Initial Aspects of Low-Cycle Fatigue Fracture of Martensitic Steels

2007 ◽  
Vol 348-349 ◽  
pp. 385-388 ◽  
Author(s):  
Tamaz Eterashvili ◽  
T. Dzigrashvili ◽  
M. Vardosanidze
Keyword(s):  
Plastic Deformation ◽  
Stainless Steel ◽  
Plastic Zone ◽  
Fatigue Fracture ◽  
Main Crack ◽  
Low Cycle Fatigue ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Slip Bands ◽  
Number Of Cycles

This study deals with the SEM and optical microscopic characterization of fatigue plastic deformation process during fatigue crack initiation to understand where, why and how cracks initiate under conditions of low cycle fatigue. Samples were prepared from the 13Х11Н2В2МФ high-chromium stainless steel used for fusion power applications. The low-cycle tests were conducted at room temperature with the standard V-notched samples prepared from conventional stainless steel. The following characteristics were studied during fatigue tests: 1 macrocrack propagation, 2. interaction between macrocrack and isolated microcracks, 3. interaction between macrocrack and slip bands, 4. interaction between macrocrack and microstructure elements of the steel. The above experiments show that during macrocrack propagation a plastic zone is formed around it, where isolated microcracks and slip bands of 2-3 different directions are observed. Measurement of plastic zone dimensions after different number of cycles of deformation show that plastic zone size increases during the first stage of cyclic deformation (until definite number of cycles are completed), and then remains unchanged. The observations show that main crack is composed of individual micro-components, the lengths of which are in a good correlation with the dimensions of microstructure elements of the steel (former austenite grains, martensite crystals). It was revealed that during growth, as a rule, macrocrack rarely propagates along isolated microcracks and slip bands. Direction of macrocrack propagation changes while passing from one microstructure element to another, so that main direction is the same. No preferable transcrystalline or intercrystalline propagation of macrocrack has been observed in the investigated steel. It is shown that after subsequent fatigue tests, dimensions of the previously created slip bands increase, and additional new slip band are also formed. The sites and frequency of slip bands’ formation in plastic zone are also studied. It was observed that the boundaries and mainly the sites of intersection of martensite crystals are the sites of isolated (rough) microcracks’ formation. The dimensions of slip bands are comparable with those of martensite crystals. The angles between the main crack propagation direction and slip bands varied from 30o to 60o, however, most of the slip bands were oriented at 45o to the main crack. Based on the obtained results a conclusion is made that plastic deformation in samples go inhomogeneously. In plastic zones, along with the heavily deformed areas, almost non-deformed areas are also observed. The speed of fatigue fracture increases with the increase in frequency and amplitude of deformations. Generally, the annealed samples are destructed prematurely in comparison with non-annealed ones of the investigated steel.

Structure Changes in Stainless Steels Used in Nuclear Reactors and Turbo Generators after Minor Low Cycle Fatigue Deformation

2005 ◽  
Vol 475-479 ◽  
pp. 3505-3508
Author(s):  
Tamaz Eterashvili ◽  
T. Dzigrashvili ◽  
M. Vardosanidze
Keyword(s):  
Room Temperature ◽  
Nuclear Reactors ◽  
Low Cycle Fatigue ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Slip Bands ◽  
Structure Changes ◽  
Before And After ◽  
Number Of Cycles ◽  
Local Plastic Deformation

The structure of austenitic steel before and after 25% of total number of cycles of low cycle fatigue tests conducted at room temperature is studied using TEM. It is shown that the cyclic deformation of the steel proceeds heterogeneously. The microstructure of the steel is investigated in the area between the deformed and undistorted parts of the samples. The crystallography of the observed twins and the slip bands is specified. The value of local plastic deformation within a micro area of a grain is measured, and the influence of microstructure on crack initiation is discussed.

