Crack growth in conventional fatigue testing

With the emergence of new technologies combining S-N and crack growth rate data, has come the need for accurate stress intensity values and J-integral solutions specifically applicable to conventional S-N test fatigue specimens. Evaluations of stress intensity values to date have not explicitly considered the geometry and end constraints of these specimens. Further, three-dimensional J-integral solutions for circular geometries do not currently exist. Analysis of flaws in piping and pressure vessel components has historically been based on Linear Elastic Fracture Mechanics principles, and in particular on cracktip stress intensity values. An analogous idea is used in cases of inelastic behavior. Elastic-plastic fracture mechanics approaches have been developed in which the governing parameter is the range in the J-integral, or energy available to drive the crack. The growth of flaws is predicted based on experimental crack growth rate data which is correlated vs. the range in stress intensity or J-integral. A more recent application of crack growth rate data involves ''correcting'' existing S-N failure data obtained in air to account for environmental effects such as reactor water, using crack growth rate data obtained in a reactor water environment. Sample results from preliminary work in this area are depicted. S-N fatigue data from the ASME Code have been ''corrected'' for reactor water environmental effects based on crack growth rate curves given in Section XI of the Code. The results in these figures indicate cycles-to-failure up to an order of magnitude fewer when environmental effects are included. Data presented are based on stress intensity factors, K, and J-integrals which are state-of-the art to date. To improve these estimates, more accurate solutions for K and J are needed for the full range of crack depths in conventional SN fatigue specimens