Load Creep Tests Research Articles

Experimental Study on Reloading Creep Characteristics of Marble After Unloading–Freezing–Thawing Damage

ABSTRACTThe reloading characteristics of slope projects such as water conservancy and hydropower in cold regions after excavation and unloading are the key to the long‐term stability problem of bank slopes in reservoirs. After pre‐peak unloading and freeze–thaw, uniaxial gradient loading creep test was carried out with marble, and the unloading creep mechanical behavior of marble under different temperature ranges and different numbers of freeze–thaw cycles was studied. The results show that the temperature range of freeze–thaw and the number of cycles control the reduction of creep damage stress and long‐term strength as well as the increasing trend of creep strain of marble, and the time‐mechanical response of unloaded marble under the action of freeze–thaw with large temperature difference is more obvious. Based on the traditional Burgers model, a non‐constant Burgers model considering freeze–thaw damage was proposed, and the applicability and rationality of the model were verified.

Effect of Load Cycling on High Temperature Creep of 316L Stainless Steel

For components in concentrating solar thermal plants, the creep mechanism will cause a significant portion of material damage to receivers, storage tanks, turbines and pipework. These components will undergo conditions where loads, temperature, or both load and temperature are not constatnly applied. This creep damage process will not resemble the constant load creep tests used to characterise creep of materials. Therefore, creep tests incorporating a load cycle were undertaken to obtain a better understanding of how high temperature materials respond to these cyclic conditions. These tests showed that when time under load was considered, creep undertaken under load cycling conditions were accelerated relative to constant load conditions. A modified Larson-Miller approach was used to assess this effect and determine an equivalent stress where constant load test data agrees with the cycled test data. This found that cycling the load was equivalent to if the system were under 3 and 7 MPa higher stress, for tests which were loaded for 12 hrs and 6 hrs per 24 hrs respectively. This could be a potential approach to simply and easily consider these load cycling effects for design and life analysis.

Experimental study on mechanical damage and creep characteristics of Gonghe granite under real-time high temperature

It is significant to study the mechanical properties of hot dry rock (HDR) for the development of deep geothermal energy. At present, the creep behavior of granite under real-time high temperature is not fully understood. The creep behavior of granite at 25 ∼ 800°C was investigated by real-time high-temperature uniaxial compression and graded load creep tests, and the thermal damage mechanism of granite was studied by scanning electron microscopy (SEM) experiments. The paper systematically analyzes the evolution of mechanical indexes such as uniaxial compressive strength (UCS), elastic modulus, creep deformation, steady creep rate and long-term strength of granite under thermal-force coupling. The results show that the UCS and elastic modulus of granite increase with increasing temperature in the range of 25 ∼ 200 °C, and decrease with increasing temperature in the range of 200 ∼ 800 °C. The damage speed of granite is the fastest in the temperature range of 400 ∼ 600 °C. The steady creep rate of granite increases with the increase of temperature and stress level. The ratio of long-term strength to UCS decreases with increasing temperature, from 93.6% at 25 °C to 73.2% at 800 °C. The research results provide relevant thermal damage mechanical parameters and theoretical basis for the development of HDR.

Laboratory Testing and Numerical Modeling of Creep Behavior of HDPE Geomembranes under Multistage Loading

Abstract Geomembranes are essential impermeable materials commonly employed in the fields of hydraulic engineering and geoenvironmental engineering. However, their long-term operation may result in significant creep deformation, particularly under high loads. This paper mainly presents the effect of loading mode on the creep characteristics of high-density polyethylene (HDPE) geomembranes. A series of creep tests of geomembranes under multistage loading were conducted and compared with single loading creep tests. Under multistage loading, creep strain increased in a step-like manner. When the cumulative load level is lower than 50 %, the creep of geomembranes can attain a stable state. When the cumulative load level attains 60 %, the creep may enter a constant-rate creep stage. Compared with single loading creep, the creep under multistage loading attained a stable state earlier for a relatively low load level, and the deformations were smaller. The creep strain rate was significantly reduced at a constant-rate creep stage for a relatively high load level. The multistage loading was beneficial to the creep deformation control of geomembranes. Furthermore, an enhanced creep model was proposed to describe the creep behavior under multistage loading, including the stable state and constant-rate creep stage. The predictions were in good agreement with the experimental data.

