scholarly journals Very high cycle fatigue crack initiation mechanism according to a 3D model of persistent slip bands formation in α-ferrite

2015 ◽  
Vol 38 (11) ◽  
pp. 1324-1333 ◽  
Author(s):  
C. Wang ◽  
D. Wagner ◽  
Q. Wang ◽  
Z. Huang ◽  
C. Bathias
Keyword(s):  
Fatigue Crack ◽  
3D Model ◽  
High Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
Cycle Fatigue ◽  
Initiation Mechanism ◽  
Slip Bands ◽  
Very High Cycle Fatigue ◽  
Persistent Slip Bands ◽  
Very High

Partition of Cyclic Plasticity in an Austenitic-Ferritic Duplex Stainless Steel and its Significance for Fatigue Crack Initiation under High and Very High Cycle Fatigue Loading Conditions

2014 ◽  
Vol 891-892 ◽  
pp. 1424-1429 ◽  
Author(s):  
Benjamin Dönges ◽  
Marcus Söker ◽  
Alexander Giertler ◽  
Ulrich Krupp ◽  
Claus Peter Fritzen ◽  
...  
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
High Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
Cycle Fatigue ◽  
Slip Bands ◽  
Very High Cycle Fatigue ◽  
Microcrack Initiation ◽  
Austenite Grains ◽  
Very High

The present study documents that at loading amplitudes close to the fatigue limit, cyclic irreversible plastic deformation in form of slip band generation in the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) mainly takes place in few austenite grains without any microcrack initiation in these grains. This was shown by means of focused ion beam (FIB) cutting in combination with high resolution scanning electron microscopy (SEM) at pronounced extrusion-intrusion-pairs in several austenite grains. Investigations by means of confocal laser scanning microscopy (CLSM) revealed that the slip band density in these grains increases with the number of loading cycles and remains constant in the very high cycle fatigue (VHCF) regime. Under such loading conditions, fatigue cracks frequently initiate in the ferrite phase due to anisotropy stresses which are strongly superimposed by stress intensifications at the tip of austenite slip bands. TEM investigations revealed that austenite slip bands, which are piling up against phase boundaries, cause localized dislocation generation and motion in neighboring ferrite grains. The cyclic irreversible motion of these dislocations on several parallel slip planes is correlated with the stage of fatigue crack initiation. A crystal plasticity model based on a finite element program, which considers anisotropic elasticity, allows for the determination of crack initiation sites in real microstructures according to the above mentioned mechanisms. Crystallographic orientations, measured by means of the electron back scatter diffraction (EBSD) technique, serve as input parameters for the calculations regarding microcrack initiation as well as for the analysis of the subsequent short fatigue crack propagation, which is strongly affected by microstructural barriers such as grain and phase boundaries.

Damage Behavior of Metallic Materials under Very High Cycle Fatigue

2007 ◽  
Vol 348-349 ◽  
pp. 237-240 ◽  
Author(s):  
Guo Cai Chai
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Surface Defect ◽  
High Cycle Fatigue ◽  
Cycle Fatigue ◽  
Damage Behavior ◽  
Slip Bands ◽  
Very High Cycle Fatigue ◽  
Defect Area ◽  
Very High

The fatigue damage behavior of three two-phase steels in the very high cycle fatigue regime (VHCF >108cycles) has been studied by both fatigue testing and microstructural investigation using SEM and TEM. The results show that the S-N curves can vary from a single to multi S-N curves, and there is also a transition of fatigue crack initiation from surface defect, subsurface defect such as inclusion to subsurface non defect area or matrix depending on the steel grades and its conditions. The surface crack initiation is caused by formation of irreversible slip bands at the free surface or around surface defect. Subsurface inclusion crack initiation is mainly caused by strain localization (slip bands) emanating at subsurface inclusion. Crack initiation in the subsurface non defect area occurs in the areas that are physically weak. It is also a fatigue damage process caused by micro cyclic plastic deformation. Formation of subsurface non defect fatigue crack origin is a crack initiation and propagation process.

On Fatigue Crack Initiation in the Matrix in Very High Cycle Fatigue Regime

2014 ◽  
Vol 783-786 ◽  
pp. 2266-2271 ◽  
Author(s):  
Guo Cai Chai
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Fatigue Damage ◽  
Strain Localization ◽  
High Cycle Fatigue ◽  
High Strain ◽  
Fatigue Crack Initiation ◽  
Cycle Fatigue ◽  
Very High Cycle Fatigue ◽  
Very High

In very high cycle fatigue, VHCF, regime, fatigue crack initiation can occur at subsurface defects such as inclusion or subsurface non-defect (matrix) origin. This paper provides a study on the fatigue crack initiation mechanisms at subsurface non-defect (matrix) origin in two metallic materials using electron backscatter diffraction and electron channeling contrast imaging. The results show that the strains in the material in the VHCF regime were highly localized, where the local maximum strain is greatly higher than the average strain value. This high strain localization can lead to the formation of fine grain zone and also fatigue damage or fatigue crack initiation at grain boundaries or twin boundaries by impingement cracking. High strain localization is caused by strain accumulation of each very small loading, and also increases the local hardness of the material. This may start quasi-cleavage crack origin, and consequently the formation of subsurface fatigue crack initiations. The results also show that fatigue damage and crack initiation mechanisms in the VHCF regime can be different in different metals due to the mechanisms for local plasticity exhaustion.

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