Abstract
This chapter details the development of a complete ultrasonic array imaging system for the inspection of a specific single-crystal turbine blade. The system is developed to detect cracks that have initiated on the internal surface of the turbine blade root. The root section of the turbine blade experiences the highest operational stress and is therefore most prone to in-service cracking. The system is also designed to perform in situ inspections to avoid the significant time and cost implications of removing the turbine blades from the engine prior to inspection. As such, the design of the system is constrained by the limited access to the turbine blades within the engine. The system utilises the corrected imaging algorithm described in Chap. 3 and the ultrasonic crystallographic orientation method developed in Chap. 4. The developed system is shown to be able to successfully detect artificial defects in turbine blades. The detection performance of the ultrasonic array system as the size and orientation of defects varies is demonstrated on test specimens. In addition, the sensitivity of the inspection system due to variations in beam amplitude and misalignment in the crystallographic orientation are also evaluated experimentally.
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Notes
- 1.
To explain this point further, consider depth amplitude correction (DAC) curves used commonly for single element inspections. The beam amplitude decreases with penetration depth in the far-field of the transducer due to beam spread and attenuation. DAC is used to increase the gain of signals from reflectors that occur at greater depths within the specimen so that equivalent reflectors give the same amplitude response. This allows for accurate sizing of small defects. However, if the reflector is too deep, its amplitude response will be similar to the noise level and will be un-detectable, regardless of whether DAC is applied or not.
- 2.
The experiment on the 0° slots with different through-going extents was repeated numerous times. Every experiment showed that the 50% through-going slot gave a significantly larger response over the 80 and 90% slots. A 3D X-ray CT scan was used to show that the slot angle, depth and through-going extent were correct. Replicas of the slots were taken and showed the roughness of the slots were very similar. Simple 2D FE models were run to show that the slot response, for 0° slots with increasing through-going extent, increased up to 60% through-going extent and then plateau. Therefore, the cause of this drop off in signal at 80 and 90% is not fully understood. Possibly, the effect is due to interference from the 3D scattering off the tips of the slots. However, this could only be verified from full 3D FE simulations, which are not currently possible on computers available for this project.
References
Drinkwater BW, Wilcox PD (2006) Ultrasonic arrays for non-destructive evaluation: a review. NDT E Int 39(7):525–541
Velichko A, Wilcox PD (2010) Strategies for ultrasound imaging using two-dimensional arrays. Rev Prog Quant Nondestr Eval 29:887–894
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Lane, C. (2014). The Development of an in Situ Ultrasonic Array Inspection System. In: The Development of a 2D Ultrasonic Array Inspection for Single Crystal Turbine Blades. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-02517-9_5
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DOI: https://doi.org/10.1007/978-3-319-02517-9_5
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