Creep Phenomena Research Articles

Creep behavior of Grade 10.9 high-strength bolts under and after fire

• Experiments were conducted on creep behaviour of Grade 10.9 high-strength bolts under and after fire. • The Field & Field model is calibrated for simulating creep behavior of high-strength bolts at elevated temperatures. • The temperature, load ratio and loading duration are key factors affecting the creep behavior of T -stub connections. • High-strength bolts have more obvious creep deformation at high temperatures than high-strength steels. • A reduction factor is recommended to consider the creep effect on determining strength of T -stub connections in Eurocode 3. High-strength bolts are widely used in beam-column connections of high-rise steel framed buildings. At the event of a fire, high-strength bolts may experience significant creep in a state of both high stress and high temperature, leading to fracture of connections and even progressive collapse of structures . This paper experimentally and numerically investigates the creep behavior of Grade 10.9 high-strength bolts under and after fire. The creep strain–time curves at different temperatures and stress ratios are obtained using DIC systems. The micromorphology and failure mechanism of high-strength bolts is examined by SEM. The Field & Field creep model is calibrated against test results, and parametric studies are conducted on creep behavior of T -stub connections. The experimental results show that the creep deformation of high-strength bolts is significant at high temperatures, and the creep strain is positively correlated with temperatures and stress ratios. While the creep effect of high-strength bolts after fire can be ignored. The creep-induced fracture of high-strength bolts under fire is in a ductile manner, and that after fire is in a brittle manner. High-strength bolts have more obvious creep phenomenon at high temperatures than high-strength steels, and it is unsafe to directly apply creep data of high-strength steels to high-strength bolts. The numerical results show that the Field & Field model is suitable for characterizing the creep behavior of high-strength bolts under fire with an applicable temperature range of 20–600 °C and stress ratio range of 0.4–0.8. The temperature, load ratio and loading duration significantly affect the creep behavior of T -stub connections under fire conditions. A reduction factor of 0.8 for 500 °C and 0.6 for 600 °C is recommended to consider the effect of creep on the strength of bolts in determining bearing capacity of T -stub connections in Eurocode 3. It is necessary to consider the effect of creep for predicting the fire behavior of high-strength bolts and steel connections, ensuring a safe design.

Modeling and Observation of Rate-Dependent Hysteresis and Creep Phenomena in Magnetorheological Clutch

Magnetorheological clutches (MRCs) are potential to provide jointed robots with the capability of behaving compliantly and safely. However, before achieving those behaviors, the MRCs should be well modeled to capture their complicated nonlinear dynamics including rate-dependent hysteresis and creep phenomena. This article develops a multistate fractional-order MRC model that captures such complicated dynamics. Compared to the classical models, this new model offers better estimation accuracy for the hysteresis and creep phenomena, simultaneously. To further improve the modeling performance, an observer based on the fractional-order model and the supertwisting algorithm is designed to compensate for the model uncertainties. With an implicit-Euler implementation method, the observer can exactly estimate the model uncertainties and then further improve the model prediction. The effectiveness of the proposed fractional-order model and observer is demonstrated with experiments and comparisons with the fractional-order Bouc–Wen model.

Study on Surrounding Rock Deformation Laws of an Argillaceous Soft Rock Roadway Based on the Creep Damage Model

The indexes of argillaceous soft rocks in Western China, such as rock strength and softening coefficient, are much smaller than those of soft rocks commonly seen in other areas. Such argillaceous soft rocks are susceptible to a more serious creep phenomenon if encountering water. Accordingly, the study theoretically constructed a constitutive creep model considering the weakening coefficient and damage of surrounding rocks. Next, the viability of this model was verified by combining numerical simulation and field monitoring. Accordingly, this study conducted a comparative analysis on the creep range and cross-sectional deformation of argillaceous surrounding rocks of a roadway in a titled stratum under dry and damp states. Results showed that the Cvisc model considering the weakening coefficient of surrounding rocks was applicable to the decay creep and uniform creep stage, and that considering the damage of surrounding rocks was applicable to the accelerated creep stage. During the creep process, the argillaceous surrounding rocks were prone to viscoelastic deformation. The damp state had an obvious time effect on the damage of surrounding rocks. Specifically, the creep range was expanded under a damp state at 18 days compared with that under a dry state, and the change in the left and upper displacements was especially significant. The roadway cross-section changed asymmetrically under both dry and damp states, where it was changed into an approximate deflected rhombic shape under the dry state and into an approximate deflected trapezoidal shape under the damp state. Additionally, the roadway cross-sectional change under the humid state was evidently greater than that under the dry state.

