved.

In order to improve the load-bearing efficiency of the stiffened conical shell in large launch vehicle, the lightweight design of the stiffened conical shell was carried out via a data-driven multi-fidelity approximate modeling optimization method. Aiming at the problems such as low efficiency and insufficient accuracy of the single fidelity approximate modeling optimization method, a data-driven multi-fidelity approximate modeling optimization framework was built based on variable-fidelity expected improvement (VF-EI) point criterion，and accordingly the optimization design of stiffened conical shell structure was carried out. Based on the finite element models of stiffened conical shells with different mesh sizes, a Co-Kriging multi-fidelity approximate model for the collapse load of stiffened conical shells was established. In the optimization iteration, multi-fidelity sampling points were generated by using VF-EI point criterion, and the global and local approximation accuracy of Co-Kriging multi-fidelity approximation model was improved sequently. Moreover, the optimization efficiency and accuracy of the proposed method were demonstrated by comparing with radial basis function approximation model and Kriging model. Besides, 11. 5% weight reduction of the optimized stiffened conical shell structure is obtained compared with the initial design, which has certain engineering application value.

Dichloromethane solution was introduced as the corrosion medium by the liquid drop method.Based on the regular quadrilateral, regular hexagon and concave structures with different porosities，the effects of different corrosion cycles and areas on the porous structural mechanical properties of polylactic acid (PLA) were analyzed by comparing with the finite element simulation results.The results show that concave porous structure plate is the least affected by the corrosion cycle,while the other two structures are affected by it depending on the size of the porosity.The hexagonal structure with a porosity of 72% has the weakest corrosion defect bearing capacity, while the concave structure has the strongest bearing capacity.The regular quadrilateral structure with a porosity of 54% is most affected by the single cell corrosion.The corrosion bearing capacity of the expanded area of a single cell in a regular hexagonal structure is weaker than that of a single cell, while the corrosion bearing capacity of the expanded area of a single cell in a regular quadrilateral structure is stronger than that of a single cell.

High-speed machining technology by improving the cutting speed and feed rate to improve the material cutting rate, machining accuracy and machining quality, is one of the main ways of modern processing. Therefore, to ensure the stability of high-speed machining is the basis of application of high-speed machining. Firstly, based on the traditional stability analysis, the influence of feed rate on the static cutting thickness was further considered, the stability model related to feed rate and radial cutting depth ratio of tool was established, and the stability of high-speed milling was analyzed by combining stability variance ratio. Secondly, based on the single factor variable feed rate test, a filter was designed to filter the frequency component of the spindle speed. The variance ratio between the filtered signal sequence and the original signal sequence was used to analyze the milling stability changes of the continuous variable axial depth test, and the validity of the analysis method considering the feed rate to affect the stability of high-speed milling was verified. The results show that the proposed method can determine milling stability more accurately for high-speed machining with small radial cutting depth. And the axial cutting depth of unstable cutting limit changes slightly with the increase of feed rate, and the feed rate will aggravate the instability of milling system.

In order to eliminate the restriction of the return spring stiffness on the power generation and system reliability of the energy harvesting speed bump with the full wave mechanical rectifier, a dual-speed bump energy harvester (DBEH) based on the half-wave mechanical rectifier was proposed. The device utilized a mechanical transmission module to convert the downward linear movement of two speed bumps, driven by wheels, into the one-way rotary movement of a single generator shaft, thereby converting mechanical energy into electrical energy. The unidirectional motion conversion reduced the demand for the load of the return spring. The theoretical model of the wheel excitation and speed bump dynamics was established and verified by road tests. Based on the theoretical model, the power generation performance of DBEH was studied by numerical simulation. The research results indicate that the root mean square (RMS) power and output electrical energy of the system increase with the increase of external excitation amplitude and the decrease of the spring stiffness. When the load resistance value is 7-9 Ω, the output performance of the system is optimal. Compared to reducing the gear radius, increasing the speed of the gearbox has a more significant impact on the improvement of system output. A small inertia flywheel can effectively improve the system’s capture of the mechanical energy and the conversion rate from the mechanical energy to the electrical energy.

Taking multicellular square tubes as the research object, the influence of the evolution of cross section shape on the energy absorption of the structure of multicellular square tubes was studied by using the verified finite element model. The results show that, under the same mass conditions, the multicellular tubes with the best energy absorption k=0.25 increased by 26.34% compared with those with the worst energy absorption k=0.40. Under the same wall thickness, the energy absorption with k=0.25 and the specific energy absorption of the multicellular tubes were 311.69% and 73.80% higher respectively than that of the ones with k=0, and the crushing force efficiency was increased by 52.51%. Finally, the parametric study of shape coefficient and wall thickness on the structural crashworthiness was carried out systematically. The research results can provide a reference for innovative design of multicellular square tubes.

To explore the impact of key parameters of brake pads on the dynamics characteristics of the braking system,the test research based on a friction testing machine with a slider-disc structure was conducted.The test research of the impact of key parameters of brake pads (rotation speed, pressure, mass, braking radius, etc.) and the braking environment (dry friction, wetness, sand) on the stability of the braking system was carried out. At the same time, a dynamics model of the brake pad braking system was established，compared with test results using the mathematical tool of autocorrelation coefficient. The results show that the key parameters of brake pads have a significant impact on the dynamics characteristics of the braking system; under different conditions of brake pad mass, tribological parameters, brake pad radius, and braking environment; the chaotic characteristics of the braking friction force signal show a trend of expansion or contraction; the chaotic vibration can be suppressed by adjusting the key parameters of brake pads. This study can provide reference for optimizing the braking strategy and reducing noise and vibration in brake pad systems.

In order to construct a bone scaffold structure with good biological properties, P-type, FRD-type homogeneous multi-porous scaffolds and P&FRD gradient multi-porous scaffolds with different porosities and unit cell scales were designed based on the triply periodic minimal surface（TPMS）.The effects of unit cell types, porosities and unit cell scales on the mechanical and biological properties of TPMS multi-porous scaffolds were investigated by tests and finite element simulation.The results show that the internal pores of homogeneous and gradient multi-porous scaffolds based on TPMS have good connectivity.The mechanical properties of multi-porous scaffolds decrease with the increase of porosity, but the permeability increases with the increase of porosity.Increasing the unit cell scale can significantly improve the permeability of multi-porous scaffolds.The permeability of the gradient multi-porous scaffold is also affected by the seepage direction.The gradient multi-porous scaffolds can synthesize the performance characteristics of each homogeneous unit cell structure, and show different mechanical and biological properties in different regions of the scaffold, which is closer to the structure and biological properties of human bone.

To solve the problems of high-power downstream belt conveyors in large inclination angles, high belt speed conditions which are very prone to flying cars and belt breakage, a kind of disk-type adjustable permanent magnet damping roller device was put forward by adjusting the size of the area of engagement between the permanent magnet and the coil to realize the braking adjustment. The Maxwell software was used to study the transient magnetic density distribution law of the disk-type permanent magnet damping device under stable conditions and the changing law of damping torque by different air gap thicknesses, and the test bench was built for test verification. The results show that with the increase of the air gap thickness, the magnetic density gradually increases, up to 2.1 T. The damping moment increase when air gap thickness is from 1 mm to 2 mm, the damping moment decrease when from 2 mm to 3.5 mm. The research can provide data support for improving and optimizing high-power downstream belt conveyors.