Abstract
Surfaces of technical components rarely appear in perfectly smooth condition. During fatigue loading, stress concentrations at surface asperities cause localized plastic deformation that can lead to crack initiation. Therefore, we have established a computer-aided method based on material ratio curves to investigate the possibility to predict the crack initiation site in fatigue tests by using detailed information on the local surface topography.The present study shows the results of investigations on the mutual influence of the average grain size and the surface condition on the fatigue behavior of commercially pure Titanium (cp-Ti) miniature specimens. Three cp-Ti states were investigated: two types of coarse-grained cp-Ti Grade 2 with 35 µm and with 100 µm average grain size and one ultrafine-grained cp-Ti Grade 4 state with less than 2.5 µm average grain size. Confocal microscopy provided the surface topography data of all specimens and data post-processing was applied to the topography in order to locate critical areas where crack initiation may preferentially occur. These areas were compared with the actual crack initiation areas in fatigue test. Finally, scanning electron microscopy (SEM) images of the fracture surfaces were studied to analyze fatigue crack initiation site and crack path of the three microstructural states.
Highlights
It is well known that the local surface condition plays a major role during fatigue crack initiation
The coarse grained commercially pure Titanium (cp-Ti) state with 100 μm average grain size and micro milled notches of 30 μm depth shows fatigue failure at low stress amplitude values
In case of cp-Ti with 2.5 μm average grain size, specimens with face milled surfaces revealed fatigue failure at lower stress amplitude values compared to the specimens with polished surfaces
Summary
It is well known that the local surface condition plays a major role during fatigue crack initiation. If a crack is initiated at the surface, its ability to grow depends on the grain size and orientation of the crystallographic planes, namely those with a high Schmid factor In consequence, these factors (surface asperities, surface grain size and grain orientation) significantly influence the fatigue limit of materials. Thereby, we showed that surface asperities do not necessarily influence the fatigue limit, when its size is considerably lower than the average grain size of the material [1,2,3]. This was shown in a study where theoretical approaches, e.g. critical distance approaches, were used to predict the fatigue limit of notched specimens [4]
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