Enhanced Fatigue Crack Growth Research Articles

Microstructural and oxidation effects on fatigue crack initiation mechanisms in a turbine disc alloy

Effects of microstructure and oxidation on fatigue crack initiation and early propagation processes were investigated in RR1000 turbine disc alloy with different γ′ distributions and carbide distributions on the grain boundary. Fatigue tests were carried out under three-point bending and trapezoidal waveform loading (with a 90 s dwell) at 650 °C in air. The failure mode in both γ′ variants is clearly characterised by intergranular features. A number of fatigue cracks are seen to initiate at grain boundaries with bulged Co-rich oxides at the surface and/or interfaces between carbides and grain boundaries, resulting from oxidation damage assisted by applied loading. Reduced lifetime is closely linked to significant intergranular crack initiation and frequent consequent crack coalescence events, which results in enhanced fatigue crack growth (FCG) rates. The extent of intergranular features and enhanced FCG are more marked where more continuous carbides exist at the grain boundary.

Analysis of Hydrogen-Induced Changes in the Cyclic Deformation Behavior of AISI 300–Series Austenitic Stainless Steels Using Cyclic Indentation Testing

The locally occurring mechanisms of hydrogen embrittlement significantly influence the fatigue behavior of a material, which was shown in previous research on two different AISI 300-series austenitic stainless steels with different austenite stabilities. In this preliminary work, an enhanced fatigue crack growth as well as changes in crack initiation sites and morphology caused by hydrogen were observed. To further analyze the results obtained in this previous research, in the present work the local cyclic deformation behavior of the material volume was analyzed by using cyclic indentation testing. Moreover, these results were correlated to the local dislocation structures obtained with transmission electron microscopy (TEM) in the vicinity of fatigue cracks. The cyclic indentation tests show a decreased cyclic hardening potential as well as an increased dislocation mobility for the conditions precharged with hydrogen, which correlates to the TEM analysis, revealing courser dislocation cells in the vicinity of the fatigue crack tip. Consequently, the presented results indicate that the hydrogen enhanced localized plasticity (HELP) mechanism leads to accelerated crack growth and change in crack morphology for the materials investigated. In summary, the cyclic indentation tests show a high potential for an analysis of the effects of hydrogen on the local cyclic deformation behavior.

Controlling factors and mechanisms of fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron

This paper proposes an extended discussion of experimental results previously published by the authors and from literature about fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron. Firstly a summary of these results obtained for different exposure conditions in terms of gas pressure and loading frequency is presented. Two characteristic regimes, the first at low ΔK values characterized by a moderated fatigue crack growth enhancement and a nearly fully intergranular mode, and a highly-accelerated regime with a quasi-cleavage failure mode with the formation of coarse striations. The mechanisms controlling the different regimes of Hydrogen-Assisted Fatigue Crack Growth are discussed on the basis of observations of fracture surfaces and of deformation substructures within the plastic zone while assessing the capability of different models from literature to account for these observations. In a last section, the dependency of the transition between the different regimes as well as the crack growth rate enhancement in the highly-accelerated regime with respect to the hydrogen gas pressure and loading frequency is analyzed.

Hydrogen enhanced fatigue crack growth rates in a ferritic Fe-3 wt%Si alloy and a X70 pipeline steel

It is well known that ferrous materials can be damaged by absorption of hydrogen. If a sufficient quantity of hydrogen penetrates into the material, static fracture and the material's fatigue performances can be affected negatively, in particular causing an increase in the material crack growth rates. The latter is often referred as Hydrogen Affected-Fatigue Crack Growth Rate (HA-FCGR). It is therefore of paramount importance to quantify the impact, in terms of hydrogen induce fatigue crack growth acceleration in order to determine the life of components exposed to hydrogen and avoid unexpected catastrophic failures. In this study, in-situ fatigue crack growth rate testing on Compact Tension (CT) specimens were carried out to determine the fatigue crack growth behaviour for a Fe-3 wt%Si alloy and X70 pipeline steel. Tests were carried out in two environmental conditions, i.e. laboratory air and in-situ electrochemically charged hydrogen, and different mechanical conditions in terms of load ratio (R = 0.1 and R = 0.5, for the Fe-3 wt%Si, R = 0.1 for the X70 steel) and test frequency (f = 0.1 Hz, 1 Hz and 10 Hz) were adopted under electrochemically charged hydrogen conditions. The results show a clear detrimental effect of H for the specimens tested in hydrogen when compared to the specimens tested in air for both materials and that the impact of hydrogen is test frequency-dependent: the hydrogen induced acceleration is more prominent as the frequency is decreased. Post-mortem surface investigations consistently relate the global crack growth acceleration to a shift from transgranular to Quasi-cleavage fracture mechanism. Despite such consistency, the acceleration factor strongly depends on the material: Fe-3wt%Si features acceleration up to 1000 times while X70 accelerates up to 76 times when compare to the material fatigue crack growth rate recorded in air. Observation of the deformation activities in the crack wake in relation to the transition into hydrogen accelerated regime in fatigue crack growth show a tendency toward restricted plastic activity in presence of hydrogen.

