Accelerated Fatigue Crack Growth Research Articles

Mechanistic model for hydrogen accelerated fatigue crack growth in a low carbon steel

Fracture by hydrogen accelerated fatigue crack growth is a severe type of environmental failure. Although fatigue crack growth has been the subject of intense investigation over several decades, a complete mechanistic and predictive model is still lacking. Such a crack growth model is even more rare in the case of hydrogen in view of the lack also of constitutive models for material deformation under cycling loading that account for the hydrogen effect. In this study, we present a model for fatigue crack propagation induced by alternating crack tip plastic blunting and re-sharpening, which in the presence of hydrogen can be accelerated by hydrogen enhanced dislocation motion and generation. The Chaboche constitutive model, which is a nonlinear kinematic hardening model capable of capturing many features of material behavior under cyclic loading, is used for the calculation of the stress and strain fields at the propagating crack tip. The Chaboche model is calibrated using a sequence of experimental data from uniaxial strain-controlled cyclic loading tests and uniaxial stress-controlled ratcheting tests with a low carbon steel, JIS SM490YB, in the absence and presence of hydrogen. The numerical simulation results indicate that the proposed crack propagation model can predict Paris law behavior and can successfully demonstrate acceleration of fatigue crack growth in the presence of hydrogen. Significantly, the profiles of the steady-state opening stress and strain ahead of the fatigue crack tip in a compact tension (C(T)) specimen were found to have sections over which they vary as ln(1/r) with distance r from the crack tip, consistent with the crack-tip strain field for a non-stationary crack.

Effects of pressurized water reactor environment and cyclic loading parameters on the low cycle fatigue behavior of 304L stainless steel

Austenitic stainless steels used in light water reactor coolant environments are susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. This study investigates the effects of a high-temperature pressurized water reactor (PWR) water environment and cyclic loading parameters on the low cycle fatigue behavior of austenitic 304L stainless steel. Prolonged exposure to a PWR environment and cyclic loading conditions such as a lower strain rate or a higher fraction of slow strain rate enhances the initiation and accelerates the crack growth rate of fatigue cracks, resulting in decreased fatigue life. The deformation-induced α'-martensite is observed in proximity to fatigue crack tips primarily in specimens tested in simulated PWR primary water, while cellular dislocation structures are more frequently observed near crack tips in specimens tested in high-temperature air. The deformation-induced martensitic transformation from γ-austenite to α'-martensite, occurring via the precursor ε-martensite phase, contributes to the accelerated fatigue crack growth rate in a PWR environment with hydrogen.

Mechanical Characterization of Fatigue and Cyclic Plasticity of 304L Stainless Steel at Elevated Temperature

BackgroundThe mechanical characterization of the cyclic elastoplastic response of structural materials at elevated temperatures is crucial for understanding and predicting the fatigue life of components in nuclear reactors.ObjectiveIn this study, a comprehensive mechanical characterization of 304L stainless steel has been performed including metallography, tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain tests.MethodsIsothermal tests were conducted at both room temperature and 300 °C for both the rolling direction and the transverse direction of the hot rolled steel. Mechanical properties were extracted from the uniaxial experiments by fitting relevant material models to the data. The cyclic plasticity behavior has been modelled with a radial return-mapping algorithm that utilizes the Voce nonlinear isotropic hardening model in combination with the Armstrong-Frederick nonlinear kinematic hardening model. The plasticity models are available in commercial FE software and accurately capture the stabilized hysteresis loops, including a substantial Bauschinger effect.ResultsThe material exhibits near isotropic properties, but its mechanical performance is generally reduced at high temperatures. Specifically, in the rolling direction, the Young’s modulus is reduced by 16 % at 300 °C, the yield strength at 0.2 % plastic strain is lower by 23 %, and the ultimate tensile strength is lower by 30 % compared to room temperature. Fatigue life is also decreased, leading to an accelerated fatigue crack growth rate compared to room temperature. A von Mises radial return mapping algorithm proves to be effective in accurately modelling the cyclic plasticity of the material. The algorithm has also been used to establish a clear correlation between energy dissipation per cycle and cycles to failure, leading to the proposal of an energy-based fatigue life prediction model.ConclusionsThe material exhibits reduced mechanical performance at elevated temperatures, with decreased monotonic strength, compared to room temperature. Fatigue life is also compromised, resulting in accelerated fatigue crack growth. The material’s hardening behavior differs at room temperature and elevated temperature, with lower peak stress values observed at higher temperatures. The radial return mapping algorithm can be used to determine the dissipated energy per cycle which together with fatigue testing has been used to propose a low cycle fatigue life prediction model at both temperatures.

