Fatigue growth of a surface crack in a welded T-joint

Structural component failures due to fatigue loading often occur in welded T-joints, the geometry of which gives raise to a local stress increase that makes them preferential sites for crack initiation and propagation. In the present paper, the fatigue behaviour of a T-jointed blade in a hydraulic turbine runner is analysed both experimentally and numerically. For relatively small cracks, it is shown that the structural component geometry does not remarkably influence the stress-intensity factor (SIF) values, provided that the stress field in the vicinity of the crack is approximately the same. Therefore, in order to simplify the problem, a cracked finite-thickness plate is examined instead of the actual T-jointed blade. The SIFs along the front of a semi-elliptical surface flaw in such a plate are determined through a finite element analysis for different elementary loading conditions that can be employed to model the actual stress field at the expected crack location in the examined T-joint. Then, by applying the superposition principle and the power series expansion of the actual stress field due to the load applied to the T-joint being considered, an approximate SIF for the cracked T-joint is evaluated and used for fatigue crack growth simulations by also employing a two-parameter model based on the Paris law. The numerical results deduced are compared with those obtained from experimental tests. The agreement between the different results is quite satisfactory.