Papers by Author: Gui Jie Liu

Abstract: The stress concentration factors (SCFs) is used in the fatigue design for calculating hot-spot stress. However a major issue can be noted that the majority of research results are focused on the SCF distribution of uni-planar tubular joints subjected to the single basic load. By aiming to find the solution of this problem, the distribution of SCFs at the weld toe of a multi-planar tubular DX-joint which is subjected to the two set of the balanced combined loading components at the end of in-plane braces is studied by the finite element method. Thus it is concluded that for the axial plus in-plane bending load case, hot-spot stress location varies between saddle and crown position; while the location is invariably at the saddle position under combined axial plus out-of-plane bending loads. At last the API RP2A equation for predicting hot-spot stress is used for comparison with the finite element analysis results. Meanwhile the distribution of SCFs is also provided, that information indicates the-hot spot location along the weld toe affects the crack initiation.

Abstract: In recent decades jacket platforms are widely used for facilitating the offshore oil industries in order to reach their demands of producing oils. In this regard given analyses shows the influence of seasonally based environmental loads acting on the offshore platform structure situated at Bohai Bay northeast of China. This paper studies and summarizes the structure reliability theory with jacket platform as the research object; it is the first time to aim and finds the most dangerous season effecting Bohai Bay offshore platforms by considering seasonal environmental loads. Finite element analysis ANSYS software is used for model analyses and optimisation of the structure spatial correlation seasonal loads. Environmental design data is extracted according to its geographical conditions in the form of wave, wind and current forces. Since most of the control management nodes work below the surface of the water so the distribution of stress and fatigue life of the pipe node under multi-axial loading is in different forms. The analysis mainly focuses on the stress and fatigue life appropriate characteristics to the actual situation of multi-axial loading case of the K-joint welding location and mainly environmental effect on the platform.

Abstract: In the capricious marine environment, the offshore platform structure is exposed to the multiaxial fatigue loading in which damage would be formed in different directions and planes. Evolution of the structural damage physical mechanism caused by marine environment load is more complicated. Based on the analysis of a large number of literatures, this paper reviews the research status of the fatigue crack mechanism in China and abroad, and predicts the development direction in the future.

Abstract: Ocean environment loads such as wave, current and wind load shows random distribution. The coupling of different kinds of load can easily lead to the fatigue damage of offshore platform structure. The damage presented the multiaxial characteristics would be formed in different directions and planes. K-joint is the key but weak part of the platform structure. This paper associated macro ocean environment factors with material interior micro structure change to reveal the microscopic mechanism of fatigue damage evolution and fatigue damage evolution regularity. By building finite element model of the offshore platform K-joint and physical model, an experiment is done under the function of wave and current load and makes a good result. In this way, the relationship between them and fatigue life was established, and the result agreed with the theoretical predictions.

Abstract: To identify the fatigue crack propagation of tubular joints in the offshore platform, we applied the wavelet coefficient energy theory to monitor and analyze the acoustic emission(AE) signal of marine structures with typical fatigue crack propagation. Based on typical tubular joints model fracture experiments of the offshore platforms, we successfully extracted the characteristics of AE signals during crack propagation. Further analysis results demonstrated that the elastic wave generated by the crack extension was superimposed by a variety of high frequency signals, and it changed with the evaluation of crack. The variation of wavelet coefficient energy and the change of crack propagation state showed a strong correlation. Thus the characteristics of AE signals can be used to identify different stages of crack propagation accurately, intuitively, and clearly, so as to label the development process of crack.

Abstract: In offshore jacket platforms, the structural steels are widely used and usually welded connected. This paper presents the welding of 45# steel plate by LER-3 YAG Pulse Laser. Experiments are carried out to investigate the performance of the welded joint under different process parameters and working conditions. In details, microstructure and macro-morphology in the laser-affected area are analyzed and the micro hardness of the specimens is tested. The results show that: (a) the performance of the welded joint can be improved by the optimization of process parameters, (b) the welding area has the closest grained, the heat affected zone is isometric crystal, (c) the martensite can be reduced by increasing the heat, (d) the grain can be greatly refined by properly increasing the scan speed and decreasing the laser power supply, and that (e) the mean value of the hardness (420HV) in the welded area is obviously higher than that of the substrate (205.12HV) and the highest hardness (492.32HV) appears in the heat affected area.

Abstract: To improve the comprehensive properties of titanium alloy, an experiment of laser-clad (Ti+Cr3C2+Al) has been carried out, using pulsed YAG laser as the radiated source. By optimizing the process parameters, we get uniform, continuous and flawless cladding layer with the composition changes gradient. It is found that cladding layer and the substrate show excellent metallurgical bonding. The microstructure transforms from coarse columnar grains to fine tree-like grains from substrate to top, and a small amount of TiAl and TiC ceramic hardening particles exists. The hardness of the cladding zone increases obviously, the maximum is 1650HV, 2-3 times of the substrate. A white narrow metallurgical bonding zone is formatted at interface between the cladding layer and the substrate.

Abstract: Engineering ceramics are hard and brittle materials, that is very easy to crack when grinding, and processing mechanism is rather complex than that of metallic materials. One kind of AE signal based grinding contact detection and feeding control method for ceramic material processing is presented in this paper based on ceramic grinding mechanism study. Through a large number of experimental data analyzing, the relationship between grinding touch and the feature of AE signals is obtained, and the automatic and intelligent grinding process can be realized by grinding contact signal driving grinding numeral program. Application results indicate this method can prove grinding efficiency and process quality.

Abstract: A processing method based on wavelet transform for the monitoring signals of grinding wheel dull is presented. The noise-falling method based on wavelet transform is used to process AE signals, grinding force signals and the electric current signals of main axis motor produced in grinding process, and the processing results can be used to identify grinding wheel state. Test result indicates that the grinding wheel state can be identified exactly by these three kinds of signal characteristics, and have higher identified precision.

Abstract: The grinding quality assessing is a complex decision-making process, which must optimize and balance multi-influence factors. A grinding quality assessing method based on fuzzy synthetic evaluation theory combined with analytic hierarchy process (AHP) is presented in this paper, and used to quality assessing for grinding process. The result of analyzing example indicates that this method can be used to estimate grinding quality based on the monitoring result of grinding process and grinding condition, and to assist grinding worker to select optimum grinding parameters for the steady grinding quality.