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

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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.