Creep Periods Research Articles

Study on creep behaviors of nickel-based single-crystal alloys considering microstructure evolution

Purpose The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution.Design/methodology/approachThe mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM).FindingsWith the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress.Originality/valueA creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band.

Design of a constitutive model based on the evolution of creep cavities damage for 9Cr-1Mo steel

Considering the impact creep cavity evolution damage of 9Cr-1Mo steel, a new creep constitutive model was established based on the damage-coupled Norton equation and the strain hardening term in the theta projection method model, and the parameters were solved by optimization technique. The creep behavior was compared with that of the Kachanov-Robotnov model. According to the simulation results, the proposed constitutive model can represent the strengthening phenomenon during the first creep stage of 9Cr-1Mo steel. It provides a more accurate description of the behavior during the three creep periods.

The long-term creep behavior and damage mechanism of RAFM steel for fusion reactors

The reduced-activation ferritic-martensitic (RAFM) steel serves as the structural material for the blanket of the International Thermonuclear Experimental Reactor (ITER) and future fusion reactors. In order to achieve a design lifetime of 2–3 years for the blanket in the fusion reactor, it is imperative to thoroughly understand the long-term (>20,000 h) creep behavior of RAFM steel. In this regard, the present study investigated the long-term creep damage mechanism of RAFM steel under creep periods greater than 20 000 h. The results indicated that the creep exponent decreases as the creep rupture time increases from several thousand hours to more than 20,000 h. The microstructure analysis revealed that dislocation slip was effectively pinned by the precipitates in the long-term creep process. Additionally, the M23C6 and Laves phase played a critical role in inhibiting interfacial slip and preventing lath coarsening. However, a few micron-scale M23C6 and Laves phases led to the formation of cavities in the boundary. These cavities subsequently polymerized and aggravated into cracks, ultimately becoming the primary mechanism for the long-term creep fracture.

Modeling of the cyclic visco-elasto-plastic one-dimensional behavior of polyamide-based woven straps

This study focuses on modeling the mechanical behavior of threadlike woven materials or those shaped in uniaxial form, such as wires, ropes, woven lines, cables, straps, slings, etc. The proposed one-dimensional model is based on the superimposition of two stress contributions: a non-Newtonian visco-elastic stress and a time-independent stress. The time-independent stress represents a particular irreversible behavior, linked to the loading history. This model neglects the thickness of the time-independent hysteresis loops during the unloading–reloading processes while preserving the irreversible character of elasto-plastic-type behavior. The model's predictions are compared to a set of experimental results, carried out on polyamide 6-6 straps, available from previous experimental studies in the literature. The model describes the shape of the stress–strain hysteresis loops very well and predicts perfectly the direction of the strain and stress evolutions during the creep and relaxation periods, regardless of their position in the first load or in the load–unload branches.

Use of a ‘Creep’: Zoo Animal Management to Encourage Reproductive Behaviours in Captive Bornean Orangutans (Pongo pygmaeus)

The number of endangered species held within zoos is increasing; therefore, zoo staff need to be able to understand factors that influence reproduction in captivity, ensuring sustainable populations. However, a variety of factors that are often unknown can cause suitable breeding set-ups to fail. This study aimed to determine whether the use of a ‘creep’, a partially closed sliding door allowing only smaller female individuals passage into enclosure areas to enhance their control over proximity to a male, has the potential to stimulate copulatory behaviours within zoo-housed orangutans Pongo pygmaeus. During observations, when the creep was in use, the male individual spent significantly less time observing people, less time resting and more time feeding and foraging. Female conspecifics also spent less time resting and more time feeding and foraging. However, when given control over their proximity to the male during creep periods, no difference was recorded in their proximity to the male. It is concluded that the use of a creep did not aid in stimulating reproductive related behaviours in this group of orangutans.

Rheological parameters, thixotropy and creep of 3D-printed calcium sulfoaluminate cement composites modified by bentonite

The structural instability and poor thixotropy of three-dimensional (3D) printing of calcium sulfoaluminate (CSA) cement composites hinder their applicability in the construction field. In this study, the rheological parameters, thixotropy and creep of 3D-printed CSA cement composites containing bentonite are investigated based on shear-rate- and shear-stress-controlled protocols to achieve a stable three-dimensional structure and excellent thixotropy. Experimental results show that a stable 3D structure with excellent thixotropy can be achieved when the content of bentonite is 2%. In this case, the plastic viscosity and dynamic yield stress are lower than 2.50 Pa s and 645.54 Pa, respectively. The correlation between the rheological parameters and thixotropy of 3D-printed CSA cement composites suggests that the actual thixotropy can be predicted by the yield stress. Furthermore, the shear-stress-controlled protocol was implemented to simulate the effect of gravity and test different creep periods of 3D-printed CSA cement composites with bentonite. The improved static yield stress exhibited better effectiveness in resisting structural deformation over time.

