A B S T R A C TThe surface crack opening displacements are characterised by digital image correlation for a (thin) plane stress 316 stainless steel compact tension sample subjected to an overload event. This supports a traditional plasticity-induced closure interpretation showing a knee in the closure response prior to overload, an absence of closure in the accelerated growth regime followed by accentuated closure in the retardation regime. By contrast, measurement of the mid-thickness elastic strain field behind and ahead of the crack made by synchrotron X-ray diffraction shows no evidence of significant crack face contact stresses behind the crack tip on approaching minimum loading. Rather the changes during loading and overloading can mostly be explained by a simple elastic plastic analysis using a value of the yield stress intermediate between the initial yield stress and the UTS. This shows very significant compressive reverse plastic strains ahead of the crack that start to form early during unloading. At the moment it is not clear whether this difference is because of the increasing stress intensity applied as the crack grows, or for some other reason, such as prevention of the crack faces closing mid-thickness due to the reverse plastic zone.Keywords overload effect; synchrotron X-ray diffraction.
N O M E N C L A T U R Ea = crack length COD = crack opening displacements da/dN = crack growth rate K I = opening mode stress intensity factor K max = maximum stress intensity factor K min = minimum stress intensity factor N = number of fatigue cycle applied OL = overload P max = maximum load P min = minimum load R = load ratio (P min /P max ) r p = size of the monotonic plastic zone r c = size of the cyclic plastic zone t = thickness UTS = ultimate tensile strength x = crack growing direction y = crack opening direction K = range of stress intensity factor (K max -K min ) P = load range (P max -P min )
Hydrofluoric acid (HF) or fluoride ion corrosion issues are often encountered in many fields, which have attracted extensive research due to its strong corrosiveness. In this paper, a critical review is presented based on recent progress on HF corrosion. In view of the discrepancy of fluoride ion compared with other ions, the special attack characteristics of fluoride ion are firstly discussed. Afterwards, the corrosion mechanisms of stainless steels, nickel-based alloys, and titanium alloys in HF solution or fluoride ion-containing environment are reviewed, and three typical corrosion behaviors are summarized, which are essentially weakening process of passivation of metallic materials. The effects of influencing factors (e.g. alloying elements, environmental factors, and stress, etc.) on HF corrosion are also discussed, which involve changes in passivation mechanism, the influence of HF attack mode and multiple damage mechanisms due to mechanical–chemical coupling. Finally, future research works on HF corrosion are proposed.
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