Integrated study of damage after overloads in single crystals

金属损伤预测是工程师们几个世纪以来面临的重大挑战。这个问题的复杂性来自于多尺度损伤机制的相互作用。例如，远离微观缺陷施加的力可以促进原子尺度缺陷的产生。在这两者之间，中尺度属性（例如，大多数建模方法没有明确考虑所有这些长度尺度，它们依赖于经验公式和宏观实验之间的唯象相关性。这种方法对于用于校准的实验的测试条件内的预测是可靠的，但是不确定性在这些条件之外是不受约束的。这种预测能力的缺乏在关键应用中是有问题的，在这些应用中，机械响应应该在大量的工作条件下得到保证，远远大于材料认证所需的典型实验。为了减轻建模的不确定性，领先的科学家[1-3]提出了在各种长度尺度下验证的多尺度方法。这些方法增加的一个关键价值依赖于较小尺度下损伤对载荷条件的较低依赖性（例如，两个位错之间的相互作用的强度不受影响）。位错结构（例如，细胞和持久滑移带（PSB））处于对载荷条件敏感的边缘。许多研究[4]表明，中尺度结构（例如，1微米大小）控制疲劳载荷下的应力-应变响应。过载后，响应取决于介观结构的变化和晶体学取向。一个关键的属性是，只有少数位错结构是晶体学稳定的，而同样的结构出现在无数的金属上。因此，我们认为，各种单晶和多晶的应力应变响应后，一个未知的加载序列可以绑定的模型，探测所有可能的稳定结构。此外，我们假设意外事件后的瞬态行为可以通过稳定结构之间的参数化过渡来重现。本研究建议通过整合中尺度力学测试和计算建模来减轻评估过载的不确定性。在实验工作方面，我们将使用克兰菲尔德大学专有的中尺度加工平台制造中尺度单晶样品。我们将测量FCC单晶和多晶试样的应力应变响应的各种尺寸（50至500微米）后，过载与不同的强度和模式。此外，我们将表征超载后的介观位错结构，以确定结构的演变及其在介观尺度上的形态。在建模方面，我们将运用PI [5]最近开发的晶体塑性模型，该模型是循环加载本构模型的最新技术，并已被Ni，Cu，和不锈钢。我们将提出一个基于物理学的结构演化来解释和约束过载后单晶和多晶的应力应变响应。我们将匹配过载的情况下，与他们的中尺度结构，独立预测的机械响应。最后，我们将通过比较晶体学Fatemi-Socie疲劳指示参数（FIP）来量化过载对微观结构小疲劳裂纹的作用。[1]Bo供稿：Jiang，Dunne. J. Mech. Phys. Sol. 106（2017）：15-33. [2]Sweeney，Vorster，Leen，et al. J. Mech. Phys. of Sol. 615（2013）：1224-40. [3]Zhu，Basoalto，Warnken和Reed。Acta Mat. 60，12（2012）：4888-4900。[4]Li，Li，Wang，Zhang，Prog. Mater. Sci. 56（2011）：328-377。[5]Castelluccio，McDowell，Int. J. Plast. 98（2017）1-26。

Damage prognosis in metals is a grand challenge that engineers have faced for centuries. The complexity of this problem arises from the interaction of damage mechanisms at multiple scales. For example, a force applied far from a microscopic imperfection can promote the production of defects at the atomic scale. In between, mesoscale attributes (e.g., heterogeneous dislocation density, grain morphology, texture) regulate the exchange of damage across micro- and macro-scales.Most modelling approaches do not explicitly consider all these length scales and they rely on phenomenological correlations between empirical formulations and macroscopic experiments. This approach is reliable for predictions within the testing conditions of the experiments used for calibration, but the uncertainty is unbound outside these conditions. This lack of predicting power is problematic in critical applications in which the mechanical response should be guaranteed for a large number of working conditions, much larger than the typical experiments required for material certification.To mitigate modelling uncertainty, leading scientists [1-3] have proposed multiscale approaches validated at various length scales. A key value added by these approaches relies on the lower dependence of damage on loading conditions at smaller scales (e.g., the strength of the interaction between two dislocations is unnafected). Dislocation structures (e.g., cells and persistent slip bands (PSBs)) lay on the verge of sensitivity to loading conditions. Much research [4] has shown that mesoscale structures (e.g., 1 micron in size) control the stress-strain response under fatigue loading. After an overload, the response depends on the changes of the mesoscale structures and on the crystallographic orientation. A key attribute is that only a few dislocation structures are thermodynamically stable and the same structures arise on myriads of metals. Hence, we argue that the stress-strain response of various single- and poly-crystals after an unknown loading sequence can be bound by models that probe all possible stable structures. Moreover, we hypothesize that the transient behaviour after unexpected events can be reproduced with a parameterized transition between stable structures.This research proposes to mitigate the uncertainty of assessing overloads by integrating mesoscale mechanical tests and computational modelling. In terms of experimental effort, we will manufacture mesoscale single crystal specimens using Cranfield University proprietary Mesoscale Machining Platform. We will measure the stress-strain response of FCC single- and poly-crystalline specimens of various sizes (50 to 500 microns) after overloads with different intensity and patterns. Additionally, we will characterize mesoscale dislocation structures after overloads in order to identify the evolution of the structures and their morphology at the mesoscale.In terms of modelling, we will exercise the crystal plasticity model recently developed by the PI [5], which is the state of the art in constitutive models for cyclic loading and has been extensively validated with single- and poly-crystal data for Ni, Cu, and stainless steel. We will propose a physics-based evolution of structures to explain and bind the stress-strain response of single- and poly-crystals after overloads. We will match overload scenarios with their mesoscale structures to predict independently the mechanical response. Finally, we will quantify the role of overloads on microstructurally small fatigue cracks by comparing the crystallographic Fatemi-Socie fatigue indicator parameter (FIP).[1]Bo, Jiang, Dunne. J. Mech. Phys. Sol. 106 (2017):15-33. [2]Sweeney, Vorster, Leen, et al. J. Mech. Phys. of Sol. 61 5 (2013):1224-40. [3]Zhu, Basoalto, Warnken, and Reed. Acta Mat. 60, 12 (2012):4888-4900. [4]Li, Li, Wang, Zhang, Prog. Mater. Sci. 56 (2011):328-377.[5]Castelluccio, McDowell, Int. J. Plast. 98 (2017) 1-26.