Thermal Creep Effects Research Articles

Experimental study on fire resistance of high strength Q960 steel columns under axial compression

In order to obtain the mechanical behavior of H-shaped high-strength Q960 steel column under axial compression in fire, the bearing capacity tests of two Q960 steel columns at ambient temperature and fire resistance tests of six Q960 steel columns at elevated temperature were carried out. By considering different slenderness ratios, load ratios, and heating rates of the test columns, the furnace temperature, column temperature, lateral displacement, and axial displacement were measured and recorded. Then, critical temperature, fire resistance, and failure mode of the columns were obtained. The thermal-mechanical coupling analysis model for the Q960 steel column was established by using ABAQUS finite element analysis software. After verifying the accuracy of the model, parameter analysis was carried out to investigate the effects of load ratio, slenderness ratio, geometric imperfection, thermal creep, residual stress, and steel strength on the fire resistance of steel columns. Based on the test data and analysis results, a fire resistance design method for high-strength Q960 steel columns under axial compression is proposed.

Development of design equations for ultra-high strength steel CHS columns under transient fire conditions considering thermal creep and axial restraint effects

Despite their potential for widespread structural applications, the use of ultra-high strength steel (UHSS) members in buildings is currently hindered by the lack of suitable design relationships for these components. In response to this technical gap, this paper presents design equations for Grade 1200 UHSS circular hollow section (CHS) columns under transient fire conditions. The design relationships are derived from an extensive parametric study on the in-fire behavior of UHSS tube columns using advanced finite element (FE) simulations in ABAQUS software. However, to verify the FE modeling framework, experiments are also conducted on axially-compressed UHSS tube specimens. The thermal creep of the UHSS tubes is explicitly included as a user-defined subroutine in the modeling by a robust creep model exclusively developed for the UHSS material under transient fire conditions. In addition to the utilization factor, the effects of non-dimensional slenderness, heating rate (or thermal creep), and axial restraint stiffness ratio are also studied and considered in the design equations obtained for the UHSS CHS columns. Finally, it is shown that the present design relationships can predict the columns' buckling and failure temperatures with a maximum absolute error (MAE) of 4%.

Thermal creep of hybrid steel columns comprising ultra-high strength steel tubes and stainless steel plates under transient fire conditions

This paper investigates the behavior of an innovative steel column section both at room temperature and under the transient heating regime of building fires. The column consists of Grade 1200 ultra-high strength steel (UHSS) tubes welded to the corners of Grade 304 stainless steel plates, forming a square-shaped hybrid cross-section. First, through performing compressive tests on 780 mm long stub columns of the hybrid section at room temperature, its vulnerability to local buckling is assessed; accordingly, decision is made on the appropriate width of the plates. Then, six transient fire tests are conducted on the hybrid stub columns under three different compressive load levels. Due to the very fast heating rate of standard fire curves such as the ISO 834, the thermal creep of steel columns may be overlooked using these heating regimes, which may lead to an overestimation of the column’s resistance against building fires. Thus, a distinctive heating regime is adopted for the fire tests in this study; at any load level, two fast and slow heating rates are applied to the specimens. In this way, by comparing the results obtained, the effect of thermal creep on the fire resistance of the columns can be assessed. More importantly, the fire tests are used to verify finite element (FE) models developed for these hybrid columns in ABAQUS software; specifically, the creep model already developed for the UHSS tube and stainless steel plate materials is verified for component-level simulations. The verified FE modeling framework is then employed to perform an extensive parametric study on the effects of length, utilization factor, axial restraint stiffness ratio, and heating rate on the hybrid columns’ fire resistance. Finally, based on the results obtained from the parametric study, simplified design equations in the temperature domain are obtained for the axially-restrained hybrid steel columns under transient fire.

