Long-term Creep Tests Research Articles

Long-term creep and microscopic deformation mechanisms of sandstone using rock rheology creep equipment and scanning through high-resolution 3D XRM

Abstract Since rock is heterogeneous, hence its behavior is always variable. The governing parameters in rock deformations are time, applied load, composition, water content, temperature and loading conditions including confinement and loading rate. Time-dependent deformation study is extremely important for the prevention of hazards such as rockburst, roof fall and collapse. In the current study, sandstone samples from Kouzidong Mine China are analyzed using long-term rheological creep tests and advanced microscopic scanning using 3D X-ray microscopy (XRM). Time is the main governing factor in rock rheological creep study. The current study focuses on the microscopic deformation of selected rock using rheological creep and microscopic analysis. Rock behaves differently under different applied loads; therefore, two different loads are applied on two sandstone samples. Two samples are scanned before and after rheological creep testing. The uniaxial constant load applied on the first sample is 40% of the uniaxial compressive strength of the rock (σ1 = 40%σc) for one month. The behavior of the first sample shows an instantaneous creep and stable creep. The maximum uniaxial strain is 0.0307. The scanning results show evidence of microfractures in sandstone sample 1 at a lower load. There is evidence of grain movement and boundary alteration. The grain movement depends on the bonding between different grains. The second sample is loaded at 50% of the uniaxial compressive strength (σ1 = 50%σc). The maximum uniaxial strain for sample 2 is 0.0408. Creep behavior is the same and the microscopic deformation is enhanced with increasing applied load.

Long-term performance of packaged fiber Bragg grating sensors for strain monitoring inside creep medium

ABSTRACT To investigate the long-term performance of the packaged fiber Bragg grating (FBG) sensors embedded in civil infrastructure for strain monitoring, in this paper, the influence of host matrix’s creep effect on the behavior of the FBG sensors was systematically studied through theoretical, numerical, and experimental analysis. A theoretical strain transfer analysis between the optic fiber, packaging layer, and host matrix to consider the creep effect of the host matrix was performed accordingly for long-term strain monitoring. Parametric studies were carried out using numerical analysis for FBG sensors packaged with glass fiber reinforced plastic (GFRP), also known as FBG-GFRP sensors in concrete, as an example. The results show that embedded in a creep medium, an acceptable long-term performance of packaged FBG sensors requires the packaging layer to have a minimum length and maximum thickness. Laboratory long-term creep tests using epoxy resin and concrete as host matrix for FBG-GFRP sensors also clearly demonstrated that the influence of creep effect cannot be ignored for strain measurements if the host matrix has a creep potential and the developed correction model performed well to reduce measurement errors of such sensors in creep medium.

A study on the creep behavior of alloy 709 using in-situ scanning electron microscopy

In this research, an experimental evaluation of creep properties of Alloy 709 in the temperature range of 750–850 °C was undertaken. Alloy 709 is a novel austenitic stainless steel with 20% Cr and 25% Ni by wt% that was developed for application in structural components of nuclear power plants. Creep rupture tests were conducted in an in-situ heating-loading and Scanning Electron Microscope (SEM) unit equipped with Electron Backscatter Diffraction (EBSD) detector and Energy Dispersive Spectroscopy (EDS). “Real-time” creep damage mechanisms of Alloy 709 at various stresses and temperatures using a flat, un-notched sample with continuously reducing cross-section is studied so that the failure and maximum creep damage occurred at the center of the sample where the in-situ SEM imaging could be focused. Accelerated creep tests at temperatures and stresses above service conditions were performed by employing multiple blocks of constant loads where the loads were increased once the sample attained constant creep rate, indicating a secondary creep regime. This technique ensures multiple data points can be obtained from the same test, saves the time required for an otherwise long-term creep test and usage of SEM. Coincident Site Lattice (CSL) boundary maps were collected as control maps before testing, and the grain boundaries were observed during the creep test to understand the effect of grain boundary character on the creep damage mechanism. Void growth, grain boundary separation, and sliding were found to be the main creep mechanisms whose rate is dependent on stress and temperature. Failure mechanisms studied on the fracture surface using SEM fractography were correlated to the sample surface observations to create complementary information to better understand the underline creep mechanism of Alloy 709.

