Use of fracture mechanics to evaluate the bearing capacity and remaining life of rotors in turbomachinery

Abstract

1. The risk of damage or brittle failure of rotors owing to the growth of crack-type macroscopic defects in them increases as the effective time to overhaul of turbomachinery increases with the expanded use of steels possessing increased creep resistance. According to statistical data, rotors of low and moderate pressure in steam turbines are especially subject to this risk. 2. Nondestructuve methods should provide for the reliable detection of defects with characteristic sizes\(\Delta<< {{K_{1_C }^2 } \mathord{\left/ {\vphantom {{K_{1_C }^2 } {n\beta \sigma _y^2 }}} \right. \kern-\nulldelimiterspace} {n\beta \sigma _y^2 }}\), where n is the safety factor, and β depends on the form of stress state at the most critical point of the rotor. The following methods of flaw detection are promising for practical use: magnetoscopic (external defects), ultrasound (internal defects), and vibrodiagnostics (for continuous in-service). 3. For equal characteristic dimensions, defects located in the axial channel or near it, and also on the surface of stress concentrators are the most dangerous: the first may give rise to brittle failure, and the second to vibration and curvature of the rotor. 4. To evaluate the stress intensity factors near cracks located on the surface of the axial channel, it is expedient to use Eqs. (2) and (4). Such a relation between the material's crack resistance KIc and the loading parameters ω, Δ, T, and p, which reduces the risk of the rotor's brittle failure, may exist for a number of rotor designs and loading conditions. 5. The selection of the material for turbomachine rotors should ensure the best compromise relationship between the material's capacity to resist creep and crack development. This selection should be made on the basis of analysis from positions of fracture mechanics and with allowance for the effective time to overhaul, as well as to the potentials of flaw-detection methods. The trend toward increased creep resistance for rotor steels increases the risk of brittle failure with decreasing KIc.