Characterization of microstructural damage due to low-cycle fatigue by EBSD observation

Electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy was used to assess the damage due to cyclic or uniform strain. Samples of Type 316 stainless steel after fatigue and tensile tests were prepared for EBSD observation and the misorientation angle between neighboring points (local misorientation) was evaluated. It was shown that the local misorientation developed due to the cyclic and uniform strain and that its spatial distribution was not uniform. In fatigue samples, the area of large local misorientation tended to form clusters, whereas it localized to the grain boundaries in the tensile samples, and the magnitude of local misorientation and the degree of the localization increased with the strain amplitude. The degree of localization was quantified via statistical processing of the measured data. It was also shown that the source of damage (cyclic or uniform strain) and the loading direction could be deduced from the EBSD observations of the damaged sample.     

Fatigue crack initiation in Hastelloy X – the role of boundaries

In polycrystalline metals, microstructural features such as grain boundaries (GBs) influence fatigue crack initiation. Stress and strain heterogeneities, which arise in the vicinity of GBs, can promote the nucleation of fatigue cracks. Because of variations in grain size and GB types, and consequently variations in the local deformation response, scatter in fatigue life is expected. A deeper quantitative understanding of the early stages of fatigue crack nucleation and the scatter in life requires experimental and modelling work at appropriate length scales. In this work, experiments are conducted on Hastelloy X under fatigue conditions, and observations of fatigue damage are reported in conjunction with measurements of local strains using digital image correlation. We use a recent novel fatigue model based on persistent slip band–GB interaction to investigate the scatter in fatigue lives and shed light into the critical types of GBs that nucleate cracks. Experimental tools and methodologies, utilizing ex situ digital image correlation and electron backscatter diffraction, for high resolution deformation measurements at the grain level are also discussed in this paper and related to the simulations.     

Propagation behaviour of microstructural short fatigue cracks in the high-cycle fatigue regime

In the high-cycle fatigue regime, it is assumed that crack initiation mechanisms and short fatigue crack propagation processes govern fatigue life of a component. Moreover, it is now becoming accepted that the conventional fatigue limit does not imply complete reversibility of plastic strain and is connected to crack initiation. However, interaction of the crack tip with microstructural barriers, such as, e.g. grain boundaries or second phases, leads to a decrease and eventually to a stop in the crack propagation. In the present contribution, examples for propagating and non-propagating conditions of short fatigue cracks in the microstructure of a duplex steel are given, quantified by means of automated EBSD. To classify the results within the scope of predicting the service life for HCF- and VHCF-loading conditions, a numerical model based on the boundary element method has been developed, describing crack propagation by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.     

Experimental characterization of short crack nucleation and growth during cycling in lean duplex stainless steels

Considering that many applications of Lean Duplex Stainless Steels (LDSSs) involve cyclic loading, the aim of this paper is to study short crack initiation and growth during low (LCF) and high cycle fatigue (HCF) in AL 2003 (UNS S32003). Electron Backscattered Diffraction (EBSD) analysis of plastically active grains allows to determinate the slip systems and their associated Schmid factor (SF). Additionally, the dislocation structure developed during cycling is observed by transmission electron microscopy (TEM). Whereas in HCF cracks nucleate at grains boundaries, during LCF cracks nucleate along intrusion/extrusions in ferritic grains and as they reach austenitic grains grow along active slip systems or by double slip system. Moreover, phase boundaries and grain boundaries act as effective barrier against crack propagation.     

Investigation of small crack behaviour under cyclic loading in a dual phase steel with an FIB tomography technique

Damage accumulation is investigated in the early stage of fatigue life in a ferritic martensitic dual phase steel. Microcrack initiation and propagation are influenced by microstructure, such as grain boundaries, grain orientation and/or phase distribution morphology. The dominant crack initiation pattern is one in which microcracks are generated in a ferrite grain along slip bands inclined at a certain angle with respect to the loading direction. Subsurface observation with a focused ion beam (FIB) device and additional crystallographic characterization by means of the electron backscatter diffraction (EBSD) technique show that a slip system having a high Schmid factor value is activated and results in a crack nucleus. The FIB tomography technique with the help of EBSD measurement allows a three‐dimensional investigation of small crack behaviour to be performed.     

