Inspection of the undulation structure with an in-plane differential flexible array probe

Abstract

Blade is a crucial part in steam turbine that chronically exposes to an atmosphere of high temperature, humidity, and pressure, which prone to cause fatigues in the structure. The root of a turbine blade is in undulation shape giving rise to the difficulties for inspection with conventional probes. This work proposes a novel flexible array eddy current testing (ECT) probe for the turbine blade root inspection. The proposed probe employs an in-plane differential scheme to reduce the common mode noise and increase the sensitivity to small defects. It works with three modes to improve the reliability of detecting defects in different orientations. It keeps the lift-off almost constant when scans the undulation shape structure, as the shape of the sensor array is flexible and is bended according to the sample surface. A prototype probe has been developed and tested by inspecting of a blade sample with artificial and nature defects. The probe can identify defect with dimensions length×wldth×depth=z.u mmxu.z mm×0.2 mm with various orientations and locations. The experiment results demonstrate that the probe has high sensitivity for defects at the root of turbine.