•Red sandstone containing pre-existing crack was tested under fatigue-creep tests.•Combined AE, SEM and DIC techniques for combined measurement analysis.•A macro and meso multi-scale analysis of the ...rock fracture mode was carried out.•A damage evolution model was established based on the LURR theory.•The improved Harris distribution was used to establish the damage model.
To study the effect of creep loading on the fracture mode and damage evolution process of jointed rock masses during fatigue loading, a series of multilevel cyclic loading and unloading tests (MCT) and fatigue-creep tests (FCT) were carried out on jointed sandstone samples. First, a combined detection platform was built using acoustic emission (AE), scanning electron microscopy (SEM) and digital image-related (DIC) technologies. Then, the fracture mode of the sample is analyzed at the macro- and mesoscale. Finally, based on the loading and unloading response ratio (LURR) theory, assuming that the internal damage of the sample obeys the improved Harris distribution, a damage evolution model of the sandstone sample was established, and the applicability and validity of the model were verified. The test results show that applying creep load during the fatigue loading process can increase the axial strength, reduce the axial deformation, weaken the energy dissipation, and exhibit the phenomenon of early hardening. In terms of fracture mode, applying creep load can change the type of cracks that dominate the failure of the sample, improve the shear resistance, and increase the brittleness when the sample is broken. At the same time, the dislocation movement between the grains, pressure solution and splitting phenomenon will be enhanced, and the types of micro-defects will increase. In terms of the damage evolution process, a short-term low-stress level creep load can reduce the rate of damage growth, thereby increasing the fatigue life of the rock mass.
A series of stress‐controlled fatigue–creep interaction loading tests were performed on rock–backfill composite structure (RBCS) samples. The testing results reveal the influence of cement/tailing ...(c/t) on deformation, energy dissipation, damage evolution, and macro–meso failure patterns. The equivalent lifetime of RBCS is the largest for a sample with smaller stiffness filling, a smaller stiffness filling leads to weaker volumetric expansion. Additionally, strain energy is relatively large for larger stiffness filling, indicating a larger proportion of energy release at failure. A dissipated energy‐based damage evolution model is proposed to describe damage propagation at the entire fatigue–creep process which fits well with the experimental data. Moreover, the macro–meso failure pattern reveals the interactions between surrounding rock and filling material, failure seems to change from tension splitting to shear with increasing c/t. Good agreement was found among the volumetric deformation, energy dissipation, and failure pattern. It is suggested that flexible backfilling is benefit to prevent rock spalling and collapse.
Highlights
Fatigue–creep interaction loading tests were conducted on RBCS samples.
The equivalent lifetime of RBCS first decreases and then increases with increasing c/t.
RBCS sample with smaller stiffness filling can absorb much more energy during failure.
It is suggested that flexible backfilling material is beneficial to prevent rock spalling.
A unified viscoplastic constitutive model with wide temperature adaptability is developed based on the basic framework of the Chaboche type model to capture the intricate cyclic viscoplastic ...behaviors of Haynes 230 subjected to isothermal low-cycle fatigue and fatigue-creep loads. By introducing effective aging time into drag stress in the hyperbolic sine viscosity function, the critical value of back stress and the additional hardening model, a rate-dependent model based on the dimensionless concentration of solute atoms or precipitates is further improved to realize simulation of dynamic strain aging effects. A multistage evolutionary approach to determining asymptotic value of back stress influenced by strain range and cumulative plastic strain is proposed to respond to cyclic hardening or softening evolved differently with the variation of temperature. For the fatigue-creep interaction, a rate or time dependent directional hardening cumulative model combined with the existing mean stress model is established. After the multi-step material parameter determination method is given, the capabilities of the proposed model are fully validated to accurately depict the mechanical behaviors of Haynes 230 for five representative temperatures.
•A systematically modified viscoplastic constitutive model with wide temperature adaptability has been developed.•A rate-dependent model to fully reflect the effect of dynamic strain aging is given.•The idea of the multistage asymptotic value evolution of back stress is proposed to respond to cyclic hardening and softening.•A rate-dependent directional hardening accumulation model is proposed for fatigue-creep interaction.•The capabilities of the proposed model are fully validated to accurately depict the mechanical behaviors of Haynes.