Adaptive Friction Compensation for a Novel Two-Axis Differential Microfeed System

The conventional drive feed system inevitably enters the nonlinear creep region at very low speed. This seriously affects its low-speed performance and tracking accuracy. The new two-axis differential microfeed system (TDMS) can overcome the low-speed crawling and creep phenomenon caused by the inherent characteristics of the traditional electromechanical servo system structure. In this study, the dynamic model of TDMS is first established, and the LuGre friction model is improved according to its motion characteristics. The friction model of the screw nut whole assembly of TDMS is established. On this basis, two nonlinear observers are designed to estimate the internal state of the friction model, and the TDMS adaptive friction compensation controller is designed to estimate the uncertain parameters in TDMS through the parameter adaptive law. The global asymptotic stability of the TDMS controller is proved by Lyapunov theorem. Finally, the experimental verification is carried out by using dSPACE system. The results showed that the proposed TDMS adaptive friction compensation controller has good control accuracy. And it can effectively compensate the friction torque of lead screw nut differential drive. Compared with the application of traditional PID controller in TDMS, the tracking accuracy is improved by an order of magnitude.

Understanding on TDS Creep Phenomena of Reverse Osmosis Membranes in Water Purifiers

Understanding on TDS Creep Phenomena of Reverse Osmosis Membranes in Water Purifiers

Investigation on Creep Behavior of Aggregates of Loess by a Discrete Element Method

In loess the aggregate is the basic structural unit, and its stability is an important factor affecting the composition, water stability and strength of loess. However, due to the difficulty of sample preparation, few scholars have done independent research on it. In this manuscript, a numerical model of aggregate is constructed by the discrete element method. Under the continuous action of certain stress, the uninterrupted development process of sample deformation with time was observed, that is, the creep of aggregate structures. The results show that the creep of aggregates is closely related to the relative movement, rotation and rearrangement of internal structural elements, and the most intuitive mesoscopic evolution of the adjustment process of structural elements is the change of contact number, namely the coordination number. The microscopic parameters and evolutionary characteristics of fabric can reveal the microscopic mechanism behind the macroscopic creep phenomenon. With the creep process, the creep stress is gradually borne by the normal contact force rather than the tangential contact force and has anisotropic characteristics. As a result of creep, the contact points of particles increase, and the interaction between aggregates changes from point contact to overlap contact. The constraint between aggregates increases, and the skeleton tends to be a more stable structure, which can bear a larger load.

Thermal–Mechanical FEM Analyses of a Liquid Rocket Engines Thrust Chamber

The Italian Ministry of University and Research (MIUR) funded the HYPROB Program to develop regeneratively cooled liquid rocket engines. In this type of engine, liquid propellant oxygen–methane is used, allowing us to reach very good performances in terms of high vacuum specific impulse and high thrust-to-weight ratio. The present study focused on the HYPROB final ground demonstrator, which will be able to produce a 30 kN thrust in flight conditions. In order to achieve such a thrust level, very high chamber pressures (up to 50 bar) and consequently high thermal fluxes and gradients are expected inside the thrust chamber. Very complex and high-fidelity numerical FEM models were adopted here to accurately simulate the thermal–mechanical behavior of the thrust chamber cooling channels, accounting for plasticity, creep, and low-cycle fatigue (LCF) phenomena. The aim of the current work was to investigate the main failure phenomena that could occur during the thrust chamber’s service life. Results demonstrated that LCF is the main cause of failure. The corresponding number of loading cycles to failure were calculated accordingly.

