PSI - Issue 66

María Moreno-Rubio et al. / Procedia Structural Integrity 66 (2024) 362–369 Author name / Structural Integrity Procedia 00 (2025) 000 – 000

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It is noteworthy that the values for ultimate tensile strength, yield strength, and Young’s modulus at room temperature were obtained through tensile testing in our laboratory. The additional values presented were sourced from the material data sheet supplied by S+D Metals. The fatigue strength coefficient ( σ f ´ ), fatigue ductility coefficient ( ε f ´ ), fatigue strength exponent ( b ), and fatigue ductility exponent ( c ) at both temperatures were obtained from the simple fatigue curves of the tests conducted in this work. The friction coefficient was obtained from Jahanmir (1993), the mode I SIF threshold at RT from Yang Hua et al. (2019), and at HT from Connolley (2002) 2. Experimental data In this study, two types of experimental tests were conducted: simple fatigue and fretting fatigue. Both tests were performed at room temperature (RT) and at 650ºC (HT). Specimens with four different geometries were used, all having a rectangular cross-section of 5 mm in thickness and 7 mm in width, as described by Moreno-Rubio (2023). The setup used for the fretting fatigue tests was identical to that described in my previous work, by Moreno Rubio (2023). This setup employed a fretting bridge in conjunction with a ring. In that study, the standard fretting bridge was modified by introducing two additional elements: a cap and two cylinders, which enhanced the transmission of the normal force to the fretting bridge without bending it. A schematic of this setup can be seen in Fig. 1, where N corresponds to the applied normal load. This load is applied through two screws threaded into the fretting ring and transmitted to the caps, cylinders, and the fretting bridge. The fretting bridge is responsible for applying the normal load to the specimen, ensuring contact between the elements. Subsequently, a cyclic axial load ( σ ) is applied to the test specimen using a servo-hydraulic machine with a capacity of 100 kN.

Fig. 1. Fretting fatigue setup geometry

Each fretting bridge has two pads; whose contact surfaces have a radius of 25 mm. This results in a cylindrical contact pad when the fretting bridge is pressed against the flat surface of the test specimen. The fretting bridge, cylinders, and cap all have thickness of 5 mm. It is also important to note that all components used throughout this work were manufactured from Inconel 718. On the one hand, the simple fatigue tests at RT were carried out in two rounds: the first with an axial stress ratio of R =-1, and the second with R =0.1, while the simple fatigue tests at HT used an R =0.1. In both cases, for RT and HT the tests were performed at a frequency of 10 Hz. The variation in the stress ratio between different temperatures was due to the specific high temperature setup; which can be seen in a photograph provided by Moreno-Rubio (2023). On the other hand, all fretting fatigue tests, both at RT and HT, were performed with R =0.1 and 8 Hz. The reduction in frequency was necessary to avoid undesirable dynamic effects at higher frequencies. The results of these experimental fatigue test are presented numerically in Tables 3 to 6, and graphically in Fig. 2 using S-N curves. In this graph, all values represented are based on an R =0.1 stress ratio. For this purpose, the