In situ high energy X-ray diffraction microscopy (HEDM) provides new opportunities to identify deformation and failure mechanisms in polycrystalline materials, which are needed to advance computational modeling in this domain. In this talk, two case studies will be discussed in Ni-based superalloys and Ti alloys. First, multi-modal experimental techniques are presented to measure and track the complete micromechanical state, evolving during loading, of each and every grain within the regions of interest. Specifically, a combination of HEDM and digital image correlation coupled with electron backscatter diffraction are conducted on two specimens, Haynes 282 and Ti7Al. Higher resolution imaging is used for identification of the slip system activity and subsequently used to study slip transmission events in the Ti7Al specimen. An accurate knowledge of the resolved shear stress in adjacent grains (grain interactions) is demonstrated to be a key descriptor of the slip transmission events. Next, direct observation of the crack closure mechanism of a naturally occurring, tortuous, 3D microstructurally small fatigue crack (SFC) are presented based on HEDM experiments on an IN718 specimen. Specifically, the evolution of the stress state was analyzed for grains at the crack tip during cyclic loading and compared to observations of crack opening/closing events enabled by tomography. The mechanisms governing crack closure are discussed based on the associated grain-scale stress state.
Michael D. Sangid received his B.S. (2002) and M.S. (2005) in Mechanical Engineering from the University of Illinois at Urbana-Champaign (UIUC). After his Master’s degree, Dr. Sangid spent two years working in Indianapolis, IN for Rolls-Royce Corporation, specializing in material characterization, fatigue, fracture, and creep of high temperature aerospace materials before resuming his education in 2007. He received his PhD in Mechanical Engineering from UIUC in 2010 and continued as a post-doctoral associate. In the spring of 2012, Dr. Sangid started as an assistant professor at Purdue University in the School of Aeronautics and Astronautics with a courtesy appointment in Materials Engineering, where he continues his work on building computational materials models for failure of structural materials with experimental validation efforts focused at characterization of the stress/strain evolution at the microstructural scale during in situ loading. He is a recipient of the TMS Young Leaders Award, the ASME Orr Award, TMS Early Career Faculty Fellow, the NSF CAREER Award, and the AFOSR, ONR, and DARPA Young Investigator/Faculty Awards. He is currently serving as an editor of the International Journal of Fatigue. Dr. Sangid has started and serves as the Executive Director of the Hypersonics Advanced Manufacturing Technology Center as the first contract within the Purdue Applied Research Institute.