Time-dependent Fatigue Crack Growth Research Articles

Martensite Boundary Characteristics on Cycle- and Time-Dependent Fatigue Crack Growth Paths of Tempered Lath Martensitic Steels in a 90 MPa Gaseous Hydrogen Atmosphere

Martensite Boundary Characteristics on Cycle- and Time-Dependent Fatigue Crack Growth Paths of Tempered Lath Martensitic Steels in a 90 MPa Gaseous Hydrogen Atmosphere

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.

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).

Computational modelling of poro-visco-hyperelastic effects on time-dependent fatigue crack growth of hydrogels

In newly developed hydrogel-based devices, hydrogels are commonly used in load-bearing components subjected to prolonged cyclic deformations. The anti-fatigue capability of hydrogels is crucial for extending the service life of the devices. While recent developments in the synthesis and characterisation of tough hydrogels have facilitated continuous improvements of the fatigue resistance, the underlying mechanisms that dominate the fatigue fracture of hydrogels are still inconclusive. This work aims to model the complex constitutive response and predict the fatigue crack behaviour of hydrogels. The contribution of this work is twofold. (i) A physically-based poro-visco-hyperelastic model is developed within the framework of the Theory of Porous Media (TPM) at finite strains to describe the solid–fluid-coupled material behaviour of hydrogels. The fluid transport in hydrogels is governed by Darcy’s law. The non-equilibrium mechanical response induced by polymer chain rearrangement is considered by introducing internal variables based on a multiplicative decomposition of the solid deformation gradient tensor into elastic and inelastic parts. The time-dependent breaking/reforming kinetics of physical chains is described by a Bell model-based chain evolution law. (ii) An energy-based fatigue crack growth model is proposed to predict the fatigue crack growth of hydrogels. A volume averaging method is used to calculate the elastic energy density surrounding the crack tip as the driving force. In particular, the cyclic evolution of the averaged energy density due to the breaking/reforming kinetics of physical chains is incorporated in the crack growth model for hydrogels with physical networks. The computational results demonstrate that the predicted fatigue crack growth of different hydrogels reasonably agrees with experimental data. Moreover, the effects of fluid transport, time-dependent deformations and chain kinetics on the fatigue fracture behaviour of hydrogels are systematically analysed. The prediction results indicate that these time-dependent mechanisms cannot be ignored in modelling the fatigue behaviour of anti-fatigue hydrogels.

Mechanistic modelling of time-dependent fatigue crack growth in Ni-based superalloys

Advanced Ni-based superalloys for aero-engine disk applications can experience multiple damage mechanisms that include oxidation, creep and stress corrosion in additional to cycle-dependent fatigue crack initiation and growth. Interactions of these various damage mechanisms can lead to the occurrence of frequency, temperature, heat dwell and environmental effects on the fatigue crack growth response and service life of Ni-based superalloys. In this overview paper, the development of a set of generic microstructure-based fatigue crack growth models for treating concurrent time-dependent and cycle-dependent damage mechanisms at the microstructural levels is summarised. Key features of the mechanistic models are highlighted and utilised to assess the variability of time-dependent fatigue crack growth response in the threshold and power-law regimes due to variations in microstructure such as grain size, and γ′ size. This methodology provides a pathway for evaluating microstructural effects on multiple damage modes and extending the service lives of hot-section components.

A Hydrogen-Induced Decohesion Model for Treating Cold Dwell Fatigue in Titanium-Based Alloys

Cold dwell fatigue in near-alpha Ti alloys is a time-dependent fracture process at ambient temperature that involves fatigue in the presence of creep to produce cracking on low-energy fracture (e.g., cleavage) facets in hard alpha grains. In this article, cold dwell fatigue is treated as a hydrogen-induced decohesion process by using a nonlinear cohesive stress-strain relation to describe the decrease in the cohesive strength with increasing local hydrogen contents. It is postulated that during cold dwell fatigue, time-dependent deformation occurs by 〈a〉 slip that results in dislocation pileups in soft alpha grains. The stress and dilatational fields of the dislocation pileups assist the transport of internal hydrogen atoms from soft grains to neighboring hard grains. The accumulation of internal hydrogen atoms at the trap sites leads to decohesion along crystallographic planes, which can be slip or hydride habit planes. The decohesion model is applied to treat cold dwell fatigue in Ti-6Al-4V with a basal-transverse texture by modeling the effects of hydrogen-induced decohesion on the stress-fatigue life (S-N f) response, the time-dependent crack growth response (da/dt), and the fracture toughness (K c) as functions of grain orientation. A probabilistic time-dependent fatigue crack growth analysis is then performed to assess the influence of microtexture on the dwell fatigue life of a Ti-6Al-4V ring disk subjected to a long-duration hold at the peak stress of the loading cycle. The results of the probabilistic life computations indicate that dwell fatigue resistance in Ti-6Al-4V may be improved and the risk of disk fracture may be reduced significantly by controlling the microtexture or reducing the size and volume fraction of hard alpha grains in the microstructure.

