The Use of an Advanced Automated Crack Monitoring System to Investigate High Temperature Fatigue Crack Growth in a Nickel-Based Superalloy

Abstract

This paper describes a new technique designed for testing surface short crack growth behaviour of metallic materials at elevated temperatures using a fully automated optical system. Short cracks as small as lOJlm initiated on smooth specimens have been detected and recorded both at room and elevated temperatures up to 700°C. The application of this computer controlled system offers a reliable, time-saving and cost­ effective method for the in situ study of the initiation and propagation of short cracks in materials subjected to both baseline and dwell fatigue loading sequences at high temperatures. Short crack growth behaviour has been studied in a Nickel-based superalloy. The crystallographic features of short crack growth and the effect of holding time at maximum load ('dwell' loading sequence) on cracking mode change from transgranular to intergranular at high temperature were investigated. 2. Experimental Technique An automated short crack monitoring and testing system for high temperature use has been developed with the support of Rolls-Royce. This system uses a personal computer to control the optics, the testing machine and data recording units through purpose written software. The monitoring of surface short crack growth was made possible by using a microscope with high magnification lens which accessed a split type furnace. Since the system is fully automated and can carry out the test continuously, it overcomes the difficulties associated with recording high temperature short fatigue crack growth by techniques such as replication. The system is capable of testing and monitoring the growth of freely initiated short cracks as small as 10 Jlm on a smooth specimen surface, and operating under baseline and complex (dwell) loading sequences at temperatures up to 700°C. Higher testing temperature is possible by using a larger furnace.