Creep Loading Research Articles

Influence of Processing Technology on Creep Damage of Ultrafine-Grained Titanium

This work deals with the influence of the processing technology of ultrafine-grained titanium by the application of severe plastic deformation (SPD) on its degradation processes in creep at elevated temperatures. Commercial grade titanium heats with different content of carbon were processed by multi-stage rolling. Creep tests under constant tensile stress were performed in a protective atmosphere of argon at a temperature of 673 K with application of different stress levels. The microstructure of the fractures creep specimens were analysed by metallographic and fractographic analyses using scanning and transmission electron microscopy (SEM, TEM). Under the same creep loading conditions the heat with higher content of carbon exhibited better creep resistance due to stronger effect of precipitation hardening by titanium carbides.

Damage and Fracture in Creep of a Cast B1914 Superalloy

Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)3B2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M3B2 boride and matrix.

Effect of the Welded Zone Property on Behavior of a Dissimilar Superalloys Friction Joint under Creep Loading at Elevated Temperature - A Simulation Approach

The forged superalloy Inconel 718 and cast superalloy Mar-M247 were joined via friction welding. The welded zone was recrystallized and formed a thin representative layer on the forged superalloy side adjacent to the interface. The effect of the creep property of the welded layer on the behavior of the joint under creep loading was investigated via numerical simulations. Despite the welded layer being relatively narrow (approximately 200 mm), it significantly effects the stress state and the creep damage parameters in and around the welded zone. When the creep rate of the welded layer is lower than that of the base alloy, the stress concentration is dramatically increases adjacent to the interface. However, the creep rate of the welded layer creep rate is higher than that of the base alloy, the interface stress concentration is reduced. The creep strain and stress exhibited an out-phase relation.

Experimental Investigation of a Transparent Interface Material for Glass Compression Members

In this experimental research a transparent thermoplastic manufactured by the DOW Corporation and known as Surlyn is investigated for use as an interface material in fabrication of an all-glass pedestrian bridge. The bridge is modular in construction and fabricated from a series of interlocking hollow glass units (HGU) that are geometrically arranged to form a compression dominant structural system. Surlyn is used as a friction-based interface between neighbouring HGUs preventing direct glass-to-glass contact. An experimental program consisting of axial loading of short glass columns (SGC) sandwiched between Surlyn sheets is used to quantify the bearing capacity at which glass fracture occurs at the glass-Surlyn interface location. Applied load cases include 100,000 cycles of cyclic load followed by 12 hours of sustained load followed by monotonic load to cracking, and monotonic loading to cracking with no previous load history. Test results show that Surlyn functions as an effective interface material with glass fracture occurring at bearing stress levels in excess of the column-action capacity of an individual HGU. Furthermore, load cycling and creep loading had no effect on the glass fracture capacity. However, the load history had a nominal effect on Surlyn, increasing stiffness and reducing deformation.

Study on Rheological Properties of Coal Rock Containing Gas under Disturbance Conditions

In view of the dynamic phenomenon that coal and rock are susceptible to external impact disturbance in the mining process, combined with the rheological hypothesis mechanism of coal and gas outburst, the RLSS-II-type triaxial loading creep test system of gas-containing coal and rock developed by ourselves is used to carry out the conventional triaxial rheological test of gas-containing coal and rock and the rheological disturbance effect test of gas-containing coal and rock under impact disturbance. The strength limit neighborhood of gas-containing coal and rock is determined, and the gas-containing coal and rock entering the strength limit neighborhood are subjected to different impact disturbances. The experimental results show that, (1) under the confining pressure of 0 MPa, 2.5 MPa, and 5 MPa, the longitudinal deformations of gas-bearing coal and rock are 32 mm, 27 mm, and 22 mm, respectively, indicating that the deformation of coal and rock will be affected by confining pressure, and with the increase of confining pressure, the deformation will decrease. (2) In the test, the deformation of coal and rock in the late stage of uniform creep stage can be regarded as a strain threshold. Before this threshold, the strain of coal and rock is not obvious, and the deformation is only 1.1 mm. After exceeding a threshold, the deformation of coal and rock is 9 mm, and the deformation increases significantly. Then, it enters the accelerated creep stage quickly and finally damages. The vicinity of this threshold is called the strength limit neighborhood of coal and rock containing gas. (3) The gas-bearing coal and rock without entering the strength limit neighborhood and entering the strength limit neighborhood are changed by confining pressure and different impact disturbance, respectively. It is found that, whether in the strength limit neighborhood or outside the strength limit neighborhood, the confining pressure has an effect on the strain, but the influence is not large. Under different impact disturbance, the deformation of coal and rock within and outside the strength limit neighborhood is 8 mm and 0.4 mm, respectively, and the deformation changes obviously, indicating that the impact disturbance has a great influence on the deformation of coal and rock within the strength limit neighborhood, and the coal and rock with large impact disturbance are destroyed before the coal and rock with small impact disturbance, indicating that the greater the impact disturbance, the shorter the time required for destruction.

