Creep Strain Rate Research Articles

Investigation into the time-dependent mechanical behavior of pre-stressed anchor bolts and fractured rock specimens under synchronized tensile loads

Pre-stressed anchor bolts serve as an effective means to reinforce fractured rock masses. The long-term efficacy of their anchoring function significantly impacts the safety throughout the entire lifecycle of rock engineering projects. Over time, fractured rock masses undergo creep deformation, which interacts synergistically with the time-dependent changes in the pre-stress of anchor bolts. In this work, we conduct uniaxial tensile tests and tensile creep tests on fractured rock specimens anchored by pre-stressed bolts, analyzing the coordinated deformation between the pre-stressed anchor bolts and the fractured specimens. Firstly, conventional uniaxial tensile tests were conducted on the pre-stressed anchorage specimen. The study found that the tensile strength of the anchored specimens was significantly higher than that of the unanchored specimens. Additionally, the ability of the specimens to withstand tensile stresses and deformation improved as pre-stress increased. Secondly, uniaxial tensile creep tests were conducted on the prestressed anchored specimens. The results indicate that, as the stress level increases, the creep strain continues to increase. The application of prestress can effectively limit the tensile deformation of the specimen and delay its damage time. The greater the pre-stress, the smaller the instantaneous strain and creep strain rate during the graded loading test. Finally, based on the synergistic deformation of pre-stressed anchor bolts and the creeping rock mass, we establish a constitutive model reflecting the creep properties of fractured rock mass and derive a theoretical viscoelastic creep formula for anchored rock mass under uniaxial tension. Comparing the creep model with the test results shows that this model is highly applicable and accurate in verifying the tensile creep deformation of prestressed anchorage specimens.

Study on the creep behavior and mechanisms of Ti3AlC2 MAX phase under ion irradiation

The creep properties of structural materials under extreme irradiation are crucial for the safety and stability of nuclear facilities. This work investigates the effect of irradiation on creep behavior of Ti3AlC2 MAX phase material, focusing on irradiation fluence (dose) and temperature. Interestingly, we discover for the first time that the relationship between the creep stress exponent (n) and irradiation fluence (ɸ) in Ti3AlC2 satisfies n=A∙lnɸ+B, enhancing the understanding of irradiation's impact on the Ti3AlC2 MAX phase. Additionally, the creep strain rate (ε̇) in Ti3AlC2 correlates with irradiation temperature (T) as ε̇=E∙(e−1T)F+G. Results show that the creep stress exponent in irradiated Ti3AlC2 increases with fluence and decreases with temperature, surpassing unirradiated samples, while the strain rate follows the opposite trend. Compared to other candidate materials, Ti3AlC2 MAX phase demonstrates excellent creep resistance under irradiation. These findings provide valuable insights for developing advanced nuclear structural materials.

Effect of Time and Stress on Creep Damage Characteristics of Cement-Based Materials

In the realm of daily life, ensuring the safety of building structures and civil engineering projects remains a paramount research focus. The creep properties of materials significantly influence their long-term loading process. Specifically, creep load and creep time are pivotal factors that impact material creep damage, thereby playing a crucial role in assessing the safety of engineering endeavors and estimating aspects such as housing construction. This study undertakes creep damage tests on cement-based materials, subjecting them to varying creep loads and creep times, and subsequently conducts uniaxial compression tests on the specimens post-creep damage. The refined Nishihara model is employed for data fitting, facilitating the construction of a creep damage time-stress model. Concurrently, a Neural Network model is utilized to validate the experimental data. The findings indicate that both steady-state creep strain and steady-state creep rate exhibit discernible trends relative to creep load and creep time, effectively mirroring the alterations in creep damage experienced by the specimens. The refined Nishihara model proves adept at predicting and equating creep damage under diverse creep loads and creep times. Similarly, the trained Neural Network model demonstrates capability in measuring and estimating various creep damages. The study successfully explored the correlation between creep time and creep load, enabling the simulation of long-term creep damage within a shorter creep time and facilitating an analysis of its physical and mechanical properties, which is pivotal in predicting the safety of large-scale engineering projects. Concurrently, it advances research on material damage equivalence, offering insights and theoretical groundwork for developing a system to assess material damage equivalence under various damage conditions.

