Because of the inhomogeneity, deterioration and defects of the welded metal materials, there are three basic assumptions in materials science; The continuity hypothesis, uniformity hypothesis and isotropy hypothesis are not satisfied, which requires the application of fracture mechanics theory. The task of fracture mechanics is to use linear elastic fracture mechanics and elastic-plastic fracture mechanics to solve the crack problem of components based on the fact that there are macro microcracks in the components. Namely, the crack size, working stress and the ability of materials to resist cracks (that is, the crack tip opening displacement CTOD fracture toughness value) in the component are quantitatively linked, and the safety and life test analysis and evaluation are carried out for the components containing microcracks and joints with poor quality tissue.

Of course, the difference between control valve and ordinary manual valve is that it cannot be regarded as an isolated element, but must be considered as a part of the whole automatic control system. There are also many problems in the use of control valves, which may not be the problem of model selection and configuration. Because the user's understanding of the control valve is obviously insufficient, the control valve cannot be debugged and coordinated with the control system. In the process of use, we need to recognize the key point of the problem. Only in this way can we make the right choice for the valve. At the same time, in the system commissioning phase, if you want to debug the control valve well, it will certainly help you to greatly reduce the failure rate and make the automatic control system operate in a long-term stability.