Abstract: For ultra-supercritical boiler of a super-heater tube with cracks, the stress-strain field at the crack tip is analyzed by numerical simulation under high temperature and high pressure for a long time. According to the theory of plastic zones, the size of the plastic zone at the crack tip is estimated, and a finite element method for calculating,the size of plastic zone is proposed. The research results indicate that the stress and strain, as well as the size of the plastic zone at the crack tip are related not only to the internal pressure but also to the temperature. At a certain temperature, the plastic strain increases with the increase of internal pressure and when the internal pressure is constant, the plastic strain increases with the increase of temperature. Under high temperature and high pressure for a long time, there is some difference between the result of finite element method and that of plastic zone theory, and the reason of the difference is analyzed. The plastic zone at the crack tip is evaluated by using a small range of yield conditions under high temperature and high pressure for a long time, it is shown that the stress intensity factor K is an appropriate fracture parameter for the fatigue crack growth of the T92 superheater tube.

Key words: ultra supercritical boiler, stress and strain, plastic zones, high temperature and high pressure, numerical research