Prior Creep Research Articles

Creep-fatigue characteristics of rock salt under different loading paths

The salt behavior subjected to creep-fatigue load is meaningful to ensure the stability of the salt cavities used for underground energy storage. The salt specimens are tested under three paths of creep-fatigue load, including creep followed by fatigue (C + F), fatigue followed by creep (F + C) and creep-fatigue (CF). The stress-strain curves, deformation, loading modulus and acoustic emission (AE) count were systematically analyzed. The axial, lateral and volumetric deformations present a four-phase and two-phase change trend at C + F (F + C) and CF loading, respectively. The deformation curve for an individual creep or fatigue stage at C + F and F + C loading resembles that in pure creep or fatigue tests. The prior creep or fatigue load decreases the strain rate in the subsequent fatigue or creep phase. The prior creep load decreases the loading modulus of rock salt in the subsequent fatigue stage, and the loading modulus presents a downward trend with increasing the cycle numbers for the salt samples at C + F loading. The hold time during the loading-unloading stage at CF loading exerts little effect on the variation trend of loading modulus. The hold time increases the irreversible deformation for each cycle and induces the salt samples to be prone to dilate at CF loading. Continuous AE activities were monitored in the creep stage at C + F loading, while minor AE responses develop in the creep phase at F + C loading and the hold time stage at CF loading. These results suggest that the combined creep and fatigue load in sequence and simultaneously result in a negative and positive interaction between them, respectively. The deformation behavior is explained from the viewpoint of micro-mechanism.

The role of prior creep duration on the acoustic emission characteristics of rock salt under cyclic loading

This study aimed to investigate the acoustic emission (AE) behavior of rock salt under combined creep and fatigue. The loading path was in a stress-controlled mode and contained a constant stress load with different durations, followed by cyclic loading. The AE counts, peak frequency, amplitude, b-value, and RA (rise time/amplitude)-AF (average frequency) distribution were systematically analyzed. The results show that the AE counts gradually increased during loading and existed sporadically within the prior creep stage and frequently in the subsequent cyclic loading phase. The peak frequency exhibited a zonal distribution and was divided into low, medium, and high parts. A longer prior creep duration induced a wider frequency distribution range, a higher average amplitude, and a reduction in b-value. According to the RA-AF distribution, the tensile cracks constituted more than 90% of the total cracks, and the prior creep duration decreased the shear crack propagation compared with the cases without prior creep. The results indicate that prior creep increases the crack dimensions and damage severity and makes the salt sample prone to fracture in the subsequent cyclic loading.

Local creep damage effects on subsequent low temperature fatigue crack growth behaviour of thick-walled pressure vessels

The influence of prior creep damage on the subsequent fatigue cracking behaviour of thick-walled pressure vessels has been thoroughly investigated in this study. Creep damage was introduced into fracture mechanics specimens by performing creep tests at 550 °C and stopping them after reaching a desired crack length. Using this experimental methodology, creep damage was successfully introduced ahead of the crack tip in the form of intergranular voids and microcracks. In order to assess the creep damage effects, in the absence of significant plasticity, on the structural integrity of thick-walled pressure vessels the material was uniformly pre-strained under compressive plastic deformation prior to the specimen manufacture, thus hardening the material. Fatigue tests were performed on specimens with and without creep damage and the results were compared with each other. Moreover, metallurgical investigations were conducted to understand the influence of microstructural damages, as a result of creep loading conditions, on the fatigue cracking behaviour of the material. The results from this study show a clear reduction in the fatigue cracking rate in the presence of creep damage local to the crack tip. The findings have been discussed in terms of the effects of static creep loading conditions and microstructural damages on the subsequent fatigue cracking behaviour under cyclic loading conditions.

Superior Creep Resistance and Remnant Strength of Novel Tempered Ferritic-Martensitic Steels Designed by Element Addition.