Structure Changes in Stainless Steels Used in Nuclear Reactors and Turbo Generators after Minor Low Cycle Fatigue Deformation

2005 ◽  
Vol 475-479 ◽  
pp. 1487-1490 ◽  
Author(s):  
Tamaz Eterashvili ◽  
T. Dzigrashvili ◽  
M. Vardosanidze
Keyword(s):  
Room Temperature ◽  
Nuclear Reactors ◽  
Low Cycle Fatigue ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Slip Bands ◽  
Structure Changes ◽  
Before And After ◽  
Number Of Cycles ◽  
Local Plastic Deformation

The structure of austenitic steel before and after 25% of total number of cycles of low cycle fatigue tests conducted at room temperature is studied using TEM. It is shown that the cyclic deformation of the steel proceeds heterogeneously. The microstructure of the steel is investigated in the area between the deformed and undistorted parts of the samples. The crystallography of the observed twins and the slip bands is specified. The value of local plastic deformation within a micro area of a grain is measured, and the influence of microstructure on crack initiation is discussed.

Modelling of Low Cycle Fatigue Initiation of 316LN Based on Crystalline Plasticity and Geometrically Necessary Dislocations

2010 ◽  
Vol 636-637 ◽  
pp. 1137-1142 ◽  
Author(s):  
Julien Schwartz ◽  
Olivier Fandeur ◽  
Colette Rey
Keyword(s):  
Lattice Structure ◽  
Low Cycle Fatigue ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Lattice Curvature ◽  
Crystalline Plasticity ◽  
Numerical Studies ◽  
Slip Bands ◽  
Persistent Slip Bands ◽  
Number Of Cycles

Initiation of intragranular cracks during low cycle fatigue is governed by complex microstructural phenomena. Depending on the loading amplitude, number of cycles, lattice structure and/or chemical composition, different dislocation structures (veins, cells or Persistent Slip Bands) develop and induce heterogeneous localization of strain and stress in the material. For a better comprehension of crack initiation in 316LN stainless steel, low cycle fatigue tests and numerical simulations were performed. Specimens of 316LN steel with polished shallow notch were cycled with constant loading amplitude and Persistant Slip Bands were identified by SEM observations. In parallel, numerical studies were carried out with the model of cristalline plasticity CristalECP. Simulations were performed on 3D polycristalline aggregates of 316LN steel with the finite elements code Abaqus® and Cast3m®. The results show a heterogeneous localization of strain in bands. For a more precise computation of the mechanical fields and to introdruce a grain size effect, Geometrically Necessary Dislocations were introduced in CristalECP. The GNDs are directly related and computed with the lattice curvature.

TEM Study of Microstructure Changes, Formation and Distribution of Slip Bands in Austenitic Steels after Low-Cycle Fatigue (LCF) Deformation - II

2015 ◽  
Vol 665 ◽  
pp. 141-144
Author(s):  
Tamaz Eterashvili ◽  
T. Dzigrashvili ◽  
M. Vardosanidze
Keyword(s):  
Plastic Deformation ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Austenitic Steels ◽  
Cycle Fatigue ◽  
Localized Deformation ◽  
Microstructure Changes ◽  
Slip Bands ◽  
Micro Deformation ◽  
Number Of Cycles

The microstructure changes, development of micro plastic deformation and formation and distribution of slip bands were studied. It is shown that development of micro deformation during LCF depends on loading conditions (amplitude and number of cycles) and microstructureIt is shown that as non-localized as well as localized micro plastic deformation takes place because of structural inhomogeneity. Supposedly, the localized deformation is related to the sites of internal stress concentration accumulated during the LCF.The effect of microstructure of structural steels on the rate of local cyclic deformation, leading to nucleation and growth of slip bands of fatigue cracks, was studied. The interaction of slip bands with precipitates, grain boundaries and low-angle boundaries were also analyzed.The sites of nucleation of primary and secondary slip bands were identified, and the following aspects were considered: 1. the possibility of microcrack nucleation on (or in) slip bands, 2. The kind of slip bands the slip bands may nucleate in, 3. The potential sites (except the slip bands) and reasons of nanocrack formation are specified.

