PSI - Issue 13

Evgeniia Georgievskaia / Procedia Structural Integrity 13 (2018) 971–975 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 1. Relative dynamic stresses in the runner

In the author's opinion, the scenario of safe crack development given in Fig. 2а is not always implemented in practice, as during crack extension stress redistribution occurs in the area of its tip. As a result the crack can change direction, moreover, several times, that results in blade part tearing off as schematically indicated in Fig. 2b, where arrows show direction of crack development. Torn off part getting in narrow gaps between rotating and fixed parts of the unit can cause serious damages of the unit up to emergency shutdown.

Fig. 2. Scenario of crack development in runner

3. Crack hazard criteria

For practical purposes it is important to adequately forecast occurrence of hazardous fatigue cracks in load bearing elements of hydraulic turbine (runners, shafts, critical fastener) under conditions of real operation. The process of crack origin and growth in hydraulic turbine elements can be conditionally divided into two stages differing by crack development mechanism. The first stage is origin and stage of slow crack growth, when primary motive power is low-frequency (LF) loads characterized by great amplitudes and accompanying operation of hydraulic turbine in off-design modes (refer to Fig. 1). Loads up to double rotational frequency inclusively usually belong to low-frequency ones. Under influence of LF-loads initial defects of material (cavities, inclusions, discontinuities in the welding zone, etc.) gradually develop into macro-cracks. Then slow stable crack growth occurs. This stage can continue for several years, sometimes, for several decades. Such cracks can relate to safe ones, i.e. not significantly influencing reliability and safety of hydraulic unit operation. At this stage high-frequency (HF) loads don’t almost influence crack kinetics as both defects and amplitudes of stresses from high -frequency loads are still too low. Crack development by this mechanism occurs up to some threshold value of crack length ℓ th determined by material specifications, design features and load conditions.