High Cycle Fatigue Threshold in the Presence of Naturally Initiated Small Surface Cracks

The high cycle fatigue (HCF) crack growth threshold of Ti-6Al-4V is investigated when naturally initiated small surface cracks of depths c = 16 μm to 370 μm are present. Small surface cracks are initiated in notched specimens using two different LCF loading histories, R = 0.1 and R = -1.0, at room temperature at a frequency of 10 Hz. Direct current potential difference (DCPD), infrared thermal imaging, and an electrochemical fatigue sensor (EFS) are used to detect crack initiation. Surface crack measurements are made using a scanning electron microscope prior to HCF testing. Heat tinting prior to HCF testing is used to mark the crack front to allow for post fracture crack measurements. HCF thresholds at R = 0.1 are determined for each specimen using step loading at room temperature and 600 Hz. Additionally, the HCF threshold is measured for some specimens that have been stress relief annealed (SRA) to eliminate residual stresses and load history. Long crack thresholds are determined using a similar step loading procedure for specimens which have been precracked using a range of values of K_max, including some that have been stress relief annealed prior to threshold testing. Results show that HCF threshold measurements, when naturally initiated small cracks are present, are dependent on the load histories that are used to initiate the cracks. Analysis shows that the measured small crack thresholds, which follow similar trends for load history effects which occur in the long crack threshold data, can be predicted from a simple model developed for long crack thresholds. Specimens precracked at R = -1.0 and SRA specimens precracked at R = 0.1 produce similar threshold values in this material, indicating that R = -1.0 under LCF has no effect on subsequent threshold under HCF.