High-Frequency Fretting Fatigue Experiments

The fretting problem is of particular interest to the damage tolerant design of turbine blades in today's gas turbine engines. The exotic environment, high-frequency, and variable amplitude load history associated with the dovetail blade/disk connection create a critical location for fretting induced crack nucleation. With little work having been done on investigating fretting contact behavior at high-frequencies and variable amplitude load spectra, sufficient impetus has been generated to better characterize these two currently ambiguous fretting factors. The threat of early crack nucleation and propagation due to these fretting conditions has led to several major research efforts aimed at explicating the high cycle fatigue (HCF) and low cycle fatigue (LCF) interaction and behavior of advanced materials used in modern aircraft turbomachinery. As a part of this effort, a well-characterized experimental setup has been constructed to aid the observation and analysis of the aforementioned frequency and loading factors in fretting. A detailed description of the designed high-frequency fretting rig is presented. Significant vibration and bending results observed during high frequency operation suggest further design modification for improved specimen and pad alignment. Preliminary experimental observations illustrate crack nucleation and failure in specimens subjected to operational frequencies between 100 and 350 Hz. A stress invariant equivalent stress life model is employed for comparison of predicted and observed experimental crack nucleations. The paper concludes with suggested future work aimed at experimentally explicating the frequency and variable amplitude factor effects in fretting fatigue.