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. (C) 2004 Elsevier Ltd. All rights reserved.