Cyclic deformation and fatigue of polycrystalline Cu under pure compressive cyclic loading condition

本项目的目的是研究和表征工程结构材料在纯压缩疲劳载荷条件下的循环变形响应和疲劳裂纹萌生。 疲劳断裂是工程构件最常见的失效形式。实际中观察到的疲劳破坏大多经历了三个阶段，即裂纹萌生、裂纹扩展和最终断裂。因此，防止裂纹萌生被认为是提高工程材料疲劳强度的重要途径之一。影响疲劳裂纹萌生的因素往往是多方面的，包括应力状态、材料性能和使用条件。结果表明，绝大多数疲劳裂纹的萌生是由拉伸或混合应力引起的。因此，以往对循环变形和疲劳断裂的研究大多是在拉伸或多向载荷条件下进行的。工程材料在纯压缩疲劳载荷作用下的循环变形和疲劳断裂还没有系统的研究，尽管这种失效形式确实发生在关键的工程部件上，如飞机起落架的疲劳断裂。因此，为填补知识库的空白，提出了本项目。研究结果将有助于了解材料在纯压缩载荷条件下循环变形和疲劳破坏的基本机理，并为提高疲劳寿命的工业应用提供更好的途径。 在此之前，本课题组研究了纯压循环载荷作用下所选钢材的循环变形和疲劳响应。研究发现，即使施加的疲劳载荷是完全压缩的，只要有一个非常适中的应力集中部位，仍然可以形成疲劳裂纹。实际上，定义了一个以冯·米塞斯应力表示的裂纹萌生的临界条件。对于无任何应力集中部位的试件，观察不全面。这进一步表明了本次调查的重要性。 该项目是针对压缩疲劳条件下循环变形和疲劳裂纹形成的微观机制进行的更基础性的研究。对于本研究的主要部分，将使用一种模型材料，即多晶铜。为了验证基础研究的结果，还将对结构钢样品进行试验，以获得更多工业适用的结果。 拟议研究的实验方法论如下。(1)循环应力-应变响应的研究：这部分工作将研究材料在压缩循环应力条件下的循环硬化和/或软化响应，以及非对称加载条件下的循环蠕变行为。(2)跟踪表面微观结构的演变：这将包括半原位观察表面形貌的变化，为此，疲劳试验将在选定的循环次数停止，并且不仅将使用光学和扫描电子显微镜，而且将使用原子力显微镜来跟踪表面。(3)循环过程中内部微观结构变化的检测：将通过透射电子显微镜来研究与应力应变响应相对应的位错结构的演变。最后，根据上述试验的结果，将用类似的方法测试结构钢的样品，但仅在选定的加载条件下进行。因此，其基本原理和实际适用性可能会得到进一步发展。

The objectives of the present project are to investigate and characterize the cyclic deformation response and fatigue crack initiation of engineering structure materials under pure compressive fatigue loading conditions. Fatigue fracture is the most frequently observed failure form of engineering components. The majority of fatigue failures observed in practice are through three stages, i.e. crack initiation, crack propagation and final fracture. Thus, prevention of crack initiation is considered one of the most important approaches to improve fatigue strength of engineering materials. Factors that affect fatigue crack initiation are often multiple including stress state, material property, and service condition. It has been shown that the overwhelming majority of fatigue crack initiation is due to tensile or mixed stress mode. Accordingly, most previous studies on cyclic deformation and fatigue fracture were carried out with either tension or multiple-direction load conditions. There has been no systematic study on cyclic deformation and fatigue fracture of engineering materials under pure compressive fatigue load, although such failure form does happen with critical engineering components, such as the fatigue fracture of aircraft landing gear frames. To fill the gap in the knowledge pool, the present project is therefore proposed. The outcome of the study will provide the understanding of the fundamental mechanisms on cyclic deformation and fatigue failure of materials under pure compressive loading condition, and offer better approach for industrial applications in fatigue life improvement. Previously, the cyclic deformation and fatigue response of selected steel under pure compressive cyclic loading was investigated in our group. It was found that, even if the applied fatigue load was fully compressive, fatigue crack may still form provided that there was even a very moderate stress concentration site. In fact, a critical condition in terms of von Mises stress was defined for crack initiation. For the samples without any stress concentration site, the observation was not comprehensive. This indicates further the significance of the present investigation. The proposed project is a more fundamental study aiming at the micro mechanisms of the cyclic deformation and fatigue crack formation under the compression fatigue condition. For the major part of this study, a model material, i.e. polycrystalline copper, will be used. In order to verify the findings from the fundamental study, tests will also be carried out with samples made of structure steels so that more industry-applicable results can be obtained. The experimental methodology for the proposed study is listed as follows. (1) Investigation of the cyclic stress-strain response: This part of the work will examine the cyclic hardening and/or softening response of the materials under the compressive cyclic stress conditions as well as cyclic creep behavior due the asymmetrical loading condition. (2) Motoring of the evolution of surface microstructure: This will include semi in situ observation of surface morphology changes, for which fatigue tests will be stopped at selected cycle numbers and the surface will be traced using not only optical and scanning electron microscopes but also atomic force microscope. (3) Examination of interior micro-structure change upon cycling: This will be done through transmission electron microscope to explore the evolution of dislocation structure corresponding to the stress-strain response. Finally, based on the results of the above tests, samples of structural steels will be tested with similar methodology but only for selected loading conditions. Thus, the fundamentals and their practical applicability may be further developed.