PSI - Issue 74

Mitra Delshadmanesh et al. / Procedia Structural Integrity 74 (2025) 9–16 Mitra Delshadmanesh / Structural Integrity Procedia 00 (2025) 000–000

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entropy to the Gibbs free energy. These alloys were considered from a theoretical viewpoint by Vincent and Cantor (1981) long before they were first fabricated by Yeh et al. (2004) and by Cantor et al. (2004). Indeed, the Cantor alloy CoCrFeNiMn crystallizing in FCC lattice was among the first high entropy alloys that were manufactured. This alloy is known for its high strength, ductility, and fracture toughness, leading to excellent fatigue life, as reported by Zare Ghomsheh et al. (2020). The present investigation is devoted to the measurement and theoretical interpretation of its fatigue notch sensitivity. The conventional evaluation of the notch effect on fatigue life is based on Peterson’s stress concentration factors. It is, however, long known that some materials show only low notch sensitivity. Sharp notches do not reduce the fatigue life for that amount, which is expected from predictions based on the stress concentration factor. This effect is material-dependent: Hu et al. (2013) established a method where an optimal parameter can be found for every material. Hamano et al. (2016) compared the notch sensitivity of different steels. Chapetti and Guerrero (2013) argued that the notch sensitivity is related to a material threshold for crack initiation. McEvily et al. (2008) suggested an explanation based on fatigue crack closure. In the present study, the reduced sensitivity against sharp notches is explained by analogy to a size effect, which can be evaluated in the frame of strain gradient elasticity. σ max maximal stress σ smooth endurance stress limit of a smooth sample σ notch endurance stress limit of a notched sample, whose stress was measured at its smooth region q fatigue notch sensitivity factor w energy density Λ first Lame elastic constant Μ second Lame elastic constant, equivalent to shear modulus A 1 - A 5 material parameters of strain gradient elasticity ε ij strain tensor η ijk second gradient of displacements u displacement x coordinate α rotation angle 2. Experimental To study the notch effect on the fatigue life of the high entropy alloy CoCrFeNiMn, samples with different notch radii were designed, and Wöhler curves were measured for each sample type. 2.1. Sample preparation and experimental setup Six types of notched samples were designed with different notch radii in the range from 2 mm to 125 µm. The geometry of these sample types A, B, C, D, E, and F is depicted in Fig. 1. The sample lengths varied from 10 mm to 5 mm, and the remaining sample width at the height of the notches was between 0.3 and 0.4 mm. The thickness of the samples was typically 200 µm. The samples made of CoCrFeNiMn were supplied in customized shapes by the company ATT Advanced Elemental Materials Ltd. For fatigue testing, the specimens were glued onto a bar of Ti6Al4V with cyanide-acrylate adhesive. This bar, which served as a sample holder, had a length of 125 mm, equivalent to the resonance length of λ /2 at the testing frequency of 20 kHz. A schematic picture of the ultrasonic fatigue resonance testing system is shown in Fig. 2. In the middle of the sample holder, there was a hole with a Nomenclature K t stress concentration factor fatigue notch factor K f σ 0 reference stress