PSI - Issue 58

M.R.A. Rahim et al. / Procedia Structural Integrity 58 (2024) 9–16 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Power facilities are critically important in producing electricity and transforming raw components into finished products. Research studies by Cui et al., (2019) and Rahim et al. (2023) indicate that in the past, these installations had an average lifetime operation of around 402,960 hours. Low-cycle fatigue stresses are frequent in these facilities, especially in high-temperature and vibrations-prone situations. A crucial method to improve machinery efficiency, especially steam boiler, is by elevating the operating temperature within acceptable limits. However, this requires a highly durable structure (Salifu et al., 2019). The equipment must be subjected to rigorous structural integrity monitoring and proactive maintenance to remain operational. Moreover, it is imperative to understand the fatigue characteristics of materials when subjected to diverse loading conditions. Structural failures can happen even at stress levels that fall below the yield stress of the material. Numerous investigations have been conducted on P91 steel's high-temperature cracking behavior under continuous load circumstances (Saxena & Narasimhachary, 2018; Speicher et al., 2013). Saad et al. (2011) developed a viscoplasticity model to simulate power plant mechanical characteristics, particularly during thermo-mechanical fatigue. According to their investigation, viscoplasticity models can provide precise forecasts regarding the behavior of P91 steel across a temperature range of 400°C to 600°C. Similarly, Zhou et al. (2021) investigated the factors that lead to the formation of elevated temperature fatigue cracks in P91 steel. Significantly, a 74% increase in the intensity of high-grain boundaries was observed when the heat-affected zones of fine-grain and coarse-grain materials were compared. The idea of fatigue is mainly based on the continuous buildup of damage caused by recurrent loading, making it a complicated phenomenon (Bassi et al., 2017). Regarding engineering structures, fatigue life is a process that can be conceptualized as occurring in multiple stages. This complicated process is usually divided into three main stages for better modeling and analysis: fatigue crack initiation, fatigue crack propagation, and the start of unstable crack growth. These stages are frequently considered individually for a more effective evaluation. The implementation of the Tanaka-Mura model (TMM) as a computational instrument to comprehend crack initiation mechanisms has been the subject of numerous studies (Jezernik et al., 2010; Mlikota and Schmauder, 2020). An improved version of the TMM, first introduced by Brückner-Foit and Huang (2006), was used to model fatigue induced fracture progression in martensitic steels, notably F82H. In contrast to the principal emphasis of the study of Kramberger et al. (2010), which was the mechanics of segmental crack nucleation, Brückner-Foit's research emphasized on the quantification of fatigue cycles. Mlikota et al. (2018) developed a multiscale modeling and simulation approach for metal fatigue utilizing an enhanced version of the TMM. They used dual-scale and multiscale frameworks to estimate the cycles required for the final fracture. Experiments on different steel varieties, such as AISI 1141, high-strength steel S960, and martensitic steel, were conducted to validate the simulation estimates. A thorough examination of the development of minor metal cracks was carried out in a comprehensive study by Santus and Taylor (2009). Their research identified a three-crack evolution process. At the onset of the microstructurally short crack (MSC) phase, concerns regarding the applicability of continuum mechanics emerge, particularly in cases where the crack size is congruent with or less than the grain dimension. Following this, the fracture undergoes minimal expansion during the physically short crack (PSC) phase, which can be ascribed to reduced closure and additional contributing elements. Dogahe et al. (2023) studied the fatigue crack initiation in Oligocrystalline materials within the stent micro-structure with difference grain sizes. A dislocation model incorporating a double pile-up positioned on a single slip band accurately depicts the preliminary phases of cracking formation, as established by Tanaka and Mura (1981, 1982). The cumulative effect of both forward and backward loading is fatigue, which reveals imperfections in the material, such as dislocations. Once more cycles have accumulated, eventually newly formed or already present crystals show indications of slip bands. Simultaneously, bands that are already present undergo expansion, and some of them advance into short cracks (Mlikota et al., 2017). Limited investigations have been dedicated exclusively to 9Cr-1Mo (P91) in fundamental research to investigate the influence of microstructural morphology on the emergence of short fatigue cracks. The fatigue life of the material in terms of cycle performance has been demonstrated to be influenced by the use of the Voronoi Tessellation (VT) technique to simulate different microstructures.