Creep Conditions Research Articles

High-throughput map design of creep life in low-alloy steels by integrating machine learning with a genetic algorithm

Creep-oriented alloy design is a long-standing interesting topic in the field of metal structural materials. However, the high cost for creep testing limits the development efficiency of new alloys using traditional trial-and-error methods. Additionally, the complex mechanism and influencing factors significantly increase the difficulty of physical modeling and simulation-guided design. In this study, an alloy design framework for creep life improvement is established, including two modules: creep life prediction and high-throughput design. For the first module, based on the creep dataset used in this research, the best machine learning model for creep life prediction was obtained by comparison of various machine learning strategies. By this way, the limitation of complex creep mechanism was partially removed and an accurate and generic model was established. For the second module, a genetic algorithm with a filter was used to obtain promising new alloy plans with optimal composition and processing parameters under specific creep conditions. After proving the rationality of this design framework and related design results, map design was used to guide the new alloy development for different creep conditions, and a new multifunctional alloy plan was proposed. This design system could provide preliminary guidance for high-efficiency alloy designs with complex target properties.

Mathematical modeling of relaxation of residual stresses in thin-walled pipelines in the delivery state and after bilateral surface hardening at creep

A mathematical model of reconstruction of the fields of residual stresses and plastic deformations in thin-walled cylindrical tubes in the delivery state and after bilateral surface plastic hardening was developed. It included a method for identification of the model parameters using the example of thin-walled mm tubes made of X18N10T steel based on experimental data for the axial and circumferential components of the residual stress tensor for samples in the delivery state and after bilateral mechanical ultrasonic hardening. The adequacy of the mathematical model for the reconstruction of residual stresses in the thin-walled tubes made of X18N10T steel was verified by the experimental data in the state of delivery and after bilateral surface plastic hardening, taking into account the anisotropy of the distribution of plastic deformation after hardening in the axial and circumferential directions. A method for calculating the two-way relaxation of residual stresses on the outer and inner surfaces of thin-walled tubes under creep conditions was developed based on the generalization of the corresponding method for unilateral hardening. The relaxation process in the thin-walled tubes made of 08X18N9 steel (an early analogue of X18N10T steel) under conditions of thermal exposure, axial tension, internal pressure and the combined action of axial tension and internal pressure was investigated on the basis of the constructed phenomenological creep theory for this steel. A detailed analysis of the kinetics of the fields of residual stresses during creep in the thin-walled samples in the delivery state and after bilateral hardening at different times was performed. It is shown that under these conditions, almost a complete relaxation of residual technological stresses occurs both in the samples in the delivery state and after bilateral surface plastic deformation within 50 hours.

Mathematical model of the stress-strain state of a rod with an inhomogeneous layered structure

The stress-strain state (SSS) of a rod with an inhomogeneous layered structure is considered. On the basis of a brief review and analysis of the current state of research of rod systems, the relevance of the study of the SSS of layered-heterogeneous wooden structures is substantiated, taking into account the presence of different resistance of layers to tension and compression. On this basis, the authors solve the problem of determining the SSS of layered-heterogeneous wooden rods in creep conditions, where factors such as humidity and temperature, as well as the difference in the resistance of wood layers to stretching and compression are taken into account. When solving the problem, the mechanical-sorption creep of wood is also taken into account.

Crack growth in rotor P2M steel at elevated temperature

The paper presents a comprehensive computational and experimental study of the crack growth rate during the creep-fatigue interaction in compact specimens of P2M steel at a temperature of 550 °C. The theoretical part of the study consisted in the formulation of the fracture resistance parameters through the classical and new constitutive equations of the cracked body state taking into account the accumulation of damage. Numerical calculations included the determination of the stress-strain state fields for the conditions of elasticity, plasticity, and creep, as well as the distributions of nonlinear stress intensity factors and the C*-integral along the crack length and along the crack front for each tested compact specimen. The interpretation of the experimental results for specimens of the same shape in plan but different thicknesses is given in terms of elastic and nonlinear stress intensity factors, taking into account the accumulation of creep damage. It was found that the crack growth rate under the fatigue and creep interaction increases monotonically as the crack size increases in comparison with harmonic fatigue by an order of magnitude or more on specimens of the same geometry. Taking into account the damage through the creep stress intensity factors causes differences in the cyclic fracture diagrams. The superposition of fatigue and creep contributions in terms of load holding time shows an order of magnitude increase in the total crack growth rate compared to the interpretation of the experimental data in terms of pure creep.

Estimation of C* Integral for Mismatched Welded Compact Tension Specimen.

