A Dislocation Model for Fatigue Crack Initiation

1981 ◽  
Vol 48 (1) ◽  
pp. 97-103 ◽  
Author(s):  
K. Tanaka ◽  
T. Mura
Keyword(s):  
Grain Size ◽  
Crack Initiation ◽  
Slip Band ◽  
Fatigue Crack Initiation ◽  
Strain Range ◽  
Material Surface ◽  
Dislocation Model ◽  
Plastic Strain Range ◽  
Stress Cycles ◽  
Distributed Dislocations

The slip band formed in a grain on the material surface is a preferential site for crack initiation during low strain fatigue of polycrystalline metals. The forward and reverse plastic flow within the slip band is modeled in the present study by dislocations with different signs moving on two closely located layers, and it is assumed that their movement is irreversible. Based on the model, the monotonic buildup of dislocation dipoles piled up at the grain boundary is systematically derived using the theory of continuously distributed dislocations. This buildup is associated with the progress of extrusion or intrusion. The number of stress cycles up to the initiation of a crack of the grain size order is defined as the cycle when the stored strain energy of accumulated dislocations reaches a critical value. The relation between the initiation life and the plastic strain range derived theoretically is in agreement with a Coffin-Manson type law, and that between the fatigue strength and the grain size is expressed in an equation of the Petch type.

scholarly journals Interactions between geometrical defects and microstructure during high cycle fatigue of polycrystalline aluminium with different grain sizes

2018 ◽  
Vol 165 ◽  
pp. 14004
Author(s):  
Benoît Bracquart ◽  
Charles Mareau ◽  
Nicolas Saintier ◽  
Franck Morel
Keyword(s):  
Grain Size ◽  
Fatigue Crack ◽  
Crack Initiation ◽  
High Cycle Fatigue ◽  
Fatigue Behavior ◽  
Fatigue Crack Initiation ◽  
Cycle Fatigue ◽  
Grain Sizes ◽  
Average Grain Size ◽  
Geometrical Defects

In this work, the influence of the geometrical defect size on the high cycle fatigue behavior of polycrystalline aluminium with different grain sizes is investigated, to better understand the role of internal length scales. Two sizes of grains and defect are used: 100 μm and 1000 μm, the grain size being controlled with thermomechanical treatments. Fully reversed stress-controlled fatigue tests are then carried out. According to fatigue test results, surface crack initiation is delayed when the grain size is reduced, while an approximation of the fatigue limit shows that it is not much influenced by the average grain size. The relative defect diameter (compared to the grain size) seems to be the leading parameter influencing fatigue crack initiation from a defect. Finally, Electron BackScattered Diffraction (EBSD) maps are collected for specimens with large grains and small defects. Fatigue crack initiation from a defect is found to be strongly impacted by the crystallographic orientation of the surrounding grain, crack initiation preferably occurring in crystals being favorably oriented for plastic slip.

scholarly journals Forecast of the fatigue crack initiation site of commercially pure Titanium miniature specimens with local surface topography data

2020 ◽  
Vol 321 ◽  
pp. 11008
Author(s):  
L. Böhme ◽  
F. Ströer ◽  
A. Keksel ◽  
J. Seewig ◽  
E. Kerscher
Keyword(s):  
Grain Size ◽  
Crack Initiation ◽  
Surface Topography ◽  
Fatigue Crack Initiation ◽  
Pure Titanium ◽  
Initiation Site ◽  
Commercially Pure Titanium ◽  
Crack Initiation Site ◽  
Average Grain Size ◽  
Cp Ti

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.

Fatigue Crack Initiation and Growth in a High Loaded Bolted Bar Connection

2009 ◽  
Vol 417-418 ◽  
pp. 737-740
Author(s):  
Srečko Glodež ◽  
Srđan Podrug ◽  
Marko Knez ◽  
Janez Kramberger
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Computational Analysis ◽  
Low Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
High Strength ◽  
Cycle Fatigue ◽  
Calculated Stress ◽  
Final Failure ◽  
Stress Cycles

The fatigue crack initiation and growth in a high loaded bolted bar connection made of high strength steel S1100Q is presented. The material parameters for the fatigue crack initiation f’, f’, b and c are determined using low cycle fatigue test according to ASTM E 606 standard. The fracture mechanics parameters C and m are determined according to ASTM E 647 standard. Based on low cycle fatigue parameters the computational analysis is performed to determine the number of stress cycles required for the fatigue crack initiation. The remain service life up to the final failure is than determined using the known parameters C and m and calculated stress intensity factor, where 3D numerical analysis is performed. The bolted bars are also experimentally tested. Comparison of computational and experimental results shows a reasonable agreement.

