A generator rotor was retired from service due to a large transverse crack. It was clarified that the crack started at a surface defect placed at the root of a tooth, critical under the anomalous high flexural stresses induced by particular problems typically occurring at the beginning of rotor life. During the following years, the crack grew mainly during transients and not during operation; that was the reason why the crack could not be detected by on-line vibration monitoring for most of the time.
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