The C* integral for the compact tension (CT) specimen is calculated using the estimation equation in ASTM E1457-15. This equation was developed based on the assumption of material homogeneity and is not applicable to a welded CT specimen. In this paper, a modified equation for estimating the C* integral for a welded compact tension (CT) specimen under creep conditions is proposed. The proposed equation is defined on the basis of systematically conducted extensive finite element (FE) analyses using the ABAQUS program. A crack in the welded CT specimen is located in the center of the heat-affected zone (HAZ), because the most severe type IV cracks are located in the HAZ. The results obtained by the analysis show that the equation for estimating the C* integral in ASTM E1457-15 can underestimate the value of the C* integral for creep-soft HAZ and overestimate for creep-hard HAZ. Therefore, the proposed modified equation is suitable for describing the creep crack growth (CCG) of welded specimens.

Out-of-plane effect on the sharp V-notch tip fields in power-law creeping solids: A three-dimensional asymptotic analysis

Understanding the three-dimensional (3D) effect on the notch tip field is important for the fracture mechanics analysis of engineering materials and structures. This paper presents an asymptotic solution for 3D sharp V-notched structures subjected to mode Ⅰ creep loading condition. The asymptotic solution, including the singularities and stress field distribution functions, is obtained by introducing the out-of-plane constraint factor. It is found that the singular behavior is related to the notch angle, material creeping exponents, and the out-of-plane constraint factor. Specifically, singularity changes sharply near the free surface. KNt-Tz solution is presented to describe the mode Ⅰ stress field around the tip of 3D V-notch under creep condition. Its accuracy and advantages compared with plane strain or plane stress solutions are validated by finite element analysis. The difference between effective ranges of KNt-Tz solution and that of plane strain solution is clarified. Moreover, the effect of the creep time on the stress field is also shown. The solutions given in this paper could be beneficial for understanding the 3D effect on notch considering creep loading conditions.

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.

Estimation of creep and damage accumulation in thin-walled concrete structures

Long-term strength investigations of construction elements made of reinforced concrete (RC) under creep conditions are considered. A calculation method that allows determination of the load-bearing capacity and long-term strength of RC thin-walled elements under short- and long-term loading is proposed. The method is based on a combination of the finite-element method for solving boundary value problems at each time step and the finite-difference method for integrating the initial value problems. The numerical results are compared with experimental data obtained in the loading–unloading mode during long-term deformation. An example of creep and damage calculations in the RC coating and recommendations for parameter selections are given.

Computer Modeling of the Deformation of Structural Elements Under the Conditions of Creep in the Course of Hydrogenation of the Metal Under Complex Loading

Computer Modeling of the Deformation of Structural Elements Under the Conditions of Creep in the Course of Hydrogenation of the Metal Under Complex Loading

Determination of technological parameters of rock freezing systems based on the condition of maintaining design thickness of ice wall

Artificial freezing ensures the formation of a temporary ice wall around the shaft under construction, which prevents groundwater penetration into the shaft and increases the strength of rocks around the unsupported walls of the shaft until the permanent support is erected. The purpose of the study is to carry out thermotechnical calculation of ice wall with subsequent theoretical analysis of changing ice wall thickness with shifting to the passive freezing stage. The idea of the study is to determine these technological parameters based on the condition of maintaining the design ice wall thickness at the stage of passive freezing. The methodology and results of thermotechnical calculation of ice wall for the clay layer as applied to the case of the shafts under construction of a potash mine in the Republic of Belarus are presented. The thermal calculation of the ice wall was carried out numerically in the ANSYS software package using the finite element method. The findings of the numerical multiparameter modeling allowed theoretical analysis of ice wall thickness decrease with shifting to the passive freezing stage with higher brine temperature. The decrease in ice wall thickness was studied both during normal operation of the freezing station and at emergency operation mode caused by the failure of one of the freezing columns. Special attention in the analysis was paid to studying the influence of the duration of the active freezing stage and the distance between the columns on the decrease in the ice wall thickness. When analyzing changes in ice wall thickness at different distances between the freezing columns, it was found that the most common column spacing in the range from 1.1 to 1.3 m requires observing restrictions on the duration of active freezing to prevent a critical decrease in ice wall thickness during the passive freezing stage or decreasing the distance between the freezing columns. In this case, preservation of positive dynamics of ice wall thickness growth is ensured. For the clay layer considered in the study and the distance between the columns from 1.1 to 1.3 m, the minimum time of active freezing is also about 4.3 months. As a result of the analysis, the technological parameters of the freezing system (duration of the active freezing stage and the distance between the freezing columns) were determined, at which the ice wall thickness at the passive freezing stage did not become lower than the minimum permissible values calculated based on the strength and creep conditions.

