Dwell-fatigue Tests Research Articles

Assessment of dwell-fatigue properties of nickel-based superalloy coated with a multi-layered thermal and environmental barrier coating

A three-layered experimental thermal and environmental barrier coating (TEBC) was deposited using air plasma spraying technology on cylindrical specimens of nickel-based superalloy MAR-M247. TEBC consists of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) upper layer, Y2O3 stabilised ZrO2 interlayer and CoNiCrAlY bond coat deposited on the grit-blasted surface of MAR-M247. The cyclic plastic response and damage mechanisms in uncoated and TEBC-coated MAR-M247 have been studied in isothermal dwell-fatigue tests conducted under strain control with constant strain amplitude at 900 °C. In each cycle, 5-minute dwells were introduced in both tensile and compression peaks of the hysteresis loop. Fatigue hardening/softening curves, stress relaxation curves, cyclic stress-strain curves and fatigue life curves are reported. Ceaseless mild softening has been found in both uncoated and TEBC-coated MAR-M247. TEBC-coated MAR-M247 showed an improved lifetime in the whole range of tested strain amplitudes. Data obtained from stress relaxation curves were used to assess the fraction of creep damage. The generalised damage accumulation rule was used to evaluate damage due to fatigue-creep-environment interaction. A study of the surface relief and internal microstructure using SEM and TEM helped to interpret the specifics of fatigue behaviour of uncoated and TEBC-coated material. The effectiveness of this newly developed TEBC, together with the dwell sensitivity of MAR-M247, were discussed from the perspective relevant to dwell-fatigue cyclic straining.

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.

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.

A study on dwell-fatigue behavior of additively manufactured Ti-alloy

The Ti6Al4V is a cold dwell sensitive alloy, as evident from the failure investigations of several aerospace fan discs and past laboratory research data. However, the studies on the dwell-fatigue sensitivity of additively manufactured Ti6Al4V alloy and its underlying failure mechanism at room temperature are limited. In this study, the cyclic plasticity behavior and the cold dwell sensitivity of powder-bed fusion based additively manufactured Ti6Al4V alloy has been investigated. The standard test specimens of Ti-alloy were fabricated using the selective laser melting process and were subjected to the low cycle fatigue and low cycle dwell fatigue tests, at the same strain rate. A significant reduction in the fatigue lives of specimens exposed to a dwell period was observed at relatively lower strain amplitudes, while fatigue lives were nearly the same at higher ones. The fractographic analysis of the broken specimens revealed multiple crack initiation regions under the action of dwell loadings. The cracks were initiated either from the process-induced defects or by the mechanism of facet formation in the α laths. The failure mechanism based on stress redistribution and its relaxation are discussed to explain the observed results and are presented in this study.

Crystallographic analysis of slip system activation in bimodal Ti–6Al–3Nb–2Zr–1Mo alloy under various dwell-fatigue loadings

The microscopic deformation behavior is always been of significant interest in revealing the mechanism of the dwell fatigue effect. This study systematically characterized the deformation system activation in Ti–6Al–3Nb–2Zr–1Mo (Ti6321) alloy using in situ tensile and quasi-in situ dwell fatigue tests. The critical resolved shear stresses of the basal and prismatic slip systems in the Ti6321alloy were first calculated using the slip trace analysis method. The study demonstrated that, under dwell fatigue, basal and prismatic slips consist of major microscopic deformation modes. Most of the slip deformation occurred rapidly during the early stage of dwell fatigue and was independent of the dwell loading. The basal slip was found to be the predominant slip mode regardless of the dwell conditions. The results suggested that the elastic anisotropy of the hexagonal-closed-packed (HCP) structure could lead to heterogeneous microscopic stress, thus activating more basal slip activities. Such activities were found to participate in the crack initiation through two unique cracking modes: basal plane crack and (0001) twist boundary crack.

The microstructure effect on fatigue and dwell-fatigue in a nickel-based superalloy

In this study, the fatigue behaviors of a nickel-based superalloy FGH96 with different microstructural morphologies generated from different heat-treatment processes were investigated. The fatigue and dwell-fatigue tests with the same maximum stress of 1200 MPa were conducted at 650 °C. The relationship between the γ' morphology and the fatigue properties was established. The results indicate that the critical shearing stress of dislocation movement played a key role in affecting fatigue life. No apparent difference can be observed from the fracture morphology at the macroscopic scale for normal fatigue and dwell-fatigue tests. However, in the crack initiation region, the crack was observed to grow along the grain boundary for the dwell-fatigue condition, resulting in the intergranular fracture, similar to creep failure. The introduction of dwell time in the fatigue loading brings larger cyclic strain deformation originates from massive dislocation slip. The dwell debit for the considered loading condition is about 100 times.

