Short Crack Growth Behaviour Research Articles

Modeling approach for a unified crack growth model in short and long fatigue crack regimes

In this paper, the recently proposed UniGrow model modification is further improved and a modeling framework of the proposed model is developed to account for both the short and long crack growth behavior. The modeling approach incorporates the integration of short and long propagation into the UniGrow crack growth model to address the short and long crack growth in a unified manner. A systematic study is performed to further validate the proposed modeling approach and assess its prediction capabilities and shortcomings to predict crack growth behavior of short and long cracks. The proposed crack growth model is assessed by first comparing the predicted fatigue crack growth results with long crack growth data sets of 7010-T7, 2024-T3, 2324-T3, 7050-T7 and 7075-T6 aluminum alloys at four different R ratios; The results are further assessed by comparison of short and long crack growth predictions to both short and long crack growth data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios. The model showed good correlation with long crack data sets of 7010-T7, 2024-T3, 2324-T3, 7050-T7 and 7075-T6 and predicted crack growth results matched well with the nonlinearity of four material data sets at four different R-ratios. The results revealed that even though the proposed model was in good agreement with long crack data sets of 2090-T8E41, LC9cs aluminum alloys and Ti-6Al-4V titanium alloys at two different R-ratios, the model did not correlate well with the short crack data sets of these three materials and to accurately account for the variability of short crack growth.

Initiation and short crack growth behaviour of environmentally induced cracks in AA5083 H131 investigated across time and length scales

Abstract Environmentally induced cracking (EIC) in a sensitized high-strength AA5083 H131 alloy has been investigated using time-lapse synchrotron X-ray computed tomography combined with post-mortem correlative characterization. Small corrosion features deliberately introduced in a pre-exposure step were found to be the site of initiation for over 95% of the 44 EIC cracks that developed under slow strain rate testing. Detailed analysis using three-dimensional electron backscatter diffraction and energy-dispersive spectroscopy analysis of a single crack confirmed the intergranular nature of the cracks from the start and that the pre-exposure corrosion was associated with an α-AlFeMnSi particle cluster. It also appears that several cracks may have initiated at this site, which later coalesced to form the 300-μm-long crack that ultimately developed. Of further note is the fact that initiation of the EIC cracks across the sample started below the yield strength and continued beyond the ultimate tensile strength. The most rapid crack propagation occurred during sample extension following a period of fixed displacement.

Assessment of predominant microstructural features controlling 3D short crack growth behavior via a surrogate approach in Ti-6Al-4V

Complex 3D microstructure and mechanical properties can be precisely characterized and linked by employing the X-ray micro-tomography combined with EBSD serial sectioning. Global sensitivity and principal component analysis can be used to rank and coarsen the contributing features that control the mechanical response. Afterwards, by creating surrogate models, the relationship between the most contributing features and mechanical properties can be further analyzed via support vector machines. The aforementioned novel approach was applied to study the relationship between 3D microstructural features and short crack behavior in a Ti-6Al-4V alloy. It was revealed that short crack path, growth rate, and its variation are mainly affected by the interaction with neighboring grains; however, as the short crack front becomes larger, there is a decrease and a change in the importance of those interactions. A high number of grains in contact with long cracked α grains in the loading direction may impose a constraint on the crack opening resulting in low crack growth rates. For the larger crack front, the increase in the shear stress field around the cracked grains leads to crack bifurcations and to the formation of secondary cracks, resulting in a decrease in crack driving forces with low crack growth rates. It was concluded that short crack behavior is strongly affected by the shape, size, and crystallographic features of its neighboring grains, which cause variation in the shear and tensile stress fields resulting in crack growth rate variation.

Physically short and long-crack growth behavior of MIG welded Al-5.8%Mg alloy

In this study, we investigated the physically short and long-crack growth behavior of Al-5.8%Mg alloy, a special hardened alloy used for welding of Al-5083 plates. Microstructural analysis on both longitudinal and cross-sectional plane of welded plates showed no crystallographic texture with nearly equi-axed grains of size 85 μm and gas porosities. Fatigue tests were done at 50 Hz using CT-specimens at different R-ratios (0.1,0.5 and 0.8) to characterize the long-crack growth behavior. Compliance method was used for crack-closure measurements. An increase in crack growth rates and reduction in threshold ΔK was observed with increasing R-ratio. This R-ratio effect could be rationalized after compensating for crack-closure. Special specimens for short-crack growth fatigue tests were cut from pre-cracked CT-specimens. Constant load-range fatigue tests at 50 Hz were conducted at three different R-ratios (-1,0.1 and 0.5) to characterize the effect of mean-stress on short-crack growth behavior. It appeared that the physically short-cracks initially propagated in closure-free environment and then deviated towards their long-crack growth rates. The microstructural effects on short-cracks were more pronounced at lower mean-stresses or R-ratios. The physically short-crack growth effect could not be removed completely using the crack-closure concept alone, suggesting the role of other growth-retarding mechanisms. A modified empirical model including the effect of other possible growth-retarding mechanisms was proposed to capture the behavior of physically short-cracks in this material.

