Issue 41

Y. Nadot et alii, Frattura ed Integrità Strutturale, 41 (2017) 220-226; DOI: 10.3221/IGF-ESIS.41.30 223 Figure 5 : Simplified ‘representative’ fatigue load spectrum. Figure 6 : Comparison between constant and variable amplitude loading for tension and bending loading. MODELING STRATEGY FOR THE FATIGUE LIFE t this stage, we have the experimental database that we can use to simulate the fatigue limit and fatigue life for welded structures under both constant and variable amplitude loadings. In order to keep as simple as possible the global design methodology (automotive industry context) but with the key parameters, we will address the problem using the following steps: 1. The fatigue criterion used in this study is the criterion proposed by Vu et al. [2] in order to be able to take into account complex loading using stress invariant approach; all details of this criterion applied to this variable amplitude loading are given in [1] 2. In order to take into account the complex stress distribution, a stress gradient approach is used and called ‘Welded Stress Gradient’. The idea is to compute a stress gradient related to the local geometry of the welded junction as explained in Fig. 7.  Vu  max t  T  1 J 2 ' ( t ) 2   2 J 2, moy 2   3 I f ( I 1, a , I 1, m )     ( N ) Second invariant of stress deviator Phaseshift effect (mean valueof J’ 2 ) Hydrostatic stress effect (amplitude and mean value of hydrostatic stress) Material Parameters A