CAREER: Understanding the Interaction between Mechanical Twinning and Fatigue Crack Growth

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This Faculty Early Career Development (CAREER) project will investigate the influence of a submicron microstructural feature called mechanical twin on enhancing the fatigue crack growth resistance in metals. Cyclic mechanical loading causing crack formation, known as fatigue, remains a common danger to structural metals in service. Mechanical twinning is an efficient means for intrinsically strengthening and increasing ductility in metals, but its effect on crack growth remains unclear due to its lack of formation during the typical low loads of high cycle fatigue. Insight gained from this study will promote the design of advanced, fatigue resistant materials supporting aggressive design strategies to improve energy and material efficiency. Intelligently modifying the microstructure of established, qualified metals will lead to accelerated deployment of enhanced fracture resistant materials in comparison to the cost and time investment required for new material development. In addition to the technical advances made, this award will also provide educational experiences for K-12, undergraduate, and graduate students. Graduate and undergraduate students will be intimately involved in integrated material science and mechanical engineering research. Educational modules will be incorporated into outreach for K-12 classrooms and girls’ summer camps to increase students’ science self-efficacy. Additionally, workshops will be held for students with little social capital, such as first generation and underrepresented students, to help them navigate and apply for fellowship and research opportunities.Fatigue crack growth, particularly in Stage I, is highly dependent on dislocation-microstructure interactions. Research has shown that annealing twin boundaries enhance the slip reversibility, especially in nanocrystalline alloys, but it is unclear how bundles of mechanical twins affect fatigue crack growth. Due to the localized nature and lower than yield stresses applied during high cycle fatigue, the critical resolved twinning stress to nucleate twins is typically not met. The research in this CAREER project will quantify the magnitude of slip irreversibility at pre-populated deformation twin boundaries during Stage I, high cycle fatigue crack growth to enhance the understanding of microstructure-based crack growth resistance. Experimental quantification of crack-microstructure interactions will be measured through high resolution digital image correlation and electron microscopy. Knowledge generated about the role of twin boundaries, twin density, and dislocation density will serve to connect information gathered from atomistic simulations to established mesoscale frameworks to support fatigue resistant material design.This project is jointly funded by the Division of Civil, Mechanical and Manufacturing Innovation (CMMI) and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.