Creep Transition Research Articles

Study on Creep Characteristics and Long-Term Strength of Mud-Calcareous Conglomerates in the Three Gorges Reservoir Area

The study of the time-dependent properties of engineering rock masses is a frontier topic in rock mechanics. In this study, creep tests and stress relaxation tests were conducted on mud-calcareous conglomerates from the Three Gorges Reservoir Area, and the long-term strength values of the conglomerate specimens were determined via different methods based on the test curves. By comparing these mainstream long-term strength determination methods, it was found that each of these methods have their own drawbacks. For example, the transition creep method requires a high accuracy of the test curve and only obtains an approximate strength interval rather than an accurate value. The long-term strength values determined by the isochronous stress–strain curve method are strongly influenced by subjective factors, among other things. Therefore, this paper proposes a new method for determining long-term strength, called the steady-state creep rate method, based on stress intervals. By comparison, the long-term strength values determined via this method are in good agreement with the transition creep method, the volume expansion method, and the stress relaxation method.

Real‐Time Forecast of Catastrophic Landslides via Dragon‐King Detection

AbstractCatastrophic landslides characterized by runaway slope failures remain difficult to predict. Here, we develop a physics‐based framework to prospectively assess slope failure potential. Our method builds upon the physics of extreme events in natural systems: the extremes so‐called “dragon‐kings” (e.g., slope tertiary creeps prior to failure) exhibit statistically different properties than other smaller‐sized events (e.g., slope secondary creeps). We develop statistical tools to detect the emergence of dragon‐kings during landslide evolution, with the secondary‐to‐tertiary creep transition quantitatively captured. We construct a phase diagram characterizing the detectability of dragon‐kings against “black‐swans” and informing on whether the slope evolves toward a catastrophic or slow landslide. We test our method on synthetic and real data sets, demonstrating how it might have been used to forecast three representative historical landslides. Our method can in principle considerably reduce the number of false alarms and identify with high confidence the presence of true hazards of catastrophic landslides.

Effects of nonproportional loading and residual stress on creep crack tip fields

Nonproportional loading and residual stresses effects on creep crack tip fields are important topics in fracture mechanics at elevated temperature. In this paper, nonproportional loading and residual stresses effects on creep crack tip fields are systematically investigated based on the novel interpretation of C(t)-integral considering residual stress. With the numerical implementation of the corrected C(t)-integral considering residual stress, the eigenstrain and mechanical induced residual stress effects on the creep crack tip fields are studied. We find that C(t)-integral level is affected remarkably in the presence of residual stress regardless of eigenstrain or mechanical induced residual stresses for creep crack tip under small scale creep. However, level of C*-integral is not significantly influenced under the effects of residual stresses in presence of large-scale creep although the transition creep time is remarkably changed. Otherwise, C(t)-integral is highly affected by loading path under small scale creep. However, C*-integral is independent of the loading path if the creep time is long enough. Nonproportional loading effect on C*-integral is very limited for creep crack tip field according to our study. The investigation given in this paper could deepen the understanding the effects of residual stress and nonproportional loading on the creep crack tip field.

Time-Dependent Deformation and Long-Term Strength of Carbonaceous Mudstone under Dry and Wet Cycles

Clarifying the time-dependent strength deterioration characteristics of carbonaceous mudstone under dry and wet cycles is of great significance to the design of expressway cut slopes. In this work, we conducted triaxial compression creep tests on carbonaceous mudstone specimens that had undergone different numbers of dry and wet cycles to investigate their creep properties. A function was established between the steady-state viscoplastic creep rate and axial compression. The threshold stress of the steady-state viscoplastic creep rate was assumed as the long-term strength, and the long-term strength deterioration law of carbonaceous mudstone under dry and wet cycles was studied. The results showed that the transient strain, viscoelastic creep, and viscoplastic creep of carbonaceous mudstone increased with the number of dry and wet cycles, and the creep failure stress and transient elasticity modulus decreased. Based on the steady-state viscoplastic creep rate method, the long-term strength of carbonaceous mudstone after n (n = 0, 3, 6, 9) dry and wet cycles was found to be 74.25%, 64.88%, 57.56%, and 53.16% of its uniaxial compression strength, respectively. Compared with the isochronous curve method and the transition creep method, the steady-state viscoplastic creep rate method can more accurately determine the long-term rock strength. The long-term strength of carbonaceous mudstone under dry and wet cycles decays exponentially, and the long-term strength decay rate during the first three dry and wet cycles is about 215 times the average decay rate.