Creep–fatigue crack growth behaviour based on EBSD observations for notched specimen made of directionally solidified Ni-base superalloy

Abstract

In order to characterise the creep–fatigue crack growth behaviour of a directionally solidified Ni-base superalloy, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method using notched specimens has been carried out. The average misorientation in the vicinity of notches increases linearly up to the initiation of cracks with the increasing creep strains under creep or creep–fatigue conditions, whereas under fatigue conditions fatigue cracks grew without an increase in misorientation. The stress holding time clearly influenced the growth behaviour of creep–fatigue cracks and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation and the relative notch opening displacement was independent of test conditions such as temperatures, stresses and stress wave forms.

It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiation of creep–fatigue cracks and their growth behaviours.     

Effects of grain size on cyclic plasticity and fatigue crack initiation in nickel

Fatigue experiments were conducted on polycrystalline nickel of two grain sizes, 24 and 290 μm, to evaluate the effects of grain size on cyclic plasticity and fatigue crack initiation. Specimens were cycled at room temperature at plastic strain amplitudes ranging from 2.5×10⁻⁵ to 2.5×10⁻³. Analyses of the cyclic stress–strain response and evolution of hysteresis loop shape indicate that the back stress component of the cyclic stress is significantly affected by grain size and plastic strain amplitude, whereas these parameters have little effect on friction stress. A nonlinear kinematic hardening framework was used to study the evolution of back stress parameters with cumulative plastic strain. These are related to substructural evolution features. In particular, long range back stress components are related to persistent slip bands. The difference in cyclic plasticity behavior between the two grain sizes is related to the effect of grain size on persistent slip band (PSB) morphology, and the effect this has on long range back stress. Fine grain specimens had a much longer fatigue life, especially at low plastic strain amplitude, as a result of the influence of grain size on fatigue crack initiation characteristics. At low plastic strain amplitude (2.5×10⁻⁴), coarse grain specimens initiated cracks where PSBs impinged on grain boundaries. Fine grain specimens formed cracks along PSBs. At high plastic strain amplitude (2.5×10⁻³), both grain sizes initiated cracks at grain boundaries.     

THE PROGRESSION OF BENDING FATIGUE IN NICKEL

Abstract— The progression of fully reversed bending fatigue, in a 99.6% pure polycrystalline nickel (NO2200) at room temperature, was monitored utilizing Nomarski interference contrast microscopy in conjunction with microhardness measurements. It was found that a gradual hardening, without early saturation, occurred from about 95 to 160 HV. Similar data were obtained with indenter weights of 15–500 g, from which it was concluded that a different behaving surface layer did not develop during fatigue. Cracks initiated at coarse slip bands within the first 1% of the life, when the hardness reached 140 HV. The cracking of these bands initiated away from grain boundaries. The slow growth of these slip band cracks dominated the life subsequently. Growth of the cracks both across and beyond the initial grain was difficult. The significant hardening prior to crack initiation and the confinement of the cracking mainly to coarse slip bands, contributed to a good inherent fatigue resistence of nickel. with a fatigue limit above the yield strength. Non-propagating cracks were not observed below the fatigue limit.     

Growth of short cracks during low and high cycle fatigue in a duplex stainless steel

Damage evolution during low- and high-cycle fatigue in an embrittled duplex stainless steel is characterized in this paper. Moreover, scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) were employed in order to analyze microcracks formation and propagation. During low-cycle fatigue, microcracks initiate the ferrite phase either along slip planes with the highest Schmid factor (SF) inside the grains or at the α/α grain boundary. Then, microcracks propagation take place in ferrite or austenite grains with the highest SF. An analysis of the dislocation structure in the near-surface and in ferritic grains in the bulk of the specimen has shown that dislocation microbands are associated with microcrack initiation.In the high-cycle fatigue regime, damage generally initiates in the austenite by slip band formation followed by crack initiation either at an α–α boundary or at an α–γ boundary in the intersection of slip bands in the austenite. The microstructure in the austenite consists of a low density of dislocation pile-ups while the ferrite is practically inactive or develops only micro-yielding at boundaries.Despite the differences in both fatigue regimes, phase boundaries are an effective barrier against crack propagation because they delay the advance of the crack tip.     