The High Temperature Strength of Single Crystal Ni‐base Superalloys – Re‐visiting Constant Strain Rate, Creep, and Thermomechanical Fatigue Testing

The present work takes a new look at the high temperature strength of single crystal (SX) Ni‐base superalloys. It compares high temperature constant strain rate (CSR) testing, creep testing, and out‐of‐phase thermomechanical fatigue (OP TMF) testing, which represent key characterization methods supporting alloy development and component design in SX material science and technology. The three types of tests are compared using the same SX alloy, working with precisely oriented <001>‐specimens and considering the same temperature range between 1023 and 1223 K, where climb controlled micro‐creep processes need to be considered. Nevertheless, the three types of tests provide different types of information. CSR testing at imposed strain rates of 3.3 × 10−4 s−1 shows a yield stress anomaly (YSA) with a YSA stress peak at a temperature of 1073 K. This increase of strength with increasing temperature is not observed during constant load creep testing at much lower deformation rates around 10−7 s−1. Creep rates show a usual behavior and increase with increasing temperatures. During OP‐TMF loading, the temperature continuously increases/decreases in the compression/tension part of the mechanical strain‐controlled cycle (±0.5%). At the temperature, where the YSA peak stress temperature is observed, no peculiarities are observed. It is shown that OP‐TMF life is sensitive to surface quality, which is not the case in creep. A smaller number of cycles to failure is observed when reducing the heating rate in the compression/heating part of the mechanical strain‐controlled OP‐TMF cycle. The results are discussed on a microstructural basis, using results from scanning and transmission electron microscopy, and in light of previous work published in the literature.

Creep properties and model of fractured sandstone under freezing environment

Creep is an important mechanical property of fractured rock. To explore the creep mechanical properties and damage evolution law of surrounding rock mass under long-term external load during freezing construction, triaxial graded loading creep and CT tests are conducted under a freezing environment (−10 °C) on sandstone with different fracture dip angles. The test results reveal that the prefabricated fracture have a significant impact on the creep of sandstone under freezing environment. As the fracture dip angel increases, the creep duration, creep deformation, and long-term strength all decrease first and then increase, with lower values at 15° and 45°. At 0°、 15° and 45° dip angles, the rocks exhibit integrated shear through failure, whereas rocks with a dip angle of 75° and 90° exhibit the mode of tensile shear through failure. Notably, no microcracks or secondary cracks are observed in the rock samples. Finally, a nonlinear viscoelastic–plastic constitutive model of fractured sandstone is established via fractional calculus. Fitting the experimental curve with the theoretical model reveals that the proposed creep damage model could accurately describe the creep behavior of fractured sandstone under freezing, especially in the accelerated creep stage, which validates the reliability of the parameters.

A novel creep contact model for rock and its implement in discrete element simulation

The creep behavior of rocks significantly impacts projects' structural safety, including mining, tunneling, and hydraulic engineering. Numerous creep models used in numerical calculations incorporate basic elements, such as Burger's model. However, relying solely on series or parallel combinations of these basic components fails to account for the accelerated creep stage, prompting efforts toward developing enhanced creep models. Despite noted progress, a limited number of studies on the full-process simulation of graded creep loading and the consequent generation of cracks. To overcome this problem, an innovative creep model is proposed based on both the Parallel Bond and the Kelvin-Voigt contact models. This proposed creep model simulates the graded loading creep test of rocks under different load conditions and also conducts a parameter sensitivity analysis for the impact of the proportionate discrepancy between the two contact models. The results indicate that this new creep model could successfully simulate the comprehensive creep process, effectively capture crack progression, and represent ultimate failure modes during the creep process. Notably, to ensure the precision of simulations, it is recommended that the proportion of the Kelvin-Voigt contact model within the total contact is at most 25%.