The in situ combustion (ISC) technique is promisingly applied in heavy oil recovery, whereas the operation inevitably causes high temperature and high pressure for a long duration in the thermal recovery well. As a critical component, oil casing, traditionally made of plain carbon steel in China, generally suffers from poor creep resistance and degraded remnant strength under such a harsh environment, which leads to frequent casing damage and inferior recovery efficiency. In this study, a strategy was adopted to tackle the issue by adding chromium (Cr) element into the plain carbon steel. We designed two types of novel steel with the respective addition of 1 wt.% and 13 wt.% Cr element into plain carbon steel for oil casing. Surprisingly, the trace addition of Cr element with 1 wt.% effectively lowered the creep rate in a creep test at 600 °C and 400 MPa and maintained high remnant tensile strength after creep. More significantly, prior creep history dramatically enhanced remnant strength when Cr element was added up to 13 wt.%. After a long-term creep time of 96 h, the samples were conferred by a stress increment of ~92.5 MPa (~11.0%) relative to the creep-free counterparts, whereas the value was reduced by ~158.4 MPa (~17.8%) for plain carbon steel under the same deformation conditions. Such superior mechanical performances in the Cr-doped steels are mainly ascribed to precipitation retardation of carbides and sluggish precipitate coarsening, which continuously favors a precipitation–strengthening effect in steel. These findings provide a fundamental understanding of precipitation response and creep behaviors and, more importantly, enable the development of high-performance steels used in the field of unconventional petroleum and gas resources.

Employing sequential experiments to gain insights on the interaction between creep and fatigue damage in Ni‐base superalloys

AbstractThe interaction between creep and fatigue damage in Ni‐base superalloy CMSX‐8 was explored using a sequential workflow that decouples creep and fatigue steps to quantify how accumulated creep damage interacts with subsequent isothermal low‐cycle fatigue (LCF) and vice versa. Two workflows were considered: creep followed by LCF and LCF followed by creep. For the first workflow, the subsequent LCF was evaluated at temperatures of 20°C, 750°C, and 1100°C. It was found that the extent of deleterious interaction of prior creep on LCF depended on increasing subsequent fatigue test temperature. For the LCF followed by creep, fatigue tests were first conducted at 20°C and 1100°C to 60% of life, and then creep was conducted at 800°C, where primary creep is promoted, or 900°C, where primary creep is absent. LCF damage accumulated at 1100°C accelerated creep and decreased ductility, while fatigue damage at room temperature had minimal effect on the creep behavior.

In-Feed Supplementation of Resin Acid-Enriched Composition Modulates Gut Microbiota, Improves Growth Performance, and Reduces Post-Weaning Diarrhea and Gut Inflammation in Piglets.

Simple SummaryThe weaning process can be detrimental to piglets and often leads to low feed intake, low weight gain, diarrhea, and eventually increased mortality. Resin acid-enriched composition (RAC) is an innovative natural feed ingredient that contains 90% fatty acids, mostly linoleic and oleic, pinolenic acids, and conjugated linoleic acid (CLA). It also contains 9% active component resin acid (RA), mostly abietic and dehydroabietic, and pimaric acids. It possesses antimicrobial, anti-inflammatory, and immunomodulatory properties and has been used in feed to improve performance in pigs. We hypothesized that RAC supplementation to sow and piglet diets might modulate intestinal microbiota, and improve post-weaning growth and reduce post-weaning diarrhea (PWD) and gut inflammation. Sow diets were supplemented with RAC from the last six weeks of gestation until weaning, and piglets were fed RAC trough creep feed and post weaning feed until seven weeks of age. Results indicated that supplementing sow and piglet diets with RAC during late gestation, lactation and at post weaning modulates gut microbiota, reduces inflammatory biomarkers, improves growth performance of post-weaning piglets, and prevents PWD in weaned piglets. Considering these results, RAC supplementation represents a potential strategy to improve piglet growth performance, and is a natural alternative to antibiotics for preventing PWD.The weaning process represents a delicate phase for piglets, and is often characterized by lower feed intake, lower weight gain, diarrhea, and ultimately increased mortality. We aimed to determine the effects of RAC supplementation in diets on improving piglet growth and vitality, reducing post-weaning diarrhea, and enhancing gut health. In a 2 × 2 × 2 factorial experiment, we selected forty sows and their piglets. Piglets were followed until seven weeks of age. There were no significant differences found between RAC treated and control piglets until weaning (p = 0.26). However, three weeks after weaning, RAC treated piglets had higher body weight and average daily growth (ADG) than the control piglets (p = 0.003). In addition, the piglets that received RAC after weaning, irrespective of mother or prior creep feed treatment, had lower post-weaning diarrhea (PWD) and fecal myeloperoxidase (MPO) level than control piglets. Gut microbiota analysis in post-weaning piglets revealed that RAC supplementation significantly increased Lachnospiraceae_unclassified, Blautia, Butyricicoccus, Gemmiger and Holdemanella, and decreased Bacteroidales_unclassified. Overall, RAC supplementation to piglets modulated post-weaning gut microbiota, improved growth performance after weaning, reduced post-weaning diarrhea and reduced fecal myeloperoxidase levels. We therefore consider RAC to be a potential natural feed supplement to prevent enteric infections and improve growth performance in weaning piglets.

