Experimental Investigation of Nodal Diameters of an Integrated Bladed Disk

This paper is presented how to find nodal diameters of an integrated bladed disk by experimental modal analysis (EMA) method. The case study is an integrated bladed disk with 29 blades and is made of Aluminum alloy (Al2618) and is used as first stage of compressor of gas turbine. Modal impact hammer test with 58th DOF geometry has been chosen for this case. The impulse excitation has applied on 58 points and the responses measured just for three points by miniature accelerometers, as reference points. In order to have a free-free condition for the test, the bladed disk has put on a soft foam. Natural frequencies up to 6 kHz were measured and nodal diameters were analyzed. Natural frequency, damping ratio and mode shape of each mode have been obtained by N-modal software. The results of numerical analysis and Campbell diagram have been obtained but not presented in this paper. The numerical analysis results of nodal diameters which were verified by experimental test data, are essential for further designs to improve high cycle fatigue and vibrations characteristics of the disk in order to prevent efficiency reduction and achieve lower risks of the failure. A simple and novel procedure is presented to determine the nodal diameters corresponding to the natural modes extracted from the experimental data.