ON THE NUCLEATION OF FATIGUE CRACKS IN PURE POLYCRYSTALLINE α-IRON

Abstract— This paper presents the results of an investigation of fatigue crack initiation in pure polycrystalline α-iron as a function of testing frequency (0.01–1000 Hz). Three distinct types of fatigue crack initiation mechanisms were identified:
(i) At low frequencies, <0.5 Hz, cracks were observed to nucleate at intrusions, extrusions and grain boundaries. However over the frequency range studied the cracks at the intrusions and extrusions were not associated with the final fatigue failure. The results suggest that in pure iron frequencies <0.01 Hz are required for fatigue failure to be initiated at intrusions and extrusions.
(ii) At frequencies ≤0.01 Hz at high strain amplitudes, cracks nucleated at the intersection of grain boundaries with the free surface as a result of the incompatable deformation of surface grains. The subsequent propagation of these cracks resulted in fatigue failure.
(iii) Tests performed at low strain amplitude at 5 Hz and 1000 Hz initiated surface grain boundary cracking without any significant associated change of grain shape.
The results of the present investigation indicate that fatigue failure in pure iron alloys (at frequencies >0.01 Hz) is initiated not at intrusions and extrusions but at grain boundaries.     

High resolution digital image correlation measurements of strain accumulation in fatigue crack growth

Microstructure plays a key role in fatigue crack initiation and growth. Consequently, measurements of strain at the microstructural level are crucial to understanding fatigue crack behavior. The few studies that provide such measurements have relatively limited resolution or areas of observation. This paper provides quantitative, full-field measurements of plastic strain near a growing fatigue crack in Hastelloy X, a nickel-based superalloy. Unprecedented spatial resolution for the area covered was obtained through a novel experimental technique based on digital image correlation (DIC). These high resolution strain measurements were linked to electron backscatter diffraction (EBSD) measurements of grain structure (both grain shape and orientation).Accumulated plastic strain fields associated with fatigue crack growth exhibited inhomogeneities at two length scales. At the macroscale, the plastic wake contained high strain regions in the form of asymmetric lobes associated with past crack tip plastic zones. At high magnification, high resolution DIC measurements revealed inhomogeneities at, and below, the grain scale. Effective strain not only varied from grain to grain, but also within individual grains. Furthermore, strain localizations were observed in slip bands within grains and on twin and grain boundaries. A better understanding of these multiscale heterogeneities could help explain variations in fatigue crack growth rate and crack path and could improve the understanding of fatigue crack closure and fracture in ductile metals.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

Short fatigue crack propagation during low-cycle,high cycle and very-high-cycle fatigue of duplex steel – An unified approach

The present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.     

Microstructural influence on small fatigue cracks in a ferritic–martensitic steel

Microstructure effects on fatigue crack initiation and propagation in ferritic–martensitic dual phase steel were investigated. Slip bands were formed in ferrite grains after several thousand cycles with ensuing crack initiation due to dislocation pile-up. Subsurface observations using a focused ion beam (FIB) and crystallographic analyses using electron backscatter diffraction (EBSD) measurements showed that crack initiation occurred as a result of the activation of a slip system having a high Schmid factor. Surface crack nucleation occurred quite frequently at ferrite/martensite and ferrite/ferrite boundaries, with crack propagation in the ferrite grains. This initiation mode can be attributed to the mismatch stresses at ferrite/martensite phase boundaries and at high angle grain boundaries.     