Size-dependent to size-independent transition in creep of single crystalline Cu micropillars

Micromechanical tests are performed to investigate the sample size effects on creep behavior of single crystalline Cu micropillars at room temperature. We report a transition from size-dependent to size-independent creep, where the transition size decreases with increasing holding stress. Furthermore, we find that the stress-related transition from size-dependent to size-independent creep can be explained by the competition between thermally-activated dislocation generation from sources and thermally-activated dislocation motion. Based on this competition mechanism, a quantitative model is established to further formulate the transition, where the model predictions agree well with the experimental results.

Uniaxial Compression Mechanical Properties of Foam Nickel/Iron-Epoxy Interpenetrating Phase Composites.

The damage process and failure mechanisms were analyzed by a series of quasi-static compressive experiments of seven materials including pure epoxy (EP), three different PPI (pores per linear inch) foam nickel-iron, and three different PPI foam nickel/iron-epoxy interpenetrating phase composites (IPC). Plotting the stress–strain curves of different materials, their change rules are discussed, then the effective elastic modulus and yield limit of the materials are provided, and the energy absorption properties of different materials are analyzed by the stress–strain curves. It was found that the effective elastic modulus and specific stiffness of the three IPC materials were higher than pure foam nickel-iron. The brittleness of epoxy can be obviously changed by selecting a suitable PPI foam nickel-iron composited with it. The unit volume energy absorption rate of foam nickel/iron-epoxy was significantly higher than pure epoxy and pure foam nickel-iron. It was also found that the energy absorption rate decreased with the increase in PPI. The stress relaxation rate decreased first and then increased with the increase in PPI. The creep behavior of the three composites was obvious in the creep elastic stage, and the creep rate increased with the increase in PPI. The creep rate decreased with the increase in PPI in the creep transition stage.

Creep characteristics and prediction of creep failure of rock discontinuities under shearing conditions

Shear creep is one of the most important mechanical behaviors of rock discontinuities. The creep mechanism and prediction of starting point of the accelerating creep stage are vital for establishing the creep model and predicting creep failure. In this study, a series of multi-step creep tests are conducted. The three creep stages of shear creep tests are investigated in detail, and a method for predicting the accelerating creep stage is proposed. Distinct nonlinear and local fluctuations caused by cracking are observed in the creep curve. To describe the transition creep stage and steady creep stage, an empirical creep model is established, and the creep characteristics related to the joint roughness coefficient (JRC) and the normal stress are explored in detail using the model’s parameters. The creep process can be described as involving the JRC resistance weakening and frictional resistance compensation, and a model also established to describe this process. The frictional resistance cannot compensate for the loss of JRC resistance; consequently, creep failure occurs. The starting point of the accelerating creep stage can be predicted by combining the JRC weakening and frictional mobilization model and the empirical creep model. A new method for determining long-term strength is also proposed based on the relationships between the starting point creep deformation and the shear creep stress.

Evaluation of Creep Properties in Anode Materials for Solid Oxide Fuel Cells by Using Small Punch Testing Method

To assess the mechanical reliability and durability of solid oxide fuel cells (SOFCs), a suitable creep test method needs to be developed. The objective of this study is to examine the potential of small punch (SP) method as a simple creep testing method for SOFCs. In this study, the creep properties of Ni-YSZ composites were evaluated at temperatures of 800-900˚C, and at stresses of 47.3 to 89.8 MPa under 1% hydrogen environment. The obtained creep curves by the SP method showed three stages: initial transition creep, secondary creep, and tertiary stages. The central displacement versus time data obtained by the SP method was found to be characterized by a power law relationship. Even though quantitative comparisons need to be made with conventional methods, the above result suggests the usefulness of the SP methods for characterizing the creep properties in constituent materials of SOFCs.

Elastoplastic Deformation in an Orthotropic Spherical Shell Subjected to a Temperature Gradient

The objective of this research paper is to present the study of elastoplastic deformation in an orthotropic spherical shell subjected to a temperature gradient by using Seth’s transition theory. Seth’s transition theory includes classical macroscopic solving problems in plasticity, creep and relaxation and assumes semi-empirical yield conditions. The nonlinear transition regions through which yielding occurs are neglected. Apparently, transition theory is used to solve problems in a general way, employing the concept of generalized strain measure and asymptotic solution at the transition points of differential equations. The problems of elastoplastic and creep transitions are solved through principal stress and principal stress difference and in consideration of the nonlinear part of the transition.