Predicted life of P91 steel for cyclic high temperature service

The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water–steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

Rheological Performance of Asphalt Binders Under Different Creep and Recovery Periods in MSCR Test

The present study was undertaken to evaluate the effects of creep and recovery periods on rheological responses and grading of the modified (polymer and crumb rubber modified) and unmodified binders in multiple stress creep recovery (MSCR) test. The MSCR test was conducted considering two creep periods (i.e., 1 and 2 seconds), three recovery periods (i.e., 9, 18, 27 seconds) and at three different temperatures (i.e., 52˚C, 64˚C and 76˚C). A total of 162 binder samples were tested in the laboratory using dynamic shear rheometer. The polymer modified binder showed increase in percent recovery (R) with an increase in a recovery period, while R value did not show variations with increasing creep recovery periods for the crumb rubber modified and unmodified binders. The stress sensitivity of modified binders showed mixed trend with an increase in creep recovery period. The effects of recovery period were minimal with longer creep period for modified binders. The grading based on AASHTO MP 19, could increase adequate traffic level of modified binders, when subjected to longer recovery time. The study shows that loading time in MSCR test may be relooked into for polymer and crumb rubber modified binders.

Creep Testing of One-Component Polyurethane and Emulsion Polymer Isocyanate Adhesives for Structural Timber Bonding*

Abstract Moisture hardening one-component polyurethane (PU) adhesives and emulsion polymer isocyanate (EPI) adhesives have recently gained a considerable market share in structural timber gluing, especially in Europe. The creep behavior of bond lines between solid timber adherends with said adhesives, specifically nine PU and five EPI products, along with two phenolic resorcinol-formaldehyde (PRF) adhesives used for reference purposes, were studied. Most of the adhesives investigated are approved for gluing of structural timber components in Europe. All of the reported 34 bending creep test series (with 340 specimens), were performed with two-part laminated spruce (Picea abies) specimens according to the European structural adhesive creep test standard EN 15416‐3. The standard addresses the ratio RC of creep between PU and EPI adhesives' bonds versus the minimally creeping bond lines of matched specimens glued with a reference PRF adhesive. Mainly glue line thicknesses of 0.3 mm were investigated, with some additional tests having been performed with 0.2- and 0.5-mm-thick glue lines as well. The investigations provide a substantial insight into the creep behavior of the two regarded polyaddition adhesive families. Additionally, the evaluation proves, for PUs and EPIs for test glue line thicknesses of 0.3 mm and less, that it is well justified to reduce the presently prescribed creep periods of 26 and 52 weeks down to 13 and 26 weeks, respectively. The results will serve as a basis for a change of the European test and requirement standards on structural PU and EPI adhesives.

Cyclic creep and recovery behavior of Nextel™720/alumina ceramic composite at 1200 °C

The cyclic creep and recovery behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200°C in laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The main objective of this study was to assess the influence of various sustained creep and cyclic creep loading histories on the creep lifetime, creep strain rate, accumulated creep strain as well as on the recovery of creep strain at near zero stress. Cyclic creep-recovery tests were performed for maximum stress levels of 100 and 125MPa with creep and recovery periods ranging from 3min to 1h. In laboratory air, lifetimes produced in cyclic creep and recovery tests significantly exceeded those obtained in sustained creep tests. Introduction of intermittent periods of unloading and recovery at near zero stress into the creep loading history resulted in an appreciable improvement in creep lifetime. Presence of steam considerably degraded the material performance. In steam, lifetimes produced in cyclic creep and recovery tests were close to those obtained in sustained creep tests. Composite microstructure, as well as damage and failure mechanisms were investigated.

Sluoksniuotųjų Konstrukcijų Įtempių IR Deformacijų Būvio Bei Valkšnumo Analizė, Veikiant Ilgalaikei Apkrovai