Size Effect on the Rheological Shear Mechanical Behaviors of Different Joints: A Numerical Study

Under the condition of constant external stress, the displacement and stress of the joint will be adjusted continuously with time, resulting in the creep phenomenon, that is, the continuous increase of the strain with time, which further causes the damage of the jointed rock mass. In order to study the size effect of joint shear rheological mechanical properties, this paper conducted shear rheological numerical simulations on nonpenetrating and penetrating joint models established in Particle Flow Code (PFC) and analyzed the effect of specimen size on the rheological and shear mechanical behavior of the model. The results show the following: (1) For the rheological direct shear test, the instantaneous shear displacement of the joint increases continuously as the size increases, showing a significant positive size effect. (2) For through-type joints, only the cohesion shows a certain size effect, and the shear strength and friction angle have a nonobvious relation with specimen size. (3) For anchored through-type joints, the increase of the model size leads to a decrease in shear strength, while the friction angle-size effect of joints is not remarkable.

Secondary Creep Analysis of FG Rotating Cylinder with Exponential, Linear and Quadratic Volume Reinforcement.

Creep is an irreversible time-dependent deformation in which a material under constant mechanical stress and elevated temperature for a considerably prolonged period of time, starts to undergo permanent deformation. Creep deformation occurs in three stages namely, primary, secondary and tertiary. Out of these three stages, secondary or steady state creep is particularly an area of engineering interest as it has almost a constant creep rate. Creep deformation plays a significant role in understanding effective service life of an engineering component working under high temperature conditions as such components such as super-heater and re-heater tubes and headers in a boiler, jet engines operating at temperature as high as 1200 C, usually experience a failure or rupture due to creep phenomenon. Design engineers keep a close attention on working stress conditions and elevated temperature under which an engineering component is expected to work as these conditions determine the onset of creep behavior in an engineering component. By recognizing the parameters of material response to creep behavior, engineers can analyse the useful service life and hazardous working conditions for an engineering components. Recognizing the creep phenomenon as high temperature design limitation, ASME Boiler and Pressure Vessel Code have provided guidelines on maximum allowable stresses for materials to be used in creep range. One of the criteria for determination of allowable stresses is 1% creep deformation of material in 100,000 h of service. Thus, the study of creep behavior in engineering components pertaining to high stress and temperature working conditions is very important as it affects the reliability and performance of the engineering components. The aim of our study is to understand the behavior of secondary creep deformation so that an advanced reinforced functionally graded material with better creep resistance, can be designed. In this paper, a secondary creep analysis of functionally graded (FG) thick-walled rotating cylinder under internal and external pressure is conducted. The novelty of the model intends to specify secondary creep stresses and strains by employing exponential, linear and quadratic volume reinforcement for ceramic in metal matrix in radial direction. This will help us to understand the effect of volume reinforcement in FG cylinder under internal/external pressure and rotating centrifugal body force by obtaining secondary creep stresses and strains. The response of the FG cylinder with isotropic material is analyzed and the solution for stress–strain rates in radial and tangential directions are obtained in closed form. Comparison of steady state creep stresses and strains under exponential, linear and quadratic volume reinforcement profiles are discussed and presented graphically.

A one-dimensional elasto-viscoplastic model coupled to damage for the description of creep in wooden materials

This work focuses on the development of a model for the description of the tertiary creep phenomenon in wooden materials. We stared from an extended standard solid body model capable of best describing primary and secondary creeps. We then modify this model by introducing a damage variable to explain and model the rapid growth of viscoplastic strain during tertiary creep. We obtain a model comprising a reduced number of parameters (05) all physically interpretable; which can be easily determined from the results of creep tests. The proposed model has been tested using the experimental results of creep-rupture tests and it has been shown to be very suitable for describing the three phases of creep, with a relative error of less than 1%. The breaking time proposed by the model is lower, but very close to the experimental breaking time (Err = 0.01). The time to failure is easily accessible, thanks to the simplicity of our model, without necessarily going through heavy algorithms. This represents a significant advantage of our model, which in sum offers both a more realistic way of describing the three phases of creep by fully accounting for the phenomenon of damage during the tertiary phase, and a simple and fast way to analyze the rupture time, compared to other models in the literature. Our model is therefore presented as a good alternative for modeling the behavior of wood material under creep stress.