Acoustic characterization for creep behaviors of marine sandy hydrate-bearing sediment

Marine natural gas hydrate (NGH) is a promising substitutive low-carbon energy resource, whereas NGH-production induced geoengineering concerns remain challenging. Advanced forecast of possible geoengineering risks is the fundamental for eco-friendly NGH exploitation. Reservoir creep deformation is an early symptom of the geoengineering risks. However, whether the creep deformation behaviors of the NGH-bearing strata is predictable remains controversial. In this study, a series of multi-step loading creep test are conducted for sandy gas hydrate bearing sediment (GHBS) samples, during which the ultrasonic responses are recorded simultaneously. The acoustic velocity, compression-to-shear velocity ratio, Poission’s ratio, main frequency, and main frequency amplitude are used to characterize creep failures of the GHBS for the first time. Combining analyses of the creep behaviors and acoustic responses yield the following conclusions. Firstly, the long-term strength derived from creeping test is 0.45–0.60 times of the shear strength derived from triaxial shearing. Ignoring the creep effect might underestimate the scale and intensity of possible geoengineering risks during long-term NGH exploitation. Secondly, the acoustic velocity increases gently and then decreases continuously during creeping. Once the accelerated creep appears, the acoustic velocity plummets significantly, together with a sudden decrease in the compression-to-shear velocity ratio, and fluctuations in the main frequency and its amplitude. Furthermore, the main frequency and its amplitude shall fluctuate abruptly prior to the emergence of the accelerated creep. Therefore, we anticipate that the combination of abnormal fluctuations of main frequency and its amplitude can be used as early-warning indicators for possible creep failure of the GHBS. The results might have great significance for in-situ detection and prediction of possible reservoir failure during long-term NGH exploitation.

Durability of geogrid in a sloped reinforced soil wall after 25 years in service

The durability of geogrid reinforcement exhumed after 25 years from a sloped wall structure was examined through a comparison of the material properties of exhumed samples with technical data for the original product type. Visual and microscopic inspections found no evidence of installation damage. Analysis of strain increments in rapid loading creep tests to 10 000 h showed excellent agreement between the exhumed and original materials. The results of this study lend confidence in the use of isochronous load–strain–time data for predicting the long-term strain of geosynthetic-reinforced soil structures in design to a serviceability limit state.

Does Suture Type or Configuration Matter in Percutaneous Achilles Tendon Repairs?

Category: Trauma; Sports Introduction/Purpose: The human Achilles tendon is functionally important for dynamic activities and is a common site of injury, resulting in pain, weakness, and removal from sport. While treatment options vary and the opinions regarding optimal management are not uniform, percutaneous repair through commercially available guides is one option that has been used in the United States. The purpose of this study was to test the mechanical properties of four suture permutations. Methods: This biomechanical study included Porcine toe flexor tendons as an analogue for the human Achilles tendon. Simulated rupture was performed and tendons repaired using a percutaneous guide. Technique was uniform with pre-determined allotment into one of four groups based on suture configuration (single or double locked) and type (round or flat) with a total of 10 specimens in each group. Specimen then underwent static creep test, a dynamic load creep test, and finally load to failure test. An analysis of variance test was performed, followed by pairwise comparisons using independent t-tests to assess inter-group differences if significant effects were found, adjusted for multiple comparisons. Results: No significant differences were seen between conditions for the creep tests. The suture configuration used was determined to have a significant effect on the maximum load to failure of the constructs (p = 0.018) and maximum stress in the construct during the load to failure test (p = 0.019). Double Tape had a significantly greater load to failure than the Single Round (66N (0.021, [25, 107]) and Single Tape (72N (0.037, [18, 126]) conditions, and reached greater stresses before failure (0.1N/mm^2 (0.04, [0.03, 0.17] and 0.1N/mm^2 (0.04, [0.02, 0.18], respectively). No significant differences were found between the double tape and double round techniques, nor the Double Round and either of the Single techniques for any of the load to failure variables. Conclusion: Currently, there is no compelling data to support surgical over non-surgical management of Achilles tendon ruptures. However, by patient or surgeon preference, many patients undergo operative intervention, and percutaneous repair through a guide is a popular modality. The results of this study, using a porcine model to simulate a minimally invasive technique for Achilles tendon repairs, suggest that the use of a double locked tape suture configuration leads to a stronger overall construct. The clinical implications of these findings will be the subject of future work.