The paper proposes numerical and analytical models of the time-dependent analysis of a multilayered composite. These models enable us to predict the time-dependent behaviour of composite structures made of materials having age-variable properties. The proposed numerical model deals with a numerical solution of linear Volterra integral equation implying the validity of the principle of superposition for the layers. Meanwhile, the proposed analytical approach is based on the application of the age-adjusted effective modulus method for the composite. The validity of these models has been proven by a direct calculation of the time-dependent stresses and strains for the three-layered composite wall structure comparing the obtained theoretical values with those determined numerically. In particular, it was demonstrated that the analytical approach gave the identical values of the time-dependent stress strain state parameters. Relying on the results obtained, the concluding remarks have been summarized as follows: a) Despite the fact of sustained load, acting on the composite made of layers with different time-dependent deformational properties does not always hold time-invariant layer stress. In particular, the layers possessing a higher strain tend to produce relaxation while the layers with a lower strain are undergone the stress increase to maintain the equilibrium between internal and external forces (Fig. 4, b, e). Therefore, the failure should primarily be originated from the layer with a lower strain; b) Time variation in the elastic modulii of the layers is responsible for the increased stresses at time t in a higher strain layer due to a partial recovery of instantaneous strains. This effect is more progressive at a young age of the composite loaded (cf. Fig. 4, b and e); c) The ageing coefficient between the layers is different and is mainly subjected by the history of the ageing process of the layers. However, at the age of loading exceeding 28 days, the difference in the ageing coefficient of the layers is insignificant (Fig. 4, a). Moreover, it should be emphasized that the ageing coefficients of the layers are coincident with each other for the case of the time invariant elastic properties of the layers (Fig. 4, d). d) For the most practical purposes requiring long creep periods (exceeding 360 days), an approximate value of the ageing coefficient of the layers equal to 0.8 should be recommended predicting the stress-strain state of the layered composite using the proposed analytical method. More sophisticated functions for the ageing coefficients proposed by Lacidogna and Tarantino (1996), Ghali et al. (2002), ACI 209R-92 may be implemented for the sake of analysis accuracy. e) The values of the creep coefficient for the whole composite are mainly within the values of the creep coefficients of the layers.

Rate Dependence and Short-Term Creep Behavior of a Thermoset Polymer at Elevated Temperature

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at 288°C. The effect of loading rate on monotonic stress-strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. The effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods on both loading and unloading paths. The strain-time response was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 288°C. The model was employed to predict the response of the material under monotonic loading/unloading and multistep load histories.

Effects of prior aging at 288°C in air and in argon environments on creep response of PMR‐15 neat resin

AbstractThe creep behavior of PMR‐15 neat resin, a polyimide thermoset polymer, aged in air and in argon environments at 288°C for up to 1000 h was evaluated. Creep tests were performed at 288°C at creep stress levels of 10 and 20 MPa. Creep periods of at least 25‐h in duration were followed by 50‐h periods of recovery at zero stress. Prior isothermal aging increased the elastic modulus and significantly decreased the polymer's capacity to accumulate creep strain. The aging environment had little influence on creep and recovery behaviors. However, aging in air dramatically degraded the tensile strength of the material. Dynamic mechanical analysis revealed an increase in the glass transition temperature from ∼330°C to ∼336°C after 1000 h in argon or in air at 288°C. The rise in the glass transition temperature with aging time is attributed to an increase in the crosslink density of the PMR‐15 polyimide. Increase in the crosslink density due to aging in both air and argon environments is likely behind the changes in the elastic modulus and the decreased capacity for inelastic straining. A visibly damaged surface layer of ∼0.16 mm thickness was observed in specimens aged in air for 1000 h. Results indicate that the unoxidized core material governs the overall mechanical response, whereas the oxidized surface layer causes a decrease in tensile strength by acting as a crack initiation site and promoting early failures. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Time-Dependent Behaviour and Static Liquefaction Phenomenon of Sand

Recent laboratory and field experiments have confirmed that sand does indeed exhibit time-dependent behaviour. Based on these findings, it was considered necessary to revisit some of the published experimental results on the static liquefaction phenomenon of loose anisotropic Hostun RF sand. Time-dependency might have had a significant influence on the observed undrained response of anisotropic consolidated sand specimens. Specific triaxial tests have been performed and a qualitative analysis is presented in this paper. It is shown that, despite the differences on the anisotropic consolidation path employed, different specimens show qualitatively identical undrained responses if creep periods are performed at identical test stages or if the anisotropic consolidation takes place very slowly. With time, the undrained stiffness and strength are considerably improved and this may explain why the static liquefaction phenomenon is not as common in practice as could be predicted based on an instability line concept. Whereas the original instability line concept was developed independently of time-dependency, in field situations, the liquefaction resistance of the sand can increase with time.