Study on the creep behavior and microstructure evolution of a low alloy steel in continuous casting processing

In continuous casting processing, the solidifying slabs undergo creep deformations under complex thermomechanical stresses, which may cause microstructure anisotropy and flaws within the casting slabs. Therefore, it is vital to acquire insights into the creep properties and microstructure characteristics of slabs tailored by thermomechanical conditions. To clarify above issues, an as-cast low alloy steel was crept under continuous casting processing conditions to explore the involved creep phenomena. The creep performances correlated with temperatures and stresses were evaluated, the results pronounced the dominant role of temperature scheme governing the creep procedure compared to stress. A constitutive model was established based on an empirical physics-based constitutive model to predict the creep curves. The crept microstructures were characterized to reveal the dependence of substructure characteristics on creep temperatures. The reduced LABs (low angle boundaries) and dislocations under higher temperatures indicated the deteriorated creep resistance and enhanced susceptibility to microcrack initiation and propagation, which accounted for the occurrence of internal microcracks within the continuous casting slab. The results can provide fundamental data for utilizing the creep deformation to develop straightening technology in continuous casting processing.

Sintering and creep behavior of Li2TiO3 pebble bed for breeding blanket of fusion reactor

Tritium is generated in a breeding blanket by nuclear reaction between neutron from fusion plasma and the lithium in breeder pebbles. The securing of stable flow path of the purge gas inside the pebble bed is important because the generated tritium is extracted by the purge gas, which flows through the empty spaces inside the pebble bed. The sintering and creep phenomena of the pebble bed can affect the flow path of purge gas and the tritium extraction because of the formation of necking between the pebbles by sintering and deformation of the pebbles by creep. This study aims to investigate the starting temperature of sintering on the Li2TiO3 pebble bed and the creep deformation behavior of the Li2TiO3 pebble bed under various conditions such as compression pressure and holding time. The initial stage of the sintering phenomena on the Li2TiO3 pebble bed under applied compression pressure of 1 MPa was estimated at between 850 °C to 900 °C. However, densification of the Li2TiO3 pebble bed caused by creep deformation progressed despite initial stage of sintering. Therefore, from the viewpoint of stable extraction of bred tritium from the breeder pebble bed, it was recommended, that, to maintain uniform flow path of purge gas inside the pebble bed, the maximum operating temperature and pressure of the Li2TiO3 pebble bed be 800 °C and 1 MPa.

Energy balance formation of sub-eutectoide fayalita at high pressures in particle separators

This article presents an approach to the problem of ceramic types adhesion, applying energy and matter balance to the established control volume (cyclone) with the use of mathematical formulas that are interrelated to develop mathematical calculations and establish a new mathematical model The first results are obtained by operating the energy balance considering the collision of particles, using the principle of conservation of energy, the first law of thermodynamics, in order to obtain information that allows describing the phenomena of thermoplasticity and creep, in the formation of adhesions, from a physicochemical and kinetic point of view, which will serve as the basis for understanding their effect. As a result, an energy value of 660 kJ / mol was obtained, sufficient energy to start the transformation of the solid particles to a state of thermo-flow that allows the adhesion phenomenon to be started.
 Keywords: Adhesion, energy balance, cyclones, elutriation, eutectoid, fayalite, thermoplasticity.
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Hot Deformation Behavior of PM Ni with Harmonic Microstructure

