| With the development of national industry and economy, the pressure vessels and pipes are widely used in the field of energy sources, transport and petrochemistry , playing an important role in industry production. It makes great meaning to assess their safeties. Nowadays the main assessment method, considering steady fluid leak before fracture, is based on leak-before-break. The dissertation aims to make research on crack opening area to support the"LBB"assessment.In the dissertation, the Engineering stress-strain curves and the the True stress-strain curves are given by mechanical test, and the COD values of weld metal and HAZ are obtained according to British Standard, BS7448. And the Paris formula of base metal, weld metal and HAZ are also given by fatigue experiment. So does the threshold stress intensity factor range of welded joint.For the structure of pipe, fatigue growth process of surface crack under cycle load is researched using the software NASGRO. We can gain the rule of growth, through wall and shape development of surface crack with different initial geometry under pure tension and bending loads. In addition, the method of ascertaining surface crack size and the computation of conservative critical crack size are also provided.Line elastic COA formula, which bases on the computation of line elastic fracture parameter and stress intensity factor, is deduced according to SINTAP. And its output is also compared with other known methods. Considering the effects that the plasticity at crack tip is to increase the opening of the crack and then the opening areas are greater than those in liner elastic case, available analytic methods are investigated in the dissertation.
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