1.Introduction
There are many risks in hydroelectric power plants. Analyzing these risks and taking precautions will provide the biggest contribution to the power plant in order to be ready for continuous energy production [1]. One of the most important of these risks is the failure of the turbines-generator units’ sliding bearing of hydroelectric power plants. The safe operation of the turbine-generator units of hydroelectric power plants depends to a great extent on the safe and sound operation of hydrodynamic lubricated bearings. Therefore, the standard size of the bearing gaps, the ideal viscosity value of the bearing oil and a good bearing oil cooling system are very important in the bearings of the hydraulic turbine-generator units. There are many studies in the literature on hydrodynamic lubrication bearings [2, 3, 4, 5]. One of the most important parts of hydrodynamic lubrication bearings is bearing shells. The bearing shells work together with the bearing pads by forming the oil film and carry a large amount of axial or radial loads. These very sensitive working conditions require very precise machining of the surfaces of the bearing shells and pads. In most cases, there are fractures in the bearing shells due to material faults and this failure prevents the turbines of hydroelectric power plants from producing electricity for a long time and causes great financial losses. Metal fractures may be caused by many reasons. Fatigue fracturing damage is often dealt with under cyclic loading, but it is also important to assess the under monotonic overload condition. This is due to the fact that fatigue fracturing increases the strength of the structure under certain conditions [6]. Two boundary conditions are taken into account for fracture structures: The Ultimate-Mate Limit State (ULS) and the Fatigue Limit State (FLS). The final boundary state corresponds to the maximum load-bearing capacity that takes into account the monotonic loading, whereas the fatigue boundary state corresponds to deterioration due to the effect of the cyclic loading over time [7]. Da-wei et al. have analyzed the ultimate strength of fracture progression in two different categories of research on the ultimate strength of fractured structures. Fractures in materials are often seen as a result of impact and welded areas, accidental overloading can potentially lead to fracturing. The onset of fractures may also be caused by the effect of repeated loads in stress concentration zones. If such initial fracturing is left undetected or not immediately repaired, it may turn into a fracturing that continues to spread under repeated loads, thus causing an aging regimen [8]. When a fracture is spread, the material tearing events in areas near the fractures are slightly larger. In the final power analysis, the mechanical properties of the structural elements are considered rather than the fracture end. Therefore, material tearing events can be seen as fracture propagation criteria [9]. Paik has performed a series of tensile and compression tests on plates with predetermined fracturing damage. He has investigated the effects of different geometric parameters on the final strength of the boards such as fracture length, crevasse, gap between fractures, fracture size and plate thickness [10].
Seifi and Khoda Yari have conducted an experimental research on plates with three different aspect ratios. They have evaluated the results by taking into account the fixed plate length and different aspect ratios obtained with variable widths. For plates with equal fracture length (the ratio of fracture length to plate width), they have concluded that the buckling load was proportional to the plate size ratio [11].
Hu et al. have examined the final tensile/compressive strength of unhardened and hardened plates. In addition to their material strength and fracture length, they have mentioned a parameter that influenced the softness of the plate and the final compressive strength [9]. Paik, et al. have stated that if the fracture size exceeds a critical fracture length, a fractured plate reaches the final boundary state. They assess the triple fracture length as a criterion for evaluating fracture progression [12]. Babazadeh and Khedmati have said final strength of fractured plate are affected by the fracture length, orientation and location. They have examined how these parameters affect plate behavior. As a result of a study on the final strength of fractured plates and the results of research in this area, the fracture length it is understood that it had the most important effect on the final strength of the plates [13]. At the same time, since the fracturing orientation changes in the longitudinal direction (perpendicular to the loading) in the transverse direction, the impact of the fracture on the highest strength of the plates becomes negligible [14].
Paik and Thayamballi have performed a series of tensile and compression tests on pre-fractured plates. They investigate the effects of different geometric parameters, such as fracture length, fracture position, gap size between fracture faces, and plate thicknesses on the final strength of the plates. The test specimen is box-type coating structures comprising a pre-broken fracture obtained by welding four similar steel plate adhesions as mounting. They have presented the results as stress curves [15]. Rycerz ve diğerleri, yatak yüzeylerinde yorulma sonucu oluşan çatlakların yayılmasını incelemişlerdir. İnceleme sonucunda, gözlenen çatlak morfolojisi, mühendislik bileşenlerinde bulunan ve literatürde rapor edilen, yüzeydeki v-şekilli görünüm, yüzeye 20-30 ° eğim açısı ve sürtünme kuvvetinin yönüne doğru yayılma dahil olmak üzere, tipik RCF çatlaklarının tüm özelliklerini sergilemiştir[16]. Maya ve diğerleri, iki perlitli yay çeliğinin yorulma çatlağı büyüme oranını incelemişlerdir. İnceleme sonucunda her iki ray çeliğinde perlitin daha küçük interlaminar aralığı düşük değerleri için yorulma çatlağı büyüme oranında bir azalmaya yol açtığını belirtmişlerdir[17].
The fracture origin is caused by a fatigue mechanism with surface defect [18]. Although some authors have reported a change in final strength about the fracture position, it can be said that there is not a significant influence on the final strength of the longitudinal and transverse locations of the fracture in general. [19]. The stress is very sensitive to material defects that provide strain points that can initiate stretch fractures [20].