High-strength and low-dwell-sensitivity titanium alloy showing high tolerance to microcracking under dwell fatigue condition

Dwell fatigue failure of titanium alloy components has been one of the major threats to aircraft safety for the last 50 years. Numerous studies have focused on the identification of critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. In this work, a microstructure with a low primary α phase (αp) content was designed to avoid the known weaknesses in the Ti-5Al-7.5 V alloy to achieve high-strength and low-sensitivity to 2-min dwell loading. Conventional and dwell-fatigue tests confirmed the efficiency of such an approach. The deformation occurred primarily by planar slip in the αp grains, resulting in possible cracking along preexisting slip bands in the cycling process. Fatigue and dwell-fatigue failure was dominated by surface crack initiation at facet matching basal planes. Moreover, numerous internal microcracks were formed along basal slip bands and occasionally along basal twist grain boundaries. Load holds were found to facilitate cracking along prismatic planes, which highlights a mechanism switch between basal-dominant cracking mode and collective basal-prismatic mode. The mechanism switch suggests that the prismatic dislocation activity considerably increases due to the low temperature creep induced by load holding. Thanks to the low αp phase microstructure, the alloy shows remarkable tolerance to microcrack growth under dwell condition. Limited growth of the internal microcracks across transformed β regions was identified as a key feature of high dwell-fatigue resistance of the investigated material.

The low cycle fatigue behaviour of MAR-M247 superalloy under different strain rates and cycle shapes at 750 °C

High-temperature low-cycle fatigue experiments were performed at different strain rates on nickel-based superalloy MAR-M247. Furthermore, slow-fast, fast-slow and dwell-fatigue tests were conducted. The deformation, damage and lifetime behaviour as a function of strain rate and cycle shape were observed. Results show that the lifetime decreases with decreasing strain rate, indicating the negative influence of time-dependent creep and oxidation mechanisms. The lifetimes of slow-fast and dwell-fatigue tests are lower than those for fast-slow tests, although the mean stress is compressive in slow-fast and dwell-fatigue tests and tensile in fast-slow tests. Slow-fast and dwell-fatigue tests lead to grain boundary cavitation.

Microstructural Effects on Thermal-Mechanical Alleviation of Cold Dwell Fatigue in Titanium Alloys

Cold dwell fatigue is a well-known problem in the titanium components of aircraft engines. The high temperature and low dwell stress of in-service conditions have been reported to give rise to dwell fatigue resistance through a thermal-mechanical alleviation process. Here, dwell fatigue tests at room temperature and the component operating temperature were performed on IMI834 titanium alloy to assess the microstructural effects on thermal-mechanical alleviation of cold dwell fatigue while eliminating the effect of chemical composition. The ratcheting strain rates under different loading conditions were quantitatively investigated to aid the understanding of thermal-mechanical alleviation.

Investigation of the damage and fracture of Ti-6Al-4V titanium alloy under dwell-fatigue loadings

Ti-6Al-4V alloy turbine components for aeronautical applications operate at a steady state after reaching the maximum stress during each duty cycle. The load holds combined with the fatigue process causes a phenomenon called the dwell effect. The dwell periods reduce the fatigue strength of disks and blades of aeronautical turbine engines made of titanium alloys. In this work, trapezoidal dwell-fatigue and triangular fatigue tests were performed with a fatigue ratio of 0.1 at room temperature. The trapezoidal waveforms (dwell-fatigue tests) had a 10-second dwell period and 1-second loading and unloading rates for each fatigue cycle. The triangular waveforms (fatigue tests) were equivalent to trapezoidal waveforms with zero seconds at the maximum stress. The Weibull distribution was used to analyze the dwell-fatigue data statistically. The experimental results showed that the dwell-fatigue life debits for dwell periods of 10 seconds at stress levels of 1.0, 0.975, and 0.95 of the material yield strength were 10.2, 10.0, and 4.5, respectively. The results suggest that the dwell sensitivity of Ti-6Al-4V alloy increases at high-stress levels. The fracture of dwell-fatigue specimens occurred at a high-cumulated plastic strain and after a significantly lower fatigue life than at pure fatigue tests, indicating a substantial dwell-life debit of the Ti-6Al-4V alloy. The damage mechanism responsible for reducing the fatigue life when the dwell time was introduced was the plastic deformation accumulation observed in dwell-fatigue tests, possibly as a result of the stress redistribution mechanism in the α phase grains that leads to slip of dislocations and, consequently, early plastic deformation processes that instigate crack nucleation.

