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There are many engineering applications that involve the use of high speed rotating components such as turbine, compressors and generators. In general, industrial materials do not have uniform composition, and generation of defects such as voids, cavities, and cracks in their substructure is inevitable. Flaws or crack-like defects presented in the components under high centrifugal force can cause catastrophic failures. If a gas compressor impeller bursts during operation, the hazard to operators due to gas spreading and to plant is very considerable. The 2-stage gas compressor of National Iranian South Oil Company (NISOC) failed during operation. Initial observation revealed the presence of some macroscopic cracks at the junction of the blade and disc of the second stage impeller (Figure 1).
Figure 1. Crack at the junction of blade and disc in an industrial gas compressor impeller
The impellers are made of low alloy steel by quenched and tempered treatments. The impellers have been constructed by welding the blades to the cover, and all of these parts were made of forged ASTM A514 steel grade F. The general loading conditions of compressor are as follow; compressor rotation speed: 900 rad/s; inlet pressure of second impeller: 134 bars which compressed to 140 bars.
As it is seen in in Figure 2 and Figure 3, stress concentration is the main reason for crack initiation and the growing of cracks has caused the catastrophic failure of impeller.
Figure 2. Finite element analysis of a gas compressor impeller
Figure 3. The crack growth in a failed gas compressor impeller
In order to assess the structural reliability of cracked components, it is necessary to know the strength of parts and the fatigue-crack-growth rate. Both the strength and crack growth depend upon the stress intensity factor K. Therefore, an accurate fracture mechanics analysis is of utmost importance to ensure the in-service safety of rotary equipments. In spite of the wide ranging application of high speed rotating discs, relatively few works on the resulting stress intensity factor are presented in the literature and for three dimensional fatigue crack growth, there is no valuable comprehensive information reported for rotational discs.
Determination of strength and life of the cracked parts in industrial rotating machines always depends on the study of crack propagation in basic rotating discs. Unfortunately, works in this area are mainly limited to two-dimensional cases. For example, Tweed and Rooke (1973; 1977) determined the stress intensity factor of a radial crack and an edge crack in a finite elastic rotating disk. Isida (1981) considered the problem of a rotating disk containing an internal crack at an arbitrary position using the eigenfunction expansions of the complex stress potentials through boundary collocation technique. A rigorous elastodynamic hybrid displacement finite element procedure for a safety analysis of fast rotating discs with mixed mode cracks is considered by Chen and Lin (1983). The three dimensional crack problems in rotating equipments are also considered by a few researchers. Xi et al. (2000) did failure investigation of blade and disc in first stage compressor in an aero engine. Metallurgical examination and stress analysis revealed that the design shortcoming resulted in over-compensation of centrifugal bend moment and bad contact condition. Hou et al. (2002) performed a series of mechanical analysis and examination on a failed blade in a gas turbine engine. They utilize the nonlinear finite element method to determine the stress of the blade, in order to identify the case of blade failure. Barlow and Chandra (2005) simulated a three-dimensional fatigue cracks in a typical military aircraft engine fan-blade attachment. The problem of fatigue fracture of the compressor blade was described by Lourenco et al. (2009).