Issue 9

T. Marin et alii, Frattura ed Integrità Strutturale, 9 (2009) 76 - 84; DOI: 10.3221/IGF-ESIS.09.08 81 b m b r        (4) that represents the content of bending stress over the total structural stress, t is the section thickness of the plate and m =3.6 represents the slope of a Paris-like crack propagation curve. It has been shown in several publications by Dong and co-workers that, using this procedure, it is possible to define a single S-N curve for many different weld geometries and loading configurations, therefore proving its robustness. This master S-N curve, which has also been incorporated in the ASME Boiler and Pressure Vessel Codes (2007) Section VIII Division 2 as an alternative prediction method, has the following form: h s NC S    (5) where C and h are parameters of the material and are tabulated for different prediction intervals. Contrary to most of the standards, this norm does provide neither a cut-off limit (fatigue strength for infinite life) nor any knees in the curve: all the cycles (after rainflow filtering) are considered damaging. This is consistent with the recommendations given in [11]. The equivalent structural stress is the parameter that provides the estimate of the life via Eq. (5). These concepts can again be applied to the example from Section 3. Accepting that the combined loading is proportional and in-phase, and assuming that the load history is constant amplitude with a stress ratio R =0, the structural stress range is  s =  s . The number of cycles to failure is then obtained through Eq. (3)-(5) and the results are given in the graphs of Figure 6. The point with the maximum  S s is the location where the fatigue cracks would most probably propagate in the through thickness of the plate. In this case the plots suggest that a fatigue crack would take place at 0.4* L of the toe 1 ( L : total length of the toe line) and that the failure of the part would occur after about 4.0E+6 cycles based on the mean master S- N curve. Figure 6 : Life prediction (with different scatter bands) and equivalent structural stress range  S s for the weld toes in Fig. 3. A PPLICATIONS ome experimental tests were performed in this work to further validate the structural stress approach and ASME master S-N curve. Several specimens of three different geometries were subjected to pulsating tensile constant amplitude loading (stress ratio R =0). To assess the predictive capabilities of the method, the maximum load was set to values corresponding to a given number of cycles. From the target life, the structural stress was deduced using Eq. (3)- (5) and compared to the maximum structural stress found in a finite element simulation of the specimen subjected to the known load. The linearity of the solution then allowed an easy scaling of the applied load to determine the force required in the test. The target life of this experimental campaign ranged from N=1.0E+5 to N=5.0E+5. S