PSI - Issue 68

Ilia Nikitin et al. / Procedia Structural Integrity 68 (2025) 24–31 I. Nikitin et al. / Structural Integrity Procedia 00 (2025) 000–000

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This work aims to build such a link via numerical simulation of the SLM procedure and corresponding fatigue properties prediction. Fatigue life prediction is carried out using the multimodal fatigue failure model by Nikitin (2022). In the model, the full fatigue life consists of several regions: low cycle fatigue (LCF), high cycle fatigue (HCF), and very high cycle fatigue (VHCF), Fig. 1. Since the structure produced by SLM is mainly designed for in-service use under elastic loading conditions, two branches of the fatigue curve (HCF and VHCF) are considered in the model. Both branches show a continuous decrease in fatigue strength with the number of applied cycles. The similarity between these curves allows them to be described by a similar Basquin-type equation with different parameters. This approach enables a seamless calculation method during the numerical implementation of the mathematical model by Nikitin (2019).

Fig. 1. The typical full fatigue curve for metallic materials.

The branches are linked by a bifurcation area introduced by Shanyavskiy (2013), where the transition from one branch to another has a probabilistic nature. In the present model, this region is indicated by a given stress range (width of the bifurcation area) Δ . This range is introduced to eliminate the discontinuity in the relationship between stress amplitude and the number of cycles to failure. Studies on the fatigue behavior of metallic materials have shown the important role of microstructure on the fatigue strength, as demonstrated by Bathias (2004). Crack initiation in the VHCF region is associated with internal defects (non-metallic inclusions) in the material, as reported by Palin-Luc (2018). It has also been shown that crack initiation in materials without evident defects is sensitive to internal microstructural features, such as macro-zones and lamellas agglomerations, as noted by Nikitin (2016). SLM technology produces numerous internal boundaries between neighboring layers, which could be a potential source of internal microstructural imperfections. In the present study, the focus is on the VHCF behavior of SLM materials. The study is conducted on typical hourglass specimens subjected to high frequency loading. High frequency fatigue loading is described by the equation of elastic vibrations: ∇ + ! = 0 (1) = ( )( : ) + 2 ( ) (2) = (∇⨂ + (∇⨂ ) " )/2 (3) Where is density, is stress tensor, is displacement, is loading frequency, and are elastic moduli (Lame), is strain tensor, is unit tensor, is damage function. The system of equation (1)-(3) is supplemented by the initial and boundary conditions of free vibrations: ∙ # | ∈&' !" = #∗ ( , ) (4)