Gongcheng Kexue Yu Jishu/Advanced Engineering Science

In order to study the vibration characteristics of reinforced concrete (RC) structures during fire and to develop the method of damage assessment after fire, 4 simply-supported RC beams (L1~L4) were designed. Firstly, the finite element model of the simply-supported beams (L1~L4) were calibrated by picking up the modal information before exposure to fire. Then fire tests were conducted on L1~L4 for 60min, 90min, 120min and 150min respectively. During the fires, the structural modal information were collected, and the attenuation formula of frequency was obtained by fitting the test results. In order to study the residual stiffness and bearing capacity of L1~L4 after the fire, the dynamic tests and bearing capacity tests were carried out. Finally, based on the modal information of specimens after the fire, by taking equivalent explosion time as the damage index, the support vector machine intelligent algorithm was used to predict the damage degree of the RC beams. Based on the Standard for Appraisal of Building Structure After Exposure to Fire (CECS252-2009), a comprehensive grading of damage index for simply-supported RC beams exposure to fire was established, and the damage index grading of L1~L4 were evaluated at the end.

The DZ125 alloy has excellent mechanical, fatigue, and high-temperature behaviors. It has been widely used in high-temperature gas turbine components. Although a lot of researches have been carried out on the fatigue problem of the DZ125 alloy, there are still few systematical studies on the fatigue crack initiation mechanism and its failure mechanism during very high cycle fatigue (VHCF) by microscopic fracture analysis. It is found that the scatter of fatigue data increases as fatigue stress decreases, and the fatigue life does not increase significantly even at a low fatigue stress (<220 MPa). The above characteristics are directly related to the change of micro-crack initiation mechanism. Under a high fatigue stress, micro-crack tends to initiate from surface or sub-surface of specimen, and the large secondary crack is the main feature of its fracture morphology. At a low fatigue stress, micro-crack is likely to initiate from interior material defects, and the existence of these defects can seriously affect the fatigue life. Persistent slip bands (PSB) lead to the formation of rough surfaces, which is the main feature at the crack tip. The main crack competes with other secondary cracks and eventually causes fatigue fractures. The results of the Murakami equation show that the average stress intensity factor at micro-crack initiation stage is 3.15 MPa·m1/2, the average stress intensity factor at the onset of unstable crack propagation is 7.7 MPa·m1/2, and the average fracture toughness (KIC) is 15.70 MPa·m1/2.

With the increase of hydropower units and installed capacity in China,the fault problem during the transient process occurs frequently.The signal analysis method is widely used to deal with the problem.The variational mode decomposition (VMD) method,a relatively new self-adaptive method for processing non-linear and non-stationary signals,was adopted to investigate the vibration signals during the transient process of a hydropower unit in the Gezhouba power plant.The first step was to use VMD to analyze the simulated vibration signals of the transient process,and to verify the effectiveness of the VMD method in dealing with non-linear and unstable signals.The Hilbert-Huang transform (HHT) was then used to calculate the instantaneous frequency of each intrinsic mode functions (IMF) decomposed by VMD to obtain the corresponding Hilbert spectrum.These results were compared with those from empirical mode decomposition (EMD) method.From the final vibration signal analysis,the IMF decomposed by VMD could be divided into three parts.The first part was the trend of signal development.The second part was the basic decomposition of the signal with a strong regularity of "spindle" signal reflecting the change law of the transient process.The third part was high-frequency characteristic signals mainly caused by interference noise or unit failure.The corresponding Hilbert spectrum had the characteristics of small fluctuation and stable amplitude.The results showed good consistency between the frequency change of each component and the time-dependent rule of the rotational speed.This suggested that VMD was capable of extracting characteristic frequencies to facilitate the analysis of vibration characteristics.Compared with EMD,the VMD outperformed in self-adaptivity.It provided more accurate and effective analysis results and a better representation of the vibration rule during the transient process of the hydropower unit.