Isothermal Creep Tests Research Articles

Temperature-gradient and isothermal creep tests on 2024 aluminum alloy: A novel approach to creep failure evaluation aided by image processing

In high-temperature working conditions, the temperature difference between parts is common, and it is necessary to perform a temperature gradient test to predict creep behavior of alloy in real working conditions. In this study, a novel temperature gradient creep (TGC) test aided by an image processing method (IPM) was designed and performed on 2024-T3 aluminum alloy. The tests were performed under the conditions of isothermal tensile creep (ITC) and TGC in the temperature range of 285–315 °C under two stresses of 90 MPa and 125 MPa. Several experimental investigations were conducted to evaluate the creep failure of the specimens. The results showed that the temperature gradient during the TGC test leads to changes in strain, strain rate, stress exponent, microstructure and damage amount in the samples. The creep strain and strain rate of the TGC are greater than those of the ITC at 300 °C and 285 °C, and the rupture life is shorter. Finally, the Larson–Miller parameter is calculated, and the mathematical relationships between the strain rates of the TGCs and ITCs in 90 MPa and 125 MPa, which can be used to predict the creep strain rate under isothermal conditions, are proposed.

Modelling of Strain-Controlled Thermomechanical Fatigue Testing of Cast AlSi7Cu3.5Mg0.15 (Mn, Zr, V) Alloy for Different Aging Conditions

Thermomechanical fatigue loadings (TMF) applied on components in a certain temperature range with a variable state of stress (tensile and/or compression) produce a localized concentration of plastic strains that results in crack initiation and propagation. The time evolution of plastic strains must be known a priori to predict the lifetime of a part submitted to TMF loadings, which requires an extensive campaign of mechanical characterization conducted at different temperatures and aging conditions. Such a campaign was proposed for the aluminum alloy AlSi7Cu3.5Mg0.15 (Mn, Zr, V), which is recognized as being creep resistant. Combined isothermal low-cycle fatigue and isothermal creep tests were performed on this alloy to determine the constitutive parameters based on the Lemaître and Chaboche (LM&C) viscoplastic model. These laws were implemented within the finite element simulation software (Z-set) to model the response of the alloy to a thermomechanical fatigue test. The results of TMF Z-Set simulations, using the LM&C model adapted for two aging conditions, were then compared with results obtained from “Out of Phase” thermomechanical fatigue testings (OP-TMF) performed on a Gleeble 3800 machine. The modelling of the OP-TMF test revealed the complexity of the mechanical behavior of the material induced by the temperature gradient prevailing along with the cylindrical specimen. It was found that a better prediction of the evolution of plastic strains requires taking into account a larger range of plastic strain rates conditions for the determination of the constitutive law and eventually includes the role of the microstructure in the evolution of the material behavior, starting first with the yield stress.

Time-temperature superposition of flexural creep response of carbon fiber PEKK composites manufactured using different prepreg stacking sequence

In this work, the long-term creep response of high-performance carbon fiber PEKK (CF/PEKK) composites was evaluated by performing extrapolated short-term flexural creep tests at various temperatures. The time-temperature superposition principle (TTSP) with vertical as well as horizontal shifting was used to generate master curves at reference temperatures of 120°C. Satin weave-based CF/PEKK prepregs were used to manufacture eight-layer composites via compression molding, with three different stacking sequences: (a) zero-direction [0]8 (b) cross-ply [0, 90]4 and (c) quasi-isotropic [90, −45, 45, 0]2 s. The flexural properties under three-point bending arrangement in a universal testing machine were also evaluated. A dynamic mechanical thermal analyzer (DMTA) in three-point bending mode was used to evaluate the temperature-dependent viscoelastic properties of the three types of composites. The creep and creep-recovery behavior was evaluated at 40°C, 80°C, 120°C, 160°C and 200°C. To construct a master curve, extrapolated short-term isothermal creep tests were performed from 120°C to 180°C at the intervals of 10°C. The predicted master curve represents the creep behavior of composites over more than 10 years. It was shown that the quasi-isotropic CF/PEKK composites exhibited 27% and 12% higher creep resistance at 120°C as compared to zero-direction and cross-ply laminates, respectively. Higher flexural modulus (23%) and flexural strengths (33%) were also exhibited by the quasi-isotropic CF/PEKK composites. The final thickness of quasi-isotropic laminates was 8% lower than the 0o laminates. After analyzing the cross-sections of the composites, it was proposed that the superior mechanical properties of the quasi-isotropic laminates could be due to enhanced nesting between neighboring prepreg layers during the compression molding process, which resulted in closer packing of the fibers. It has been shown that the prepreg stacking sequence could affect the creep behavior and flexural properties of the compression-molded CF/PEKK composites.

