科创中国

Influence of geometric and cutting parameters of cemented carbide cutting tool on reliability of cutting tool has become more and more mature,yet influence of its physical and material parameters on reliability is still blank.In view of this,cutting test and fatigue crack growth test of YT05 cemented carbide cutting tool are conducted to measure such data as the original crack size,growth size,times of impact loading,number and time of cutting tool in failure,and stress distribution of cutting tool is also obtained by simulating cutting process of tools.Mathematical models on dynamic reliability and dynamic reliability sensitivity of cutting tool are derived respectively by taking machining time and times of impact loading into account,thus change rules of dynamic reliability sensitivity to physical and material parameters can be obtained.Theoretical and experimental results show that sensitive degree on each parameter of tools increases gradually with the increase of machining time and times of impact loading,especially for parameters such as fracture toughness,shape parameter,and cutting stress.This proposed model solves such problems as how to determine the most sensitive parameter and influence degree of physical parameters and material parameters to reliability,which is sensitivity,and can provide theoretical foundation for improving reliability of cutting tool system.

Current research on the operational reliability of centrifugal pumps has mainly focused on hydrodynamic instability. However, the interaction between the fluid and structure has not been sufficiently considered; this interaction can cause vibration and dynamic stress, which can affect the reliability. In this study, the dynamic stresses in a single-blade centrifugal pump impeller are analysed under different operating conditions; the two-way coupling method is used to calculate the fluid–structure interaction. Three-dimensional unsteady Reynolds-averaged Navier-Stokes equations are solved with the SST k–ω turbulence model for the fluid in the whole flow passage, while transient structure dynamic analysis is used with the finite element method for the structure side. The dynamic stresses in the rotor system are computed according to the fourth strength theory. The stress results show that the highest stress is near the loose bearing and that the equivalent stress increases with the flow rate because the dynamic stresses are closely related to the pressure load. The stress distributions on the blade pressure side, suction side, leading edge, and trailing edge are each analysed for different flow rates; the highest stress distribution is found on the pressure side. On the blade pressure side, a relatively large stress is found near the trailing edge and hub side. Based on these results, a stress distribution prediction method is proposed for centrifugal pumps, which considers the interaction between the fluid and structure. The method can be used to check the dynamic stress at different flow rates when optimising the pump design to increase the pump reliability.

It is a common phenomenon that the cracks originating from a hole can cause structural damage in engineering.However,the fracture mechanics studies of hole edge crack problems are not sufficient.The problem of an elliptical hole with two collinear edge cracks of unequal length in an infinite plate under uniform tension at infinity is investigated.Based on the complex variable method,the analytical solutions of stress functions and stress intensity factors are provided.The stress distribution along the axes and the edge of the elliptical hole is given graphically.The numerical results show that there is obvious stress concentration near the hole and cracks,and the stresses tend to applied loads at distances far from the defect,which conform to Saint-Venant's principle.Hence,the stress functions are proved to be right.Under special conditions,the present configuration becomes the Griffith crack,two symmetrical cracks emanating from an elliptical hole,two cracks of unequal length emanating from a circular hole,a crack at the edge of a circular hole,or a crack emanating from an elliptical hole.Compared with available results,stress intensity factors for these special shapes of ellipses and cracks show good coincidence.The stress intensity factor for two cracks of unequal length at the edge of an elliptical hole increases with the crack length and the major-to-minor axis ratio of the elliptical hole.The stress distribution in an infinite plate containing an elliptic hole with unsymmetrical cracks is given for the first time.

Parallel kinematic machines have drawn considerable attention and have been widely used in some special fields.However,high precision is still one of the challenges when they are used for advanced machine tools.One of the main reasons is that the kinematic chains of parallel kinematic machines are composed of elongated links that can easily suffer deformations,especially at high speeds and under heavy loads.A 3-RRR parallel kinematic machine is taken as a study object for investigating its accuracy with the consideration of the deformations of its links during the motion process.Based on the dynamic model constructed by the Newton-Euler method,all the inertia loads and constraint forces of the links are computed and their deformations are derived.Then the kinematic errors of the machine are derived with the consideration of the deformations of the links.Through further derivation,the accuracy of the machine is given in a simple explicit expression,which will be helpful to increase the calculating speed.The accuracy of this machine when following a selected circle path is simulated.The influences of magnitude of the maximum acceleration and external loads on the running accuracy of the machine are investigated.The results show that the external loads will deteriorate the accuracy of the machine tremendously when their direction coincides with the direction of the worst stiffness of the machine.The proposed method provides a solution for predicting the running accuracy of the parallel kinematic machines and can also be used in their design optimization as well as selection of suitable running parameters.

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue(VHCF)behavior,which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects.Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types,size,shape,location,and residual stress influences.Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects,and higher strength materials are more sensitive to soft inclusions(elastic modulus lower than the matrix).The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions(elastic modulus higher than the matrix).It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter.There exists an inclusion size independent region for crack incubation,outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one.As for elliptical inclusions,reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation,whereas the long-axis length controls the site of peak stress concentration.The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions.Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime.The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime,and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

As a redundant drive mechanism,twin ball screw feed system has the advantage of high stiffness and little yaw vibration in the feeding process,while leads to increased difficulty with vibration characteristics analysis and structure optimization.Only low-dimensional structure and dynamics parameters are considered in the existing research,the complete and effective model for predicting the table's vibrations is lacked.A three-dimensional(3D)mechanical model of twin ball screw driving table is proposed.In order to predict the vibration modes of the table quantitatively,an analytical formulation following a comprehensive approach is developed,where the drive system is modeled as a lumped mass-spring system,and the Lagrangian method is used to obtain the table's independent and coupled axial,yaw,and pitch vibration modes.The frequency variation of each mode is studied for different heights of the center of gravity,nut positions and table masses by numerical simulations.Modal experiment is carried out on the Z-axis feed table of the horizontal machining center MCH63.The results show that for each mode,the error between the estimated and the measured frequencies is less than 13%.The independent and coupled vibration modes are in accordance with the experimental results,respectively.The proposed work can serve a better understanding of the table's dynamics and be beneficial for optimizing the structure parameters of twin ball screw drive system in the design stage.