Temperature compensation for quartz flexible accelerometer based on nonlinear auto-regressive improved model

Temperature variation is an important factor affecting the performance of Quartz flexible accelerometer (QFA). Performance deterioration of QFA degrades the navigation accuracy of inertial navigation system (INS). Normally dramatic change in temperature causes both thermal effect and severe creep effect on the performance of QFA. Previous papers have proved that part of errors caused by thermal effect can be restrained through simple temperature compensation. However, error caused by severe creep effect is seldom considered. In this paper, creep effect and thermal effect in QFA are detailed analyzed, respectively. Furthermore, based on the analysis of thermal effect and creep effect, the novel temperature model based on nonlinear auto-regressive with external input (NARX) improved by wavelet transform (WT) is proposed to address the retardation problem caused by thermal effect. Creep error causing the ruleless deformation of QFA’s structure is separated from overall errors, and only the thermal error whose effect has strong relationship with temperature is compensated by proposed model. Moreover, a 4-point dumpling experiment in temperature control oven is conducted to train and verify the proposed temperature model. The result of the comparative experiments shows that the performance of proposed method is the best among the comparative models. The compensation result based on proposed method improves stability of 1 g output from 776 to 14.6 µg. The proposed temperature compensation method improves the performance of QFA effectively and feasibly, which could be promoted to other applications of INS in temperature changing environment.

On the nonlinear thermal stress, thermal creep, and thermal edge flows in triangular cavities

Here, rarefied thermally driven flow is investigated in two-dimensional equilateral triangular cavities with different uniform wall temperatures. We used three different solvers, i.e., the direct simulation Monte Carlo solver, discrete unified gas kinetic scheme solver, and continuum set of equations of a slow non-isothermal flow solver. Two main cases were considered; in the first case, the cavity's base is considered hot, and the other sides were set cold. In the second case, the right half of the bottom wall was regarded as a diffuse reflector with high temperature, while the left half of the bottom border was set as a specular reflector. The adjacent side walls were set cold with diffuse reflector boundary conditions. The imposed temperature difference/wall boundary condition induces various vortices in the geometry. In case 1, we observe that principal vortices appearing in the triangle are due to nonlinear thermal stress effects, and the thermal creep effects cause other smaller, confined ones. In case 2, a thermal edge flow is set up from the specular wall on the way to the diffusive hot wall, creating a large vortex in the geometry. As the Knudsen number decreases, another small vortex appears near the left cold border.

Thermo-viscoelastic homogenization of 3D woven composites with time-dependent expansion coefficients

Stress relaxation in the viscoelastic matrix influences the evolution of thermal expansion and chemical shrinkage of polymer composites. Its effects are included into a viscoelastic model of the generalized Maxwell type by means of time-dependent thermal expansion and chemical shrinkage coefficients. A homogenization strategy is proposed to obtain these coefficients of the composite from the constituent behaviors, taking into account the viscoelastic effects. The homogenized behavior is validated by full-scale finite element simulations. It reproduces features such as sign changes of the thermal strain rate close to the glass transition temperature and thermal creep effects, which can have an important impact on the residual stress formation in composites, but are not taken into account by classical thermo-elastic homogenization methods.

Thermal creep effects on fluid flow and heat transfer in a microchannel gas cooling

This work presents a numerical analysis of the effect of thermal creep on gas flow and heat transfer in a microchannel under gas cooling conditions. The hydrodynamic equations in vorticity-stream function form are solved together with the energy equation using a finite difference scheme. Axial conduction, pressure work, and viscous dissipation effects are included in the analysis. Solutions are obtained with and without thermal creep effects to illustrate the thermal creep effect on gas cooling. The results show that thermal creep becomes more pronounced as the wall temperature decreases relative to the inlet gas temperature. The results also show that the thermal creep effect on gas cooling becomes more significant as Reynolds number decreases. Furthermore, increasing Knudsen number has caused thermal creep effects to extend further into the channel. Additionally, the work has concluded that thermal creep causes an increase in the hydrodynamic entrance length. Moreover, the work also showed that for low enough wall temperature, Knudsen number, and Reynolds number, the slip velocity can reach negative values resulting in a significant change in the wall velocity allowing vortices to form. The work has also concluded that, at least for most practical purposes, thermal creep has no tangible effects on the thermal part of the problem, where the solution with and without thermal creep effect, is producing almost the same results for temperature field, total heat transfer, and thermal entrance length.

