FATIGUE THRESHOLDS OF ADDITIVELY MANUFACTURED AND WROUGHT METAL MATERIALS WITH NOTCH-DEFECT INTERACTION SUBJECTED TO MULTIAXIAL LOCAL STRESSES

Among the possibilities AM offers, lattice structures (or cellular structures, metamaterials) stand out. These structures are obtained by the periodic repetition of a Unit Cell (UC) of struts or surfaces in space. The possibility to obtain stiffness-to-weight and strength-to-weight ratios competitive with composite materials make such structures particularly suitable for the aerospace, automotive, and biomedical industries. However, the potential of AM, especially for lattice structures, is currently limited by significant concerns related to their structural integrity due to mechanical peculiarities closely linked to the production process. These challenges underscore the urgent need for further research and standardization in this field. In particular: (i) the pronounced surface roughness and the internal defects that may favor fatigue crack initiation and growth, (ii) the intense residual stresses in As Built (AB) conditions, and (iii) the high defect sensitivity of AMed microstructures in AB conditions. These factors collectively contribute to the poor fatigue strength of AMed materials compared to their counterparts obtained by conventional processes. Moreover, the fatigue assessment is complex considering: (i) the potentially significant discrepancy between the As Designed (AD) and As Built (AB) geometry of the component, (ii) the inherently notched nature of lattice structures and the manufacturing defects leading to severe stress concentrations and local multiaxial stress fields, (iii) the unpredictability of the defect distribution based solely on process parameters and the geometry of the part, and (iv) the lack of regulations standardizing either the experimental testing of lattice structures and their fatigue assessment. In this context, the present work aims to estimate the fatigue limit of lattice structures produced via Powder Bed Fusion (PBF) of metallic powders, assessing the effect of the AM characteristic defects and the local multiaxiality with a unified approach. The key novelty of this Ph.D. project is to consider lattice structures as defective and notched materials and to estimate the fatigue limit using non-conventional extensions of the Linear Elastic Fracture Mechanics (LEFM), starting from the mechanical properties of the base material. It will be possible to develop optimization strategies for lattice structures that are robust to the actual geometry of the lattice structure and the remote load actions since the fatigue assessment is performed locally at the scale of the UC, where the fatigue strength is included as a constraint. However, the design approach needs to be validated. To address the intricate nature of the fatigue problem for lattice structures, this work has methodologically explored the following aspects before analyzing the fatigue strength of lattice structures: (i) the interaction between defects and notches, particularly in the presence of sharp notches, an aspect poorly documented in the literature, and (ii) the development of a unified criterion for estimating the multiaxial fatigue limit for sharp notches and defects.