What types of non-destructive testing can be performed on large hydro-generator rotors to detect subsurface cracks?

Several non-destructive testing (NDT) methods can be used to detect subsurface cracks in large hydro-generator rotors, without damaging the rotor. These methods include ultrasonic testing (UT), magnetic particle testing (MPT), and eddy current testing (ECT). Ultrasonic testing uses high-frequency sound waves to detect internal flaws. A transducer emits sound waves that travel through the rotor material and are reflected back to the transducer. Cracks or other discontinuities will reflect the sound waves differently than the surrounding material, allowing them to be detected. Magnetic particle testing is used to detect surface and near-surface cracks in ferromagnetic materials. The rotor is magnetized, and then fine magnetic particles are applied to the surface. Cracks will disrupt the magnetic field, causing the magnetic particles to accumulate at the crack location, making it visible. Eddy current testing uses electromagnetic induction to detect surface and near-surface flaws. An alternating current is passed through a coil, which generates an electromagnetic field. When the coil is brought near the rotor surface, eddy currents are induced in the material. Cracks will disrupt the flow of eddy currents, which can be detected by the coil. For large rotors, phased array ultrasonic testing (PAUT) is often used, as it provides a more detailed image of the subsurface. The choice of NDT method depends on the type of material, the size and orientation of the cracks being sought, and the accessibility of the rotor surface. Each method requires trained personnel and specialized equipment to perform and interpret the results accurately.