Short Fatigue Crack Growth Research Articles

Microstructure-interacting short crack growth in blocky alpha Zircaloy-4

Microstructurally short fatigue crack growth in blocky alpha Zircaloy-4 is experimentally investigated in cyclic three-point bend testing. The short crack propagation is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries. Polycrystals with predominant c-axis texture aligned out-of-plane and normal to loading give alternating crack paths along prismatic planes. Samples with c-axis texture aligned in plane and normal to loading typically show straight paths along prismatic planes, sometimes tortuous paths, but always crystallographic. Prismatic <a>-direction crack growth rate is low compared to that for prismatic <c>-direction growth for given loading. Hence the crystallographic plane within which cracks grow is important for determining overall growth rate. For tortuous cracks, with the predominant c-axis texture in plane and normal to loading, crack growth occurs along basal, prismatic and pyramidal planes, deflecting from one slip plane to another during transgranular propagation.

An iterative technique to assess the fatigue strength of notched components

The present work provides an efficient formulation to assess the growth of short fatigue cracks in metallic components. The proposed technique consists on the iterative combination of a micromechanical short-crack growth model and the Finite Elements Method. The interaction of the crack with the microstructure of the material is evaluated through the dislocations distribution technique. The finite elements analysis of the problem is needed to obtain the stress gradient ahead of the notch. The division of the main problem into simpler scenarios makes the resolution of the method easier since cases with known solutions are required exclusively. The iterative method formulation is properly described and application examples are given in order to show its usefulness.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

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

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.

Short review on multiscale short fatigue crack growth model

Short review on multiscale short fatigue crack growth model

Hydrogen induced blister cracking and mechanical failure in X65 pipeline steels

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm2 for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.

Role of prior austenite grain boundaries in short fatigue crack growth in hydrogen charged RPV steel

Short crack propagation under cyclic loading is compared experimentally in hydrogen charged and un-charged SA508 Gr. 3 Cl. I low alloy steel (LAS). This LAS is used in manufacturing of pressure vessels installed in nuclear power plants and is susceptible to hydrogen embrittlement (HE) during operation. Single edge notch tension (SENT) specimens with an initial notch of 85 μm–90 μm are used for this short fatigue crack propagation study. The short crack growth from the notch of SENT specimen subjected to cyclic loading is measured using moving digital microscope fitted on the servo-hydraulic fatigue testing machine. The short fatigue crack growth rate in hydrogen charged SA508 Gr. 3 Cl. I LAS is found to be one order higher as compared to the un-charged subject reactor pressure vessel (RPV) steel. Both trans-granular and inter-granular crack propagation is observed in un-charged and hydrogen charged specimens. In un-charged specimens, inter-granular crack propagation along the prior austenite grain boundaries (PAGBs) occurred only when the short fatigue crack encounters the martensite/austenite (M/A) island present along the PAGB. Whereas, in hydrogen charged specimens, inter-granular crack propagation occurred along the PAGBs of large size prior austenite grains and trans-granular crack propagation through the small size prior austenite grains. Strong resistance to short fatigue crack propagation is provided by PAGBs in un-charged specimens whereas, these PAGBs offered negligible resistance to short fatigue crack growth in hydrogen charged subject RPV steel. A new perspective of studying HE in materials by conducting short fatigue crack growth experiments is demonstrated.

A novel microstructure-based model validated experimentally for simulating short fatigue crack growth in 3 dimensions in planar slip alloys

A novel microstructure-based model was developed to quantify the 3-D growth behaviors of short fatigue cracks by considering both the local driving force and resistance along their irregular fronts in planar-slip alloys. The driving force at a point on the crack front varied with its distance from and the size of the reference semi-circle which covered the area same as that of the crack, and the total resistance at the point was the summation of the dragging forces as a Gaussian function of distance from those grain boundaries that fell behind the point on the crack front. The resistance of a grain boundary to crack growth was a Weibull function of twist angle of crack plane deflection at the boundary. The growth rate at each point on the crack front was subsequently quantified using a microscopic-scale Paris’ equation using an effective driving force which was the driving force minus the total resistance at the point. The crack growth behaviors simulated by this model was consistent with the growth rate measured on the surface of a naturally occurring short fatigue crack in AA8090 aluminum alloys. The anomalous growth behaviors of short cracks could all be quantitatively explained using the model, for the first time. The model was also able to quantify the relationships between the stochastic behaviors of short crack growth, and the microstructure and texture in the alloys.