Published studies in regard to coupler systems have been mainly focused on the manufacturing process or coupler strength issues. With the ever increasing of tonnage and length of heavy haul trains, lateral in-train forces generated by longitudinal in-train forces and coupler rotations have become a more and more significant safety issue for heavy haul train operations. Derailments caused by excessive lateral in-train forces are frequently reported. This article studies two typical coupler systems used on heavy haul locomotives. Their structures and stabilizing mechanism are analyzed before the corresponding models are developed. Coupler systems models are featured by two distinct stabilizing mechanism models and draft gear models with hysteresis considered. A model set which consists of four locomotives and three coupler systems is developed to study the rotational behavior of different coupler systems and their implications for locomotive dynamics. Simulated results indicate that when the locomotives are equipped with the type B coupler system, locomotives can meet the dynamics standard on tangent tracks; while the dynamics performance on curved tracks is very poor. The maximum longitudinal in-train force for locomotives equipped with the type B coupler system is 2000 kN. Simulations revealed a distinct trend for the type A coupler system. Locomotive dynamics are poorer for the type A case when locomotives are running on tangent tracks, while the dynamics are better for the type A case when locomotives are running on curved tracks. Theoretical studies and simulations carried out in this article suggest that a combination of the two types of stabilizing mechanism can result in a good design which can significantly decrease the relevant derailments.

The methods of improving the dynamic performance of high speed on/off solenoid valve include increasing the magnetic force of armature and the slew rate of coil current, decreasing the mass and stroke of moving parts. The increase of magnetic force usually leads to the decrease of current slew rate, which could increase the delay time of the dynamic response of solenoid valve. Using a high voltage to drive coil can solve this contradiction, but a high driving voltage can also lead to more cost and a decrease of safety and reliability. In this paper, a new scheme of parallel coils is investigated, in which the single coil of solenoid is replaced by parallel coils with same ampere turns. Based on the mathematic model of high speed solenoid valve, the theoretical formula for the delay time of solenoid valve is deduced. Both the theoretical analysis and the dynamic simulation show that the effect of dividing a single coil into N parallel sub-coils is close to that of driving the single coil with N times of the original driving voltage as far as the delay time of solenoid valve is concerned. A specific test bench is designed to measure the dynamic performance of high speed on/off solenoid valve. The experimental results also prove that both the delay time and switching time of the solenoid valves can be decreased greatly by adopting the parallel coil scheme. This research presents a simple and practical method to improve the dynamic performance of high speed on/off solenoid valve.

Quadruped robots consume a lot of energy,which is one of the factors restricting their application.Energy efficiency is one of the key evaluating indicators for walking robots.The relationship between energy and elastic elements of walking robots have been studied,but different walking gait patterns and contact status have important influences on locomotion energy efficiency,and the energy efficiency considering the foot-end trajectory has not been reported.Therefore,the energy consumption and energy efficiency of quadruped robot with trot gait and combined cycloid foot trajectory are studied.The forward and inverse kinematics of quadruped robot is derived.The combined cycloid function is proposed to generate horizontal and vertical foot trajectory respectively,which can ensure the acceleration curve of the foot-end smoother and more successive,and reduce the contact force between feet and environment.Because of the variable topology mechanism characteristic of quadruped robot,the leg state is divided into three different phases which are swing phase,transition phase and stance phase during one trot gait cycle.The non-continuous variable constraint between feet and environment of quadruped robot is studied.The dynamic model of quadruped robot is derived considering the variable topology mechanism characteristic,the periodic contact and elastic elements of the robot.The total energy consumption of walking robot during one gait cycle is analyzed based on the dynamic model.The specific resistance is used to evaluate energy efficiency of quadruped robot.The calculation results show the relationships between specific resistance and gait parameters,which can be used to determine the reasonable gait parameters.

The current design of hydro-viscous clutch(HVC)in tracked vehicle fan transmission mainly focuses on high-speed and high power.However,the fluid torque under the influence of fluid temperature can not be predicted accurately by conventional mathematical model or experimental research.In order to validate the fluid torque of HVC by taking the viscosity-temperature characteristic of fluid into account,the test rig is designed.The outlet oil temperature is measured and fitted with different rotation speed,oil film thickness,oil flow rate,and inlet oil temperature.Meanwhile,the film torque can be obtained.Based on Navier-Stokes equations and the continuity equation,the mathematical model of fluid torque is proposed in cylindrical coordinate.Iterative method is employed to solve the equations.The radial and tangential speed distribution,radial pressure distribution and theoretical flow rate are determined and analyzed.The models of equivalent radius and fluid torque of friction pairs are introduced.The experimental and theoretical results indicate that tangential speed distribution is mainly determined by the relative rotating speed between the friction plate and the separator disc.However,the radial speed distribution and pressure distribution are dominated by pressure difference at the lower rotating speed.The oil film fills the clearance and the film torque increases with increasing rotating speed.However,when the speed reaches a certain value,the centrifugal force will play an important role on the fluid distribution.The pressure is negative at the outer radius when inlet flow rate is less than theoretical flow,so the film starts to shrink which decreases the film torque sharply.The theoretical fluid torque has good agreement with the experimental data.This research proposes a new fluid torque mathematical model which may predict the film torque under the influence of temperature more accurately.