Dwell-fatigue Conditions Research Articles

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 sustained load on fatigue crack growth behavior of FGH96 at elevated temperature

Accurate prediction of fatigue crack growth behavior is crucial to damage tolerance assessment of turbine disk. Powder nickel-based superalloy is a kind of preferred material of turbine disk. For the purpose of better understanding the characteristics of its crack growth behavior under dwell fatigue conditions at elevated temperature, dwell fatigue crack growth tests of powder nickel-based superalloy FGH96 were conducted with compliance method at three temperatures (600℃, 650℃, 700℃) and multiple dwell time conditions (from 0 s to 60 s). The results show that sustained load can accelerate fatigue crack growth rate of FGH96 significantly at high temperature, and it is more pronounced with higher temperature and prolonged dwell time. At higher ΔK level, the acceleration effect brought by sustained load seems to diminish, this is ascribed to rapid crack growth rate which leads to insufficient time for time-dependent damage occurring at crack tip zone. The effect of temperature on dwell fatigue crack growth rate is more remarkable from 650℃ to 700℃, compared with that from 600℃ to 650℃. The fatigue crack growth resistance of FGH96 is sensitive to sustained load at 700℃, just 1 s dwell loading can produce remarkable acceleration effect on fatigue crack growth rate. The time-dependent crack extension in a loading cycle increases with dwell time in a decelerated manner. Finally, A phenomenological-based crack growth model considering time-dependent damage was proposed, the comparation between the proposed model and the experiment data confirms that it works well for a wide range of temperature and dwell time.

Effect of Temperature and Dwell Time on Fatigue Crack Growth Behavior of CP-Ti

In this paper, the effects of temperature and dwell time on the Fatigue Crack Growth (FCG) behavior of commercial pure titanium were studied under high and low load ratios. Besides, combined with the fracture surface morphology, the specific characteristics of FCG were analyzed under pure fatigue and dwell fatigue conditions. The experiment results show that the FCG rate of commercial pure titanium (CP-Ti) increases with the temperature under low load ratio, and the dwell time increases the FCG rate. Also, the enhancement of the dwell time increases as the temperature rises. The dwell effect tends to be saturated when the temperature rises to 200 °C. Under high load ratio, the FCG rate of CP-Ti also exhibits a temperature-sensitive enhancement. The enhancement effect of the dwell time on the FCG rate under high load ratio is more significant. However, the effect of the hold time on the FCG rate does not increase at 300 °C. The da/dN–ΔK/E FCG curves for CP-Ti have a tendency to approach each other under different load ratios, which indicates that the E-modulus is an important factor for the difference. The effect of dwell time on the FCG behavior of CP-Ti is dominated by the creep deformation mechanism under different load ratios from room temperature to 300 °C. At the same time, the oxidation effect gradually becomes significant as the load ratio increases to 300 °C. The fracture surface morphology shows that the secondary cracks and the roughness increase with temperature or dwell time under low load ratio condition, while, under high load ratio, the effect of creep deformation on the FCG behavior is more obviously enhanced, and plastic deformation is gradually significant with increase of the dimples.

Effects of oxygen-related damage on dwell-fatigue crack propagation in a P/M Ni-based superalloy: From 2D to 3D assessment

Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725°C. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior.

Time- and cycle-dependent crack propagation in Haynes 282

Haynes 282 is a promising superalloy candidate for several high-temperature applications in both aero and land-based gas turbine engines. To study the crack growth behaviour under time-dependent conditions relevant to such applications, a test program was carried out at room temperature up to 700°C with conditions ranging from pure cyclic to sustained tensile loading. At 650°C and high stress intensity factors the crack growth was fully time-dependent for dwell-times of 90s and longer. At lower stress intensities, the behaviour was mainly controlled by the cyclic loading, even under dwell conditions. The behaviour under dwell-fatigue conditions was well described by a liner superposition model. The main crack growth occurred transgranularly at room temperature and there was a transition in cracking behaviour from cycle dependent transgranular growth to time-dependent intergranular propagation at ∼45MPam for the high temperature tests. No effect of cyclic frequency could be observed at room temperature, and at lower frequencies the crack growth rate increased with temperature.

On rate-dependent polycrystal deformation: the temperature sensitivity of cold dwell fatigue.

A temperature and rate-dependent crystal plasticity framework has been used to examine the temperature sensitivity of stress relaxation, creep and load shedding in model Ti-6Al polycrystal behaviour under dwell fatigue conditions. A temperature close to 120°C is found to lead to the strongest stress redistribution and load shedding, resulting from the coupling between crystallographic slip rate and slip system dislocation hardening. For temperatures in excess of about 230°C, grain-level load shedding from soft to hard grains diminishes because of the more rapid stress relaxation, leading ultimately to the diminution of the load shedding and hence, it is argued, the elimination of the dwell debit. Under conditions of cyclic stress dwell, at temperatures between 20°C and 230°C for which load shedding occurs, the rate-dependent accumulation of local slip by ratcheting is shown to lead to the progressive cycle-by-cycle redistribution of stress from soft to hard grains. This phenomenon is termed cyclic load shedding since it also depends on the material's creep response, but develops over and above the well-known dwell load shedding, thus providing an additional rationale for the incubation of facet nucleation.

