A Multiscale Study of Ratcheting Failure Mechanisms in Austenitic and Ferritic Steel Welded Joints

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This award by the Division of Materials Research to North Carolina State University Raleigh is to study failures in austenitic and ferritic steel welded joints subjected to displacement controlled low-cycle fatigue. The progressive accumulation of strain with cycle known as ratcheting is believed to result in unexpected failures of defect-free joints. With this award, Professors Hassan and Murty will investigate the micro-structural processes of welded joint ratcheting failures in both austenitic and ferritic steel pipes. This project research will perform transmission electronic microscopy (TEM) studies of dislocation substructures of the heat affected zone (HAZ) and base metals. The TEM studies will be carried out at various stages of ratcheting response. A systematic set of uniaxial and biaxial cyclic loading experiments of the base and weld metals will be performed on smooth-tubular specimens. These tests on smooth specimens with simulated microstructures and loading conditions make it feasible to study the metallurgical processes of the HAZ metal in a more controlled manner. In addition, efforts will be made to develop a numerical scheme for simulating the ratcheting responses of welded joints using both macroscale and multiscale based constitutive models. The numerical scheme will include detailed residual stress calculations for investigating its influence on ratcheting failure mechanisms for two types of steels (austenitic and ferritic) with varied crystal structures (FCC and BCC).The broader impacts of the project lie in the fact that the new knowledge gained from this research may reveal inherent reasons for the unexpected welded joint failures of steel buildings and bridges during earthquakes, and of components and structures in chemical, nuclear, offshore, shipbuilding, automotive and aerospace industries during regular operation. Outcomes from this integrated multiscale investigation will facilitate improving the designs for all these industries. Since the research will develop fundamental knowledge of failure mechanisms as well as numerical scheme for its simulation, the design improvement can be made scientifically towards bringing a paradigm shift in design methodologies. Students and teachers will realize the educational impacts through the College of Engineering program on partnerships with the primary and high schools by their participation in the experimental studies of the proposed research.