Effect of Surface Finish and Loading Conditions on the Low Cycle Fatigue Life of Stainless Steel Welded Piping in PWR Environment

2013 ◽  
Author(s):  
Yuichi Fukuta ◽  
Yuichiro Nomura ◽  
Seiji Asada
Keyword(s):  
Stainless Steel ◽  
Fatigue Life ◽  
Austenitic Stainless Steel ◽  
Surface Finish ◽  
Loading Condition ◽  
Low Cycle Fatigue ◽  
Complex Loading ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Effect Of Surface

NUREG/CR-6909 of USA and JSME of Japan proposed new rules for evaluating environmental effects in fatigue analyses of reactors components. These rules were established from a lot of fatigue data with polished specimens under simple loading condition. The effects of surface finish or complex loading condition were reported in some papers, but these data were obtained with the simple shaped specimens. In order to evaluate the effects of surface finish and loading condition and to confirm the applicability of the proposed rules to actual components, Low Cycle Fatigue tests are performed in PWR environment with the specimens cut from 316 austenitic stainless steel welded piping. The pipes are machined to have three levels of surface finish condition and the load pattern simulating the thermal stress is applied to specimens. In this study, the effect of surface finish on fatigue life is included to be small for 316 austenitic stainless steel welded piping. Considering the insensitive region in the current evaluation rule, predicted accuracy is increased and possibility of improving the current rule is indicated.

Damage Assessment on Low Cycle Fatigue Properties of Cyclic Pre-Strained Austenitic Stainless Steel

2011 ◽  
Author(s):  
Nao Fujimura ◽  
Hiroyuki Oguma ◽  
Takashi Nakamura
Keyword(s):  
Surface Roughness ◽  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Fatigue Damage ◽  
Low Cycle Fatigue ◽  
Strain Ratio ◽  
Fatigue Properties ◽  
Cycle Fatigue ◽  
Damage Law ◽  
Number Of Cycles

The effects of cyclic pre-strain on low cycle fatigue properties of austenitic stainless steel were investigated, and the fatigue damage was assessed based on several parameters such as the full width at half maximum (FWHM) of diffracted X-ray profile and surface roughness of specimens. The strain-controlled tests were conducted under strain ratio Rε = −1 and various constant total strain ranges. Also the change in remnant fatigue lives were investigated when the cyclic pre-strain were applied to the specimens under the different number of cycles which were determined with reference to the usage factor UFpre ranged from 0.2 to 0.8. As a result, the remnant fatigue life of the pre-strained samples became shorter than that of the sample without pre-strain as the UFpre increased. The relationship between the pre-strain damage expressed in UFpre and the remnant fatigue damage in UFpost was roughly described by the cumulative linear damage law: UFpre + UFpost = 1. Namely, the cyclic pre-strain affected the remnant fatigue lives. In order to evaluate the effects of cyclic pre-strain on fatigue lives more precisely, the damage in the cyclic pre-straining processes was estimated by using FWHM and surface roughness. The FWHM of the specimens with pre-strain once decreased with increase in UFpre, and then increased after showing a minimum value. The surface roughness of specimens increased linearly with an increase of the number of pre-straining cycles. These results suggested that the damage due to pre-strain can be assessed by means of FWHM and surface roughness of specimens.

Low Cycle Fatigue Tests and Simulations on Steel Elbows

2013 ◽  
Author(s):  
Patricia Pappa ◽  
George E. Varelis ◽  
Spyros A. Karamanos ◽  
Arnold M. Gresnigt
Keyword(s):  
Finite Element ◽  
Low Cycle Fatigue ◽  
Local Strain ◽  
Fatigue Behaviour ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Element Analysis ◽  
Out Of Plane ◽  
Strain Gauge Measurements ◽  
Number Of Cycles

In this paper the low cycle fatigue behaviour of steel elbows under strong cyclic loading conditions (in-plane and out-of-plane) is examined. The investigation is conducted through advanced finite element analysis tools, supported by real-scale test data for in-plane bending. The numerical results are successfully compared with the experimental measurements. In addition, a parametric study is conducted, which is aimed at investigating the effects of the diameter-to-thickness ratio on the low-cycle fatigue of elbows, focusing on the stress and strain variations. Strain gauge measurements are compared with finite element models. Upon calculation of local strain variation at the critical location, the number of cycles to fracture can be estimated.