Hydrogen isotope effect on the embrittlement and fatigue crack growth of steel

The deleterious effect of hydrogen on steel is a long-standing problem. Embrittlement in the presence of hydrogen is, in part, a consequence of the fast diffusion of hydrogen in ferritic steels. Because of the identical chemical properties but large differences in mass between hydrogen isotopes, interaction of hydrogen isotopes with steel structures are of interest as a tool to study the role of diffusion in hydrogen embrittlement under all-else-equal conditions. In this paper, we present tensile and fatigue data for steel samples measured in air, in hydrogen, and in deuterium. Tensile measurements show deuterium causes a loss of ductility comparable to the loss due to hydrogen. In fatigue, a large pressure effect on deuterium-assisted fatigue crack growth was observed, an effect which is not exhibited in uniaxial tensile measurements. With sufficient pressure to show an enhanced fatigue crack growth effect, the fatigue crack growth rate is well-predicted assuming a scaling of the diffusion coefficient by 1/2. These results suggest surface adsorption kinetics play a large role in hydrogen-assisted fatigue crack growth at low pressure. At higher pressures, hydrogen-assisted fatigue crack growth is primarily affected by the kinetics of diffusion of hydrogen within the steel lattice.

Noise isolation with phononic crystals to enhance fatigue crack growth detection using acoustic emission

The background noise imposes difficulties in monitoring crack activities in real-time monitoring of structures using the acoustic emission (AE) method. Here, a two-dimensional phononic crystal (PC) is designed to enhance fatigue crack growth detection capability of acoustic emission technique in plate-like structures, such that a single AE sensor is sufficient to monitor the crack activity. PCs, also known as acoustic metamaterials, are artificial composite, man-made periodic systems creating new responses through physical constraints in the constituent materials such that certain frequencies are prohibited by local the resonance of the periodic system. The integration of PC design with AE method can be utilized to block unwanted noise signals and enhance the crack detection ability of the AE method. The proposed PC structure to block certain frequencies is numerically studied using COMSOL Multiphysics software through applying two approaches: the unit cell analysis and the transmission loss. It is concluded that the primary reason of the band gap formation through the designed PC structure is the local resonance phenomenon. The final geometry is selected to provide a band gap near 150 kHz, which is a common frequency to monitor crack growth in metallic structures. The stubs forming the PC structure are placed on the compact specimen periodically, except along the crack growth path and sensor position, to block the friction emissions coming from the grip locations. The performance of the designed PC structure is demonstrated experimentally through the fatigue testing of aluminum standard compact tension specimens. The cumulative numbers of hits and AE energy obtained from the specimen integrated with the PC structure demonstrate the successful implementation of noise blocking in the AE testing of detecting active cracks.

Rolling contact fatigue performance evaluation of Ti–6Al–4V parts processed by selective laser melting

Selective laser melting (SLM) of titanium components is gradually gaining popularity in various fields, such as aerospace, automobile, and medical for the purpose of saving material and reducing the carbon footprint. To capitalize on their benefits, the mechanical properties of parts fabricated by SLM must meet engineering requirements. Fatigue properties are particularly important for the performance of safety-critical structures. In this research, rolling contact fatigue (RCF) tests were performed for SLM fabricated Ti–6Al–4V samples in differently post-treated conditions to investigate the effect of different post-processing methods on the fatigue properties and, as a comparison, forgings of Ti–6Al–4V were also tested. It was found that the RCF properties of the Ti–6Al–4V generated by SLM were superior to those of the forged specimens and strengthened after polishing and heat treatment. The worn surfaces were observed by using an optical microscope, and the results indicated that surface and subsurface defects are the main causes of fatigue crack initiation and the residual stresses enhance fatigue crack growth.

