Correlating Nonlinear Wave Response with Mesoscale Dislocation-Based Damage to Understand Fatigue Evolution

疲劳损伤会导致金属的纳米和微观结构变化，最终导致结构失效。虽然材料表征技术可以在非常小的范围内对这种损伤进行分类，但在损伤发生时和在大规模样品中测量这种变化仍然是一个挑战。该奖项支持通过研究非线性高频波如何在遭受疲劳损伤的金属中传播来应对这些挑战的基础研究。这一知识不仅将提高我们对疲劳等损伤如何演变的理解，还将为高频波在金属中的传播提供重要的洞察。由于许多结构和材料会经历疲劳，这将通过提高结构的安全性和效率而造福社会，例如用于机身、交通系统和能源基础设施的结构。该奖项还将通过一个综合的K-12推广计划，包括以项目为基础的本科生课程和研究生导师，支持学生在多个层次上进行关键工程技能的教育。该项目的目标是确定非线性高频波传播与中尺度位错损伤之间的关系。为此，非线性波测量技术和模型将与数字图像相关、电子背散射衍射和扫描电子显微镜相结合，以表征多晶面心立方材料的循环加载。基于位错的损伤的空间变化将被捕获并与非线性高频波测量相关联，并用于预测疲劳损伤。引入一种新的本构关系，将超声测量的声学非线性参数与通过数字图像相关测量的累积应变在细观尺度上捕捉基于位错的损伤相关联。最后，非线性超声将被用来表征样品整个疲劳寿命的位错参数，然后用于验证或更新多晶金属疲劳演化的细观力学模型。这一基本知识将揭示基于位错的损伤如何在中尺度上表现出来，这如何与疲劳演化有关，以及非线性高频波如何在空间不均匀的非线性弹性介质中传播。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Fatigue damage causes nano- and micro-structural changes in metals that ultimately cause structures to fail. While materials characterization techniques can classify this damage at very small scales, measuring such changes while damage is occurring and in large-scale samples remains a challenge. This award supports fundamental research to address these challenges by studying how nonlinear high-frequency waves propagate in metals undergoing fatigue damage. This knowledge will not only improve our understanding of how damage such as fatigue evolves, it will also provide important insight into how high-frequency waves propagate in metals. Because many structures and materials undergo fatigue, it will benefit society by improving the safety and efficiency of structures, such as those used in airframes, transportation systems, and energy infrastructure. This award will also support student education in critical engineering skills at multiple levels, through an integrated K-12 outreach program that incorporates a project-based undergraduate course and graduate student mentors.The objective of this project is to determine the relationship between nonlinear high-frequency wave propagation and dislocation-based damage at the mesoscale. To do this, nonlinear wave measurement techniques and models will be integrated with digital image correlation, electron backscatter diffraction, and scanning electron microscopy, to characterize cyclic loading of polycrystalline FCC material. Spatial variations in dislocation-based damage will be captured and correlated with nonlinear high-frequency wave measurements and used to predict fatigue damage. A new constitutive relationship will be introduced relating the acoustic nonlinearity parameter measured by ultrasound and accumulated strain that captures dislocation-based damage at the mesoscale measured with digital image correlation. Finally, nonlinear ultrasound will be used to spatially characterize dislocation parameters throughout the fatigue life of samples, and then used to validate or update micromechanical models for fatigue evolution in polycrystalline metal. This fundamental knowledge will uncover how dislocation-based damage manifests at the mesoscale, how this relates to fatigue evolution, as well as how nonlinear high-frequency waves propagate through spatially heterogeneous nonlinear elastic media.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.