Utilization of Waste Glass Powder and Glass Composite Fillers in Asphalt Pavements

Today, researchers around the globe are looking for suitable alternatives of conventional fillers which can form flexible pavements with satisfactory engineering performance in an environmental friendly and cost-effective manner. This study investigated the engineering, economical, and environmental viability of recycling waste glass powder (GP) and glass-hydrated lime (GL) composite as alternative fillers, in place of stone dust (SD). All fillers were characterized, and asphalt concrete mixes incorporating them at different proportions (4–8.5%) were designed using the Marshall mix design method. The engineering performance of asphalt mixes was analyzed using the static creep analysis, indirect tensile fatigue test, Cantabro test, modified Lottman test, resilient modulus test, mixing time analysis, and boiling water test. Additionally, the design of single km of two-lane flexible pavements utilizing aforesaid mixes was done as per the mechanistically empirical method suggested in IRC 37 guideline. Finally, the economic and environmental analysis was done by comparing their material cost and global warming potential (GWP). GL and GP mixes exhibited better resistance against rutting, fatigue, and low temperature cracking at lower optimum asphalt content than SD mixes. However, GP mixes also displayed poor moisture resistance and adhesion due to the high amount of silica in GP. GL mixes had satisfactory moisture resistance up to 7% filler content due to the fine nature and anti-stripping properties of hydrated lime. The pavement containing GL and GP fillers also reduced material cost and GWP up to 35% while consuming up to 74 tons of GP.

Experimental study on aluminum bronze coating fabricated by electro-spark deposition with subsequent ultrasonic surface rolling

Aluminum bronze coatings were prepared on the as-cast substrate of the same material by electro-spark deposition (ESD), and the coatings were treated by ultrasonic surface rolling (USR). The effects of USR on the microstructural, mechanical, and tribological properties of the ESD coatings were investigated. The microstructure, phase composition, nanoindentation, microhardness, surface residual stress, and tribological properties of the coatings were experimentally determined and characterized in detail. The USR treatment with different static pressures (1.0 MPa, 2.5 MPa, 4.0 MPa) was employed to reveal the mechanism of microstructure evolution and surface modification. The results showed that the original columnar crystal structure of the coatings treated by USR was deflected, elongated, and even broken, resulting in the grain refinement of the coatings. The hole defects were healed after USR2.5 treatment. The phase composition of the coatings changed from β′ (Cu3Al) phase to α (Cu) and κ (Fe3Al) phase because of the heat released at USR. The nanohardness, static creep resistance, and microhardness of the ESD coatings were effectively promoted by USR. The nano-hardness of the ESD coating after USR4.0 treatment (4.4 GPa) was 1.38 times and 2.2 times that of the ESD coating and the as-cast alloy respectively, and the creep displacement (5.193 nm) was 27.2% and 18.5% that of the ESD coating and the as-cast alloy respectively. The residual tensile stress of the ESD coatings was eliminated after USR treatment and converted into the residual compressive stress. The surface residual stress of the ESD coating after USR2.5 was −337.16 MPa. After USR2.5 treatment, the wear loss of ESD coating reduced by about 1/2, which was attributed to the healing of hole defects, residual compressive stress, and the increase of the hardness.

Effects of bentonite, diatomite and metakaolin on the rheological behavior of 3D printed magnesium potassium phosphate cement composites

In this study, the effects of bentonite, diatomite, and metakaolin on the rheological behavior of 3D printed magnesium potassium phosphate cement composites (3D printed MKPCs) were investigated through characterization of the material properties, including yield stress, plastic viscosity, thixotropy, and creep. The thixotropy was studied by assessing the hysteresis area and thixotropic parameters under dynamic shear tests. The structural stability was evaluated by assessing the creep recovery, modulus of the fresh cement pastes, and deformation rate of the printed specimens. The results showed that diatomite played a key role in enhancing the thixotropy of the 3D printed MKPCs. Bentonite was found to significantly enhance the static yield stress, creep resistance, and shape stability owing to its high water absorption in the 3D printed MKPCs. Isothermal calorimetry testing showed that all additives accelerated the hydration reaction, which was confirmed by the increased compressive strength. Finally, a schematic representation of the yield stress evolution with time was proposed, revealing the beneficial effects of three additives in MKPCs during different stages of 3D printing.