The effect of surface damage and residual stresses on the fatigue life of nickel superalloys at high temperature

A methodology for evaluating the effect of surface damage in the fatigue life of nickel superalloys is presented in this paper. Dents generated due to low velocity impacts of hard objects were simulated using a finite element (FE) model. The residual stress distribution underneath the dent root obtained numerically was compared with the measurements on experimentally simulated damaged specimens using ring-core milling at the micron scale through a combined Focused-Ion Beam and Digital Image Correlation technique (FIB-DIC). The numerical and experimental results for the residual stress show good agreement in terms of residual stress trends and magnitudes. The residual stress distribution obtained via the FE model was subsequently used in a fatigue short crack growth model for an estimation of the fatigue life of dented specimens. The fatigue life predictions were then compared with experimental fatigue results for the nickel superalloy at high temperatures. The comparison shows a significant improvement in the prediction of fatigue life of parts with superficial damage due to careful consideration of the residual stresses around the damage.

Fatigue Fracture Stages of Metals and Alloys and Stage-to-Stage Transition Criteria

Basic mechanisms of scattered and localized fatigue damage of metals and alloys are analyzed. The methods of evaluating the criteria of scattered-to-localized fatigue damage transition are proposed and substantiated based on the analysis of short and long crack propagation behavior. One of the methods is based on the analysis of short fatigue crack growth kinetics defined by their size or growth rate against the number of load cycles. Crack sizes and a number of cycles, corresponding to a more intensive increase in the crack growth rate, are taken to be the transition criteria. With the scattered-to-localized fatigue damage transition at the stresses above the endurance limit, the form and parameters of the equation describing the crack size against the number of load cycles are changed. Another method is built upon the analysis of variation of the short crack propagation rate against the stress intensity factor. In this case, the stress intensity factor span and a corresponding crack size at which the stress intensity factor starts varying at a higher rate are taken as the transition criteria. The above methods are used to assess crack sizes and a number of load cycles, corresponding to the scattered-to-localized fatigue damage transition, for carbon, alloy, austenitic steels and an aluminum alloy accounting for stress levels and properties of examined materials. Fatigue crack sizes on the transition at the stresses above the endurance limit are established to decrease with stresses and remain smaller than the crack sizes at the endurance limit. Main crack sizes are shown to correlate with the endurance limit of examined materials, decreasing with its increase.

A modification of UniGrow 2‐parameter driving force model for short fatigue crack growth

AbstractIn this paper, a modification of the UniGrow model is proposed to predict total fatigue life with the presence of a short fatigue crack by incorporating short crack propagation into the UniGrow crack growth model. The UniGrow model is modified by 2 different methods, namely the “short crack stress intensity correction method” and the “short crack data‐fitting method” to estimate the total fatigue life including both short and long fatigue crack propagations. Predicted fatigue lives obtained from these 2 methods were compared with experimental data sets of 2024‐T3, 7075‐T56 aluminium alloys, and Ti‐6Al‐4V titanium alloy. Two proposed methods have shown good fatigue life predictions at relatively high maximum stresses; however, they provide conservative fatigue life predictions at lower stresses corresponding high cycle fatigue lives where short crack behaviour dominates total fatigue life at lower stress levels.

Hydrogen Embrittlement Mechanism in Fatigue Behavior of Austenitic and Martensitic Stainless Steels

In the present study, the influence of hydrogen on the fatigue behavior of the high strength martensitic stainless steel X3CrNiMo13-4 and the metastable austenitic stainless steels X2Crni19-11 with various nickel contents was examined in the low and high cycle fatigue regime. The focus of the investigations were the changes in the mechanisms of short crack propagation. Experiments in laboratory air with uncharged and precharged specimen and uncharged specimen in pressurized hydrogen were carried out. The aim of the ongoing investigation was to determine and quantitatively describe the predominant processes of hydrogen embrittlement and their influence on the short fatigue crack morphology and crack growth rate. In addition, simulations were carried out on the short fatigue crack growth, in order to develop a detailed insight into the hydrogen embrittlement mechanisms relevant for cyclic loading conditions. It was found that a lower nickel content and a higher martensite content of the samples led to a higher susceptibility to hydrogen embrittlement. In addition, crack propagation and crack path could be simulated well with the simulation model.

A grain boundary interaction model for microstructurally short fatigue cracks

In this paper, we present a phenomenological model for simulating the effect of a grain boundary on crack growth along crystallographic planes. This model combines various geometrical features of the interaction between the crack plane and the grain boundary plane. The tilt and twist misorientations, calculated at a grain boundary, between a crack plane and a favorable plane in the next grain are incorporated into this model, as are the Schmid factor of the next grain and a critical crack transmission stress. A model calibration procedure is demonstrated based on experimental short fatigue crack growth data measured in a high performance wrought magnesium alloy. The proposed combined GB interaction model is shown to accurately predict the short fatigue crack growth retardation and arrest at grain boundaries in this alloy.

Description of the Kinetics of Short Surface Fatigue Crack Growth Using the Parameters of Fatigue Curves

To describe the kinetics of short surface crack nucleation and growth in metals and alloys, an approach has been developed, which is based on deformation curves in the low-cycle fatigue region. In view of the known Ramberg–Osgood stress–strain relationship, the deformation fatigue curves have been replaced by traditional fatigue curves. Analysis of known experimental data shows that the kinetics of short surface fatigue crack initiation and growth can be described by the parameters of fatigue curves for both the low- and high-cycle fatigue region, which makes such results more informative.

