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

材料研究部授予北卡罗来纳州州立大学罗利的这一奖项是为了研究奥氏体和铁素体钢焊接接头在位移控制低周疲劳下的失效。应变随循环的逐渐积累被称为棘轮效应，被认为会导致无缺陷接头的意外失效。有了这个奖项，教授哈桑和Murty将研究焊接接头棘轮失效的奥氏体和铁素体钢管的微观结构过程。本研究将利用透射电子显微镜（TEM）对热影响区（HAZ）和母材的位错亚结构进行研究。TEM研究将在棘轮响应的各个阶段进行。将在光滑管状试样上进行一组系统的母材和焊缝金属的单轴和双轴循环载荷试验。这些测试光滑试样模拟微观结构和负载条件下，使其能够研究的HAZ金属的冶金过程中，以更可控的方式。此外，将努力开发一个数值方案，用于模拟棘轮响应的焊接接头使用宏观和多尺度的本构模型。数值方案将包括详细的残余应力计算，以研究其对两种钢棘轮失效机制的影响（奥氏体和铁素体）具有不同的晶体结构（FCC和BCC）。该项目更广泛的影响在于，从这项研究中获得的新知识可能揭示地震期间钢建筑和桥梁意外焊接接头失效的内在原因，以及化学、核、海上、造船、汽车和航空航天工业中的部件和结构。这种综合多尺度调查的结果将有助于改善所有这些行业的设计。由于这项研究将开发故障机制的基础知识，以及其模拟的数值方案，设计改进可以科学地实现设计方法的范式转变。学生和教师将通过参与拟议研究的实验研究，通过与小学和高中合作的工程学院计划实现教育影响。

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.