Creep properties and constitutive model for salt rock subjected to uniaxial trapezoidal cyclic loading

Uniaxial trapezoidal cyclic loading creep tests with different loading and unloading rates are performed on salt rock to study its creep properties. The results indicate that the axial strain produced by salt rock under trapezoidal cyclic loading occurs mainly in the 1st cycle, and other cycles generate less axial strain. The existence of constant high pressure stage leads to an obvious horizontal segment in the stress-strain curve, while constant low pressure stage has little effect on its stress-strain curve. The stress-strain curve in the 1st cycle includes elastic stage, plastic stage, creep stage and deformation recovery stage. The axial strain generated at plastic deformation stage gradually tends to 0 with increasing cycle numbers, then the curve in a single cycle includes only elastic stage, creep stage and deformation recovery stage. When the cycle numbers is small, the irreversible deformation produced by salt rock in a single cycle mainly consists of plastic deformation produced at the loading stage and creep deformation generated at the constant high pressure stage. After several cycles, the irreversible deformation in a single cycle mainly consists of creep deformation generated at the constant high pressure stage. With increasing cycle numbers, the minimum value of creep rate in a single cycle basically does not change, and the maximum value of creep rate decreases firstly and then keeps constant. Moreover, with increasing loading rate, the mean value of minimum creep rate basically remains unchanged and the mean value of maximum creep rate gradually increases. With increasing unloading rate, both the mean values of minimum and maximum creep rate decrease gradually. Finally, the creep equation for the Burgers model subjected to trapezoidal cyclic loading is deduced, and the reasonableness of which is checked.

Influence of Moisture Content on Creep Mechanical Characteristic and Mic-Fracture Behavior of Water-Bearing Coal Specimen

The most challenging aspect of constructing an underground reservoir is ensuring that the safety coal pillar has long-term stability. It is of great significance to study the creep mechanical behavior of coal pillars under water-rock interaction for the construction of underground reservoirs. Five kinds of coal specimens with different moisture contents (dry, 2.42%, 5.53%, 7.55%, and saturated) were prepared. The effect of moisture content on the creep mechanical behavior of coal specimens was studied by graded loading, and the acoustic emission signals during the creep loading of coal specimens were monitored. The results showed that with the increase in moisture content, the instantaneous strain and creep strain of coal specimens gradually increase, and the total time of the creep test, creep failure stress, and long-term strength generally decreases. The creep failure stress to peak strength and the ratio of long-term strength to peak strength were 73%~85% and 57%~65%, respectively, which are basically independent of water content. As the water content of the coal increased, the cumulative acoustic emission energy decreased, and the failure form of the coal specimen gradually shifted from tension failure ( ω = 0 % and 2.42%), tension shear composite failure ( ω = 5.53 % ) to shear failure ( ω = 7.55 % and 10.08%). The research results can be used as a reference for the design and strength evaluation of coal pillars for underground reservoirs in coal mines.