Fractional description of creep behavior for fiber reinforced concrete: Simulation and parameter study

Fiber reinforced concrete (FRC) exhibits enhanced strength and excellent durability, and the creep behaviors of FRC are major concerns for its application in structural analysis and design process. In this work, a novel fractional creep model for FRC is developed by introducing the fractional flow rule into the classical elastoplastic constitutive model. Specifically, a hierarchical single-surface (HISS) type yield function is adopted and an isotropic hardening function is introduced as hardening variables. Without using plastic potential, the proposed model exhibits higher accuracy than several conventional creep models with relatively simple form which is validated by comparing the numerical predictions with experimental results from short-term and long-term creep tests. Furthermore, a parameter study on the fractional order suggests that the order can adjust the magnitude and rate of the creep deformation, and the consistency between macroscopic deformation and the material formulation of FRC could be reflected by the tendency of the fractional order.

Structural Stability of Sandvik 3R60™ After 240 131 Hours Ageing and Creep Test at 700 °C

Sandvik 3R60™ is an AISI 316/316L type of stainless steel. In this paper, the structural stability of the material under long-term ageing or creep test has been studied. The material had been creep tested with a stress of 45 MPa at 700 °C. The predicted rupture time for the creep specimen was about 100,000 h; however, the specimen broke first after 240,131 h. The oxidation behavior and structural stability in both aged and creep-tested samples were studied using SEM/EDS, EBSD and ECCI techniques. Thin oxide layers near the sample surface are mainly spinel oxides and eskolaite (Cr2O3). Sigma phase, χ-phase, Eta phase, M23C6 and Cr2N have been observed in the matrix of the samples. In the crept sample, the amount of sigma phase has increased, so has Eta phase and χ-phase as well. Thermo-Calc evaluation can reasonably predict precipitation of sigma phase, Eta phase and M23C6, but not χ-phase and Cr2N phases. Creep crack initiation behavior has been studied. It is mainly noticed to start at surface oxide layer or coarse sigma particles at grain boundary or triple point. Additionally, it is also observed that the presence of a thin Cr2O3 layer between the oxide and matrix along with discontinuous sigma phase distribution at grain boundary that will reduce the risk for creep crack initiation. Further, the crack propagation behavior has also been discussed.

Compaction of crushed salt for safe containment – overview of the KOMPASS project

Abstract. In Germany, rock salt formations are possible host rock candidates for a repository for heat-emitting radioactive waste. The safety concept of a repository in salt bases on a multibarrier system consisting mainly of the geological barrier salt and geotechnical seals ensuring safe containment. Crushed salt will be used for backfilling of cavities and sealing measures in drifts and shafts due to its favourable properties and its easy availability (mined-off material). The creep of the rock salt leads to crushed salt compaction with time. Thereby, the crushed salts' porosity is reduced from the initial porosity of 30 %–40 % to a value comparable to the porosity of undisturbed rock salt (≤1 %). In such low porosity ranges, technical impermeability is assumed. The compaction behaviour of crushed salt is rather complex and involves several coupled THM processes (Kröhn et al., 2017; Hansen et al., 2014). It is influenced by internal properties like humidity and grain size distribution, as well as boundary conditions such as temperature, compaction rate or stress state. However, the current process understanding has some important gaps referring to the material behaviour, experimental database and numerical modelling. It needs to be extended and validated, especially in the low porosity range. The objective of the KOMPASS project was development of methods and strategies for the reduction of deficits in the prediction of crushed salt compaction leading to an improvement of the prognosis quality. Key results are as follows (KOMPASS Phase 1, 2020): selection of an easily available and permanently producible synthetic crushed salt mixture, acting as a reference material for generic investigations; development and proof of different techniques for producing pre-compacted samples for further investigations; establishment of a tool of microstructure investigation methods to demonstrate the comparability of grain structures of pre-compacted samples with in-situ compacted material for future investigations; execution of various laboratory experiments using pre-compacted samples, e.g. long-term creep tests which deliver reliable information about time- and stress-dependent compaction behaviour; development of a complex experimental investigation strategy to derive necessary model parameters considering individual functional dependencies. Its technical feasibility was successfully verified; benchmarking with various existing numerical models using datasets from three different triaxial long-term tests. The result was not entirely satisfactory; however, the number of influencing factors is small and further validation work has to be done. Overall, the KOMPASS project has made significant progress in the approaches to solving the outstanding question, building the basis for further investigations.