Short crack growth behavior and its transitional interaction with 3D microstructure in Ti-6Al-4V

Currently, short crack growth behavior and rate variations are not well understood in the literature. This is due to lack of studies regarding the interaction between 3D short crack and microstructure and its effect on crack growth. In order to study this interaction, in situ computed tomography was performed to measure crack growth at sub-grain level (every 5 µm) during a fatigue test in a bimodal Ti-6Al-4V alloy for two crack front regions. This was followed by serial sectioning coupled with electron backscattering diffraction (EBSD) to identify the short crack growth in the microstructure, i.e. α, α + β phases and interface. Results show that crack growth has the highest rate in α phase as compared to the α + β phase and the interface in both regions. The crack grows preferably into α phase when compared to the average microstructural fraction in the first region, but it decreases below this fraction in the second region. The crack grows mainly close to crystallographic planes in α grains with the maximum shear stress (favorable planes) in the first region. As the short crack grows into the second region, there is an increase in number of grains enclosed in the plastic zone size. As a result, there is a decrease in the mismatch angle between neighboring cracked grains, which leads to higher deviation from favorable planes causing a local variation in crack growth rate.

Tracking hydrogen embrittlement using short fatigue crack behavior of metals

Understanding hydrogen embrittlement phenomenon that leads to deterioration of mechanical properties of metallic components is vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour using conventional CT specimens. Extensive literature is available on hydrogen effect on long fatigue crack growth behaviour of metals that leads to the change in crack growth rate and the threshold stress intensity factor range (ΔKth). However, it is the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as a strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs a curves. Various metallic materials ranging from austenitic stainless steel (AISI 316L) to reactor pressure vessel steel (SA508 Grade 3 Class I low alloy steel) and line pipe steels (API 5L X65 & X80) are studied in this work.

Transitional effect of 3D microstructural features on short crack growth behavior in Ti-6Al-4V alloy

Transitional effect of 3D microstructural features on short crack growth behavior in Ti-6Al-4V alloy

On the transition of short cracks into long fatigue cracks in reactor pressure vessel steels

Short fatigue cracks, having dimension less than 1 mm, propagate at much faster rates (da/dN) even at lower stress intensity factor range (da/dN) as compared to the threshold stress intensity factor range obtained from long fatigue crack growth studies. These short cracks originate at the sub-grain level and some of them ultimately transit into critical long cracks over time. Therefore, designing the components subjected to fatigue loading merely on the long crack growth data and neglecting the short crack growth behavior can overestimate the component’s life. This aspect of short fatigue cracks become even more critical for materials used for safety critical applications such as reactor pressure vessel (RPV) steel in nuclear plants. In this work, the transition behaviour of short fatigue crack gowth into long fatigue crack is studied in SA508 Grade 3 Class I low alloy steel used in RPVs. In-situ characterization of initiation, propagation and transition of short fatigue cracks is performed using fatigue stage for Scanning Electron Microscope (SEM) in addition to digital microscopes fitted over a servo-hydraulic fatigue machine and correlated with the microtructural information obtained using electron backscatter diffraction (EBSD). SA508 steel having an upper bainitic microstructure have several microstructural interfaces such as phase and grain boundaries that play a significant role in controlling the short fatigue crack propagation. Specially designed and prepared short fatigue specimens (eletro-polished) with varying initial crack lengths of the order of tens of microns are used in this study. The transition of such short initial cracks into long cracks is then tracked to give detailed insight into the role of each phase and phase/grain boundary with an objective of establishing Kitagawa-Takahashi diagram for the given RPV steel. The behavior of the transited long cracks is then compared with the crack propagation behavior obtained using conventional CT specimens. The outcome of this research will enhance information on the integrity of the components made from RPV steel used in Indian nuclear power plants.

Stress corrosion cracking initiation and short crack growth behaviour in Alloy 182 weld metal under simulated boiling water reactor hydrogen water chemistry conditions

The effect of dissolved hydrogen on the stress corrosion cracking initiation and short crack growth behaviour of Alloy 182 weld metal was evaluated in 274°C hydrogenated high-purity water using accelerated crack initiation and growth tests with sharply notched fracture mechanics specimens and in-situ crack initiation and growth monitoring. A maximum in initiation susceptibility and crack growth rates was observed at the Ni/NiO phase transition line. Grain boundary misorientations and mismatch in Schmid factor were measured by electron backscattered diffraction along intergranular stress corrosion cracks and at crack initiation sites. Low-angle boundaries seem to be particularly resistant to cracking.

