Single Crystal Turbine Blade Inspection Using a 2d Ultrasonic Array

In this paper a non-destructive method for inspecting jet-engine turbine blades for root cracking is developed. This method utilises 2D ultrasonic arrays to enable the detection and characterisation of sub-surface cracks in three-dimensions. The majority of modern turbine blades are manufactured from single crystals of nickel based superalloys which exhibit high elastic anisotropy. This complicates the inspection due to the complex behaviour of ultrasonic wave propagation in anisotropic media and the variation of the crystal orientation in turbine blades. This paper, therefore, addresses these issues to develop a reliable inspection for a typical jet-engine single crystal turbine blade. Introduction Turbine blades are the components within gas-turbine engines that exact mechanical power from the hot, high-pressure combustion gases. During operation these components are highly stressed, due to the aerodynamic and centrifugal loadings that act upon them, and are surrounded by gases that may exceed the incipient melting temperature of the bulk blade material. It is therefore desirable to have a sensitive non-destructive evaluation method to inspect these components to ensure safe operation under these extreme conditions. 2D ultrasonic phased arrays are one method which would potentially enable the volumetric inspection of turbine blades for accurate detection and characterisation of sub-surface defects (1) . Ultrasonic array systems are also portable enough to allow in-situ inspections, which removes the costly and time consuming process of stripping the turbine blades from the engine. However, modern turbine blades are cast from single crystals of nickel based superalloys for the excellent mechanical properties these materials exhibit at elevated temperatures (2) . Single crystal materials are elastically anisotropic which causes difficulties when inspection these materials with ultrasound, in particular causing a significant defocusing of the images produced by the phased array. Recently the anisotropic propagation of ultrasound in single crystals has been modelled and these models were used to correct a volumetric imaging algorithm (3) . In this current paper the corrected algorithm is presented and is then applied to the inspection of cracking in a single crystal turbine blade. Bulk Wave Propagation in Anisotropic Media The phenomenon of anisotropic wave propagation has been discussed extensively in the literature, and is therefore only briefly presented here. A thorough discussion and derivation of governing equations can be found in Musgrave (4) and Červený (5) . For a more concise overview of the subject consult Rose (6) or Auld (7) . The most important aspect of wave propagation in anisotropic media for ultrasonic array inspection is that waves propagate with different velocity depending on the direction. This effect can be illustrated by referring to Figure 1. This figure shows images from finite element simulations that modelled waves propagating from a cylindrical point-source in isotropic and anisotropic media. In the isotropic case, Figure 1 a, the individual longitudinal and slower shear wave modes propagate with a constant velocity in all directions and hence have a circular wave surface. The waves in the anisotropic case, Figure 1 b, propagate with different velocities depending on the direction and therefore the wave surface is noncircular.