The effects of prior creep–fatigue on the strain rate sensitivity of a P92 welded joint

The high-temperature creep–fatigue (CF) interaction on the service damage of P92 welded joint was uncovered based on the evolution of local strain rate sensitivity of strength by nanoindentation. The individual creep, fatigue and creep–fatigue loadings were performed to reveal the CF interaction on both the structure and mechanical properties of the welded joint. And CF loadings interrupted at lifetime fractions of 10%, 20%, 30%, 60%, 100% were adopted to acquire the high-temperature service damage evolution. The structure changes of the heterogeneous welded joint during service damage were characterized by using electron backscatter diffraction method. According to structure evolution including the changes of grain size and dislocation density during high-temperature deformation, we systematically discussed the local variation mechanism of hardness and SRS at different microstructural zones.

On the microstructural evolution and room‐temperature creep behaviour of 9%Cr steel weld joint under prior creep–fatigue interaction

AbstractThe high‐temperature creep–fatigue (CF) interaction on the service damage of P92 welded joint was uncovered based on the evolution of local creep behaviour by nanoindentation. The creep resistance and high‐angle grain boundary (HAGB) distributions of the different regions of welded joint were investigated for the specimens under individual creep (stress relaxation [RS]), fatigue and CF loadings interrupted at lifetime fractions of 10%, 20%, 30%, 60% and 100%. According to structure evolution including the changes of HAGB and dislocation densities during high‐temperature deformation, the local variation mechanism of creep behaviour at different microstructural zones was systematically discussed.

Effect of prior oxidation and creep damages on subsequent tensile properties of low alloy steels

Low alloy steels find extensive use in thermal recovery well oil casing, while the degradation on tensile properties is ineluctable operating at high temperature. So this study focuses on evaluating the effect of prior oxidation and creep damages on subsequent tensile properties of C–Mn steel and 3Cr–Mo steel. Prior oxidation tests interrupted at different time are conducted at 450 °C, 600 °C and 800 °C, respectively. And prior creep tests interrupted at different time are implemented at 600 °C and 400 MPa. Then subsequent tensile tests are carried out at room temperature. Results indicate that the remnant tensile strength (RTS) of 3Cr–Mo steel changes slightly after prior oxidation or creep tests at 600 °C, while that of C–Mn steel decreases significantly. As prior oxidation temperature increases to 800 °C, the RTS of both steels presents dramatic deterioration. Moreover, the remnant elastic modulus of C–Mn steel and 3Cr–Mo steel exhibit no significant changes after prior oxidation or creep at 600 °C, while show sharp decline after prior oxidation at 800 °C. Microstructure observations reveal that the deterioration of remnant tensile properties is mainly attributed to the microstructure instability such as coalescence and coarsening of carbides. Besides, oxide film cracking or peeling may also promote the decline of RTS.

Effect of Prior Creep Strain on High Cycle Fatigue Life of TI 834

Room-temperature fatigue tests were conducted on Ti 834 with prior creep strains accumulated under constant load at 550 °C and 600 °C, respectively. Microstructural and fractographic examinations on specimens with prior creep strain > 3% revealed the failure process consisting of multiple surface crack nucleation and internal void generation by creep, followed by fatigue crack propagation in coalescence with the internally distributed damage, leading to the final fracture. The amount of prior creep damage increased with creep strain. The fatigue life of Ti 834 was significantly reduced by prior creep straining. The behavior is rationalized with the integrated creep-fatigue theory.

Generation of creep-fatigue interaction diagram for an indigenous reduced activation ferritic martensitic steel (IN-RAFMS) at 823 K based on sequential tests

A novel method of generating creep-fatigue interaction diagram is presented. Creep-fatigue interaction diagram for reduced activation ferritic–martensitic steel (RAFMS) is generated at 823 K by sequential tests decoupling fatigue and creep loading modes. Prior exposure to fatigue was found to be the life limiting factor compared to prior creep under creep-fatigue interaction, on account of the extensive cyclic softening occurring during the prior fatigue exposures.