Cr-Mo casting steels are widely used as casings and valves in thermal power plants. Creep fatigue damage gradually proceeds in these high-temperature components during operation resulting in creep ...voids and micro crack initiation in which repair welding is considered to be applied for life extension. In this study, a temper bead welded joint of Cr-Mo casting steel as well as fine and coarse grain simulated materials were prepared. Creep and fatigue, creep-fatigue tests were performed using specimens taken from base and weld metals of the welded joint, the simulated materials and cross welded specimens. Material constants of creep deformation and cyclic stress-strain properties for constituted materials of the welded joint were obtained from the creep and fatigue tests. Minimum creep strain rate of the base metal at the same stress was the highest than that of other materials. Plastic deformation resistance is higher in the order of the base metal, fine grain, coarse and weld metal. Creep-fatigue life of the welded joint, which failed at the base metal 2mm away from the boundary between the base metal and fine grain, was shorter than that of the uniform base metal specimen indicating that the temper bead welding may reduce the creep-fatigue life. Finite element analysis of the cross welded specimen showed that elastic-plastic and creep strains accumulated at the base metal near the boundary are higher than those at other regions causing life reduction and failure at the base metal. The creep-fatigue life prediction by using the analysis results indicated that the nonlinear damage accumulation model gave more accurate life prediction results than the time fraction and ductility exhaustion rules.
Recently, the high-efficiency gas turbine combined cycle power generation has become essential in operation as a backup power supply of renewable energy sources. In turbine blades and vane of the gas ...turbine operated as a backup power supply, there is growing concern about the damage due to thermo-mechanical fatigue (TMF) loading because of expecting increased starting/stopping cycle and frequent load fluctuation. This study investigated the small crack propagation behavior in single crystal Ni-based superalloy under the in-phase TMF loading cycle. The creep-fatigue interaction on the small crack propagation behavior was discussed based on the relationship between the crack growth rate and the creep J-integral, considering the small-scale creep at the crack tip. The experimental results indicated that the crack growth rates under the in-phase TMF loading cycle were significantly higher than those under the out-of-phase TMF condition at a given fatigue J-integral range. In addition, introducing the tension holding at the maximum temperature in the in-phase TMF loading cycle accelerated the crack growth rates. The small-scale creep stress field at the crack tip might dominate the small crack propagation behavior under the in-phase TMF loading with the high-temperature holding as the creep-fatigue interaction.
•Evaluation of modeling features in simulating creep and fatigue responses.•Need of coupling continuum damage model to unified constitutive model (UCM).•Issues in simulating creep strains at low ...temperatures by the modified UCM.•Issues in simulating fatigue responses at high temperatures by the modified UCM.•Challenges of UCM in simulating elevated temperature creep and fatigue responses.
A unified constitutive model (UCM) specifies that its flow rule for inelasticity computes both the plastic and creep strains as a single state variable. A Chaboche framework based UCM with the modeling features of strain range-dependence, strain rate-dependence, static recovery and mean stress evolution was developed and experimentally validated against a broad set of fatigue and fatigue-creep responses of Haynes 230 (HA 230) under isothermal and anisothermal temperature conditions. This article demonstrates that this advanced Chaboche-based UCM can simulate the secondary minimum creep strain rates reasonably, but is unable to predict the tertiary creep strain responses. To simulate the tertiary creep strain responses a continuum damage model is needed to be coupled to the UCM. This study also evaluated three different unified flow rules, Norton's power law, exponential Norton and sine-hyperbolic Norton for calculating the inelastic strain rates. It is found that the choice of flow rule is important in simulating the stress amplitude saturation rate of fatigue responses, but has minimal effect in simulating the tertiary creep strains. However, the damage coupled UCM independent to the unified flow rules listed above can adequately simulate fatigue, fatigue-creep including the stress relaxation during strain dwell, and creep strain up to the tertiary range for HA 230. The drawbacks of the damage coupled UCM are the hysteresis loop softening at very high temperatures and asymptotic simulation at low creep temperatures, which are identified as challenges to be overcome towards developing a universal UCM for robust design and analysis of high temperature components.