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).

Effects of seawater absorption and desorption on the long‐term creep performance of graphene oxide embedded glass fiber/epoxy composites

AbstractThis study is primarily focused on analyzing the consequences of sea water absorption and desorption on the long‐term durability of glass fiber/epoxy (GE) and 0.5 wt% graphene oxide (GO) dispersed glass fiber/epoxy (GO‐GE) composites. Both GE and GO‐GE composite specimens were initially exposed to natural seawater for a duration of 9 months. Desorption was also carried out for some of the seawater saturated samples. Before ageing, flexural test and dynamic mechanical analysis were carried out to understand the impact of GO incorporation on the characteristics of GE composite. Using gravimetric analysis, the seawater uptake kinetic curves were determined for both the composites. Long‐term durability was assessed by performing stepwise isothermal creep and recovery tests in 30°C to 120°C temperature range and analysis of the results involved the use of time‐temperature superposition principle for constructing the creep master isotherms in different conditions (before ageing or dry state, saturated state, and desorbed state).

Numerical multiscale homogenization approach for linearly viscoelastic 3D interlock woven composites

This work aims at modeling the homogenized behavior of a polymer matrix composite reinforced with three dimensional (3D) woven fabric. The effective warp and weft tows’ properties were determined by numerical homogenization with Abaqus considering elastic fibers, a viscoelastic matrix and periodic boundary conditions. The temperature- and cure-dependent linearly viscoelastic model previously developed by the authors for this particular polymer matrix was implemented into a subroutine using a differential strategy. A second homogenization procedure was carried out to obtain the mesoscopic structure homogenized behavior. Moreover, rectangular composite plates were manufactured by Resin Transfer Molding (RTM) and isothermal creep tests were carried out to study the material’s viscoelastic behavior at high temperatures below the resin’s glass transition temperature. Experimental results were compared to the temperature-dependent homogenized linearly viscoelastic model predictions. This model is a step forward for the accurate prediction of the residual stresses developed during the manufacturing of structural parts made out of 3D woven interlock composites.

Lifetime Prediction of Nano-Silica based Glass Fibre/Epoxy composite by Time Temperature Superposition Principle

The incorporation of nano fillers in Fibre reinforced polymer (FRP) composites has been a source of experimentation for researchers. Addition of nano fillers has been found to improve mechanical, thermal as well as electrical properties of Glass fibre reinforced polymer (GFRP) composites. The in-plane mechanical properties of GFRP composite are mainly controlled by fibers and therefore exhibit good values. However, composite exhibits poor through-thickness properties, in which the matrix and interface are the dominant factors. Therefore, it is conducive to modify the matrix through dispersion of nano fillers. Creep is defined as the plastic deformation experienced by a material for a temperature at constant stress over a prolonged period of time. Determination of Master Curve using time-temperature superposition principle is conducive for predicting the lifetime of materials involved in naval and structural applications. This is because such materials remain in service for a prolonged time period before failure which is difficult to be kept marked. However, the failure analysis can be extrapolated from its behaviour in a shorter time at an elevated temperature as is done in master creep analysis. The present research work dealt with time-temperature analysis of 0.1% SiO2-based GFRP composites fabricated through hand-layup method. Composition of 0.1% for SiO2nano fillers with respect to the weight of the fibers was observed to provide optimized flexural properties. Time and temperature dependence of flexural properties of GFRP composites with and without nano SiO2 was determined by conducting 3-point bend flexural creep tests over a range of temperature. Stepwise isothermal creep tests from room temperature (30°C) to the glass transition temperature Tg (120°C) were performed with an alternative creep/relaxation period of 1 hour at each temperature. A constant stress of 40MPa was applied during the creep tests. The time-temperature superposition principle was followed while determining the Master Curve and cumulative damage law. The purpose of a Master Curve was to determine the variation of compliance with respect to increase in time and temperature of the specimen. The shift factors at any reference temperature were determined by Arrhenius activation energy method at a far lower temperature than Tg (Glass transition temperature) and by manual shift method at a temperature near Tg (Glass transition temperature).