3D analysis of the 174-m high Quxue asphalt-core rockfill dam in a narrow canyon

The site for the high Quxue Dam has a narrow canyon with very steep abutments and complex geology in Sichuan Province, China. Various types of embankment and concrete dams were considered and an asphalt-core embankment dam (ACED), 174 m high, was selected. The asphalt core is a slender impervious element in an embankment dam and some concerns have been raised about the reliability and safety of the asphalt-core type of dams (ACEDs). This paper presents the case study of the highest asphalt core dam ever built and presents measured and numerical analyses of the core deformations. A 3D non-linear FE analysis has been performed for the Quxue Dam, and the results have been compared with those computed by a 2D analysis to study the effects of the narrow canyon. The computed results are compared with the monitoring data from dam construction and first impoundment. The maximum settlement measured inside the embankment during construction was only about 0.7 m due to the good basaltic rockfill used and the heavy compaction in layers of only 1.0 – 1.2 m thickness. The results of the 3D analysis agree well with the measured downstream displacements and the deflected shape of the thin asphalt core during impoundment, while the 2D analysis overpredicted the maximum displacement by a factor of about two. At a reservoir level 3 m below full supply level, the maximum measured downstream core displacement was only about 80 mm due to the stiff rockfill and the restraining effects of the abutments in the narrow valley.The rockfill behaviour was modelled by the constitutive relationship proposed by Duncan and Chang (1970). The rockfill parameters were determined by laboratory tests prior to construction and were adjusted to better match field observations of vertical settlements during construction. As asphalt concrete exhibits pronounced visco-elastoplastic behaviour, the material modelling was based on long-term triaxial creep tests on samples drilled out of the dam core. The performance monitoring and numerical analysis results for the record-high Quxue ACED document that the concerns expressed about the asphalt core safety and behaviour is not warranted. The asphalt core type of dam is suitable even for high ACEDs in narrow valleys with steep abutments.

Study on very long-term creep tests and nonlinear creep-damage constitutive model of salt rock

To better understand the creep mechanical behavior of salt rock under very long-term tests and more accurately predict the long-term deformation of the salt caverns , a series of long-term uniaxial creep tests lasting for 21000 h (875 days) were carried out. The results show that the creep strain and creep rate of salt rock increase with the increase of axial stress . The initial creep stage under lower stresses lasted for 4.3–8.3 months, which was much higher than that under high stresses. Predicting the true creep rate of the salt cavern based on the test results of the creep rate in this paper is more accurate than the existing research results. A new nonlinear creep-damage constitutive model is proposed by introducing the nonlinear damage body and using the fractional derivative theory. Using the presented model and the classic creep constitutive models to fit the experimental data, it is found that the presented model best characterizes the creep evolution characteristics of salt rock. The fitting function obtained from the experimental data at different times predicts the final creep results, revealing that the longer the test duration, the more accurate the predicted results. The research in this paper provides the necessary basis for intensive investigation and prediction of the long-term creep mechanical behavior of salt storage caverns.

Microstructure changes occurring in heat-resistant steels during severe plastic deformation and subsequent creep

This work investigates creep and microstructure characteristics in P92 martensitic and 304L austenitic stainless steel after the imposing of large value of plastic strain. The as-received coarse-grained steels were strongly deformed at room temperature (RT) before tensile creep testing. The flat specimens were tested at 873 K and 923 K under different applied stresses. Microstructures were analysed by means of scanning electron microscope with EBSD (electron back-scatter diffraction) camera and transmission electron microscope. It was observed that application of severe plastic deformation (SPD) techniques at RT significantly reduces grain size and changes phase structure of 304L austenitic steel. The microstructure investigations revealed that the growth of new phases is enhanced in plastically deformed P92 martensitic and 304L austenitic stainless steels compared to their undeformed states. Creep results showed that the mean grain size of SPD-processed steels is still near ultrafine-grained region even after long-term creep testing and the coarsening of microstructure is predominantly influenced by creep time and strain.