The hot deformation behavior of a harmonic-structured pure nickel has been studied and compared with the hot deformability of a homogeneously structured nickel. Both materials were produced via the powder metallurgy route through the Spark Plasma Sintering (SPS) of mechanical milled and un-milled powders. Hot deformation was evaluated through compression tests at three different temperatures (400 °C, 800 °C, and 1300 °C), covering a wide range in the homologous temperature spectrum for Ni (from 0.39 to 0.91), and at three different strain rates (0.001, 0.01, and 0.1 s−1). The evaluation of the stress–strain curves showed a higher hot compression resistance for the harmonic-structured nickel, together with higher strain hardening and strain rate sensitivity, thanks to the peculiar microstructural features of this material. Through the metallographic analysis of the specimens after hot compression, different mechanisms were identified as responsible for the deformation behavior in relation to the temperature of testing. While at 400 °C dynamic recrystallization has slightly started, at 800 °C it is widely diffused, and at 1300 °C it is replaced by grain growth and diffusion creep phenomena.

МЕТОД СВЧ АКУСТИЧЕСКОГО ИССЛЕДОВАНИЯ МАТЕРИАЛОВ ПОД ВЫСОКИМ ДАВЛЕНИЕМ

The arrangement of a new integrated measuring system (IMS) "HBAR + DAC" and its application to study changes in the acoustic properties of a paper and polycrystalline molybdenum samples under the influence of high pressure are discussed. When creating an IMS for the synthesis of piezoelectric films of aluminum-scandium nitride, the magnetron deposition method was used. The quality and thickness of the films were controlled by X-ray diffraction analysis and atomic force microscopy. For the first time, it is shown that the HBAR in the IMS content has good operational characteristics in the microwave band up to 8.8 GHz under the high-pressure impact on the samples. A decrease in the Q-factor of an HBAR as a sensitive element occurred to values of ~2500 – 3000 at the upper pressure limit, which is quite sufficient for accurate measurements of the resonant frequencies of acoustic overtones. The possibilities of IMS for studying the features in the behavior of solids under the high-pressure impact and the registration of plastic deformations in metals are demonstrated. It is found that the pressure dependence of the relative frequency shift of overtones during compression of a paper sample has a linear character. Thanks to this, the calibration of the high-pressure chamber on diamond anvils was performed using the RAMAN method. It is found that the frequency shift of overtones allows more accurately determining the stresses in the diamond anvil up to 5 GPa. When compressing a sample of polycrystalline molybdenum, the phenomena of plastic deformation and creep, as well as the transit flight of an acoustic wave through the sample, starting from a pressure of 3 GPa, were detected. The results obtained will be useful for researchers, students and postgraduates in the fields of high pressure physics and physical acoustics.

Creep and thermal ratcheting characterization of polytetrafluoroethylene-based gasket materials

Characterization of Teflon polymer based gaskets under expedited aging is the objective of this work. Teflon gaskets are exploited frequently as a replacement to asbestos fiber gaskets because of their excellent leak tightness and nonhazardous physical degradation properties. The research focuses profoundly on the adverse influence of temperature and thermal cycles on the creep and cumulative damage phenomenon under compressive load. Virgin and expanded PolyTetraFluoroEthylene (PTFE) are tested under 28 and 41 MPa of gasket stress at different temperatures. Intricate analysis of creep under coalesces of thermal ratcheting and principal stress is achieved through Universal Gasket Rig (UGR). The instigated cumulative damage is distinguishable into upper and lower bound temperature region indicating the escalation and decrease of thickness change during cycling which saturates after 12 thermal cycles for expanded PTFE while no saturation is reached for virgin PTFE in even after 20 thermal cycles. Percentage of thickness reduction at different applied stress is nearly the same for virgin PTFE whereas expanded PTFE shows largest reduction under lower stress. Compressive creep bespeaks the impact of temperature and load, thereby dictating the magnitude of ratcheting damage and contrariwise. Finally, the creep and thermal ratcheting has a proliferating effect on value of the coefficient of thermal expansion for all chosen gaskets. Copyright © 2017 VBRI Press.

Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood. Although laboratory creep tests have been carried out to determine the creep deformation of various rocks, these tests are expensive and time-consuming. Nanoindentation creep tests, as an alternative method, can be performed to investigate the mechanical and viscoelastic properties of granite samples. In this study, the reduced Young’s modulus, hardness, fracture toughness, creep strain rate, stress exponent, activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results. It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift, and then it decreased slowly. The stress exponent values obtained were in the range from 4.5 to 22.9, which indicates that dislocation climb is the creep deformation mechanism. In addition, fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties. Last but not least, both three-element Voigt model and Burgers model fit the nanoindentation creep curves well. This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a micro-scale perspective, which is of great significance to the understanding of localized deformation processes of rocks.

Investigating the effective stress function algorithm for the thermostructural analysis and design of liquid rocket engines thrust chambers

The thermostructural behavior of a regeneratively cooled thrust chamber for aerospace applications is strongly non-linear as plastic and creep phenomena come into play. Indeed, the fuel combustion inside the thrust chamber generates high heat fluxes and, then, elevated temperatures are expected in the inner copper structure of the chamber which is made of cooling channels separating the hot gases from the coolant flow. Since the analysis of those non-linear phenomena requires the adoption of very complex and heavy numerical models, it is crucial to employ very efficient numerical algorithms such as the Effective Stress Function algorithm; such an algorithm allows to obtain accurate results in terms of stresses in finite element thermostructural analyses taking into account both plastic and creep phenomena. The aim of the current paper is, then, to demonstrate that the Effective Stress Function algorithm is well suited for the analysis of the thrust chamber providing accurate results in a reasonable amount of computation time. The numerical results obtained are very similar to those achieved with the classical approach adopted by finite element commercial codes with significant computation time savings.

Creep behaviour of two-component grout and interaction with segmental lining in tunnelling

The complexity of the two-component grout behavior significantly affects the interaction between the support system and the tunnel. The loading of the filling material (and also of the segmental lining) takes place during the curing phase. At the end of it a creep phase develops which shows secondary deformations over time which result in a relaxation of the segmental lining and an increase in the deformation of the tunnel wall. In this work, a laboratory experiment campaign allowed to study the two-component material in detail during the creep phase. The typical trend of deformations over time, obtained for different stress load values and material curing age, was identified. The maximum stress was also evaluated which still allows to avoid the failure of the material over time, when a persistent state of stress is applied. The information obtained from the experimentation allowed to provide considerations on the support system-tunnel interaction, using the convergence-confinement method. The approach followed was then applied to a real case of a tunnel excavated in Northern Italy, for which it was possible to estimate the decrease in the load applied to the support system and the increase in deformations of the tunnel wall due to the creep phenomenon of the two-component material.

Stress-Based topology optimization under the stress relaxation effect

One of the effects of creep phenomena in structures is stress relaxation. Owing to this effect, the stiffness of each point is inversely related to the stress at that point, which causes a stress redistribution, a reduction of the maximum stress, and an increment in the displacement field. By contrast, for structural optimization, it is a practical and challenging problem to consider the maximum stress as a failure criterion or objective function. In the present study, the stress relaxation effect was investigated in stress-based topology optimization. The so-called solid isotropic material with penalization method and a nonlinear finite element analysis are utilized for this purpose. The maximum von Mises stress and the total volume of the structure are selected as the objective function and constraint, respectively. The main novelty of this research is to find the optimized topology with the least maximum stress by developing and implementing a time-dependent adjoint method for sensitivity analysis. Implementing the calculated sensitivity for stress-based problems in a gradient-based optimization method, such as the optimality criteria, is quite challenging, and some numerical instabilities in the optimization process are addressed. Several examples are solved with and without considering the stress relaxation effect. The results reveal that the influence of stress relaxation in the reduction of stress concentration makes the optimized layout more similar to the optimized layout obtained by minimizing the compliance for the linear structures.