Effect of Roller-Working on Fatigue Strength Improvement of Notched Structural Stainless Steel

2006 ◽  
Vol 306-308 ◽  
pp. 151-156
Author(s):  
Priyo Tri Iswanto ◽  
Shinichi Nishida ◽  
Nobusuke Hattori ◽  
Yuji Kawakami
Keyword(s):  
Plastic Deformation ◽  
Stainless Steel ◽  
Fatigue Limit ◽  
Fatigue Behavior ◽  
Fatigue Tests ◽  
Mean Stress ◽  
Increase With Increase ◽  
Fatigue Strength Improvement ◽  
Rolled Specimen ◽  
Number Of Cycles

In order to study the effect of plastic deformation on fatigue behaviors of plastically deformed specimen, bending fatigue tests had been performed on notched deformed stainless steel specimens. Also pulsating fatigue tests were done on notched non-deformed specimens to evaluate the influence of mean stress on fatigue behavior of notched non-deformed specimens. The result showed that according to increase of deformation value, the fatigue limits of these specimens also significantly increase. Fatigue limit of rolled specimen does not linearly increase with increase in plastic deformation value. Based on fatigue limit diagram, the effect of compressive residual stress on fatigue limit improvement of stainless steel is higher than that of work-hardening. In case of non-deformed specimen, when the compressive mean stress increases, the fatigue limit and the number of cycles to failure increase. In case of tensile mean stress, this kind of mean stress decreases the fatigue limit.

Crack Initiation Under Low-Cycle Multiaxial Fatigue in Type 316L Stainless Steel

1983 ◽  
Vol 105 (2) ◽  
pp. 138-143 ◽  
Author(s):  
B. Jacquelin ◽  
F. Hourlier ◽  
A. Pineau
Keyword(s):  
Stainless Steel ◽  
Crack Initiation ◽  
Low Cycle Fatigue ◽  
Cyclic Stress ◽  
Fatigue Tests ◽  
Multiaxial Fatigue ◽  
Stage I ◽  
Type B ◽  
Strain Behavior ◽  
Number Of Cycles

Low-cycle fatigue tests corresponding to fatigue life range between 103 and 105 cycles were carried out at room temperature on one heat of 316 L austenitic stainless steel. These tests included: (i) reversed tension-compression, (ii) reversed tension-compression with a superimposed steady torque, (iii) pulsated tension-compression with a stress ratio (Rσ) such that −0.5<Rσ<0, (iv) reversed and pulsated tension-compression with a superimposed steady internal pressure. In tests (ii), the torsional ratcheting effect was measured. SEM observations were used to determine the number of cycles corresponding to Stage I crack initiation and the orientation of Stage I microcracks. It was observed that the in-depth growing Type B shear microcracks were most damaging. A simple criterion is proposed Ni=No(Δγp B)α•(σnB)β where Ni is the number of cycles to crack initiation, Δγp B is the range of plastic shear strain on Type B planes, σnB is the maximum normal stress acting on these planes, No,α and β are parameters adjusted from the Manson-Coffin law and reversed cyclic stress-strain behavior.

Evaluation of Low-Cycle Fatigue in Simulated PWR Environment

2006 ◽  
Vol 326-328 ◽  
pp. 1011-1014 ◽  
Author(s):  
Ill Seok Jeong ◽  
Sang Jai Kim ◽  
Taek Ho Song ◽  
Sung Yull Hong
Keyword(s):  
Fatigue Life ◽  
Nuclear Power ◽  
Power Plants ◽  
Low Cycle Fatigue ◽  
Water Environment ◽  
Fatigue Tests ◽  
Cycle Fatigue ◽  
Fatigue Design ◽  
Number Of Cycles ◽  
Measured Length

For developing fatigue design curve of cast stainless steel that is used in piping material of nuclear power plants, a low-cycle fatigue test rig was built. It is capable of performing tests in pressurized high temperature water environment of PWR. Cylindrical solid fatigue specimens of CF8M were used for the strain-controlled environmental fatigue tests. Fatigue life was measured in terms of the number of cycles with the variation of strain amplitude at 0.04%/s strain rates. The disparity between target length and measured length of specimens was corrected by using finite element method. The corrected test results showed similar fatigue life trend with other previous results.

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