Creep and Hardening Characteristics of Anthracite under Graded Static–Dynamic Coupled Loading

Remaining coal pillars in remining areas exhibit clear creep characteristics, and dynamic pressure accelerates their instability and failure. The creep and hardening characteristics of coal under dynamic pressure are of great engineering significance for the stability of remaining coal pillars in remining areas and their control. To investigate the creep and hardening characteristics of anthracite under static–dynamic coupled loading, graded loading creep tests with different loading rates were conducted. In this research, the creep strain, instantaneous elastic modulus, and creep rate of anthracite were studied under different graded loading rates. The results showed that the hardening effect of the samples manifested as an increasing instantaneous elastic modulus at the loading stage and a decreasing strain rate at the creep stage. When the graded loading rate increases from 0.01 to 0.1 mm/s, the instantaneous elastic modulus increases by 0.16–2.32 times. The sudden increase in the instantaneous elastic modulus at the failure stress level explains the instantaneous failure of the samples well. The actual yield levels corresponding to the peak instantaneous elastic modulus of the samples linearly decreased with increasing graded loading rate. The functional relationship between the graded loading rate and the elastic modulus hardening coefficient, the actual yield stress, and the strain rate decay coefficient were established, which could quantitatively describe the influence of different graded loading rates on the creep and hardening characteristics of anthracite and predict its creep damage.

Study on creep behaviors and nonlinear creep constitutive model for sandy marine hydrate-bearing sediments

Marine natural gas hydrate (NGH) production is critically affected by reservoir damage, which is usually presented as creep deformation. Clarifying the creep behaviors of hydrate-bearing sediments (HBS) is essential for predicting potential geoengineering risks during long-term NGH mining. In this study, a series of multi-step loading creep tests on HBS samples are conducted and a modified creep model specialized for sandy HBS is proposed. The creep strain, creep rate, and long-term strength are experimentally evaluated under various hydrate saturation (Sh) and/or effective confining pressure (Pc). Three typical creep stages of deceleration creep, stable creep, and accelerated creep are observed in all experiments. The results show that the creep deformation of HBS is enhanced with the decrease in Sh under the same axial load. The long-term strength decays linearly with decreasing Sh and Pc. The creep model is established by modification of both the Kelvin element and the ideal viscoplastic element in the conventional Nishihara model. A nonlinear viscous coefficient and a time-damaged elastomer considering are introduced to describe the nonlinear creep and damage characteristics of HBS. The model is able to describe the whole failure process, especially the accelerated creep stage. The results might have some significance on the prediction of nonlinear failure behaviors of the reservoir during long-term NGH mining.

Creep properties and energy evolution characteristics of weakly cemented rock under step loading

