Demonstration of an in situ microscale fatigue testing technique on a titanium alloy

Under high cycle and very high cycle fatigue (HCF and VHCF) conditions, scatter in fatigue lifetimes is substantial; often 2–3 orders of magnitude. Characterization of fatigue crack initiation sites in laboratory scale fatigue specimens has led to the identification of characteristic initiation sites and microstructural arrangements. Despite these observations, in some cases, it is still unclear how apparently similar initiation sites exhibit such different total fatigue lifetimes. Differences in crack-initiation mechanisms can be further revealed if specific microstructural arrangements are isolated within a micro-specimen. Towards this end, an in situ microscale tension testing technique was adapted to enable microscale fatigue testing on tensile dog-bone specimens. Microscale tensile fatigue specimens with approximate gage diameters of 20μm were prepared with a focused ion beam (FIB) microscope. Initial tensile experiments were conducted to characterize the mechanical behavior of microscale specimens for this microstructure. The microscale tensile specimens were observed to exhibit reduced yield and flow stresses in comparison to bulk tensile specimens. However, in this feasibility demonstration of in situ microscale fatigue testing, microscale specimens exhibit enhanced fatigue properties relative to conventional specimens, which may result from the differences in strain rate and, potentially, the method of test control (load vs. displacement) between these two testing methods. Both tensile and fatigue specimens have been characterized with electron backscatter diffraction to identify the neighborhood and the specific slip systems that were activated under local deformation conditions within the microstructure. Both basal and prism slip were observed, although prism slip was more prevalent. Results of the tension and fatigue experiments are discussed in the context of the growing body of literature on microscale testing, and avenues for future work are highlighted.