Predicted life of P91 steel for cyclic high temperature service

The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water–steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

Effect of creep phenomena on room-temperature tensile properties of cast & forged P91 steel

Abstract In present research work, the as-received cast & forged (C&F) P91 steel was subjected to the creep test at temperature of 620 °C and 650 °C for applied stress of 120 MPa. The room temperature tensile test was conducted after normalizing and tempering (N&T) treatment of the ruptured creep specimen. The standard tensile-test specimen was prepared from the gauge section of creep ruptured specimen. The N&T treatment was performed to restore the microstructure and mechanical properties of virgin P91 steel (N&T P91 steel). The microstructure of creep fractured specimen in ruptured state and N&T condition were characterized by using field-emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS). The fracture surface morphology of crept specimen and the tensile tested specimen was also studied. The effect of prior creep deformation on the mechanical strength was more significant in the sample with longer creep rupture life.

The Enigma of Large-Scale Permeability of Gas Shale: Pre-Existing or Frac-Induced?

The existing commercial programs for simulation of hydraulic fracturing (aka fracking, or frac) of gas (or oil) shale predict parallel vertical cracks to spread in vertical parallel planes, with no lateral branching. These cracks emanate from the perforation clusters on the horizontal wellbore casing, typically spaced 10 m apart or more. For such a large spacing, the rate of gas production observed at the wellhead can be explained only upon making the hypothesis that the large-scale (or regional) permeability of shale is (even at 3 km depth) about 10,000 times higher than the gas permeability of shale measured in the lab on drilled (nondried) shale cores under confining pressures corresponding to shale at the depth of about 3 km. This hypothesis has recently been rendered doubtful by a new three-phase medium theory that takes into account the body forces due to pressure gradients of pore water diffusing into the pores. This theory predicts the fracking to produce a dense system of branched vertical hydraulic cracks with the spacing of about 0.1 m. This value matches the crack spacing deduced from the gas production rate at wellhead based on the actual lab-measured permeability. It is calculated that, to boost the permeability 10,000 times, the width of the pre-existing open (unfilled) natural cracks or joints (whose ages are distributed from one to several hundred million years) would have to be about 2.8 μm (not counting possible calcite deposits in the cracks). But this width is improbably high because, over the geologic time span, the shale must exhibit significant primary and secondary creep or flow. It is shown that the creep must close all the cracks tightly (except for residual openings of the order of 10 nm) even if the cracks are propped open by surface asperities. The inevitability of secondary creep (or steady-state flow) is explained theoretically by activation of new creep sites at stress concentrations caused by prior creep deformation. The time of transition from primary to secondary creep is taken equal to the Maxwell time estimate from geology. The overall conclusion is that the 10,000-fold increase of large-scale permeability is most likely not pre-existing but frac-induced. Although this conclusion will make little difference for long-term forecasts, it would make a major difference for the understanding and control of the frac process.

Fatigue and Fracture Resistance of 316H Stainless Steel With Prior Creep Damage

316H stainless steel is widely used in high temperature components for advanced gas cooled reactors. Some plats have been de-rated to temperatures below the creep regime, due to the presence of creep damage in some components. However, such damaged or defected components must still satisfy structural integrity criteria for fatigue crack growth and fracture toughness behaviour. Therefore work has been performed to examine the fatigue and fracture toughness resistance of prior creep damaged material. A large block of material has been globally creep damaged (GCD) at 550 °C to the onset of tertiary creep behaviour. In this work, half sized compact tension C(T) specimens have been extracted from 316H blocks which were both pre-compressed and prior creep damaged at 550 °C. Fatigue crack growth and fracture toughness tests were subsequently performed on them. Note that prior to creep, this block was pre-compressed (PC) to 8% plastic strain at room temperature in order to work harden the material and limit the influence of crack tip plasticity in subsequent creep crack growth tests. The results, when compared to those previously obtained from as-received (AR), pre-compressed (PC) and locally creep damaged (LCD) standard sized C(T) samples, show an overall reduction of the fracture energy J. However, the global creep damage method does not introduce substantial changes on the fatigue crack growth behaviour of the material.

Quantification and Prediction of Residual Stresses in Creep Crack Growth Specimens

An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.