Experimental study on creep characteristics of cemented coal gangue backfill under uniaxial compression

ABSTRACT The uniaxial compression creep test is used to study the cemented coal gangue backfill body specimens with different cement-sand ratios (1:4,1:6,1:8, respectively). Through the processing and analysis of the creep curve, we discussed the influence of different cement-sand ratios, stress load and loading time on the strain, creep rate and long-term strength of the backfill body. These studies have revealed the creep law of the backfill body and obtained accurate conclusions. The results show that: (1) Different proportions of cemented coal gangue backfill body under the action of the set step by step axial loading stress, the strain is positively correlated with the change of loading stress. (2) With the increase of axial stress, the instantaneous deformation of cemented coal gangue backfill with three kinds of cement–sand ratio increases with the decrease of cement–sand ratio under the same axial stress. (3) With the increase of the axial stress of the three kinds of cement–sand ratio cemented coal gangue backfill body, the strain rate of the deceleration creep speed and the constant creep stage increases, and the duration of the deceleration creep stage increases. The average creep strain rate of the three kinds of cement–sand ratio cemented coal gangue backfill materials before entering the accelerated creep stage is determined. (4) Using an improved steady-state creep rate inflection point method, the long-term strength of three kinds of cement–sand ratio of cemented coal gangue backfill is determined, and it is concluded that the long-term strength decreases with the decrease of cement–sand ratio, which is positively correlated.

Comparison of inelasticity creep failure evaluation codes for elevated-temperature non-nuclear pressure equipment

PurposeTo ensure the reliable and safe operation of elevated-temperature pipes and equipment in the long term, it is essential to thoroughly assess the creep rupture life. Nevertheless, there is currently no design code that specifies a creep rupture life evaluation method for non-nuclear elevated-temperature equipment. The paper aims to discuss this issue.Design/methodology/approachAn analysis was conducted to compare the differences and conservativeness in calculating creep strain using three major codes (ASME-CC-2843, API-579 and BS-7910) based on the results of the 316H creep constitutive model and creep strain prediction. In addition, the creep resistances of 316H, 304H and 347H were compared. Subsequently, the ANSYS Usercreep subroutine was developed to compare the discrepancies between different codes under multiaxial stress conditions using numerical simulations.FindingsBS-7910 employs the Norton creep model with calculation parameters for the average creep strain rate, which is not applicable for the engineering design stage. ASME-CC2843 code primarily focuses on the primary and secondary creep stages, making it more suitable for non-nuclear pipeline and equipment design. For 316H, the creep strain curves predicted by ASME-CC2843 and API-579 typically intersect at a specific point. By combining the creep strain predicted by ASME-CC2843 and API-579, 347H exhibits superior predicted creep resistance compared to 316H, whereas 316H exhibited better predicted creep resistance than 304H.Originality/valueThis study provides a guide for future evaluation methods and material choices for non-nuclear equipment and pipelines operating at elevated temperatures.

An Advanced Sine-Hyperbolic Creep-Damage Model Incorporating Threshold Strength

Abstract This study aims at improving the classic sine-hyperbolic (Sinh) creep-damage model to predict minimum-creep-strain-rate (MCSR), rupture, damage, and creep deformation. The Sinh model employs a continuum-damage-mechanics-based framework to model secondary and tertiary creep regimes. In Sinh, the creep strain and damage rate equations exhibit an implicit threshold stress that arises during numerical optimization. Herein, the Sinh model is modified to include an explicit threshold strength as a material property and the tensile strength. Threshold strength is defined as the lower limit for creep activation at a given temperature. Stresses are applied below the threshold, resulting in infinite life. The advanced Sinh offers several advantages including a physical significance of stress ratios where the onset of creep is defined by threshold strength, a closed-form solution where the rate equations remain finite at any combination of stress and temperature, and adaptability in finite element analysis where the solution space remains numerically stable. Experimental creep data of 304 SS at multiple isotherms are gathered from prior literature. The advanced Sinh is calibrated to the MCSR and SR data of 304 SS. The calibration of threshold strength follows a standard procedure from literature and is observed to be realistic for stainless steel at elevated temperatures. The MCSR and SR predictions illustrate the Sigmoidal bend and demonstrate zero creep rate and infinite life at or below threshold strength. The creep deformation and damage predictions exhibit agreement with experimental data. The advanced Sinh is validated by employing finite element simulation to ensure the applicability of the model across a range of applications. The advanced Sinh improved creep response prediction and added physical realism to the model's framework.

