Issue 41

Y. Nadot et alii, Frattura ed Integrità Strutturale, 41 (2017) 220-226; DOI: 10.3221/IGF-ESIS.41.30 222 Figure 3 : Fatigue life under tension for different load ratio. Under bending loading the results are different from tension (Fig. 4). Two different initiation sites can be activated, the slope of the curve if different and the amplitude of the loading is not the unique governing parameter. Under bending, R = -1 and 0 when the initiation site is ‘Higher root’ the fatigue curve is a function of the load ratio with ‘m’ higher than 3 probably because initiation and propagation is mainly on the base material, so that the welded junction is less involved in the final mechanism. For bending R = 0, when the initiation site is the root and the propagation inside the welded part, the slope is close to 3. For bending R = 0.5 when the initiation site is the root and the propagation inside the welded part, there is a similar fatigue life compared to bending R = 0 (same initiation site) so that the amplitude seems to be the governing parameter; the slope of the curve is lower than 3 but only a few points are involved for this load ratio so that the important point to discuss is the similar result in term of fatigue life (controlled by the amplitude). Figure 4 : Fatigue life under bending for different load ratio. The variable amplitude loading is presented in Fig. 5. This spectrum is a very strong uniaxial simplification of an automotive fatigue spectrum. This spectrum is a combination of a large number of small cycles plus a very small number of overloads close to the yield stress of the material. Fig. 6 presents the results obtained under variable amplitude loading compared to constant amplitude one. Under tension loading the crack initiation site is the same and the slope of the curve is similar. Under bending, the crack initiation site under variable amplitude loading is the same that the one obtained at R = -1 so that it seems that the part of the spectrum spent at R = -0.8 should play a major role in the spectrum.