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

AbstractThe inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Viscous behaviour of dry sand

AbstractTests on air‐dried Hostun sand were performed on a newly developed triaxial apparatus, which allows static and dynamic measurements at ‘Département Génie Civil et Bâtiment’ (DGCB) of ENTPE. The prototype was designed to perform tests in order to investigate the viscous effects of sand from small strain (some 10−5m/m) up to intermediate strain (some 10−3m/m). The accuracy of the measuring system is ensured by the use of local non‐contact displacements transducers and by the use of an internal load cell. The viscous behaviour of sand is experimentally studied for loose and dense specimens for confining pressure values from 80 up to 400 kPa and considering different histories of loading. Three types of viscous loading: creep periods; stress relaxation periods; and paths with stepwise changes in the strain rate are specifically analysed. The proposed viscous modelling is issued from a three‐component general framework and is able to reproduce the experimental observations. More specifically, the peculiar behaviour of sand observed during a stepwise increase or decrease in the strain rate is detailed and modelled by the viscous evanescent (VE) model, developed at DGCB/ENTPE. Based on the three types of viscous tests, a simple relation is finally proposed for the viscous parameters of the VE model. Comparisons between data and simulations validate the approach. Copyright © 2007 John Wiley & Sons, Ltd.

Gas Diffusion in Polyolefin Foams during Creep Tests. Effect on Impact Behaviour and Recovery after Creep

A method to obtain the effective diffusion coefficient of the gas contained in closed cell polyolefin foams under static loading is presented. This property is obtained from pressure decrease with time using an analytical solution of the diffusion equation. The effect of density, type of base polymer, crosslinking and sample size on the diffusion coefficient is analysed.It has been shown that the impact behaviour of low density closed cell polyethylene based foams deteriorates after compressive creep periods and that this reduction of the cushion capabilities is directly related with the diffusion coefficient of the foams.Moreover, the recovery of the foams after creep showed a peculiar non-homogeneous behaviour, which has been analysed. Gas diffusion during creep is the main responsible for this particular behaviour.

Three Dimensional Creep Model for Wood Under Variable Humidity-Numerical Analyses at Different Material Scales

This paper presents a qualitative explanation of the creep phenomenon based on the physical and chemical mechanisms that occur at micro and ultra-structural levels of wood during moisture diffusion. This part is then completed by the formulation of a 3-dimensional non linear hydro-visco-elastic model, combined with hygro-expansion effects, and able to describe creep and recovery phenomena under variable humidity conditions. The constitutive relation is based on a generalised Maxwell model whose relaxation time functions depend on the moisture content rate, the history of accumulated moisture variations and the stress level. The model was implemented in a Finite Element (FE) program. Several applications based on external experimental tests and with creep periods ranging from 1 to 2735 days were carried out in order to prove the relevance of the approach at different structure scales. A local strain energy density criterion, associated with a flow law, allows the representation of the rupture phase initiated by the tertiary creep and is incorporated into the model in order to open up new horizons for the service life estimation of timber structures.

Anelastic creep behaviour of RR-58 aluminum alloy at 180 °C: Phenomenological aspects and analysis based on the unbowing of dislocation segments

The strain relaxation behaviour after full stress removals during creep of RR-58 Hiduminium alloy has been observed. Creep tests were carried out at 180 °C, with stresses of 120, 170 and 230 MPa. Tests involved a series of unloading cycles after different creep periods involving the primary and secondary creep stages. The results were interpreted according to a model based on the thermally activated unbowing of dislocation segments. Activation parameters were calculated and an interpretation of the phenomena is made in terms of microstructural details of the material. The appearance of the fast and slow anelastic creep stages may be just a consequence of the high non-linearity existing between the line tension and the area swept by the segments during their unbowing movement.

Heterogeneous blends of recycled poly(ethylene terephthalate) with impact modifiers: phase structure and tensile creep

AbstractSeveral types of photomicrographs have concurrently shown that impact modifiers (IMs) form particles (on a micrometre scale) which are evenly dispersed in a recycled poly(ethylene terephthalate) (PET) matrix; their adhesion to the latter is high enough that fracture surfaces (produced in liquid nitrogen) do not follow the interface. An essential part of the tensile creep of PET corresponds to the elastic time‐independent component; the time‐dependent component is rather limited, even if relatively high stresses are applied. Thus, the tensile compliance of PET is virtually independent of the applied stress, which indicates linear viscoelastic behaviour. The compliance of PET/IM blends (93/7, 90/10, 85/15 (by weight)) grows with the IM content and its time‐dependence becomes more visible. The effects of the two types of IM used in this study seem to be practically identical from the viewpoint of dimensional stability of the blends. The logarithm of compliance grows with the logarithm of time faster than linearly, and this tendency becomes more apparent with increasing fraction of IM. Even if the strain‐induced free volume is taken into account, a noticeable upswing of the compliance for longer creep periods (t > 1000 min) is evident. This cannot be interpreted as a consequence of the flow, because the recovery following the creep has proved the complete reversibility of the previous deformation. A simple empirical equation is proposed, which provides a plausible prediction of the creep behaviour of PET with dispersed impact modifiers. Copyright © 2004 Society of Chemical Industry