Scattered light scanning for fatigue damage precursor detection on turbine components

A laser scanning technique was developed to monitor in situ surface damage on nickel based superalloy turbine components under low cycle fatigue conditions. This technique characterizes a surface state using a discontinuity frequency parameter that was designed to minimize memory requirements and data processing time. As a result, the present technique is capable of scanning speeds that are substantially greater than those achieved with image processing methods. Sections of turbine rotors made of a nickel based superalloy were tested using a servohydraulic machine at ambient temperatures under load control conditions. The fatigue damage was monitored by scanning a laser beam along the rotor section in situ and during periodic interruptions of the cyclic loading. Acetate replicas of the gage section surface were also made to examine the surface morphology using scanning electron microscopy. Comparisons of the results demonstrate the capabilities of the present light scanning technique for characterizing fatigue damage precursors on the surface of turbine components. In particular, a rapid rise in the mean discontinuity frequency is shown to correspond to surface relief features that correspond to grain boundaries that intersect the surface in the areas of greatest stress. The presence of this surface relief can be attributed to the presence of relatively soft precipitate free zones along the grain boundaries that preferentially deform under fatigue loading conditions, leading to the formation of microcracks.