The contact mode of particles in weakly cemented mudstone (WCM) and the persistence of cemented materials, as well as the cemented particulate matter composed of them, lead to different mechanical response characteristics from general soft rock during loading, that is, rheological failure occurs under low stress. In order to study the creep characteristics and energy evolution characteristics of weakly cemented rock (WCR) in the western mining area of China, based on the triaxial graded loading creep test of WCR, the creep deformation and failure law of WCM under graded loading was studied, and the energy evolution characteristics of WCR in each creep stage were revealed by energy dissipation theory. The results show that with the increase of confining pressure, the fracture angle decreases, but the failure strength and volume expansion point show the opposite trend. In addition, due to the strong heterogeneity of WCR samples, the contact force between particles is different after loading, which makes the slip between particles and cements and then produces a completely different fracture form from other rocks. The energy growth in the deceleration and steady creep stages is mainly affected by the confining pressure. With the increase of confining pressure, the growth rate of dissipation energy Ud and total energy U gradually tends to be constant, but the growth rate of strain energy Ue remains at a high level. In the accelerated creep stage, the growth rate of strain energy Ue decreases rapidly, and the residual energy storage capacity does not change significantly with the increase of confining pressure. These results revealed the unique creep properties and energy evolution characteristics of the WCR, providing a theoretical basis for long-term stability control of roadways that are built with WCR.

Design of an Anthracite Creep Model Based on Fractional Order Theory: Experiments and Simulations

Fractional order theory was used to characterize the accelerated creep phase of a nonlinear creep model. To accurately describe each stage of the anthracite creep model, the “gyroscope” unit was introduced by combining the Heaviside function and the creep damage definition. The effect of damage on anthracite creep was determined by designing and completing three-axis graded-separation loading creep tests on anthracite. The test curves were combined to classify anthracite into five stages: transient deformation, pseudo-acceleration, deceleration, isothermal, and acceleration creep. Each stage was combined with suitable components to form a combined fractional-order creep model. The one-dimensional equation of the state of the model was extended to three dimensions. The Levenberg–Marquardt optimization algorithm for fitting origin rheological curves was used to complete the fitting of the basic parameters. Finite differences were performed on the model equation of state, and a secondary development of a combined fractional-order creep model (NEG) was completed based on the built-in Burgers model in FLAC3D. A comparison of the numerical simulation results shows that the combined fractional-order creep model is important for accurately predicting the full creep stage of anthracite.

Characterization and evaluation of coarse aggregate wearing morphology on mechanical properties of asphalt mixture

To evaluate the impact of coarse aggregate morphology on the mechanical performance of the asphalt mixture, the coarse aggregates with diverse wearing times were prepared and the corresponding asphalt mixture samples were compacted. The morphology characteristics of coarse aggregate under diverse wearing times were evaluated by an image measurement system. The dynamic modulus test and repeated loading creep tests were carried out to evaluate the viscoelastic properties and permanent deformation resistance of the asphalt mixture. The results indicated that the angularity index and three-dimensional sphericity could well characterize the wearing morphology. With the increase of wearing time, the angularity index of coarse aggregate gradually decreased, and the three-dimensional sphericity gradually increased. The balanced complex modulus Ge* and rheological parameter RG could effectively characterize the viscoelastic properties of asphalt mixture. As the angularity index increased, Ge* increased linearly, and RG decreased linearly. However, the Ge* and RG showed the opposite trend with the increase of three-dimensional sphericity. The flow number FN and the combination parameter εp/FN were the indexes to evaluate resistance to permanent deformation of the asphalt mixture. As the angularity index increased, the value of εp/FN decreased linearly, and FN increased linearly. With the increase of three-dimensional sphericity, εp/FN increases linearly and FN decreases linearly. The research results can provide a reference for the selection of coarse aggregate considering the morphological characteristics, and then improve the performance of asphalt mixture.

Study on High Temperature Properties of Fiber-Free SMA Mixture

The high temperature performance of asphalt mixture is an important factor to determine pavement life. Through rut test, dynamic modulus test and repeated load creep test, the high-temperature properties of two kinds of SMA asphalt mixtures were compared by different indexes, and the performance characteristics of non-fiber SMA asphalt mixtures were evaluated. The dynamic stability of the mixture and the slope of the creep curve showed that the non-fiber SMA mixture had an advantage in rut development rate compared with ordinary SMA. The estimated road life and flow number Fn based on rutting showed that the non-fiber SMA mixture could bear nearly 80% more standard axle load than the normal SMA flow number Fn by 11%. When the permanent deformation of the mixture reaches the failure stage, the non-fiber SMA can bear more loads. The main curves of dynamic moduli of the two SMA mixtures at different reference temperatures are basically the same, and the dynamic moduli characterizing the mechanical properties of the mixtures have little difference. Compared with ordinary SMA mixture, the asphalt film thickness on the surface of non-fiber SMA mixture is thinner, and the skeleton effect between coarse aggregates is more prominent, which is an important reason for the improvement of high temperature stability of the mixture.

