PSI - Issue 33

1098 Riccardo Caivano et al. / Procedia Structural Integrity 33 (2021) 1095–1102 Riccardo Caivano et al./ Structural Integrity Procedia 00 (2019) 000–000 propagation is that average applied stress is higher, and this effect is already evaluated in the limit by Eq.(4). It is enough to ensure that the limit on the maximum first principal stress is respected in the worst case, i.e. ��� , to conclude that no defects will let propagate the crack. Reversely, if a negative R is considered, then the applied minimum force ��� is in the opposite sense of the maximum one ��� , changing completely the stress field. In this condition, two different load cases must be considered: the first one with the ��� downward force and the second with the ��� upward force. Indeed, the change in sense of the applied force put in traction other zones of the material, previously in compression with the downward force. The two load cases are needed to ensure that in all the parts of the material under traction during the historic cyclic load the first principal stress is not larger than the Murakami fatigue limit. In both load cases, Eq.(4) provides the limit of the maximum first principal stress. Therefore, when the stress ratio R is negative, it is crucial to impose the limit in both the two different stress distributions to ensure that no crack will propagate from the defects. 4

Fig. 1 – Corbel design domain and geometrical dimensions More generally, given an estimated defect population such as that supposed for AM AlSi10Mg reported in Table 1, it is possible to map the limit in the alternate first principal stress limit with respect to the variable stress ratio R according to Eq.(3), as shown in Fig. 2a.

Fig. 2 – Defects and related fatigue limits: a) first principal alternate stress limit [MPa] depending on stress ratio R; b) maximum first principal stress limit [MPa] depending on stress ratio R; c) LEVD cumulative distribution probability; d) Gumbel plot of the defect size (same as [34])