Papers by Author: Yun Xin Wu

Abstract: Because neural network has the advantages of fast parallel processing, associative memory, self-organizing and self-learning, it is widely applied in the fault diagnosis of hydraulic system. Present in this paper is a fault diagnosis approch to a typical failure in hydraulic system which is leakage of hydraulic cylinder.The fault diagnosis approch is based on monitoring preesure singal,time domain feature and neural network. According to the method, the time domain feature is extracted from the pressure singal and costitutes the eigenvectors at first, then these eigenvectors are input into neural network to identify faults. The experimental results show that three modes of no leakage, slighter leakage and severe leakage are correctly identified and it can be used in the fault diagnosia of hydraulic syetem.

Abstract: Different distributed residual stresses were introduced by quenching and two shot-peening treatments on 7075 aluminum alloy. The residual stress distributions and micro-hardness profiles in surface layers were measured. Pre-stress coefficient characterizing contribution of local residual stresses to local yield strength is introduced to analyze residual stress relaxation under cyclic loading. Load testing shows that re-distribution of residual stresses and proportional decrease of the pre-stress coefficient would occur in the non-uniform structural residual stresses introduced by quenching, while great stress relaxation and non-linear decrease of the pre-stress coefficient would occur in the uniform surface residual stresses introduced by shot-peening. Additionally, advantages of surface compressive residual stress and micro yield strength on anti-fatigue property and on restraining initiation and propagation of surface micro cracks should be considered in the usually conservative engineering design.

Abstract: To predict the three-dimensional residual stress distribution of 7075 aluminum alloy thick plates, the quenching and pre-stretching process were simulated by using MARC finite element software. In order to simulate quickly according to different reality conditions, the parametric model was realized by using defining parameters and selecting objects by inputting coordinate value with the box mode. In the quenching process, a non-linear thermo-mechanical direct coupled analysis of the aluminum alloy thick plates was carried out, by taking into account of the temperature dependence of the thermo-physical properties. In the pre-stretching process, the clamp and release of the clamp of pre-stretch machine were simulated by using life and death element technology. The study demonstrates that the theoretical predictions agree well with the experimental values of residual stresses measured by the X-ray diffraction method. The simulation results provide the user a tool for analyzing and studying on quenching and pre-stretching technique using virtual reality method.

Abstract: The model of ultrasonic propagation at a solid-solid contact interface was established. Higher-order harmonic wave generation and waveform distortion take place when the ultrasonic wave propagation at the solid-solid contact interface. Aluminum wire bond experiments were performed on a laboratory test bench. The relation of bond strength, nonlinear coefficient and contact interface pressure was studied. The experiment results show that when contact interface pressure is less than 6 kPa, higher-order harmonic wave component and the nonlinear coefficient decrease and bond strength increases with contact interface pressure increasing, when contact interface pressure is in range of 6 kPa to 10 kPa, higher harmonic wave and the nonlinear coefficient is the least and bond strength is the highest, however, when contact interface pressure is more than 10 kPa, the nonlinear coefficient increases and bond strength decreases with pressure increasing. The nonlinear coefficient of ultrasonic is a method of forecast bond strength.

Abstract: For improving the bonding strength of chip and vibration stability, propagation characteristics of ultrasonic wave across contact interface in wire bonding transducer was investigated in this paper. The rough contact interface is studied. The propagation characteristic is equivalent to the spring-mass model. The spring-mass model of ultrasonic propagation through contact interface was established. The reflection coefficient and transmission coefficient of ultrasonic is related to the stiffness of the contact interface. Aluminum wire bond experiments were performed on a laboratory test bench. The vibration velocities of the capillary and bond strength of chip were obtained at different tightening torque. The results show that the tightening torque in a moderate condition can the highest velocity of capillary and the largest bond strength be reached. Measuring of velocity of capillary can be a method of forecasting the bond strength of chip

Abstract: Concrete pump truck is a kind of mobile construction machinery, with the characteristic of complex structure, poor working condition and difficult to maintenance. Therefore, adopting appropriate health monitoring methods, and accurate grasping the running status information of the pump truck is significant to the pump truck’s safety use and pre-judgment maintenance plan arrangement. In this paper, the traditional structural health monitoring methods was studied. And the structure, load and work environment characteristics of concrete pump truck were analyzed. Taking into account the economy and reliability of the structural health monitoring system, the technical route of the concrete pump truck structural health monitoring system and health evaluation criteria were proposed. The evaluation criteria takes into account both the cumulative health effect and timely health status of concrete pump truck structure.

Abstract: Inner leakage of hydraulic cylinder is a very serious failure in the hydraulic system and it can lead to many problems.A important fault diagnosis way is to detect the pressure signal.But the pressure signal is seriously influenced by pressure fluctuation and other noises.It is difficult to extract features from pressure singal. Aiming at the difficulty in extracting feature from pressure singal in fault diagnosis for leakage of hydraulic cylinder,a fault diagnosis approch based on monitoring preesure singal and wavelet energy is proposed in this article.According to the method, the enegry of different frequency bands after wavelet decomposition costitutes the eigenvectors at first, then these eigenvectors are input into BP network to identify faults. The experimental results show that three modes of no leakage, slighter leakage and severe leakage were correctly identified.This approach can be used in the leakage fault diagnosis of hydraulic cylinder.

Abstract: Using Finite Element Method, the changing rules of residual stresses in the test specimen cut out from a quenched plate were analyzed, and suitable dimensions of the specimen for crack compliance method test was recommended. The results show that cutting has great influences on residual stresses in the specimen edge areas to a range of about one thickness, while it has little influences on stresses at the center part of the specimen; cutting mainly influences stresses normal to the cutting plane, while it has little effect on stresses parallel to the cutting plane; specimen with length bigger than 2.67 times the thickness and width bigger than 2 times the thickness is suitable for crack compliance method test, under these conditions peak stresses losing in crack compliance method results is less than 5% compared with surface stress X-ray diffraction results.

Abstract: A mathematical model has been developed to predict the residual stresses level in pre-stretched aluminum alloy plate. This is based on force balances of the residual stress, theory of plastoelasticity and a new conception of free length. The model is relatively simple because only rolling direction residual stress is taken into account, but provides a clear illustration of stress relief mechanism in stretching process. With this model, residual stress distributions of stretched beam can be determined directly by knowing the specimen dimensions, material properties and the original stress. The model offers an useful tool to show the effect of varying tension ratio on the final residual stress level, thus makes it possible to predict stress relief and control residual stresses. An example of using the model is presented by applying published data while showing mechanism of stress relief during stretching. Analysis indicates that it is stretch-caused convergence of the free lengths of strips in beam that lead to reduction in the residual stresses.

Abstract: The cut, a main preparing means of specimen, will cause a difference in stress distribution between thick plate and the specimen, so this article focuses on the topic of evaluating impact of the cut on stress measurement results. Firstly, the real surface stress distribution of the plate and specimen were determined by X-ray diffraction (XRD) and FEM. Secondly, according to the specimen dimension and machining procedure, the influence of the cut on internal stress measurement is obtained and compared with the FEM results. The conclusions are as follows: (1) the normal stress of cut plane steeply decreases and the stress gradient descends along with distance away from the cut. The range of distance is approximately one-thickness of specimen. (2) In order to obtain stable and reliable results, the plane size of specimen is taken three times larger than the thickness. (3) The milling processing of layer removal method (LRM) can change the surface stress state of specimen, but the machining effects on residual stresses can be removed by properly choosing milling parameters.