Mechanical investigation of rock-proppant interaction with focus on geothermal applications

Since decades proppants have been used to enhance hydraulic pathways for the exploitation of hydrocarbon reservoirs. These technologies may also be adapted for geothermal applications, but geothermal reservoirs rocks strongly differ from those typically targeted by hydrocarbon industries. Therefore, the mechanical interaction between conventional proppants and geothermal reservoir rocks and their effect on hydraulic properties need to be investigated in more detail to maximize the success of such method adaption. In principle, two mechanical requirements must be fulfilled: (1) proppants must withstand the stresses that are present in a reservoir, and (2) proppants must not penetrate the rock formation of the reservoir, because this would reduce the fracture conductivity. Fine mobilized rock- or proppant particles can also clog the fracture. For the mechanical suitability of proppants, uniaxial compressive strength tests and long-term creep tests were carried out in the laboratory. The test setup consists of a proppant monolayer placed between two cylindrical rock samples that were axially loaded. We determined Young’s moduli and creep rates for sample stacks with various proppants under different stress conditions. The laboratory tests are part of the ZoKrateS Project, which aims at showing the feasibility of enhancing fractured carbonate rock mass by proppant placement for geothermal applications.

Behavior of RC beams strengthened with CFRP sheets under sustained loads

This paper investigates the long-term response of reinforced concrete (RC) beams strengthened with CFRP subjected to sustained static loads. The beams were strengthened using two different configurations of CFRP with different steel reinforcement ratios and web reinforcement. Two RC beams and five CFRP-RC beams were evaluated under static short-term bending, and five additional CFRP-RC beams were evaluated under sustained loads for 332 days. The sustained loads were maintained at around 60% of the static capacity of the beams strengthened with CFRP. The residual strength of the CFRP-RC beams after long-term creep testing was evaluated experimentally under static loads to failure. CFRP sheets were attached to the transverse and tension faces of the beams. The test results show that CFRP strengthening of beams is effective in enhancing their static capacities. The residual strength of the long-term creep-tested beams was 5–18% lower than the strength of the short-term beams except for the steel over-reinforced beam. In addition, the residual stiffness did not change significantly. However, the residual toughness was 22% to 64% of the toughness of the short-term beams due to creep damage. The web steel reinforcement played an important role in improving the overall performance and failure mode of the strengthened beams.

Prediction of Long-Term Creep Properties of Zirconium-2.5 % Niobium Alloy Using Wilshire Method

Abstract Zirconium-2.5 % niobium alloy, used as a pressure tube material in pressurized heavy water reactors, have to experience extreme stress, temperature, and radioactive conditions to serve within the designed dimensional tolerance limit. Prolonged duration under high stress and high temperature causes the creep deformation, ultimately leading to deformation beyond the permitted limit or interference with other components, or both. Hence, a priori using particular material for a specific application, mechanical testing has to be done to investigate its deformation behavior. However, creep tests of material can last from a few hours to several months or even years. The cost of testing the material can eventually impact the cost of the component. Instead of carrying out long-term creep tests, an extrapolation technique named the Wilshire method is applied to predict the creep behavior of the material under any given test conditions. This method is also capable of reconstructing the complete creep curve using the data from accelerated creep tests. Such an approach can drastically scale down the cost and time required for the long-term creep tests. In the present study, zirconium 2.5 % niobium alloy, which is used as pressure tube material in pressurized heavy water reactors, is employed as a test material for the analysis using the Wilshire method.

