CAREER: A Micromechanics-Based Approach to Ductile Fracture Simulation in Additively Manufactured Steels for Seismic Structural Fuse Design

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This Faculty Early Career Development Program (CAREER) award will advance knowledge and innovation to improve the seismic performance of the nation's civil infrastructure through the development of a micro-mechanics based framework for ductile fracture prediction in additively manufactured (AM) steels. Emerging AM technologies, such as 3-D metal printing, are promising for performance-based optimization of seismic steel systems, as they can accommodate highly irregular component designs through highly controlled weld-free geometry formation. In order for AM steel parts to transition to functioning components in seismic structural systems, the ability to predict damage limit states, such as ductile fracture and low-cycle fatigue, is needed. Existing ductile fracture models lack the ability to accurately capture fracture processes in AM steel alloys due to the complex micro features formed during fabrication. This research will develop an innovative framework for upscaling micro-scale material measurements in AM steel alloys to predict macro-scale behavior in seismic structural fuse components. These micro-scale measurements and up-scaling framework can lead to the creation of a hybrid analysis-AM framework, allowing iteration and optimization of seismic structural fuse performance through probabilistic fracture predictions prior to fabrication. The capability to optimize the seismic performance of steel buildings through AM structural fuse design will promote national welfare and prosperity through safer and more resilient and sustainable building construction to better protect life and property during earthquakes and to maintain essential services and business continuities during response and recovery. Integral with this research are an innovative middle school outreach program and a graduate-level international research collaboration with the Swiss Federal Institute of Technology. The middle school outreach, in the form of engineering songwriting workshops, will couple music education with science, technology, engineering, and math (STEM) curricula. Termed STEMusic, the outreach plan aims to promote creativity, understanding, and retention of engineering principles through the engagement of alternative cognitive processes. The international research collaboration will include the exchange of graduate students. The objective of this CAREER award is to test the hypothesis that measured micromechanical material behavior can be scaled to accurately predict macroscale ductile fracture in steel alloys created through common AM processes such as selective laser melting. In the research, modern technologies such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and modified nano-indentation will be used to measure the fundamental localization processes driving ductile fracture in AM steels and create a new simulation framework to allow the future creation of more generalizable fracture models. Steel specimens approximately 0.002 millimeters in diameter will be fabricated using focused ion beam milling and mechanically tested using a modified in-SEM nano-indentation device. The measured micromechanical behavior, coupled with TEM spatial characterizations of the fracture surface microstructure, will be used to inform statistical volume element simulations for upscaling to larger material volumes. This micro-mechanics based framework will be used to investigate the ductile fracture performance of free-form structural fuse geometries (geometries created using AM processes). Potential impacts of this award outside the described application to seismic design include advances in materials engineering and AM fabrication. The micromechanical experiments have the potential to provide fundamental insights into the effects of underlying material morphology and chemistry on macro-mechanical AM steel alloy response, allowing material response (e.g., fracture, yielding, and deformation) to be designed into the AM geometry creation process. Coupling this within the field of nonlinear topology optimization, the framework to be developed will be essential for deriving optimum design solutions that satisfy complex performance criteria involving high plastic strains that lead to fracture or fatigue under various complex loadings.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.