To explore the fracture characteristics of rock under the interaction of fatigue load and creep load, fatigue–creep interactive loading experiments were performed on red sandstone with prefabricated ...cracks. The crack evolution process and failure mode were analyzed using acoustic emission technology and digital image correlation. The results showed that crack growth mainly occurred in the fatigue loading stage; the crack evolution of the sample could be divided into three stages: nucleation and initiation (
SI), stable expansion (
SII), and unstable fracture (
SIII). There were distinct differences in the crack propagation modes of the rock samples with different prefabricated crack angles. The relationship between the crack initiation angle and prefabricated crack angle was analyzed based on the maximum circumferential stress theory. Moreover, with an increase in the prefabricated crack angle, the rock sample gradually changed from compression–shear failure to tension–shear failure.
Highlights
Red sandstone with prefabricated cracks was tested under fatigue–creep interaction.
The AE characteristics and crack evolution process of the samples were analyzed.
The influence of the crack angle on the failure mode is revealed.
•LCF and TMF tests were performed with and without strain dwell for 42CrMo4 steel.•TMF tests were performed for three different phase angles between 300 °C and 600 °C.•The employed phase angle ...significantly affects the dominant damage mechanism.•The damage model incorporates the effect of frequency, phase angle and mean stress.
Strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on a low-alloy martensitic steel in the temperature range between 300 °C and 600 °C, with and without a strain dwell. TMF tests were performed for three different phase angles between thermal strain and mechanical strain: as in-phase, as out-of-phase and as counter clockwise-diamond. The results show that the phase angle that is employed has a significant influence on the dominant damage mechanism and on the observed lifetime. Finally, a modified damage model is proposed in order to take into account the effect of the phase angle.
We elucidate here the effect of low oxygen partial pressure on the dwell fatigue crack growth rate (DFCGR) of 718Plus alloy at high temperature. In comparison under the air and vacuum conditions with ...lower oxygen partial pressure, the DFCGR of 718Plus alloy reduced by three orders of magnitude at 704 °C at lower oxygen partial pressure than that of in air condition. By analysis of microstructure, oxide, and crack path under different conditions, it was considered that the oxygen-induced dynamic embrittlement (DE) was the dominant impact mechanism for fatigue crack propagation at high-temperature in 718Plus alloy. Meanwhile, the stress-accelerated grain boundary oxidation (SAGBO) mechanism also played a significant role, especially in the long step-wise period. In addition, the types of oxide formed in creep process were directly related to the oxygen partial pressure and oxidation time.
•Fatigue–creep interaction tests were carried out on red sandstone with prefabricated cracks.•The characteristics of deformation and energy under fatigue–creep interactive loading was analyzed.•The ...macro–micro coupling damage evolution model was established.•The influence of crack angle on acoustic emission characteristics was investigated.•The failure modes of samples under FCI loading were discussed.
Most engineering rock masses are actually under the interaction of fatigue load and creep load. Currently, the understanding of the fatigue or creep characteristics of rock is relatively mature, but the damage and failure characteristics of rock under fatigue–creep interaction (FCI) are rarely studied. In this study, FCI loading tests were carried out on red sandstone with prefabricated cracks at different angles. The results showed that the elastic modulus tended to be stable with the loading. While the deformation modulus and secant modulus decreased nonlinearly, and a relatively significant decrease occurred after the creep loading. The strain evolution could be divided into deceleration stage (P1), stable stage (P2) and acceleration stage (P3). Creep loading would increase the energy dissipation of the sample, thereby accelerating the accumulation of rock damage. A macro–micro coupling damage model was established, which reflected the damage evolution and strength variation of samples. In addition, acoustic emission (AE) was mainly generated in the fatigue loading stage, and the AE level increased with the increase of the crack angle. The crack propagation process of samples with different angles of crack was analyzed by b value of AE, and the failure modes of samples were further discussed. The research results have certain theoretical reference value for understanding the damage and failure characteristics of fractured rock mass under interaction.
PDF