A Numerical Study of Creep Crack Growth in an Aero-engine Turbine Disc using XFEM

In this paper, a turbine disc of an aero-engine having a macro crack under creep loading is studied to predict the creep crack growth. Extended finite element method (XFEM) is used as it does not require conformal meshing and remeshing due to change in the topology during crack growth. The elasto-plastic behavior of the material is modelled by Ramberg-Osgood model and von-Mises yield criterion. The creep behavior is modelled by coupling of spatial and temporal dimensions to capture the effect of stress relaxation and redistribution due to creep strain. The creep crack growth rate is computed by the C(t)-integral which includes small-scale creep, transition creep and extensive creep. The crack growth direction is estimated by the maximum principal stress criterion using mode-I and mode-II stress intensity factors (SIFs). The interaction integral approach is implemented for the evaluation of stress intensity factors of different modes. The history fields of plasticity and creep are transferred properly from the old configuration to new configuration after the crack growth which is the main challenge in this analysis. This proposed numerical scheme is then utilized to obtain the creep crack growth in the component made of elasto-plastic-creeping material. The creep crack growth variation with time is estimated by the XFEM to evaluate the life of an aero-engine turbine disc under creep conditions.

Short-Term Creep Data Based Long-Term Creep Life Predictability for Grade 92 Steels and Its Microstructural Basis

Long-term creep life (5000–100,000 h) predictabilities, based on the short-term creep life data (~ 5000 h), for Grade 92 steel were investigated among major creep life prediction models, Larson–Miller parameter (LMP), normalized power law (NPL) and Wilshire models. The NPL and Wilshire models showed superior short-term creep data based long-term creep life predictabilities to the LMP model. In particular, the Wilshire model showed relatively accurate predictions (within an error range of 7%–10%), which seemed to be due to reasonable coupling of the normalized stress with the temperature-dependent rupture life term in a form of the cumulative distribution function. Both NPL and Wilshire models, calibrated by the short-term creep life data, showed a transition in creep mechanism at a similar normalized stress. Thermodynamics-based kinetic simulation (MatCalcTM) results for major precipitates (M23C6, MX and Z phases) of Grade 92 steel suggested that the creep transition is associated with coarsening of M23C6 precipitates at high temperatures (above 600 °C), which led to the degradation of the creep property.

Mixed mode crack growth in elasto-plastic-creeping solids using XFEM

In this work, an extended finite element based fully implicit algorithm is proposed to predict the creep deformation and creep crack growth in elasto-plastic-creeping solids. The effect of stress relaxation and redistribution due to creep strain is captured through elasto-plastic-creep analysis. In creep crack growth analysis, mostly C∗ is used to compute the creep crack growth which includes only extensive creep whereas C(t)-integral includes small-scale creep, transition creep and extensive creep. Thus, in the present study, creep crack characterization parameter C(t)-integral is evaluated using XFEM to compute creep crack growth rate. The mixed mode stress intensity factors are evaluated using domain based interaction integral approach to estimate the crack growth direction. Various numerical issues related to history dependency of plasticity and creep are properly addressed. To validate the present approach, various numerical problems are solved and the predicted creep deformation and creep crack growth results are compared with the experimental observations.

Shear creep tests and creep constitutive model of marble with structural plane

The paper aims at investigating the shear creep property and creep model of marble with structural plane. Marble specimens were taken from diversion tunnel engineering of Jinping II hydropower station. A series of shear creep tests were conducted for two types of marbles with weak structural plane and hard structural plane. The results showed that the creep process of marble with weak structural plane included three creep phases when shear stress was higher. However, shear creep process of marble with hard structural plane only included transition creep phase and steady creep phase and roughness of hard structural plane affected on the shear creep property. Based on the experimental results, a time-dependent statistical damage constitutive model was proposed in terms of fractional calculus theory and damage evolution law to describe the time-dependent damage characteristics. The proposed model was analysed by utilising results of marble creep testing. The results of the analysis showed that the proposed model could describe not only the transient and steady creep phase of marble with weak or hard structural plane, but also the acceleration creep phase of marble with weak structural plane when shear stress is higher.

Experimental creep behavior of porcine liver under indentation with laparoscopic grasper for MIS applications

Abstract Mechanical response of soft tissues behaved disparate due to fast and large deformation during surgical grasping, so there is a need for experimental databases of biomechanical characteristics of soft tissue under the contact of MIS tool, which are more useful for designing new surgical instruments, training inexperienced surgeons, improving surgical simulations and developing surgical robotics system. A novel indentation test to simulate the real-time surgical operation condition was present in this paper. The creep behavior of porcine liver in vitro was studied under uniaxial indentation by using MIS grasper. The nominal stress between the grasper and the liver was 0.02 to 0.1 MPa, the loading velocity was 1 to 3 mm/s, and the holding time was 300 s to simulate clamping tissue operation. Results showed that the creep process of the liver during 300 s of duration can be divided into three stages: loading stage I, transition creep stage II and steady creep stage III. The creep characteristic of liver behaves time-dependent, load-dependent and strongly loading velocity-dependent due to its nonlinear viscoelastic characteristics and hysteresis characteristics. These creep behavior might also be associated with the deformation, migration and biochemical reaction of the liver cells. The phenomenological model derived in this paper may describe the creep behavior of the liver. The results would provide experimental databases and phenomenological models for investigating biomechanical characteristics of soft tissue under the contact of MIS tool.

