Abstract
Single-crystal nickel-base superalloys are often used in the hot sections of gas turbines due to their good mechanical properties at high temperatures such as enhanced creep resistance. However, the anisotropic material properties of these materials bring many difficulties in terms of modelling and crack growth prediction. Cracks tend to switch cracking mode from Mode I cracking to crystallographic cracking. Crystallographic crack growth is often associated with a decrease in crack propagation life compared to Mode I cracking and this must be taken into account for reliable component lifing. In this paper a method to evaluate the crystallographic crack propagation rate related to a crystallographic crack driving force parameter is presented. The crystallographic crack growth rate is determined by an evaluation of heat tints on the fracture surface of a specimen subjected to fatigue loading. The complicated crack geometry including two crystallographic crack fronts is modelled in a three dimensional finite element context. The crack driving force parameter is determined by calculating anisotropic stress intensity factors along the two crystallographic crack fronts by finite-element simulations and post-processing the data in a fracture mechanics tool that resolves the stress intensity factors on the crystallographic slip planes in the slip directions. The evaluated crack propagation rate shows a good correlation for both considered crystallographic cracks fronts.
Highlights
There is always a strive towards higher combustion temperatures when designing industrial gas turbines used for power generation, due to the resulting increase in efficiency and reduction in terms of pollution [1]
The aim of this paper is to evaluate and model the crack growth rate for crystallographic cracking
The equivalent Resolved Stress Intensity Factor (RSIF) for the crystallographic crack fronts for the two considered crack instances are evaluated along the parts of the crack fronts, which are visible in the optical microscope
Summary
There is always a strive towards higher combustion temperatures when designing industrial gas turbines used for power generation, due to the resulting increase in efficiency and reduction in terms of pollution [1]. These ever increasing temperatures put high requirements on many components in the hot sections of the gas turbines. One crucial component that is exposed to this harsh environment is the blade in the first turbine stage These blades are often manufactured from nickel-base superalloys cast as single-crystals, due to their great material properties under these conditions. The aim of this paper is to evaluate and model the crack growth rate for crystallographic cracking. The parameters in the crack growth law have been calibrated by evaluating heat-tinted fracture surfaces of performed experiments
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