Plastic Creep Research Articles

On the prediction of creep behaviour of alloy 617 using Kachanov-Rabotnov model coupled with multi-objective genetic algorithm optimisation

The accurate prediction of elevated-temperature creep behaviour of alloys is important for preventing catastrophic failure of systems operating under prolonged elevated temperature-stress conditions. Here, we couple the Kachanov-Rabotnov (K-R) creep model with a multi-objective genetic algorithm (MOGA) to predict the creep behaviour of Alloy 617 at 800°C, 900°C, and 1000°C, under various stress conditions. It is shown that the MOGA-optimised K-R creep model can capture the overall elevated-temperature behaviour of the alloy at 800°C under a wide range of stress conditions. However, at 900°C and 1000°C, oxidation leads to the atypical accumulation of creep plasticity, which the K-R model cannot account for. Nevertheless, it is shown that the proposed methodology of optimising the K-R model with a MOGA can consistently provide accurate results within the limits of the K-R model.

On the transition of failure control from material-intrinsic defects to defects forming during monotonically increasing and cyclic mechanical loading in WC-Co hard metal at elevated temperature

WC-Co hard metals are composite materials with extraordinary mechanical properties especially at elevated temperatures which make them common materials for metalworking tools. The current work investigates damage and fracture behavior of hourglass-shaped specimens made of a WC-Co hard metal with submicron-sized WC grains and 12 wt.% Co binder. All investigated specimens were isothermally heated inductively to 700°C in a servo-hydraulic testing machine equipped with a vacuum chamber to avoid surface oxidation. The nature of origins of fracture and the evolution of bulk material damage features was studied via scanning electron microscopy for experiments under monotonically increasing load as well as cyclic loading under a stress ratio R = σmin / σmax = -1. A formation of small cavities that formed in the material bulk during loading was observed for all investigated loading conditions. For monotonically increasing load, the coalescence of these cavities was identified as the dominant damage mechanism. For cyclic loading, the number and size of the mentioned cavities did rise with the applied number of load cycles and rising stress amplitude. The coalescence of the cavities was found to be a main mechanism controlling the fatigue crack propagation process at elevated temperature. The found results imply that a certain combination of stress amplitude and temperature exists, at which a transition of failure control occurs from: (i) the size of material-inherent defects present prior to loading, to (ii) the kinetics of the coalescence of cavities that form due to plastic deformation and creep in the Co matrix during loading.

Identification of texture characteristics for improved creep behavior of a L-PBF fabricated IN738 alloy through micromechanical simulations

Additive manufacturing (AM) of nickel-based superalloys, due to high temperature gradients during the building process, typically promotes epitaxial growth of columnar grains with strong crystallographic texture in form of a 〈001〉 fibre or a cube texture. Understanding the mutual dependency between AM process parameters, the resulting microstructure and the effective mechanical properties of the material is of great importance to accelerate the development of the manufacturing process. In this work, a multi-scale micromechanical model is employed to gain deeper insight into the influence of various texture characteristics on the creep behavior of an IN738 superalloy. The creep response is characterized using a phenomenological crystal plasticity creep model that considers the characteristic γ–γ′ microstructure and all active deformation mechanisms. The results reveal that the creep strength increases with decreasing texture intensities and reaches its maximum when the 〈001〉 fibre and cube textures are misaligned to the specimen building direction by 45°. The simulations also predict that the uncommon 〈111〉 and 〈110〉 fibres offer significantly higher creep resistance than the typically observed 〈001〉 fibre, which provides a further incentive to investigate AM processing conditions that can produce these unique textures in the material. As the intensities and the alignment of 〈001〉 fibre and cube textures can be attributed to the laser energy density and the scan strategy employed and as the formation of distinct fibre textures depends on the geometry of the resulting melt pool, the laser powder bed fusion process parameters can be optimized to obtain microstructures with features that improve the creep properties.

Experimental investigation into the shakedown state of unsaturated road base aggregate at various fine contents and matric suctions

The road base aggregate usually reaches the shakedown state under large number of cycles of traffic loads. But this shakedown state is expected to depend on the inclusion of fines and the matric suction. In this study, monotonic and cyclic triaxial tests were performed under controlled matric suctions on the road base aggregate with different fine contents (FCs) and matric suctions. Results show that the plastic creep limit of unsaturated road base aggregate is affected by the two-side effects of fines: reduction of friction angle and enhancement of matric suction. For the coarse-grain controlled soil (FC range from 0% to 4%) and fine-grain controlled soil (FC range from 8% to 16%), the plastic creep limit decreases with increasing FC due to the friction angle reduction caused by the inclusion of fines. For the transition soil (FC range from 4% to 8%) under unsaturated state, the plastic creep limit increases with the increase of FC as the dominant effect of fines shift to the enhancement of matric suction. By adopting an allowable cyclic stress ratio (CSR), a novel method is proposed, allowing prediction of the plastic creep limit based on the results from standard monotonic triaxial test for coarse-grain controlled soil and fine-grain controlled soil, and the results from post-cycle monotonic triaxial test for the transition soil. This method can be applied to road base aggregate with different FCs and matric suctions.