Generation of intergranular strains during high temperature creep fatigue loading of 316H stainless steel

This paper investigates the development of intergranular strains and stresses in AISI type 316H austenitic stainless steel during cyclic loading at high temperature. Isothermal cyclic creep tests at 650°C with 2 h displacement controlled creep dwells at maximum strain are conducted with in situ neutron diffraction monitoring at a time of flight facility. The evolution of intergranular strains in five independent {hkl} grain families were successfully measured during consecutive cycles. The grain family with {111} lattice planes normal to the loading direction exhibited linearly reversible behaviour with cyclic load whereas the {200} planes deformed in a non-linear manner forming a hysteresis loop. Intergranular strains during the first dwell remained unchanged with time, but relaxed with time during later dwells. The start of dwell intergranular strains increased from cycle 1 to cycle 2, but markedly less moving from cycle 2 to cycle 3.

Using Stress Relaxation Data to Predict Creep Behavior

An estimation method to predict creep performances of high temperature structural materials has been proposed. A Stress relaxation equation is obtained by fitting stress relaxation testing curves and modifying Tanaka-Ohba reloading stress relaxation constitutive equation. Based on the relationship between stress relaxation and creep, a unified prediction equation of creep is deduced. The method is to use the unified equation to derive creep strain rates or creep strain vs. time curves from stress relaxation measurements through some specified time increments. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal creep tests conducted on 1Cr10NiMoW2VNbN steel. Good agreement between results of creep tests and the predicted results indicates that the developed method can be recommended in the creep behavior evaluation of high temperature materials.

Prediction Methodology of Creep Performance from Stress Relaxation Measurements

An estimation method to predict creep performances of high temperature structural materials has been proposed. The method is to use a simplified and normalized model of stress relaxation to derive creep strain rates and creep strain vs. time curves from stress relaxation measurements through an integrated analytical procedure according to the relationship between stress relaxation and creep. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal creep tests conducted on 1Cr10NiMoW2VNbN steel with the same temperature of stress relaxation tests. Good agreement between results of relaxation tests and the predicted results indicates that the developed method can be recommended in the creep behavior evaluation of high temperature materials.

Phenomenological modeling of the effect of specimen thickness on the creep response of Ni-based superalloy single crystals

Isothermal creep tests on single-crystal Ni-based superalloy sheet specimens show a thickness-dependent creep response that is known as the thickness debit effect. A size-dependent creep response at similar length scales has also been observed in a wide variety of other materials. We focus on Ni-based single-crystal superalloys and present a phenomenological nonlinear parallel spring model for uniaxial creep with springs representing the bulk and possible surface damage layers. The nonlinear spring constitutive relations model both material creep and evolving damage. The number of springs and the spring creep and damage parameters are based, as much as possible, on recent experimental observations of the thickness debit effect under two creep test conditions: a low-temperature, high-stress condition, 760°C/758MPa, and a high-temperature, low-stress condition, 982°C/248MPa. The bulk damage mechanisms accounted for are the nucleation of cleavage-like cracks from pre-existing voids and, at the higher temperature, void nucleation. The surface damage mechanisms modeled at the higher temperature are an oxidation layer, a γ′-precipitate-free layer and a γ′-precipitate-reduced layer. Model results for the creep response and for the thickness debit effect are in close quantitative agreement with the experimental results. In addition, the model predicts qualitative features of the failure process that are in good agreement with experimental observations. The simplicity of the model also allows parameter studies to be undertaken to explore the relative roles of bulk and surface damage as well as the relative roles of cleavage-like cracking and void nucleation in the bulk.