Creep life estimation of reformer alloy using θ-projection method - A neuro fuzzy approach

This paper deals with creep life assessment and deformation behavior of 11 years service exposed primary hydrogen reformer tube made up of HP40 grade of steel using the θ-projection method. Constant load creep tests have been performed at 870 °C under the stress range of 50–68 MPa. The generated creep data exhibit a significant amount of scatter under identical stresses. The existence of scatter in the creep deformation data leads to substantial amount of uncertainty in the estimation of θ-parameters and hence in the assessment of creep life. An adaptive neuro fuzzy based model is proposed to study the effect of variation of these parameters on creep life prediction. The model has been compared with the experimental findings.

Predicting the behavior of commercially available building foundation isolation materials using a four-parameter fractional Zener model

In this paper we apply a four-parameter fractional Zener model to describe the short and long term creep behavior of a viscoelastic building foundation isolating material currently in use globally in noise and vibration mitigation applications. Building from the classical rheological models composed of springs and dashpots suitably combined (Maxwell, Kelvin-Voight, Zener), we include a fractional derivative component in the form of a Scott-Blair element composed of a Riemann-Liouville derivative of non-integer order. The parameters are identified empirically through measured stress-strain results and hysteresis curves for the material in compression. The model is then compared to the results of a 168h constant load creep test of three commercially available materials.

A Novel Design of Constant Load Creep Test Machine

Polymer materials have been used in a wide area from our homes to industry today. For this reason, creep curves from the mechanical properties of polymer materials are an important factor for the correct use of polymers. This article presents a new constant load creep test machine design which is a mechatronic system to perform creep testing of polymers. A low-cost creep test machine was developed by mounting clamps, pulleys, weights and steel cable on top of a main frame. With the measurement unit, the linear movement of the steel cable was detected and recorded with the data logging system together with the ambient temperature. Tests were made with a 15% calcium carbonate reinforced polypropylene specimen. It was observed that the experimental data obtained were compatible with a standard creep test curve. These findings confirm that the presented constant load creep test machine design works efficiently.

Viscous Behavior and Constitutive Modeling of Peaty Soil in Kunming, China

The Kunming peaty soil exhibits distinct physical and mechanical properties from the inorganic clay due to the special soil-forming process and material composition. To better understand the viscous behavior of peaty soil, a series of laboratory tests, including four types of oedometer tests (one- and multistage loading creep tests, constant rate of strain tests, and constant rate of stress) and constant rate of strain undrained triaxial compression tests, are conducted on the Kunming peaty soil. The experimental results demonstrate that the Kunming peaty soil shows significant secondary compression during oedometer creep tests. Meanwhile, the variations of the measured preconsolidation pressure and undrained shear strength with the strain rate are essentially linear in log-log plot for the examined range of strain rate. Based on the generalized power law overstress viscoplastic theory, it is confirmed that the rate-sensitivity and time-dependency of the Kunming peaty soil can be uniformly described using the sensitivity parameter involved in the theory, regardless of the applied loading conditions. The average value of 0.06 for the sensitivity parameter is generally greater than those for inorganic clay, indicating that the Kunming peaty soil exhibits more significant viscous behavior. Finally, by using the yield surface of the modified Cam–clay model, an overstress elastic-viscoplastic model is established and numerically implemented. The numerical model with a unique sensitivity parameter can well predict the rate-sensitive and time-dependent response of Kunming peaty soil under different loading conditions.