Hydrogen Enhanced Fatigue Crack Growth Rates in a Ferritic Fe-3wt%Si Alloy

It is well known that the presence of hydrogen in ferrous materials promotes both static fracture and affect the material fatigue crack growth rates. The latter is often referred to as Hydrogen Enhanced Fatigue Crack Growth Rate (HE-FCGR) which defines the reduction of crack growth resistance of the material under cyclic stresses when hydrogen is present. When it comes to the determination of the life of components exposed to hydrogen it is therefore of paramount importance to establish such hydrogen induced variation in crack speed in the material in order to avoid unexpected catastrophic failures. In this study the fatigue crack growth rate was determined for a Fe-3wt%Si alloy. Compact tension specimens were used to determine the Paris regime of the fatigue crack growth rate curve of the material. Two environmental conditions were investigated: laboratory air and in-situ electrochemically charged hydrogen. Different mechanical conditions, in terms of load ratio (R=0.1 and R=0.5) and test frequency (f=0.1 Hz, 1 Hz and 10 Hz), were used under electrochemically charged hydrogen conditions. The results show that compared to the specimens tested in air, there is a clear detrimental effect of H for the specimens tested in hydrogen, in terms of accelerated crack growth. The strength of the impact of hydrogen in enhancing the fatigue crack growth rates of the Fe-3wt%Si alloy clearly depends on the test conditions. Fractographic investigations were also used to unveil the mechanisms involved in the process leading to accelerate crack growth in presence of hydrogen.

Illuminating the chemo-mechanics of hydrogen enhanced fatigue crack growth in aluminum alloys

The presence of elemental hydrogen is known to accelerate fatigue crack growth in aluminum alloys. However, a direct link between experimental data and the governing atomistic mechanisms has remained elusive. Here we present a series of computational studies, across multiple length scales, directly linking an atomistic mechanism to experimental data for a specific aluminum alloy. Starting with an ab initio investigation of hydrogen bonding near the (1 1 1) aluminum surface, we quantify the effects of hydrogen surface impurities on slip and decohesion. We then modify an aluminum-only interatomic potential to reproduce ab initio trends by strategically shielding critical surface bonds in accordance with the environmental exposure level. The strategic shielding approach is used within a coupled atomistic-continuum discrete dislocation framework to predict the effect of hydrogen on near threshold fatigue crack growth rates. The predicted trends agree with published experimental data, suggesting that hydrogen enhanced surface deformation is a key failure mechanism for aluminum alloys in humid environments. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Atomistic mechanisms of fatigue in nanotwinned metals

We investigate the fatigue behavior of nanotwinned Cu using a combination of molecular statics and molecular dynamics simulations. The presence of nanoscale twins is found to enhance fatigue crack growth resistance. For the twin-free nanocrystalline samples, the fatigue crack propagates by linking the nanovoids that are formed ahead of the crack tip. In the case of the nanotwinned samples, however, it advances as the crack tip alternately blunts and re-sharpens due to dislocation emission and slip. Both detwinning and crack closure are observed in the path of the fatigue crack in nanotwinned samples with a high density of twin boundaries. As the twin number per grain (quantified by the ratio of the mean grain size to the twin boundary spacing d/λ) increases, detwinning increases the dissipated energy of fatigue cracking, leading to enhanced fatigue resistance. The atomistic simulations show that fatigue crack growth in nanotwinned Cu conforms to Paris’ law. In conjunction with the experimental results, we obtain a quantitative estimation of the Paris’ law exponent (∼4.0), which is in agreement with the theoretical predictions from the damage accumulation model.

Enhanced Model for Describing Total Fatigue Rate Curve considering Stress Ratio Effects

An enhanced fatigue crack growth model is proposed to approximate fatigue crack growth rate in all three regions of fatigue rate curve. The model uses Collipriest's crack driving force for accounting stress ratio effect on fatigue crack growth. Seven fatigue crack growth models, namely Priddle, Mcevily, Weertman, Collipriest, Broek, Walker and Forman are examined. Performance of the crack driving force of these models in depicting stress ratio effects in fatigue crack growth rate is evaluated by fitting a lowess curve in transformed FCG data. Non-uniform B-spline is used to describe fatigue rate curve which allows local movements of the fatigue crack growth data that can be matched well in the vicinity without extending their effects to distant parts of the curve. Experimental FCG data of Al 2024 T351 from literature are used for validation of the model.