Effects of compressive residual stress on short fatigue crack growth in a nickel-based superalloy

The effects of compressive residual stress on short fatigue crack growth in Inconel 718 have been investigated. Using two different indentation procedures, controlled plane-strain compressive residual stress fields were applied to short through-thickness cracks, which had been generated by machining away wakes of long cracks that had been grown down to threshold levels at a stress ratio of 0.1. Short fatigue crack growth tests were conducted at stress ratios of 0.1 and 0.7. At a stress ratio of 0.1, the residual stress-free testpieces demonstrated typical short fatigue crack growth behaviour, indicated by a reduction of threshold value and increases in fatigue crack growth rates compared to those of long fatigue cracks. A significant decrease in short crack growth rates was observed within the compressive residual stress region of indented testpieces, together with an increase of threshold values. At a high stress ratio of 0.7, any similar decrease in rate is barely observable. This indicates that the effects of compressive residual stress on short fatigue crack growth are monotonic in sign and could perhaps be simulated by a crack closure approach similar to that applied for long fatigue cracks. However the superposition principle which incorporates the compressive residual stress as a negative stress intensity factor, despite many successful applications to long fatigue cracks, is found to be inappropriate here. Hence one should be cautious when extending such a methodology to short fatigue cracks as it may result in optimistic life estimations.

Short cracks growth in low cycle fatigue under multiaxial in-phase loading

Crack propagation in full plastic regions is one of the main aspects of fatigue life design for components subjected to high strain concentrations. Residual life assessment for those components, in which high stress concentrations cause cyclic yielding of the material, can be considered as a crack propagation problem by assuming crack growth from the first load cycle. The aim of this paper is to study the crack growth behaviour of short cracks in low cycle fatigue under a multiaxial loading condition. In particular, a series of experiments in LCF regime at room temperature was performed to determine crack growth during axial, torsional and axial-torsional tests. Crack advancement was checked with the plastic replica technique, during test interruptions. Experimental results were compared, in terms of crack growth rates and fatigue life assessment, with those analytically calculated, considering different multiaxial fatigue parameters introduced in an exponential crack growth law and an approach based on the multiaxial cyclic J-Integral concept.

Numerical modelling of the fatigue crack shape evolution in a shot-peened steam turbine material

In this study, the short crack initiation and growth behaviour in a notched sample under low-cycle fatigue (LCF) was investigated in a low-pressure steam turbine material. Different crack initiation mechanisms and crack shape evolution processes were experimentally observed in samples subjected to different surface treatments: polished, T0 (industrially applied shot peening process) and T1 (a less intense shot peening process). To better understand the effects of shot peening on fatigue, a 3D finite element (FE) model was developed to investigate the interaction between crack growth and the effects induced by shot peening. Firstly, residual stress redistribution caused by both mechanical loading and the presence of a crack was numerically investigated. This model was also used to successfully predict the differences in crack shape evolution between varying surface conditions, and quantified the retardation of short crack growth behaviour resulting from shot peening. Finally, the 3D model introduced in this study was compared with a previously developed 2D model with plane strain assumptions to demonstrate the limitation of the 2D model in simulating the crack growth behaviour, and to emphasise the importance of taking the 3D crack shape into account when evaluating the short crack growth behaviour.

On the Short Surface Fatigue Crack Growth Behavior in a Fine-Grained WC-Co Cemented Carbide

In the present study, the fatigue crack growth (FCG) behavior of short surface cracks in a fine-grained cemented carbide with a length of less than 1 mm was investigated. The rotating bending and the four-point bending fatigue tests were carried out at stress ratios of R = −1 and R = 0.1 (R = maximum stress/minimum stress). It was found that a short surface crack had a longer stable fatigue crack growth area than a long through-thickness crack; the FCG behaviors of the two types of crack are clearly different. Furthermore, the FCG path of short surface cracks was investigated in detail to study the interaction between fatigue cracks and microstructures of the cemented carbide such as WC grains and the Co phase. At a low Kmax (Kmax = the maximum stress intensity factor), it was found that fatigue crack growth within WC grains is difficult because of a small driving force; instead, crack growth is along the brittle WC/WC interface. On the other hand, at a high Kmax, WC grain breakage often occurs, since the driving force of FCG is large, and the fatigue crack grows linearly.

