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

纯压缩循环加载条件下多晶Cu的循环变形与疲劳

• 批准号: RGPIN-2014-06545
• 负责人: Wang, Zhirui
• 金额: $ 2.11万
• 依托单位: University of Toronto
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2015
• 资助国家: 加拿大
• 起止时间: 2015-01-01 至 2016-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=589077
• 关键词: Cyclic; deformation; fatigue; polycrystalline; Cu

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.

本项目的目标是调查和表征在纯压缩疲劳载荷条件下工程结构材料的环状变形响应和疲劳裂纹的启动。 疲劳骨折是工程组件最常观察到的故障形式。在实践中观察到的大多数疲劳失败是通过三个阶段，即裂纹开始，裂纹繁殖和最终断裂。因此，预防裂纹开始被认为是改善工程材料疲劳强度的最重要方法之一。影响疲劳裂纹引发的因素通常是多重的，包括压力状态，材料特性和服务状况。已经表明，绝大多数疲劳裂纹开始是由于拉伸或混合应力模式引起的。因此，大多数先前关于循环变形和疲劳断裂的研究是通过张力或多个方向载荷条件进行的。尚无对纯压缩疲劳负荷下工程材料的循环变形和疲劳断裂的系统研究，尽管这种失败形式确实发生在关键的工程组件中，例如飞机起落架齿轮框架的疲劳断裂。为了填补知识池中的空白，因此提出了本项目。该研究的结果将提供对纯压缩负荷条件下材料的循环变形和疲劳失败的基本机制的理解，并为疲劳生活改善的工业应用提供更好的方法。 以前，在我们的组中研究了纯压缩环状载荷下选定钢的环状变形和疲劳反应。已经发现，即使施加的疲劳负荷完全压缩，疲劳裂纹仍然可能形成，只要甚至有一个非常中等的应力浓度位点。实际上，定义了von Mises应力的危急条件以进行裂纹启动。对于没有任何应力浓度位点的样品，观察结果并不全面。这进一步表明了本研究的重要性。 拟议的项目是一项更基本的研究，旨在在压缩疲劳条件下循环变形和疲劳裂纹形成的微观机制。对于本研究的主要部分，将使用模型材料，即多晶铜。为了验证基本研究的发现，还将使用由结构钢制成的样品进行测试，以便可以获得更多可观的效果。 拟议研究的实验方法如下列出。 （1）研究环状应激 - 应变响应：这部分工作将检查材料在压缩循环应力条件下的循环硬化和/或软化响应，以及由于不对称的载荷条件而循环蠕变行为。 （2）表面微观结构演化的发展：这将包括半原位观察表面形态的变化，为此，将在选定的循环数量下停止疲劳测试，并不仅使用光学和扫描电子显微镜，还可以使用原子力显微镜来追踪表面。 （3）骑自行车时的内部微结构变化的检查：这将通过透射电子显微镜进行，以探索与应力 - 应变反应相对应的位错结构的演变。 最后，基于上述测试的结果，结构钢的样品将使用类似的方法进行测试，但仅用于选定的加载条件。因此，可以进一步发展基本面及其实际适用性。