PSI - Issue 57

Rando Tungga Dewa et al. / Procedia Structural Integrity 57 (2024) 762–771 Dewa et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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arrows), crack propagation area, and final fracture zone in dashed line. In the crack initiation zone, the fracture surface shows cleavage-like facets. The crack propagation mainly shows a type of flat surface with dense striations and small number of secondary cracks. The sharp feature of surface, indicating a large tension during fracture, contributes a small portion of final fracture zone. However, the failure modes generally exhibit the transgranular crack in nature with cleavage-like facets for typical fatigue loadings at RT. This type of crack indicates the fatigue failure occurred due to the fatigue slips at the surface. No indications of premature failure or defect failure are noticed during the examination. Furthermore, Fig. 8 shows SEM micrographs (with zoomed images on each zone) for LCF specimen tested at low strain rate. For low strain rate specimen, however, the crack propagation can be characterised into two stages (separated by yellow line). The first stage indicates a typical transgranular fracture with a bigger step of striations. It is related to the lower cycle it has. Subsequently, the second stage shows the domination of secondary cracks. It is believed that somehow, the secondary crack occurred before the crack tip reached. These secondary cracks can be originated from typical cavities/voids. At the final fracture zone, intergranular dimple fracture somehow is even more obvious. The failure mode shows intergranular fracture mechanism with creep has. Therefore, it can be illustrated that the intergranular crack-fatigue interaction somehow can be promoted at a very low strain rate LCF loading even at RT, as the deformation of grain boundary sliding is more prevalent at this condition and resulting in lower cycles to failure. Therefore, the lower fatigue life at the very low strain rate can be expected from this mechanism during fatigue. The result reveals that longer tensile loading rate per cycle is needed for the type of experiment which results in increase of this cavities-driven crack interaction. These findings can be confirmed with the lower strain rate condition, in which the strength is lower with a higher ductility and slow softening rate so that the dislocation mobility is limited compared to the high strain rate specimen. Additionally, opinions that can be taken with continuing rapid assessment of the environmental test behavior for the studied material, since the material is greatly related to the time-dependent behavior. 6. Conclusions The LCF resistance of Alloy 617 is found to be time-dependent over a range of 5E-4 ~ 1E-2 s -1 strain rates. Even though the fatigue life was almost completely governed by plasticity of materials during testing, the authors consider the time-dependent factor is best represented the fatigue life evaluation and the linearity agreement proved it. All materials show similar trend with initial hardening phase, followed by cyclic softening and rapid drop of stress or failure. However, it is noticed at lower strain rate that the specimen has lower stress range compared to other specimens and the macrocrack initiation for specimen with the fastest strain rate occurred earlier. The influence of strain rates on fatigue life can be evidenced through fractography images. At the lowest strain rate, the specimen fractured with additional failure mode in intergranular manner. The domination of secondary cracks is obvious at the surface. These secondary cracks might be originated from cavities or voids. A typical mixed intergranular dimple fracture for the very low strain rate specimen similarly indicates to the interaction of cavities driven cracking mechanism during fatigue loading at RT, attributing to the lower LCF life. It is coinciding well with the result at lower strain rate condition, in which the strength is lower with a higher ductility and slow softening rate so that the dislocation mobility is limited compared with the high strain rate specimen. The LCF resistance of Alloy 617 is found to be time-dependent. It is recommended that factor of safety must be considered in designing phase to evaluate the fatigue life. The fatigue life variation can be extremely broad at millions of cycles, especially out of time-dependent damage towards the catastrophic failure.