Enhancement in creep resistance of pristine polystyrene with incorporation of exfoliated 2D graphene nanosheets at low filler loading

Determination of the reliability and durability of polymers in structural applications is highly dependent on the resistance to time-dependent plastic deformation. In this study, creep behaviour and enhancement in creep resistance of polystyrene-graphene nanocomposites is investigated at low filler loading. Herein, 2D material (graphene) is synthesised by the liquid-phase exfoliation method ( LPE) using the tetrahydrofuran ( THF) solvent via batch sonication. Samples are characterised by Scanning Electron Microscopy ( SEM) for morphology analysis and X-rays Diffraction ( XRD) for identification of polystyrene-graphene (PS-G) peaks in nanocomposites. Additionally, Atomic Force Microscopy ( AFM) is employed to quantify sheet’s thickness and Optical Microscopy ( OM) to corroborate the dispersion of 2D sheets. The creep resistance of PS-G nanocomposites is measured at room temperature (25°C) by incorporating 2D sheets of flake length ∼359 μm with 0.1, 0.3, 0.5, 0.7 and 0.9 wt.%. A significant enhancement in the time to withstand constant creep load (1 N) is observed. The creep resistance of the samples exhibits a maximum increase of 79%, 258.24%, 647.25%, 417.58% and 760.44%, respectively, compared to pristine polystyrene. This increase in creep resistance resulted from intense interfacial contact between polymer chains and filler accompanied by adequate scattering/dispersion in the polymer matrix.

Influence of dynamic disturbance on rock creep from time, space and energy aspects

In order to explore the influence of dynamic disturbance on creep behavior of rock, the creep damage evolution in rock under the dynamic disturbance are reproduced with creep damage model. In this model, the constitutive law of rock under combined creep and dynamic loading condition is implemented based on elastic damage principle and Norton-Bailey equation in COMSOL Multiphysics. The numerical results show that the dynamic disturbance alters the creep behavior of rock in three aspects, i.e., time, space and energy. In temporal aspect, dynamic disturbance not only accelerates the creep damage evolution of rock, but also leads to the tensile damage accounting for the main damage mode. The tensile damage mode mainly occurs during the unloading stage of dynamic disturbance, and the tendency of transition becomes evident under more dynamic disturbances. In spatial aspect, the dynamic disturbance may facilitate the development of rock damage, resulting in unstable rock failure. In energy aspect, the dynamic disturbance causes creeping rock to absorb and release energy in a short time, which can be quantified with the energy dissipation rate. The residual deformation caused by the dynamic disturbance is also exponentially related to the energy dissipation rate.

Unified creep constraint parameter solutions for surface cracks in plates under tensile and bending loads

AbstractIn order to accurately assess creep crack initiation (CCI) and growth (CCG) life for elevated temperature cracked structures, both out‐of‐plane and in‐plane constraints need to be considered, and the unified creep constraint parameter solutions should be investigated. In this work, the new unified creep constraint parameter Ac values for plate structures with various surface crack sizes subjected to tensile and bending loads have been investigated and obtained using three‐dimensional (3D) finite element (FE) analysis. The effects of material creep properties, crack sizes, loading modes, and plate thickness on the parameter Ac have also been analyzed. The estimation equation of Ac for materials with different creep exponent n has been fitted. The parameter Ac solutions can be used in CCI and CCG life evaluation of the C*‐Ac two‐parameter approach for cracked plates subjected to tensile and bending loads.

Long-term creep behavior of NC filled UHPC tube composite column

Because normal concrete (NC) filled ultra-high-performance concrete (UHPC) tube (NFUT) composite column can avoid additional formwork, save construction time, and cost, and increase bearing load capacity, it shows great potential in the application of engineering structures. This study aims to investigate long-term creep behavior of the NFUT composite column. The creep test was conducted on the NC column, UHPC tube, and composite column with different axial loads, which were applied by the steel strands with a double-tensioned technology to reduce the prestress loss and achieve the creep loading. Also, a numerical model was established for reproducing the test results, and several assumptions (such as perfect bond between the UHPC tube and NC core, and computing models for material creep and shrinkage) were adopted in the numerical model. It is found that the nominal creep coefficient of the NFUT composite column after 433-day loading is 36% of that of the NC column due to the contribution of the UHPC tube. Although slight differences can be observed in the local region between the numerical and test nominal creep coefficient curves probably due to the varying temperature and relative humidity, they generally show good agreement. The validated numerical model was used for parametric studies. The effect of the longitudinal reinforcement ratio, UHPC area ratio, loading age, and NC core stress level on the long-term deformation and stress redistribution was discussed. As the reinforcement and UHPC area ratios increase, the elastic and total strains decrease, and their decreasing degrees gradually become small. Moreover, they show significant effect on the UHPC and NC stress changes due to the stress redistribution (i.e., UHPC and NC stress changes decrease as the two factors increase). Also, properly increasing loading age is beneficial for decreasing the creep deformation and the stress level does not affect the long-term total strain development trend.

