Microstructure-Sensitive Investigations of Fatigue of Magnesium Alloys

材料疲劳是一个重要的工程科学课题，因为它是从航空航天到汽车、海洋和工业领域中观察到的机械故障的主要原因之一。该奖项支持基础研究，以提供必要的知识，以了解疲劳损伤的孕育、启动和演变。这项研究特别令人感兴趣的是镁的情况，因为它的合金具有可以在减轻重量和节约能源方面取得重大进展的特性。因此，这项研究的结果有望造福美国经济和社会，因为它有可能改善这种非常轻的金属合金的使用。这项研究的更广泛影响针对的是学术代表性不足的群体参与STEM研究和培训。具体地说，费城社区学院的学生将被选中与调查小组一起进行研究，调查小组利用一个全面的外联计划，重点是使用多尺度力学和基于模拟的工程。费城社区学院是该市最大的公共机构，人口以少数族裔为主。该奖项支持的研究方法是基于多尺度实验力学和基于物理的计算建模的使用。具体地说，识别疲劳损伤的可靠先兆的问题是通过在一个尺度上进行实验来解决的，在该尺度上，晶级信息是重要的，并且与诸如变形等力学领域以及与包括孪生-去孪晶、位错活动、开裂及其相互作用在内的微观结构参数及其演变相关联。在颗粒尺度上的实验测量将包括全场光学和声学无损数据集，这些数据集可以与在宏观尺度上进行的机械行为和类似的无损测量直接相关。这些实验信息可以为宏观上观察到的疲劳行为效应提供微观结构敏感的解释，包括塑性各向异性、拉压不对称和变形条带。实验信息还将被用来形成新的计算程序，能够模拟疲劳引起的材料微观结构的变化。具体地说，分子动力学将被用来研究单晶水平上的孪晶成核和增厚。此外，晶体塑性模拟将用于模拟空间分辨的孪晶及其与滑移系和晶界的相互作用，以验证在镁多晶中所做的观察。此外，晶体塑性和连续相场模型将被用来模拟实验观察到的和疲劳诱导的与损伤起始相关的局部应变形成。

Fatigue of materials is an important engineering science topic since it is one of the predominant causes of mechanical failure observed in applications ranging from aerospace to automotive, marine and industrial fields. This award supports fundamental research to provide needed knowledge to understand fatigue damage incubation, initiation and evolution. Of particular interest in this research is the case of magnesium since its alloys have properties that could lead to significant advances in weight reduction and energy savings. Therefore, results of this research are expected to benefit the U.S. economy and society as it has the potential to improve the use of this very light metal alloys. The broader impact of this research targets the participation of academically underrepresented groups in STEM research and training. Specifically, students in the Community College of Philadelphia, which is the largest public institution in the city with a predominantly minority-based population, will be selected to conduct research with the investigators' group leveraging a comprehensive outreach plan focusing on the use of multiscale mechanics and simulation-based engineering. The research approach supported by this award is based on the use of multiscale experimental mechanics coupled with physics-based computational modeling. Specifically, the problem of identifying reliable precursors to fatigue damage is addressed by performing experiments at a scale at which grain level information is important and is linked to mechanical fields such as deformation, as well as to microstructural parameters and their evolution including twinning-detwinning, dislocation activity, cracking and their interactions. Experimental measurements at the grain scale will include full field optical as well as acoustic nondestructive datasets, which can be directly correlated with the mechanical behavior and similar nondestructive measurements made at the macroscale. Such experimental information could provide a microstructure-sensitive explanation of macroscopically observed fatigue behavior effects including plastic anisotropy, tension-compression asymmetry and deformation banding. The experimental information will be also used to form novel computational procedures capable to model fatigue-induced changes in the material microstructure. Specifically, molecular dynamics will be used to study twin nucleation and thickening at the single crystal level. Furthermore, crystal plasticity simulations will be used to simulate spatially resolved twins and their interactions with slip systems and grain boundaries to validate observations made in magnesium polycrystals. Furthermore, crystal plasticity and continuum phase field models will be used to model experimentally observed and fatigue-induced localized strain formations which are related to damage initiation.