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In the recently accident-investigation report on blade failure, almost 70% of blade failureswas found at low pressure turbine blades, and it is wel known that main cause is due to the vibration modes. This paper describes the systematic approach on the root cause of the blade failure at L-0 stage, 30MW single flow industrial steam turbine which had tripped by high vibration after ten-month commercial operation. A fracture was found at the only one damping wire hole of 59 blades, and crack was detected at three damping wire holes by NDT. According to the analysis result for the crack fracture surface and the chain of the sequential operational events, we come to the conclusion that a typical high cycle fatigue is the most dominant factor caused to the blade failure, the resonance frequency margin was narrowed by the cut damping wire and the high cycle vibration was amplified, and then the blade was broken at once by the centrifugal force when the crack reached the critical size. 접수일 : 2007년 1월 26일, 채택확정 : 2007년 2월 10일김성봉(책임저자) : 한전KPS 기술연구원E-mail : sbkim2@kps.co.kr Tel 031-710-4393 김인철, 한승우, 전채홍 : 한전KPS 기술연구원