3D Imaging Using X-Ray Tomography and SEM Combined FIB to Study Non Isothermal Creep Damage of (111) Oriented Samples of γ / γ ' Nickel Base Single Crystal Superalloy MC2

An unprecedented investigation consisting of the association of X-Ray tomography and Scanning Electron Microscopy combined with Focus Ion Beam (SEM-FIB) is conducted to perform a 3D reconstruction imaging. These techniques are applied to study the non-isothermal creep behavior of close (111) oriented samples of MC2 nickel base superalloys single crystal. The issue here is to develop a strategy to come out with the 3D rafting of γ’ particles and its interaction whether with dislocation structures or/and with the preexisting voids. This characterization is uncommonly performed away from the conventional studied orientation [001] in order to feed the viscoplastic modeling leading to its improvement by taking into account the crystal anisotropy. The creep tests were performed at two different conditions: classical isothermal tests at 1050°C under 140 MPa and a non isothermal creep test consisting of one overheating at 1200°C and 30 seconds dwell time during the isothermal creep life. The X-Ray tomography shows a great deformation heterogeneity that is pronounced for the non-isothermal tested samples. This deformation localization seems to be linked to the preexisting voids. Nevertheless, for both tested samples, the voids coalescence is the precursor of the observed damage leading to failure. SEM-FIB investigation by means of slice and view technique gives 3D views of the rafted γ’ particles and shows that γ corridors evolution seems to be the main creep rate controlling parameter.

Effect of consolidation on the flexural creep behaviour of all-polypropylene composite

The long-term viscoelastic behaviour of self-reinforced polypropylene composites (SRPPC) was studied by short-term flexural creep tests at different temperatures. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethyl- ene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures (5, 20, 35°C above the melting temperatures of the matrices) keeping the holding time and pressure constant. The manufactured specimens were subjected to isothermal creep tests at different temperatures rang- ing from -20 to 80°C under an applied load. The time-temperature superposition principle was verified for the creep data. An Arrhenius type relationship described the shift data obtained from the creep tests. It was found, that with improving con- solidation (increasing processing temperature) the creep compliance decreased and good correlation was found between creep compliance and density/peel strength.

Time and temperature effect on the linear–nonlinear viscoelastic transition threshold of a polymeric system

AbstractIn the present study, the transition of a polymeric material from the linear to the nonlinear viscoelastic behavior and the determination of the nonlinearity stress threshold variation with time and temperature are investigated. For this purpose a systematic experimental program consisted of thermal and mechanical characterization of certain polymeric material followed by isothermal creep tests at different stress levels and temperatures was conducted. Through isochronal curves that occurred from creep tests, the nonlinearity threshold dependence on both time and temperature is presented. The reported results provide information that is critical for the design and development of polymer structures and components. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Flexural Creep Behavior of Unidirectional and Cross‐Ply All‐Poly(propylene) (PURE®) Composites

AbstractThe long‐term viscoelastic behavior of reinforced all‐poly(propylene) composites was studied by flexural creep tests. Both unidirectional and cross‐ply laminates were prepared from PURE® coextruded tapes by vacuum bag molding in an autoclave. The specimens were subjected to isothermal creep tests at different temperatures ranging from 20 to 80 °C under an applied load. The time‐temperature superposition principle was verified for the creep data. An Arrhenius type relationship was found to better describe the shift data obtained from the creep tests. The activation energies relating to the different reinforcement architecture and different relaxation process were calculated.magnified image