The roles of rise and fall time in load shedding and strain partitioning under the dwell fatigue of titanium alloys with different microstructures

Dwell fatigue failure of titanium alloys has threatened flight safety for over five decades. To quantitatively evaluate the component life, experimental dwell fatigue tests are generally conducted in the lab. However, the loading profile in the lab is generally shorter than those of the realistic in-service conditions by several orders of magnitude, not only for the stress hold but also for the stress rise and fall periods. Although the dependence of fatigue life on the dwell period has been extensively studied, the effect of the rise and fall time has been ignored. Investigating such a topic is extremely time-consuming, especially when the applied stress is lower than the yield strength under the in-service loading. The fatigue tests could require years of loading when the rise and fall periods reach the order of magnitude of 103s, for example, which is experimentally infeasible. Modeling techniques provide a solution to systematically study the effect of the rise and fall on dwell fatigue responses benefit from the high computing power availability. In this study, crystal plasticity models representing equiaxed α and dual-phase α-β lamellar microstructures have been developed and used to study the effects of the rise and fall time over a wide range (10−1-103s) on the load shedding behaviors of IMI834 titanium alloy under dwell fatigue loadings. The time for the peak stress at the soft-hard grain boundary reaching equilibrium under different load profiles is quantitatively investigated. The rise and fall time have been demonstrated to influence the load shedding at different degrees. The microstructural features in the soft grains of dual-phase titanium alloys play a significant role in affecting the local stress evolutions. The correlations between the load shedding and the rise and fall time are also influenced by the microstructure. The strain partitioning between the α and β phases in the soft grains under cyclic dwell fatigue of titanium alloys has been examined to elucidate the underlying mechanisms of the rise/fall time dependence.

Dislocation-based crystal plasticity modelling of a nickel-based superalloy under dwell-fatigue: From life prediction to residual life assessment

In this work, a dislocation-based crystal plasticity finite element (CPFE) framework was implemented to investigate the effects of loading conditions on dwell-fatigue crack initiation life. Experimentally, a large number of strain-controlled dwell-fatigue tests were carried out at 650 °C in a nickel-based superalloy. The combinations of CPFE simulations and post-test examinations were used to reveal the dwell-fatigue crack initiation mechanisms. Then, a life prediction approach was presented on the basis of accumulated energy dissipation at half-life cycle. Good agreement between the experimental and simulated lives verifies the robustness as well as the accuracy of the present approach. Finally, a new three-dimensional (3-D) damage tolerance diagram was proposed by introducing a CP-based physical parameter to describe the degradation levels and evaluate the residual dwell-fatigue life.

Low cycle fatigue and dwell-fatigue of diffusion coated superalloy Inconel 713LC at 800 °C

The charm of diffusion coatings is mainly in their protective function of the surface of gas turbine components, which are often subjected to fatigue-creep loading in extremely hostile environments. Their impact gains in importance with the increasing time that the components are exposed to high temperatures and aggressive environments. This motivates the authors to study the low cycle fatigue behaviour of Al-Cr coated nickel-based superalloy Inconel 713LC. Polycrystalline diffusion Al-Cr coating was produced by out-of-pack Cr-modified aluminising chemical vapour deposition technique on the surface of cylindrical fatigue specimens manufactured by investment casting. Continuous low cycle fatigue (CLCF) tests and dwell-fatigue (DF) tests where a 10-minute tensile dwell was included in each cycle were conducted in strain control mode with constant total strain amplitude at 800 °C. The fatigue behaviour was characterised by cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves. The Al-Cr coating provided excellent protection against surface oxidation of Inconel 713LC, especially during dwell-fatigue tests. The substrate as well as the Al-Cr coating were examined in as-coated conditions and after fatigue loading through scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Dislocation arrangement in the substrate material was investigated in a transmission electron microscope (TEM). The microstructural observations and the discussion on the degradation mechanisms of surface-treated superalloy have been provided for both regimes of cycling.

The effect of loading direction on the dwell fatigue properties of Ti-6Al-4V forged bar with highly oriented texture

The purpose of this study was to improve our understanding of the effect of macro/micro texture on dwell fatigue properties. The Ti-6Al-4V forged bar with highly oriented texture and fine equiaxed microstructure was used. Dwell fatigue, cyclic fatigue and creep tests were conducted at room temperature by using specimens taken in the longitudinal (L) direction and the transverse (T) direction of the bar. The effects of loading direction on dwell fatigue life and fracture surface morphology were examined in detail. The dwell fatigue life in T direction was shorter than that in L direction. The fracture surface morphologies were characteristically varied by loading waveform, amplitude and directions. In the range of 93 - 95% of 0.2%PS, the characteristic large facets were observed in the T direction in dwell fatigue. Detailed analyses revealed that the large facet consists of multiple initiation facets and propagation facets. The propagation facet plane and crack propagation direction appeared to correspond to alpha (0001) <10-10>. Furthermore, the relationship between strain rate and life time was compared to that for room temperature creep. The deviation from the Monkman-Grant relationship corresponded to the change of the fracture surface morphology.

