Calculation of critical crack length in a pressure vessel for mass diffusion coupled with stress

In pressure vessels containing hydrogen, the role of hydrogen will be important because of hydrogen cracking problem. It is difficult to predict what is happened in metallurgical field in spite of a lot of studies have been searched. Embrittlement of hydrogen influences on mechanical properties and degrades it, also this embrittlement causes to enhance crack growth and then fast fracture is happened. The main role incontrolling the mass diffusion as driving force is related to stress. In this study, a finite element analysis is implemented to estimate material’s behavior associated with hydrogen embrittlement. For this purpose, one model of pressure vessel is introduced that it has definite boundary and initial conditions. In fact, the finite element method is employed to solve the sequentially coupled mass diffusion problem under the action of stress near a blunting crack tip in X-750 alloy pressure vessel for different crack lengths. Moreover, the stress intensity factor distribution for semicircular crack under the action of tension is extracted assuming linear elastic fracture mechanics (LEFM) is considerations. On the other hand, embrittlement of the material leads to fracture toughness decrease. This embrittlement is due to the effect of diffusion of the hydrogen in the metal. In other words, the fracture toughness of the metal decreases with the increase of the concentration of hydrogen through the metallic bands. The critical crack length corresponding to the decreased fracture toughness has then been computed. It was observed that the maximum accumulation of hydrogen occurs near the front due to the effect of applied stresses.

Keywords:

Fracture Toughness , Embrittlement , Mass Diffusion , FEM , Pressure Vessel