Fatigue Behavior Comparisons Between Ultrasonic and Servohydraulic Axial Testing Procedures

Abstract

An investigation of cycling rate effects on fatigue life behavior is being conducted on Aluminum (Al) alloys. This effect, along with specimen diameter, highlights the major difference between ultrasonic and servohydraulic fatigue test procedures. Ultrasonic fatigue testing is conducted on a 3.2 mm diameter hourglass specimen operating at 26 kHz, and servohydraulic axial testing is conducted on an ASTM E 466-07 standard dogbone specimen with a 25.4 mm gage length and 4.45 mm diameter operating at 35 Hz. Previous works have shown that cycling rate increases of 100–1000 times can reduce the fatigue crack growth rates in the stress intensity region between threshold and critical by at least an order of magnitude for aluminum and steel. For high cycle fatigue (HCF), however, where the majority of cycles to failure are accumulated before crack initiation, not during propagation, the effects of cycling rate on total loading cycles to failure needs further understanding, which may lead to more accurate and/or less conservative HCF design of critical gas turbine engine components. Fatigue behavior of Al 6061-T6 was assessed between the failure range of 104–1010 cycles using ultrasonic and servohydraulic testing procedures. Though aluminum is not a widely used alloy in gas turbine engine applications, understanding frequency-based fatigue life discrepancies associated with the choice of empirical methods is of paramount importance to component design; Aluminum 6061-T6 is the most cost-effective way to gaining this understanding. Comparisons were made between the fatigue behavior results using cycling rate (also stated as strain-rate or frequency) comparisons for fatigue crack growth study. The comparisons show promising results correlating the fatigue behavior trends of servhydraulic and ultrasonic fatigue data.