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.

Missing data interpolation and multi‐sensors integration and its application in accelerated degradation data

AbstractMultiple sensors are commonly used for accelerated degradation monitoring. Since different sensors may be sensitive at different stages of the accelerated degradation process and each sensor dataset may contain only partial information of the unit degradation, then integration approaches of the accelerated degradation data from multiple sensors can effectively improve degradation modeling and life prediction accuracy. We present a non‐parametric approach that assigns weights to each sensor based on the dynamic clustering of the sensors' observations. Missing data are common in degradation data acquisition, especially when multiple sensors are used. We provide two approaches for data interpolation: the nonlinear Brownian bridge and the inverse Gaussian bridge when the underlying degradation paths follow the Brownian motion process and inverse Gaussian process, respectively. The stochastic bridges capture the nonlinearity and uncertainties of the degradation processes. The data integration model and stochastic bridge models are validated with real accelerated fatigue crack growth data monitored with multiple NDT sensors. The proposed models provide an accurate accelerated degradation path and reliability prediction.

Antagonistic fatigue crack acceleration/deceleration phenomena in Ni-based superalloy 718 under hydrogen-supply

Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogenating environment. Hence, the mechanisms behind such acceleration phenomenon in FCG should be understood comprehensively toward developing promising alloys resistant to hydrogen occlusion. In particular, Alloy718 has a meager resistance to HE, even regularly displaying superior mechanical and physical performances. Notwithstanding, the present study unveiled that the FCG acceleration by dissolved H in Alloy718 can be negligible. An abnormal deceleration of FCG can instead be pronounced by optimizing the metallurgical state, a hopeful prospect in Ni-based alloys applied to the hydrogenating environment.

Fatigue crack threshold and crack growth behavior of 17–4 PH steel determined with internal hydrogen

Effect of hydrogen on the fatigue crack threshold and fatigue crack growth (FCG) behavior of 17–4 PH steel was investigated using a circumferentially notched specimen with a stress concentration factor of 6.6 precharged in a 100 MPa hydrogen gas environment. Fatigue life tests were performed in air at room temperature at a stress ratio of −1 and test frequencies of 0.001–10 Hz. Although the FCG rate was accelerated, it was discovered that the fatigue crack threshold was not degraded by the hydrogen. The H-induced FCG acceleration ratio was predicted by a new model considering cycle- and time-dependent FCGs.

Resistance of pearlite against hydrogen-assisted fatigue crack growth

Recent progress in realizing hydrogen society requires higher-strength steels having decent hydrogen embrittlement resistance via optimized microstructural controls. The present communication first reports the fatigue crack growth (FCG) property of pearlitic steel in a high-pressure, i.e., 90 MPa, hydrogen gas environment to probe this standard microstructure's potency for becoming a constructive reinforcing agent in future hydrogen-compatible structural materials. As-transformed eutectoid steel with 1080 MPa-tensile strength exhibited hydrogen-induced FCG acceleration to a lesser extent relative to martensite with an equivalent strength level and occasionally to pure ferritic iron. Furthermore, the superiority over martensite was augmented under slow loading-rate conditions, suppressing time-dependent cracking, a critical issue in implementing defect tolerant design on high-strength steels for hydrogen gas applications.

The difference in fatigue crack growth induced by internal and external hydrogen in selective laser melted 304L stainless steel

The mechanical properties of selective-laser-melted (SLM) 304L steel under hydrogen charging and hydrogen environment were investigated by tensile and fatigue crack growth (FCG) tests. SLM samples showed higher mechanical properties and weaker hydrogen embrittlement compared with wrought samples, and the FCG rates of SLM 304L tested in a hydrogen environment were higher than hydrogen-charged samples. More obvious cleavage platforms and lamellar structures appeared and cracking tended to the multi-directional feature in the hydrogen gas environment. External hydrogen promotes slip planarity, deformation twinning, and plastic localization near the crack tip, and then causes the acceleration of FCG in SLM 304L steel.

