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The quasi-stationary nature of ‘steady-state’ cyclic deformation

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The cyclic stress-strain (CSS) behaviour of fatigued metals is fundamental to formulations of fatigue life prediction. The CSS curve of so-called wavy slip materials is considered to be unique and history-independent. In ‘mechanical’ cyclic saturation in the sense that the loading parameters (stress and strain) remain constant, it is generally assumed that so-called ‘steady-state’ conditions prevail. Accordingly, the deformation-induced dislocation microstructure is assumed to attain a constant state, resulting from a dynamic equilibrium between defect production and annihilation. Here, it will be shown, based on earlier experimental studies, including microstructural observations and measurements of different defect-sensitive physical properties, that mild but non-negligible changes of the microstructure persist well into ‘mechanical’ saturation. It follows that a distinction must be made between ‘mechanical’ and ‘microstructural’ saturation and that so-called ‘steady-state’ cyclic deformation is actually only quasi-stationary. The consequences of this behaviour will be discussed.

This talk is part of the Surfaces, Microstructure and Fracture Group series.

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