Effect of creep fracture mode transition of P91 steel welded joints on long-term creep fracture life

The formation mechanism of the two fracture behaviors and their effects on the creep life of P91 steel welded joints during creep were investigated, and the morphology and microstructure of the two fractures were characterized by SEM and EBSD, and the results were as follows: the change of fracture modes was affected by the creep strain rate and cavity nucleation rate in the joint. When the applied stress is large, the creep strain rate of BM is higher, and brittle fracture occurs at BM; when the applied stress is low, the cavity nucleation rate of FGHAZ is higher, and brittle fracture occurs at FGHAZ, known as type IV cracking. The occurrence of type IV cracking will reduce the accuracy of creep life prediction methods, and life prediction considering fracture mode can effectively improve the prediction accuracy.

Shear creep deformation of rock fracture distrubed by dynamic loading

The long-term stability of jointed rock masses is usually dominated by fault activation, which may be triggered by the dynamic disturbance generated by blasting during mining activities, leading to the occurrence of disasters such as landslides in open-pit and rockbursts in deep mining. The initial stress and dynamic disturbance are key factors that strongly affect the shear creep behavior of rock fractures. In this work, the shear failure instability of rock fractures of sandstone under creep-impact loading was experimentally investigated by using a creep-impact test machine, which allows for applying creep loading and an additional dynamic disturbance on rock fractures. Three stages of shear creep deformation, creep strain rate, and time-to-failure are examined under different creep stress levels and impact energies. Experimental results show that the tangential and normal creep rates increase with the increase of creep stress and impact energy, but the increment of tangential creep rate is higher than that of the normal creep rate. The time-to-failure of the creeping specimen is shortened under high creep stress and large impact energy, while the time-to-failure after the last dynamic disturbance of the specimen is determined by the total impact energy and creep stress level. By using high-speed photography, it is found that the failure types of rock depend on the magnitude of impact energy and creep stress level; that is, rock mainly slides with low stress levels and shears off with high stress levels. In addition, under different impact energy and creep stress levels, the variation of height is between 0.38 and 0.52, while the defined fracture factor, which describes the degree of failure of serrations, is between 0.30 and 0.54. The findings can provide deep insight into the fault sliding mechanism caused by mining activities, which provides theoretical support for the safe mining of ore in fault fracture zones.

Understanding the deformation creep and role of intermetallic compound-microstructure in Sn-Ag-Cu solders

Tin-based alloys are commonly used in lead-free solder joints in electronic interconnection applications, where creep can limit joint reliability. In this work, directionally solidified bulk solder alloys of different compositions (pure tin, SAC105 and SAC305) are mechanically tested under a constant load for each composition at room temperature to understand mechanical creep performance and quantify the role of different content of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in terms of secondary creep strain rate, microstructural evolution, and total amount of accumulated strain. In this work, microstructures are fabricated using directional solidification to promote a systematic variation in IMC sizes, shapes, and distributions within similar crystal orientations of the primary β-Sn matrix. Analysis of creep data indicates that secondary creep is dominated by obstacle-controlled dislocation motion. This is further confirmed by electron backscatter diffraction (EBSD) analysis, which reveals that the formation of subgrains and internal structure, correlating to the initial microstructure of the sample, i.e. changes in secondary dendrite arm spacing (λ2) and eutectic intermetallic spacing (λe). The experimental observations are supported further by crystal plasticity simulations that explicitly model the presence of IMCs within the Sn-based samples and show the dependence of the secondary creep rate upon the IMC content, and therefore be beneficial for the possible future composition design in solders.

Thermal creep strain test and model of Q460GJ steel at elevated temperatures

Q460GJ steel is a typical high strength and high-performance structural steel. The thermal creep test on two thicknesses (8 mm and 12 mm) of Q460GJ steel plates at elevated temperatures (400–800 °C) was carried out. The test results showed that the thermal creep strain increases with the increases of temperature and stress. When the temperature exceeds about 500 °C and the stress ratio is greater than about 0.55, the Q460GJ steel plate specimens has obvious creep deformations, so it is necessary to consider the thermal creep deformation of steel at elevated temperatures for steel structural design. The difference in plate thickness does not affect the creep properties of the 8 mm and 12 mm Q460GJ steel plates at elevated temperatures. When the temperature is no more than 600 °C, the second stage creep strain rates of Q460 steel are obviously higher than that of Q460GJ steel while the stress levels are close. The Fields & Fields creep model is suitable for fitting the thermal creep strain-time curves for Q460GJ steel. The findings will contribute to providing theoretical support for design of high-performance steel engineering structures under fire.

