T he U.S. Air Force operates and maintains approximately 24,000 turbine engines, and many components in these engines are limited by fatigue life. Current strategies for estimating engine component lifetimes gener- ally rely on extrapolating mean fatigue lifetime behavior from extensive experimental data- bases. Although these extrapolations may yield overly conservative predictions [1-3] , such strate- gies are critical for developing preventive main- tenance schedules that ensure safety at an affordable cost. Significant savings may be real- ized by further developing and implementing microstructurally based mechanistic models of fatigue behavior [2-4] . Fatigue behavior is weak link Fatigue behavior is known to be a weak-link process significantly influenced by local mi- crostructural configurations [5,6] . Typically, lab- oratory scale tests are used to determine a material’s fatigue capability because these tests are relatively inexpensive and easily run to fail- ure. Fractography is often performed after specimen failure to determine which mi- crostructural features are associated with fa- tigue crack initiation and growth. These tests provide lifetime-to-fatigue failure data under specific loading conditions and microstructural configurations. Often, it is impossible to unambiguously identify the mechanism of fatigue crack initia- tion and cyclic damage accumulation through post-mortem investigations, as many different microstructural configurations can lead to damage accumulation and subsequent crack initiation [6] . Fatigue properties of titanium alloy microstructures are of particular interest be- cause of their use in critical aircraft and engine aerospace components. Aerospace titanium alloys usually consist of two phases at service temperatures. The first phase (beta) has a bcc structure and is more ductile, with lower strength, than the second phase (alpha). The alpha phase is an hcp struc- ture with either lath-like or equiaxed grains, de- pending on the thermomechanical processing of the material. In laboratory scale fatigue tests of alpha + beta titanium alloys, fatigue cracks typically initiate via a facet formation on the basal plane of primary alpha (α p ) grains [6-10] . The microstructural neighborhood surround- ing these grains also plays an important role in the crack initiation process, and specific attrib- utes of these neighborhoods were identified. However, there are numerous paths for cyclic strain to accumulate and various types of mi- crostructural neighborhoods may lead to simi- lar fatigue lifetimes [6] . Traditionally, researchers use in situ testing methods to characterize mechanisms of damage accumulation due to cyclic loading, and these studies have historically been limited to contin- uum length scales. Advances in micromachining and small-scale testing capabilities [11,12] enable in- situ mechanical testing at the microstructural scale [13-16] . Other advances facilitated quantitative 3D analysis of microstructures, including high- fidelity crystallographic scale and orientation information [17-19] . The current work describes modifications of an in-situ monotonic mechani- cal testing device [20,21] to enable in situ microscale fatigue testing [22] , coupled with subsequent serial sectioning methods to interrogate tested sam- ples. The long term goal of this work is to develop a fatigue damage metric capable of representing and predicting fatigue damage accumulation within specific microstructural neighborhoods using a crystal plasticity finite element modeling (CPFEM) approach. Development of a Microscale Fatigue Testing Technique C.J. Szczepanski P.A. Shade M.A. Groeber J.M. Larsen, FASM* U.S. Air Force Research Laboratory, AFRL/RX Wright-Patterson Air Force Base, Ohio S.K. Jha Universal Technology Corp. Dayton, Ohio R. Wheeler UES Inc. Dayton, Ohio A microscale fatigue testing technique for characterizing mechanical response — and relating this response to microstructural features contained within the tested volume — enables more accurate approaches to predicting mechanical behavior in larger fatigue volumes. ADVANCED MATERIALS & PROCESSES • JUNE 2013 18 *Member of ASM International Fig. 1 — A schematic of the microtesting rig used for in-situ testing. Adapted from [20] . Piezoelectric actuator Alignment flexor Attocube x/y/z 50 mm piezoelectric Sample positioning stage 50 mm Load cell Silicon grip 150 mm