Characterization of Low-cycle Fatigue Damage in Inconel 718 by Laser Light Scanning

A technique for in situ laser light scanning (LLS) was developed to monitor surface damage on nickel-base superalloy specimens under low-cycle fatigue conditions. This technique characterizes the surface state with a parameter called the defect frequency which minimizes memory requirements and data processing time since it does not involve image processing. As a result, the present technique is capable of scanning speeds that are substantially greater than those achieved with image processing methods. Cylindrical Inconel 718 specimens were tested using an automated servo-hydraulic machine at ambient temperature under fully reversed strain control conditions for constant strain amplitudes ranging from 0.3% to 1%. The fatigue damage was monitored by scanning a laser beam along the gauge section of the specimens during periodic interruptions of the cyclic loading. Acetate replicas of the gauge section surface were also made on some of the specimens to characterize the damage using SEM and image analysis techniques. Comparisons of the results demonstrate the capabilities of the present light-scanning technique for characterizing fatigue damage on the surface of the Inconel 718 specimens. In particular, a rapid rise in the mean defect frequency is shown to correspond to an initial increase in microcrack density that saturates at approximately 20% of the fatigue life. This transient behavior is followed by a plateau in defect frequency which corresponds to crack propagation and interlinkage until failure occurs. The number of cycles to microcrack density saturation as indicated by the defect frequency is found to be linearly related to the number of cycles to failure. Accordingly, the present system provides a characterization of microcrack damage that may be used to predict the low-cycle fatigue life of Inconel 718 specimens long before failure occurs.