Analysis of Thermal Creep Effects on Fluid Flow and Heat Transfer in a Microchannel Gas Heating

Abstract Thermal creep effects on fluid flow and heat transfer in a microchannel gas flow at low velocities are studied numerically. The continuity and Navier–Stokes equations in vorticity–stream function form, coupled with the energy equation, are solved, considering the thermal creep effect due to the longitudinal temperature gradient along the channel wall in addition to the combined effects of viscous dissipation, pressure work, axial conduction, shear work, and nonequilibrium conditions at the gas–wall interface. The governing equations are also solved without thermal creep, and comparisons between the two solutions are presented to evaluate the thermal creep effect on the flow field in the slip flow regime at relatively low Reynolds numbers. The results presented show that the thermal creep effect on both velocity and temperature fields become more significant as the Reynolds number decreases. Thermal creep effect on the velocity field also extends a longer distance downstream the channel as the Reynolds number decreases, hence increasing the hydrodynamics entrance length. Thermal creep can cause high positive velocity gradients at the upper channel wall for gas heating and hence reverse the flow rotation in the fluid layers adjacent to the wall. Thermal creep also results in a higher gas temperature in the developing region and higher heat exchange between the fluid and the channel wall in the entrance region. Thermal creep effect on heat exchange between the gas and the channel wall becomes more significant as the Knudsen number decreases.

Failure mechanism of plasma-sprayed thermal barrier coatings under high-temperature isothermal aging conditions

The failure of thermal barrier coatings (TBCs) is caused by the stress generated during service owing to the different physical properties and high-temperature behavior of the ceramic top coat made of Y2O3–ZrO2, thermally grown oxide (TGO) primarily composed of β-Al2O3 that forms and grows at high temperature, and metallic bond coat and substrate materials. The operating environment of TBCs varies depending on the objective of use and the operation stage of the gas turbine to which it is applied; therefore, it is important to understand the stress state by considering the operation environment. In this study, we analyzed the failure of TBCs in an operation environment when they were subjected to high-temperature conditions for a long time. For this, the effects of thermal expansion misfit, creep, and TGO growth on the high-temperature stress states were evaluated and analyzed systematically through finite element analysis(FEA.) The failure mechanism under a high-temperature isothermal condition was presented, considering degradation phenomena such as TGO growth rate decrease and creep rate reduction. It reveals that in the early stage of degradation at high temperatures, a valley-to-valley crack propagation was induced; however, at relatively low temperatures, changes in behavior during long-term exposure caused a peak-to-peak crack propagation. Additionally, the proposed mechanism was verified through analysis of the residual top coat area of the surface of the TBC specimen that was delaminated during the isothermal aging tests performed at different temperatures; this is important because the results of the analysis could not be explained by the mechanisms of previous studies in which cooling stresses under thermal cyclic conditions were focused.

Modelling of effective irradiation swelling for inert matrix fuels

The results of effective irradiation swelling in a wide range of burnup levels are numerically obtained for an inert matrix fuel, which are verified with DART model. The fission gas swelling of fuel particles is calculated with a mechanistic model, which depends on the external hydrostatic pressure. Additionally, irradiation and thermal creep effects are included in the inert matrix. The effects of matrix creep strains, external hydrostatic pressure and temperature on the effective irradiation swelling are investigated. The research results indicate that (1) the above effects are coupled with each other; (2) the matrix creep effects at high temperatures should be involved; and (3) ranged from 0 to 300 MPa, a remarkable dependence of external hydrostatic pressure can be found. Furthermore, an explicit multi-variable mathematic model is established for the effective irradiation swelling, as a function of particle volume fraction, temperature, external hydrostatic pressure and fuel particle fission density, which can well reproduce the finite element results. The mathematic model for the current volume fraction of fuel particles can help establish other effective performance models.