Experimentelle Charakterisierung und zweidimensionale Simulation der mikrostrukturbestimmten Ermüdungsrissausbreitung in einer β‐Titanlegierung

In this project the initiation and propagation of short fatigue cracks in the metastable β‐titanium alloy TIMETAL®LCB is investigated. By means of an interferometric strain/displacement gauge system (ISDG) to measure the crack opening displacement (COD) and the electron back scattered diffraction technique (EBSD) to determine the orientation of individual grains the microstructural influence on short crack initiation and growth can be characterized. Finite element calculations show a high influence of the elastic anisotropy on the initiation sites of cracks. Crack propagation takes place transgranulary along slip planes as well as intergranulary along grain boundaries. The crack growth rate depends strongly on the active mechanism at the crack tip which in turn is influenced by crack length, the applied stress and the orientation of the grains involved. The value of the steady state crack closure stress changes from a positive value at low applied stresses (roughness induced) to a negative one at higher applied stresses (due to plastic deformations at the crack tip). The crack growth simulation is realised by a two‐dimensional boundary element technique, which contains the ideas of Navarro und de los Rios. The model includes the sequence of the applied stress amplitude as well as the experimental measured roughness induced crack closure.     

Investigation of crack initiation and propagation behavior of AISI 310 stainless steel up to very high cycle fatigue

Fatigue behavior up to very high cycles for AISI 310 stainless steel has been investigated. The fatigue crack initiated from the surface of the material. It was found that up to 10⁶ cycles, cracks initiated from the carbide precipitates at grain boundaries. However, above 10⁶ cycles, the cracks initiated from persistent slip bands found at the surface of the specimen. At lower stress levels, slip bands were developed without initiating the cracks. The horizontal asymptote S–N curve from 10⁶ to 10⁹ cycles was attributed to the development of slip bands all over the surface of the specimen, before crack initiation.     

HIGH TEMPERATURE FATIGUE BEHAVIOUR OF A CAST COBALT-BASE SUPERALLOY

Abstract The initiation and the propagation of fatigue cracks and the low cycle fatigue life of a cast cobalt base superalloy was studied at 293K, 973K and 1173K by optical and scanning electron microscopy. A substantial decrease in fatigue life occurred at 973 and 1173K when compared to room temperature life. A time-dependent bulk damage was evidenced at 1173K which was determined by quantitative microscopy for two plastic strain levels. High strain fatigue crack propagation experiments were carried out at room temperature and at 1173K. From these experiments the decrease of the overall fatigue life at high temperature was shown to result from a considerable reduction of the initiation period due to oxidation and also from a significant acceleration of the crack propagation rate in the presence of oxidation and bulk damage.     

SHORT CRACK FATIGUE BEHAVIOUR IN A MEDIUM CARBON STEEL

The initiation stage and short crack behaviour in torsional fatigue of a 0.4% C steel was investigated by a replication technique. The fatigue cracks initiated and propagated in the ferrite phase which is located at the prior austenite grain boundaries in the form of long allotriomorphs. At this stage of crack development it is proposed that crack growth rate depends on the extent and intensity of plasticity at the tip of the crack. Crack growth per cycle is correspondingly proportional to the strength of the slip band. The ferrite-pearlite boundaries are strong barriers to crack propagation, which is manifested by a deceleration of growth and possible arrest. On raising the stress level the previously non-propagating cracks may continue to grow by branching or joining with other cracks in the ferrite phase. This process is repeated until the stress fields of one or more dominant cracks attain a critical value to sustain continued growth that leads to failure.     

核电主管道不锈钢在高温高压水环境下的疲劳裂纹萌生行为

利用腐蚀疲劳测试系统研究了高温高压水环境下两种压水堆核电站一回路主管道用不锈钢的腐蚀疲劳裂纹萌生行为。结果表明,316LN奥氏体不锈钢的裂纹主要在材料表面的驻留滑移带处萌生,少量裂纹在两簇驻留滑移带交界的亚晶界面处。含有少量铁素体的Z3CN20.09M奥氏体不锈钢的疲劳裂纹依次在试样表面的驻留滑移带处、相界处和点蚀坑处萌生,但主要是在驻留滑移带处。通过研究高温高压水环境下氧化膜的组成和腐蚀疲劳试样横截面的形貌,分析了疲劳裂纹在滑移带处萌生的机理。最后对比分析两种不锈钢裂纹萌生机制的异同,并讨论了铁素体对材料腐蚀疲劳性能的影响。