Transition mechanism of cycle- to time-dependent acceleration of fatigue crack-growth in 0.4 %C Cr-Mo steel in a pressurized gaseous hydrogen environment

Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth distance was proportional to load duration. Several observations and analyses revealed that such time-dependent FCG was due to stress-driven cracking along hierarchical martensite boundaries, stemming from the hydrogen-induced degradation of their cohesive strengths as a result of competition between mechanically-determined crack-tip stress (driving stress) and statistically-distributed boundary strength (resistance stress).

Fatigue crack growth acceleration in S355 steel under a single and periodic underload

To consider the acceleration effect induced by the underloads in the design stage, the fatigue crack growth rates in S355 steel were measured based on the compliance method. Three different load sequences, constant amplitude loading (CAL), single underload (UL), and periodic underloads (PULs), were tested. Moreover, several methods were used to study the relevant mechanisms from the macroscopic and microscopic perspectives, including the digital image correlation technique, X-ray diffraction, transmission electron microscopy, and finite element method, etc. Compared with the crack growth during CAL, the UL induced a notable crack growth acceleration in subsequent cyclic loadings. When the amplitude of UL was constant, the effect of UL enhanced with the lower stress ratio of the fatigue baseline loading. In the macroscopic view, the acceleration effect was primarily attributed to the decreasing plasticity-induced crack closure and induced tensile residual stress at the crack tip. A higher amplitude underload led to an increase in the crack acceleration, which gradually degenerated within the following cycles. The PULs were also affected by the amplitude of underloads. The relevant crack growth acceleration mechanisms were closely related to the cycles affected by UL and the crack opening level. Another main parameter affecting PULs was associated with the number of cycles between two consecutive underloads: the frequency of PULs, which reflects the degree of interaction and accumulation. In the microscopic view, the accumulation of dislocation around the crack tip induced by PULs contributed to the crack acceleration, along with the corresponding periodic striations in the fractography. This study showed the crack growth acceleration effect caused by underloads and demonstrated the interactions between the UL and PULs, offering a better understanding of the acceleration mechanisms.

Elasto-Plastic Fatigue Crack Growth Behavior of Extruded Mg Alloy with Deformation Anisotropy Due to Stress Ratio Fluctuation.

Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (Kmax), the stress ratio R was changed from 0.1 to −1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = −1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = −1 (|(σmin)R = −1|) exceeded the compressive yield strength of the material (σcy), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σmin)R = −1| < σcy, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the Kmax value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress–strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJeff.

Fatigue crack acceleration and retardation induced by high temperature load hold in Inconel 718

The effect of high temperature tension hold on fatigue crack growth in Inconel 718 is investigated. A single tension hold at maximum load was applied during cyclic loading at 650 °C, and the effect of load level and hold time are examined. In all experiments, a rapid acceleration of fatigue crack growth is observed immediately after tension hold. A retardation is observed after the initial acceleration at lower stress intensity factors, while only an acceleration occurs at higher intensity factors. These results are discussed by considering two competed mechanisms: grain boundary damage related to oxygen in front of the crack tip, and stress relaxation due to creep deformation.

Effect of machining on near surface microstructure and the observation of martensite at the fatigue crack tip in PWR environment of 304L stainless steel

This work highlighted that a ground surface finish and the exposure to a pressurised water reactor (PWR) environment result in a decreased low-cycle fatigue lifetime, an enhanced fatigue crack initiation and an accelerated fatigue crack growth rate of 304 L austenitic stainless steel. A ground surface finish promotes fatigue crack initiation and short crack growth especially in a water environment, due to the highly deformed underlying microstructure with high-angle grain boundaries and the grinding marks on surface. Martensite was observed in the vicinity of secondary crack tips in specimens tested in a simulated PWR primary side environment. The aggregated presence of α′- and ε-martensite in the vicinity of the fatigue crack tip can enhance the material's susceptibility to hydrogen-assisted fatigue cracking. Martensite formation was rarely observed in specimens exposed to high temperature air. The phase transformation from γ-austenite to αꞌ-martensite in the PWR primary environment occurred via the intermediate ε-martensite phase.

