FATIGUE STRENGTH CURVES OF THICK WALLED TUBES UNDER CONSIDERATION OF AUTOFRETTAGE

摘要

It is concluded, that life time of high pressure components is significantly improved by increasing autofrettage depth. Fatigue strength and life time of pressurized thick walled cylinders is strongly influenced by the tangential stress. Fig. shows, that due to autofrettage, the tangential stress on inner surface is changed to compressive stress. The increased life time can be explained by the reduction of tangential stress at inner diameter. But it has to be also considered, that the stresses in wall thickness and on outside diameter are increased due to autofrettage and limited by reverse yielding. Depending on autofrettage pressure cracks might be generated at both, inner and outer surface. Especially on heat exchangers, stress risers and stress crack corrosion may also occur on outside surface. Therefore too high stresses on outside diameter should be avoided in order to avoid cracks. Nevertheless for other high pressure parts, which have fatigue or life time problems due to pulsating pressure, the life time can be increased by considering a higher autofrettage depth, limited by the indicated criteria. Basically the material strength is the main parameter for design and calculation. Generally the test results indicate higher life time at heat treatment to higher strength. In practice a smaller wall thickness is selected if high strength material is used. In this case the advantage of longer life time is eliminated, but of course the material costs are lower. For further investigation and fatigue testing of thick walled autofrettaged cylinder, a general standardization of test specimen and testing parameter is strongly requested, in order to allow the comparison of different materials and treatments. In this paper the trend of increasing life time is shown and the investigation can be an impulse for further tests to demonstrate optimum material selection and treatment for different applications.