Mechanisms for fatigue crack growth in meso/micro and nano specimens - crack initiation and short crack growth under geometrical and mechanical constraints

更广泛的研究背景/理论框架：几十年来，断裂力学的概念已经为人所知，并且在许多应用领域变得越来越重要，以优化零件和组件中材料的效率，这对于例如轻量化结构以节省资源和能源至关重要。尺寸效应是断裂力学中的一个固有的和众所周知的问题，甚至在宏观尺度上也是如此，这导致即使对于相应测试标准中定义的样品也有尺寸要求。当使用复杂的试样几何形状时，情况变得更加复杂，例如用于MEMS测试和/或在裂纹尖端前方存在非常大的塑料和工艺区，这通常是小规模断裂测试的情况。此外，例如在多层中发现的复杂微观结构可以通过机械和微观结构约束强烈影响裂纹的产生和扩展。这些影响和挑战过去没有详细讨论过，将是本提案的重点。假设/研究问题/目标：关于小尺度试样的断裂力学概念的适用性，必须如何考虑试样尺寸、边界条件（约束）、材料行为和不均匀性，以及微米尺寸试样（例如疲劳预裂纹）的测试是否有必要的预设？方针/方法：为了分析机械约束下的短裂纹扩展，我们不仅要改变试样尺寸，而且要通过电沉积设计具有不同晶粒尺寸的定制多层结构。我们提出了一个规模桥接调查。萨尔兰大学测试中微米尺寸的样品，Montanuniversität Leoben提供纳米和微米样品的测试。对于准静态和低周疲劳物理短裂纹，我们将改变试样尺寸、层系和晶粒尺寸。结合我们在微观和纳米力学方面的经验，我们将通过塑性区的位移和应变场来表征裂纹。独创性/创新水平：到目前为止，工作主要集中在理解静态尺寸效应的起源，但没有详细研究循环塑性和断裂的尺寸依赖性演化。 此外，从微观结构和机械约束所产生的基本机制的知识是至关重要的缺失。该项目的目的是获得一个更好的理解物理短裂纹的驱动力，无论是准静态和低周疲劳制度（LCF），在几何，微观结构和机械的边界条件和约束条件下，开发一个全面的模型疲劳和断裂的非均匀材料在微米尺寸的尺寸。

Wider research context / theoretical framework: Fracture mechanical concepts are known since many decades and become more and more important in many fields of applications to optimise the efficiency of materials in parts and components, which is essential for e.g. lightweight constructions to save resources and energy. Size effects are an intrinsic and commonly known problem in fracture mechanics even at the macroscopic scale which leads to size requirements even for the samples defined in the according test-standards. The situation becomes even more complicated when complex specimen geometries are used e.g. for testing of MEMS and/or comparably large plastic and process zones are present in front of the crack tip which is typically the case in small-scale fracture testing. In addition, a complex microstructure, found e.g. in multilayers, can strongly influence crack initiation and propagation through mechanical and microstructural constraints. These effects and challenges have not been addressed in past in detail and will be the focus of the current proposal. Hypotheses/research questions /objectives: How must specimen size, boundary conditions (constraints), material behaviour and inhomogeneities be considered regarding the applicability of the fracture mechanical concepts for small-scale specimens and are there necessary pre-sets for testing of micron-sized specimens (e.g. fatigue pre-cracks)? Approach/methods: To analyse short crack growth under mechanical constraints we will not only vary the specimen size, but also design tailored multilayer structures with varying grain sizes via electrodeposition. We propose a scale-bridging investigation. Meso to micron-sized samples are tested at the Saarland University, Montanuniversität Leoben provides testing of nano and micro samples. We will vary the sample size, the layer system and the grain size regarding quasi-static and LCF physically short cracks. Combining our experience in micro and nano-mechanics we will achieve a characterization of the crack via the displacements and strain fields in the plastic zone. Level of originality / innovation: Up to now, work has mostly focussed on understanding the origin of static size effects, but there is no detailed study of size dependent evolution of cyclic plasticity and fracture. Furthermore, knowledge of the underlying mechanisms arising from microstructural and mechanical constraints is crucially missing. The aim of this project is to gain an improved understanding of the driving force of physically short cracks, both quasi-statically and in the low cycle fatigue regime (LCF), under geometrical, microstructural and mechanical boundary conditions and constraints to develop a comprehensive model of fatigue and fracture in inhomogeneous materials in micron-sized dimensions.