Dwell Fatigue Research Articles

Predicting crack nucleation in commercially pure titanium using orientation imaging microscopy and machine learning

Due to the prohibitively long experimental and simulation times, dwell fatigue (DF) failure prediction in titanium and its alloys is a challenging task. Since most of these failures have a microstructural level origin, this work focusses on utilizing minimal experiments and machine learning for predicting failure initiation points in a given microstructure. Failure initiation points in commercially pure titanium were identified using interrupted tensile and DF tests. Orientation imaging data was used to train a Random Forest model to calculate the relative importance of various grain orientation-based features to crack nucleation. Subsequently a predictive model for identifying locations that are likely to form a DF crack in a microstructure is developed.

Exploring microcrack formation in cyclic and dwell fatigue in MTR-lean Ti 6242 alloy

This study examines cyclic and dwell fatigue crack initiation and growth in well forged, low microtexture intensity Ti 6242 alloy. The statistics of crack occurrence have been explored in cross-sections of samples that failed in cyclic and dwell fatigue. Cracks are observed dominantly on the basal planes of the primary α grains with a high Schmid factor for basal slip in both cyclic and dwell fatigue and also in hard α grains in dwell fatigue. The crystallography of early crack initiation and extension is described. Electron channelling contrast imaging shows that basal slip dominates low strain behaviour in this alloy, leading to the observed crack formation.

Investigating dwell fatigue crack growth in welded titanium alloy structures

ABSTRACT In this paper, dwell-fatigue crack growth tests under complex loading spectra were conducted for a titanium alloy used in marine structures. The influence of upper and lower peak dwell time on the dwell-fatigue crack growth behavior of the welded structure were comprehensively investigated. The results show that the longer the upper and lower peak dwell times were, the faster the crack growth rate was. Still, the impact of the upper and lower dwell times on the crack growth rate of titanium alloy had a specific saturation value. Based on the above findings, a new model was developed that combined the fracture mechanics theory, considering the effects of welding and load dwell on crack growth. By predicting the dwell-fatigue behavior of titanium alloy welded structures using the model, it was confirmed that the model has a strong prediction ability for the dwell-fatigue behavior of welded structures under complex loads..

Effect of CrN coating on the hot salt corrosion fatigue behavior of titanium alloy

The dwell fatigue behavior of CrN coated titanium alloys were carried out in air and hot salt corrosive environment at 500 ℃. The results indicate that the decrease of the elastic modulus difference between the coating and the substrate alleviates the deformation mismatch issue at high temperature. Stress relaxation at the crack tip caused by Cr transition layer oxidation can improve the fatigue properties of the coated samples. Furthermore, the CrN coating enhances the corrosion fatigue properties, which is related to the barrier effect of the coating on the corrosive medium. The crack growth mode in the substrate transition from stress corrosion crack to fatigue damage crack.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

In-plane biaxial low-cycle dwell fatigue behavior of CP-Ti based on DIC method

This study investigates the biaxial dwell fatigue behavior of commercial pure titanium (CP-Ti) under different load ratios and dwell times. The evolution of mean strain, ratcheting strain, and creep strain was systematically analyzed. The findings reveal that with the decrease of load ratio or the increase of dwell time, both ratcheting and creep strain exhibit an upward trend. The interaction between ratcheting strain and creep strain promotes crack initiation and propagation. The statistical analysis of crack paths by the digital image correlation (DIC) method indicates that crack initiation occurs between 0.7Nf and 0.8Nf, and fatigue failure is governed by the maximum principal strain. Moreover, the electron backscatter diffraction (EBSD) analysis results indicate that as the load ratio increases, the activity of prismatic slip systems decreases, while the activities of other slip systems and twinning modes increases. To enhance predictive accuracy, a modified SWT model by incorporating time-dependent creep damage and anisotropy is proposed for life prediction under biaxial dwell fatigue conditions.

