S3E1

The occurrence of premature creep failures in welded heat resistant steel components used for advanced fossil and nuclear power plants has caused significant structure integrity issues and property loss. It is essential to understand the underlying mechanisms responsible for the high-temperature fracture behavior and thus to enhance the performance of power plant systems. In this work, the constitutive models regrading both high-temperature deformation and fracture mechanics have been purposely developed. This allow us to investigate the roles of key micromechanical and microstructural factors, e.g., strength, grain boundary, grain morphology and orientation, and their interactions during high-temperature deformation, in the formation of voids governing the creep fracture behavior. In this talk, the dominated failure mechanism, i.e., creep-controlled or diffusion-controlled creep fracture, together with key results will be discussed to predict the creep rupture life.

A micromechanical model is developed to determine the failure strain of high temperature alloys, accounting for various governing deformation mechanisms at different length scales, including the nucleation of grain boundary cavities, their growth by competition of grain boundary diffusion and grain interior creep, viscous grain boundary sliding, and the emergence of microcracks by coalescence and their evolution to the ultimate failure. We will thus present a mechanistic analysis of the ductility at elevated temperatures, in commentary to the necking studies, which finds interesting applications in superplasticity, weldment failure, and forging limit analysis. Contrasts to ductility analysis at room temperature are also discussed.

Most shape-morphing strategies introduced so far rely on the compliant nature of the materials involved. Thus, the application of these strategies to the realization of load bearing structures presents challenges that might require a complete re-design of the system, and the selection of unusual, yet structurally-relevant materials. In this work, we explore the applicability of twisted bulk metallic glass (BMG) ribbons as building blocks for shape-changing structures. First, we analyze the finite-twisting mechanics of various ribbon geometries. We find that ribbons with undulated edges allow to better localize the deformation onto desired regions, without reaching stresses that would lead to failure. After selecting a suitable shape, we cut ribbons out of a BMG sheet and thermoform them above their glass-transition temperature, obtaining stress-free twisted ribbons. Finally, the packaging efficiency provided by this design is demonstrated by bonding multiple ribbon building blocks into tabletop-scale prototypes of deployable systems.

With the ever-increasing demand for the long-term reliability of graphene-based devices and structures, the fatigue behavior of graphene necessitates careful investigation. We enabled the intrinsic fatigue study of suspended two-dimensional (2D) materials based on a modified atomic force microscopy technique. We discovered that monolayer and few-layer graphene have a remarkable fatigue life of more than one billion cycles at large stress levels, which is higher than any materials reported to date. Unexpectedly, monolayer graphene did not reveal any obvious progressive damage during cyclic loading, which differs from macroscopic fatigue mechanisms. Despite the record-high intrinsic fatigue life, we also observed significant interfacial fatigue damage when introducing graphene-polymer contact. Cyclic loading through the interface resulted in the generation and propagation of graphene buckles, which was revealed to follow an inverse Paris’ law. Moreover, the weak vdW interfaces at the contact could also induce substantial fracture of graphene even in tens of cycles. These studies provide fundamental insights into the dynamic reliability of graphene.

Prof. Yanfei Gao teaches at the Department of Materials Science and Engineering, University of Tennessee. He received his BS in Engineering Mechanics and Dual BS in Computer Science from Tsinghua University in 1999, and PhD in Mechanical and Aerospace Engineering from Princeton University in 2003 (advised by Prof. Zhigang Suo, now at Harvard), followed by a post-doc experience at Brown University in 2003-2005. His group (https://gao.utk.edu) works mostly on mechanical behavior of materials, especially on bridging metallurgy and mechanics. Out of more than 150 journal papers, he published 19 articles in Acta Materialia and 15 in Journal of the Mechanics and Physics of Solids. He was the winner of Gold Medal in Hammer Throw in John Ma Campus Games, Tsinghua University (Spring 1999).

Paolo Celli is an Assistant Professor in the Department of Civil Engineering at Stony Brook University. Prior to joining SBU in January 2020, he was a postdoc at Caltech. There, he worked in the lab of Chiara Daraio and collaborated extensively with NASA JPL’s Materials Development and Manufacturing Technology Group. Trained as a mechanical engineer in Italy, he obtained his PhD in civil engineering from the University of Minnesota in 2017.

Paolo’s research interests are in experimental and computational aspects of solid mechanics, structural dynamics and smart structures.

Dr. Teng Cui is currently a postdoctoral fellow in the Department of Mechanical and Industrial Engineering at the University of Toronto, where he also received his M.Sc. and Ph.D. in 2016 and 2020 respectively. In 2014, he obtained his B.Eng. studying Engineering Mechanics (in Class Qian Weichang) from Nanjing University of Aeronautics and Astronautics, and he was granted the University Achievement Award and Presidential Special Award. His research interest lies in the intersection of solid mechanics, nanomaterials, and nanotechnology. He currently works on the nanomechanics of two-dimensional (2D) materials with a focus on the fatigue, fracture, and interfacial properties.