Creep Behavior of Polystyrene/Fluorohectorite Micro‐ and Nanocomposites

AbstractSummary: Pristine FH is incorporated into a PS matrix by melt blending with and without latex precompounding of PS and FH. Direct melt blending results in microcomposites, whereas the latex‐mediated (masterbatch) technique results in PS/FH nanocomposites. The tensile creep response of the micro‐ and nanocomposites are determined in short‐term creep tests. The resistance to creep is improved with increasing dispersion of FH in the PS matrix. Master curves (creep compliance vs. time), constructed based on isothermal creep tests performed in the temperature range between 5 and 45 °C, show that the FH reinforcement affects mostly the initial creep compliance (interphase effect). On the other hand, the stable creep is matrix (bulk) dominated. It is established that the Williams‐Landel‐Ferry equation is fairly applicable to the creep results.Scheme of the change of creep compliance as a function of time for micro‐ and nanocomposites with an amorphous matrix.magnified imageScheme of the change of creep compliance as a function of time for micro‐ and nanocomposites with an amorphous matrix.

Activation Energy Spectrum of Irreversible Structural Relaxation of Finemet Glassy Alloy

The apparent activation energy spectrum (AES) of irreversible structural relaxation of Finemet Fe73.5Cu1Nb3Si13.5B9 metallic glass is reconstructed using the analysis of (i) linear heating and (ii) isothermal creep tests within the framework of the Directional Structural Relaxation (DSR) model. It is shown that the two independent ways of AES reconstruction used give consistent results indicating the validity of the corresponding DSR model flow equations. The AES determined, in accordance with previous findings, takes the form of a monotonously increasing function indicating that the potential profile for irreversible atomic rearrangements during irreversible structural relaxation might have more than two local minima.

Thermomechanical behaviour of a copper–chromium in situ composite

The mechanical response of an in situ copper–chromium composite was investigated over a range of temperatures by means of tensile and isothermal creep tests. Scanning electron microscopy was used to characterise the extent, type, and distribution of damage. It was found that the failure mechanisms fell into distinct regimes. At cryogenic temperatures damage tended to occur in the form of reinforcement fracture. Around room temperature, very little damage was observed in the composite. At temperatures of about 400°C, extensive damage was again observed in the form of reinforcement failure and cavitation. Further increase in the test temperature resulted in a transition from a local to a global load sharing, with damage distribution becoming more homogeneous. These experimental observations were rationalised by considering the relative extent of deformation within the two phases as a function of temperature.

An investigation of the isothermal creep response of Al-based composites by neutron diffraction

Discontinuously reinforced aluminium-based composites containing particulate or whisker SiC reinforcements at different volume fractions, were produced via a powder-route. Isothermal creep tests have been carried out at 270, 300 and 320°C. During these tests axial and transverse strains were monitored using a non-contacting laser scanning extensometer arrangement. At the same time neutron diffraction has been used to measure the elastic phase strain evolution with composite creep deformation. It was found that after the initial loading the phase strains remained essentially constant during primary and secondary creep. This indicates that the initial matrix/reinforcement inelastic misfits remain largely unchanged, probably due to the setting up of a dynamic equilibrium between misfit generation and stress relaxation. For each system, the neutron data has been combined with Eshelby modelling to obtain a measure of the load transfer. It was found that while the reinforcement often bears the larger share of the load, under certain conditions this is not the case. This would suggest that under these conditions the presence of the reinforcement is deleterious to the composite creep response.

The effect of processing route and reinforcement geometry on isothermal creep behaviour of particulate and short fibre MMCs

Isothermal creep tests were performed on discontinuously reinforced Al composites at 270 and 300°C. Axial and transverse strain were monitored simultaneously during testing using laser scanning extensometry. Damage mechanisms were found to dominate the creep behaviour rather than load transfer considerations. The effect of powder-processing versus squeeze infiltration on composite creep behaviour was characterised. The effect of interfacial bond strength was investigated by comparing carbon and Saffil (alumina) short fibre reinforcements. The effect of reinforcement shape, size and aspect ratio was also investigated by comparing short fibre, whisker and particulate reinforcements.