Fatigue crack growth at stress concentrators under spectrum loading

2005 ◽  
Vol 40 (2) ◽  
pp. 117-127
Author(s):  
R. V Prakash
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Surface Defects ◽  
Fatigue Cracks ◽  
Mapping Technique ◽  
Critical Dimensions ◽  
Small Cracks ◽  
Stress Raisers ◽  
Kinetics Of

Fatigue cracks initiate at stress raisers such as notches, discontinuities, and surface defects. Many of the field failures that indicate the presence of a fatigue crack at failure can be traced to crack initiation from one or more crack initiation sites and merger of cracks over a period of service. Substantial service life is spent in the growth of small cracks from an initial size of few micrometres before they coalesce and grow to critical dimensions that cause fracture. This paper summarizes research that was carried out in order to understand the kinetics of crack growth of small cracks at notches under simulated FALSTAFF service loading. This paper also presents a method used to understand crack growth kinetics in a pin-loaded lug joint through a crack-front-mapping technique.

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.

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 1. Statement of problem. The effect of the initial notch acuity on the fatigue crack rate on small-sized specimens

2019 ◽  
pp. 191-204 ◽  
Author(s):  
V. I. Smirnov ◽  
A. J. Minkin ◽  
B. Z. Margolin ◽  
V. I. Kokhonov
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth ◽  
Growth Kinetics ◽  
Fatigue Cracks ◽  
Stress Concentration Zone ◽  
Concentration Zone ◽  
Methodical Approach ◽  
Reactor Materials ◽  
Kinetics Of

The paper considers methodical issues in the experimental research of fatigue crack growth kinetics when testing irradiated small-sized specimens. The effect of the initial notch acuity is studied on the long crack growth rate. The stress concentration zone sizes are estimated for notches of various types. A brif literature review of the main problems in the study of the growth kinetics of short fatigue cracks has been performed. The tasks of further research are formulated. 

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.

Setup for studying the kinetics of crack growth in cyclic bending tests

2020 ◽  
Vol 86 (7) ◽  
pp. 59-64
Author(s):  
V. N. Pustovoit ◽  
S. A. Grishin ◽  
V. V. Duka ◽  
V. V. Fedosov
Keyword(s):  
Experimental Data ◽  
Fatigue Crack ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Fatigue Tests ◽  
Computer Processing ◽  
Crack Development ◽  
Potential Difference ◽  
Fatigue Crack Development ◽  
Kinetics Of

The goal of the study is analysis of the features of fatigue cyclic fracture of steels. An installation has been designed to induce fatigue cracks and to study the kinetics of fatigue crack development. Crack growth is recorded by the method of potential difference. The data on the crack growth kinetics were processed on a computer using LGraph2 programs and Excel spreadsheets. When studying the kinetics of the fatigue crack development, the electrodes were soldered to the edges of the initial notch of the sample and time dependence of the potential difference was recorded on a computer during crack growth. To interpret the experimental data, a calibration chart in the coordinates «potential difference (U) – the crack length (Lcr)» constructed on the basis of the millivoltmeter readings was used, with due regard for the size of the sample section, current flow and length of the fatigue crack. Cyclic loading of the sample resulted in a stepwise character of the crack growth: first occurred zone of plastic deformation of the metal is then followed by accumulation of stresses of a certain size, their relaxation in the form of a crack and sudden crack growth. An abrupt crack growth is clearly visible on a graph of the fatigue crack growth rate obtained upon computer processing of experimental data. Using a graphical editor KOMPAS, a graph was constructed which characterized the growth of the fatigue crack against the number of cycles of fatigue tests for bending. The experimental setup provides the possibility of fatigue crack formation for impact tests, determination of the work of crack propagation, as well as studying of the kinetics of crack development and computer processing of experimental data.

scholarly journals Effect of Rod-like Structure on Fatigue Life, Short Surface Crack Initiation and Growth Characteristics of Extruded Aluminum Alloy A2024 (Analysis via Modified Linear Elastic Fracture Mechanics)

Materials ◽  
2021 ◽  
Vol 14 (24) ◽  
pp. 7538
Author(s):  
Kenichi Masuda ◽  
Sotomi Ishihara ◽  
Hiroshi Shibata ◽  
Noriyasu Oguma
Keyword(s):  
Fatigue Crack ◽  
Fatigue Life ◽  
Fatigue Crack Growth ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Linear Elastic ◽  
Al Matrix

In the Al alloy A2024-T3 extruded material, a rod-like structure is generated parallel to the extrusion direction. In this study, the effects of rod-like structures on fatigue crack initiation and growth behavior were comprehensively investigated. Two types of specimens were used in a fatigue experiment, in which the direction of the load stress amplitude was parallel (specimen P) and perpendicular (specimen V) to the rod-like structure. Based on the experimental and analytical results, the following findings were obtained regarding the fatigue life, location of crack initiation, and fatigue crack growth behavior. Because the fatigue life of specimen P was longer than that of specimen V, it is inferred that the rod-like structure significantly affects the fatigue life. In specimen P, fatigue cracks were generated from the grain boundaries of the Al matrix. By contrast, in specimen V, cracks were generated from the Cu–Mg-based intermetallic compound in the Al matrix. In specimen P, fatigue cracks were more likely to propagate across the rod-like structure, which decreased the fatigue crack growth rate. In specimen V, fatigue cracks did not propagate across the rod-like structure; instead, they propagated through the Al matrix. Therefore, the fatigue crack growth resistance of specimen V was lower than that of specimen P. The relationship between the fatigue crack growth rate and the modified linear elastic fracture mechanics parameter could be used to predict the S–N curve (stress amplitude vs. fatigue life) and fatigue crack growth behavior. The predicted results agreed well with the experimental results.

Guided Wave Acoustic Emission from Fatigue Crack Growth in Aluminium Plate

2006 ◽  
Vol 13-14 ◽  
pp. 23-28 ◽  
Author(s):  
C.K. Lee ◽  
Jonathan J. Scholey ◽  
Paul D. Wilcox ◽  
M.R. Wisnom ◽  
Michael I. Friswell ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Elastic Waves ◽  
Fatigue Cracks ◽  
Guided Wave ◽  
Experimental Results ◽  
Multiple Reflections ◽  
Alloy Plate

Acoustic emission (AE) testing is an increasingly popular technique used for nondestructive evaluation (NDE). It has been used to detect and locate defects such as fatigue cracks in real structures. The monitoring of fatigue cracks in plate-like structures is critical for aerospace industries. Much research has been conducted to characterize and provide quantitative understanding of the source of emission on small specimens. It is difficult to extend these results to real structures as most of the experiments are restricted by the geometric effects from the specimens. The aim of this work is to provide a characterization of elastic waves emanating from fatigue cracks in plate-like structures. Fatigue crack growth is initiated in large 6082 T6 aluminium alloy plate specimens subjected to fatigue loading in the laboratory. A large specimen is utilized to eliminate multiple reflections from edges. The signals were recorded using both resonant and nonresonant transducers attached to the surface of the alloy specimens. The distances between the damage feature and sensors are located far enough apart in order to obtain good separation of guided-wave modes. Large numbers of AE signals are detected with active fatigue crack propagation during the experiment. Analysis of experimental results from multiple crack growth events are used to characterize the elastic waves. Experimental results are compared with finite element predictions to examine the mechanism of AE generation at the crack tip.

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.

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