Proposed Material Properties, Allowable Stresses, and Design Curves of Diffusion Bonded Alloy 800H for the ASME Code Section III Division 5

AbstractIncreased interest in compact heat exchangers (CHXs) to serve as intermediate heat exchangers of very high temperature reactors resulted in significant research and development on their design, analysis, and construction. Printed circuit heat exchangers are a type of CHXs with high thermal efficiency and compactness achieved through diffusion bonding a stack of etched plates with millimeter scaled channels. The diffusion bonding process changes the microstructural and mechanical properties of the wrought metal plates. The current nonnuclear design code ASME section VIII, division 1 captures the material property change through a “joint efficiency factor.” However, the current nuclear design code ASME section III, division 5 does not address or support the diffusion bonded material properties. Hence, there is a need to develop allowable stresses, isochronous curves, and fatigue life curves for various diffusion bonded alloys. In this study, Alloy 800H material was selected to establish the diffusion bonded material properties under tension, creep, fatigue, and creep-fatigue loads at elevated temperatures in the range 550–760 °C. A set of tests on diffusion bonded Alloy 800H (DB 800H) were performed and the acquired data are used in developing allowable stresses Sy, Su, Sr, Sm, St, Smt, So, isochronous curves and fatigue life curves according to the ASME section III, division 5 requirements. This paper also presents detailed procedures used in developing the ASME code section III division 5 design provisions for diffusion bonded Alloy 800H.

A small‐scale creep test for calibrating an efficient lifetime model for high pressure turbine blades

AbstractJet engines of airplanes are designed such that in some components damage occurs and accumulates in service without being critical up to a certain level of damage. Since maintenance, repair, and component exchange are very cost‐intensive, it is necessary to predict efficiently the component lifetime with high accuracy. A former developed lifetime model, based on interpolated results of aerodynamic and structural mechanics simulations, uses material parameters estimated from literature values of standard creep experiments. For improved accuracy, an experimental procedure is developed for the characterization of the short‐time creep behavior, which is relevant for the operation of turbine blades of jet engines. To consider microstructural influences resulting from the manufacturing of thin‐walled single crystal turbine blades, small‐scale specimens from used turbine blades are extracted and tested in short‐ and medium‐time creep experiments. Based on experimental results and literature values, a creep model, which describes the fracture behavior for a wide range of creep loads, is calibrated and is now used for the lifetime prediction of turbine blades under real loading conditions.

Flexural Creep Response of Hybrid GFRP-FRC Sandwich Panels.

This work was developed within the scope of the research project “Easyfloor—Development of composite sandwich panels for building floor rehabilitation”, which aims at developing an innovative hybrid sandwich panel as an alternative construction system to conventional floor solutions, mainly for building rehabilitation. The developed hybrid sandwich panel is composed of a top face layer of steel-fibre-reinforced self-compacting concrete (FRC), a core of polyurethane (PUR) closed-cell foam, a bottom face sheet, and lateral webs of glass-fibre-reinforced polymer (GFRP). Full-scale experimental tests on the developed sandwich panels were carried out to characterize their short- and long-term (creep) flexural behaviour. The present work includes a detailed description of the developed panels and the experimental programme carried out and presents and discusses the relevant results. The experimental results showed an almost linear behaviour up to failure. The creep tests were carried out for a period of 180 days, using a creep load equal to 20% of its ultimate loading capacity. An average creep coefficient of 0.27 was obtained for this period. The composed creep model used to simulate the sandwich panel’s creep deflections by considering the individual viscoelastic contributions was able to predict the observed structural response with good accuracy.