A new mechanism in hydrogen-enhanced fatigue crack growth behavior of a 1900-MPa-class high-strength steel

This paper presents a new mechanism controlling the acceleration of fatigue crack growth of a hydrogen-charged high-strength steel (bearing steel SAE52100, σ ult > 1, 900MPa, HV = 569). Three- dimensionally complicated shape of a primary crack and secondary cracks were observed in hydrogen- charged specimens. Marked acceleration of fatigue crack growth in the presence of hydrogen was observed particularly at low test frequency, and was attributed to the initiation and successive coalescence of secondary cracks formed ahead of primary crack. These secondary cracks were produced along prior-austenite grain boundaries and carbide boundaries, or by direct cracking of carbides. Surprisingly, secondary cracks were observed outside the ordinary plastic zone ahead of the crack tip. TEM observation elucidated that the secondary cracks outside the crack tip plastic zone were produced by hydrogen-induced deformation twins impinging on grain boundaries and carbides. These results suggest a new mechanism of the acceleration of fatigue crack growth rates in high-strength steels caused by hydrogen-induced deformation twins, rather than due to hydrogen- enhanced localized plasticity. The phenomena associated with time dependent fatigue crack growth are presumed to be correlated with the initiation and coalescence of secondary cracks in the presence of hydrogen.

Investigation on behavior of trapezoidal through-wall cracks under constant pressure loading

In general, a crack found in a steam generator tube has an elliptical shape, and as soon as it penetrates through the wall, it is assumed to be rectangular for conservatism. However, the leak and crack growth behavior have not been clearly understood after the elliptical crack penetrates the tube wall. One steam generator tube failure showed the tube rupture behavior without any significant prior indication. Several experiments using steam generator tubes with a trapezoidal crack were performed under constant pressure loading by Argonne National Laboratory and exhibited crack growth behavior much faster than expected. It has been found to be caused by time-dependent fatigue crack growth. No K I solution is available for a trapezoidal crack which can be used for fatigue crack growth analysis. The objective of this study is, therefore, to develop a new K I solution for a trapezoidal crack using FEM.

A unified time dependent model for low cycle fatigue and ratchetting failure based on microcrack growth

In this paper, a unified time dependent model for low cycle fatigue and ratchetting failure has been developed based on the microcrack growth. The model utilized fracture mechanics theory using J-integral under creep–fatigue loading and assumed that the microcrack propagation determines the failure life. The microcrack rates are separated into three parts: (1) the time independent fatigue crack growth, (2) the time dependent fatigue crack growth and (3) the time dependent ratchetting crack growth. The cyclic failure criteria under different loading conditions were derived from the microcrack growth. Some simplified models were obtained and they can characterize the time dependent low cycle and ratchetting failure lives with hold time and loading frequency effects.

Effect of aging treatment on fatigue crack growth in eutectic Sn–Pb alloy

The current work investigated the influence of aging treatment on fatigue crack growth behavior of eutectic Sn–Pb alloy. After aging treatment, the eutectic grain structure changed to particle structure. This seriously influenced the time-dependent fatigue crack growth behavior, as well as the cyclic dependent behavior resistance in near-threshold regime at high frequency, while having slight influence on the crack propagation in Paris regime.

Interfacial Composition Measurements in Complex Ni-Base Superalloys

ABSTRACTIn this study, three different Ni-base superalloy / heat treatment combinations are studied in an attempt to assess the role of grain boundary morphology, composition, and phase distribution on mechanical properties, particularly time-dependent fatigue crack growth. The alloys chosen include one in which crack growth can be slowed by slow-cooling, and one in which crack growth is slow in the fast-cooled state. Both x-ray spectroscopy and energy-filtered imaging in the analytical electron microscope were used to measure grain boundary composition. The x-ray spectroscopy showed some enhancement of Cr, Mo, and W in the γ matrix at grain boundaries in the fast-cooled state, which was not present after slow cooling. Energy-filtered imaging showed no significant enhancement in alloying elements at interfaces in any of the samples studied. The results did show the tendency for the γ matrix to quickly equilibrate by second-phase precipitation, and a preference to avoid γ ‘- γ’ interfaces. The conclusions of this study are that time-dependent fatigue crack growth behavior in these alloys cannot be completely explained on the basis of grain boundary composition of major alloying elements.

Fatigue Crack Growth Behavior of Alloy 800H at Elevated Temperature

High temperature crack growth behavior is investigated over a wide range of R-ratios, frequencies, and temperatures in Alloy 800H. It is found that high R-ratio, low frequency, or high temperature can enhance creep damage and thus induce an intergranular crack growth mode. At low frequencies, the nonlinear fracture mechanics parameter, C*, is found to correlate time-dependent fatigue crack growth rate well if the applied mean stress is used in calculating C*. On the other hand, the Paris crack growth law using Keff is proven to be an adequate expression to use when fatigue (time-independent) damage dominates. These conclusions correlate well with damage mechanisms observed from sample fracture surfaces.