Effect of retrogression and reaging treatment on the microstructure and fatigue crack growth behavior of 7050 aluminum alloy thick plate

Effect of Retrogression and Reaging (RRA) treatment on the microstructure and fatigue crack growth behavior of 7050 aluminum alloy thick plate was investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results showed that the RRA microstructure is similar to that for T6 treatment inside the grains, but is slightly denser and coarser and more stable in nature. Also grain boundaries precipitates (GPBs) coarsen obviously, distributing discontinuously and isolated. The precipitation free zones (PFZs) are broadened than that of the T6 sample, and narrower than that of the T7451 sample. And maintaining strength equal to that for the T6 peak-aged treatment and enhanced fatigue crack growth resistance to that for the T7451 overaged treatment can be achieved for RRA treatment.

Hydrogen enhanced fatigue crack growth in an HSLA steel

Fatigue crack growth rate (FCGR) for an HSLA steel was studied with in-situ hydrogen charging. The effects of different parameters, like stress intensity range (ΔK), frequency, load ratio (R), stress values and crack length have been investigated. The hydrogen contribution to FCGR decreased with increasing ΔK. FCGR was inversely dependent on frequency. The role of loading strain rate on the hydrogen enhanced FCGR was explained. The effect of load ratio on hydrogen enhanced FCGR was not monotonic. A dependence of FCGR of hydrogenated samples on crack length was observed and explained.

Role of Sour Environments on the Corrosion Fatigue Growth Rate of X65 Pipe Steel

Corrosion fatigue of C-Mn steels in sour environments is a concern in various offshore applications, particularly for flow-lines and risers. The commonly accepted mechanism of the enhanced fatigue crack growth rate (FCGR) behavior in these environments is hydrogen embrittlement. Hydrogen enters in the metal exposed to a sour environment through corrosion reactions occurring on the metal surface (referred to as bulk-charged hydrogen). During crack propagation, the fresh exposed metal at the crack tip under fatigue loading can also react with the corrosive environment and therefore generate hydrogen (referred to as crack tip hydrogen). Potentially, both of these two sources of hydrogen can impact the FCGR of the pipe steel. In this work, it was found that the bulk-charged hydrogen played a more dominant role in enhancing the FCGR of carbon steel in sour environments. The contribution to the FCGR from the crack tip hydrogen is not appreciable. The interaction of the freshly exposed metal with the corrosive environment could lead to the formation of iron(II) sulfide (FeS) film at the crack tip. This could lead to crack closure and decrease the effective ΔK and thus decrease the FCGR at low frequency, particularly at high H2S concentration and low pH environments, which is likely the cause for the observation of a plateau in FCGR in the low-frequency regime for carbon steel in the sour environment. Additionally, the experimental results suggest that although diffusible hydrogen was primarily responsible for the enhanced FCGR in the sour environment, the trapped hydrogen also played some role.

Fatigue crack propagation under gaseous hydrogen in a precipitation-hardened martensitic stainless steel

A test device has been developed at P′ Institute in order to investigate the mechanical behaviour of structural materials under high pressure of gaseous hydrogen. In this paper, preliminary results on fatigue crack propagation in a martensitic stainless steel are presented. A tremendous fatigue crack growth enhancement is observed at high pressure (9 MPa). This enhancement is dependent of pressure. It is noticed that the maximum enhancement is associated with a brittle fracture mode. However no intergranular decohesion is noticed in this regime.

Synergistic action of fatigue and corrosion during crack growth in the 2024 aluminium alloy

This study was undertaken with the aim of identifying the domains where synergistic interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024, as well as the rate controlling factors. Corrosion fatigue crack propagation tests have been carried out at different frequencies and/or waveforms, grain orientations and tempers, in air, distilled water and 3.5% NaCl solution at free potential. In saline solution in the S-L orientation, crack growth rates are shown to decrease with decreasing frequency. In addition, at high frequencies, these growth rates are substantially higher than in air and in distilled water. This enhancement is accompanied by a typical change in the shape of the dadN curve. It is demonstrated that this enhancement, that is only observed in metallurgical conditions of high sensitivity to stress corrosion cracking, is controlled by the loading rate, and not by the duration of exposure to the corrosive medium. The behaviour is discussed on the basis of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. It is finally proposed that the fatigue crack growth enhancement observed is related to a crack-tip embrittlement mechanism by hydrogen produced by anodic dissolution in relation with film rupture periodicity.