Effect of thermal creep on the behavior of flush end-plate connection under transient conditions of fire

The properties of steel material become dependent on the temperature and duration of heating during a fire event. Thermal creep strain is a time-dependent plastic deformation that becomes significant under constant load and high temperatures. The goal of this study is to understand the thermal creep effect on the behavior of a beam-column flush end-plate connection subjected to transient fire conditions. Finite element models were developed and validated against experimental work available in the literature. Parametric studies were conducted to study the overall response of the connection with and without the explicit consideration of thermal creep. The parameters included are heating/cooling rate, beam length, load ratio, plate thickness, bolt size, column axial compressive force, and creep effect in bolts. The thermal creep effect decreases the beam thermal induced compressive forces and simultaneously causes larger mid-span deflections at high heating temperatures. At slow heating rates, the effect of thermal creep can significantly reduce the compressive force at the end of heating while generating high tensile forces at the end of cooling. Neglecting the thermal creep in the analysis may cause inaccurate prediction of the tying forces and rotational demand of the connection.

Numerical analysis of nonlinear thermal stress flow between concentric elliptical cylinders

The present study numerically investigates nonlinear thermal stress in a rarefied gas flow between two coaxial elliptic cylinders with a hot outer wall. Monatomic argon is considered, and isothermal boundary conditions are implemented on the walls. Three different numerical methods are used to solve this problem, i.e., first, the direct simulation Monte Carlo (DSMC) method is used and validated against the numerical solution of the Boltzmann equation available in the literature. The results of the DSMC algorithm are compared with the results of the discrete unified gas kinetic scheme and the continuum set of equations of slow non-isothermal flows, which includes nonlinear thermal stress terms compared to the Navier–Stokes equations. For the first time, the flow is examined in a wide range of Knudsen numbers in the slip regime from the Knudsen number (Kn) of Kn = 0.005 to Kn = 0.1. At a minimal Knudsen number, there is just one vortex due to the nonlinear thermal stress flow; however, as Kn increases, the velocity peak near the inner cylinder is replaced with another vortex, which is created due to the second-order thermal creep effects on the inner wall of the cylinder. We show that as Kn increases, the inner vortex increases in size, while the outer vortex shrinks. The mechanisms of the formation of both vortices are described in detail. Other flow characteristics, including walls’ velocity slip, shear stress, and vorticity, are discussed.

Transient creep analytical modeling for shear endplate assemblies in fire

PurposeThe purpose of this study is to develop an analytical model that is capable of predicting the behavior of shear endplate beam-column assemblies when exposed to fire, taking into account the thermal creep effect.Design/methodology/approachAn analytical model is developed and validated against finite element (FE) models previously validated against experimental tests in the literature. Major material and geometrical parameters are incorporated in the analysis to investigate their influence on the overall response of the shear endplate assembly in fire events.FindingsThe analytical model can predict the induced axial forces and deflections of the assembly. The results show that when creep effect is considered explicitly in the analysis, the beam undergoes excessive deformation. This deformation needs to be taken into account in the design. The results show the significance of thermal creep effect on the behavior of the shear endplate assembly as exposed to various fire scenarios.Research limitations/implicationsHowever, the user-defined constants of the creep equations cannot be applied to other connection types. These constants are limited to shear endplate connections having the material and geometrical parameters specified in this study.Originality/valueThe importance of the analytical model is that it provides a time-effective, simple and comprehensive technique that can be used as an alternative to the experimental tests and numerical methods. Also, it can be used to develop a design procedure that accounts for the transient thermal creep behavior of steel connections in real fire.

Improved MPS model for concrete creep under variable humidity and temperature

The creep of concrete is significantly affected by ambient temperature and humidity. In design codes, the influence of ambient temperature and humidity on concrete creep is approximately considered by correction coefficient terms. However, this method is not suitable for creep analysis of specific structure under complex temperature and humidity environments. At present, the Microprestress-Solidification (MPS) theory is widely adopted to analyze the complex influence of temperature and humidity on concrete creep. However, the MPS theory still has some shortcomings (The MPS theory fails to reflect the size effect in drying creep. Besides, the humidity dependence of transient thermal creep cannot be correctly modeled.). On the basis of MPS theory, this paper makes some appropriate corrections and modifications: firstly, the evolution equation of micro-prestress S is reasonably updated by redefining the relationship between the long-term creep viscosity and micro-prestress S. Meanwhile, based on the thermodynamics, the influence of temperature and humidity variation on the evolution of micro-prestress S is reformulated and the transient thermal creep effect is modified to be humidity-dependent. In order to verify the improved MPS model, experiment data in literatures are adopted to make verifications. Results prove the correctness of the modifications and the practicability of the improved model.

