Total Life Estimation of a Compressor Blade with Corrosion Pitting, SCC and Fatigue Cracking

Since compressors have components rotating at high speed, the most common cause of failure in compressors is their high cycle fatigue. In the present study, one of the rotating blades on the first row of an axial flow compressor was prematurely fractured. The crack position and direction were determined, using nondestructive tests. The results indicated that fatigue cracking initiated at two corrosion pits on the surface of the pressure side of the blade in response to corrosive compounds in the site atmosphere. The total life of the blade, from corrosion pit initiation to unstable fatigue crack growth, was calculated. The blade life had three stages: pit initiation and pit growth, pit transition to initial crack and stable crack growth reaching a critical value where the final fracture occurred. Pit growth and its transition to initial cracking were described. The time an equivalent pit requires to reach initial fatigue cracking was subsequently calculated. The number of cycles was calculated via two methods until initial cracking reached unstable growth. The first calculation was performed, using ZENCRACK fracture mechanics code. These results demonstrated that the rate of fatigue crack growth was at first slow and stable. After a period of time, the stress intensity factor reached a critical value where the crack began to propagate unstably. A MATLAB code was written based on Paris model as the second method for estimating fatigue life. The results obtained from drawing a comparison between these methods revealed that ZENCRACK crack shape simulation was in agreement with the fatigue beach marks.