Effects of size and spacing of uniformly distributed pearlite particles on fatigue crack growth behavior of ferrite–pearlite steels

Fatigue crack growth tests of three ferrite–pearlite steels with different size and spacing of pearlite particles, which were uniformly distributed in the ferrite matrix, were carried out to investigate the effect of microstructure on fatigue crack growth behavior in the Paris regime. The fatigue crack growth rates for the three materials did not coincide with each other, even when the crack growth curves were arranged by the effective stress intensity factor range. From in situ observations, crack tip stress shielding phenomena, such as interlocking and branching, were found on the crack wake, which enhanced fatigue crack growth resistance. A small size and spacing of pearlite particle seemed to induce small but frequent crack deflections, which resulted in crack closure phenomena. On the other hand, the large size of the pearlite particle seemed to induce stress shielding phenomena and, thus, contribute to high crack growth resistance, which was the main reason for the higher fatigue crack growth resistance of the large size and spacing of pearlite particles compared to the small size of pearlite particles.

Near-threshold fatigue crack growth in bulk metallic glass composites

A major drawback in using bulk metallic glasses (BMGs) as structural materials is their extremely poor fatigue performance. One way to alleviate this problem is through the composite route, in which second phases are introduced into the glass to arrest crack growth. In this paper, the fatigue crack growth behavior of in situ reinforced BMGs with crystalline dendrites, which are tailored to impart significant ductility and toughness to the BMG, was investigated. Three composites, all with equal volume fraction of dendrite phases, were examined to assess the influence of chemical composition on the near-threshold fatigue crack growth characteristics. While the ductility is enhanced at the cost of yield strength vis-à-vis that of the fully amorphous BMG, the threshold stress intensity factor range for fatigue crack initiation in composites was found to be enhanced by more than 100%. Crack blunting and trapping by the dendritic phases and constraining of the shear bands within the interdendritic regions are the micromechanisms responsible for this enhanced fatigue crack growth resistance.

Influence of frequency and waveform on corrosion fatigue crack propagation in the 2024-T351 aluminium alloy in the S–L orientation

This study was undertaken in order to identify the domains of possible synergistic effects between corrosion and fatigue damage during crack growth in aluminium alloys. In this aim the 2024-T351 aluminium copper alloy was selected and tested in the S–L orientation. The effects of load frequency and waveform have been investigated at a load ratio R = 0.7 in a 3.5% NaCl solution and compared to data obtained in air and distilled water. In saline solution, a fatigue crack growth enhancement associated with transitions in the d a/d N curve is observed at high frequencies under sinusoidal signals, while low frequencies provide results comparable with those obtained in air. It is furthermore shown that this behaviour is primarily controlled by the rising time (RT), i.e. the duration of the rising load part of a cycle, and not by the period. A competition between passivation of the crack tip and hydrogen embrittlement caused by localized anodic dissolution may account for the frequency effect observed in this study.

Influence of frequency and exposure to a saline solution on the corrosion fatigue crack growth behavior of the aluminum alloy 2024

The aim of this study is to identify domains where the interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024. The scope considered encompasses the influence of frequency and of alternate immersion in saline solution. Corrosion fatigue crack propagation tests have been carried out under sinusoidal and saw-tooth waveforms, and at different frequencies, load ratios, grain orientations and tempers, in air, distilled water and 3.5% NaCl in permanent and alternate immersion. The stress corrosion cracking behavior of the alloy 2024 has also been considered in order to evaluate the possible contribution of this type of damage during corrosion fatigue crack growth. In 3.5% NaCl, growth rates were found to decrease with decreasing frequency. In alternate immersion, growth rates were increased by up to an order of magnitude for the Δ K values considered compared to permanent immersion and air. The possible mechanisms that govern the corrosion fatigue behavior of the 2024 alloy are discussed in terms of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. Finally, it is proposed that the fatigue crack growth enhancement observed during permanent immersion is related to a crack-tip hydrogen embrittlement mechanism. Hydrogen would be produced by anodic dissolution in relation with film rupture periodicity and then be dragged into the process zone. In alternate immersion, precipitate-free zone dissolution would govern crack advance, as during stress corrosion cracking.