Thermal creep effects of aluminum alloy cladding on the irradiation-induced mechanical behavior in U–10Mo/Al monolithic fuel plates

Three-dimensional finite element simulations are implemented for the in-pile thermo-mechanical behavior in U–Mo/Al monolithic fuel plates with different thermal creep rates of cladding involved. The numerical results indicate that the thickness increment of fuel foil rises with the thermal creep coefficient of cladding. The maximum Mises stress of cladding is reduced by ∼85% from 344 MPa on the 98.0th day when the creep coefficient of cladding increases from 0.01 to 10.0, due to its equivalent thermal creep strain enlarged by 3.5 times. When the thermal creep coefficient of Aluminum cladding increases from 0 to 1.0, the maximum mesoscale stress of fuel foil varies slightly. At the same time, the peak mesoscale normal stress of fuel foil can reach 51 MPa on the 98.0th day for the thermal creep coefficient of 10, which increases by 60.3% of that with the thermal creep un-occurred in the cladding. The maximum through-thickness creep strain components of fuel foil differ slightly for different thermal creep coefficients of cladding. The dangerous region of fuel foil becomes much closer to the heavily irradiated side when the creep coefficient of cladding becomes 10.0. The creep performance of Aluminum cladding should be optimized for the integrity of monolithic fuel plates.

Thermal creep of shear tab connection assemblies under transient-state temperatures of fire

Purpose This study aims to investigate the thermal creep behavior of steel frame assemblies with shear tab connections subjected to transient-state fire temperatures. Different key parameters are investigated to study their effect on the global response of the steel frames in fire. Design/methodology/approach Finite element (FE) models of connection assemblies are first analyzed using Abaqus under transient-state temperature conditions and validated against experimental work available in the literature. Upon acquiring the validated conditions, parametric studies are carried out to study the effect of key geometric and heating parameters on the overall response of the frame assembly to fire temperatures. Thermal creep material is also incorporated in the analyses through a user-defined subroutine, and a comparison between including and excluding creep material is illustrated to show the effect of thermal creep on the structural behavior. Findings The results reported herein indicate that having a rigid column increases the thermal-induced axial forces, thus increasing the development of thermal creep strains. Slow heating rates can cause axial stress relaxation in the restrained beam and increase the mid-span deflection and consequently the development of beam catenary action. The results also show that reaching higher initial cooling temperatures and having longer cooling phase durations result in more tensile forces at the end of the cooling phase. Originality/value Previous studies were limited to isolated steel connections under steady-state conditions. This study investigates the creep behavior of shear tab connection assemblies under transient-state conditions of fire when creep effects are explicitly considered. This can provide a rational and realistic assessment of the steel behavior in fire events.

Thermal creep behavior of shear tabs in fire using modified burgers model

A mechanical model is developed to predict the time-independent (fast) and time-dependent (due to thermal creep) force-rotation characteristics of shear tab connections subjected to fire. The time-independent behavior of the connection is modeled using a group of springs that can predict the rotational stiffness and the strength of the connection when subjected to fire temperatures. A modified Burgers model is incorporated in the mechanical model to include the thermal creep effect. The modified Burgers model is composed of linear springs and viscous dashpots that can predict the time-dependent behavior of the shear tab connection before and after beam-column contact, and bolt bearing. The proposed model shows excellent agreement when compared with experimental results and finite element (FE) simulations. Furthermore, isochronous curves are developed to study explicitly the effect of different parameters, on the strength and rotational capacity of shear tab connections. The proposed model can be considered an important step toward the inclusion of thermal creep in fire design of shear tab connections.

