Fatigue precracking of a titanium alloy

The extensive use of Ti-6AI-4V forgings in aircraft gas engine compressor discs and blades has generated considerable interest in its crack growth behaviour at elevated temperatures. Before crack growth studies can be performed on test specimens it is necessary to precrack the specimen to provide consistently sharp fatigue cracks of adequate size and straightness. Initially the stress intensity amplitude used to grow a precrack is considerably higher than that used in the actual crack growth studies but it is progressively reduced as the required length of precrack is achieved. The reductions in stress intensity amplitude are usually made manually by observing the crack length as stress cycling proceeds and decreasing the stress amplitude on the test machine as the crack length grows. In this study, a method of performing this precracking operation automatically by the use of computer control of the test machine was developed, and tested. The results confirm the method used to be successful.