Effect of peak stress and dwell time on the fatigue damage behavior of Ti-22Al-23Nb-2(Mo, Zr) alloy with lamellar microstructure

Ti2AlNb alloy is one of the candidate high strength titanium alloy materials under consideration for compressor disc, which will endure dwell loading at high peak stress level during service. In this work, low cycle fatigue (LCF) and low cycle dwell fatigue (LCDF) deformation behaviors and damage mechanism of Ti2AlNb alloy with lamellar microstructure under high loading level were investigated. The results show that fatigue life is highly dependent on the peak stress (σpeak) and dwell time (tdwell). With increasing the σpeak, the LCF life decreases gradually since higher loading stress can accelerate plastic strain accumulation and lead to faster crack propagation. When at the same σpeak, the LCDF life is lower than the LCF life due to greater plastic strain accumulation caused by dwell effect, indicating a dwell life debit of the Ti2AlNb alloy. Moreover, dwell fatigue sensitivity exhibits an increasing trend as the tdwell increases. Under different loading waveforms, the fractography shows remarkably different features. Crack source of LCF appears on sample surface, while crack source of LCDF appears on surface and sub-surface of sample. The research of LCDF behavior is relevant to aviation industry to prevent premature failure of rotating components made of Ti2AlNb alloys.

Cyclic deformation behaviors and damage mechanisms in P92 steel under creep-fatigue loading: Effects of hold condition and oxidation

P92 steel has gained plenty of attention as a top contender for steam generator components and main steam tubes due to the growing need to increase the energy conversion efficiency. However, a major contributor to the premature failure of these components remains the interaction of creep and fatigue damage caused by typical dwell fatigue loading conditions. In this study, creep-fatigue tests of P92 steel were conducted under strain-controlled cyclic loading including hold durations up to 3600 s at 873 K in air. Special attention was given to the influence of hold conditions and oxidation on cyclic deformation and damage mechanisms. The results revealed that introducing hold time substantially decreased the failure life, with compressive hold causing more severe damage than tensile hold. Additionally, oxidation became severer with longer hold time, resulting in increased surface roughness, crack initiation sites and density of secondary cracks. Fatigue dominated the failure in the case of the short-term tensile hold, and the interaction of creep, fatigue and oxidation dominated the failure in long-term tensile hold tests. In contrast, the failure in compressive hold case was dominated by fatigue and severe oxidation. Furthermore, a three-dimensional oxidation-creep-fatigue damage interaction diagram was proposed for quantitative assessments in practice.

Data-driven machine learning approach for predicting dwell fatigue life in two classes of Titanium alloys

Predicting dwell fatigue life accurately in Titanium (Ti) alloys is vital for designing reliable components such as aero-engine discs and blades. This study proposes a data-driven machine learning (ML) approach for precisely predicting dwell fatigue life in two classes of Ti alloys: near α and α + β alloys. To build a robust model, the data-driven approach tackles this multi-dimensional problem by training a comprehensive dataset of various input features, including alloy composition, monotonic properties, and fatigue testing parameters. Advanced ensemble-based algorithms, Gradient Boosting (GBOOST) and Extreme Gradient Boosting (XGBOOST) are employed. The split-insensitive best-performing XGBOOST model achieved a high accuracy (R2) score of 89.9 %, indicating impressive prediction capability. The model’s robustness is further assessed using an unseen validation dataset to draw qualitative and quantitative conclusions. ML model’s validation further involved comparing its predictions with actual experimental trends across relationships, such as dwell life versus dwell time and dwell-debit versus dwell time. Dwell-debit is also predicted as a function of varying stresses normalized to 0.2 % yield strength, highlighting the susceptibility of investigated Ti alloys to dwell fatigue. Additionally, the SHapley Additive exPlanations (SHAP) analysis was conducted to gain deeper insights into various input features affecting dwell fatigue life.