Effect of twins and plastic-deformation anisotropy of extruded magnesium alloy on fatigue crack growth and crack closure behavior

To study the effect of twins and plastic deformation anisotropy on the fatigue crack growth (FCG) behavior and crack closure behavior of extruded AZ31B magnesium alloys, two types of single-edge notched tensile specimens were prepared, with loading directions parallel (specimen E) and perpendicular (specimen T) to the extrusion direction (ED). FCG experiments using these specimens were performed at stress ratios R = 0.1 and −1, and cyclic speed of 10 Hz.In extruded magnesium alloy materials, the c-axis of the crystal is radially oriented on a plane perpendicular to the ED, indicating the generation of a strong texture.In the FCG experiment at R = −1 for specimen E, elongation in the c-axis direction occurred under compressive load, which generated {101‾2} <101‾1> tensile twins. The tensile twins increased the material deformation and fatigue crack opening distance. As a result, the effective stress intensity factor range ΔKeff increased, leading to the acceleration of the FCG velocity. At R = 0.1, tensile twins were not generated because the load waveform had no compressive load component; there was no change in ΔKeff and no acceleration of FCG.For specimen T at R = 0.1 and −1, tensile twin generation was expected because elongation in the c-axis direction occurred under tensile load. However, tensile twins did not occur, which is attributed to the effect of plastic deformation anisotropy. Therefore, for specimen T, the increase in ΔKeff and FCG acceleration did not occur, and no difference due to the R value was observed.

Short fatigue crack growth behavior in 18Ni marageing steel

In recent decades, it has been found that the fatigue crack growth rate could be slowed down by the zigzag crack growth path in some materials, but the mechanism was not well clarified. In this study, the effect of crack growth path on the fatigue crack growth rate of 18Ni maraging steel under three different heat treatment states was studied, and two factors influencing the fatigue crack growth rate were outlined. It was found that, firstly, the main factor affecting the fatigue crack growth rate was the strength of the material; secondly, the zigzag crack growth path did not slow down the crack growth rate but increased the crack growth rate. Therefore, a two-phase model was established to understand the influence of crack growth path on the growth rate. In different phases, their resistances to the crack growth are different, and the difference in the strength between the two phases determines whether the zigzag cracking path can depress the fatigue crack growth or not. Specifically, when the difference in the strength between the second phase and the matrix is greater than a certain value, the zigzag cracking path leads to the acceleration of fatigue crack growth. The consequences show that not all the zigzag crack propagation paths are beneficial for the higher fatigue cracking resistance, and the specific analysis should be combined with the material microstructure in detail.

On the role of crack tip creep deformation in hot compressive dwell fatigue crack growth acceleration in aluminum and nickel engine alloys

Ambient and Hot Compressive Dwell (HCD) fatigue experiments were carried out to understand the effects of HCD on the fatigue crack growth behavior of cast 319 Al and Inconel 718 engine alloys. HCD was found to reduce endurance and increase rates of crack growth at engine service temperatures, while far less crack growth acceleration was found at room temperature. A fracture mechanics-based model was developed, considering tensile residual stress contributions due to compressive creep in order to predict crack growth with HCD. Good agreement was found between predictions and experimental results.

Crack growth direction effects on corrosion-fatigue behaviour of offshore wind turbine steel weldments

In this study corrosion-fatigue tests have been conducted on fracture mechanics specimens extracted from an S355 G10+M structural steel welded plate. The tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. The corrosion-fatigue test results from this study have been compared with the data available on the base metal as well as air tests on the same material. Moreover, the obtained results have been compared with the corrosion-fatigue data available in the literature on a wide range of steels and also the fatigue trends for welded joints in free-corrosion condition recommended in the BS7910 Standard. The effect of the specimen orientation, with respect to the weld region, is also examined in this study and it has been found that higher corrosion-fatigue crack growth rates are generally observed in the tests with 0° orientation. The results have also shown that the corrosive environment has significant effects on the fatigue crack growth acceleration at the beginning of the tests; however, as the crack propagates, the environmental damage effect on crack growth behaviour becomes less pronounced. The results presented in this study are discussed in terms of improvement in the structural integrity assessment of offshore wind turbine monopiles.

A mechanism behind hydrogen-assisted fatigue crack growth in ferrite-pearlite steel focusing on its behavior in gaseous environment at elevated temperature

Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations.

Evaluation of Fatigue Crack Growth Acceleration Behavior in Alloy 617 Based on Evaluation of Oxides in the crack

Evaluation of Fatigue Crack Growth Acceleration Behavior in Alloy 617 Based on Evaluation of Oxides in the crack