1. Introduction
The centrifugal fan blades are mainly used for cooling of traction
motors, main transformer cabinets and auxiliary transformer cabinets of
high-speed trains. As one of the main components of rail transportation
equipment, the fan blades’ safety and reliability deserves more
attention and study. The phenomenon of fatigue is one of the main
reasons for material failure. Due to the complex operating environment
of the high-speed-train system, the blades are subjected to periodic
changes in alternating stress, vibration excitation and so on when
working. These loads usually can be described by deterministic formulas
or stochastic theory [1]. On the other hand, when the fan starts and
stops, the alternating stress will increase suddenly. High cycle fatigue
life consumption [2], caused by alternating stress during starts and
stops, will be considered and examined. In short, the centrifugal fan
blades will be damaged by fatigue working long hours, which will affect
the normal operation of the high-speed-train group system. Therefore,
the assessment of fatigue life is necessary for the design of fan blade
and an important index for evaluating reliability. It has important
theory meanings and practical applying value for centrifugal fan blades
fatigue life forecasting.
At present, many investigations on the fatigue performance of fan blades
have been conducted. In general, fatigue life prediction for fan blades
is based on nominal stress [3-5]. Static and dynamic stress analyses
and fracture mechanics criterion were accomplished to simulate fatigue
crack growth for compressor blade [6, 7].The most commonly adopted
approach to study fatigue performance of fan blades is finite element
technique, which makes the analysis of coupling of fluid and solid and
is the key technique of understanding fatigue characteristics and
failure mechanism [8-10]. Finite element analysis and Neu/Sehitoglu
(N/S) TMF model provided a life assessment tool for aero jet engine
blades by Abu, AO [11]. Finite element analysis could be used to
assess low cycle fatigue life of steam turbine blade [12], which
could predict the catastrophic failure of turbine blade arisen from
fatigue [13]. The influence of interaction of low cycle fatigue
(LCF) and high cycle fatigue (HCF) on aerodynamic and structural
properties of fan blades were also studied [14-16]. The fatigue
caused by combined cycle loadings was noticed [17-20]. Miner’s rule
was used to calculate the fatigue life of helicopter main rotor blade
according to the classical life estimation method under random load
[21]. In actual railway projects, the Miner’s rule [22] had been
frequently used to assess combined cycle loadings, however, it ignored
the effects of load interactions and the coupling damage [23].
Considering the impact of structural dynamic response, it is very
important to verify relevant models for the development of fan blades
[24-26]. Meanwhile, The research on the dynamic characteristics of
fan blades mainly focuses on gas turbine blade [27], compressor
blade [28], steam turbines [29], wind turbine blade [30,
31], wings [32], helicopter blade [33] and there are few
studies on fan blades in the ventilation and cooling system of railway
locomotives. It’s critical to design the fan blades of many railway
vehicles, but fatigue design experiments of each type of fan structure
are very costly and time consuming. Therefore, reliable fatigue life
prediction is an important requirement for new product development.
This paper aimed to develop a model for the fatigue life prediction of
fan blades in the ventilation cooling system of high-speed-train.
Focusing on the causes of the dynamic stress of the fan blade, a typical
centrifugal fan blade was taken as the research object. Chemical
composition analysis, static tensile test and dynamic fatigue test were
carried out on the blade material DC51D+Z, the basic composition and
mechanical property parameters of the material were obtained, and theS-N curve of the material DC51D+Z was fitted. By using the finite
element model of fan, considering the influence of friction and contact
force of its structural parts, the stress distribution at the critical
point of blade components was calculated. Considering the loading form,
size effect, surface quality and stress concentration factor (SCF) of
the fan blade comprehensively, this paper introduced an improved method
based on the concept of nominal stress. Finally, in order to prevent the
fan blade from damaging caused by fatigue under unexpected
circumstances, this paper established a new model for predicting the
fatigue life of fan blades in the ventilation cooling system of
high-speed-train. It was combined with the Palmgren-Miner cumulative
damage criterion and S-N curve to further studying details
concerning the feasibility of the calculation model.