The fracture behavior transitions in cold dwell fatigue of Ti-6Al-4V

Ti-6Al-4V forged materials which had fine grain size and commercial billet which had moderate grain size and micro texture were prepared. Cold dwell fatigue tests were conducted using both materials, and dwell time condition was up to 1800s. Fracture cycles and elongation were almost same on each specimen. Number of cycle to failure decreased with increase of dwell time. Fracture elongation increased up to dwell time of 10s, and it was constant in over 10s. Fracture surface were observed. In short time dwell condition up to 2s, fracture surface showed fatigue type. Fracture surfaces changed to dimple in over 10s dwell. In billet material, facets were observed in inner area of fracture surface.

Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy

Frequency is an important factor influencing the fatigue behavior. Regarding to the dwell fatigue, it corresponds to the effect of rise and fall time, which is also an important issue especially for the safety evaluation of structure parts under dwell fatigue loading, such as the engines of aircrafts and the pressure hulls of deep-sea submersibles. In this paper, the effect of rise and fall time (2 s, 20 s, 110 s, and 200 s) on the dwell fatigue behavior is investigated for a high strength titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X with basket-weave microstructure. It is shown that the dwell fatigue life decreases with increasing the rise and fall time, which could be correlated by a linear relation in log–log scale for both the specimen with circular cross section and the specimen with square cross section. The rise and fall time has no influence on the crack initiation mechanism by the scanning electron microscope observation. The cracks initiate from the specimen surface and all the fracture surfaces present multiple crack initiation sites. Moreover, the facet characteristic is observed at some crack initiation sites for both the conventional fatigue and dwell fatigue tests. The paper also indicates that the dwell period of the peak stress reduces the fatigue life and the dwell fatigue life seems to be longer for the specimen with circular cross section than that of the specimen with square cross section.

Deformation and rupture behaviors of SiC/SiC under creep, fatigue and dwell-fatigue load at 1300 °C

High temperature creep and fatigue properties are important characteristics of SiC/SiC composites. In this work, dwell-fatigue tests were conducted at 1300 °C for 3D-braided SiC/SiC with and without coatings, and experimental results of dwell-fatigue tests were analyzed together with results of fatigue and creep tests. Then, general quantitative method was adopted to explain the deformation and rupture behaviors of SiC/SiC in creep, fatigue and dwell-fatigue tests. Micromechanical creep model could effectively simulate deformation behavior when the maximum load was under matrix cracking stress. Stress transfer behavior of constituents during creep and fatigue tests provided insights into the difference in macro time/cycle dependent deformation. Next, by incorporating damage evolution model which represents oxidation assisted unbridged crack growth, deformation acceleration in tertiary stage of creep was successfully simulated. Finally, SiC/SiC lifetimes under all fatigue, creep, and dwell-fatigue loads were predicted considering matrix crack propagation and fiber strength degradation. Simulated results correlated well with experimental data, verifying the effectiveness of micromechanical model to predict the creep and fatigue behaviors of SiC/SiC composites at high temperatures.

Dwell-fatigue crack propagation in additive manufactured Hastelloy X

Additively manufactured Hastelloy X by laser-powderbed fusion is a superalloy used in for example burners and non-rotating parts in gas turbines. Turbines are often subjected to dwell-fatigue as a result of an operating profile including load cycles with long constant power output. The effect of building direction and heat treatments on dwell-fatigue crack propagation in additively manufactured Hastelloy X has not yet been thoroughly investigated. Crack propagation behaviour was characterized using compact tension samples cut from as-built and heat treated material blocks. Samples were machine with the notch parallel and perpendicular to the building direction enabling the investigation of building direction on crack behaviour and crack propagation rates. The samples were subjected to dwell-fatigue tests at 700 °C with 90 s or 2160 s dwell-times at maximum load. Microstructural characterization was conducted using light optical microscopy and scanning electron microscopy techniques such as electron channelling contrast imaging and electron backscatter diffraction. The additively manufactured alloy exhibits anisotropic behaviour caused by the directionally solidified microstructure. Cracks propagated intergranularly and preferably through streaks of topologically close-packed phases.

Comparison of slip system activation in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo under tensile, fatigue and dwell-fatigue loadings

The deformation systems operating in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo at the onset of plastic slip were characterized by means of monotonic in-situ tensile tests performed in a scanning electron microscope. A slip traces analysis was carried out using the local crystalline orientation characterized by the electron backscattered diffraction technique. The activation of the involved deformation processes were then characterized step by step and the critical resolved shear stress was estimated. Interrupted fatigue and dwell fatigue tests were conducted on both alloys as well to study the possible specificities associated to the operating slip system distribution under cyclic loadings. The main challenge of this work is thus to detect any significant difference in deformation behavior between both alloys that could explain, even partly, the difference they exhibit in dwell fatigue regime. A potential relationship between the operating slip systems and dwell-fatigue life debit is finally discussed.