Prediction of creep crack initiation behaviour in 316H stainless steel using stress dependent creep ductility

The creep crack initiation behaviour of Type 316H stainless steel at 550°C has been predicted by implementing a stress dependent creep ductility and average creep strain rate model in finite element analyses. Simulations were performed on five specimen geometries: C(T), CS(T), DEN(T), M(T) and SEN(T). The predicted results have been characterised using the C* fracture mechanics parameter and the short-term, long-term and transition creep crack initiation trends are predicted for each of the specimen geometries examined. The prediction results have been validated through comparison with experimental data available in the literature. The predicted short-term and long-term creep crack initiation trends have also been compared with NSW prediction lines. The predicted results, from each specimen geometry, are compared to each other and the differences in crack initiation trends have been discussed in terms of the specimen geometry, in-plane constraint and stress level effects on the creep crack initiation behaviour of the material. A mesh sensitivity analysis has also been performed to find the optimum mesh size for performing crack initiation simulations.

Estimation of C(t) and the creep crack tip stress field of functionally graded materials and verification via finite element analysis

Functionally graded materials (FGMs) are increasingly being used in high-temperature structural components. The time-dependent C-integral, C(t), is a significant parameter in characterizing the crack tip stress field from the small-scale to the extensive steady-state creep stages. In this paper, the creep crack behavior of FGMs with a crack parallel to the material property gradients was studied by using the finite element method. It was proven that C(t) remained valid for FGMs. An engineering method was proposed to estimate C(t) in the small scale and transition creep stages and estimate the crack tip stress field. Finite element results verified that the method could estimate C(t) in the small scale and transition creep stages and estimate the crack tip stress field. For cases with creep properties that increased in the crack growth direction, the effect of constraint on the estimated crack tip stress field had to be considered. The constraint parameter of Q was significantly affected by the creep property gradients. The method was independent of the gradient variation law of material properties.

Estimation of Ct for weld cracks including HAZ softening region

Ct is an important parameter in predicting creep crack growth life from the small-scale creep stage to the extensive steady state creep stage. In this paper, weld interface and non-interface creep cracks for compact tension (CT) specimens with heat-affected zone (HAZ) were studied by finite element method (FEM) to systematically research the estimation method of Ct for weld creep cracks. Simulation results showed that Ct of weld interface cracks could be estimated by Yoon’s method. For weld non-interface cracks, Ct under the small-scale and transition creep stages could be exactly estimated by a proposed method. The accuracy of the proposed method was not affected by HAZ, but Yoon’s method was slightly affected by HAZ.

Thermal creep transition stresses and strain rates in a circular disc with shaft having variable density

PurposeThe purpose of this paper is to present study of thermal creep stresses and strain rates in a circular disc with shaft having variable density by using Seth’s transition theory.Design/methodology/approachSeth’s transition theory is applied to the problem of thermal creep transition stresses and strain rates in a thin rotating disc with shaft having variable density by finite deformation. Neither the yield criterion nor the associated flow rule is assumed here. The results obtained here are applicable to compressible materials. If the additional condition of incompressibility is imposed, then the expression for stresses corresponds to those arising from Tresca yield condition.FindingsThermal effect increased value of radial stress at the internal surface of the rotating disc made of incompressible material as compared to tangential stress and this value of radial stress further much increases with the increase in angular speed as compared to without thermal effect. Strain rates have maximum values at the internal surface for compressible material.Originality/valueThe model proposed in this paper is used in mechanical and electronic devices. They have extensive practical engineering application such as in steam and gas turbines, turbo generators, flywheel of internal combustion engines, turbojet engines, reciprocating engines, centrifugal compressors and brake disks.

Determine variation of poisson ratios and thermal creep stresses and strain rates in an isotropic disc

Seth's transition theory is applied to the problem of thermal creep transition stresses and strain rates in a thin rotating disc with shaft having variable density by finite deformation. Neither the yield criterion nor the associated flow rule is assumed here. The results obtained here are applicable to compressible materials. If the additional condition of incompressibility is imposed, then the expression for stresses corresponds to those arising from Tresca yield condition. Thermal effect decreased value of radial stress at the internal surface of the rotating isotropic disc made of compressible material as well as incompressible material and this value of radial stress further much increases with the increase in angular speed. With the introduction of thermal effects, the maximum value of strain rates further increases at the internal surface for compressible materials as compare to incompressible material.

On the creep transition from superplastic behavior to nanocrystalline behavior

The present analysis shows that the creep transition from superplastic behavior to nanocrystalline behavior can be defined by an expression, which relates the normalized grain size, d/b (b is the Burgers vector) to the normalized shear stress, τ/G (G is the shear modulus).