Mechanisms of fatigue crack initiation in annealed, quenched and tempered 4340 steel

2000 ◽  
Vol 214 (9) ◽  
pp. 1151-1161 ◽  
Author(s):  
F Yang ◽  
A Saxena
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Fatigue Damage ◽  
Fatigue Crack Initiation ◽  
Strain Range ◽  
Remaining Life ◽  
Structural Components ◽  
Atomic Force ◽  
Slip Bands ◽  
4340 Steel

The mechanisms of fatigue crack initiation in annealed and quenched and tempered 4340 steel were characterized. Several axial fatigue specimens were tested at a strain range of 1.5 per cent to various fractions of fatigue life. The tested specimens were thoroughly examined using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The latter technique provides a better resolution and is also capable of providing quantitative surface topographical information. In annealed 4340 steel, the initial fatigue damage is shown to accumulate in the form of steps between ferrite and cementite laths in the pearlitic microstructure. Subsequent damage accumulation occurs by formation of slip bands which are formed by joining several adjoining steps. Cracks initiate from the slip bands. In quenched and tempered steels, the fatigue damage accumulates at discontinuities at a more rapid rate than in the remaining regions of the specimen, leading to crack initiation and growth emanating primarily from the discontinuities. The roughness of the specimen surface increases with fatigue damage in both microstructures, as demonstrated from the measurements obtained from AFM studies. Such correlations may be useful in predicting the remaining life of cyclically loaded structural components.

Effect of Grain Size on Stress Corrosion of Type 302 Austenitic Stainless Steel

CORROSION ◽  
1966 ◽  
Vol 22 (9) ◽  
pp. 261-264 ◽  
Author(s):  
V. L BARNWELL ◽  
J. R. MYERS ◽  
R. K. SAXER
Keyword(s):  
Grain Size ◽  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Crack Initiation ◽  
Stress Corrosion ◽  
Incubation Period ◽  
Stress Corrosion Cracking ◽  
Dislocation Model ◽  
Size Dependent ◽  
Propagation Period

Abstract Stress-corrosion cracking of Type 302 austenitic stainless steel exposed to boiling 42 w/o MgCl2 was determined to be grain-size dependent Decreased resistance to cracking observed for the larger grain-sized material was associated tentatively with a decreased incubation period. Incubation period was discussed in terms of a dislocation model for crack initiation. Possibility of a grain-size dependency for the propagation period was considered.

Fatigue Behaviour of AZ91 Magnesium Alloy in as-Cast and Severe Plastic Deformed Conditions

2014 ◽  
Vol 891-892 ◽  
pp. 397-402 ◽  
Author(s):  
Ludvík Kunz ◽  
Stanislava Fintová
Keyword(s):  
Fatigue Strength ◽  
Crack Initiation ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Ion Beam ◽  
Fatigue Crack Initiation ◽  
Az91 Alloy ◽  
Material Surface ◽  
Fine Grained ◽  
Ultrafine Grained

Fatigue strength, crack initiation and microstructure were experimentally investigated in an as-cast AZ91 alloy and in ultrafine-grained (UFG) AZ91 alloy processed by equal channel angular pressing (ECAP). The microstructure after ECAP is bimodal, consisting of fine-grained regions and clusters of larger grains with lower density of intermetallic particles. It has been found that the ECAP substantially increases the tensile strength (factor of two), improves ductility (factor of five) and improves the fatigue strength in low-cycle fatigue region. The improvement of the endurance limit based on 107 cycles is weak. The cyclic slip bands, as sites of the fatigue crack initiation on material surface, were investigated. Focussed ion beam technique (FIB) was applied to reveal the surface relief and the microstructure in the vicinity of early fatigue cracks. No grain coarsening was observed in the close vicinity of the initiated cracks. Fatigue cracks in ultrafine-grained structure develop both in the regions of larger grains and also in the fine grained areas. Two types of crack initiation were observed.

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