High Frequency Vibration Based Fatigue Testing of Developmental Alloys

Many fatigue test methods have been previously developed to rapidly evaluate fatigue behavior. This increased test speed can come at some expense; since these methods may require non-standard specimen geometry or increased facility and equipment capability. One such method, developed by George et al, involves a base-excited plate specimen driven into a high frequency bending resonant mode. This resonant mode is of sufficient frequency (typically 1200 to 1700 Hertz) to accumulate 107 cycles in a few hours. One of the main limitations of this test method is that fatigue cracking is almost certainly guaranteed to be surface initiated at regions of high stress. This brings into question the validity of the fatigue test results, as compared to more traditional uniaxial, smooth-bar testing, since high stresses are subjecting only a small volume to fatigue damage. This limitation also brings into question the suitability of this method to screen developmental alloys, should their initiation life be governed by subsurface flaws. However, if applicable, the rapid generation of fatigue data using this method would facilitate faster design iterations, identifying more quickly, material and manufacturing process deficiencies. The developmental alloy used in this study was a powder metallurgy boron-modified Ti-6A1-4V, a new alloy currently being considered for gas turbine engine fan blades. Plate specimens were subjected to fully reversed bending fatigue. Results are compared with existing data from commercially available Ti-6A1-4V using both vibration based and more traditional fatigue test methods