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].