About the creep-fatigue interaction on the fatigue behaviour of off-axis woven-ply thermoplastic laminates at temperatures higher than Tg

The influence of plasticity and viscous effects on the fatigue behaviour of off-axis C/PPS laminates was investigated at temperatures higher than glass transition temperature. The obtained results clearly show that creep and fatigue are mutually influencing phenomena. Compared to the reference fatigue behaviour (with no prior loading), the fatigue life can be significantly extended with prior creep depending on loading conditions. Indeed, the strain accumulation seems to slow down after a long time creep preload, as if the time-dependent mechanisms were “evacuated” during this preload. The same conclusion can be drawn for the damage accumulation when the prior creep stresses are higher than the damage threshold or when the hold time is long enough, inducing significant plastic deformations. In angle-ply laminates, such deformations are associated with the reorientation of fibres. They contribute to the reduction of stress intensities, which results in increasing both fatigue life and maximum strain ɛmax at failure during fatigue loadings.

Evaluación de la longitud de grietas por fatiga mediante la deformación plástica en muestras compactas de tracción empleando el ruido magnético de Barkhausen

In this paper the influence of different parameters on crack length is determined via plastic deformation using magnetic Barkhausen noise; the compact specimens steel used in this study were 12Cr1MoV and 11Cr1Mo, which are used for steam transportation on power plants. The main objective of this paper is to determine the crack length and its incubation time using magnetic Barkhausen noise, which lets to improve the pre-cracking process prior creep crack growth rate test simulating the real conditions of high pressure and temperature of main steam lines. Results showed that mean root square and maximum pick of voltage decrease with plastic deformation for each steel analyzed in this work, on the other hand, with the increasing of plastic deformation, a logic relationship between plastic deformation and spectrum density is observed for mid frequencies values, as well as, a variation of the amplitude, width and shape of the Barkhausen signal envelope.

Impact induced failure of cartilage-on-bone following creep loading: A microstructural and fracture mechanics study

Cartilage-on-bone samples obtained from healthy bovine patellae, with or without prior static compression (i.e. creep) at 2MPa for 3h, were delivered a single impact via an instrumented pendulum indenter at a velocity of 1.13m/s and an energy of 2.2J. Mechanical data was obtained and microstructural assessment of the region of failure was carried out using differential interference contrast (DIC) optical imaging. In addition, a fibrillar-level structural analysis using scanning electron microscopy (SEM) was conducted on a control batch of non-impacted samples that were subjected to either creep or non-creep loading protocols. Arising from the impact event the deepest levels of crack penetration into the articular cartilage occurred in those samples subjected to prior creep loading. Further the crack depth was inversely proportional to the rebound velocity of the indenter. By contrast, those impacted samples not subjected to prior creep loading had only short obliquely patterned microcracks confined to the upper one-third of the full cartilage depth. Ultrastructurally the creep-loaded cartilage matrix exhibited a substantial radial collapse or compaction of the fibrillar network in its primary radial zone. The increase in crack length in the prior creep-loaded cartilage is consistent with a reduction in its dissipative properties as indicated by a reduction in rebound velocity. An interpretation is offered in terms of classical fracture mechanics theory.

Does prior sustained compression make cartilage-on-bone more vulnerable to trauma?

BackgroundThis study investigated how varying levels of prior creep deformation in cartilage-on-bone samples influences their mechanical response and vulnerability to structural damage following a single traumatic impact. MethodsBovine patellae were subjected to varying intervals of prior creep loading at a constant stress of 4MPa. Immediately following removal of this stress the samples were impacted with a pendulum indenter system at a fixed energy of 2.2J. FindingsWith increasing prior creep, the peak force on impact rose, the duration of impact and time to reach peak force both decreased, and both the energy dissipated during impact and the magnitude of impulse were both unchanged by the level of prior creep. With increasing prior creep, the severity of impact-induced osteochondral damage increased: articular cartilage cracks penetrated to a greater depth, extending to the calcified cartilage layer resulting in hairline fractures or articular cartilage delamination and associated secondary damage to the vascular channels in the subchondral bone. InterpretationThe study shows that exposure of the cartilage-on-bone system to prior creep can significantly influence its response to subsequent impact, namely force attenuation and severity of damage to the articular cartilage, calcified cartilage and vascular channel network in the subchondral bone.