The Effect of Low Cycle Fatigue Cracks and Loading History on High Cycle Fatigue Threshold

Abstract

The high cycle fatigue (HCF) resistance of Ti-6Al-4V for gas turbine engine pplications is studied when the material is first subjected to low cycle fatigue (LCF). The high cycle fatigue (HCF) threshold is determined after small LCF surface cracks are formed in notch tension specimens. LCF loading at two stress ratios, R = 0.1 and R -1.0, is used to initiate the LCF cracks, which are detected using direct current potential difference (DCPD). The surface crack sizes are measured under load using a static loading fixture and a scanning electron microscope (SEM). In addition to the SEM surface measurements, heat tinting is used to mark the crack profiles before HCF testing so that fractography can be used after failure to measure the 2D crack geometry. The LCF surface-cracked specimens are tested at room temperature in lab air at 600 Hz using a step-loading procedure at two stress ratios, R = 0.1 and R = 0.5. The LCF loading history is found to affect the HCF threshold compared to what is predicted from long crack threshold values obtained from other crack geometries. Variations in HCF crack growth thresholds obtained on specimens with LCF crack sizes from 25 to 175 μm are attributed to overload and underload effects from the LCF precracking.