A physically short fatigue crack growth approach based on low cycle fatigue properties

This work proposes an improved and simplified fatigue life equation called LAPS, formulated from low cycle fatigue data of smooth specimens and the Rice-Kujawski-Ellyin asymptotic field, with proper crack opening functions for closure effects. The model captures both long and physically short fatigue crack growth behavior, but the emphasis of this contribution is on the modifications for physically short cracks based on empirical models found in the literature. The predictions for physically short cracks from this model coincide well with experimental data for the railway axle used steel 25CrMo4. The predictions for long cracks match well with data from a variety of different metals. This makes the model a suitable alternative, e.g. to NASGRO, for engineering applications.

A numerical study of the effects of shot peening on the short crack growth behaviour in notched geometries under bending fatigue tests

The current paper presents a numerical analysis of the effects of shot peening on short crack growth in a low pressure (LP) steam turbine material, FV448. The fatigue behaviour of this material has been experimentally evaluated using a U-notched specimen (representing the fir tree root geometry of the turbine blade) under 3-point bend tests. Two different shot peening intensities were considered in this study: an industrially applied shot peening process and a less intense shot peening process.In the modelling work, a 2-D finite element (FE) model with static short cracks has been developed, incorporating both compressive residual stress and strain hardening distribution effects caused by shot peening. Both linear-elastic (LEFM) and elasto-plastic (EPFM) fracture mechanics were used to characterise the crack driving force in the un-peened and shot-peened conditions, taking into account the effects of stress redistribution caused by residual stress relaxation and crack opening. The stress intensity factor used in the LEFM approach was calculated using the weight function method, and the equivalent stress intensity factor used in the EPFM approach was calculated from the J-integral, which was evaluated using the cracked FE model. These results could explain the mechanism of (experimentally observed) retardation of crack growth through the shot-peening-affected layer and also quantified this influence on fatigue life. The relative contributions of compressive residual stresses and strain hardening were assessed by investigating them separately. The sub-surface compressive residual stress distribution produced by shot peening could effectively reduce crack propagation but the strain hardening distribution, in contrast, can accelerate it. However, strain hardening is expected to hinder the crack initiation process by restricting the plastic deformation during cyclic loading. Predictions of the fatigue life of the shot-peened notched specimens were made based on this numerical analysis. Acceptable results were obtained using both the LEFM and EPFM approaches and the difference between them is discussed.

Does a nucleated crack propagate?

Conditions for propagation or non-propagation of an incipient crack from stress concentrations such as notches, holes, etc., are evaluated using elastic-plastic fracture mechanics methods and the Kitagawa-Takahashi diagram, hence linking smooth specimen behavior to fracture mechanics. The analysis differs from all other previous models which are either empirically based or assume that short crack growth behavior differs from that of long cracks due to crack closure or lack of it. It is shown that the stress intensity factor of an incipient crack decreases and then increases as it moves away from the internal stress field of a notch. Crack propagation is ensured only when the minimum is equal to or greater than the unique fatigue threshold for crack growth. Otherwise non-propagation conditions prevail. Equations are developed to establish the conditions for the minimum stress required for continuous propagation of the originated crack. It is shown that by analyzing published results on notch-fatigue that specimen failure occurs only if the applied stresses exceed the minimum stress required for crack propagation. Otherwise crack arrest occurs resulting in non-propagating cracks. The analysis can be readily used to determine the conditions for failure of a notched specimen if the elastic stress concentration factor, kt, the notch-tip radius, ρ, and fatigue crack threshold, Kth, for the material are known. The analysis, in principle, is applicable to all subcritical crack growth processes.

Effects of dissolved hydrogen and surface condition on the intergranular stress corrosion cracking initiation and short crack growth behavior of non-sensitized 316 stainless steel in simulated PWR primary water

The effects of dissolved hydrogen (DH) and inner surface finish condition of the hollowed specimen on stress corrosion crack (SCC) initiation and small crack growth behaviors of 316 stainless steel were investigated in high temperature water with various DH levels. The drilled specimen showed higher surface hardness and more enhanced SCC initiation and short crack growth behavior than honed specimen but both showed a similar trend in the water chemistry with different DH levels. Fractographic examinations showed mixed modes of transgranular and intergranular cracking. The influences of DH and surface condition on SCC behaviors are discussed.

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

AbstractThe short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Short and long crack growth behavior of welded ferritic stainless steel

To analyze the influence of welding on fatigue crack growth, several geometrical configurations have been tested using notched specimens with notch located within base metal and welded joint. This methodology has shown that fatigue crack growth rate was similar for base metal and welded part for the ferritic stainless steel F18TNb (corresponding to AISI 441 or EN 1.4509 grades) considering long crack. Whereas for short crack, the microstructure induced by the welding process was evidenced to drastically increase the resulting crack growth rate. The tested configurations have been systematically modeled by finite element analysis to obtain reliable shape function and SIF assessment for the chosen geometries. Then fatigue crack growth behavior is discussed on the basis of both SIF values and the influence of microstructure in crack path and crack growth rate.