Experimental study on creep of double-rock samples disturbed by dynamic impact

During underground mining, rock deformation may be time-dependent and is reflected in rheological behavior. These changes may be also accelerated by dynamic disturbances, such as rock blasting, which can trigger mining-induced hazards such as rockburst and a falling roof. A combined double-rock sample was analyzed to clarify the rockburst mechanism resulting from interaction between roof and pillar. In this work, a double-rock sample was subjected to a combined creep and dynamic loading condition. Based on the results, an ideal mechanical model was developed to describe the interaction between double rock samples loaded in series to simulate interaction between the pillar and roof. To test the model, a double-rock sample composed of granite and sandstone was loaded in series under creep and dynamic loading and the effect of dynamic disturbance on the creep process was quantified. Under the same creep stress, a higher impact dynamic disturbance resulted in shorter failure time. Under the same dynamic disturbance, the specimen failed faster with increased strain energy of rebound of the granite sample under increasing creep stress. The energy dissipation rate and energy transfer efficiency are defined to quantify the energy dissipation of double-rock sample and energy transfer between strong and weak sample. Based on the quantification of energy transfer efficient, it confirms that the ratio of stiffness of the double-rock sample should be a indicator for the energy transfer between strong and weak samples. The granite sample had higher uniaxial compressive strength, so it did not fail during the loading process. For the plaster-granite sample, the granite released most energy to accelerate the unstable failure of plaster. The granite-granite sample failed simultaneously and released most strain energy violently. The results can provide a reference to evaluate the delayed rockburst of rock subjected to creep stress.

Influence of All-Round Forging under Short-Term Creep Conditions on the Structure and Mechanical Properties of the Al7075/10SiCp Composite with an Aluminum Matrix

Influence of All-Round Forging under Short-Term Creep Conditions on the Structure and Mechanical Properties of the Al7075/10SiCp Composite with an Aluminum Matrix

Long-term stiffness prediction of particle filled polymers by dynamic mechanical analysis: Frequency sweep versus creep method

The long-term service life of polymers can be estimated with much shorter experiments by applying the time temperature superposition principle (TTS). In this approach, data is obtained at different temperatures, usually through a stepped isothermal method (SIM) on the same sample. Dynamic mechanical analysis (DMA) instruments offer two different measurement methods to obtain SIM data: (i) static creep tests and (ii) dynamic frequency sweeps. This paper compares both methods on highly graphite filled polypropylene. Our studies on reproducibility of each method show that the uncertainty for 20 year prediction can be lower than 6% for both methods. While creep-based tests require a shorter experimental time, frequency sweep based tests show a lower scatter on the final result. The two main factors introducing uncertainty on the end results are related to (i) the reproducibility of the experimental raw data and (ii) the TTS optimisation using shift factors. The optimisation of the shift factors by a numerical method improves the accuracy of the master curve. By comparing creep and frequency sweep SIM, it shows that for predictions of one decade, the methods deliver very comparable results (less than 10% difference). For longer predictions, the methods differ and are not interchangeable. Furthermore, DMA was also effectively used as a three-point bending setup, providing information about strain rate sensitivity and the linear visco-elastic region using the same test setup and same sample dimensions as for TTS.

Creep damage and interaction behavior of neighboring notches in components at elevated temperature

Previous studies have focused on creep damage evolution behavior of components with one isolated notch, but two or more notches may be placed at the components due to the function requirements, the limitation of the structural size, or may be induced by chemical/physical damage processes. In this work, numerical analyses and creep tests on creep damage and interaction behaviors of neighboring notches in components under creep conditions are investigated. Effects of structural configurations on creep damage and interaction behavior are discussed. Results indicate that the notch still indicates a strengthening effect on rupture life of components despite the interaction behavior of the two adjacent notches. A negative interaction effect on rupture life of components with two notches is found. Creep damage and interaction behaviors of the components with two neighboring notches of various distance values are related to structural configurations of the components, including notch acuity ratio, notch open angle and notch depth.

The regularities of creep deformation and failure of the VVER’s pressure vessel steel 15Kh2NMFA-A in air and argon at temperature range 500–900 °C”