Prediction of Cold Dwell-Fatigue Crack Growth of Titanium Alloys

The dwell effect of the material can reduce the fatigue lives of titanium alloys at room temperature. A unified fatigue life prediction method developed by the authors’ group is modified in this paper to predict dwell-fatigue crack growth taking into account the effects of dwell time and maximum stress. The modified model can be successfully used to predict the crack growth rate and calculate the fatigue life of different titanium alloys under pure fatigue and dwell-fatigue conditions. It is validated by comparing prediction results with the experimental data of several titanium alloys with different microstructures, dwell time, hydrogen contents, stress ratios and stress levels .

The crack growth behavior of Incoloy 800H under fatigue and dwell-fatigue conditions at elevated temperature

Fatigue crack growth rate (FCGR) tests with different load ratios and dwell-fatigue crack growth rate (DFCGR) tests with different dwell times were conducted at 750°C for Incoloy 800H. As the load ratio increases from 0.1 to 0.5, the crack growth rate increased and the transition ΔK value from region I to region II (Paris regime) shifted leftward. In DFCGR tests with dwell time of 10 and 30 seconds, the Paris regime started at relatively lower ΔK level and the crack grew much faster than in FCGR tests. However, the crack growth rates between the 10 sec and 30 sec dwell times were relatively similar. The higher crack growth rates in the DFCGR tests compared to FCGR tests was associated with the reduction of the M23C6 precipitates in the vicinity of the advancing crack by the Cr depletion, suggesting the crack propagation in DFCGR conditions was environmentally assisted. The crack growth rate was controlled by trans-granular mode regardless of the dwell time because the dwell time was not enough to cause creep damage.

Texture heterogeneities in αp/αs titanium forging analysed by EBSD‐Relation to fatigue crack propagation

The microstructure and the local texture of a large IMI 834 forging were characterized using the Electron Back Scattered Diffraction (EBSD) technique. Crystallographic domains called macrozones and formed by a majority of primary alpha(p) grains with their axes in nearly the same direction were found. They had a band-like structure, parallel to the axial direction of the forging. The influence of these macrozones on the cold dwell-fatigue properties was studied. Several samples were tested under cold dwell-fatigue conditions. The crack initiation and the short-distance propagation region optically matched a bright region that contained numerous quasi-cleavage facets. The analysis of the EBSD measurements showed that this bright region was enclosed within a sharp textured region with axes at less than 30 degrees from the loading axis. The crystallographic features of the crack nucleation site and the crack propagation path were also analysed.

Measuring Stress Distributions in Ti-6Al-4V Using Synchrotron X-Ray Diffraction

This article presents a quantitative strain analysis (QSA) study aimed at determining the distribution of stress states within a loaded Ti-6Al-4V specimen. Synchrotron X-rays were used to test a sample that was loaded to a uniaxial stress of 540 MPa in situ in the A2 experimental station at the Cornell High Energy Synchrotron Source (CHESS). Lattice-strain pole figures (SPFs) were measured and used to construct a lattice strain distribution function (LSDF) over the fundamental region of orientation space for each phase. A high-fidelity geometric model of the experiment was used to drastically improve the signal-to-noise ratio in the data. The three-dimensional stress states at every possible orientation of each α (hcp) and β (bcc) crystal within the aggregate were calculated using the LSDF and the single-crystal moduli. The stress components varied by 300 to 500 MPa over the orientation space; it was also found that, in general, the crystal stress states were not uniaxial. The maximum shear stress resolved on the basal and prismatic slip systems of all orientations within the α phase, $$ \hat \tau _{{\text{rss}}} , $$ was calculated to illustrate the utility of this approach for better identifying “hard” and “soft” orientations within the loaded aggregate. Orientations with low values of $$ \hat \tau _{{\text{rss}}} , $$ which are potential microcrack initiation sites during dwell fatigue conditions, are considered hard and were subsequently illustrated on an electron backscatter diffraction (EBSD) map.

Understanding the contributions of normal-fatigue and static loading to the dwell fatigue in a near-alpha titanium alloy

This article presents the results of a recent study of the response of an α/β-forged Ti-6Al-2Sn-4Zr-2Mo alloy during static, normal-fatigue, and dwell-fatigue loading. The plastic-strain accumulation under different loading conditions is reported. The failure modes and associated fractographic features under static, normal-fatigue, and dwell-fatigue loading conditions are also discussed. These results are used to obtain a better understanding of the relative contributions of the cyclic and creep processes to the overall damage under dwell-fatigue conditions.