Application of modified Locati method in fatigue strength testing of a turbo compressor blade
Sanjin Braut, Marina Tevčić, Mirko Butković, Željko Božić, Roberto Žigulić
Abstract
The blades of axial turbomachines (steam and gas turbines, axial compressors) are highly stressed by centrifugal force, temperature, and alternating fluid forces, which leads to vibrations and fatigue cracking. This requires the studious engagement of designers and technologists. In this paper, a procedure for accelerated fatigue testing of a gas turbine compressor blade based on the modified Locati method is presented. The tests were performed on a vibration shaker with the frequency that equal the blade’s first natural frequency. S-N curves were estimated according to two approximate representation namely one-slope and two-slope models. Then, the blades were tested at different stress levels starting at a level below foreseen fatigue strength at the specified fatigue life. In each loading block, a blade was subjected to 2ˑ107 cycles. After the first block was completed, the stress level in the second loading block was increased by 20 MPa and a new 2ˑ107 cycles were applied. A procedure was repeated until the blade failure occurred. It was considered that a blade has failed when its natural frequency drops by more than 2%. After the failure, the fatigue strength was calculated using Palmgren -Miner rule.