Energy evolution and failure characteristics of red sandstone under discontinuous multilevel fatigue loading

This paper explores the energy storage and dissipation, damage evolution and failure mode characteristics of red sandstone with prefabricated cracks under discontinuous multilevel fatigue (DMLF) loading. It was found that the three energy density parameters increase quadratic polynomial with the increase of stress level. The insertion of creep loading increased energy dissipation during fatigue deformation, but had no significant effect on elastic energy. There was a linear energy storage and dissipation law in the sample loading process, and the strength variation laws of samples with different angles were explained in terms of energy. Fatigue damage accumulated slowly and then increased rapidly with loading, and a damage evolution model was established based on dissipated energy. Moreover, the 0° and 30° samples showed shear failure, while the 45°, 60°, and 90° samples showed tensile-shear composite failure. Combined with macro-micro analysis, the damage mechanism of the samples under DMLF loading was further revealed.

Effect of thermal exposure on microstructure evolution and mechanical properties of cast beta-solidifying TiAl-based alloy doped with Gd

Evolution of an initial nearly-lamellar (γ+α2+β0) structure of a new cast β-solidifying TiAl-based alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd (at.%) after isothermal exposures at 800 °C for up to 10,000 h has been systematically investigated using SEM, TEM, EDS, and XRD analyses. During the exposures the alloy microstructure degraded by the α2 laths and β0 phase dissolution, resulting in coarsening of lamellar structure and rising of the γ phase volume fraction. A 2.5- and 3-fold increase in the number of nano-sized gadolinium oxide (Gd2O3) particles have been observed after 10,000-h exposure in lamellar colonies and in grain boundaries, respectively. The gadolinium oxides had monoclinic (C2/m) and cubic (Ia-3) structures and precipitated predominantly along γ/α2 and γ/β0 interfaces and inside the γ lamellae. Furthermore, the applied creep loading was found to contribute significantly to the precipitation process of Gd2O3 nano-particles, since dislocation networks being the sites for their preferred formation. Nanohardness of the γ phase increased from 4.4 ± 0.3 to 5.0 ± 0.4 GPa after the exposures, but no changes were observed for the β0 phase. As a result of structure evolution the tensile strengths reduced by 10–12 and 11–14 % measured at 20 and 750 °C testing temperatures, respectively. Meanwhile, the plastic elongation values remained essentially unchanged. We interpret this as insignificant effect of the precipitation of Gd-rich oxide nano-particles on the alloy ductility.

Effect of creep load on fatigue behavior and acoustic emission characteristics of sandstone containing pre-existing crack during fatigue loading

To study the effects of creep loading on the fatigue behavior and acoustic emission characteristics of jointed rock masses during fatigue loading, a series of multilevel cyclic loading and unloading tests and fatigue-creep tests were carried out on jointed sandstone specimens. Experimental studies were carried out using acoustic emission (AE), digital image correlation (DIC), and scanning electron microscopy (SEM) techniques. Combined with the evolution trend of the b value and Felicity ratio, the acoustic emission characteristics of rock masses were studied. The damage evolution model of the specimens was established based on the load-unload response ratio (LURR) theory, and the fatigue behavior of the specimens was analyzed at a macro-meso multiscale. The results show that the application of a short-term, low-stress level of creep load during fatigue loading can reduce the increase in deformation parameters, slow down the attenuation of elastic parameters, and restrain the decline in response level. Effectively controlling the growth rate of damage, the phenomenon of early hardening occurs, thereby increasing the fatigue life of the rock mass. In terms of microstructure, under the influence of creep loading, most of the grains exhibit split failure, the area of folds caused by fatigue loading is reduced, the number of fatigue cracks is decreased, the roughness of the fracture is reduced, and the brittleness of the specimen increases when it fails. In addition, due to the application of creep loading, the Felicity effect appears later, and the Felicity ratio decays more slowly, leading to interlocking of particles within the rock mass and inhibiting the crack propagation process, which causes the b value to rise.

