scholarly journals Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth

2015 ◽  
Vol 373 (2038) ◽  
pp. 20140132 ◽  
Author(s):  
Haël Mughrabi
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Damage ◽  
Cyclic Deformation ◽  
Damage Evolution ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Cycle Fatigue ◽  
Cubic Metals

In this survey, the origin of fatigue crack initiation and damage evolution in different metallic materials is discussed with emphasis on the responsible microstructural mechanisms. After a historical introduction, the stages of cyclic deformation which precede the onset of fatigue damage are reviewed. Different types of cyclic slip irreversibilities in the bulk that eventually lead to the initiation of fatigue cracks are discussed. Examples of trans- and intercrystalline fatigue damage evolution in the low cycle, high cycle and ultrahigh cycle fatigue regimes in mono- and polycrystalline face-centred cubic and body-centred cubic metals and alloys and in different engineering materials are presented, and some microstructural models of fatigue crack initiation and early crack growth are discussed. The basic difficulties in defining the transition from the initiation to the growth of fatigue cracks are emphasized. In ultrahigh cycle fatigue at very low loading amplitudes, the initiation of fatigue cracks generally occupies a major fraction of fatigue life and is hence life controlling.

Plastic Slip and Early Stage of Fatigue Damage in Polycrystals: Comparison between Experiments and Crystal Plasticity Finite Elements Simulations

2014 ◽  
Vol 891-892 ◽  
pp. 1711-1716 ◽  
Author(s):  
Loic Signor ◽  
Emmanuel Lacoste ◽  
Patrick Villechaise ◽  
Thomas Ghidossi ◽  
Stephan Courtin
Keyword(s):  
Fatigue Crack ◽  
Finite Elements ◽  
Crack Initiation ◽  
Fatigue Damage ◽  
Crystal Plasticity ◽  
Early Stage ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Plastic Slip

For conventional materials with solid solution, fatigue damage is often related to microplasticity and is largely sensitive to microstructure at different scales concerning dislocations, grains and textures. The present study focuses on slip bands activity and fatigue crack initiation with special attention on the influence of the size, the morphology and the crystal orientation of grains and their neighbours. The local configurations which favour - or prevent - crack initiation are not completely identified. In this work, the identification and the analysis of several crack initiation sites are performed using Scanning Electron Microscopy and Electron Back-Scattered Diffraction. Crystal plasticity finite elements simulation is employed to evaluate local microplasticity at the scale of the grains. One of the originality of this work is the creation of 3D meshes of polycrystalline aggregates corresponding to zones where fatigue cracks have been observed. 3D data obtained by serial-sectioning are used to reconstruct actual microstructure. The role of the plastic slip activity as a driving force for fatigue crack initiation is discussed according to the comparison between experimental observations and simulations. The approach is applied to 316L type austenitic stainless steels under low-cycle fatigue loading.

Modeling of Fatigue Crack Propagation

2004 ◽  
Vol 126 (1) ◽  
pp. 77-86 ◽  
Author(s):  
Yanyao Jiang ◽  
Miaolin Feng
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Initiation ◽  
Crack Propagation ◽  
Crack Growth ◽  
Fatigue Damage ◽  
Fatigue Crack Propagation ◽  
Fatigue Crack Initiation ◽  
Cyclic Plasticity ◽  
R Ratio

Fatigue crack propagation was modeled by using the cyclic plasticity material properties and fatigue constants for crack initiation. The cyclic elastic-plastic stress-strain field near the crack tip was analyzed using the finite element method with the implementation of a robust cyclic plasticity theory. An incremental multiaxial fatigue criterion was employed to determine the fatigue damage. A straightforward method was developed to determine the fatigue crack growth rate. Crack propagation behavior of a material was obtained without any additional assumptions or fitting. Benchmark Mode I fatigue crack growth experiments were conducted using 1070 steel at room temperature. The approach developed was able to quantitatively capture all the important fatigue crack propagation behaviors including the overload and the R-ratio effects on crack propagation and threshold. The models provide a new perspective for the R-ratio effects. The results support the notion that the fatigue crack initiation and propagation behaviors are governed by the same fatigue damage mechanisms. Crack growth can be treated as a process of continuous crack nucleation.

Crack Initiation Life Model of Stiffened Plates Based on Damage Mechanics

2014 ◽  
Vol 904 ◽  
pp. 508-512
Author(s):  
Hong Wang ◽  
Ping Yang ◽  
Jun Lin Deng ◽  
Qin Dong
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Plastic Strain ◽  
Fatigue Damage ◽  
Damage Evolution ◽  
Damage Mechanics ◽  
Low Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
Stiffened Plate ◽  
Crack Initiation Life

Based on the continuum damage mechanics theory, according to the development of the fatigue damage evolution equation, and combining the interaction coefficient of stiffener and plate, with plastic strain as the control quantity of damage evolution, the stiffened plate low cycle fatigue damage mechanics model is established, and the calculation method of the fatigue crack initiation life is obtained. This method for the initiation life of fatigue crack is divided into the life before the damage and the life of the damage evolution. The model results are compared with those of the finite element results. Conclusions show that the model can reflect the regularity of axial plastic strain evolution of stiffened plate, and can be directly used for fatigue loads analysis under the mechanism of initiation life.