Coupling Characteristics of Creep Fracture of Rock Foundation on Wind Turbine under Wind-Induced Vibration

In view of the nonlinear mechanical characteristics of rock foundation creep fracture and its influence on the stability of wind turbine under wind load, based on the influence of wind load, this paper proposed the elastoplastic creep fracture and rock foundation bearing capacity on wind turbine. Considering the superstructure concrete with rock foundation and its boundary conditions, the wind load standard value was obtained and wind turbine system composition was constructed. The two grades of freedom system of viscous damping vibration model was proposed. Furthermore, the frequency characteristic equations and the relations of the first- and second-order natural frequencies were obtained. Considering plastic yield theory on power hard rock base material, the analytical expressions of principal stress distribution, plastic zone, and plastic state of I-II composite crack were obtained and used for coupling Mohr–Coulomb plastic yield condition and creep fracture characteristics. Furthermore, the nonlinear creep model equation and accelerated creep fracture time were also obtained, to be used for the modified Kelvin nonlinear accelerated creep model. Combined with the calculation examples, it is verified that the accelerated creep displacement and crack propagation of rock foundation are obvious, taking full account of the wind bracing and creep characteristics of rock foundation. Final, the failure mode of rock foundation is of compressive shear, local shear, and bending-shear; so, it is necessary to reinforce the interface of the rock foundation in a timely manner.

Experimental investigation on dynamic and static rock mechanical behavior, failure modes, and sequences of frequent interbedded sand and shale reservoirs

High variability in diagenetic strength and rock mechanical properties in interbedded sand and shale necessitates a sustained interest in the study of the dynamic and static rock mechanical behavior and failure modes. We have analyzed the rock mechanical behavior, rupture characteristics, and sequences of the clastic reservoirs of the Xu 5 member, Western Sichuan Depression, China. The results indicate that the Xu 5 shale can produce longer plastic creep behavior than tight sandstone at the same load rate. This causes greater stresses to build up inside the shale than in adjacent sandstone formations. The average internal friction angle of sandstone is 43°, whereas the average internal friction angle of shale is 33°. The failure modes of the Xu 5 member sandstone are mainly brittle-tensile failure and brittle X-type shear failure. We observed that the tensile rupture is dominant, accounting for approximately 75.2%. Shale failure forms mainly include plastic-tensile failure and X-type shear failure, in which shear failure accounts for approximately 67.9%. The sequence of failures of similar clastic reservoirs is generally tensile failure or tensile-shear failure to extension failure. We found that the Xu 5 shale has high plasticity, and the stress conditions required for its failure are higher and more complicated. In addition, our test results indicate that, for the same lithology, the tensile failure is the initial rupture rather than shear failure.

Constitutive and damage model for the whole-life uniaxial ratcheting behavior of SAC305

With the wide application of SAC305 solders in electronic packaging, it is important to examine their reliability. In this study, uniaxial ratcheting experiments were carried out on SAC305 solders with varying loading rates, mean stresses, and stress amplitudes at room temperature. Based on the unified creep–plasticity constitutive model applicable to SAC305 solders, a nonlinear continuum damage mechanics model was established to simulate the whole-life ratcheting behavior. The damage variable was extracted from the elastic modulus decline of the unloading section during the cyclic loading process, and its evolution was based on the damage dissipation potential, which was derived from the theory of continuum damage mechanics. In addition, the normalized life was used to correct the evolution of the damage variable. For a more accurate description of their evolution, the energy method was used to modify the damage evolution parameters under different loading conditions. Finally, the uniaxial ratcheting behavior of SAC305 solder could be accurately described by the modified damage-coupled constitutive model under different loading conditions.

