Analysis of multiple gas turbine failures due to high order (6e) resonant vibrations
The results and conclusions are presented of a study concerning the durability problems experienced with gas turbine engines at a processing plant. The investigation encompassed the design and failure history of hot gas path components in the engines, focusing upon the power turbine (PT) blade failures responsible for repeated wrecks. Two of the wrecks followed shortly after replacement of bushings in the power turbine variable vanes. The root causes of these PT blade failures were resonant vibration, PT variable vane alignment and, in at least two instances, fabrication defects. Resonant blade vibration occurred in a sixth-order harmonic (6e) when operating at 4650 rpm, the mode shape being a six-nodal diameter coupled blade-disk bending of the blades predominantly in the axial direction. The dominant forcing function was due to misalignment of the 24 variable inlet vanes immediately upstream from the PT blades because of deviations in adjustable angle from the desired average setting. High amplitude vibratory bending stresses were predicted analytically for variations in adjustable static vane angles. The resulting stress amplitude was sufficient to propagate fatigue cracks from pre-existing fabrication defects (cracks) at corners of the PT blade root seal plate locating holes. It was also determined that high cycle fatigue cracks could be initiated in the trailing edges of the blades just above the airfoil-to-root platform fillet radius. From fractographic observations of the failed root sections it was possible to identify the size of cracks that initiated failures and to estimate growth rates of the cracks. Calculations of stress intensity factor and observations of the fracture surfaces confirmed the presence of a high vibratory stress and accelerated crack growth rate under resonant conditions. Furthermore, the fracture surface topologies indicated only three or four brief periods of such rapid crack growth, totalling less than an hour. The small number of crack growth periods and the short times involved at the 6e frequency of 465 Hz indicate that specific changes of vane adjustment combined with other engine operating parameters were responsible for achieving an abnormally high level of blade excitation at this frequency.