Very high cycle fatigue (VHCF) tests were carried out under variable amplitude loading for TC21 titanium alloy. The first level of high amplitude loading was set as 950 MPa close to yield strength, and the second level of low amplitude loading was determined between 435 MPa and 500 MPa where fatigue cracks initiated at the specimen subsurface under constant amplitude. The results indicate that the high/low stress block significantly reduced the cumulative fatigue life of low stress amplitude, and the fatigue crack initiation site changed from the specimen subsurface under constant loading to the specimen surface under stress block. Based on continuum damage mechanics, the fatigue damage model of two-step stress block was established to estimate the fatigue damage process. The prediction of cumulative fatigue life generally agreed with the experimental data. The cumulative fatigue damage of the stress block was related to the stress amplitude and the cycle ratio, which determined the stress fatigue damage and its interaction damage. The surface crack initiation in the stress block accelerated fatigue damage of low stress amplitude, reducing the cumulative life.

Low cycle fatigue (LCF) crack initiation, propagation and damage behaviors of TC21 alloy with basketweave microstructure were investigated. The process of LCF damage was observed by a long-focus optical microscopic imaging system, and fatigue crack propagation was analyzed through in-situ SEM fatigue. The results indicated that LCF crack damage displayed different sensitivity to cyclic stress. LCF microcracks initiated from slip bands and propagated through the microcrack coalescences at high stress, while LCF cracks tended to initiate at the α_L/β interface and connect with these interface microcracks. Furthermore, the LCF damage model was established on the basis of Lemaitre damage theory. When the maximum stress exceeded yield stress, LCF damage increased sharply and fatigue life decreased significantly, which agreed with experiment data.

Titanium alloys additively manufactured by electron beam melting (EBM) inevitably obtained some pore defects, which significantly reduced the very high cycle fatigue performance. An ultrasonic fatigue test was carried out on an EBM TC21 titanium alloy with hot isostatic pressing (HIP) and non-HIP treatment, and the effect of pore defects on the very high cycle fatigue (VHCF) behavior were investigated for the EBM TC21 titanium alloy. The results showed that the S-N curve of non-HIP specimens clearly had a tendency to decrease in very high cycle regimes, and HIP treatment significantly improved fatigue properties. Fatigue limits increased from 250 MPa for non-HIP specimens to 430 MPa for HIP ones. Very high cycle fatigue crack mainly initiated from the internal pore for EBM specimens, and a fine granular area (FGA) was observed at the crack initiation site in a very high cycle regime for both non-HIP and HIP specimens. ΔK_FGA had a constant trend in the range from 2.7 $\mathrm{MPa}\sqrt{\mathrm{m}}$ to 3.5 $\mathrm{MPa}\sqrt{\mathrm{m}}$, corresponding to the threshold stress intensity factor range for stable crack propagation. The effect of pore defects on the very high cycle fatigue limit was investigated based on the Murakami model. Furthermore, a fatigue indicator parameter (FIP) model based on pore defects was established to predict fatigue life for non-HIP and HIP specimens, which agreed with the experimental data.

The effect of low cycle fatigue (LCF) predamage with no precracks on very high cycle fatigue (VHCF) properties, and crack initiation characteristics for TC21 titanium alloy, was investigated. The results showed that LCF predamage with less than 5% of fatigue life had little influence on fatigue limit, but reduced its fatigue life. Fatigue cracks were initiated on the surface of the specimen at high stress amplitude, whereas fatigue cracks were initiated on the subsurface of the specimens at low stress amplitude. Based on Lemaitre damage theory, a very high/low cycle combined fatigue damage model was established to analyze the fatigue damage process, which was consistent with the experimental data. It was indicated that 5% LCF predamage value was the equivalent damage value, which was close to the critical value of VHCF crack initiation. The fatigue crack initiation of the specimens with LCF predamage less than 5% took up the major components of fatigue life.