Crack initiation mechanisms of micro-textured Ti60 alloys with different dwell sensitivities subjected to fatigue and dwell fatigue loading

In this work, Ti60 alloys with different micro-texture intensity were designed to elucidate the effect of micro-texture on fatigue life and dwell sensitivity and the difference in crack initiation mechanism for low-cycle fatigue (LCF) and dwell fatigue (DF). It was found that micro-texture region (MTR) significantly reduced LCF and DF lifetimes and increased dwell sensitivity compared to no-MTR samples. However, micro-texture does not change the characteristics of the crack initiation grains, and the fracture initiation region facet of all samples matches the (0001) basal plane and is related to basal slip. Therefore, the [0001] orientation domains for crack initiation in both fatigue states are discussed, and it is found that LCF tends to nucleate along the basal slip bands or basal twist grain boundaries (BTGBs) of the high SF grains, whereas DF nucleation is independent of the basal SF value, and the mechanism of initiation of hard oriented grain cracking is also included. In situ further demonstrated that hard grains can activate basal slip as well as cracking, and the possibility of soft/hard grain pairs generating dislocation pile-up stresses leading to cracking of hard grains was determined by theoretical calculations, providing experimental support and evidence for the soft/hard grain model. Based on these findings, criteria for the angle (θ) of the c-axis with respect to the loading axis and the basal SF range are proposed to determine the differences between LCF and DF with respect to the mechanisms of hard grain cracking and BTGB crack initiation.

Crystallographic micro-mechanism of faceted crack initiation in near-α titanium alloy Ti6321 under room-temperature fatigue and dwell fatigue loadings

Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect, and it is not completely understood yet. This study conducted stress-controlled low-cycle fatigue and dwell fatigue tests on Ti-6Al-3Nb-2Zr-1Mo alloy with bimodal microstructure to reveal its microstructural characteristics and crack initiation mechanisms. The study demonstrated that the faceted primary α nodules located near the specimen surface acted as crack initiation sites during both fatigue and dwell fatigue tests. Slip trace analysis revealed that faceted cracking occurred at (0001) basal plane with the maximum Schmid factor value through a special cracking mode referred to as (0001) twist boundary cracking. Innovative criteria of parameters C1 and C2 were proposed based on experimental observation and molecular dynamics simulations, which well identify candidates for (0001) twist boundary crack nucleation. It demonstrated that grain pairs combining a moderately high Schmid factor for basal slip and a well-orientated Burgers vector in the out-of-surface plane was the preferable location for surface (0001) twist-boundary crack initiation, and grain pairs combining a high Schmid factor for basal slip and a high normal stress on basal plane are perfect candidates for subsurface cracking. Based on this, phenomenological models are proposed to explain the surface (0001) twist-boundary cracking mechanism from the perspective of surface extrusion-intrusion-induced micro-notches.

Crack nucleation and dislocation activities in titanium alloys with the strong transverse texture: Insights for enhancing dwell fatigue resistance

Cold dwell sensitivity of near α titanium alloys has posed a significant challenge to the engineering safety within the aerospace industry. However, there exists inconsistency regarding the critical facet formation mechanism between basal and prismatic nucleation. A key revelation in this paper is that the controversy surrounding the facet nucleation plane primarily arises from variations in texture: when the loading direction is parallel to the c-axis or at a 45 ° angle, it leads to basal plane cracking, whereas loading direction perpendicular to the c-axis results in prismatic plane cracking. What's more, a large anisotropy is observed in the dwell fatigue performance in descending order: rolling direction (RD) > transverse direction (TD) >45° direction. To be specific, the dwell fatigue life of RD specimens was 2 times and 4.07 times longer than that of TD and 45 ° specimens, which exhibits excellent dwell fatigue resistance. Furthermore, the high prismatic dislocation density near the facet of RD specimen was confirmed through the Transmission Electron Microscope (TEM) and Transmission Kikuchi Diffraction (TKD) map, which is evidenced that prismatic slips lead to higher strain hardening during cyclic loading. The basal plane of facet grain is generally parallel to the facet line, indicating that the elliptical facet developed by the transgranular fracture through basal plane. Twinning activation in hard grain to accommodate deformation in Ti60 alloy is firstly observed under dwell fatigue tests, which is a product of high stress level in the hard grain. This paper unveils the significant impact of texture on dwell fatigue resistance, offering novel insights into enhancing dwell fatigue performance through texture control.