Critical dynamic stress for the frozen subgrade soil

Triaxial dynamic tests at the temperature −6 °C were performed on frozen subgrade soil to study accumulative deformation and resilient deformation properties. Based on the shakedown theory, a reliable evaluation for the long-term serviceability of the frozen subgrade was further presented. The influences of confining pressure, dynamic stress amplitude, initial stress path and initial stress ratio on dynamic properties of frozen subgrade soil were analysed. The evolution features of accumulative permanent strain under cyclic loading were experimentally determinated. The boundary functions for plastic shakedown area and plastic creep area were further established. Critical dynamic stresses for the different deformation areas can be calculated by the experimental data and the proposal formula. The influences of consolidation pressure and initial stress ratio on critical dynamic stress were discussed in detail. The distribution characteristics (envelope line loci) of the critical dynamic stress were investigated in the mean stress-the shear stress space.

Effect of heat-up and oxidation on the PHWR channel under severe accident scenario-an experimental study

During unmitigated postulated severe accidents, the coolant channels of the pressurized heavy water reactor are heated to elevated temperatures. The channels may further get exposed to oxidizing media during the accident progression. Both heat-up rate and oxidation reaction affect the sagging deformation of channels. This study explores the effect of initial transient heat-up rate on the channel deformation and the contact time between heated and un-heated channels experimentally. The effect of oxidizing environment on the structural integrity of the channels is also studied. As experimentation with multiple full-length channels with bundle weights is complex and expensive, the experiments are conducted with scaled-down channels with geometric scaling of 1:3. During experiments, the sagging deformation of the channel has increased gradually with a low heat-up rate, whereas the contact between the channels has been instantaneous at a high heat-up rate. It is thus suggested that the deformation of the heated channel and the contact propagation along contacting channels may be governed by the rate-dependent plasticity (creep). The oxidative reaction in oxygen-rich corrosive media has severely deteriorated the outer calandria tube of the coolant channel, exposing the inner pressure tube to corrosion. The oxidation of the pressure tube degrades its mechanical strength resulting in the failure of the whole coolant channel. The length of central debris supported by the unheated bottom channel is long (equivalent to 3–4 fuel bundles) compared to end debris. The insight generated from this experimental study will further be used to validate the computational models for the channel deformation and its associated structural integrity.

Thermal cycling creep properties of a directionally solidified superalloy DZ125

Aero-engine turbine blades may suffer overheating during service, which can result in severe microstructural and mechanical degradation within tens of seconds. In this study, the thermal cycling creep under (950 °C/15 min+1100 °C/1 min)-100 MPa was performed on a directionally solidified superalloy, DZ125. The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950 °C/100 MPa, 950 °C/270 MPa, and 1100 °C/100 MPa. The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950 °C/100 MPa. The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep. The thermal cycling creep curve exhibited three stages, similar to the 1100 °C isothermal creep, but its minimum creep rate occurred at a lower creep strain. The overheating events caused severe microstructural degradation, such as substantial dissolution of γ' phase, earlier formation of rafted γ' microstructure, widening of the γ channels, and instability of the interfacial dislocation networks. This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life, as the thermal cycling promoted more dislocations to cut into γ' phase and more cracks to initiate at grain boundaries, carbides, and residual eutectic pools. This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design.

Visco − elastic − plastic creep model based on Burgers and Bingham model and its verification

Visco − elastic − plastic creep model based on Burgers and Bingham model and its verification

Effects of creep characteristics of natural gas hydrate-bearing sediments on wellbore stability

Natural gas hydrate (NGH) reservoirs consist of the types of sediments with weak cementation, low strength, high plasticity, and high creep. Based on the kinetics and thermodynamic characteristics of NGH decomposition, herein a heat-fluid-solid coupling model was established for studying the wellbore stability in an NGH-bearing formation to analyze the effects of the creep characteristics of NGH-bearing sediments during long-term drilling. The results demonstrated that the creep characteristics of sediments resulted in larger plastic yield range, thus aggravating the plastic strain accumulation around the wellbore. Furthermore, the creep characteristics of NGH-bearing sediments could enhance the effects induced by the difference in horizontal in situ stress, as a result, the plastic strain in the formation around the wellbore increased nonlinearly with increasing difference in in situ stress. The lower the pore pressure, the greater the stress concentration effects and the higher the plastic strain at the wellbore. Moreover, the lower the initial NGH saturation, the greater the initial plastic strain and yield range and the higher the equivalent creep stress. The plastic strain at the wellbore increased nonlinearly with decreasing initial saturation.