Research on nonlinear damage hardening creep model of soft surrounding rock under the stress of deep coal resources mining

To better understand the creep mechanical behavior of deep soft rock coal seam and more accurately predict the long-term deformation of the surrounding rock. Firstly, the electron microscope scanning test under different magnifications and a step loading creep test of deep soft rock were carried. the creep of the soft rock has the characteristics of ”sudden change”, and the propagation and combination of crystal particle cracks lead to the damage of rock mass, and the extrusion and dislocation movement of grains along the cavity hardens the rock mass. Deep rocks have both damage and hardening mechanisms in the creep deformation process, and there is a game relationship between them. Secondly, the damage variable and hardening function are introduced into Hooke’s body and Kelvin’s body, respectively, and connected in series with the adhesive dashpot with nonlinear viscosity coefficient containing power function, to establish the nonlinear damage and hardening creep model of three elements. Finally, the creep model agrees with the creep test data, which can accurately determine the opening time point of accelerated failure and the long-term strength of deep soft rock. This study provides a theoretical basis for creep laws of soft rock, which can effectively guarantee the long-term stability of deep energy extraction projects.

Time‐dependent failure of off‐axis loaded unidirectional glass/iPP composites

AbstractIn our previous study, we demonstrated that the time‐dependent failure of transversely loaded UD (unidirectional) glass/iPP is fully plasticity‐controlled and proposed a lifetime prediction method based on plasticity to predict transverse failure. In the present work, extending our previous study to other off‐axis angles, we aim to investigate the effect of the off‐axis angle on the time‐dependent failure of UD glass/iPP, and to propose a lifetime prediction method for the off‐axis failure. Glass/iPP specimens with different off‐axis angles are tested at various strain rates, creep, and fatigue loads to characterize the anisotropic, time‐dependent mechanical response. It is demonstrated that the influences of strain rate and fiber orientation angle on tensile strength are multiplicatively separable; also referred to as factorizable, enabling one to characterize the angle dependence at a single strain rate and the strain rate dependence at a single angle. Moreover, similar to transverse loading, off‐axis failure is also observed to be plasticity‐controlled. Based on these observations, the lifetime prediction method for the plasticity‐controlled transverse failure is extended to off‐axis loading using the aforementioned factorazibility, which resulted in lifetime predictions in agreement with the experimental creep and fatigue data.

Properties of Different Waterproof Bonding Layer Systems for Cement Concrete Bridge Deck Pavement

The performance of waterproof bonding layer systems significantly affects the service life of concrete bridge deck pavement. This article aims to compare the bonding properties and composite structure mechanical properties of different waterproof bonding layer systems in bridge deck pavement to recommend appropriate structures based on service conditions. To fulfill this objective, four kinds of waterproof bonding material composite systems were designed, and test samples were fabricated. Then, the waterproof bonding layer properties were studied by penetration resistance tests, scalding resistance tests, and corrosion resistance tests. Pull-off tests were conducted to compare the bonding properties. Then, dynamic modulus tests, Hamburg wheel tracking tests, and static load creep tests of composite structure specimens were implemented to evaluate the dynamic and static mechanical performances and water-temperature stability. The findings showed that the four schemes had good impermeability. The pull-off strengths of scheme III at 25 °C were about 2.5-, 2.5-, and 4-times those of schemes I, II, and IV, respectively, and the pull-off strengths at 50 °C were about 1.6-, 2-, and 1.6-times those of schemes I, II, and IV, respectively, and the bonding performances and dynamic modulus of different schemes were ranked as follows: III > I > II > IV. The results of the Hamburg wheel tracking test were consistent with the static load creep test. The high-temperature stability of scheme IV was the best, while the stabilities of scheme III and I were better than that of scheme II. In conclusion, the scheme of concrete bridge deck pavement can be selected based on the environmental conditions and traffic load, and schemes III and I should be considered first. The findings of this research could provide technical support for the future design of bridge deck waterproof bonding systems.