Development of a scanning acoustic microscopy-based structural prior for microtexture region characterization

Nondestructive evaluation (NDE) plays a crucial role in ensuring aircraft availability. The current NDE paradigm often relies on mono-modal testing and signal-over-threshold criteria to provide robust defect or damage detection, not characterization. One example is found in the risk-based management of surface-breaking cracks in metal, where cracks of a given size can be detected by eddy current testing (ECT) with a calculable probability. Yet, there are cases where detection proves insufficient. Consider the case of microtexture regions (MTR) found in certain titanium alloys, which can increase the risk of cold dwell fatigue failure when found above a certain size and in specific orientations relative to the surrounding material. At present, the size and orientation of MTR cannot be characterized using only one NDE modality. In this work, a data fusion-based solution to MTR characterization is developed. First, two inspection methods—scanning acoustic microscopy (SAM) and ECT—are selected, where each method is individually capable of only partial characterization. Then, matching component analysis is used to develop a surrogate forward model relating MTR orientation to ECT output. This data is then inverted using boundaries provided from the SAM data as a structural prior.

Investigation of neighboring grain effects on load shedding in titanium alloys under cold dwell fatigue

Load shedding under cold dwell fatigue in titanium alloys has been a threat to the safety of the aero-engine since it is responsible for crack formation. The strain incompatibility between the soft-hard grain pair arises from the significant creep in the soft grain during the dwell period, which is fundamentally related to the load shedding. Such deformational heterogeneity is not only affected by the soft-hard grain itself but also by its neighboring grains, but the effect of the latter is rarely reported. In this paper, the effect of neighboring grains, including location, crystal orientation, texture, etc., on load shedding is quantitatively investigated using a three-dimensional rate-dependent crystal plasticity model by controlling the microstructural morphology of the grains. In particular, the mechanism that the dwell fatigue cracks tend to nucleate from the sample's interior rather than the free surface is revealed. Restriction of rigid body rotation from neighboring grains can further enhance load shedding. The crystal orientation of individual and group neighboring grains sitting at different locations with respect to the soft-hard grain pair is evaluated. The grain directly attached to the hard grain has the strongest influence. The statistics of load shedding are analyzed by conducting hundreds of simulations with different textures. Stronger textures are found to be able to enhance load shedding and thus deteriorate the service lifetime. The change of locations showing the highest stress during the dwell period arising from plastic deformation in neighboring grain may lead to the phenomenon of reverse load shedding. Understanding the neighboring grain effects from both the mechanistic basis and the statistical point of view is important in integrity evaluations and service life assessments of titanium components in aero-engines.

Low-cycle and dwell fatigue properties for a near alpha titanium alloy Ti–6Al–3Nb–2Zr–1Mo

As a new type of marine titanium alloy, Ti–6Al–3Nb–2Zr–1Mo (Ti6321) has gained attention for its comprehensive properties such as high toughness, strength, weldability, and corrosion resistance. However, there are only a few reports on fatigue properties of Ti6321. In this study, the dwell fatigue and low cycle fatigue tests were performed under various maximum load stresses with the stress ratio of R = 0. Experimental results revealed that the failure can be classified into brittle and ductile types. Fatigue life decreases with the increase of maximum load stress. The dwell time facilitates the accumulation of plastic strain in each loading-unloading cycle and significantly reduces fatigue life. The stress-strain response in the initial period of loading has a decisive effect on fatigue life for both low-cycle and dwell fatigue. The plastic strain energy density in the first loading cycle and fatigue life satisfies the power law relationship, which can be utilized to predict the load holding and low cycle fatigue life of Ti6321.