Reclaimed Asphalt Pavement (RAP) as an Unbound Base Course Material: A Mechanistic Design Approach Based on Multi-stage Repeated Load Triaxial Tests

Reclaimed asphalt pavement (RAP) and blends of RAP with virgin aggregate (VA) were characterized in this study to be utilized as base course aggregate. Multi-stage and single repeated load triaxial (RLT) tests were conducted in the laboratory to characterize the materials. The results from these tests were used to develop a mechanistic design approach that is suited not only for virgin but also for recycled materials that are prone to excessive plastic deformations under repeated loads. The design approach presented in this study allows the users to select different pavement structure configurations in terms of base course and HMA thicknesses, where the selected configuration can withstand the repeated loads without undergoing significant permanent strains. Such design approach was achieved with the laboratory tests that were conducted with variety of stress ratios (σ1/σ3) that represented different pavement layer configurations and beyond. Principles of shakedown theory were utilized to define the boundaries of plastic shakedown, creep shakedown, and incremental collapse. This study demonstrated that the use of failure and plastic shakedown boundaries were crucial to the proposed mechanistic design approach.

Size dependent creep deformation of elastically constrained compliant metallic joints

The present study proposes a novel method to predict the size dependent creep behavior of bi-material joints formed by constraining a creep compliant metal, such as Sn, having varied thickness ranging from 1.4 mm to 170 µm, between stiff elastic substrates, such as Cu. A dramatic reduction in the secondary or minimum creep rate was observed with decrease in joint thickness. Finite element (FE) analysis using continuum formulations attributed this strengthening to the geometric constraints imposed by Cu, which increases the triaxiality in the joints and hence reduces the effective stress. While FE results were in close agreement with experiment in thick joints, it significantly overpredicted the creep rate of miniature joints. Further, orientation imaging revealed that microstructure varied with length scale, from bulk Sn with multiple grains to miniature joints having a few grains. The additional strengthening was captured using dislocation-based crystal plasticity (CP) creep modeling of Sn-Cu joints by incorporating this microstructural length scale, in addition to the geometric constraints. CP simulations revealed that orientation anisotropy of Sn and the constraints imposed by substrates on dislocation motion lead to higher strength in the miniature joints. Using the insights from FE and CP modeling a unified length scale sensitive model, incorporating both geometric or continuum and microstructural factors, was developed that can accurately predict the creep response of the joints of macro and meso length scale.

ОЦЕНКА РЕСУРСНЫХ ХАРАКТЕРИСТИК ПОЛИКРИСТАЛЛИЧЕСКИХ КОНСТРУКЦИОННЫХ СПЛАВОВ ПРИ ДЕГРАДАЦИИ ПО СОВМЕСТНЫМ МЕХАНИЗМАМ УСТАЛОСТИ И ПОЛЗУЧЕСТИ МАТЕРИАЛА

The problem of assessing the resource characteristics of responsible engineering facilities, taking into account the peculiarities of operational loading conditions, is discussed. The processes of degradation of the initial strength properties of structural materials (metals and their alloys) under combined fatigue and long-term strength mechanisms are considered. A mathematical model describing the processes of cyclic viscoplastic deformation and accumulation of fatigue damage in structural alloys under multiaxial disproportionate modes of combined thermomechanical loading has been developed from the modern positions of mechanics of the damaged medium and fracture mechanics. The model consists of three interrelated components: relations determining the cyclic viscoplastic behavior of the material, taking into account the dependence on the destruction process; evolutionary equations describing the kinetics of the damage accumulation process; criteria for the strength of the damaged material. The variant of the cyclic determining relations is based on the idea of the existence of plasticity and creep surfaces in the stress space and the principle of gradiency of the velocity vectors of plastic deformations and creep deformations to the corresponding surface at the loading point. These equations of state reflect the main effects of the process of cyclic viscoplastic deformation of the material for arbitrary complex loading trajectories. The variant of kinetic equations of damage accumulation is based on introduction of a scalar damage parameter and on energy principles. Ittakes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex modes of combined thermomechanical loading. A joint form of the evolutionary equation of damage accumulation in the areas of low-cycle fatigue and long term strength is proposed. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. The results of numerical modeling of deformation processes and damage accumulation in structural alloys with degradation mechanisms combining fatigue and long-term strength of the material are presented. The results of the comparison of calculated and experimental data showed that the proposed model of the mechanics of the damaged medium qualitatively and with the accuracy necessary for practical calculations quantitatively describes the processes of destruction of hazardous areas of structural elements under the combined actions of degradation mechanisms of fatigue and long-term strength.