• Creep tension tests of VVER vessel steel 15Kh2NMFA-A at 500–900 °C in air and Ar and duration up to 100 hrs. • Test specimens were made of the fragment of unirradiated VVER-1000 RPV. • Secondary creep rate and time to failure were determined. • Singularity of creep process is due to the phase transformations in range from 770 to 820 °C. • The creep mechanism in 15Kh2NMFA-A steel is the same in temperature range from 700 to 900 °C. The paper presents the results of the determination the creep strength properties and both the microstructure and regularities of high-temperature creep behavior of unirradiated ferritic-pearlitic steel 15Kh2NMFA-A within the temperature range from 500 to 900 °C and the creep process duration up to 100 h. It is extremely necessary to obtain the such experimental data characterizing the creep strength properties of this steel under the condition of high-temperature creep to predict the behavior of the VVER’s RPV structure in the severe accident when the pressure vessel structure undergoes both force and high-intensive thermal action from the melted core materials. This ferritic-pearlitic steel 15Kh2NMFA-A (2% Cr, 1% Ni, 0.5% Mn, 0.5% Mo) is one of basic steel types used in fabrication of reactor pressure vessels of the VVER type designed in Russia. This investigation included several series of creep tension tests of the specimens made of 15Kh2NMFA-A steel. In addition, an analysis of the microstructure of the steel under study was carried out for a number of samples after the creep tests. For the creep tests, several sets of specimens made of the fragment of unirradiated VVER-1000 RPV. The specimen’s working part was 8 mm in diameter and the gauge length was ~50 mm. Isothermal creep tests under uniaxial tension were carried out in air up to 650 °C and in shielding argon medium within the temperature range from 700 to 900 °C. Basing on mathematical processing of test results, the values of steady-state creep rate and the values of corresponding parameters ( B ) and ( n ) were determined. The analysis of obtained results showed that within the temperature range from 750 to 900 °C, the creep mechanism of 15Kh2NMFA-A steel is the same. A sharp variation of parameters ( B ) and ( n ) (describing the steady-state creep rate of this steel) is observed within the temperature range from 500 to 750 °C. At temperature of more than 750 °C, the value log( B ) is varied within the range from −9.65 to −8.48 and the values of parameter ( n ) are within the range from 4.72 to 4.82. The results of creep tests show that both creep failure time and secondary creep rate values of the 15Kh2NMFA-A steel at 800 °C coincide very closely with the analogous ones for the specimen tests under analogous conditions at 850 °C. It is obvious that such singularity is due to the simultaneous presence of several phases in the 15Kh2NMFA-A steel during the phase transformations in temperature range from 770 to 820 °C. The formation of oxide layer on the specimen surface under the creep condition in air at temperature of 800 °C and more than the given one changes considerably the model of deformation and fracture of the investigated steel under the creep condition. We defined the parameters for the constitutive equation of the Larson-Miller type (LMP) for this steel by statistical processing the results of creep tests within the temperature range from 500 to 900 °C. The analysis of the obtained LMP equation for this steel revealed an important fact that within the range of temperature values Ac1-Ac3, where microstructure transformations of steel take place, we observe essential differences between experimental data and corresponding values defined by using the obtained LMP dependence. When estimating rupture time of this steel under the condition of high-temperature creep, such a difference between the specified values results in a considerable error (by more than the order of magnitude).

Effect of nickel-based filler metal types on creep properties of dissimilar metal welds between Inconel 617B and 10% Cr martensitic steel

Creep rupture behaviors of dissimilar metal welds (DMWs) between Inconel 617B and 10%Cr martensitic steel with two different filler metals Inconel 617 and 82 were comparatively investigated at 600 °C to study the effect of nickel-based filler metal types on creep properties. Both the crept DMWs ruptured on the steel side prematurely compared to steel base metal (BM). The failure mode of DMW shifts from interfacial failure at weld metal (WM)/steel interface to Type Ⅳ crack in the intercritical heat-affected zone (ICHAZ) of steel with longer rupture life at the same creep condition by replacing Inconel 617 filler metal with Inconel 82. Type Ⅳ crack was related to matrix softening and fine prior austenite grain boundaries (PAGBs) scarcely decorated by precipitates in the ICHAZ. Interfacial failure was controlled by creep voids nucleation, growth and coalescence at the interface, while oxide notch only triggered crack initiation. The failure mode shift is because Inconel 82 filler metal compared to Inconel 617 can achieve slighter creep deformation concentration in the HAZ closely adjacent to interface and avoid interfacial failure of the DMW due to the lower creep strength gradient across the interface it induced. Based on this study, creep strength compatibility should be considered when selecting nickel-based filler metals for similar DMWs to obtain prolonged creep life.

Experimental Studies of Creep of Unidirectional Composite Material of Composite Overwrapped Pressure Vessel

Experimental studies of creep of a composite material of a composite overwrapped pressure vessel are presented. Composite material was an unidirectional tape based on IMS60 carbon fibers and epoxy resin. Creep tests were carried out at various loads and temperatures, taking into account the stress level in structure and measured glass transition temperature of the material. For testing, special equipment was used to install specimens in the grips of the testing machine. According to the test results, the creep curves were obtained. Creep curves according to Findlay's model are presented. For the stage of steady-state creep, the variation of the creep rate at different temperatures were determined, and the average value of the creep rate was calculated for all tested specimens. The results of the obtained data can be used in numerical models for predicting strains of a composite overwrapped pressure vessel under creep conditions