Researching stages of destruction of titanium alloys during high and gigacycle fatigue by analyzing morphology of fracture surfaces

Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surfaces. Specimens were loaded using a resonance fatigue machine Shimadzu USF-2000 in gigacycle regime and servohydraulic machine BISS bi-00-100 in high-cycle fatigue regime. Fine grain alloys demonstrated higher fatigue resistance for both HCF and gigacycle fatigue regimes. Fractured specimens were analyzed by interferometer microscope and SEM to improve methods of monitoring damage accumulation during fatigue test and verify the models of fatigue crack kinetics. Fatigue crack initiation occurred from the surface of the sample in high-cycle fatigue regime and bulk of material with a characteristic type of fracture - “fish-eye” in gigacycle fatigue regime. Quantitative fractography is an effective method for investigating the role of the original structural heterogeneity, the accumulation of defects in various scale levels (dislocation ensembles, micropores, microcracks) in the evaluation of the critical conditions for the transition from particulate to the macroscopic fracture. The description of the stages of this transition, including the initiation and propagation of cracks, is the basis for assessing the temporary resource products in terms of fatigue. High resolution profilometry (interferometer-profiler New View 5010) data of fracture surface roughness allowed estimating scale invariance (the Hurst exponent) and establishing the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ~300 µm has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest of fracture surface; and it is considered as the trace of the fatigue crack path corresponding to the Paris kinetics.