Microstructural stability of ODS steel after very long-term creep test

Microstructure of 9Cr oxide dispersion strengthened (ODS) tempered martensitic steel (TMS) cladding and 12Cr-ODS ferritic steel cladding after very long-term internal pressurized creep tests was observed. The creep test conditions were 700°C for 46,504 h and 750°C for 45,550 h for 9Cr-ODS TMS and 700°C for 30,116 h for 12Cr-ODS ferritic steel. The stability of martensite and ferrite matrix structure, including carbide distribution, dislocation density, and lath boundary, and that of nano-particle distribution were investigated. It was confirmed that the nano-particles were stably present during the high-temperature and very long-term stress loading environment, contributing to the prevention of matrix structure degradation. The results of observation are consistent with the fact that the 9Cr-ODS TMS and 12Cr-ODS ferritic steel exhibit excellent creep strength and show no reduction in the slope of the stress versus time to rupture curve over a significantly extended period of time in the creep test.

Study on Time-Dependent Fracturing Behaviour for Three Different Hard Rock Under High True Triaxial Stress

True triaxial short-term and long-term tests were conducted on three different types of hard rocks, namely, marble, basalt and granite. During true triaxial long-term creep tests, the crack evolution information was collected by using an acoustic emission system. The micro-structures before and after creep tests were analysed by scanning electron microscopy and polarising microscopy. The time-dependent propagation of cracks in hard rocks is affected by applied stress and the mineral composition. The high axial stress makes the cracks propagate with time development during creep stage. When the axial stress exceeds the damage stress, the AE signals intensity have increased significantly. The damage stress values determined through short-term and long-term tests are different, but the difference is small so that the former can be used efficiently to test the damage stress in terms of long-term strength analysis. Accumulation of extensile cracks induced by high applied axial stress eventually can cause creep failure of hard rocks; however, the time-effect seems to only increase the micro-cracking inside the rock. The ultimate macroscopic failure surface of the rock sample appears to only be affected by the difference between σ2 and σ3. The time-dependent dilatancy of Jinping marble and Baihetan basalt exceeded that of granite specimens. Under creep loading, calcite and pyroxene, which have lower hardness values, are more prone to crack time-dependent expansion than feldspar and quartz, which have higher hardness values.

Experimental investigation of the long-term behavior of reconstituted bamboo beams with various loading levels

To study the creep behavior of bamboo beams, the long-term bending creep tests of twelve bamboo beams with different loading levels were carried out in an indoor environment. The following results were obtained. The creep of reconstituted bamboo beams under different loading levels exhibits the three-stage characteristics of short rapid creep, sustained moderate-speed creep, and stable low-speed creep; the growth ratios of creep deflection are between 30.9% and 39.1% for the reconstituted bamboo beams at the 10–50% loading level. The creep and the creep rate increase with an increase in the loading level. Meanwhile, the long-term creep prediction model of the reconstituted bamboo beam is derived based on the Burgers model; the proposed model may reflect the significant effects of the loading level and is able to accurately evaluate the deflection-time behavior of the recombinant bamboo by taking the loading level into consideration. Based on the prediction, the creep value of reconstituted bamboo beams is large and develops rapidly, and the creep deflection of the reconstituted bamboo beams at high loading level (30–50%) will exceed the maximum deflection corresponding to the ultimate bending capacity within 5 years.

Customized High-Temperature Bending with DIC for High-Throughput Determination of Creep Parameters: Technique, Instrumentation, and Optimization

Bending is emerging as an alternative to uniaxial testing for high-throughput determination of primary-cum-secondary creep parameters. This is feasible because of augmentation of cantilever bending tests with digital image correlation (DIC). Here, we provide guidelines for selecting the patterning medium, spray conditions, and other important parameters for laying high-resolution, high contrast ratio DIC speckle-patterns that are optimal for long-term creep tests performed up to 800°C. Furthermore, an experimental setup for efficiently performing creep experiments in bending at high temperatures is designed and developed. Proper choices of paint, spraying technique, high-resolution image capturing device, and the appropriate heat management system in the mechanical testing unit yield unambiguous correlation in the “DIC augmented bending creep” tests. The developed methodology and the equipment are validated by evaluating the creep behavior of a stainless steel at 750°C by obtaining multiple stress–strain rate pairs from a single test. An excellent match between uniaxial and bending data is observed, thereby paving the way for using DIC augmented bending creep for studying the creep response of materials at high temperatures in a high-throughput fashion.