Finite element simulation of real cavity closure in cast Ti6Al4V alloy during hot isostatic pressing

The healing behavior of shrinkage cavity inside the cast Ti6Al4V alloy during hot isostatic pressing (HIP) was investigated experimentally by interrupted hot isostatic pressing tests. The X-ray micro computed tomography was used to record the morphology changes before and after hot isostatic pressing. The two-dimensional geometry obtained by the microCT scan was used in simulation to study the evolution of the real shrinkage cavity during hot isostatic pressing. Shrinkage cavities, shrinkage porosity and small gas pores can be effectively eliminated under proper HIP conditions. The two-dimensional morphology in the simulation results agrees well with the experimental results. This study reveals that plastic deformation, creep and diffusion are the main mechanisms of cavity closure during hot isostatic pressing. In addition, the simplified elliptical pores with aspect ratios at different positions were used to replace the real pores to further study the factors affecting the position of dimples after HIP by simulation. It is found that the position of the dimples mainly depends on the aspect ratio of the elliptical pore and the distance between the pore surface and the external surface of the geometric model.

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.

Strength and permanent deformation properties of demolition wastes, glass, and plastics stabilized with foamed bitumen for pavement bases

Foamed bitumen (FB) stabilization is an in-place soil improvement technique that has become popular in recent times. Nevertheless, a lack of understanding exists about the strength and deformation responses of FB stabilized recycled demolition wastes in pavements. This paper aims to introduce the application of FB stabilized recycled demolition waste blends with glass and plastic in the pavement base layer. A suite of laboratory tests was undertaken to evaluate the indirect tensile strength, unconfined compressive strength, California bearing ratio, and permanent deformation behavior of FB stabilized blends. Recycled concrete aggregate (RCA) was blended with recycled glass (RG), recycled plastic (RP), and reclaimed asphalt pavement (RAP) in various percentages and the performance of the FB stabilized blends were characterized through extensive experimental testing. A multi-stage repeated load triaxial (RLT) testing procedure was adopted to evaluate the permanent deformation behavior of the unbound and FB stabilized blends under various stress levels. The ITS, UCS, and CBR values exhibited decreasing trends with increasing the RAP, RG, and RP contents. The permanent deformation of the blends was found to be reduced as a result of FB stabilization, most notably for the blends with higher percentages of RAP, RG, and RP. The permanent deformation responses of the blends were ranked using the shakedown concept that indicated the FB stabilized demolition wastes exhibited relatively stable responses under cyclic loads. The deformation response of the blends with 50%RAP and 7%RP changed from incremental collapse to plastic creep as a result of FB stabilization. The results indicated that the FB stabilization of recycled aggregates is a suitable practice for developing more sustainable roads with enhanced ductility and resistance to rutting.

Room temperature nanoindentation creep behavior of detonation sprayed Fe-based amorphous coating

The Fe-based amorphous amorphous coating with a thickness of 150 μm was prepared by detonation spraying. Under a constant strain rate (0.01 s−1 and 0.50 s−1) and a holding time of 50 s, the Fe-based amorphous coatings were subjected to room temperature nanoindentation creep tests with the penetration depth varying from 50 nm to 500 nm, then the depth-dependent creep deformation mechanism of Fe-based amorphous coating during nanoindentation tests were investigated in detail. The results show that the creep responses of coating obviously depend on the indentation penetration depth (hp) and the indentation load rate, i.e., the creep displacement of coating increases with the increase of hp and loading strain rate. This is mainly due to the rapid increase in the shear transition zone volume of the coating as the hp increases, making it easier for the shear band to be excited to nucleate, and results in an obvious creep deformation. Moreover, at high strain rates, the initiation rate of the free volume in the amorphous coating far exceeds its annihilation rate, resulting in a large amount of excess free volume in the amorphous coating, which makes it easier to excite multiple shear bands in the amorphous coating and cause creep plastic deformation.

A new thermodynamically based model for creep and cyclic plasticity

Creep and cyclic plasticity are two common mechanical behaviors in engineering, and their interaction significantly affects the performance and degradation of corresponding components and structures, especially at high temperatures. How to build a physically based model for describing materials’ mechanical response under complex loading is still a challenge. This study proposes a thermodynamically based model for describing complex creep plasticity behaviors, such as cyclic plasticity and creep. Unlike existing models, the proposed model can describe time-dependent cyclic plasticity and clarify the contribution of various creep mechanisms. Starting from the fundamental thermodynamic laws, Helmholtz free energy and dissipation analysis considering different creep mechanisms are introduced. Then, constitutive equations are derived according to the physical hypothesis of normality of dissipation space. In the proposed model, plastic deformation is thought to depend on static plasticity and creep, which is further decomposed into three parts based on diffusion and dislocation activities, and the contribution of each creep mechanism is explicitly described. Finally, a series experimental data of Ti-6-4 is used to verify the utility of the proposed model. Satisfactory modeling results are obtained, including primary creep, steady creep, power-law breakdown, and creep cyclic plasticity interactions, confirming the validity of the proposed model.