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.

Fatigue Crack Initiation and Propagation in SuperCMV Hollow Shafts with Transverse Holes

2011 ◽  
Vol 70 ◽  
pp. 141-146 ◽  
Author(s):  
Ran Li ◽  
W. Sun ◽  
Thomas H. Hyde ◽  
Edward J. Williams ◽  
Xing Guo Wang
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Three Dimensional ◽  
Axial Loading ◽  
Maximum Principal Stress ◽  
Experimental Results ◽  
Three Dimensional Finite Element

A fatigue crack growth test program has been carried out on hollow, SuperCMV shaft specimens, with transverse holes, under combined torsional and axial loading. The experimental results show that fatigue cracks always initiated in the stress concentration areas, i.e., in the transverse holes. Up to four cracks were observed to be initiated at different positions near the holes in the shafts. The fatigue crack propagation was initially found to occur under Mode I conditions, followed by a period of mixed Mode II/III crack growth. Three dimensional, finite element, elastic-plastic analyses have also been conducted, in an attempt to predict the crack-initiation locations and lives. The predicted crack-initiation sites agree with the experimental observations, for a range of loading conditions. The initiation sites were found to be approximately on the planes of maximum principal stress. The predicted, torque-dominated, fatigue lives of the shafts, obtained by use of a stress-life (S-N) approach, correlate reasonably well with the experimental results.

Initiation and Growth of Small Fatigue Cracks of Steels Used for Nuclear Power Plants Under Low Cycle Regime

2014 ◽  
Author(s):  
Shota Hasunuma ◽  
Takeshi Ogawa
Keyword(s):  
Fatigue Crack ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Nuclear Power ◽  
Power Plants ◽  
Nuclear Power Plants ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Cycle Fatigue

Low cycle fatigue tests were conducted for carbon steel, STS410, low alloy steel, SFVQ1A, and austenitic stainless steel, SUS316NG, which were used for nuclear power plants, in order to investigate the mechanism of fatigue damage when the plants were subjected to huge seismic loads. In these tests, the surface behavior of fatigue crack initiation and growth was observed in detail using cellulose acetate replicas, while the interior behavior was detected in terms of fracture surface morphology developed by multiple two-step strain amplitude variations with periodical surface removals. Fatigue crack growth rates were evaluated by elasto-plastic fracture mechanics approach. For SFVQ1A and SUS316NG, the fracture mechanics approach is available in order to predict the crack growth life from the metallurgical crack initiation size to the final crack length of the specimens. For STS410, numerous small cracks initiated, grew and coalesced each other on the specimen surface under low cycle fatigue regime.

Modeling the Fatigue Damage Evolution in Welded Joints

2017 ◽  
Author(s):  
Zbigniew Mikulski ◽  
Vidar Hellum ◽  
Tom Lassen
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Damage ◽  
Damage Evolution ◽  
Threshold Value ◽  
Phase Model ◽  
Two Phase ◽  
Initiation Phase ◽  
Weld Toe ◽  
Small Cracks

The present paper presents a two-phase model for the fatigue damage evolution in welded steel joints. The argument for choosing a two-phase model is that crack initiation and subsequent crack propagation involve different damage mechanisms and should be treated separately. The crack initiation phase is defined as the number of cycles to reach a crack depth of 0.1 mm. This phase is modelled based on the Dang Van multiaxial stress approach. Both a multiaxial stress situation introduced by the acting loads and the presence of the multiaxial welding residual stresses are accounted for. The local notch effect at the weld toe becomes very important and the irregular weld toe geometry is characterized by extreme value statistics for the weld toe angle and radius. The subsequent crack growth is based in classical fracture based on the Paris law including the effect of the Stress Intensity Factor Range (SIFR) threshold value. The unique fatigue crack growth rate curve suggested by Huang, Moan and Cui is adopted. This approach keeps the growth rate parameters C and m constant whereas an effective SIFR is calculated for the actual stress range and loading ratio. The model is developed and verified based on fatigue crack growth data from fillet welded joints where cracks are emanating from the weld toe. For this test series measured crack depths below 0.1 mm are available. The two-phase model was in addition calibrated to fit the life prediction in the rule based S-N curve designated category 71 (or class F). A supplementary S-N curve is obtained by the Random Fatigue Limit Method (RFLM). The test results and the fitted model demonstrated that the crack initiation phase in welded joins is significant and cannot be ignored. The results obtained by the Dang Van approach for the initiation phase are promising but the modelling is not yet completed. The fracture mechanics model for the propagation phase gives good agreement with measured crack growth. However, it seems that the prediction of crack retardation based on a threshold value for the SIFR gives a fatigue limit that is overly optimistic for small cracks at the weld toe. The threshold value has been determined based on tests with rather large central cracks in plates. The validity for applying this threshold value for small cracks at the weld toe is questioned. As the present two-phase model is based on applied mechanics for both phases the parameters that have an influence on the fatigue damage evolution are directly entering into the model. Any change in these parameters can then be explicitly taken into account in logical and rational manner for fatigue life predictions. This not the case with the rule based S-N curves that are based on pure statistical treatment of the bulk fatigue life.

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