Algorithms and software for processing images of the structure of metallic materials for the purpose of determining creep characteristics

The paper contains a description of the approach, algorithms, and software tool for image analysis of deformed material structures. The developed approach is based on the analysis of the microstructure of the material to identify important factors that affect high-temperature deformation during creep in a heat-resistant nickel-based alloy, namely, the dimensions of the channels and their orientation. The developed software tool uses algorithms for converting images into binary black and white using the Otsu method. The intensity of the gradient at each point of the image is visualized using the Sobel operator. Fragment boundaries are determined using the Canny edge detector. Straight line segments are found using the Hough transformation. The software tool is implemented in the Python programming language using the OpenCV library. The main components are described and a flow chart of the program is provided. With the use of the developed software, the transformation of the known experimentally obtained images of the structures of the specimens from heat-resistant nickel-based alloy CMSX-4 deformed at a temperature of 1273K and in a wide range of stresses at different time moments was performed. The results of the analysis of the dimensions of the g-phase channels in the alloy using quantitative evaluation of transformed binary images are discussed. The found characteristics were matched to the value of the creep strain rate, which was determined by calculation based on known experimental data. The possibility of determining the transition from the secondary creep to the stage of avalanche-like growth of strains and hidden damage is shown. For the considered example, the location of the channels in the representative image was determined. A technique for correcting creep curves with the involvement of material structure image processing data is proposed.

Experimental study on the mechanical properties of red sandstone with fractures under different loading rates

In order to study the effects of crack inclination angle and loading rate on rock mechanical properties, creep characteristics, and failure characteristics. Taking homogeneous red sandstone with different fracture angles as the research object, uniaxial compression tests and uniaxial compression creep tests were conducted at different loading rates. The results showed that under the same fracture angle, the loading rate was positively correlated with the peak strength, elastic modulus, instantaneous strain, creep strain, and steady-state creep rate of the sample, while negatively correlated with the peak strain. At the same loading rate, the mechanical properties and creep properties of the sample were controlled by the crack inclination angle α. With the increase of α, the peak strength, peak strain, instantaneous strain, creep strain and steady-state creep rate decreased first and then increased, and the elastic modulus increased. On the basis of rock creep testing, it is also important to establish a creep model that conforms to the actual test situation for studying rock creep characteristics. However, many models currently used cannot accurately describe the three stages of rock creep, especially the accelerated creep stage. Therefore, based on Burgers elements, this paper introduces plastic damage bodies based on damage rates and software components based on fractional calculus, A new creep model was obtained and its rationality was verified through experimental results. The results showed that the fit between the model and experimental data was above 0.97, indicating that the model can better describe the three stages of rock creep, especially reflecting the non-linear characteristics of the accelerated creep stage.

Incorporation of uranium nitride fuel capability into the ENIGMA fuel performance code: Model development and validation

Uranium dioxide (UO2) is the primary fuel form for nuclear reactors but its moderate uranium density and low thermal conductivity have prompted the exploration of alternative materials. Uranium nitride (UN) has emerged as a promising candidate for a variety of reactors, offering higher uranium density and thermal conductivity. This paper details the development and implementation of UN fuel capabilities within the ENIGMA fuel performance code for Light Water Reactor (LWR) applications. The new UN capability in ENIGMA includes correlations for theoretical density at room temperature, thermal conductivity, specific heat capacity, enthalpy, thermal expansion strain, Young’s modulus, Poisson’s ratio, thermal creep strain rate, irradiation creep strain rate and emissivity. Additionally, it incorporates models for densification, solid fission product swelling, fission gas bubble swelling, and fission gas release, along with a modified RADAR model for determining the pellet radial power profile and helium generation. Validation of the UN model was conducted using data from the L414 pin irradiation in the JOYO fast reactor in Japan. Further validation efforts are planned using datasets from JOYO and the Siloé thermal reactor in France. The paper also outlines areas of future work to address experimental data gaps and enhance model accuracy to cover a broader range of cladding materials, manufacturing parameters (including porosity volume fraction) and operating conditions (including fuel temperatures and burnups). Although focused on LWR applications, the work outlined supports the use of UN fuel across various reactor systems.

