The Research On The Prediction Of Fatigue Crack Initiation For The Structure Of Oblique Nozzles Which Was Subjected To Internal Pressure

With the increase of the proportion of overseas energy in domestic energy market, the construction of oil and gas pipelines beyond 10000 kilometres on land is increasingly common. And the structure of oblique nozzles is an inevitable structural style of oil and gas pipelines. Though it is a low proportion of overall system of oil and gas pipelines, it is one of the most weak links of overall system of oil and gas pipelines. The weak link usually engender the various lose efficacies and destructions. Its safe operation relates to not only the safety of national energy strategy but also the important responsibility of environmental conservation in related region.Therefore the thesis investigated the prediction of fatigue crack initiation for the ideal structure of oblique nozzles which was subjected to internal pressure. And the thesis analysed the relationship between structure parameters, stress and strain distribution, the prediction of fatigue crack initiation. Meanwhile the thesis provide some suggestions about structural optimization designs in order to improve its fatigue life in the actual environment.The prediction of fatigue crack initiation for the ideal structure of oblique nozzles was investigated by using finite element methods and theoretical arithmetic. And the thesis access to data mainly from some finite element numerical simulations. The main work rests with the following aspects.1. Some typical crack initiation prediction approaches for notched components was studied. And the advantage and disadvantage of each approach was discussed. And according to the existing conditions, the research was chosen a feasible method.2. The relationship between elastic stress concentration factor and structure parameters for the ideal structure of oblique nozzles was studied. The research showed that the stress concentration conditions was increased with the smaller angle of inclination, the larger ratios of nozzle diameter/shell diameter, the larger ratios of shell thickness/nozzle thickness, the larger ratios of shell diameter/shell thickness. The results of the stress concentration conditions was remarkably decreased when the angle of inclination was less than 45 degree.3. The region of crack initiation was studied. Meanwhile the internal intersecting line, the nozzle weldtoe, the shell weldtoe were researched. The research showed that the region of crack initiation was occured at the sharp angle side of the internal intersecting line.4. The prediction of crack initiation for the structure of oblique nozzles was studied by the local notch stress and strain approach. And the effect laws of structural parameter to predict crack initiation was given. The research showed that the result of the crack initiation prediction was decreased with the smaller angle of inclination, the larger ratios of nozzle diameter/shell diameter, the larger ratios of shell thickness/nozzle thickness, the larger ratios of shell diameter/shell thickness. The results of the crack initiation prediction was remarkably decreased when the angle of inclination was less than 45 degree.5. The probabilistic analysis result of crack initiation for the structure of oblique nozzles was studied. And the probabilistic analysis in the limited cycle load range was given.