Numerical simulation of surface crack paths in fatigued T-joints

In the present paper, the fatigue behaviour of welded T-joints, such as those used in Francis hydraulic turbine runners, is analysed both experimentally and numerically. To simplify the problem, a finite-thickness cracked plate is examined instead of the actual T-jointed blade: as a matter of fact, for relatively small cracks, the geometry of the component does not remarkably influence the Stress-Intensity Factor (SIF) values provided that the stress field near the crack is approximately the same. Then, an approximate SIF expression for the cracked T-joint is evaluated by applying the finite element method and the superposition principle. Finally, by means of a numerical model based on the Paris law, fatigue crack paths are predicted. The results of such simulations are compared with those obtained from experimental tests. The agreement between the different results is quite satisfactory.