Characterization of Microstructure of Crept Samples of Dissimilar Weld Joint Using Standard and Advanced Electron Microscopy Techniques

Conventional long-term creep test (CCT) to the rupture and so called accelerated creep test (ACT) of the dissimilar weld joint made of FB2 and F martensitic steels and of the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa. Assessment of microstructure development and changes of hardness was correlated with the creep strength. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centers for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and cause failure of samples. The aim of this paper is to compare results and possibilities of the “standard” methods and advanced scanning electron microscopy performed by instrument equipped with a concentric backscatter electron detector (CBS). Filtering of the signal enables improving and/or diminishing of selected type of contrast caused by various types of particles of secondary phases. The images were used as an input data for image analysis and developments of microstructures during CCT and ACT were compared. Results have shown that specimens after ACT contains significantly lower content of the Laves phase.

Experimental evaluation of localized creep deformation in grade 91 steel weldments

Spatially resolved measurement of localized creep deformation in heterogeneous creep resistant steel weldments is crucial but challenging for lifetime assessments of critical steam components in power plants. In this work, experimental approaches were established to quantitatively evaluate commonly observed localized creep deformation in multi-pass Grade 91 steel weldments. An in-situ digital image correlation (DIC) system was utilized with a creep testing frame to monitor and measure both full-field strain and localized strain accumulation across the weldments during long-term creep testing at elevated temperatures. The in situ DIC method measured not only the creep deformation behavior of the weld metal, heat affected zone (HAZ), and base metal, but also creep strain evolution for each sub-region within the HAZ itself, including coarse-grained HAZ, fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ). The DIC results revealed that local creep strain in the ICHAZ reached up to 90% strain before final rupture, whereas nominal creep strain measured by the standard extensometer of the tested cross-weld specimen was below 10%, indicative of Type IV cracking of the Grade 91 weld. Microstructural analyses revealed that the faster creep degradation/deformation in the HAZ was caused mainly by accelerated matrix grain recrystallization/growth and a reduced pinning effect from the segregated and coarsened precipitates in the FGHAZ and ICHAZ. The ultimate creep rupture occurred in the ICHAZ owing to its lowest creep resistance induced by the largest recrystallized grain size, the lowest fraction of coincidence site lattice, and the lowest local strain energies/dislocation densities.

Analyzing the long-term creep behavior of composite pipes: Developing an alternative scenario of short-term multi-stage loading test

Glass Fiber Reinforced Polymer pipes (GFRP) are used in a variety of industries such as water, oil and gas, aerospace and chemical industries. In the present paper, the creep response of GFRP pipes undergoing compressive transverse loading is analyzed through experimental observations and numerical modeling. Firstly, the long-term creep response of a GFRP pipe is investigated under single-stage loading till more than 400 days. Then, the creep response of another sample from the same pipe is evaluated under multi-stage loading with four 720-h steps (totally 2880 h). Theoretical modeling is performed using a two-level model based on short-term viscoelastic behavior of resin. A procedure is proposed to replace a long-term creep test under a single stage loading with short-term creep tests with multi-stage loading. The proposed procedure is validated through experimental study. The proposed multi-stage creep test scenario is very valuable to reduce the duration of experimental creep test. Utilizing the proposed method during the design process of a GFRP pipe, the creep response of the pipe can be estimated and its long-term properties can be ensured by performing equivalent short-term tests. The proposed procedure is validated through experimental study.

Research on Treatment of Retaining Wall Foundation with Geosynthetics Based on BP Neural Network

Through the long-term load creep test of CE131 geonet and SD L25 retaining wall foundation, which are widely used in reinforced earth engineering, a large number of experimental data are obtained. On this basis, the least-squares and BP neural network are used to predict its creep variables. The principle of least squares is to find a curve in the curve family to fit the experimental data. From the sum of the squared errors σ = 0. 001 16, the fitting accuracy is higher. The BP neural network has adaptive learning and memory capabilities, especially the three-layer BP neural network model. The maximum error between the predicted value and the actual value is 0.91%, which is a lot better than the error of the least square 3.4%. This method Found a new way for creep prediction.