The influence of crack and interface oxidation on dwell fatigue crack propagation behavior of the nickel-base superalloy ATI A718Plus

The crack propagation properties are an important and often limiting factor for the commercial application of superalloys in aerospace applications where the oxidation ahead of the crack tip plays an important role. Previous research on the Ni-base superalloy A718Plus revealed that the orientation and the volume fraction of the high temperature grain boundary phases δ and η have a major influence on the dwell fatigue crack propagation rate at 650 °C in ambient air but not in vacuum. In this work, the effect of internal oxidation at the η/δ–matrix interface and its effect on dwell fatigue crack propagation and mechanical properties are examined by advanced microscopic methods and micro-cantilever testing. During crack propagation, the crack tip is exposed to air where a Nb-rich oxide layer forms and embrittles the η/δ–matrix interface causing the cracks to deflect and propagate along the oxidized interfaces. Micro-cantilever tests on the oxidized interface show that these oxide layers also significantly reduce the local strength and fracture toughness of the material. This proves that interfacial oxide layers are the underlying reason for the reduction of the dwell fatigue crack growth resistance at 650 °C, particularly in microstructures whose η/δ–matrix interfaces are oriented parallel to the crack growth direction.Graphical

The effect of anisotropic rate dependency on the deformation response of Ti-6242 during dwell fatigue loading

The α phase of titanium alloys is known to exhibit both elastic and plastic anisotropic mechanical behavior during deformation loading. Recent experimental studies indicate that single crystal rate dependency may also be anisotropic, though these studies are limited in scale and scope. Consequently, we present a study in which anisotropic rate dependency is studied on polycrystalline samples via crystal plasticity finite element (CPFE) simulations, which are compared against previously-published data from electron backscatter diffraction/digital image correlation (EBSD-DIC) experiments. Cold dwell fatigue loading is utilized, owing to the propensity for transient behavior during constant-load dwells, which is hypothesized to be sensitive to the relative rate dependencies on different slip families (i.e., basal, prismatic, and pyramidal) in hexagonal close packed crystals. A suite of CPFE simulations is conducted on polycrystalline samples in which the slip family rate dependency values are parameterized. We analyze simulation results by focusing on the slip activity and accumulation of slip on the various slip families as a function of dwell cycles, as well as the transient effect during singular dwell cycles, for hundreds of crystals simultaneously. Results are further discussed through the lens of findings from EBSD-DIC experiments.

Damage-failure transition in titanium alloy Ti-6Al-4V under dwell fatigue loads

The role of structural mechanisms responsible for the consequent staging of damage-failure transition as the combination and continuity of ductile and creep kinetics of the structure evolution and the modeling in dwell fatigue regime. Damage-failure transition is considered as critical phenomena, the structural-scaling transition, when the damage develops as specific phase with characteristic stages: nucleation of new phase and the phase growth kinetics. In the case of dwell fatigue, the nucleation stage is associated with slip localization, faceting, voids and microcrack initiation; the phase growth kinetics has the relation to specific non-linearity of the free energy release responsible for the staging of damage-failure transition. Statistically based phenomenological model of damage-failure transition specified the links of macroscopic material parameters with structural parameters responsible for the influence of microstructure on the structure sensitive mechanical properties. The developed conception of modeling of Ti alloys based on the duality of damage kinetics in dwell fatigue loads allowed us to propose the strategy of structural study to provide in perspective the links of structural parameters of /phases with phenomenological parameters responsible for different mechanisms of damage accumulation at LCF and stress hold regimes.

The effect of heat treatment temperature on the mechanical properties of TIMETAL® 575

TIMETAL® 575 is an α + β alloy reported to possess superior dwell fatigue capabilities over the conventionally used Ti–6Al–4V. The increased performance of this alloy has in part been linked to the presence of fine-scale tertiary α precipitates, which are present in the microstructure following suitable post-forging heat treatments. However, there is currently little understanding as to the extent that these precipitates influence the mechanical properties. When this is coupled with limited information regarding the temperatures over which tertiary α is expected to form, and other microstructural changes which may be occurring concurrent to the evolution of tertiary α in the microstructure, an incomplete picture arises to the true nature of tertiary α on mechanical behaviour. Therefore, this study aims to characterise the mechanical properties of TIMETAL® 575 across a number of different heat treatment temperatures to assess the sensitivity of the mechanical properties on the microstructure.

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti-6Al-4V Alloy

Crack Tip Deformation during Dwell Fatigue and Its Correlation with Crack/Fracture Surface Morphologies in a Bi-Modal Ti-6Al-4V Alloy