Numerical Studies on the Creep Behavior of Shear Endplate Connection Assemblies UNDER Transient Heating

Correct assessment of the steel connection performance under fire requires including the time-dependent response of steel material in the structural analysis. Failure to do so might impose critical threats on the stability and integrity of steel structures. To this aim, the objective of this study is to investigate the effect of thermal creep on the behavior of shear endplate beam-column connections subjected to transient-state fire temperatures. First, finite element models of shear endplate assemblies are developed using ABAQUS and validated against experimental work available in the literature. Parametric studies are then carried out to study the effect of key geometrical, thermal, and material parameters on the overall response of the frame assembly in fire while explicitly including creep. This includes heating and cooling rates, initial cooling temperature, column size and height, load ratio, plate thickness, and steel grade. The results show that including thermal creep causes a reduction in the induced compressive forces and an increase in the mid-span beam deflection, for about six times higher in some cases, thus earlier development of beam catenary action. It is also concluded that lower heating and cooling rates result in larger beam tying forces on the shear end plate connections, which can reach values around ten times larger than when creep is neglected. This study shows that the current practice of neglecting creep in fire analyses, especially in slow heating, may underestimate the forces that are exerted on the shear endplate connections during fire and thus leads to unsafe structural design.

Study of Flow Characteristics of Gas Mixtures in a Rectangular Knudsen Pump.

A Knudsen pump operates under the thermal transpiration effect or the thermal edge effect on the micro-scale. Due to the uneven temperature distribution of the walls in the channel axis direction or the constant temperature of the tips on the walls, directional thermally-induced flow is generated. In this paper the Direct Simulation Monte Carlo (DSMC) method is applied for N2–O2 gas mixtures in the ratios of 4:1, 1:1, and 1:4 with different Knudsen numbers in a classic rectangular Knudsen pump to study the flow characteristics of the gas mixtures in the pump. The results show that the changing in the gas physical properties does not affect the distribution of the velocity field, temperature fields, or other fields in the Knudsen pump. The thermal creep effect is related to the molecular mass of the gas. Even in N2 and O2 gas mixtures with similar molecular masses, N2 can be also found to have a stronger thermal creep effect. Moreover, the lighter molecular weight gas (N2) can effectively promote the motion of the heavier gas (O2).

Flush Endplate Connections in Fire: Time-Dependent Behavior of Tension Bolts

Steel connections play a crucial role in maintaining the integrity and stability of steel building frames especially when exposed to fire temperatures. The behavior of flush endplate connections in fire is shown to be governed by tension bolt failure as bolts lose their strength and stiffness more rapidly at higher temperatures. As a result, the ability to predict the development of stresses in tension bolts of flush endplate connections at different stages of fire is of special importance. One of the factors influencing bolt stresses in fire is the thermal creep or time-dependent inelastic response of steel to elevated temperatures. Therefore, time- and temperature-dependent behavior of tension bolts of flush endplate connections in fire is the focus of this study. Stress-time histories in tension bolts are obtained by explicit consideration of thermal creep of steel in FE models of flush endplate connections at elevated temperatures. To better understand the effect of thermal creep on tension bolt behavior, the correlation between time-dependent rotational deformation of flush endplate connections and bolt stresses is also investigated. Further, the isochronous representation is utilized to study the rotational deformation and the tension bolt stresses under various applied moments ranging from 50% to 95% of the moment capacity and fire temperatures ranging from 450°C to 600°C with 25°C increment. Through such representation, it is indicated that the connection behavior is not only dependent on bolt strength degradation and applied moment, but also affected by the time duration of applied moments and temperatures. Also, with the inclusion of thermal creep of steel, the connection experiences higher rotation and excessive endplate deformation with stress relaxation leading to top tension bolt failure at earlier stages of fire. More specifically, for time exposure greater than or equal to 60 min, the failure temperature of the connection decreases from 600°C to around 550°C. Therefore, neglecting thermal creep of structural steel may result in an unsafe prediction of the overall response of flush endplate connections in fire.