The Assessment of the Developed Inspection Capability

Abstract

In the previous chapter, the development of an ultrasonic array system for the inspection of a specific single-crystal turbine blade was presented. The inspection was designed to be performed in situ and detect cracks which initiate from the internal surface of the blade root. The performance of the system was evaluated using artificial defects. This chapter evaluates the inspection capability of the developed array system using turbine blades containing real cracks. The evaluation is performed using probability of detection (PoD) analysis. This is a statistical method that is used widely to evaluate NDE processes in a rigorous manner. A PoD analysis provides a quantitative measure of the probability of detecting a defect of any given size. To perform the PoD analysis for the array system, the ultrasonic response was recorded from cracked blades installed inside an engine mock-up which represented the in-service inspection conditions as accurately as possible. This process was also performed using a conventional single-element probe. After inspection, the cracked turbine blades were cut open to measure the geometry of the cracks. The results of the PoD analysis for the two ultrasonic systems are compared and the array system is shown to offer significant improvements in crack detection capability. However, it is shown that the developed array system suffers from poor reliability. The factors affecting the inspection reliability are discussed.