Initiation and growth of short fatigue cracks in austenitic Sanicro 25 steel

AbstractAustenitic heat‐resistant stainless steel Sanicro 25, often used in energy production installations, was cyclically strained at room temperature, and initiation and growth of short fatigue cracks have been studied. Cylindrical specimens with pre‐existing shallow notch were cyclically strained under constant total strain amplitudes, while the surface relief, crack initiation, and short crack growth were observed, in‐situ, using optical and electron microscopes. Persistent slip markings were found to be the principal locations where natural fatigue cracks initiated. The character of the crack paths of short cracks was studied using SEM and EBSD techniques. The results exhibit prevailing transgranular crack growth, ie, along the persistent slip markings within the grains. The growth of the largest cracks in the central part of the shallow notch was measured for different applied strain amplitudes. Because the coalescence of initiated cracks plays a significant role during crack growth, the use of the “equivalent crack” concept gives adequate results in the description of the crack growth rate. The crack growth law of short cracks was established and compared with the Manson‐Coffin law of the material.

Hydrogen Embrittlement Mechanism in Fatigue Behaviour of Austenitic and Martensitic Stainless Steels

In the present study, the influence of hydrogen on the fatigue behaviour of the high strength martensitic stainless steel X3CrNiMo13-4 and the metastable austenitic stainless steels X2Crni19-11 with various nickel contents was examined in the low and high cycle fatigue regime. The focus of the investigations was the changes in the mechanisms of short crack propagation. The aim of the ongoing investigation is to determine and quantitatively describe the predominant processes of hydrogen embrittlement and their influence on the short fatigue crack morphology and crack growth rate. In addition, simulations were carried out on the short fatigue crack growth, in order to develop a detailed insight into the hydrogen embrittlement mechanisms relevant for cyclic loading conditions.

Short fatigue crack growth in welded joints described by the effective cyclic J-integral

The purpose of the present contribution is to predict the fatigue life of welded joints by using the effective cyclic J-integral as crack driving force. The plasticity induced crack closure effects and the effects of welding residual stresses are taken into consideration. Here, the fatigue life is regarded as period of short fatigue crack growth. The node release technique is used to perform finite element based crack growth analyses. For fatigue lives calculations, the effective cyclic J-integral is employed in a relation similar to the Paris (crack growth) equation. For this purpose, a specific code was written for the determination of the effective cyclic J-integral for various lifetime relevant crack lengths. The effects of welding residual stresses on the crack driving force and the calculated fatigue lives are investigated. Results reveal that the influence of residual stresses can be neglected only for large load amplitudes. Finally, the predicted fatigue lives are compared with experimental data: a good accordance between both results is achieved.

Modeling of hydrogen effects on short crack propagation in a metastable austenitic stainless steel (X2CrNi19-11)

The experimentally observed short fatigue crack growth rate of an uncharged specimen tested in air is compared with results obtained from specimens tested in 10 MPa hydrogen atmosphere and specimens previously charged with hydrogen. To further discuss the hydrogen related short propagation mechanisms, a simulation approach for predicting short fatigue crack growth is presented. The boundary element method is used for calculating stresses and displacements in an anisotropic elastic solid. The hydrogen concentration is assumed to be homogeneously distributed in the microstructure. Based on this modelling approach, it could be concluded that hydrogen leads to an increasing short fatigue crack growth rate due to increasing irreversible deformation processes at the crack tip and also promotes grain boundary cracking in specimens tested in 10 MPa hydrogen atmosphere.

Short Fatigue Crack Behavior of LZ50 Axle Steel Under Rotating-Bending Cyclic Loading

The short fatigue short crack replica tests using LZ50 axle steel hourglass-shaped specimens of were carried out under rotating-bending cyclic load. Seven sets of effective test data were obtained. The analysis demonstrated that short fatigue cracks generally originate in ferrite grains or on their boundaries, and then short cracks start propagating, which is strongly influenced by the microstructure of the material. The crack growth rate increases serratically, followed by its decrease double-fold because of the limit of ferrite grain boundaries and the banded pearlite structure. Based on the three principles of the best fit, which include the overall fitting effect, correlation with fatigue physics, and safety of residual strength prediction, the statistical evolution of the dominant effective short fatigue crack length and life share were analyzed. The statistical results show that the minimum value distribution exhibit the best fit of the dominant effective fatigue short crack length and the life share. The short fatigue crack growth model, which includes various microstructure barriers, provides a good fitting result and reflects the cyclic effect of the microstructure on the short fatigue crack behavior.

Fatigue Lives of Power Plant Structures Due to Load Sequence Effects Originating from Fluctuating Production of Renewable Energy

The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudo-random sequence derived from measurements in power plants. The developed non-linear fracture mechanics based model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.