Crystallographic orientation dependence in creep deformation of micron-thick silicon films for 3-D microstructures

In this study, the steady-state creep deformation properties of 5 μm-thick single crystal silicon (Si) films at elevated temperatures were investigated using punch creep-forming tests for the design of three-dimensional (3-D) microstructured MEMS. The relationship between the creep strain rate and stress of the Si films with {100}, {110}, and {111} planes at temperatures of 1223–1323 K was derived using finite element analysis following punch creep-forming tests, and a power-law creep constitutive equation for Si films was determined. The steady-state creep properties of the Si film samples varied clearly with crystallographic orientations. Among the three crystallographic orientations, the creep strain rate of the micron-thick Si films was the fastest for the {011} plane, followed by the {111} and {001} planes, in the applied stress range of 110 MPa to 220 MPa. This is consistent with the increasing order of magnitude of the thermal activation energy. The crystallographic orientation dependence of the steady-state creep properties of Si films is discussed based on the mobility of dislocation gliding per unit lattice and scanning electron microscope observations. The sample-size dependence of the creep strain rate of Si with the {001} plane was clearly observed between the micron- and millimeter-thick samples, which was attributed to the difference in the frequency factor of the power-law creep. However, the thermal-activation energy remained constant. This study successfully revealed the steady-state creep properties of Si films and provided useful engineering data for the design of 3-D Si-MEMS fabrication by the plastic processing of Si films.

Estimating Time-to-Failure and Long-Term Strength of Rocks Based on Creep Strain Rate Model

Estimating Time-to-Failure and Long-Term Strength of Rocks Based on Creep Strain Rate Model

Accelerated discovery of lead-free solder alloys with enhanced creep resistance via complementary machine learning strategy

Minor alloying is an effective method to improve the performance of lead-free solder alloys. In this study, we propose a complementary Machine Learning (ML) strategy for minor alloying to design solder alloys with enhanced creep resistance. Two ML models, leveraging compositional and knowledge-aware features, respectively, were constructed to predict the creep stress exponent of Sn–Ag–Cu (SAC)-based solder alloys. Five new solder alloys were designed and experimentally evaluated by screening the virtual sample space consisting of the critical elements, including Bi, In, and Ni, affecting the creep resistance of SAC solder alloys with the ML model. The creep resistance of the designed alloys was characterized using nanoindentation tests. Notably, the creep stress exponent of SAC387-3 wt % Bi-0.4 wt% Ni was determined to be 12.82 at room temperature, indicating a significant 33.9% decrease in creep strain rate compared to the SAC387 alloy. This study demonstrates the potential of the ML approach to design solder alloys with enhanced creep resistance.

Nanoindentation Creep Behavior of Additively Manufactured H13 Steel by Utilizing Selective Laser Melting Technology.

Nowadays, H13 hot work steel is a commonly used hot work die material in the industry; however, its creep behavior for additively manufactured H13 steel parts has not been widely investigated. This research paper examines the impact of volumetric energy density (VED), a critical parameter in additive manufacturing (AM), and the effect of post heat-treatment nitrification on the creep behavior of H13 hot work tool steel, which is constructed through selective laser melting (SLM), which is a powder bed fusion process according to ISO/ASTM 52900:2021. The study utilizes nanoindentation tests to investigate the creep response and the associated parameters such as the steady-state creep strain rate. Measurements and observations taken during the holding phase offer a valuable understanding of the behavior of the studied material. The findings of this study highlight a substantial influence of both VED and nitrification on several factors including hardness, modulus of elasticity, indentation depth, and creep displacement. Interestingly, the creep strain rate appears to be largely unaltered by these parameters. The study concludes with the observation that the creep stress exponent (n) shows a decreasing trend with an increase in VED and the application of nitrification treatment.

Investigation on the room temperature compressive creep behavior of TC4 titanium alloy joints with vacuum electron beam welding

The room temperature compressive creep behavior is a key issue for the titanium alloys welded structural components served in deep-sea environment. In the present work, the compressive creep behavior of different areas for TC4 titanium alloy joints with vacuum electron beam welding was systematically studied, and it is found that the fusion zone (FZ) of TC4 titanium alloy joints has the minimum creep strain rate. This is mainly because the creep deformation of TC4 titanium alloy is mainly controlled by the movement of dislocations in the α or α′ phases, the size of α′ phase in FZ is small, and thus the dislocation slip is limited. MD simulation results have further revealed that the room temperature compressive creep nature of α-Ti can be regarded as a dynamic hysteresis phenomenon in plastic strain and is dominated by the dislocation movement.

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.

Thermo-mechanical creep-fatigue damage evolution and life assessment of TiAl alloy

Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys.